CROSS-REFERENCE TO RELATED APPLICATIONS

BACKGROUND

[0001] On-demand fabrication of parts using three-dimensional (3D) computer-assisted design (CAD) data, also referred to as 3D printing, has been improving and becoming more prevalent. 3D printing technologies can include several different technology methods. One such method is ink-jet printing.

SUMMARY

[0002] The present disclosure describes a system and method for three-dimensional (3D) printing of reactive polyimide precursor compounds onto a substrate to provide for rapid prototyping of one or more polyimide layers.

[0003] The present inventors have recognized, among other things, that a problem to be solved can include that rapid prototyping of polyimide materials typically requires the use of aggressive organic solvents. The present subject matter described herein can provide a solution to this problem, such as by providing for 3D inkjet printing of reactive solutions of polyimide precursor compounds in relatively mild solvents, such as water or an aliphatic alcohol such as methanol or ethanol.

[0004] The present inventors have recognized, among other things, that a problem to be solved includes that solutions of polyimide materials in the aggressive organic solvents or in a molten form have too high of a viscosity for inkjet printing methods. The present subject matter described herein can provide a solution to this problem, such as by providing for a polyimide precursor solution with a low solution viscosity, e.g., below 50 centipoise (cP) for a 50% solids concentration.

[0005] The present inventors have recognized, among other things, that a problem to be solved can include that polyimide parts formed by rapid prototyping tend to have anisotropic properties, such as less mechanical strength in a depth direction (sometimes referred to as the Z-direction) compared to the length and width directions (sometimes referred to as the X- and Y-directions). The present subject matter described herein can provide a solution to this problem, such as by providing for a 3D inkjet printing system for printing reactive polyimide precursor compounds that, upon polymerization, form chemical bonds through the part in all three dimensions resulting in a substantially isotropically even part.

[0006] The present inventors have recognized, among other things, that a problem to be solved can include that present inkjet printing methods require curing of the printed material with ultraviolet light, which requires line-of- sight for the UV light and can sometimes result in uneven curing. The present subject matter described herein can provide a solution to this problem, such as by providing for 3D inkjet printing of reactive polyimide precursor compounds that can be polymerized by the application of heat, which can be applied evenly and reliably.

[0007] The present inventors have recognized, among other things, that a problem to be solved can include adhesion between layers of a part fabricated by 3D inkjet printing. The present subject matter described herein can provide a solution to this problem, such as by providing for printing of reactive solutions of polyimide precursor compounds that can react and crosslink between layers.

[0008] The present inventors have recognized, among other things, that a problem to be solved can include limited ability to control the final properties of a polyimide formed by rapid prototyping, such as molecular weight, density, tensile strength and other physical properties. The present subject matter described herein can provide a solution to this problem, such as by providing for control over physical properties of a final polyimide material by controlling initial properties of the polyimide precursor solution that is printed.

BRIEF DESCRIPTION OF THE FIGURES

[0009] FIG. 1 is a schematic diagram of an example system for fabricating a polyimide part via inkjet printing.

[0010] FIG. 2 is a schematic diagram of another example system for fabricating a polyimide part via inkjet printing.

[0011] FIG. 3 is a flow diagram of an example method of fabricating a polyimide part via inkjet printing.

[0012] FIG. 4 is a flow diagram of another example method of fabricating a polyimide part via inkjet printing.

DETAILED DESCRIPTION

[0013] 3D inkjet printing is a method for fabricating structures by selectively printing droplets of a build material onto a substrate in a build area. 3D inkjet printing is gaining popularity due to its versatility and high speed of construction. Polymers used in 3D inkjet printing typically have been dissolved in an organic solvent to adjust the solution viscosity to the requirements of inkjet process. Some materials, such as polyimides, have typically been dissolved in very aggressive organic solvents, such as tetrahydrofuran, chlorinated solvents including methylene chloride, chloroform, and dichlorobenzene, or solvents with high boiling points greater than 150 °C such as N-methyl pyrrolidone, dimethylacetamide or dimethylformamide. These solvents can have undesirable environmental or health effects. Even with the use of such aggressive solvents, polyimides typically have not been good candidates for 3D inkjet printing due to their high molecular weights and corresponding high viscosities.

[0014] The present disclosure describes 3D inkjet printing systems and methods for fabricating structures by selectively printing droplets of one or more solutions comprising one or more polyimide precursor compounds onto a substrate in a build area, permitting the formation of structure including polyimides using 3D inkjet printing systems and methods. One or more print heads can be used to print the polyimide precursor compound onto one or more target locations corresponding to a cross- sectional area of a structure including polyimide. A layer of the one or more polyimide precursor solutions can be selectively printed onto the one or more target areas within a build area. The build area can be defined by a selected coordinate system, such as Cartesian and polar coordinate systems, so that CAD data can be used to direct the one or more print heads. The one or more print heads can selectively print the one or more polyimide precursor solutions onto the one or more target locations corresponding to the cross-section of a layer. The printed polyimide precursor solution or solutions can be subjected to environmental conditions, such as a selected temperature and pressure, that results in polymerization of the polyimide precursor compound at the target locations where the one or more precursor solutions were printed, forming a polyimide layer. This process optionally can be repeated to print further layers, such as a second layer on the first layer, a third layer on the second layer, a fourth layer on the third layer and so on, until the selected structure is completed.

