CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Patent Application No. 63/226,574, filed on July 28, 2021, and U.S. Provisional Patent Application No. 63/226,588, filed on July 28, 2021, each of which is hereby incorporated herein by reference in its entirety for all purposes.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No. 2036339 awarded by the National Science Foundation. The Government has certain rights in this invention.

TECHNICAL FIELD OF THE INVENTION

The present inventions relate to the technical field of resin compositions, methods for forming an article, and articles thereof.

BACKGROUND OF THE INVENTION

Three dimensional printing (3D printing), in particular resin-based 3D printing, allows rapid fabrication of computer-designed objects with complex shapes, high geometric accuracy, and high surface finish quality while requiring minimal or no tooling or subtractive processes such as drilling, tapping, cutting, grinding, etc. 3D printing is promising as a manufacturing technique for a wide range of industries, including consumer devices (smartphones, headphones), microelectronics components (chip interposers, interconnects, antennae, waveguides, etc.), and medical devices (micro fluidics, microneedles, dental appliances, drug delivery). Traditional 3D printing resin chemistries (acrylates, epoxies) lack materials properties (mechanical, thermal, electrical, etc.) suitable for such devices and components. Thus, there is a need for resins and hardenable compositions suitable for use in 3D printing, including 3D printing suitable for manufacturing and prototyping, methods including same, and products made therefrom.

BRIEF SUMMARY OF THE INVENTION

The present invention includes benzoxazine compounds, benzoxazine resins, hardenable resin compositions, methods including a hardenable resin composition for forming an article, and articles produced from such hardenable resin compositions, and or methods.

In accordance with one aspect of the invention, there is provided a benzoxazine compound represented by the following structure (I): wherein each of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ can be the same or different, and wherein each of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups.

As used herein, "Structure I" refers to above structure I.

For example, any of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ can independently comprise hydrogen, a substituted or unsubstituted straight-chain, branched, or cyclic aliphatic group having from 1 to 20 carbon atoms, substituted or unsubstituted aromatic groups, substituted or unsubstituted unsaturated hydrocarbons, an amine, or a halogen. Adjacent or nonadjacent groups in the set of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ may optionally connect to form cycles or macrocycles. Optionally one or more of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ can include a heteroatom, which can be the same or different as a heteroatom included in any other R group.

In accordance with another aspect of the present invention, there is provided a benzoxazine compound represented by the following structure (IX): wherein R ¹ and R ² can be the same or different, and wherein each of R ¹ and R ² is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups.

As used herein, "Structure IX" refers to above structure IX.

For example, R ¹ and R ² can independently comprise hydrogen, a substituted or unsubstituted straight-chain, branched, or cyclic aliphatic group having from 1 to 20 carbon atoms, substituted or unsubstituted aromatic groups, substituted or unsubstituted unsaturated hydrocarbons, an amine, or a halogen. Optionally R ¹ and R ² may connect to form a cycle or macrocycle. Optionally one or both of R ¹ and R ² can include a heteroatom, which can be the same or different as a heteroatom included in the other.

In accordance with another aspect of the present invention, there is provided a benzoxazine resin comprising a benzoxazine compound represented by the following structure (I): wherein each of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ can be the same or different, and wherein each of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups.

A benzoxazine resin can further include one or more photohardenable components. A benzoxazine resin can optionally further include one or more additives.

In accordance with another aspect of the present invention, there is provided a benzoxazine resin comprising a benzoxazine compound represented by the following structure (IX): wherein R ¹ and R ² can be the same or different, and wherein each of R ¹ and R ² is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups.

A benzoxazine resin can further include one or more photohardenable components. A benzoxazine resin can optionally further include one or more additives.

In accordance with another aspect of the present invention, there is provided a hardenable composition for forming a three-dimensional object, the composition comprising: (i) a first resin component that is photohardenable by exposure to light, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) a photoinitiator configured to create reactive species for initiating hardening of the first resin component included in the hardenable resin composition when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a benzoxazine resin including a benzoxazine compound represented by Structure I. In accordance with another aspect of the present invention, there is provided a hardenable composition for forming a three-dimensional object, the composition comprising: (i) a first resin component that is photohardenable by exposure to light, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) a photoinitiator configured to create reactive species for initiating hardening of the first resin component included in the hardenable resin composition when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a benzoxazine resin including a benzoxazine compound represented by Structure IX. A hardenable resin composition in accordance with one or more aspects of the present invention optionally further includes one or more of the following additives: a thixotrope; a stabilizer; a light absorbing pigment or dye; a non-reactive solvent diluent; a filler; a defoamer; an oxygen scavenger; a catalyst; and a thermal radical initiator. Other additives additionally or alternatively be included. In accordance with another aspect of the present invention, there is provided a method of forming an article, comprising: (a) providing a volume of a hardenable resin composition comprising a first resin component comprising a photohardenable components and a second resin component comprising a benzoxazine compound represented by Structure I or Structure IX; (b) forming an intermediate article from the hardenable resin composition, wherein formation of the intermediate article comprises: (i) irradiating one or more locations in the hardenable resin composition with excitation light at one or more selected wavelengths for initiating local hardening of the hardenable resin composition at the one or more selected locations in the volume of the hardenable resin composition; and (ii) optionally repeating step (i) at one or more of the same or different selected locations within the volume of the hardenable resin composition until the intermediate article is formed; and (c) hardening (e.g., further reacting, further polymerizing, further chain extending) the second resin component in the intermediate article by a thermally driven reaction or mechanism to form the article. In accordance with another aspect of the present invention, there is provided a hardenable resin composition useful for forming an article, the composition comprising: (i) a first resin component that is photohardenable by exposure to light in the presence of a photoinitiator, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) the photoinitiator configured to create reactive species for initiating hardening of the first resin component when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) an upconverting component configured to upconvert excitation light at one or more wavelengths in a first range of wavelengths to shorter wavelength light at one or more wavelengths in a second range of wavelengths, wherein the upconverted light includes radiation within the range of wavelengths absorbed by the photoinitiator; and (iv) a second resin component that is hardenable by a thermally driven reaction or mechanism, a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing. Preferably the second resin component comprises a benzoxazine resin in accordance with one or more aspects of the present invention. In accordance with yet another aspect of the present invention, there is provided a method of forming an article, the method comprising: (a) providing a volume of a hardenable resin composition, the composition comprising: (i) a first resin component that is hardenable by exposure to light in the presence of a photoinitiator, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) the photoinitiator configured to create reactive species for initiating hardening of the first resin component included in the hardenable resin composition when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) an upconverting component configured to upconvert excitation light at one or more wavelengths in a first range of wavelengths to shorter wavelength light at one or more wavelengths in a second range of wavelengths, wherein the upconverted light includes radiation within the range of wavelengths absorbed by the photoinitiator; and (iv) a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (b) forming an intermediate article from the hardenable resin, wherein formation of the intermediate article comprises: (i) irradiating one or more locations in the hardenable resin composition with excitation light at one or more wavelengths in the first range of wavelengths for upconversion by the upconverting component and initiating local hardening of the first resin component included in the hardenable resin composition at the one or more selected locations in the volume of the hardenable resin composition; and (ii) optionally repeating step (i) at one or more of the same or a different selected locations within the volume of the hardenable resin composition until the intermediate article is formed; and (c) subjecting the intermediate article to a thermally driven reaction or mechanism to further harden the hardenable resin composition and form the article. Preferably the second resin component comprises a benzoxazine resin in accordance with one or more aspects of the present invention. In accordance with additional aspects of the present invention, there are provided articles formed from hardenable resin formulations and/or a methods in accordance with aspects of the present invention. The terms article and object are used interchangeably herein. It should be appreciated by those persons having ordinary skill in the art(s) to which the present invention relates that any of the features described herein in respect of any particular aspect and/or embodiment of the present invention can be combined with one or more of any of the other features of any other aspects and/or embodiments of the present invention described herein, with modifications as appropriate to ensure compatibility of the combinations. Such combinations are considered to be part of the present invention contemplated by this disclosure. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Other embodiments will be apparent to those skilled in the art from consideration of the description, from the claims, and from practice of the invention disclosed herein. DETAILED DESCRIPTION OF THE INVENTION Various aspects and embodiments of the present invention will be further described in the following detailed description. Benzoxazine resins have numerous uses as high performance thermosetting resins and exhibit properties including high glass transition temperature (Tg), high thermal stability, intrinsically low flammability, high char yield, low shrinkage upon cure, low thermal expansion, low moisture uptake, excellent dielectric properties, and wide molecular design flexibility. Such properties make benzoxazines attractive for use in 3D printing resins for manufacturing applications. Benzoxazine monomers are generally produced from the reaction of phenols, primary amines, and an aldehyde (typically formaldehyde). Various disclosures regarding benzoxazine monomers, polymers and compositions are set forth in the following references: U.S. Pat. Nos. 5,152,939; 5,266,695; 5,543,516; 5,900,447; 5,973,144; 6,160,079; 6,207,786; 6,225,440; 6,323,270; and 7,947,802. However, typical benzoxazine monomers, which are based on aromatic amines and phenols or polyphenols, are not ideal for incorporation into 3D printing resins due to poor solubility in acrylate monomers (which are often aliphatic), tendency to crystallize out of solution, and strongly colored impurities (due to oxidized aromatic groups) which absorb light used for photohardening of the resin. For example, P-d Type Benzoxazine available from Shikoku, which is a benzoxazine monomer based on phenol and diaminodiphenylmethane, is a yellow powder in commercially supplied form. The yellow color derives from impurities present in the commercial material, as evidenced by the isolation of the majority component benzoxazine monomer as white crystals after column purification and solvent evaporation. The impure material dissolves in acrylate monomers (e.g., N,N-dimethylacrylamide, tricyclodecanedimethanol diacrylate), but the impurities dramatically reduce the reactivity of the resin towards photopolymerization (inhibitory effect) and render it poorly suitable for 3D printing. On the other hand, the purified material is poorly soluble to insoluble in acrylate monomers, and when some dissolution is obtained by heating (e.g., to 90 deg C), precipitation of benzoxazine occurs immediately upon cooling. Thus, there is need for benzoxazine monomers which have low color, low inhibitory effect, high solubility, and low tendency to precipitate for use in 3D printing resins. A benzoxazine compound of the invention may also be referred to herein as a benzoxazine monomer. In accordance with one aspect of the present invention there is provided a class of benzoxazine compounds represented by structure (I): wherein each of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups. Non-limiting examples of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ include by way of example and without limitation hydrogen, a substituted or unsubstituted straight-chain, branched, or cyclic aliphatic group having from 1 to 20 carbon atoms, substituted or unsubstituted aromatic groups, substituted or unsubstituted unsaturated hydrocarbons, an amine, or a halogen. Optionally, a substituent group can include a heteroatom. Benzoxazine resins including a benzoxazine compound represented by any of Structures I- XI may be pure or substantially pure compounds or may comprise mixtures of, e.g., monomer, oligomers, and isomers. Benzoxazine compounds in the class represented by Structure I are preferably synthesized from one or more monophenols, bicyclo[2.2.1]heptanebis(methylamine)norbornane (BAMN), preferably as a mixture of isomers; and a formaldehyde source which is preferably formalin or paraformaldehyde. The monophenols contribute radicals (R ¹ through R ⁸ ) which may comprise any atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups. Two or more radicals may connect to form, e.g., cyclic, polycyclic, macrocyclic, or heterocyclic moieties. It has been found that bicyclo[2.2.1]heptanebis(methylamine), also referred to as bis(aminomethyl)norbornane (BAMN), is a difunctional primary amine which, in combination with one or more monophenolic species and formaldehyde, can be used to produce benzoxazine monomers with unique properties, including low color, low inhibitory effect in 3D printing resins, low melting/solidification temperatures, high solubility in acrylate monomers, and low tendency to precipitate or crystallize from resins formulated with acrylate monomers. BAMN, in commercially available form as a mixture of isomers, is a colorless, odorless, low volatility, low toxicity, low viscosity liquid at room temperature, which enables easy and safe handling and processing. Due to the highly reactive primary aliphatic amines on BAMN, benzoxazine monomers may be synthesized quickly and in high yields (typically >75%) from a wide range of monophenols. When cured, 3D printing resins containing benzoxazine monomers based on BAMN can exhibit excellent properties including high glass transition temperature (Tg) and high degradation temperature which meet or exceed the performance of typical benzoxazine resins (which are not 3D printable) and significantly exceed those of typical 3D printing materials based on acrylates or epoxies. In particular, the bulky norbornane group contributed by BAMN contributes desirable properties to the 3D printing resins including high Tg, high rigidity, high toughness, low moisture uptake, and low color. Additionally, 3D printing resins including benzoxazine monomers based on BAMN show sufficient reactivity for printing, in contrast to commercial benzoxazine monomers such as P-d. Examples of monophenols used to synthesize benzoxazine compounds in the class represented by Structure I include, but are not limited to, phenol, para-cresol, 4-(tert-butyl)-phenol, 4-cyclohexylphenol, 5,6,7,8-tetrahydro-2-naphthol, 2-cyanophenol, 3-cyanophenol, and 4- cyanophenol. Mixtures of different monophenols may be used in the same synthesis to produce a distribution of differently substituted benzoxazines. Particularly preferred benzoxazine compounds in this class include: phenol/BAMN benzoxazine monomer (P-nb type benzoxazine, Structure II (wherein each of R ¹ -R ⁸ is hydrogen)), para-cresol/BAMN benzoxazine monomer (pC-nb type benzoxazine, Structure III (wherein each of R ² and R ⁷ is a methyl group and each of R ¹ , R ³ -R ⁶ , and R ⁸ is hydrogen)), and 5,6,7,8-tetrahydro-2- naphthol/BAMN benzoxazine monomer (THN-nb type benzoxazine, Structure IV (wherein R ² and R ³ combine to form a tetrahydrobenzo group, R ⁶ and R ⁷ combine to form a tetrahydrobenzo group, and each of R ¹ , R ⁴ , R ⁵ , and R ⁸ is hydrogen). Structures II-IV are shown below: Structure I I.(Representative structure of P-nb type benzoxazine). As used herein, "Structure II" refers to above structure II. Structure III (Representative structure of pC-nb type benzoxazine). As used herein, "Structure III" refers to above structure III. Structure IV (Representative structure of THN-nb type benzoxazine). As used herein, "Structure IV" refers to above structure IV. Benzoxazine compounds represented by the following structures may also be useful for inclusion in 3D printing applications: Structure V (4-(t-Butyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer (tBP-nb Benzoxazine)). As used herein, "Structure V" refers to above structure V. Structure VI (4-Cyclohexylphenol/bis(aminomethyl)norborane benzoxazine monomer (CyP-nb Benzoxazine)). As used herein, "Structure VI" refers to above structure VI. S tructure VII (4-Adamantylphenol/bis(aminomethyl)norbornane benzoxazine monomer (AdmP-nb Benzoxazine)). As used herein, "Structure VII" refers to above structure VII. Structure VIII (4-(Maleimidomethyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer (MMP-nb Benzoxazine)). As used herein, "Structure VIII" refers to above structure VIII. In accordance with another aspect of the present invention there is provided a second class of benzoxazine compounds represented by structure IX below. Structure IX (General structure of Bisphenol AF/monoamine type benzoxazine monomer) In structure IX, R ¹ and R ² can be the same or different, and wherein each of R ¹ and R ² is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups. Examples of benzoxazine compounds of this class include: benzoxazine monomer of the general structure IX with radicals R ¹ and R ² comprising 1-adamanylamine (BAF-adm, Structure X); and benzoxazine monomer of the general structure in Structure IX with radicals R ¹ and R ² comprising 3-maleimidomethyl-3,5,5-trimethylcyclohexyl moieties (BAF-mi, Structure XI). Structure X (Bisphenol AF/1-adamantylamine benzoxazine monomer (BAF-adm)). As used herein, "Structure X" refers to above structure X. Structure XI (Bisphenol AF/1-[(5-amino-1,3,3-trimethylcyclohexyl)methyl]-1H-pyrrole- 2,5-dione benzoxazine monomer (BAF-mi)). As used herein, "Structure XI" refers to above structure XI. Benzoxazine compounds described herein may be synthesized by methods including commonly used techniques, including homogeneous solvent, heterogenous solvent, high solids, and solvent free methods. For the homogeneous solvent synthesis of the class of benzoxazine resin including a benzoxazine compound represented by structure (I), due to the difunctionality of bis(aminomethyl)norbornane, formation of an insoluble perhydrotriazine network occurs if homogeneous solvent phase synthesis is used, e.g., dioxane solution, which causes a reduction in reaction rate due to reduction in reagent diffusion; furthermore, degradation of the perhydrotriazine network may take hours. Similarly, in the solvent free synthesis of the same, the perhydrotriazine network prevents liquification of the reagent mixture which leads to poor heat transfer and the development of hot spots, which may result in thermal degradation or side reactions. The heterogeneous solvent method is preferred for synthesis of the class of benzoxazine resin including a benzoxazine compound represented by structure (I), because it avoids formation of a persistent perhydrotriazine network. For example, for a reaction media of 10:1 toluene:formalin solution at 65 deg C, with the amine and phenolic species dissolved in the toluene phase, transient gelation from formation of perhydrotriazine upon combination of the reagents lasts only seconds to minutes under rapid stirring before dissipating into a suspension of water in toluene, with the product formation occurring in the toluene phase. For benzoxazine compounds represented by Structure IX, any of the above methods would be suitable. For example, such compounds can be synthesized, for example, from 4,4'- (hexafluoroisopropylidene)diphenol (also known as Bisphenol AF); a monoamine; and a formaldehyde source such as formaldehyde gas, formalin, or paraformaldehyde. The monoamine species contribute radicals (R ¹ and R ² ) which may comprise any atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, unsaturated hydrocarbons, or aromatic groups. Optionally R ¹ and R ² may connect to form a cycle or macrocycle. Preferably the radicals are alkyl groups or derivatives thereof. The benzoxazine compounds represented by structure I and benzoxazine compounds represented by structure IX can reduce or avoid the above-mentioned challenges of coloration and reduced solubility. Further reduction of the level of colored impurities in the synthesized benzoxazine monomers is preferably conducted by washing an organic solution of the monomer (either from the reaction mixture or after solvent removal and redissolution) with strong base, such as 1 N sodium hydroxide aqueous solution. This treatment is believed to deprotonate any remaining phenolic species (such as starting material or partially reacted species) and draw them into the aqueous phase, where they can be separated as a yellow aqueous solution. Typically multiple base washing steps are used, followed by subsequent washes with brine and distilled water. The organic phase containing the product may then be dried (e.g., over sodium sulfate) and the solvent may be removed by rotary evaporation. If desired, impurity levels may be further reduced by column chromatography, as the colored impurities tend to absorb strongly to silica. The physical form of the solvent free benzoxazine monomers ranges from semisolids to glasses at room temperature. For monomers which are glasses, the material may be pulverized into a powder for handling. For monomers which are semisolid, it is useful to dissolve the benzoxazine monomer in an acrylate monomer (e.g., at loadings of 30-60 wt%) to make a pourable liquid for handling. Elevated temperature (e.g., 50-100 deg C) or volatile solvent (e.g., dichloromethane) may be used to assist in making the monomer solution. A particularly useful acrylate is SR228 (also known as PRO13443, available from Sartomer) which is a blend of tricyclodecanedimethanol diacrylate and tris(2- hydroxyethyl)isocyanaturate triacrylate. If a volatile solvent is added to ease dissolution in acrylate monomer, then the solvent is removed afterwards, e.g., by rotary evaporation, to yield an essentially solvent-free monomer blend. When removing solvent under vacuum, it is helpful to include a small amount of inhibitor (e.g., 2 ppm TEMPO free radical) to prevent autopolymerization of acrylate under anoxic conditions (such as rotary evaporation). In practice, a benzoxazine compound may, for example, be a pure or substantially pure benzoxazine compound. In practice, a benzoxazine compound may, for example, include a benzoxazine compound with associated oligomers and/or isomers produced in the synthesis of the compound. In accordance with another aspect of the present invention, there is provided a benzoxazine resin comprising a benzoxazine compound represented by the following structure (I): wherein e ach of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ can be the same or different, and wherein each of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups. Non-limiting examples of R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ include by way of example and without limitation hydrogen, a substituted or unsubstituted straight-chain, branched, or cyclic aliphatic group having from 1 to 20 carbon atoms, substituted or unsubstituted aromatic groups, substituted or unsubstituted unsaturated hydrocarbons, an amine, or a halogen. Optionally, a substituent group can include a heteroatom. Preferred benzoxazine resins including a benzoxazine compound in the class of compounds represented by Structure I include, but are not limited to, benzoxazine resins including a phenol/BAMN benzoxazine monomer (a P-nb type benzoxazine represented by Structure II), benzoxazine resins that include a para-cresol/BAMN benzoxazine monomer (a pC-nb type