[0015] FIG. 1 shows an example of an inkjet printing system 10 for fabricating a structure 12 including polyimide by printing one or more polyimide precursor compounds to form a polyimide. The printing system 10 can include a build chamber 14 enclosing a substrate 16 onto which the structure 12 is to be built. The system 10 can include a first print head 18 to print a polyimide precursor solution, also referred to as a precursor print head 18. The polyimide precursor solution can comprise, in a solvent, one or more of: one or more bisanhydride precursor compounds; one or more diamine precursor compounds; or a reaction product of one or more bisanhydride precursor compounds and one or more diamine precursor compounds. The polyimide precursor compound can be polymerized by heating the printed polyimide precursor solution, which can result in the removal of solvent from the solution and can initiate polymerization of the polyimide precursor compound to form the polyimide polymer that will make up the structure 12 including polyimide, according to an example.

[0016] The system 10 can optionally include a second print head 20 to selectively print a support material, also referred to as a support print head 20. Additional print heads beyond the precursor print head 18 and the support print head 20 can also be included in the system 10 to print other materials, such as a second polyimide precursor solution comprising one or more second polyimide precursor compounds, a catalyst for the polymerization reaction, a surface protectant, or a colorant.

[0017] One or more of the print heads 18, 20 can be moved relative to the substrate 16 so it can be selectively aimed onto any of a plurality of target locations 24 on the surface of the substrate 16 or on the surface of a top-most layer that has been built on the substrate 16. A print head 18, 20 can be coupled to a print head block 26 that is movable over the substrate 16 to selectively aim it toward a target location. The print head block 26 can be movable in any direction within a selected coordinate system, such Cartesian and polar coordinate systems. The print head block 26 can be movable over the substrate 16 in an X-direction 2. The print head block 26 can be movable over the substrate 16 in a Y-direction 4. The X-direction 2 can be substantially orthogonal to the Y-direction 4. Both the X and Y-directions 2, 4 can be substantially parallel to the top surface of the substrate 16. The print head block 26 can be moved by a printer actuator 28 according to the selected coordinate system, e.g., by moving the print head block 26 along the X-direction 2 and the Y-direction 4 over the substrate 16, such as with at least one of one or more motors, and one or more screw drives. The actuator 28 can also move the print head block 26 in a Z-direction 6 (shows as being up and down in FIG. 1). The Z-direction 6 can be substantially orthogonal to one or more of the X-direction 2, the Y-direction 4, and the top surface of the substrate 16. A print head (e.g., 18 or 20) can be moved separately, e.g., with its own actuator. The substrate 16 can be movable in one or more directions, such as one or more of the X-, Y-, and Z-directions 2, 4, 6, by a separate substrate actuator, such as one or more motors 30.

[0018] Print head 18, 20 can print droplets of the solution being fed to the respective print head 18, 20. The precursor print head 18 can print droplets 32 of a polyimide precursor solution, also referred to as precursor droplets 32. The polyimide precursor solution can comprise at least one of a first polyimide precursor compound, a second polyimide precursor compound, or a reaction product of the first polyimide precursor compound and the second polyimide precursor compound. The support print head 20 can print second droplets 34 of a support material, also referred to as support material droplets 34, to form support structures 36, e.g., to provide support for an overhang 38 of the structure 12.

[0019] The precursor print head 18 can print the precursor droplets 32 onto each of the plurality of target locations 24 where the precursor solution is to be printed to build an active layer 40 of the structure 12 being built. The term "active layer," as used herein, refers to the layer of the structure 12 currently being printed by the printing system 10, which corresponds to a cross- section of the structure 12, as described above. The support print head 20 prints the support material droplets 34 onto one or more support locations 42 to build a layer of a support structure 36.

[0020] As is further shown in FIG. 1, the printing system 10 can include one or more dispensing devices to dispense the solutions being printed by the print heads 18, 20. A first dispenser 44 can dispense a fluid (e.g., the polyimide precursor solution) to the precursor print head 18. A second dispenser 46 can dispense a fluid (e.g., the support material) to the support print head 20. The dispensers 44, 46 can include a reservoir for the fluid being dispensed to the respective print head 18, 20. The dispensers 44, 46 can also include a pump or other fluid displacement device to move the fluid from the reservoir to its respective print head 18, 20. The fluids being dispensed to the print heads 18, 20 can be fed through flexible conduits, such as flexible tubing or piping, to accommodate movement of the print head block 26 and the print heads 18, 20. A first flexible conduit 48 can carry the build material precursor solution to the precursor print head 18. A second flexible conduit 50 can carry the support material to the support print head 20.