benzoxazine represented by Structure III), and benzoxazine resin including a 5,6,7,8-tetrahydro-2- naphthol/BAMN benzoxazine monomer (a THN-nb type benzoxazine represented by Structure IV). Other benzoxazine resins that include a benzoxazine compound in the class of compounds represented by Structure I that may be useful for 3D printing include, but are not limited to, benzoxazine resins including a 4-(t-butyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer (a tBP-nb type benzoxazine represented by Structure V), benzoxazine resins including a 4-cyclohexylphenol/bis(aminomethyl)norborane benzoxazine monomer (a CyP-nb type benzoxazine represented by Structure VI)), benzoxazine resins including a 4- adamantylphenol/bis(aminomethyl)norbornane benzoxazine monomer (a AdmP-nb type benzoxazine represented by Structure VII), and benzoxazine resin including a 4- (maleimidomethyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer ( an MMP-nb type benzoxazine represented by Structure VIII). In accordance with another aspect of the present invention, there is provided a benzoxazine resin comprising a benzoxazine compound represented by the following structure (IX): wherein R ¹ and R ² can be the same or different, and wherein each of R ¹ and R ² is an atom, functional group, or moiety including, e.g., hydrogen, heteroatoms, alkyl groups, or aromatic groups. Preferred benzoxazine resins including a benzoxazine compound in this class of compounds represented by Structure IX(a general structure of a Bisphenol AF/monoamine type benzoxazine monomer) include benzoxazine resins that include a benzoxazine monomer of the general structure IX with radicals R ¹ and R ² comprising 1-adamanylamine (BAF-adm type benzoxazine, Structure X); and benzoxazine resins that include a benzoxazine monomer of the general structure in Structure IX with radicals R ¹ and R ² comprising 3-maleimidomethyl-3,5,5-trimethylcyclohexyl moieties (BAF- mi type benzoxazine, Structure XI). In practice, a benzoxazine compound included in a benzoxazine resin in accordance with the present invention may be pure or substantially pure benzoxazine monomers. A benzoxazine resin may also include oligomers and/or isomers produced in the synthesis of the benzoxazine compound. A benzoxazine resin in accordance with the invention can optionally comprise a mixture of two or more benzoxazine resins that include different benzoxazine compounds. A benzoxazine resin including a benzoxazine compound in accordance with one or more aspects of the present invention can further include a photohardenable component. Additional information concerning photohardenable components is provided below. A benzoxazine resin including a benzoxazine compound in accordance with one or more aspects of the present invention can further include a photoinitiator. Additional information concerning photoinitiators is provided below. A benzoxazine resin including a benzoxazine compound in accordance with one or more aspects of the present invention can optionally further include an upconverting component. Additional information concerning upconverting components is provided below. A benzoxazine resin including a benzoxazine compound in accordance with one or more aspects of the present invention can optionally further include one or more additives. Examples of such additives include, but are not limited to, a thixotrope; a stabilizer; a light blocker comprising a light absorbing pigment or dye; a non-reactive solvent diluent; a filler; a defoamer; an oxygen scavenger; a catalyst; and a thermal radical initiator. Other additives may alternatively or additionally be included. Additional information concerning additives that can optionally be included in hardenable resin compositions and methods in accordance with various aspects and embodiments of the present invention is provided below. In accordance with another aspect of the present invention, there is provided a hardenable composition for forming a three-dimensional object, the composition comprising: (i) a first resin component that is photohardenable by exposure to light, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) a photoinitiator configured to create reactive species for initiating hardening of the first resin component included in the hardenable resin composition when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a benzoxazine resin including a benzoxazine compound represented by Structures I. Preferred benzoxazine resins for inclusion in the second resin component including a benzoxazine compound in the class of compounds represented by Structure I include, but are not limited to, benzoxazine resins including a phenol/BAMN benzoxazine monomer (a P-nb type benzoxazine represented by Structure II), benzoxazine resins that include a para-cresol/BAMN benzoxazine monomer (a pC-nb type benzoxazine represented by Structure III), and benzoxazine resin including a 5,6,7,8-tetrahydro-2-naphthol/BAMN benzoxazine monomer (a THN-nb type benzoxazine represented by Structure IV). Other benzoxazine resins that include a benzoxazine compound in the class of compounds represented by Structure I that may be useful for inclusion in the second resin component include, but are not limited to, benzoxazine resins including a 4-(t-butyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer (a tBP-nb type benzoxazine represented by Structure V), benzoxazine resins including a 4-cyclohexylphenol/bis(aminomethyl)norborane benzoxazine monomer (a CyP-nb type benzoxazine represented by Structure VI)), benzoxazine resins including a 4- adamantylphenol/bis(aminomethyl)norbornane benzoxazine monomer (an AdmP-nb type benzoxazine represented by Structure VII)), and benzoxazine resin including a 4- (maleimidomethyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer ( a MMP-nb type benzoxazine represented by Structure VIII). A second resin component can optionally further include one or more other resin hardenable by a thermally driven reaction or mechanism. A second resin component can optionally further include two or more benzoxazine resins of the invention that include different benzoxazine compounds. A first resin can optionally include more than one first resin component that is photohardenable by exposure to light. A hardenable resin composition optionally can further include an upconverting component. Additional information concerning upconverting components is provided below. A hardenable resin composition optionally can further include one or more of the following additives: a thixotrope; a stabilizer; a light absorbing pigment or dye; a non-reactive solvent diluent; a filler; a defoamer; an oxygen scavenger; a catalyst; and a thermal radical initiator. Other additives additionally or alternatively be included. In accordance with another aspect of the present invention, there is provided a hardenable composition for forming a three-dimensional object, the composition comprising: (i) a first resin component that is photohardenable by exposure to light, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) a photoinitiator configured to create reactive species for initiating hardening of the first resin component included in the hardenable resin composition when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a benzoxazine resin including a benzoxazine compound represented by Structures IX. Preferred benzoxazine resins for inclusion in the second resin component including a benzoxazine compound in the class of compounds represented by Structure IX include benzoxazine resins that include a benzoxazine monomer of the general structure IX with radicals R ¹ and R ² comprising 1-adamanylamine (a BAF-adm type benzoxazine, Structure X); and benzoxazine resins that include a benzoxazine monomer of the general structure in Structure. IX with radicals R ¹ and R ² comprising 3-maleimidomethyl-3,5,5-trimethylcyclohexyl moieties (a BAF-mi type benzoxazine, Structure XI). A second resin component can optionally further include one or more other resin hardenable by a thermally driven reaction or mechanism. A second resin component can optionally further include two or more benzoxazine resins of the invention that include different benzoxazine compounds. A first resin can optionally include more than one first resin component that is photohardenable by exposure to light. A hardenable resin composition optionally can further include an upconverting component. Additional information concerning upconverting components is provided below. A hardenable resin composition optionally can further include one or more of the following additives: a thixotrope; a stabilizer; a light absorbing pigment or dye; a non-reactive solvent diluent; a filler; a defoamer; an oxygen scavenger; a catalyst; and a thermal radical initiator. Other additives additionally or alternatively be included. A hardenable resin composition in accordance with one or more aspects of the present invention optionally further includes one or more of the following additives: a thixotrope; a stabilizer; a light absorbing pigment or dye; a non-reactive solvent diluent; a filler; a defoamer; an oxygen scavenger; a catalyst; and a thermal radical initiator. Other additives additionally or alternatively be included. In accordance with another aspect of the present invention, there is provided a hardenable resin composition useful for forming an article, the composition comprising: (i) a first resin component that is photohardenable by exposure to light in the presence of a photoinitiator, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) the photoinitiator configured to create reactive species for initiating hardening of the first resin component when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) an upconverting component configured to upconvert excitation light at one or more wavelengths in a first range of wavelengths to shorter wavelength light at one or more wavelengths in a second range of wavelengths, wherein the upconverted light includes radiation within the range of wavelengths absorbed by the photoinitiator; and (iv) a second resin component that is hardenable by a thermally driven reaction or mechanism, a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing. A second resin component can optionally further include one or more other resin hardenable by a thermally driven reaction or mechanism. A first resin can optionally include more than one first resin component that is photohardenable by exposure to light. A hardenable resin composition can optionally further include any one or more of a thixotrope; a stabilizer; a light blocker comprising a light absorbing pigment or dye; a non-reactive solvent diluent; a filler; a defoamer; an oxygen scavenger, a catalyst, a thermal radical initiator. Other additives may alternatively or additionally be included. Information concerning a first resin component, photohardenable components, photoinitiators, and optional additives is provided below. Examples of suitable second resin components include, but are not limited to, polyurethane, polyurethane-urea, and polyurea precursors; epoxy resins and epoxy curing agents; cyanate ester resins and phthalonitrile resins; maleimide resins such as bismaleimide resins, alone or with allyl curing agents; polyimide and polyamide imide precursors including but not limited to polyamic acids (e.g. poly(pyromellitic dianhydride-co-4,4'-oxydianiline) amic acid), polyamide amic acids (e.g. Torlon AI-30 and Torlon AI-50 available from Solvay), amines, acid anhydrides, and isocyanates; norbornene resins such as nadic anhydride-terminated resins; phenolic resins; and benzoxazine resins. Polyurethane, polyurethane-urea, and polyurea precursors undergo thermally accelerated polymerization via addition of isocyanates to amines and alcohols to form urethane and urea groups, respectively. Other thermally driven reactions include addition of isocyanates to urethane and urea groups to form allophanate and biuret crosslinks, respectively. Extremely tough materials may be produced from polyurethane, polyurethane-urea, and polyurea precursors, due to hydrogen bonding and microscale phase separation, but their temperature resistance is relatively poor. Isocyanates also react with atmospheric moisture and moisture dissolved in or absorbed to raw materials to form urea crosslinks, causing liquid hardenable resin compositions containing isocyanates to advance in viscosity and eventually harden unless kept scrupulously dry. Further, the reaction of isocyanates with moisture, either from the atmosphere or dissolved in or absorbed to raw materials, produces carbon dioxide gas which may result in the deleterious formation of voids or cracks in the article upon hardening. Also, polymerization and crosslinking reactions occur readily at room temperature, greatly limiting pot life when polyamines or polyols are incorporated along with isocyanates into hardenable resin compositions (commonly minutes and as short as <1 min). International Publication No. WO 2015/200201 A1 describes polyurethane, polyurea, and polyurethane-urea copolymers containing a blocked or reactive blocked prepolymer, a blocked or reactive blocked diisocyanate, or a blocked or reactive blocked diisocyanate chain extender. Blocked isocyanates allow for a longer pot life, up to 48 hrs or more, but introduce other limitations such as the plasticizing effect of the blocking agent, low level of cure due to difficulty in evaporating the blocking agent from the bulk material, and high viscosity due to the strong hydrogen bonding of urethane or urea functional groups. Insoluble polyamines or wax-encapsulated chain extenders, such as described in U.S. Patent Nos.10,316,213 and 10,793,745 of Arndt, et al., may allow for even better shelf stability, but these have the disadvantage of making the hardenable resin composition opaque, due to the size of the particles which is typically >1 μm, which impedes or prohibits volumetric 3D printing. Examples of polyurethane, polyurethane-urea, and polyurea precursors include but are not limited to monomeric or oligomeric polyisocyanates, blocked monomeric or oligomeric polyisocyanates, isocyanate prepolymers, blocked isocyanate prepolymers, isocyanate- functional acrylates and methacrylates, aliphatic polyamines, cycloaliphatic polyamines, aryl polyamines, polyetheramines, blocked polyamines such as diaminodiphenylmethane-sodium chloride complex, insoluble polyamines such as 4,4’-diaminodiphenylsulfone, dicyandiamide, diols, triols, sucrose, saccarides, polyols, polyether polyols, polyester polyols, polycaprolactone polyols, polycarbonate polyols, polybutadiene polyols, hydrogenated polybutadiene polyols, and acrylic polyols. Epoxy resins undergo thermally accelerated ring opening reactions with suitable curing agents or catalysts to form crosslinks. Epoxy resins also co-react in a thermally accelerated manner with numerous other types of resins, e.g., isocyanates and blocked isocyanates, phenolic resins, cyanate ester resins, and benzoxazine resins. However, epoxy resins do not readily cure without the addition of a curing agent, catalyst, or co-curable resin and have the further disadvantage of reacting with curing agents at room temperature, reducing pot life. Insoluble curing agents such as dicyandiamide or wax-encapsulated curing agents, as described in U.S. Patent Nos.10,316,213 and 10,793,745 of Arndt, et al., may be used to extend pot life almost indefinitely, but these have the disadvantage of making the hardenable resin composition opaque, due to the size of the particles which is typically >1 μm, which impedes or prohibits volumetric 3D printing. Examples of epoxy resins include but are not limited to bisphenol A diglycidyl ether, bisphenol A based epoxy resins, aliphatic epoxy resins, aryl epoxy resins, cycloaliphatic epoxy resins, bisphenol F based epoxy resins, dicyclopentadiene based epoxy resin, epoxy phenol novolacs, epoxy cresol novolacs, glycidyl amine based epoxy resins, triglycidyl isocyanurate, epoxide-functional acrylates and methacrylates, brominated epoxy resins, polythiol epoxy resins, and polysulfide epoxy resins. Examples of epoxy curing agents include but are not limited to aliphatic polyamines, aryl polyamines, cycloaliphatic polyamines, acid anhydrides, dicyandiamide, imidazoles, insoluble powdered polyamines such as 4,4’-diaminodiphenylsulfone, blocked polyamines such as diaminodiphenylmethane-sodium chloride complex, thiols, disulfides, latent acids, and transition metal complexes. Cyanate ester and phthalonitrile resins undergo thermally driven cyclotrimerization of aryl cyanate groups to form triazine crosslinks. Additionally, cyanate ester and phthalonitrile resins may co-react with epoxy resins, with which they may be blended to modify viscosity or performance properties, to form triazine, isocyanurate, and oxazolidinone crosslinks. These crosslinking reactions impart superior mechanical, thermal, and dielectrical properties relative to the photocured material prior to post curing. Cyanate esters have the disadvantage of self-reacting slowly at room temperature; as a result, cyanate ester resins are typically stored at 2-8°C. Examples of cyanate ester resins include but are not limited to: bisphenol A dicyanate, bisphenol E dicyanate, bisphenol M dicyanate, novolac cyanates; AroCy L-10, AroCy XU 366, AroCy XU 371, AroCy 378 available from Huntsman Corp; Primaset BA-200, Primaset BA-230 S, Primaset BA-3000, Primaset BA-3000 S, Primaset BTP-6020 S, Primaset CE-320, Primaset DT-4000, Primaset DT-7000, Primaset HTL- 300, Primaset LECy, Primaset LVT-50, Primaset PT-15, Primaset PT-30, Primaset PT-30 S, Primaset PT-60, Primaset PT-60 S, and Primaset ULL-950S available from Lonza. Maleimide resins such as bismaleimides undergo thermally driven free radical polymerization of the maleimide double bond as well as thermally driven ene and Diels-Alder addition reactions of the maleimide double bond with suitable allyl curing agents to form additional crosslinks. Maleimide resins with two or more maleimide functionalities can be more desirable for cross-linking. Maleimide resins containing no allyl curing agents are stable indefinitely at room temperature, and yet may still be polymerized at elevated temperatures, a considerable advantage relative to cyanate ester resins. However, incorporation of allyl curing agents reduces shelf stability considerably, where a continual advancement of viscosity over time is observed. A further disadvantage of maleimide resins is poor solubility in monomers and oligomers useful for hardenable resin compositions. Examples of maleimide resins include but are not limited to: 1,1'- (methylenedi-4,1-phenylene)bismaleimide, 2,2'-bis[4-(4-maleimidophenoxy)phenyl]propane, N,N'- (1,3-phenylene)dimaleimide, N,N'-(1,4-phenylene)dimaleimide; Matrimid 5292-A available from Huntsman Corp.; Homide 116, Homide 117, Homide 121, Homide 122, Homide 123, Homide 125 available from HOS-Technik Vertriebs- und Produktions-GmbH; Compimide MDAB, Compimide TDAB, Compimide 353 A, Compimide 796, Compimide P500, Compimide 200, Compimide 1206R55, Compimide 1224L60, Compimide 1251RH60, Compimide 353RTM, Compimide 50RTM, and Compimide 50LM available from Evonik. Examples of allyl curing agents suitable for use with maleimides include but are not limited to: 2,2'-diallylbisphenol A and 4,4'-bis(o- propenylphenoxy)benzophenone; Compimide TM123, and Compimide TM124 available from Evonik. Benzoxazine resins undergo thermally accelerated ring opening polymerization to form crosslinked networks. Benzoxazine resins are stable indefinitely at room temperature, can endure short excursions to 100°C or more without curing, and possess good miscibility with a wide range of monomers, oligomers, pre-polymers, and polymers. Benzoxazine resins have numerous exemplary properties, including near-zero volume shrinkage upon thermal curing, low water absorption (<2%, compared to 3-20% for phenolic and epoxy resins), high glass transition temperatures (160 °C to 400 °C), fast cure and development of properties at elevated temperatures, and very high char yields [Ishida, H. & Agag, T. “Handbook of Benzoxazine Resins” Elsevier, B. V. (Amsterdam, The Netherlands), 2011]. Benzoxazine resins are often classified by the phenolic compounds and amines from which they are synthesized by co-reaction with formaldehyde. Examples of benzoxazine resins include, but are not limited to: 4-hydroxybenzylalcohol/p-toluidine benzoxazine monomer, phenol/aminophenyl propargylether benzoxazine monomer, p-cresol/aminophenyl propargylether benzoxazine monomer, hydroxyphenylmaleimide/allylamine benzoxazine monomer, primary amine functional phenol/aniline benzoxazine monomer, amino-functional p-cresol/aniline benzoxazine monomer, tetrachlorophthalimido-functional phenol/aniline benzoxazine monomer, phenol/aminophenyltriethoxy silane benzoxazine monomer, allyl phenol/aniline benzoxazine monomer, 2-naphthol/aniline benzoxazine monomer, 2-naphthal/allylamine benzoxazine monomer, 1-naphthol/allylamine benzoxazine monomer, bisphenol A/aniline benzoxazine monomer, bisphenol A/allylamine benzoxazine monomer, bisphenol A/aminophenyl propargylether benzoxazine monomer, 1,5-dihydroxynaphthalene/aniline benzoxazine monomer, 1,5- dihydroxynaphthalene/allylamine benzoxazine monomer, 1,5-dihydroxynaphthalene/aminophenyl propargylether benzoxazine monomer, N-trifluoroacetyl-protected amino containing benzoxazine monomer, primary amino-functional benzoxazine monomer, amide-containing benzoxazine monomer, phenol/diaminodiphenylmethane benzoxazine monomer, phenol/oxydianiline benzoxazine monomer, phenol/diaminodiphenylsulfone benzoxazine monomer, amide-containing benzoxazine monomer, triazol-containing benzoxazine monomer, hydroxyphenylmaleimide/diaminodiphenylsulfone benzoxazine monomer, hydroxyphenylmaleimide/oxydianiline benzoxazine monomer, hydroxyphenylmaleimide/4,4’- naphthalene-2,7-diylbis(oxy)dianiline benzoxazine monomer, urethane-containing benzoxazine monomer, 4-hydroxybenzyl alcohol/diaminodiphenylmethane benzoxazine monomer, 4,4’- oxydiphenol/aniline benzoxazine monomer, 4,4’-oxydiphenol/allylamine benzoxazine monomer, 4,4’-oxydiphenol/aminophenyl propargylether benzoxazine monomer, 4,4’-thiodiphenol/aniline benzoxazine monomer, 4,4’-thiodiphenol/allylamine benzoxazine monomer, 4,4’- thiodiphenol/aminophenyl propargylether benzoxazine monomer, bisphenol S/aniline benzoxazine monomer, bisphenol S/allylamine benzoxazine monomer, bisphenol S/aminophenyl propargylether benzoxazine monomer, 4,4’-(Hexafluoroisopropylidene)diphenol/aniline benzoxazine monomer, 4,4’-(Hexafluoroisopropylidene)diphenol/allylamine benzoxazine monomer, 4,4’- (Hexafluoroisopropylidene)diphenol/aminophenyl propargylether benzoxazine monomer, bisphenol B/aniline benzoxazine monomer, bisphenol B/allylamine benzoxazine monomer, bisphenol B/aminophenyl propargylether benzoxazine monomer, bisphenol C/aniline benzoxazine monomer, bisphenol C/allylamine benzoxazine monomer, bisphenol C/aminophenyl propargylether benzoxazine monomer, bisphenol E/aniline benzoxazine monomer, bisphenol E/allylamine benzoxazine monomer, bisphenol E/aminophenyl propargylether benzoxazine monomer, bisphenol F/aniline benzoxazine monomer, bisphenol F/allylamine benzoxazine monomer, bisphenol F/aminophenyl propargylether benzoxazine monomer, bisphenol G/aniline benzoxazine monomer, bisphenol G/allylamine benzoxazine monomer, bisphenol G/aminophenyl propargylether benzoxazine monomer, bisphenol M/aniline benzoxazine monomer, bisphenol M/allylamine benzoxazine monomer, bisphenol M/aminophenyl propargylether benzoxazine monomer, 4,4′-(1- phenylethylidene)bisphenol/aniline benzoxazine monomer, 4,4′-(1- phenylethylidene)bisphenol/allylamine benzoxazine monomer, 4,4′-(1- phenylethylidene)bisphenol/aminophenyl propargylether benzoxazine monomer, 4,4'-((1,2,3,3,4,4- hexafluorocyclobutane-1,2-diyl)bis(oxy))bisphenol/aniline benzoxazine monomer, 4,4'-((1,2,3,3,4,4- hexafluorocyclobutane-1,2-diyl)bis(oxy))bisphenol/allylamine benzoxazine monomer, 4,4'- ((1,2,3,3,4,4-hexafluorocyclobutane-1,2-diyl)bis(oxy))bisphe nol/aminophenyl propargylether benzoxazine monomer, 4,4’-dihydroxybenzophenone/aniline benzoxazine monomer, 4,4’- dihydroxybenzophenone/allylamine benzoxazine monomer, 4,4’- dihydroxybenzophenone/aminophenyl propargylether benzoxazine monomer, 4,4’- dihydroxybiphenyl/aniline benzoxazine monomer, 4,4’-dihydroxybiphenyl/allylamine benzoxazine monomer, 4,4’-dihydroxybiphenyl/aminophenyl propargylether benzoxazine monomer; 4,4’- dihydroxybiphenyl/aniline benzoxazine monomer, 4,4’-dihydroxybiphenyl/allylamine benzoxazine monomer, 4,4’-dihydroxybiphenyl/aminophenyl propargylether benzoxazine monomer; 4,4′-(9- fluorenylidene)diphenol/aniline benzoxazine monomer; 4,4′-(9-fluorenylidene)diphenol/allylamine benzoxazine monomer; 4,4′-(9-fluorenylidene)diphenol/aminophenyl propargylether benzoxazine monomer; phenolphthalein/aniline benzoxazine monomer; phenolphthalein/allylamine benzoxazine monomer; phenolphthalein/aminophenyl propargylether benzoxazine monomer; benzoxazines available from Shikoku under the P-d, F-a, ALP-d trade names; Araldite MT 35700, Araldite MT 35710 FST, Araldite LZ 8280 N 75, Araldite LZ 8282 N 70, Araldite XU 35610, Araldite XU 35910, and Araldite XU 35500 available from Huntsman Corp. While performance characteristics, such as those discussed above, make benzoxazines attractive for inclusion as a thermally reactive or hardenable component in hardenable resin compositions described herein that include an upconverting component, commercially available benzoxazine resins can exhibit yellow or brown coloration. Such coloration is believed to arise from oxidized impurities associated with the monomer; these impurities and the associated coloration may be partially or completely removed by purification, e.g., recrystallization or column chromatography. However, such purification of commercial benzoxazines can render the material less soluble in hardenable resin compositions, possibly due to higher crystallinity in the purified material. Benzoxazine resins of the invention are designed to maintain low color and high solubility at high purity while also possessing performance characteristics particularly well suited for inclusion in a hardenable resin composition described herein. Preferably a second resin component comprises a benzoxazine resin including a benzoxazine compound in the class of compounds represented by Structure I. Examples of particularly preferred benzoxazine resin including a benzoxazine compound in the class of compounds represented by Structure I include, but are not limited to, benzoxazine resins including a phenol/BAMN benzoxazine monomer (a P-nb type benzoxazine represented by Structure II), benzoxazine resins that include a para-cresol/BAMN benzoxazine monomer (a pC-nb type benzoxazine represented by Structure III), and benzoxazine resin including a 5,6,7,8-tetrahydro-2- naphthol/BAMN benzoxazine monomer (a THN-nb type benzoxazine represented by Structure IV). Other benzoxazine resins that include a benzoxazine compound in the class of compounds represented by Structure I that may be useful for inclusion in a second component include, but are not limited to, benzoxazine resins including a 4-(t-butyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer (a tBP-nb type benzoxazine represented by Structure V), benzoxazine resins including a 4-cyclohexylphenol/bis(aminomethyl)norborane benzoxazine monomer (a CyP-nb type benzoxazine represented by Structure VI), benzoxazine resins including a 4- adamantylphenol/bis(aminomethyl)norbornane benzoxazine monomer (an AdmP-nb type benzoxazine represented by Structure VII), and benzoxazine resin including a 4- (maleimidomethyl)phenol/bis(aminomethyl)norbornane benzoxazine monomer ( an MMP-nb type benzoxazine represented by Structure VIII). Additional preferred benzoxazine resins for inclusion in the second resin component include benzoxazine resins including a benzoxazine compound in the class of compounds represented by Structure IX. Examples of particularly preferred benzoxazine resin including a benzoxazine compound in the class of compounds represented by Structure IX include, but are not limited to, a benzoxazine monomer of the general structure IX with radicals R ¹ and R ² comprising 1- adamanylamine (a BAF-adm type benzoxazine, Structure X); and benzoxazine resins that include a benzoxazine monomer of the general structure in Structure IX with radicals R ¹ and R ² comprising 3- maleimidomethyl-3,5,5-trimethylcyclohexyl moieties (a BAF-mi type benzoxazine, Structure XI). Hardenable resin compositions and methods described herein that include an upconverting component are configured to upconvert excitation light at one or more wavelengths in a first range of wavelengths to shorter wavelength light at one or more wavelengths in a second range of wavelengths. An upconverting component can comprises one or more compositions that alone or in combination can absorb light at one or more wavelengths in a first range of wavelengths and emit light at one or more wavelengths in a second range of wavelengths, the second range of wavelengths being shorter than the first range of wavelengths. An upconverting component preferably comprises a sensitizer and an annihilator, the sensitizer being selected to absorb excitation light at one or more wavelengths in the first range of wavelengths and the annihilator being selected to emit light at one or more wavelengths in the second range of wavelengths after transfer of energy from the sensitizer to the annihilator. Methods described herein that include an upconverting component can be useful for printing three-dimensional objects or other articles from a resin composition which can be solidified at volumetric positions impinged upon by excitation light in the first range of wavelengths by upconversion-induced photopolymerization, crosslinking, or hardening. Preferably, the upconversion comprises triplet upconversion (or triplet-triplet annihilation, TTA) which may be used to produce light of a higher energy relative to light used to photoexcite the sensitizer or annihilator. An annihilator can comprise molecules capable of receiving a triplet exciton from a molecule of the sensitizer through triplet-triplet energy transfer, undergo triplet fusion with another annihilator molecule triplet to generate a higher energy singlet that emits light in a second range of wavelengths to excite the photoinitiator to initiate polymerization or cross-linking of the first resin component and the hardenable resin composition. Examples of annihilators include, but are not limited to, polycyclic aromatic hydrocarbons, e.g., anthracene, anthracene derivatives (e.g., 9,10- bis(triisopropysilyl)ethynyl)anthracene, 9,10 diphenyl anthracene (DPA) 9,10-dimethylanthracene (DMA), 2-chloro-9,10-diphenylanthracene), 2-carbonitrile-9,10-diphenylanthracene, 9,10- bis(phenylethynyl)anthracene (BPEA), 2-chloro-9,10-bis (phenylethynyl) anthracene (2CBPEA), 5,6,11,12-tetraphenylnaphthacene(rubrene), pyrene and or perylene (e.g., tetra-t-butyl perylene (TTBP). Mixtures including one or more of the foregoing can also be used. The above anthracene molecules can be substituted or unsubstituted and/or functionalized with a halogen. Preferred halogenated anthracene derivatives include, for example, DPA or 9,10- bis(triisopropysilyl)ethynyl)anthracene further functionalized with a halogen (e.g., fluorine, chlorine, bromine, iodine), more preferably at the 2 or at the 2 and 6 position. Bromine can be a preferred halogen. Fluorescent organic dyes can be preferred. Preferably the annihilator comprises a molecule capable of receiving a triplet exciton from a molecule of the sensitizer through triplet energy transfer and undergoing triplet fusion with another annihilator molecule triplet to generate a higher energy singlet that emits light in the second range of wavelengths. Preferred annihilators emit upconverted light in a second range of wavelengths from about 300 to about 600 nm, from about 400 nm to about 500 nm. More preferably the second range of wavelength is from about 360 nm to about 420 nm, about 400 nm to about 480 nm, from about 440 to about 510 nm, from about 450 nm to about 520 nm, from about 460 nm to about 530 nm. A sensitizer can comprise at least one molecule capable of passing energy from a singlet state to a triplet state when it absorbs the photonic energy of excitation light in a first range of wavelengths. Examples of sensitizers include, but are not limited to, porphyrins, metalloporphyrins (e.g., palladium tetraphenyl tetrabutyl porphyrin (PdTPTBP), platinum octaethyl porphyrin (PtOEP), octaethyl-porphyrin palladium (PdOEP), palladium-tetraphenylporphyrin (PdTPP), palladium-meso- tetraphenyltetrabenzoporphyrin (PdPh4TBP), 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine (PdPc (OBu)), 2,3-butanedione (or diacetyl), which can be substituted or unsubstituted, derivatives of any of the foregoing, or a combination of several of the above molecules. Other examples of sensitizers include osmium sensitizers. See, for example, R. Haruki, et al, Chem. Commun., 2020, Advance Article accepted 13 May 2020 and published 13 May 2020, the abstract of which is available at https://doi.org/10.1039/D0CC02240C, which paper is hereby incorporated herein by reference. Examples of sensitizers absorb excitation light in a first range of wavelengths from about 500 to about 800 nm. Preferred sensitizers absorb excitation light 500 nm to about 540 nm, from about 620 nm to about 650 nm, or from about 680 nm to about 740 nm. More preferred examples include 532 ± 10 nm and 638 ± 10 nm. By way of example, without limitation, taking into consideration the particular upconverting component, excitation light at one or more wavelengths in a range of 638 nm plus or minus 10 nm can be useful to produce upconverted light in a range from about 390 nm to about 520 nm. By way of another example, without limitation, taking into consideration the particular upconverting component, excitation light at one or more wavelengths in a range of 532 nm plus or minus 10 nm can be useful to produce upconverted light in a range from about 390 nm to about 500 nm. A consideration in selecting a sensitizer/annihilator pair may include the compatibility of the pair with the photoinitiator being used. An upconverting component can preferably comprise upconverting nanoparticles for absorbing light at one or more wavelengths in a first range of wavelengths and emitting upconverted light at one or more wavelengths in a second range of wavelengths. The upconverting nanoparticles preferably include a sensitizer and an annihilator, the sensitizer being selected to absorb light at one or more wavelengths in the first range of wavelengths and the annihilator being selected to emit upconverted light at one or more wavelengths in the second range of wavelengths after transfer of energy from the sensitizer to the annihilator. Upconverting nanoparticles preferably have an average particle size less than the wavelength of the exciting light. Examples of preferred average particle sizes are less than 100 nm, less than 80 nm, less than 50 nm, although still larger, or smaller, nanoparticles can also be used. Most preferably, the upconverting nanoparticles have an average particle size that creates no appreciable light scattering. Preferably, at least a portion of upconverting nanoparticles include a core portion including the sensitizer and annihilator and an encapsulating shell over at least a portion, and preferably substantially all, of an outer surface of the core portion. More preferably, at least a portion of upconverting nanoparticles include a core portion (also referred to herein as a core) that includes a sensitizer, an annihilator, and a medium and an encapsulating coating or a shell (e.g., silica or other metal oxide) around at least a portion, and more preferably all, of the outer surface of the core portion. A preferred medium included in a core including a medium comprises a liquid. A liquid can facilitate molecular mobility of the sensitizer and annihilator included in the core allowing for collisions that facilitate energy transfer between the sensitizer and annihilator also included in the core. A liquid included in the core can comprise a liquid that is substantially polar or substantially nonpolar. For example, the liquid can comprise a solvent that is substantially polar or miscible in water, or one that is substantially nonpolar or is substantially immiscible in water, e.g., such that it forms a separate phase when mixed with water (even if some portions of the solvent can dissolve in water). Non-limiting examples of substantially polar solvents include water, or other aqueous fluids, such as ethanol. In some embodiments, an amphiphilic solvent may be used. Non-limiting examples of substantially amphiphilic or non-polar solvents include carboxylic acids such as oleic acid, stearic acid, arachidonic acid, linolenic acid, or other similar carboxylic acids with shorter or longer aliphatic chains. In some embodiments, a non-polar solvent may be used. Additional examples of nonpolar or amphiphilic solvents include, but are not limited to, trimethlybenzene, trichlorobenzene, chloroform, toluene, or the like. Non-limiting examples of a core including a sensitizer, an annihilator, and a liquid can include a micelle or a liposome. A micelle (which may typically be formed from one or more surfactants, e.g., having a relatively hydrophilic portion and a relatively hydrophobic portion.) may have a substantially spherical shape, e.g., where the hydrophilic portion defines an exterior portion of the micelle, while the hydrophobic portion defines an interior portion of the micelle. Liposomes are similar to micelles, except that instead of a single layer, the surfactants form a bilayer, and the interior portion is typically hydrophilic rather than hydrophobic. However, the interior of the bilayer is typically hydrophobic. Preferably upconverting nanoparticles include a core portion that includes a sensitizer, an annihilator, and a liquid (e.g., oleic acid) and an encapsulating coating or a shell (e.g., silica) around the outer surface of the core portion. The core can preferably comprise a micelle, that includes the sensitizer and annihilator and a liquid in which the sensitizer and annihilator are dispersed. (A micelle is typically formed from one or more surfactants, e.g., having a relatively hydrophilic portion and a relatively hydrophobic portion.) Examples of upconverting nanoparticles include, but are not limited to, nanocapsules described in International Application No. PCT/US2019/063629, of Congreve, et al., filed November 27, 2019, which published as WO 2020/113018 A1 on June 4, 2020; which is hereby incorporated herein by reference in its entirety. Other information concerning nanocapsules that may be useful includes S. Sanders, et al., “Photon Upconversion in Aqueous Nanodroplets”, J. Amer. Chem. Soc.2019, 141, 9180-9184, which is hereby incorporated herein by reference in its entirety. Optionally other components may be included within the interior portion of the core, e.g., hydrophobic liquids or solvents, such as non-polar or amphiphilic solvents, including by way of example, but not limited to, trimethylbenzene, trichlorobenzene, chloroform, etc. When the core portion further includes a liquid, the core portion preferably includes the sensitizer at a concentration in a range from about 10 ^-6 M to about 10 ^-2 M mg/ml and the annihilator at a concentration in a range from about 10 ^-4 M to about 10 ^-1 M. A nanoparticle including a core and a shell wherein the core includes the sensitizer, the annihilator, and a medium, preferably a liquid, may also be referred to herein as a nanocapsule. Upconverting nanoparticles can further include functional groups or other surface- modifying agents on a portion or all of the surface thereof for facilitating inclusion (e.g., distribution, dispersion, etc.) of the nanoparticles in other materials and/or liquids. Surfactants and other materials useful as surface treatments are commercially available. Optionally an upconverting nanoparticle can include a combination of one or more chemically distinct functional groups or other surface-modifying agents on the outer surface thereof. Examples include, but are not limited to, polyethylene glycols, polyacrylates, polyvinyl esters, polyvinyl amides, etc. For example, inclusion of a surfactant comprising polyethylene glycol functionality on the outer surface of nanoparticles can facilitate dispersion thereof in a hydrophilic medium. Examples of additional surface treatment materials for functionalizing the nanoparticle surfaces include, but are not limited to, polyethylene glycols, silanes, for example, but not limited to, PEG-silanes, (3- aminopropyl)triethoxysilane, (3-glycidyloxypropyl)trimethoxysilane, 2- [methoxy(polyethyleneoxy) _9-12 propyl]trimethoxysilane, 3-glycidoxypropyldimethoxymethylsilane, Isooctyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N- (triethoxysilylpropyl)-O-poly(ethylene oxide)4-6 urethane, O-[Methoxy(polyethylene oxide)]-N- (triethoxysilylpropyl)carbamate, octadecyltrimethoxysilane, triethoxyvinylsilane, trimethoxy[2-(7- oxabicyclo[4.1.0]hept-3-yl)ethyl]silane, trimethoxyphenylsilane, vinyltrimethoxysilane, 3- (trimethoxysilyl)propyl methacrylate, N-[3-(trimethoxysilyl)propyl]aniline. Preferably, an excitation source for use with an upconverting component is selected to emit excitation light at one or more wavelengths in a first range of wavelengths which matches or at least overlaps a range of wavelengths in which the upconverting component absorbs and upconverts at least a portion of the absorbed light to light at one or more wavelengths in a second range of wavelengths (shorter than the first range of wavelengths) for activating a photoinitiator and initiating photohardening of the hardenable resin composition in which it is included. When an upconverting component includes a sensitizer and an annihilator, such match or overlap preferably occurs between one or more wavelengths in a first range wavelengths emitted by the excitation light and one or more wavelengths in the first range of wavelengths absorbed by the sensitizer, whereupon the sensitizer transfers energy to an annihilator for generating upconverted light in a second range of wavelengths, the second range of wavelengths being shorter than the first range of wavelengths. Excitation light may be visible light, ultraviolet light, or other suitable forms of electromagnetic radiation. Other information that may be useful in connection with aspects of the invention including upconversion, e.g., sensitizers, annihilators, and the use thereof, includes International Application No. PCT/US2019/063629, of President And Fellows Of Harvard College, filed November 27, 2019, which published as WO 2020/113018 A1 on June 4, 202, S. Sanders, et al., “Photon Upconversion in Aqueous Nanodroplets”, J. Amer. Chem. Soc.2019, 141, 9180-9184, International Application No. PCT/US2020/053765, of the President And Fellows Of Harvard College, filed October 1, 2021, each of the foregoing being hereby incorporated herein by reference in its entirety. WO 2019/025717 A1 of Baldeck, et al., published