[0021] The printing system 10 can include an environmental system to control one or more conditions that the printed materials are exposed to, such as to facilitate polymerization of the polyimide precursor compound. The environmental system can provide for reaction and polymerization of a first polyimide precursor compound (e.g., one or more bisanhydride precursor compounds) and a second polyimide precursor compound (e.g., one or more diamine precursor compounds). In some examples, the environmental system can control the conditions in order to facilitate formation of the support structures 36. The environmental system can include a heater 52 to control the temperature within the build chamber 14. The heater 52 can heat the printed droplets 60 to a reaction temperature to initiate and propagate polymerization reaction of the polyimide precursor compound, such as a polymerization between one or more bisanhydride precursor compounds and one or more diamine precursor compounds or of a reaction product of one or more bisanhydride precursor compounds and one or more diamine precursor compounds to form a polyimide material to build a solidified or substantially solidified active layer 40. The active layer can be printed and polymerized on top of the substrate 16, e.g., if the active layer is the first printed layer or if the active layer forms a single- layer structure 12, or the active layer 40. The active layer 40 can be built on a substrate 16. The active layer 40 can be formed on top of previously built layers 54, 56, 58, e.g., so that the active layer 40 is part of a multi-layer structure 12. For polymerization of one or more bisanhydride precursor compounds and one or more diamine precursor compounds, the heater 52 can be configured to heat the printed droplets 32 to a reaction temperature of from about 100 °C to about 400 °C, such as from about 250 °C to about 500 °C, for example from about 300 °C to about 450 °C.

[0022] The reaction temperature provided by the heater 52 can depend on a number of factors, including, but not limited to, the concentrations of the polyimide precursor compound in the precursor solution of the precursor droplets 32, and a desired reaction rate for the polymerization of the polyimide precursor compound. The reaction rate selected can be fast enough such that the active layer 40 polymerizes to such an extent that the active layer 40 can support printing of a subsequent layer before printing of the subsequent layer is commenced. The reaction temperature provided by the heater 52 can be selected to polymerize the polyimide precursor compound to a B-stage level of polymerization and then a subsequent layer of the one or more polyimide precursor solutions can be printed on top of the B- staged polymer layer,.

[0023] The reaction rate can be selected to be slow enough so that full polymerization and solidification of the active layer 40 does not occur until after the subsequent layer has been printed. This can allow the printed droplets 60 that make up the active layer 40 to further combine during printing to form a substantially continuous active layer 40 with a substantially flat surface. It can also allow the partially polymerized solution of the printed droplets 60 to intermix at least partially with the previously built layer 58 immediately below the active layer 40, or can allow for better diffusion of the polyimide precursor compound into the previously built layer 58, while the precursor droplets 32 are being printed and thus can allow for some crosslinking between layers 40 and 58 because the solution of the previously built layer 58 may not be fully polymerized. Similarly, a slower reaction rate can allow a subsequently-built layer that is printed on top of the active layer 40 to partially intermix with the active layer 40 and/or for better diffusion of the polyimide precursor compound into the active layer 40 to provide for at least partial crosslinking between the active layer 40 in FIG. 1 and the subsequently-built layer. Crosslinking within the same active layer 40, e.g., crosslinking among the printed droplets 60, or between layers of a multi-layer structure 12 can provide for a stronger and more unitary structure 12 than if the cross-linking did not occur.

[0024] The environmental system can include a pressure control system 62 to control a pressure within the build chamber 14. The pressure in the build chamber 14 can be controlled so that the pressure experienced by the active layer 40 can be optimized for polymerization of the polyimide precursors.

[0025] The printing system 10 can include a control system to control one or more of the components of the system 10, such as one or more of the print heads 18, 20, the printer actuator 28, the one or more motors 30, and one or more of the dispensers 44, 46. The control system can ensure that the precursor droplets 32 are printed at selected times and onto selected target locations 26.

[0026] The control system can include one or more process controllers 64. The one or more process controllers can process and provide instructions to components of the system 10. The one or more process controllers 64 can take the form of any processing or controlling device capable of providing the instructions, including, but not limited to, one or more microprocessors, one or more controllers, one or more digital signal processor (DSP), one or more application- specific integrated circuit (ASIC), one or more field-programmable gate array (FPGA), or other digital logic circuitry. The instructions provided by the process controller 64 to the components under its control can take the form of electrical signals via communication links 66. The communication links 66 can be any wired or wireless connection that can transmit signals between the process controller 64 and the device or devices receiving the signal. The one or more process controllers 64 can be configured so that the support material droplets 34 are printed at selected support locations 42 at selected times.

[0027] The one or more process controllers 64 of the control system can be configured to control the environmental system. The one or more process controllers 64 can be configured to control the heater 52. The one or more process controllers 64 can be configured to control the pressure control system 62. The one or more process controllers 64 can control the reaction conditions within the build chamber 14 to facilitate polymerization of the polyimide precursor materials. The one or more process controllers 64 can control the heater 52 through a feedback system, such as with a temperature sensor 68 that can determine the temperature within the build chamber 14 and provide a temperature reading signal to the one or more process controllers 64. The one or more process controllers 70 can provide a control signal to the heater 52 in order to reach a temperature set point. The one or more process controllers 64 can control the pressure control system 62 through a feedback system, such as with a pressure sensor 70 that can determine the pressure within the build chamber 14 and provide a pressure reading signal to the process controller 64, which can, in turn, provide a control signal to the pressure control system 62 in order to reach a pressure set point.