February 7, 2019, and International Application No. PCT/US2019/063629, of Congreve, et al., filed November 27, 2019, which published as WO 2020/113018 A1 on June 4, 2020, also provide additional information that may be useful concerning the concentration of the upconverting nanoparticles and the concentrations of the sensitizer and annihilator in a hardenable resin composition. A photoinitiator included in a hardenable resin composition described herein that includes an upconverting component is preferably activated by one or more wavelengths included in upconverted light and not activated by the excitation light being used to excite the upconverting component. Information concerning other photoinitiators that may be useful for inclusion in a hardenable resin composition described herein that includes an upconverting component includes WO 2019/025717 A1 of Baldeck et al., published February 7, 2019, and International Application No. Application No. PCT/US2019/063629, of Congreve, et al., filed November 27, 2019, which published as WO 2020/113018 A1 on June 4, 2020; each of which is hereby incorporated herein by reference in its entirety. A hardenable resin composition described herein that includes an upconverting component can optionally further include one or more additives. Examples of such additives include, but are not limited to, a thixotrope; a stabilizer; a light blocker comprising a light absorbing pigment or dye; a non-reactive solvent diluent; a filler; a defoamer; an oxygen scavenger; a catalyst; and a thermal radical initiator. Other additives may alternatively or additionally be included. Additional information concerning examples of additives that can optionally be included in hardenable resin compositions and methods in accordance with various aspects and embodiments of the present invention is provided below. As mentioned above, hardenable resin compositions in accordance with one or more aspects of the present invention are particularly well-suited for use in forming an article, including, for example, in building a three-dimensional (3D) object, such as by three-dimensional (3D) printing, especially volumetric three-dimensional-printing. Following is additional information about photohardenable components, photoinitiators, and other additives: Photohardenable Component Examples of light-activated polymerization techniques useful for photohardening a first resin component in hardenable resin compositions in accordance with the present invention include, but are not limited to, stereolithography (SLA), projection microstereolithography (PμSL), Digital Light Projection (DLP), Low-Force Stereolithography (LFS), Digital Light Synthesis (DLS), Continuous Liquid Interface Production (CLIP), tomographic 3D printing, Computed Axial Lithography (CAL), dual-wavelength photopolymerization printing, two-photon printing (2PP), and building two-dimensional patterns, such as by lithography. Preferably, a first resin component included in a hardenable resin composition described herein is selected to achieve an optically transparent or clear liquid, which is desirable in processes and systems in which light, e.g., excitation light, is directed into the composition or light, e.g., upconverted light, is emitted from species included in the composition. Hardenable resin compositions in accordance with one or more aspects of the inventions described herein can be pourable, which is particularly desirable for handling purposes and for use of the compositions in three-dimensional printing processes and systems. Preferably hardenable resin compositions in accordance with one or more aspects of the inventions described herein can have a viscosity in a range from about 0.5 centipoise to about 10,000,000 centipoise. Examples of viscosities under the conditions in which methods in accordance with the present invention can be is carried out, that may be useful include, for example, but without limitation, greater than 1 centipoise, greater than 1,000 centipoise, and greater than 5,000 centipoise. Higher viscosities may be desirable, for example, when printing an article, e.g., a three- dimensional object, that is suspended or floating within a volume of a hardenable resin composition that is included in a container or build chamber. In such instances, a higher viscosity can be desirable for keeping the object suspended or floating while being printed. A hardenable resin composition having a viscosity of about 1,000 centipoise or higher, 2,000 centipoise or higher, 4,000 centipoise or higher, or even higher can be preferred in this regard. It may alternatively or additionally be desirable in some instances for a hardenable resin composition to display non-Newtonian rheological behavior. By way of example, and without limitation, a hardenable resin composition displaying non-Newtonian rheological behavior can be preferred for inclusion in a method in which an article is printed in a suspended state within a volume of the hardenable resin composition. For a hardenable resin composition displaying non- Newtonian rheological behavior, steady shear viscosities can be, for example, less than 30,000 centipoise, less than 20,000 centipoise, less than 10,000 centipoise, less than 5,000 centipoise, or less than 1,000 centipoise. Steady shear viscosity refers to the plateau value of the viscosity measured under continuous constant-rate shear, such as at shear rates ranging from about 0.00001 s ^-1 to about 1000 s ^-1 .) Preferably a photohardenable component included in a hardenable resin composition is photohardenable by exposure to excitation light in the presence of a photoinitiator. A first component in a hardenable resin component in accordance with the present invention preferably comprises one or more photohardenable components. A photohardenable component can comprise a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing. A first resin component may comprise any resin suitable for photohardening, e.g., by free-radical photopolymerization, controlled free- radical photopolymerization, thiol-ene photopolymerization, thiol-Michael photopolymerization, photoinitiated hydrosilylation (e.g., of silicones), cationic photopolymerization, anionic photopolymerization, olefin addition photopolymerization (e.g., of norbornenes), cationic ring opening photopolymerization (e.g., of epoxides, oxetanes, lactones, cyclic carbonates, cyclosiloxanes, etc.), olefin metathesis photopolymerization, ring-opening olefin metathesis photopolymerization, and photo-induced cross-linking. Examples of first resin components that may be included in the hardenable resin composition include, for example, without limitation, monomers, oligomers such as dimers or trimers, pre-polymers, and polymers, or mixtures including one or more of the foregoing, such as, for example, without limitation, free-radically photopolymerizable or crosslinkable ethylenically-unsaturated species including, for example, acrylates, methacrylates, acrylamides, methacrylamides, vinyl ethers, vinyl esters, vinyl amides, vinyl imidazoles, vinyl oxazolidinones such as 5-methyl-3-vinyl-1,3-oxazolidin-2-one, vinyl carbazoles, maleimides, methylene malonates, allyl ethers, cyanoacrylates, cyclopolymerizable monomers such as methyl 2- ((allyloxy)methyl)acrylate, and certain vinyl compounds such as styrenes, as well as cationically- photopolymerizable monomers and oligomers and cationically-crosslinkable polymers (which are most commonly acid-initiated and which include, for example, epoxies, vinyl ethers, oxetanes, cyclic carbonates, etc.), and the like, and mixtures thereof. A non-limiting list of examples of such suitable first resin components include ethylenically-unsaturated species described, for example, by Palazzotto et al. in U.S. Pat. No. 5,545,676 at column 1, line 65, through column 2, line 26, that include mono-, di-, and poly- acrylates and methacrylates (for example, methyl acrylate, methyl methacrylate, ethyl acrylate, isopropyl methacrylate, n-hexyl acrylate, stearyl acrylate, allyl acrylate, glycerol diacrylate, glycerol triacrylate, ethyleneglycol diacrylate, diethyleneglycol diacrylate, triethyleneglycol dimethacrylate, 1,3-propanediol diacrylate, 1,3-propanediol dimethacrylate, trimethylolpropane triacrylate, 1,2,4- butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, sorbitol hexacrylate, bis[1-(2- acryloxy)]-p-ethoxyphenyldimethylmethane, bis[1-(3-acryloxy-2-hydroxy)]-p- propoxyphenyldimethylmethane, trishydroxyethyl-isocyanurate trimethacrylate, the bis-acrylates and bis-methacrylates of polyethylene glycols of molecular weight about 200-500, copolymerizable mixtures of acrylated monomers such as those of U.S. Pat. No.4,652,274, and acrylated oligomers such as those of U.S. Pat. No.4,642,126); unsaturated amides (for example, methylene bis- acrylamide, methylene bis-methacrylamide, 1,6-hexamethylene bis-acrylamide, diethylene triamine tris-acrylamide and beta-methacrylaminoethyl methacrylate); vinyl compounds (for example, styrene, diallyl phthalate, divinyl succinate, divinyl adipate, and divinyl phthalate); and the like; and mixtures thereof. Suitable reactive polymers include polymers with pendant (meth)acrylate groups, for example, having from 1 to about 50 (meth)acrylate groups per polymer chain. Examples of such polymers include aromatic acid (meth)acrylate half ester resins. Other useful reactive polymers curable by free radical chemistry include those polymers that have a hydrocarbyl backbone and pendant peptide groups with free-radically polymerizable functionality attached thereto, such as those described in U.S. Pat. No.5,235,015 (Ali et al.). Mixtures of two or more monomers, oligomers, and/or reactive polymers can be used if desired. Suitable cationically-photopolymerizable species are described, for example, by Oxman et al. in U.S. Pat. Nos.5,998,495 and 6,025,406 and include epoxy resins. Such materials, broadly called epoxides, include monomeric epoxy compounds and epoxides of the polymeric type and can be aliphatic, alicyclic, aromatic, or heterocyclic. These materials generally have, on the average, at least 1 polymerizable epoxy group per molecule (preferably, at least about 1.5 and, more preferably, at least about 2). The polymeric epoxides include linear polymers having terminal epoxy groups (for example, a diglycidyl ether of a polyoxyalkylene glycol), polymers having skeletal oxirane units (for example, polybutadiene polyepoxide), and polymers having pendant epoxy groups (for example, a glycidyl methacrylate polymer or copolymer). The epoxides can be pure compounds or can be mixtures of compounds containing one, two, or more epoxy groups per molecule. These epoxy- containing materials can vary greatly in the nature of their backbone and substituent groups. For example, the backbone can be of any type and substituent groups thereon can be any group that does not substantially interfere with cationic cure at room temperature. Illustrative of permissible substituent groups include halogens, ester groups, ethers, sulfonate groups, siloxane groups, nitro groups, phosphate groups, and the like. The molecular weight of the epoxy-containing materials can vary from about 58 to about 100,000 or more. Other epoxy-containing materials that are useful include glycidyl ether monomers. Examples are glycidyl ethers of polyhydric phenols obtained by reacting a polyhydric phenol with an excess of a chlorohydrin such as epichlorohydrin (for example, the diglycidyl ether of 2,2-bis- (2,3epoxypropoxyphenol)-propane). Other examples of suitable epoxy resins include octadecylene oxide, epichlorohydrin, styrene oxide, vinyl cyclohexene oxide, glycidol, glycidylmethacrylate, diglycidyl ethers of Bisphenol A, vinylcyclohexene dioxide, 3,4epoxycyclohexylmethyl-3,4-epoxycyclohexene carboxylate, 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methyl-cycloh exene carboxylate, bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate, bis(2,3-epoxycyclopentyl) ether, aliphatic epoxy modified from polypropylene glycol, dipentene dioxide, epoxidized polybutadiene, silicone resin containing epoxy functionality, flame retardant epoxy resins, 1,4-butanediol diglycidyl ether of phenolformaldehyde novolak, resorcinol diglycidyl ether, bis(3,4-epoxycyclohexyl)adipate, 2- (3,4epoxycyclohexyl-5,5-spiro-3,4-epoxy) cyclohexane-meta-dioxane, vinylcyclohexene monoxide 1,2-epoxyhexadecane, alkyl glycidyl ethers such as alkyl C8-C10 glycidyl ether, alkyl C12-C14 glycidyl ether, butyl glycidyl ether, cresyl glycidyl ether, p-tert-butylphenyl glycidyl ether, polyfunctional glycidyl ethers such as diglycidyl ether of 1,4-butanediol, diglycidyl ether of neopentyl glycol, diglycidyl ether of cyclohexanedimethanol, trimethylol ethane triglycidyl ether, trimethylol propane triglycidyl ether, polyglycidyl ether of an aliphatic polyol, polyglycol diepoxide, bisphenol F epoxides, and 9,9-bis[4-(2,3-epoxypropoxy)-phenyl]fluorenone. Other useful epoxy resins comprise copolymers of acrylic acid esters of glycidol (such as glycidylacrylate and glycidylmethacrylate) with one or more copolymerizable vinyl compounds. Examples of such copolymers are 1:1 styrene-glycidylmethacrylate, 1:1 methylmethacrylate- glycidylacrylate, and a 62.5:24:13.5 methylmethacrylate-ethyl acrylate-glycidylmethacrylate. Other useful epoxy resins are well known and contain such epoxides as epichlorohydrins, alkylene oxides (for example, propylene oxide), styrene oxide, alkenyl oxides (for example, butadiene oxide), and glycidyl esters (for example, ethyl glycidate). Useful epoxy-functional polymers include epoxy-functional silicones such as those described in U.S. Pat. No.4,279,717 (Eckberg). These are polydimethylsiloxanes in which 1-20 mole % of the silicon atoms have been substituted with epoxyalkyl groups (preferably, epoxy cyclohexylethyl, as described in U.S. Pat. No.5,753,346 (Kessel)). Blends of various epoxy-containing materials can also be utilized. Such blends can comprise two or more weight average molecular weight distributions of epoxy-containing compounds (such as low molecular weight (below 200), intermediate molecular weight (about 200 to 10,000), and higher molecular weight (above about 10,000). Alternatively, or additionally, the epoxy resin can contain a blend of epoxy-containing materials having different chemical natures (such as aliphatic and aromatic) or functionalities (such as polar and non-polar). Other cationically-reactive polymers (such as vinyl ethers and the like) can additionally be incorporated, if desired. Additional examples of epoxies include aromatic glycidyl epoxies and cycloaliphatic epoxies. Suitable cationically-photopolymerizable species also include vinyl ether monomers, oligomers, and reactive polymers (for example, methyl vinyl ether, ethyl vinyl ether, tert-butyl vinyl ether, isobutyl vinyl ether, triethyleneglycol divinyl ether, trimethylpropane trivinyl ether, divinyl ether resins, and mixtures thereof. Blends (in any proportion) of one or more vinyl ether resins and/or one or more epoxy resins can also be utilized. Polyhydroxy-functional materials (such as those described, for example, in U.S. Pat. No.5,856,373 (Kaisaki et al.)) can also be utilized in combination with epoxy- and/or vinyl ether-functional materials. A first resin component can comprise one or more multifunctional acrylate monomers. Dipentaerythritol pentaacrylate, a pentafunctional acrylic monomer available from Sartomer as SR399 is an example of a preferred first resin component for inclusion in a hardenable resin composition of the present invention. Aliphatic urethane acrylates are also preferred first resin components for inclusion in a hardenable resin composition described herein. Mixtures of multifunctional acrylate monomers, such as dipentaerythritol pentaacrylate (e.g., SR399 from Sartomer), and aliphatic urethane acrylates can also be used. An acrylamide monomer can also be included in a hardenable resin composition to act as a solvent for mixing the photoinitiator in the first resin component. A hardenable resin including a photohardenable component and a resin component that is hardenable by a thermally driven reaction or mechanism, preferably comprising a benzoxazine resin including a benzoxazine compound represented by any of Structures I-XI, can impart added properties to a hardened product thereof that would not be provided by the photohardenable component alone. This can facilitate forming 3D printed articles that can display characteristics and/or performance properties that can be suitable for various end-use applications. Examples of added properties that may be attained by the inclusion of the second resin component include, for example, but are not limited to, mechanical, thermal, electrical, dielectric, chemical resistance, moisture resistance, and biocompatibility properties. Examples include improved mechanical properties including increased tensile strength and modulus, flexural strength and modulus, compressive strength and modulus, impact strength, hardness, wear resistance, fatigue resistance, fracture toughness; improved thermal properties including increased glass transition temperature, increased heat deflection temperature, increased thermal degradation temperature, or reduced coefficients of thermal expansion; reduced moisture or solvent uptake; improved radiation resistance; improved fire resistance, flame retardancy, or char yield; improved dielectric performance (e.g., reduced dielectric constant, reduced dielectric loss constant, or increased breakdown voltage); or improved optical properties (e.g., increased refractive index). Photoinitiators A photoinitiator can be included in a hardenable resin composition of the present invention including a photohardenable component that is hardenable by exposure to excitation light in the presence of the photoinitiator. A photoinitiator for inclusion in hardenable resin compositions and methods in accordance with the present invention can be readily selected by one of ordinary skill in the art, taking into account its suitability for the mechanism to be used to initiate hardening (e.g., polymerization, cross- linking, curing, etc.) as well as its suitability for and/or compatibility with the photohardenable component and other components included in the hardenable resin composition of the present invention. A photoinitiator can comprise a single photoinitiator or a combination of photoinitiators or a photoinitiator system including two or more components, at least one of which is a photoinitiator. Selection of a photoinitiator is generally made taking into consideration the absorption band of the photoinitiator and the wavelength of the radiation or light that will be used to activate the photoinitiator so that there is a match or at least an overlap between the two. By way of non-limiting examples, photoinitiators are available that can be activated by UV, visible, or near-infrared wavelength light. Other factors, e.g., absorption coefficients, rate constants of the primary radicals toward the first resin component, possible side reactions, light intensity can also be taken into consideration and balanced in the selection process. See, for example, A, Eibel, et al., “Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data”, Polym. Chem., 2018, 9, 5107-5115. A preferred photoinitiator initiates polymerization or cross-linking of the first resin component by free radical reactions. For example, a photoinitiators may form free radicals and/or cations upon initiation. Examples of photoinitiators, but are not limited to, 2-isopropylthioxanthone, benzophenone, 2,2- azobisisobutyronitrile, camphorquinone, diphenyltrimethylbenzoylphosphine oxide (TPO), HCP (1- hydroxycyclohexylphenylketone), BAPO (phenylbis-2,4,6-(trimethylbenzoyl)phosphine oxide), Speedcure VLT (Bis(2,6-difluoro-3-(1-hydropyrrol-1-yl)phenyl)titanocene). Other examples include Norrish Type-1 and Norrish Type-2 initiators. An example of a photoinitiator includes a free-radical photoinitiator system including a ketocoumarin dye, an iodonium salt, and a borate salt which generates free radicals capable of initiating photohardening the first resin component upon excitation of the ketocoumarin by radiation and energy and electron transfer reactions between the ketocoumarin and its reaction products. This photoinitiator system can be activated by radiation in a range of wavelengths from about 440 nm to about 510 nm. Additional information concerning a free-radical photoinitiator system comprising: a ketocoumarin dye, an iodonium salt, and a borate salt and compositions and methods including same can be found in U.S. Application No.63/091,863 of Arndt, et al., filed October 14, 2020 and U.S. Application No.63/121,906 of Arndt, et al., filed December 5, 2020, each of which is hereby incorporated herein by reference in its entirety. An example of a photoinitiator includes a free-radical photoinitiator system including a ketocoumarin (e.g., Esacure 3644, IGM Resins), an acyl phosphine oxide (e.g., TPO), and an amine (e.g., 2-ethylhexyl 4-(dimethylamino)benzoate). A further example of a photoinitiator is Ivocerin (Ivoclar). For hardenable resin compositions which contain a photohardenable component and do not include an upconverting component, examples of photoinitiator systems include but are not limited to photoinitiator systems comprising: a dye and an amine; a dye and an onium salt (e.g., diaryliodonium or triarylsulfonium salts); a dye, an onium salt, and an amine; a dye, an onium salt, and a borate salt (e.g., Borate V (butyrylcholine butyltriphenyl borate), Spectra Photopolymers); or a dye and a borate salt. Examples of Additives As noted