[0028] FIG. 2 shows another example inkjet printing system 100 to fabricate a structure 102 by printing one or more reactive polyimide precursor solutions to form a structure 102 including polyimide. The printing system 100 can include a build chamber 104 enclosing a substrate 106 onto which the structure 102 is to be built. The system 100 can include a first print head 108 to print a first polyimide precursor solution, also referred to as a first precursor print head 108. The printing system 100 can include a second print head 110 to print a second polyimide precursor solution, also referred to as a second precursor print head 110. The first precursor print head 108 and the second precursor print head 110 are also referred to collectively as "precursor print heads 108, 110" or simply "print heads 108, 110." The printing system 100 can include a third print head 112 to dispense a support material, also referred to as a support material print head 112. Additional print heads beyond the precursor print heads 108, 110 and the support material print head 112 can also be included in the system 100 to print other materials, such as a catalyst for the polymerization reaction, a surface protectant, or a colorant

[0029] Print head 108, 110, 112 can be moved relative to the substrate 106 so that the print head 108, 110, 112 can be selectively aimed onto any of a plurality of target locations 114 on the surface of the substrate 106 or on the surface of a top-most layer that has been built on the substrate 106. The print heads 108, 110, 112 can be coupled to a print head block 116 that can be moved over the substrate 106 to aim the print heads 108, 110, 112 toward a target location 114. The print head block 116 can be movable in any direction within a coordinate system, such as Cartesian and polar coordinate systems. The print head block 116 can be movable within a build area in an X-direction 2 (shown as being from left to right in FIG. 2) and in a Y-direction 4 (shown as being into and out of the page in FIG. 2). The X-direction 2 can be substantially orthogonal to the Y-direction 4. Both the X and Y-directions 2, 4 can be substantially parallel to the top surface of the substrate 16. The print head block 116 can be moved by a printer actuator 118 according to the coordinate system, e.g., by moving the print head block 116 along the X- direction 2 and the Y-direction 4 over the substrate 106, such as with one or more motors, screw drives, or other moving mechanisms. The printer actuator 118 can also move the print head block 116 in a Z-direction 6 (shows as being up and down in FIG. 1). The Z-direction 6 can be substantially orthogonal to the X-direction 2, the Y-direction 4, and the top surface of the substrate 106. A print head (e.g., 108, 110, or 112) can be moved separately, e.g., each can be part of its own print head block that is controlled by its own separate print head positioning device.

[0030] Print head 108, 110, 112 can print droplets of the solution being fed to the print head 108, 110, 112. The first precursor print head 108 can print first droplets 120 of a first precursor solution comprising a first polyimide precursor compound (e.g., one or more bisanhydride precursor compounds) in a first solvent, also referred to as first precursor droplets 120. The second precursor print head 110 can print second droplets 122 of a second precursor solution comprising a second polyimide precursor compound (e.g., one or more diamine precursor compounds), also referred to as second precursor droplets 122. The support print head 110 can print third droplets 124 of a support material, also referred to as support material droplets 124, to form support structures 126.

[0031] At the target locations 114, one or more first precursor droplets 120 and one or more second precursor droplets 122 can be printed successively so that the one or more first precursor droplets 120 and the one or more second precursor droplets 122 combine at the target location 114 to form a reactive mixture droplet 128 at the target location 114. The term "successively," "successive," or similar language, as used herein, can refer to the one or more first precursor droplets 120 being printed separately from the one or more second precursor droplets 122. For example, the first precursor droplet or droplets 120 can be printed to a target location 114 first and then the second precursor droplet or droplets 122 can be printed to the target location 114 to combine with the printed first precursor droplet 120 to form the reactive mixture droplet 128, or vice versa, with the second precursor droplet or droplets 122 being printed to the target location 114 first and then the first precursor droplet or droplets 120 being printed to the target location 114 to combine with the printed first precursor droplet 120 to form the reactive mixture droplet 128. The first and second precursor droplets 120, 122 for a particular target location 114 can be printed at substantially the same time so that the first precursor droplet 120 arrives at a target location 114 at substantially the same time as a corresponding second precursor droplet 122.

[0032] By separating the printing of the first polyimide precursor solution and the second polyimide precursor solution, e.g., from the first and second print heads 108, 110 respectively, the printing system 100 can allow for easier control of the concentrations of the first and second polyimide precursor compounds in the reactive mixture droplets 128, which in turn can provide for easier control over the composition of the resulting polyimide polymer. Control over the composition of the final polyimide polymer can allow for some level of control over one or more physical properties of the structure 102 including polyimide. By controlling the concentration of the first polyimide precursor compound (e.g., one or more bisanhydride precursor compounds) in the first precursor droplets 120 and the concentration of the second polyimide precursor compound (e.g., the diamine) in the second precursor droplets 122, the molar ratio of the first polyimide precursor compound relative to the second polyimide precursor compound in the reactive mixture droplets 128 can be controlled. The volume of a precursor droplet 120, 122 printed onto a target location 114 can be controlled. The printing system 100 can control the volume of the first and second precursor droplets 120, 122 that are printed onto a target location 114, the number of first and second precursor droplets 120, 122 that are printed onto a target location 114, or both. Variations in the molar ratio of the first and second precursor compounds (e.g., one or more bisanhydride precursor compounds and one or more diamine precursor compounds) in the reactive mixture droplets 128 can control properties of the resulting structure 102, including, but not limited to, final molecular weight, reactive functional groups of the final polymer, and mechanical properties including flexural, tensile, and impact strengths.

[0033] The precursor print heads 108, 110 can both be aimed at the same target location 114, as shown in the example printing system 100 of FIG. 2. When the print head block 116 that carries the print heads 108, 110 is stationary, the first precursor droplets 120 can be aimed at the same target location 114 as the second precursor droplets 122 so that the droplets 120, 122 combine to form the reactive mixture droplet 128 at that target location 114. Print head 108, 110 can be aimed in the same direction and the print head block 116 can be moved so that the first and second precursor solutions can be printed onto a target location 114.

[0034] The physical co-aiming of the print heads 108, 110, as shown in the example of FIG. 2, can potentially provide for precision and accuracy to direct printing of the first and second precursor droplets 120 and 122 at the target locations 114 in order to form the reactive mixture droplets 128. The co-aiming of the print heads 108, 110 can also provide for faster mixing of the first and second precursor solutions in order to initiate the polymerization reaction, which can provide for more precision of the solutions with reduced or minimized spreading due to flow of the solution.

[0035] The remainder of the printing system 100 shown in FIG. 2 can be substantially similar or identical to the printing system 10 shown in FIG. 1. Material dispensing systems can be included to dispense the solutions or fluids to the print heads 108, 110, 112. A first dispenser 130 can dispense the first precursor solution to the first precursor print head 108. A second dispenser 132 can dispense the second precursor solution to the second precursor print head 110. A third dispenser 134 can dispense the support material to the support material print head 112. The environment in the build chamber 104 can be controlled by an environmental control system to provide for reaction between the precursors in the precursor solutions. A heater 136 can control the temperature within the build chamber 104. A temperature sensor 138 can be used to measure the temperature within the build chamber 104, which can be used to adjust the heater 136. A pressure-control system 140 can control the pressure within the build chamber 104. A pressure sensor 142 can be used to measure the pressure within the build chamber 104, which can be used to adjust the pressure-control system 140. One or more process controllers 144 can be provided to control operation of one or more of the components of the printing system 100, such as the printer actuator 118, the dispensers 130, 132, 134, the print heads 108, 110, 112, the heater 136, and the pressure-control system 140. [0036] FIG. 3 is a flow diagram of an example method 150 using inkjet printing of one or more reactive polyimide precursor to form a polyimide part. The method 150 will be described with reference to the printing system 10, as described above with reference to FIG. 1. However, the description of the method with respect to specific structures shown in FIG. 1 and described above is intended to be for illustrative purposes only, and is not meant to be limiting to the method 150. It will be understood that variations can be performed without varying from the scope of the present invention.

[0037] The method 150 can include, at 154, printing one or more droplets of a one or more polyimide precursor solutions, e.g., precursor droplets 32, as a first printed layer on a substrate 16. The one or more polyimide precursor solutions can comprise one or more of: one or more bisanhydride precursor compounds, one or more diamine precursor compounds, and a reaction product of one or more bisanhydride precursor compounds and one or more diamine precursor compounds in a solution comprising a solvent. Examples of the bisanhydride precursor compound and the diamine precursor compound are described in more detail below. The polyimide precursor solution can be printed by a precursor print head 18 that is configured to print the precursor droplets 32, e.g., with a first dispenser 44 feeding the polyimide precursor solution to the precursor print head 18 in a controlled manner.

[0038] Printing the precursor droplets 32 can include printing one or more of the precursor droplets 32 at a plurality of target locations 24. The target locations 24 can correspond to specific points, or pixels, of a section or layer of the structure 12 including polyimide to be built. A target location 24 can be identified and selected according to three-dimensional CAD data that can be used to operate a process controller 64. The process controller 64, in turn, can control the aim of the precursor print head 18 so that the precursor droplets 32 are printed at the target locations 24. The CAD data can include prepared CAD data corresponding to the location of material in a cross-section of the structure 12.

[0039] After printing a layer of the polyimide precursor solution, the first printed layer is heated, at 156, to remove solvent from the solution that forms the first printed layer. Removal of the solvent and heating can result in polymerization of the bisanhydride precursor compound and the diamine precursor compound or between molecules of the reaction product of the

bisanhydride precursor compound and the diamine precursor compound to form at least a portion of a first polyimide layer. The temperature to which the first printed layer is heated can depend on factors such as a desired level of polymerization, e.g., to achieve a selected final molecular weight for the polymerized precursor compounds, and a desired polymerization rate. The first printed layer of the polyimide precursor solution for the heating step 156 can be heated to a temperature sufficient for substantially complete polymerization of the precursor compounds, e.g., to a molecular weight of at least about 1,000 Daltons, such as at least about 5,000 Daltons, for example at least about 10,000 Daltons, such as at least about 50,000 Daltons, for example at least about 100,000, such as 150,000 Daltons or more, within a reasonable period of time. In order to achieve substantially complete polymerization, the layer of the liquid polyimide precursor can be heated to a temperature of at least about 250 °C, such as at least about 300 °C. However, the temperature and duration of the heating 156 can be selected depending on a desired final molecular weight of the structure 12. Higher temperatures will tend to result in higher molecular weight and faster polymerization. Longer heating times will also tend to result in higher molecular weight.