above, hardenable resin compositions in accordance with the present invention can further include one or more additives. Examples of such additives include, but are not limited to, thixotropes, light blockers, defoamers, oxygen scavengers, non-reactive solvent diluents, catalysts, thermal radical initiators, and fillers. Any additive can be a single additive or a mixture of additives. For example, a light blocker additive can comprise a single light blocker or a mixture of two or more light blockers. Additives are preferably selected so that they do not react with other components or additives included in the hardenable resin composition. Additional information concerning examples of certain additives follow. Thixotropes A hardenable resin composition in accordance with the present invention can further include a thixotrope. Inclusion of a thixotrope can advantageously facilitate forming an article, such as a three-dimensional object in the case of 3D printing, without the need for support structures employed in conventional layer-by-layer three-dimensional or additive manufacturing processes to provide mechanical support to the object being fabricated where, for example, a region of the object is not fully supported by previously formed layers. Avoiding the use of support structures also avoids the step of removing support structures from the object after printing which can lead to surface deformations or other flaws in the object being formed. Inclusion of a thixotrope in a hardenable resin composition can further facilitate forming a three-dimensional object at a selected position in a volume of the hardenable resin composition with a minimal displacement of the object in the volume of hardenable resin composition during formation. Preferably the minimal displacement is an amount of movement that is acceptable for precisely producing the intended object geometry during the time interval required to form the object. Most preferably, the position of the object in the volume of the hardenable resin composition remains fixed position during formation of the object. Thixotropes suitable for inclusion in the hardenable resin compositions include, for example and without limitation, urea derivatives; modified urea compounds such as Rheobyk 410 and Rheobyk-D 410 available from BYK-Chemie GmbH, part of the ALTANA Group; fumed metal oxides (also referred to as pyrogenic metal oxides) including for example, but not limited to, fumed silica, fumed alumina; zirconia; precipitated metal oxides including for example, but not limited to, precipitated silica, precipitated alumina; unmodified and organo-modified phyllosilicate clays; dimer and trimer fatty acids; polyether phosphates; oxidized polyolefins; hybrid oxidized polyolefins with polyamide; alkali soluble/swellable emulsions; cellulosic ethers; hydrophobically- modified alkali soluble emulsions; hydrophobically-modified ethylene oxide-based urethane; sucrose benzoate; ester terminated polyamides; tertiary amide terminated polyamides; polyalkyleneoxy terminated polyamides; polyether amides; acrylamidomethyl-subsituted cellulose ester polymers; polyethyleneimine; polyurea; organoclays; hydrogenated castor oil; organic base salts of a clay mineral (e.g., montmorillonite) and other silicate-type materials; aluminum, calcium, and zinc salts of fatty acids, such as lauric or stearic acid. See U.S. Patent Nos.6,548,593 of Merz, et al., issued April 15, 2003, and 9,376,602 of Walther, et al. issued June 28, 2016, which are hereby incorporated herein by reference in their entireties, for information relating to urea derivatives that may be useful as thixotropes. Thermally reversible gellants such as ester terminated polyamides, tertiary amide terminated polyamides, polyalkyleneoxy terminated polyamides, and polyether amides, and combinations thereof, may be desirable for use as thixotropes. Examples include Crystasense LP1, Crystasense LP2, Crystasense LP3, Crystasense MP, Crystasense HP4, and Crystasense HP5, Rheoptima X17, Rheoptima X24, Rheoptima X38, Rheoptima X58, Rheoptima X73, and Rheoptima X84 available from Croda. Metal oxides that have been surface-treated to impart dispersibility characteristics compatible with the hardenable resin composition and components thereof may be desirable for use as thixotropes. A thixotrope can be included in the hardenable resin composition in an amount effective to at least partially restrict movement of the three-dimensional object or one or more regions thereof in the hardenable resin composition during formation. When it is desired to print an article or portion thereof that is suspended during formation in a volume including a photohardenable component (e.g., not in contact with a surface of the build region or container in which the volume is contained), a thixotrope is preferably included in the composition in an amount effective to at least partially restrict movement of the suspended three- dimensional object in the volume during formation. More preferably the position of the object in the volume remains fixed during formation. Light Blockers A hardenable resin composition in accordance with the present invention can further include a light blocker to control the spread of light and improve the selectivity and resolution of hardening. Preferable a light blocker has an absorption wavelength range that overlaps at least partially with the absorption wavelength range of the photoinitiator and the emission wavelength range of the radiation or light used to activate the photoinitiator. Examples of preferred light blockers include azo dyes such as Sudan 1, Sudan 3, and other light blockers that can be readily identified by one of ordinary skill in the relevant art. Defoamers A hardenable resin composition in accordance with the present invention can further include a defoamer to aid in removing bubbles introduced during processing and handling. A preferred defoamer is BYK 1798 (a silicone based defoamer) available from BYK-Chemie GmbH, part of the ALTANA Group. Oxygen Scavengers A hardenable resin composition in accordance with the present invention can further include an oxygen scavenger to react with oxygen (e.g., singlet oxygen, dissolved oxygen) present in the hardenable resin composition. WO 2019/025717 A1 of Baldeck, et al., published February 7, 2019 provides information that may be useful regarding antioxidant additives. Non-Reactive Solvents A hardenable resin composition in accordance with the present invention can further include a non-reactive solvent diluent. Examples include, but are not limited to, acetone, amyl acetate, n- butanol, sec-butanol, tert-butanol, butyl acetate, cyclohexanone, decane, dimethylacetamide, dimethylformamide, dimethylsulfoxide, dipropylene glycol, dipropylene glycol methyl ether, ethanol, ethyl acetate, ethylene glycol, glycerol, heptane, isopropanol, isopropyl acetate, methyl ethyl ketone, N-methyl pyrrolidone, propylene carbonate, propylene glycol, propylene glycol diacetate, tetrahydrofuran, tripropylene glygol methyl ether, toluene, water, xylenes. Catalysts A hardenable resin composition in accordance with the present invention can further include a catalyst. Inclusion of a catalyst can accelerate the curing reaction of the thermally cured component which may be used to reduce the cure time and/or lower the curing temperature. Examples include, but are not limited to: organometallic compounds such as organotin, organozinc, organozirconium, and organobismuth compounds; metallocene compounds such as titanocenes; transition metal salts and complexes including thermally latent and photolatent metal salts and complexes; ionic photoacid generators such as diaryliodonium salts and triarylsulfonium salts; nonionic photoacid generators; photobase generators; amines and amine salts; and phenolic compounds such as thiodiphenol. Thermal Radical Initiators A hardenable resin composition in accordance with the present invention can further include a thermal radical initiator. Inclusion of a thermal radical initiator may, for example, increase the overall level of cure of the photohardenable component (e.g., increase conversion of acrylate or methacrylate groups); may provide the cure mechanism for the thermally cured component (e.g., polymerization of maleimide resins or nadic anhydride-terminated resins); may promote crosslinking; or may facilitate hardening of the hardenable resin composition through a combination of such mechanisms. Examples include, but are not limited to, organic peroxides and azo initiators. Fillers A hardenable resin composition in accordance with the present invention can further include a filler. A filler can be included in the hardenable resin composition depending on the properties desired in the part or article to be made. Fillers may be solid or liquid, organic or inorganic, and may include, for example, reactive and non-reactive thermoplastics (including but not limited to: poly(ether imides), maleimide-styrene terpolymers, polyarylates, polysulfones and polyethersulfones, etc.), preferably any such thermoplastics included as a filler has a refractive index that is the same as or substantially the same as that of the hardenable resin composition; liquid rubbers such as carboxy-terminated butadiene acrylonitrile rubbers and vinyl-terminated butadiene acrylonitrile rubbers; plasticizers such as phthalates; inorganic fillers such as silicates (such as talc, clays, silica, mica), glass, zirconia, alumina, titania, baria, barium titanium oxides, barium strontium titanium oxides, carbon nanotubes, graphene, cellulose nanocrystals, etc., and combinations including two or more of the foregoing. A hardenable resin composition in accordance with the various aspects of the present invention can include, for example, but without limitation, from about 0.5 to about 95, preferably from about 40 to about 95, weight percent first resin component; from about 0.1 to about 25, preferably from about 0.2 to about 10, weight percent photoinitiator; and from about 0.5 to about 95, and preferably from about 15 to about 80, weight percent second resin component. When a hardenable resin composition optionally further includes an upconverting component, the upconverting component can be included in an amount from about 0.1 to about 85, preferably from about 1 to about 20, weight percent. A hardenable resin composition can optionally further include, for example, any one or more of the following: from about 0.05 to about 15, preferably from about 1 to about 10, weight percent thixotrope, from about 0.005 to about 1 weight percent defoamer, from about 0.005 to about 10 weight percent light blocker, from about 0.00005 to about 5 weight percent oxygen scavenger, from about 0.05 to about 95 weight percent non-reactive solvent diluent, from about 0.00005 to about 10 weight percent catalyst, from about 0.05 to about 10 weight percent thermal radical initiator, and from about 0.5 to about 95 weight percent filler. Unless otherwise indicated, specified weight percents are based on the total weight of the hardenable resin composition. Hardenable resin compositions in accordance with one or more aspects of the present invention can have a viscosity in a range from about 0.5 centipoise to about 10,000,000 centipoise. Examples of viscosities under the conditions in which methods in accordance with one or more aspects of the present invention can be is carried out, that may be useful include, for example, but without limitation, greater than 1 centipoise, greater than 1,000 centipoise, and greater than 5,000 centipoise. Higher viscosities may be desirable, for example, when printing an article, e.g., a three- dimensional object, that is suspended or floating within a volume of a hardenable resin composition that is included in a container or build chamber. In such instances, a higher viscosity can be desirable for keeping the object suspended or floating while being printed. A hardenable resin composition having a viscosity of about 1,000 centipoise or higher, 2,000 centipoise or higher, 4,000 centipoise or higher, or even higher can be preferred in this regard. It may alternatively or additionally be desirable in some instances for a hardenable resin composition to display non-Newtonian rheological behavior. By way of example, and without limitation, a hardenable resin composition displaying non-Newtonian rheological behavior can be preferred for inclusion in a method in which an article is printed in a suspended state within a volume of the hardenable resin composition. For a hardenable resin composition displaying non- Newtonian rheological behavior, steady shear viscosities can be, for example, less than 30,000 centipoise, less than 20,000 centipoise, less than 10,000 centipoise, less than 5,000 centipoise, or less than 1,000 centipoise. Steady shear viscosity refers to the plateau value of the viscosity measured under continuous constant-rate shear, such as at shear rates ranging from about 0.00001 s ^-1 to about 1000 s ^-1 . It may be desirable for a hardenable resin composition to comprise one or more second components, preferably one or more benzoxazine resin described herein, wherein the second component includes one or more photohardenable or photo-crosslinkable functionality(ies). In such case, such functionality(ies) can serve as the first resin component in the composition. In accordance with another aspect of the present invention, there is provided a method of forming an article, comprising: (a) providing a volume of a hardenable resin composition comprising a first resin component comprising a photohardenable component and a second resin component comprising a benzoxazine resin including a benzoxazine compound represented by any of Structures I -IX; (b) forming an intermediate article from the hardenable resin composition, wherein formation of the intermediate article comprises: (i) irradiating one or more locations in the hardenable resin composition with excitation light at one or more selected wavelengths for initiating local hardening of the hardenable resin composition at the one or more selected locations in the volume of the hardenable resin composition; and (ii) optionally repeating step (i) at one or more of the same or different selected locations within the volume of the hardenable resin composition until the intermediate article is formed; and (c) hardening (e.g., further reacting, further polymerizing, further chain extending) the second resin component in the intermediate article by a thermally driven reaction or mechanism to form the article. In accordance with yet another aspect of the present invention, there is provided a method of forming an article, the method comprising: (a) providing a volume of a hardenable resin composition, the composition comprising: (i) a first resin component that is hardenable by exposure to light in the presence of a photoinitiator, the first resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing; (ii) the photoinitiator configured to create reactive species for initiating hardening of the first resin component included in the hardenable resin composition when activated by light within a range of wavelengths absorbed by the photoinitiator; (iii) an upconverting component configured to upconvert excitation light at one or more wavelengths in a first range of wavelengths to shorter wavelength light at one or more wavelengths in a second range of wavelengths, wherein the upconverted light includes radiation within the range of wavelengths absorbed by the photoinitiator; and (iv) a second resin component that is hardenable by a thermally driven reaction or mechanism, the second resin component comprising a monomer, an oligomer, a pre-polymer, a polymer, or a mixture including at least one the foregoing,; (b) forming an intermediate article from the hardenable resin, wherein formation of the intermediate article comprises: (i) irradiating one or more locations in the hardenable resin composition with excitation light at one or more wavelengths in the first range of wavelengths for upconversion by the upconverting component and initiating local hardening of the first resin component included in the hardenable resin composition at the one or more selected locations in the volume of the hardenable resin composition; and (ii) optionally repeating step (i) at one or more of the same or a different selected locations within the volume of the hardenable resin composition until the intermediate article is formed; and (c) subjecting the intermediate article to a thermally driven reaction or mechanism to further harden the hardenable resin composition and form the article. Preferably the second resin component included in a hardenable resin composition that includes an upconverting component includes a benzoxazine resin including a benzoxazine compound represented by any of Structures I-IX. A hardenable resin composition can be included in a container or build region that includes at least a portion that is optically transparent so that the hardenable resin composition is accessible by radiation or excitation light. In certain embodiments, the container or build region includes a volume of hardenable resin composition and an article is formed in the volume. In methods in accordance with one or more aspects of the inventions described in which an article is printed in a volume of a hardenable resin composition, it can be desirable for an article comprising a three-dimensional object to be fully suspended in the volume of the hardenable resin composition during formation thereof. In the systems and methods in accordance with the present invention, the selection of wavelength(s) of the excitation light for the excitation light directed into the volume of the hardenable resin composition is preferably made taking into account the photopolymerizable composition and hardening mechanism being used. For example, for hardenable resin compositions including a photohardenable component compositions that is hardenable via a hardening mechanism that involves a single wavelength of excitation light, the wavelength of the one or more excitation light projections can be the same. Optionally in such case, at least one of the two or more excitation light projections can include a different wavelength light, for example for inhibiting undesired hardening of the photopolymerizable composition. In cases in which a hardenable resin composition includes a photohardenable component that is hardenable via a hardening mechanism that involves more than wavelengths of excitation light, the excitation light projections will be selected for projecting excitation light with appropriate wavelengths (or ranges of wavelengths) for the hardening mechanism. Optionally a third wavelength light can also be used to inhibit undesired hardening of the photopolymerizable composition. Radiation within the appropriate wavelengths is preferably selectively directed to one or more selected locations in the hardenable resin composition to initiate hardening of the hardenable resin composition at the one or more selected locations. Depending on the hardening mechanism of the photohardenable component, one or more separate optical systems is preferably positioned or positionable to selectively direct one or more excitation light projections into the volume of hardenable resin composition. Excitation light can be directed into the volume as an optical projection of excitation light. The excitation light or optical projection of excitation light can be directed to one or more selected locations in the volume of a hardenable resin composition including a photohardenable component to selectively initiate photohardening at such one or more selected locations in the volume. Excitation light can be applied as an image. Examples of images include, without limitation, a line of light, a single beam of light, a two-dimensional image, a patterned image, a patterned two- dimensional image. An image can comprise a cross-sectional plane of the three-dimensional image or article being printed. Excitation light can be applied as a plane of light or a light sheet. In the methods described herein for forming an article, excitation light or radiation may be applied using any suitable light or electromagnetic radiation source or optical projection system (which typically includes a light or electromagnetic radiation source). Examples of light or electromagnetic radiation light sources include, but are not limited to, lasers (including laser diodes) and other coherent light sources, light-emitting diodes (LEDs), micro-LED arrays, vertical cavity lasers (VCLs), and filtered lamps. Such light sources are commercially available and selection of a suitable light source can be readily made by one of ordinary skill in the relevant art. LEDs of the type such as Phlatlight LEDs available from Luminus for use with DMDs may also be suitable. Optionally, the excitation light can be temporally and/or spatially modulated. Optionally, the intensity of the excitation light can be modulated by known or readily ascertainable techniques. Optionally, source drive modulation by known or readily ascertainable techniques can be used to adjust the absolute power of the light beam. Spatially modulated excitation light can be used to direct or irradiate a patterned image or a two-dimensional image at one or more selected locations in the volume of hardenable resin composition. For example, spatially modulated excitation light can be created by an optical projection system including a spatial modulation component (also referred to herein as a spatial light modulator (“SLM”)). Examples of spatial modulation components for