[0040] In another example, the heating step 156 can heat a printed layer, such as the first printed layer, to a first temperature that will partially polymerize the polyimide precursor compounds to a state that is sufficient to provide support to subsequently-printed layers, sometimes referred to as a B-stage polymer. An intermediate polymerization of the polyimide precursor compounds can result in a polymer with an intermediate polymer molecular weight of from about 2000 Daltons to about 20,000 Daltons. Heating can form a B-stage polymer by removing a portion of the solvent, resulting in a more tacky and viscous solution of an oligomeric polyimide reaction product. The heating to a B-stage can also result in a small amount of polymerization relative to the final polymerization. Heating to a B-stage can be performed by heating the polyimide precursor solution to a temperature of less than about 250 °C (above which, in some examples, ring-closing imidization reactions can begin to occur), such as 200 °C or less, for example, from about 50 °C to about 150 °C, such as from about 60 °C to about 120 °C.

[0041] After all the layers of the structure 12 have been printed and polymerized as B-stage polymer, then the intermediate B- staged part can be heated to a second temperature that is higher than the first temperature to achieve a final polymerization that is greater than the B-stage polymerization, e.g., with a final molecular weight of at least about 1,000 Daltons, such as at least about 5,000 Daltons, for example at least about 10,000 Daltons, such as at least about 50,000 Daltons, for example at least about 100,000, such as 150,000 Daltons or more. The second temperature can be at least about 250 °C, such as from 250 °C to about 500 °C, for example at least about to about 300 °C, such as from about 300 °C to about 450 °C.

[0042] This method of heating the printed layers to a first intermediate temperature to provide for a B-stage polymer, followed by heating to a second final temperature for final

polymerization can result in crosslinking between adjacent printed layers. For example, a second layer can be printed onto a B -staged first printed layer. The second printed layer, which comprises the polyimide precursor solution, can then at least partially intermix with the B- stage polymer of the first printed layer. The lower molecular weight of the B-staged layer can also allow for better diffusion of the polyimide precursor compound between the second printed layer and the first layer. The second printed layer can be heated to the intermediate temperature to B- stage the second printed layer. As the second printed layer is heated to the intermediate temperature and polymerized to a B-stage polymer, the polymer chains can grow across the boundaries between the first printed layer and the second printed layer to provide at least partial cross-linking between the B-staged first and second printed layers The B-staged first and second printed layers can continue to intermix partially, e.g., because B-staged polymers can still allow for some fluid flow or diffusion, or both. Then, when a plurality of the layers are printed and B-staged, which may or more not comprise the entre part, the plurality of layers can be heated to the final polymerization temperature, allowing the crosslinking across the layer boundaries to continue. The crosslinking across the layer boundaries can result in one or more of stronger interlayer strength for the part, better overall part strength, and higher part density due to partially reduced void space between adjacent layers.

[0043] The heating 156 can be performed by any heater 52 or heating method that can be reasonably applied to the printed layer of the precursor compounds, including, but not limited to, infrared (IR) heating, laser heating, injection of a hot gas into the build chamber 14 (e.g., hot nitrogen or hot argon), or heating the substrate 16 (e.g., with heating coils or heat exchangers).

[0044] Steps 154 and 156 can be repeated as many times as needed to build the structure 12 in a layer-by-layer manner in order to complete the structure 12, such as when a multi-layer structure 12 is being printed. When a multi-layer structure 12 is being printed, a first layer 54 can be formed on the substrate 16 by selectively printing precursor droplets 32 at the plurality of target locations 24 corresponding to the first printed layer 54 of the structure 12 (step 154). The printed first layer 54 can be heated (step 156) to initiate or continue polymerization of the polyimide precursor compound, e.g., one or more bisanhydride precursor compounds and one or more diamine precursor compounds, respectively, or a reaction product thereof. Heating the printed first layer 54 (step 156) can be performed after the precursor droplets 32 have been printed. The heating can be performed continuously as the precursor droplets 32 are being printed to form the printed first layer 54 so that polymerization of the polyimide precursor compound can proceed as the first layer 54 is being printed. Support structures 36 can also be printed, e.g., by printing support materials droplets 34 from a support print head 20, that can provide support for overhangs 38 of subsequent layers. After printing and heating the first layer 54, a second printed layer 56 can be formed on top of the first printed layer 54 by selectively printing additional precursor droplets 32 at a plurality of target locations 24 corresponding to the second printed layer 56 (step 154 repeated). The second printed layer 56 can be heated in the same way as the first printed layer 54 (step 156 repeated). The heating (step 156) can be performed after each successive printing step 154. The heating (step 156) can be substantially continuous as the layers 54, 56 is being printed (e.g., as step 154 is repeated). Successive layers can be built (e.g. a third layer on the second, a fourth layer on the third, and so on) until the structure 12 is completed. Support structures 36 can be printed along with any layer 54, 56, etc. to provide support for subsequently printed layers. If a single-layer structure 12 is being printed, than steps 154 and 156 need not be repeated to form the single-layer structure 12.