inclusion in an optical projection system include, for example and without limitation, a liquid crystal display (also referred to herein as “LCD”), a digital micromirror device (also referred to herein as “DMD”)), a micro-LED array, a vertical cavity laser (also referred to herein as “VCL” or as a Vertical Cavity Surface Emitting Laser (also referred to herein as “VCSEL”), a scanning laser system, a liquid crystal on silicon (also referred to herein as “LCoS”) microdisplay. An optical projection system comprising a spatial light modulator may be utilized with incoherent light as an amplitude modulator in combination with projection lens to form images in the photopolymerizable liquid for amplitude base projections. Optionally, an optical projection system comprising a spatial light modulator may be utilized as a wavefront encoding device to form a phase or complex amplitude modulation on the wavefront in a holographic configuration. An optical projection system can be selected to apply continuous excitation light. An optical system can be selected to apply intermittent excitation light. Intermittent excitation can include random on and off application of light or periodic application of light. Examples of periodic application of light includes pulsing. An optical system can be selected to apply a combination of both continuous excitation light and intermittent light, including, for example, an irradiation step that includes the application of intermittent excitation light that is preceded or followed by irradiation with continuous light. The methods described herein can further comprise repeating the step of irradiating or directing excitation light into the volume of the hardenable resin composition at one or more of the same, additional, and/or different selected locations within the volume until the desired article or object is partially or fully formed. An optical projection system can further include projection optics and/or additional components including, but not limited to, one or more translational stages for moving the system or components thereof. The methods described herein can also include the use commercially available projection and filtering techniques that can assist in providing a very narrow depth of focus or systems that employ two or more optical projection methods at once. A preferred optical projection system can include a light source, projection optics, and a spatial modulation component. In methods described herein, power densities or intensities of excitation light directed into the volume of hardenable resin composition to cause hardening (e.g., by polymerization, crosslinking) to occur at the one or more selected locations may be, without limitation, less than 5000 W/cm ² , less than 1000 W/cm ² , less than 500 W/cm ² , less than 100 W/cm ² , less than 50 W/cm ² , less than 10 W/cm ² , less than 5 W/cm ² , less than 1 W/cm ² , less than 500 mW/cm ² , less than 100 mW/cm ² , less than 50 mW/cm ² , less than 10 mW/cm ² , less than 1 mW/cm ² , etc. Other power densities or intensities may also be determined to be useful. In embodiments of methods including a hardenable resin component described herein that includes an upconverting component and a photohardenable component, the upconverting component may have a quadratic dependence on fluence, allowing initiation of hardening of the hardenable resin composition at a focal point or region of the light within the print volume, e.g., without causing polymerization in other regions within the print volume, due to the quadratic dependence. In some cases, due to this quadratic dependence, relatively low light intensities can be used to initiate hardening at a focal point or region. For instance, the intensity or power density of the applied electromagnetic radiation applied to the focal point or region to initiate photohardening of the photohardenable component may be less than 5,000 W/cm2, less than 3,000 W/cm2, less than 2,000 W/cm2, less than 1,000 W/cm2, less than 500 W/cm2, less than 300 W/cm2, less than 200 W/cm2, less than 100 W/cm2, less than 50 W/cm2, less than 30 W/cm2, less than 20 W/cm2, less than 10 W/cm2, less than 5 W/cm2, less than 3 W/cm2, less than 2 W/cm2, less than 1 W/cm2, less than 500 mW/cm2, less than 300 mW/cm2, less than 200 mW/cm2, less than 100 mW/cm2, etc. A hardenable resin composition may be included or contained within any build region or container which may define a print volume in some cases. Light may penetrate to at least various depths within the non-hardened or liquid composition. Preferably the non-hardened composition is optically transparent. Preferably the container or build region includes at least a portion that is optically transparent so that the hardenable resin composition is accessible to radiation or excitation light. In some embodiments, the light or other electromagnetic radiation may be focused onto one or more specific locations or regions within the print volume. As mentioned above, methods described herein can include providing a volume of a hardenable resin composition included within a build region or container wherein at least a portion of the container is optically transparent so that the hardenable resin composition is accessible by excitation light. Preferably, the entire container is optically transparent. Optically transparent portions of a container can be constructed from a material comprising, for example, but not limited to, glass, quartz, fluoropolymers (e.g., Teflon FEP, Teflon AF, Teflon PFA), cyclic olefin copolymers, polymethyl methacrylate (PMMA), polynorbornene, sapphire, or transparent ceramic. Examples of container shapes include, but are not limited to, a cylindrical container having a circular or oval cross-section, a container having straight sides with a polygonal cross-section or a rectangular or square cross-section. Preferably the optically transparent portion(s) of the container is (are) also optically flat. Optionally, one or more filters are added to at least a surface of any optically transparent portions of the container to block undesired light to prevent unintentional curing. In methods of the present invention, taking into consideration oxygen sensitivity of components including in hardenable resin composition, it may be desirable or advantageous to degas, purge or sparge with an inert gas the hardenable resin composition before or after being introduced into the container. It may further be desirable or advantageous to maintain the hardenable resin composition under inert conditions, e.g., under an inert atmosphere, while in the container. Optionally the container is closed (e.g., to ambient atmosphere) during printing. This can prevent introduction of oxygen into the container while the three-dimensional object or other article is being printed or formed. In the case that the container is sealed or otherwise closed, it may optionally be closed in an air-tight manner to prevent introduction of oxygen into the container during printing. The seal or other closing techniques that may be used should not be permanent so at least that the printed objects and unpolymerized material can be removed from the container. In certain instances, depending, for example, upon the components or materials used, the hardenable resin composition is preferably substantially oxygen free (e.g., less than 50 ppm oxygen) during printing. In the methods described herein, the container optionally may be rotated to provide additional angles of illumination or projection of excitation light into the volume of hardenable resin composition contained therein. This can be of assistance in patterning object volumes or surfaces more accurately or it can be used as a means of providing multiple exposure of a given feature from different angles. In the method described herein, the container optionally may be stationary while a beam or optical projection of excitation light is being directed into the hardenable resin composition. In the methods described herein, once the intermediate article is formed, it may be removed from the hardenable resin composition, optionally washed, any other modifications optionally made, and then subjected to a thermally driven reaction or mechanism (e.g., heated and/or microwave irradiated) sufficiently to further harden (e.g., by further reacting, further polymerizing, further chain extending) the second resin component and form the article. Additional modifications to the formed article may also be made following the heating and/or microwave irradiating step. Examples of preferred thermally driven reactions or mechanisms include, but are not limited to, heating (e.g., the direct or indirect application of heat or thermal energy, irradiation with microwaves, irradiation with UV, visible, or infrared light for purpose of heating). Other examples include autoclave processing. Optionally the thermally-driven reaction or mechanism can be carried out under pressure. A second resin component can comprise a benzoxazine resin of the invention or a mixture including a benzoxazine resin of the invention and at least one other hardenable monomer, oligomer, pre-polymer, polymer that is hardenable by a thermally driven reaction or mechanism. A printed object prior to post-curing (including, e.g., the thermally-driven reaction or mechanism to harden the second resin component) can be referred to as a “green” object; the printed object subsequent to post curing can be referred to as a “post cured” object. Washing may be carried out with any suitable organic or aqueous wash liquid, or combination thereof, including solutions, suspensions, emulsions, microemulsions, etc. Examples of suitable wash liquids include, but are not limited to water, alcohols (e.g., methanol, ethanol. Isopropanol, etc.), glycol ethers and glycol esters, benzene, toluene, etc. Wash liquids including a mixture of two or more liquid (e.g., water and an alcohol (e.g., isopropanol) may also be suitable. Such wash solutions may optionally contain additional constituents such as surfactants, etc. After the intermediate article is formed, optionally washed, etc., as described above, it is then hardened by a thermally driven reaction or mechanism. Hardening can comprise heating and/or microwave irradiation to further cure the same. Heating may be active heating (e.g., in an oven, such as an electric, gas, or solar oven), or passive heating (e.g., at ambient temperature). Active heating can be more rapid than passive heating and in some embodiments can be preferred. Passive heating, e.g., by maintaining the intermediate at ambient temperature for a sufficient time to effect further cure, can also be desirable. Optionally, heating can comprise heating at a first temperature for a first time period, and then heating at a second temperature for a second time period, and then heating at a third temperature for a third time period, and so on, for any number of temperatures and time periods. The temperatures and time periods may be selected to facilitate evaporation of volatiles from the article without causing damage (e.g., cracks); to facilitate more complete curing of lower temperature curing component(s) to stabilize the article shape prior to subsequent cure of higher temperature curing component(s); or to develop higher thermomechanical properties. Differential scanning calorimetry may assist in determining the temperatures and time periods appropriate for curing, by indicating the temperatures where curing reactions initiate and reach their maximum rates (e.g., in a temperature ramp experiment) as well as indicating how much time is required to complete a curing reaction (e.g., in an isothermal experiment). The time periods can be the same length or different. In certain embodiments, the first temperature can be ambient temperature or greater than ambient temperature, and each subsequent temperature can be greater than the previous. Preferably the maximum temperature is sufficient to completely or substantially completely harden or cure the hardenable resin composition but is less than the degradation temperature, e.g., the 5% mass loss temperature as measured by thermogravimetric analysis. When multiple temperatures are used, the temperature can be ramped, for example, by step-wise increases. For example, the intermediate may be heated in a stepwise manner at a first temperature in a first range of from about 70°C to about 150°C, and then at a second temperature in a second range from about 150°C to 200°C or 150°C to 250°C, with the duration of each heating depending on the size and shape of the intermediate article. In another embodiment, the intermediate may be cured by a ramped heating schedule, with the temperature ramped from a first temperature (e.g., ambient temperature or a temperature greater than ambient temperature) through a second temperature (e.g., a temperature in a range greater than the first temperature to about 150°C, and up to a final temperature (e.g., a temperature greater than the second temperature to about 250°C, at a change in heating rate of 0.5°C per minute, to 5°C per minute. The article formed by a method described herein can be the same as or different from the intermediate article. For example, there can be changes between the intermediate article and the article as a result of, for example, shrinkage (e.g., up to 1, 2, 5, 10, 25, or 50 percent by volume), expansion (e.g., up to 1, 2, 5, 10 percent by volume), or possible optional intervening forming steps (e.g., intentional bending, stretching, drilling, grinding, cutting, polishing, or other intentional forming after formation of the intermediate product, but before formation of the subsequent article). The intermediate article can be of the same shape as the article to be formed or can have a shape to be imparted to the article to be formed. Additional possible post curing treatments for articles at least partially formed by 3D printing methods involving photopolymerization include but are not limited to application of light, heat, electron beam, time (aging), and humidity, and combinations of multiple such treatments in tandem or sequence. Light-based post cure treatments are commonly used and involve providing additional light exposure (e.g., LEDs, sunlight, medium pressure mercury lamps, or fluorescent lamps) to a “green” printed object, often at somewhat elevated temperature (e.g., 45°C to 80°C), and often in a specially constructed device (post cure chamber) to a green object to reinitiate and further propagate photoinitiated polymerization reactions (e.g., free radical polymerization of acrylates and methacrylates; cationic ring opening polymerization of epoxides, oxetanes, and vinyl ethers) to reach higher overall conversion of photopolymerizable reactive groups in the post cured object. Incomplete or inhomogeneous penetration of light into the printed object leading to inhomogeneous cure; little or no efficacy for non-photocurable reactive groups; and requirement of specialized equipment are disadvantages of light-based post curing. The requirement of light penetration into the object is especially problematic in highly colored compositions, filled compositions, and compositions where the photoinitiator does not photobleach. The methods of the present invention can further include post-treatment of the three- dimensional object(s) formed. Examples of post-treatments include, but are not limited to, washing, post-curing (e.g., by light, e-beam, heat, non-ionizing radiation, ionizing radiation, time (aging), pressure, humidity, or simultaneous or sequential combinations of techniques), metrology, labelling or tracking (e.g., by barcode, QR code, or RFID tag), freeze-dry processing, critical point drying, and packaging. When used as a characteristic of a portion of a container or build chamber, “optically transparent” refers to having high optical transmission to the wavelength of light being used, and “optically flat” refers to being non-distorting (e.g., optical wavefronts entering the portion of the container or build chamber remain largely unaffected). Before printing, a digital file of the article or object to be printed is typically obtained. If the digital file is not of a format that can be used to print the object, the digital file is then converted to a format that can be used to print the object. An example of a typical format that can be used for printing includes, but is not limited to, an STL file. Typically, the STL file is then sliced into two- dimensional layers with use of three-dimensional slicer software and converted into G-Code or a set of machine commands, which facilitates building the object. See B. Redwood, et al., “The 3D Printing Handbook - Technologies, designs applications”, 3D HUBS B.V.2018. Other information concerning optical systems that may useful in connection with the various aspects of the present inventions includes Texas Instruments Application Report DLPA022- July 2010 entitled “DLP ^TM System Optics”; Texas Instruments “TI DLP ^® Technology for 3D Printing – Design scalable high-speed stereolithograpy [sic] systems using TI DLP ^® technology” 2016; Texas Instruments “DLP65000.651018p MVSP Type A DMD”, DLP6500, DLPS040A- October 2014 – Revised October 2016; and Y-H Lee, et al., “Fabrication of Periodic 3D Nanostructuration for Optical Surfaces by Holographic Two-Photon-Polymerization”, Int’l Journal of Information and Electronics Engineering, Vol 6, No.3, May 2016, each of the foregoing being hereby incorporated herein by reference in its entirety. In accordance with additional aspects of the invention, there are provided articles formed from a hardenable resin composition and/or a method in accordance with the present invention. The article that is formed can be, for example, a complete or finished article; a component part of a different article, machine, or system; a partial article suitable for further processing for obtaining additional and/or different features or properties; a prototype of any of the foregoing. EXAMPLES The examples provided herein are provided as examples and not limitations, wherein a number of modifications of the exemplified compositions and processes are contemplated and within the scope of the present invention. Examples of Procedures for Benzoxazine Monomer Preparation Example 1: P-nb Type Benzoxazine Synthesis. With the aid of a funnel, 12.02 g paraformaldehyde powder (95%, Sigma Aldrich) is weighed into a 125 mL Erlenmeyer flask.20 mL distilled water is added.2 mL of 1 N sodium hydroxide aqueous solution is added by syringe. A stir bar is added, and the flask is stoppered with a needle placed in a rubber septum to vent. The flask is set in a 65 deg C metal bead bath with rapid stirring to dissolve the paraformaldehyde and form formaldehyde solution. Next, to a 500 ml three-neck flask, 15.43 g bis(aminomethyl)norbornane (mixture of isomers, >98.0%, TCI) is added.18.82 g phenol (99.0-100.5%, Sigma Aldrich) is added to the flask with the aid of a funnel.200 mL toluene is added to the flask through the funnel. A magnetic stir bar is added to the flask. The flask is set in a room temperature metal bead bath with rapid stirring to dissolve the phenol. Once dissolved, the three-neck flask is transferred to a preheated 65 deg C metal bead bath. The flask is set for rapid magnetic stirring. The formaldehyde solution is poured into the flask. There is very rapid coagulation of the aqueous phase into a white gel which subsequently breaks up within minutes, and once stirring resumes the reaction mixture is now a suspension of aqueous phase in toluene solution. After 2 hr stirring at 65 deg C, the flask is lifted out of the heating bath to cool. The reaction media quickly separates into organic and aqueous layers. The organic phase, containing the product, is typically light yellow or straw colored. The mixture may be worked up immediately after cooling or may be left for some period, e.g., overnight. For workup, the contents of the flask are poured into a 500 mL separatory funnel. The organic phase is washed with 3x 150 mL 1N sodium hydroxide aqueous solution and 3x brine. The aqueous layers are discarded after each wash. The organic layer after washing is decanted into a 600 mL beaker, and anhydrous sodium sulfate (Sigma Aldrich) is stirred in to dry. The mixture is filtered through fluted filter paper into a 1 L round bottom flask. The solvent is substantially removed by rotary evaporation, yielding the product as a viscous oil. The approximate yield is 32 g product (80%). The flask is moved to a vacuum oven set to 60 deg C, and the residual solvent is evaporated overnight under high vacuum. The product is a highly viscous liquid at room temperature. Example 2: pC-nb Type Benzoxazine Synthesis. A 125 mL Erlenmeyer flask is equipped with a funnel.12.02 g paraformaldehyde powder (95%, Sigma Aldrich) is weighed into the flask.20 g distilled water is added.2 mL of 1 N sodium hydroxide aqueous solution is added. The flask is set in a 65 deg C metal bead bath to stir rapidly to dissolve the paraformaldehyde to form formaldehyde solution. A 500 mL three-neck flask is equipped with a funnel and a magnetic stir bar.15.43 g bis(aminomethyl)norbornane (mixture of isomers, >98.0%, TCI) is weighed into the flask.21.63 g p-cresol (99%, Sigma Aldrich), which has been preheated to 40 deg C, is added to the flask.200 mL toluene is added to the flask. The flask is placed in a metal bead bath preheated to 65 deg C to stir. The formaldehyde solution is poured in. The mixture coagulates immediately. The funnel is replaced with a waterless condenser. The two side necks and the condenser top are stoppered with rubber septa. A needle is placed in the