[0045] As described in more detail below, the bisanhydride precursor compound can comprise one or more aromatic bisanhydride precursor compounds, such as one or more bisphenol bisanhydrides, for example, bisphenol A. The diamine precursor compound can comprise one or more aromatic diamine precursor compounds, such as metaphenylene diamine. The solvent can comprise at least one of water and an aliphatic alcohol, such as at least one of methanol and ethanol. The polyimide precursor solution can comprise a secondary or tertiary amine. The secondary or tertiary amine can comprise at least one of dimethylethanolamine and

trimethylamine.

[0046] Before printing the precursor droplets 32 (step 154), the method 150 can include, at 152, preparing the polyimide precursor solution that will form the precursor droplets 32. The polyimide precursor solution can be prepared by one of three processes. The process of preparing the polyimide precursor solution (step 152) can include dissolving the bisanhydride precursor compound and the diamine precursor compound in water in the presence of a secondary or tertiary amine to provide a water-based polyimide precursor solution. Dissolving the bisanhydride precursor compound and the diamine precursor compound in water can include first dissolving the diamine precursor compound and the secondary or tertiary amine in water at a water refluxing temperature, e.g., at least about 140 °C, which can be performed under pressure. The bisanhydride precursor compound can be ground into fine particles, e.g., particles having a particle size of 100 micrometers or less, in order to optimize dissolution. After dissolution of the bisanhydride precursor compound, the diamine precursor compound can be added to the mixture and dissolved in the water. The diamine precursor compound can be added in a substantially equimolar ratio relative to the bisanhydride precursor compound. The water- bisanhydride-diamine solution can be kept at the water refluxing temperature for a period of time to provide for a selected level of reaction between the bisanhydride precursor compound and the diamine precursor compound to provide the polyimide precursor solution that can be printed in step 154. The water-bisanhydride-diamine solution can be kept at the water refluxing temperature, e.g., 140 °C, for at least about 1 hour, such as at least about 2 hours, to provide for a selected reaction between the precursor compounds to provide the selected polyimide precursor solution for printing 154. A chain-stopping agent, such as pthalic anhydride, can optionally be added to the dissolved mixture of the bisanhydride precursor compound and the diamine precursor compound. The secondary or tertiary amine can comprise at least one of dimethylethanolamine and trimethylamine.

[0047] The process of preparing the polyimide precursor solution (step 152) can include dissolving one or more bisanhydride precursor compounds and one or more diamine precursor compounds in an aliphatic alcohol to provide an alcohol-based polyimide precursor solution. Dissolving the bisanhydride precursor compound and the diamine precursor compound in an aliphatic alcohol can include first dissolving the bisanhydride precursor compound in the aliphatic alcohol at an alcohol refluxing temperature, e.g., at least 100 °C, which can be performed under pressure. The bisanhydride precursor compound can be ground into fine particles, e.g., particles having a particle size of 100 micrometers or less, in order to optimize dissolution. After dissolution of the bisanhydride precursor compound in the alcohol, the diamine precursor compound can be added to the mixture and dissolved in the aliphatic alcohol. The diamine precursor compound can be added in a substantially equimolar ratio relative to the bisanhydride precursor compound. The alcohol-bisanhydride-diamine solution can be kept at the alcohol refluxing temperature for a period of time to provide for a selected level of reaction between the bisanhydride precursor compound and the diamine precursor compound to provide the polyimide precursor solution that can be printed in step 154. The alcohol-bisanhydride- diamine solution can be kept at the alcohol refluxing temperature, e.g., at least 100 °C, for at least about 1 hour, such as at least about 2 hours, to provide for a selected reaction between the precursor compounds to provide the selected polyimide precursor solution for printing step 154. The aliphatic alcohol can comprise at least one of methanol and ethanol. A chain- stopping agent, such as pthalic anhydride, can optionally be added to the dissolved bath of the bisanhydride precursor compound and the diamine precursor compound. Optionally, a secondary or tertiary amine can be added to the alcohol-dissolved mixture of the bisanhydride precursor compound and the diamine precursor compound, e.g., the alcohol-based polyimide precursor solution, to provide a water-reducible polyimide precursor. The secondary or tertiary amine can comprise at least one of dimethylethanolamine and trimethylamine.

[0048] The process of preparing the polyimide precursor solution (step 152) can include dissolving one or more bisanhydride precursor compounds and one or more diamine precursor compounds in a mixture of water and an aliphatic alcohol to produce the polyimide precursor solution. Dissolving the bisanhydride precursor compound and the diamine precursor compound in a water-alcohol mixture can include first dissolving the bisanhydride precursor compound in a mixture comprising the aliphatic alcohol and 50 wt.% or less water at a mixture refluxing temperature, e.g., at least 100 °C, which can be performed under pressure. The bisanhydride precursor compound can be ground into fine particles, e.g., particles having a particle size of 100 micrometers or less, in order to optimize dissolution. After dissolution of the bisanhydride precursor compound in the alcohol-water mixture, the diamine precursor compound can be added to the mixture and dissolved in the alcohol-water mixture. The diamine precursor compound can be added in a substantially equimolar ratio relative to the bisanhydride precursor compound. The alcohol-water-bisanhydride-diamine solution can be kept at the mixture refluxing temperature for a period of time to provide for a selected level of reaction between the bisanhydride precursor compound and the diamine precursor compound to provide the polyimide precursor solution that can be printed in step 154. The alcohol- water- bisanhydride-diamine solution can be kept at the mixture refluxing temperature, e.g., 100 °C, for at least about 1 hour, such as at least about 2 hours, to provide for a selected reaction between the precursor compounds to provide the selected polyimide precursor solution for printing 154. Upon cooling to room temperature (e.g., about 23 °C), the precursor solution can separate into two fractions, a water fraction and an alcohol fraction. The fractions can be converted back to a homogeneous solution by heating below the boiling point of the aliphatic alcohol used in the formulation. The aliphatic alcohol can comprise at least one of methanol and ethanol. A chain- stopping agent, such as pthalic anhydride, can optionally be added to the dissolved bath of the bisanhydride precursor compound and the diamine precursor compound.