septum atop the condenser to vent. After 10 min, the coagulation has broken up and the mixture stirs freely. The mixture has no perceivable color at this stage, indicating good purity of starting materials. The flask is wrapped loosely with aluminum foil to insulate the flask. After 2 hrs, the flask is raised out of the heating bath. The mixture has slight straw color, and the aqueous phase is more colored than the organic phase. The mixture is left to cool and stand overnight. The reaction mixture is poured into a 500 mL separatory funnel. The yellow aqueous layer is discarded. The organic layer is washed with 150 mL 1 N sodium hydroxide aqueous solution. Upon shaking, an emulsion forms. The mixture is left to separate. The aqueous layer forms rapidly, but the funnel is left to stand for ~1 hr to get the highest quality separation possible. The aqueous phase is discarded. The organic phase is washed a second time with 150 mL 1 N sodium hydroxide aqueous solution. The mixture separates rapidly. The aqueous phase is discarded. The organic phase is washed with 150 mL brine. The mixture separates rapidly. The organic phase is washed a second time with 150 mL brine. The mixture separates rapidly. The aqueous phase is discarded. The organic phase is washed with 150 mL distilled water. The mixture separates rapidly. The aqueous phase is discarded. The organic phase is washed a second time with 150 mL distilled water. The mixture separated rapidly, but the mixture is left for 30 min to get as good of a separation as possible before moving on to next step. The aqueous phase is discarded. The organic phase is decanted into a 600 mL beaker, and sodium sulfate is stirred in. After 5 min, the mixture is passed through filter paper into a tared 1 L round bottom flask. The sodium sulfate is rinsed with dichloromethane which is filtered into the flask. The solvent is removed by rotary evaporation until a very viscous oil remained (mass 37 g). The flask is then placed in the vacuum oven set to 60 deg C under vacuum to remove the residual solvent. After overnight in the vacuum oven, the product is completely degassed and has collected at the bottom of the flask. The yield is 35.5 g (85%). Example 3. THN-nb Type Benzoxazine Synthesis.6.00 g paraformaldehyde powder (95%, Sigma Aldrich) is weighed out and added through a funnel to a 125 mL Erlenmeyer flask.10 g distilled water is added.1.3 mL of 1 N sodium hydroxide aqueous solution is added. A magnetic stir bar is added. The flask is stoppered with a rubber septum and placed in a 65 deg C metal bead bath to dissolve the paraformaldehyde to form formaldehyde solution.7.71 g bis(aminomethyl)norbornane (mixture of isomers, >98.0%, TCI) is weighed into a 250 mL three-neck flask.14.82 g 5,6,7,8- tetrahydro-2-naphthol (97%, AmBeed) is weighed into the flask with the aid of a funnel.100 mL toluene is poured into the flask. A stir bar is added to the flask. The flask is placed in a preheated metal bead bath with 65 deg C set point. The mixture is stirred to dissolve. The solution is dark brown due to impurities in the 5,6,7,8-tetrahydro-2-naphthol. The formaldehyde solution is poured into the flask with the aid of a funnel. The residual solution in the Erlenmeyer flask is rinsed with distilled water into the three-neck flask. The three-neck flask is equipped with a waterless condenser. The flask and condenser are stoppered with rubber septa. A needle is used to vent the stopper atop the condenser. The flask is set to stir rapidly. A brown suspension forms. After 2 hr reaction time, the flask is raised out of the bead bath to cool. The reaction mixture is left to stand overnight. The contents of the flask are poured into a 500 mL separatory funnel.150 mL 1 N sodium hydroxide aqueous solution is added, and the funnel is shaken. An emulsion forms.50 mL brine is added, and the funnel is shaken. An emulsion forms but starts to separate.50 mL toluene is added, and the funnel is shaken. The mixture separates. Both the organic and aqueous phases are brown. There is a dark brown layer at the top of the aqueous phase. The aqueous phase is discarded. 150 mL 1 N sodium hydroxide aqueous solution is added, and the funnel is shaken. The mixture separates easily. The aqueous phase is light brown and the organic phase remains brown. The aqueous layer is discarded.200 mL 1 N sodium hydroxide aqueous solution is added and the funnel is shaken. The mixture separates easily and the aqueous phase is discarded. The organic phase is washed a further 3 times with 150 mL brine, discarding the aqueous phase after each wash. The organic phase is decanted into a 600 mL beaker. Sodium sulfate is stirred in to absorb the water. The mixture is passed through filter paper into a 1 L round bottom flask to remove the sodium sulfate, which is rinsed with dichloromethane to collect residual product. The solvent is removed by rotary evaporation. The product is a glass. The yield is approximately 20 g (80%). The flask is placed in the vacuum oven set to 60C to remove residual toluene. Example 4: P[0.6]THN[0.4]-nb Type Benzoxazine (Mixed Phenol Benzoxazine Synthesis). To a 125 mL Erlenmeyer flask is added a magnetic stir bar, 0.24 g sodium hydroxide pellets, 12.02 g paraformaldehyde prills (Electron Microscopy Sciences), and 20 mL distilled water. The flask is stoppered with a rubber septum pierced with a needle for a vent and placed in a 65 deg C metal bead bath to stir rapidly to dissolve the paraformaldehyde and form formaldehyde solution. To a 500 mL three neck flask is added a magnetic stir bar, 11.86 g (0.080 mol) 5,6,7,8,-tetrahydro-2-naphol (97%, Sigma Aldrich), 11.29 g (0.12 mol) phenol (99.0-100.5%, Sigma Aldrich), 15.43 g (0.10 mol) bis(aminomethyl)norbornane (mixture of isomers, >98.0%, TCI), and 200 mL toluene. The flask is set in a 65 deg C metal heating block to stir vigorously and dissolve. The side necks are stoppered with rubber septa. The funnel is left on the center neck. Upon dissolution, the solution is yellowish. The formaldehyde solution is poured into the three neck flask under rapid stirring and rinsed in with a few milliliters of distilled water. The mixture immediately forms a white opaque suspension which becomes off-white and translucent after a few moments. At no point does the reaction mixture set to a gel or stop stirring. The funnel is replaced with a waterless condenser. The top of the condenser is stoppered with rubber septa which is pierced with a needle to vent. The flask is wrapped loosely in foil. After two hours, the reaction is stopped by raising the flask out of the heating block. The reaction mixture is left to cool and stand overnight. The reaction mixture is poured into a 500 mL separatory funnel. The aqueous layer is discarded.150 mL 1 N sodium hydroxide aqueous solution is added. An emulsion forms immediately. The emulsion is left to separate. The aqueous layer is discarded. The organic layer is washed with a further 1x 150 mL 1 N sodium hydroxide aqueous solution, 2x 150 mL brine, and 2x 150 mL distilled water, discarding the aqueous layer after each wash. The organic phase is decanted into a 600 mL beaker, and sodium sulfate is stirred in. After 5- 10 min, the solution is passed through filter paper into a tared 1 L round bottom flask. The solvent is removed by rotary evaporation. The flask is placed in a vacuum oven set to 80 deg C under high vacuum to remove residual solvent. The yield is 36.1 g (83%). Example of Procedure for Upconverting Nanocapsule Synthesis Distilled water, is titrated to pH 10.5 with sodium hydroxide (200 mL), is chilled over an ice bath and then poured into to a Vitamix Blender (Amazon.com) in an inert atmosphere. The stock solution containing sensitizer and annihilator (1.45 mL) is carefully dispensed into the water in one portion (stock solution: PdTBTP (0.5 mg/mL and Br-TIPS anthracene (10 mg/mL) in 99% oleic acid)). The solution is blended for 60 s at the maximum speed, and the emulsion is transferred to the flask and immediately stirred at high speed. (3-aminopropyl)triethoxysilane (0.75 mL, Acros Organics) is added until the mixture becomes transparent, and then 5K MPEG-Silane (4 g, Nanosoft Polymers) is immediately added to prevent capsule aggregation. After 10 minutes, tetraethyl orthosilicate (TEOS, 36 mL, Sigma Aldrich) is added in one portion. The flask is sealed with a septum and the solution is stirred vigorously for 30 minutes at room temperature. Then, the flask is heated to 65 ºC at constant pressure for 2 days. The reaction crude is allowed to cool to room temperature, poured into a centrifuge tube, and centrifuged at 7000 rpm for one hour at room temperature (18-22 ºC), after which the pellet is discarded. The solution is then centrifuged at 7000 rpm for 14 hours at room temperature. After the second centrifuge, the Upconverting Nanocapsule (UCNC) paste is transferred from the glovebox to a round bottom flask where 100 mL of ethanol and 10 mL of water, as well as 2 mL of 30% NH3OH and is stirred until a homogeneous solution is formed. To this solution is added at 60 degrees Celsius 6 mL of 3-(trimethoxysilyl)propyl methacrylate. The solution is stirred 24 hours at 60 degrees Celsius, then centrifuged at 6000 RPM for 8 hours to obtain the solid capsule paste, discarding the ethanol. This paste is redispersed in 100 mL of N,N-dimethylacrylamide and stirred 4 hours at 60 degrees Celsius to remove any external sensitizer that might remain, and centrifuged one more time at 6000 RPM for 8 hours to obtain the final capsule paste that is dispersed at 60 wt% in N,N-dimethylacrylamide for subsequent dispersion into resin. Example of Procedures for Capsule Dispersion Example 1. To make the dispersion of capsules suitable for mixing into resin, a quantity of capsule slurry (typically 2 g to 100 g) obtained from centrifugation is transferred to a suitable plastic jar. The jar is mixed in a speedmixer (model DAC 150.1 FVZ-K, Flacktek) to mix the capsules and distribute the capsules evenly in the bottom of the jar. A small quantity (about 0.1-1 g) of mixed capsule slurry is placed in a disposable aluminum pan. The masses of the sample and pan are recorded. The mass of the remaining capsule slurry in the jar is recorded. The pan is placed in a vacuum oven set 130°C to dry under vacuum for at least 2 hr. The pan is removed from the oven and the combined mass of dried samples and pan is recorded. The solids fraction of the capsule slurry is calculated as the dried capsules mass (difference in final and empty pan masses) divided by the initial sample mass times 100%. A typical range of solids fraction in the capsule slurry is about 35 wt% to about 70 wt%. A quantity of N,N-dimethylacrylamide (99.5%, Sigma Aldrich) is added to the jar containing the remaining capsule slurry to adjust the solids content to 35 wt%. The jar is speedmixed until a homogeneous dispersion is obtained. Mixing time and speed is adjusted based on the consistency of the material. Typical mixing speeds and times are 3500 rpm and 3 min. The mixing may be conducted multiple times at the same or different speeds and times. If any non- dispersible material remains, the dispersion is poured or pressed through a disposable paint strainer (190 μm mesh) into a new jar for storage. Example 2. To make the dispersion of capsules suitable for mixing into resin, a quantity of capsule slurry (typically 20 g to 200 g) obtained from centrifugation is transferred to one or more suitable plastic jars, the tare mass of which has been recorded. The jar is mixed in a speedmixer (model DAC 150.1 FVZ-K, Flacktek) to mix the capsules and distribute the capsules evenly in the bottom of the jar; generally this step requires 3 min at 3500 rpm but longer mixing times may be required if the slurry is tightly packed. A small quantity (about 0.1-1 g) of mixed capsule slurry is placed in a disposable aluminum pan, the tare mass of which has been recorded. The combined mass of the sample and pan is recorded. The combined mass of the remaining capsule slurry and the jar is recorded, and the remaining capsule mass is calculated by subtracting the jar tare mass from this value. The aluminum pan is placed in a vacuum oven set 130°C to dry under high vacuum for at least 2 hr. The pan is removed from the oven and the combined mass of dried sample and pan is recorded. The solids fraction of the capsule slurry is then calculated as the dried capsules mass (difference in final and empty pan masses) divided by the initial sample mass times 100%. A typical range of solids fraction in the capsule slurry is about 35 wt% to about 70 wt%. A quantity of benzyl acrylate (Komerate A003, Green Chemical) is added to the jar containing the remaining capsule slurry to adjust the solids content to 35 wt%. The jar is speedmixed until a homogeneous dispersion is obtained. Mixing time and speed is adjusted based on the consistency of the material; generally 3 min at 3500 rpm is required. The mixing may be conducted multiple times at the same or different speeds and times. The dispersion is obtained as a medium to low viscosity liquid that may exhibit significant thixotropy. If any non-dispersible material remains, the dispersion is poured or pressed through a disposable paint strainer (190 μm mesh) into a new jar for storage. Examples of Procedures for Resin Mixing Example 1.100 mg Esacure 3644 (purified by column chromatography, IGM Resins) is weighed out and added to a 20-mL amber septum-top scintillation vial.200 mg diphenyl(2,4,6- trimethylbenzoyl)phosphine oxide (>98%, TCI) is weighed out and added to the vial.0.4 g 2- ethylhexyl 4-(dimethylamino)benzoate (98%, Sigma Aldrich) is added to the vial by disposable plastic pipet.0.6 g 1,4-cyclohexanedimethanol divinyl ether (mixture of isomers, 98%, Sigma Aldrich) is added to the vial by disposable plastic pipet.2.0 g capsules dispersion (35.0 wt% capsules dry weight in N,N-dimethylacrylamide; see Capsule Dispersion Procedures: Example 1 below) is added to the vial by disposable plastic pipet. The vial is mixed in a speedmixer (model DAC 150.1 FVZ-K, Flacktek) for 1 min at 3100 rpm to complete dissolution of the photoinitiators. The vial is set aside.1.8 g P-d type benzoxazine (Shikoku) is weighed out and added to a 40-mL amber septum-top scintillation vial.7.0 g SR228 high Tg acrylate monomer (alternative name PRO13443, Sartomer) is added to the vial by disposable plastic pipet, and the vial is placed in a heating block at 105°C for 5 min. The vial is speedmixed for 1 min at 3100 rpm. If undissolved benzoxazine resin remains, the vial is briefly reheated and then remixed; this is continued until all the benzoxazine resin has dissolved.0.6 g benzyl acrylate (Komerate A003, Green Chemical) is added to the vial. The vial is speedmixed for 1 min at 3100 rpm. The contents of the first vial are transferred by syringe into the second vial. The vial is speedmixed for 1 min at 3100 rpm.0.48 g thixotrope (Rheobyk 410, BYK) is added by disposable plastic pipet. The vial is speedmixed for 1 min at 2100 rpm. The vial is transferred to the glovebox, and 90 μL 0.1 w/v% 2,2,6,6- tetramethylpiperidinooxy free radical (98%, Sigma Aldrich) solution in N,N-dimethylacrylamide is added by micropipette. The vial is resealed, placed in a heating block at 45°C, and then sparged with pure nitrogen for 12 min. Example 2. To a 40 mL amber septum top scintillation vial, 0.14 g thixotrope (Crystasense HP-5, Croda) is added.100 μL Sudan I solution (30 mg Sudan I dissolved in 5.00 g N,N- dimethylacrylamide) is added by capillary piston pipette.0.40 g N,N-dimethylacrylamide (99.5%, Sigma Aldrich) is added by disposable plastic pipet. The vial is placed in a 105 deg C heating block for 5 min and then vortexed to dissolve the thixotrope.600 mg Ivocerin photoinitiator (Ivoclar) is weighed out and added to the vial. The vial is returned to the heating block for 1-2 min. The vial is vortexed to dissolve the photoinitiator.1.0 g capsules dispersion (35.0 wt% capsules dry weight in benzyl acrylate; see Capsule Dispersion Procedures: Example 2 below) is added to the vial, and the vial is mixed in a speedmixer (model DAC 150.1 FVZ-K, Flacktek) for 1 min at 3100 rpm. The mixture is slightly hazy.6.0 g P-nb type benzoxazine resin pre-blended with acrylate monomer for easier handling (33 wt% P-nb in Sartomer SR228) is added to the vial, and the vial is placed in a 65 deg C heating block for 5 min. The vial is speedmixed for 1 min at 3100 rpm. The vial is returned to the 65 deg C heating block for 1-2 min and then speedmixed again. The resin remains slightly hazy. 2.5 g dendritic thioether acrylate oligomer (BDT-4330, Dymax) and 1.0 g vinyl ether monomer (1,4- cyclohexanedimethanol divinyl ether, mixture of isomers, 98%, Sigma Aldrich) are added to the vial, and the vial is speedmixed for 1 min at 3100 rpm. The resin is completely haze free. The vial is transferred to the glovebox, and 90 μL 0.1 w/v% 2,2,6,6-tetramethylpiperidinooxy free radical (98%, Sigma Aldrich) solution in N,N-dimethylacrylamide is added by micropipette. The vial is resealed, placed in a heating block at 45°C, and then sparged with pure nitrogen for 12 min. Examples of Procedures for Hardening Steps Hardening of the above-described mixed resin can include a first photo-cure step and a second thermal cure step. Photo-cure Step Example 1. After sparging the hardenable resin composition with nitrogen gas to remove dissolved oxygen (see Resin Mixing Procedures above), a quantity of the hardenable resin composition is transferred to a cuvette constructed of a suitable transparent and chemically resistant material (e.g., glass, cyclic olefin copolymer, quartz). The cuvette is sealed with a plastic lid and the cuvette is removed from the glovebox. If bubbles are present, the cuvette is centrifuged to bring the bubbles to the surface and remove them from the resin. The cuvette is affixed in a plastic holder on the three axis linear stage of the 3D printer.3D printing is accomplished by focusing long wavelength (e.g., 640 nm) laser or light emitting diode light to one or more focal points within the volume of the resin, at which focal point or points the capsules emit shorter wavelength (e.g., 440 nm) light into the resin which is absorbed by the photoinitiator to initiate photocuring. Suitable long wavelength light power densities at the focal point(s) may be, e.g., 1 W/cm ² , 5 W/cm ² , 10 W/cm ² , 50 W/cm ² , 100 W/cm ² , or more, depending on the capsules characteristics and the desired curing characteristics (e.g., voxel size, resolution). The time required to photocure resin at a focal point may be, e.g., 0.001 ms, 0.01 ms, 0.1 ms, 1 ms, 10 ms, 100 ms, 1 s, 10 s, 100 s, or more, and may elapse in a continuous exposure or a series of exposures. Exposure at multiple adjacent or nearby focal points may combine to provide a higher cumulative exposure. At any given time, the locations of the focal points (which cause photocuring) may be selected by moving the sample relative to the projected light (e.g., by translating the linear stage), by changing the locations where the long wavelength light is projected (e.g., using a digital micromirror device to select projected locations), or by a combination of these or other methods. When the desired region(s) of the hardenable resin composition have been photo- cured (solidified), the solid material (which comprises the intermediate article) is separated from the remaining uncured liquid and then optionally subjected to a post treatment (e.g., washing with a solvent, additional photocuring, trimming, etc.). Thermal Cure Step Example 1. The intermediate article is placed in a vacuum oven (model 1410M, Shel Lab) equipped with a rotary vane vacuum pump (model E2M1.5, Edwards Vacuum). The oven begins at room temperature. The oven is set to 80 deg C and vacuum is applied. After 1 hr, the oven set point is increased to 90 deg C. After 1 hr, the oven set point is increased to 100 deg C. After 30 min, the oven set point is increased to 105 deg C. After 1 hr, the oven set point is increased to 110 deg C. After 30 min, the oven set point is increased to 115 deg C. After 30 min, the oven set point is increased to 120 deg C. After 30 min, the oven set point is increased to 125 deg C. After 30 min, the oven set point is increased to 130C. After 30 min, the oven set point is increased to 135 deg C. After 30 min, the oven is turned off. The article is left to cool under vacuum. After standing overnight, and while remaining under vacuum, the oven is set for 120 deg C. After 1.5 hr, the oven set point is increased to 140 deg C. After 1 hr, the oven set point is increased to 160 deg. After 1.5 hr, the oven set point is increased to 180 deg C. After 2.5 hr, the oven set point is increased to 200 deg C. After 40 min, the oven set point is increased to 220 deg C. After 2 hrs, the oven is turned off. The article is left to cool under vacuum. The article, now fully cured, is removed from the oven after standing overnight. As used herein, the singular forms "a", "an" and "the" include plural unless the context clearly dictates otherwise. Thus, for example, reference to an emissive material includes reference to one or more of such materials. Applicant specifically incorporates the entire contents of all cited references in this disclosure. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range. Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the present specification and practice of the present invention disclosed herein. It is intended that the present specification and examples be considered as exemplary only with a true scope and spirit of the invention being indicated by the following claims and equivalents thereof.