[0049] FIG. 4 is a flow diagram of another example method 160 using inkjet printing of one or more reactive polyimide precursor compounds to form a polyimide part. The method 160 of FIG. 4 will be described with reference to the printing system 100, as described above with reference to FIG. 2. However, the description of the method 160 with respect to specific structures shown in FIG. 2 and described above is intended to be for illustrative purposes only, and is not meant to be limiting to the method 160. It will be understood that variations can be performed without varying from the scope of the present invention.

[0050] The method 160 can include, at 164, printing a plurality of first droplets 120 of a first polyimide precursor solution comprising one or more bisanhydride precursor compounds in a first solvent at a plurality of target locations 114 on a substrate 106, such as with a first precursor print head 108. The method 160 can include, at 166, printing a plurality of second droplets 122 of a second polyimide precursor solution comprising one or more diamine precursor compounds in a second solvent at the plurality of target location 114 on the substrate 106, such as with a second precursor print head 110. The first and second droplets 120, 122 can combine to from a reactive mixture droplet 128 at a target location 114 to form a printed layer on the substrate 106. The precursor droplets 120, 122 can be printed in any order, e.g., the first precursor droplets 120 can be printed first followed by the second precursor droplets 122 or vice versa, or the first and second precursor droplets 120, 122 can be printed at substantially the same time.

[0051] The first polyimide precursor solution can comprise a first concentration of the bisanhydride precursor compound and the second polyimide precursor solution comprises a second concentration of the diamine precursor compound. The first and second concentrations can be selected and controlled in order to provide for a selected material property for the part being printed. For example, the relative concentration of the bisanhydride precursor compound in the first liquid precursor compared to that of the diamine precursor compound in the second liquid precursor can be selected to provide for selected properties of the resulting polyimide, including, but not limited to, final molecular weight, reactive functional groups of the final polymer, and mechanical properties including flexural, tensile, and impact strengths. The volume of the polyimide precursor solution printed at a target location can be selected and controlled to provide for a selected relative concentration of the bisanhydride precursor compound relative to the diamine precursor compound in the reactive mixture droplets 128 for the printed layer.

[0052] After printing the first and second droplets 120, 122 to form a layer made up of reactive mixture droplets 128, the method 160 includes, at 168, heating the printed first and second polyimide precursor solutions to remove the first and second solvents from the reactive mixture droplets 128. Removal of the solvent and heating can initiate polymerization of the

bisanhydride precursor compound and the diamine precursor compound to provide at least a portion of a polyimide layer. The heating (step 168) of the reactive mixture droplets 128 can be substantially similar to the heating step 156 described above with respect to FIG. 3, such as to the temperatures and polymer molecular weights described above.

[0053] Steps 164, 166, and 168 can be repeated as many times as needed to build the structure 102 in a layer-by-layer manner in order to complete the structure 102, such as when a multilayer structure 102 is being printed. When a multi-layer structure 102 is being printed, the first precursor droplets 120 can be selectively printed at a plurality of target locations 114

corresponding to the desired locations of a first printed layer 146 of the structure 102 (step 164). The second precursor droplets 122 can be selectively printed at the target locations 114 (step 166) so that the first and second precursor droplets 120, 122 combined to form at least one reactive mixture droplet 128 at the target locations 114. The reactive mixture droplets 128 can form the first printed layer 146. The printed first layer 146 can be heated (step 168) to initiate or continue polymerization of the first polyimide precursor compound and the second polyimide precursor compound, e.g., the bisanhydride precursor compound and the diamine precursor compound, respectively, or a reaction product thereof. Heating the printed first layer 146 (step 168) can be performed after the precursor droplets 120, 122 have been printed. The heating 168 can be performed continuously as the precursor droplets 120, 122 are being printed to form the printed first layer 146 so that polymerization of the first and second polyimide precursor compounds can proceed as the first layer 146 is being printed. Support structures 126 can also be printed, e.g., by printing support materials droplets 124 from a support print head 112, that can provide support for overhangs of subsequent layers.

[0054] After printing and heating the first layer 146, a second printed layer 148 can be formed on top of the first printed layer 146 by selectively printing additional first precursor droplets 12 at a plurality of target locations 114 corresponding to the second printed layer 148 (step 164 repeated) and by selectively printing additional second precursory droplets 122 at the plurality of target locations 114 (step 166 repeated). The first and second precursor droplets 120, 122 can combine to form the reactive mixture droplets 128, which in turn can form the second printed layer 148. The second printed layer 148 can be heated in the same way as the first printed layer 146 (step 168 repeated). The heating (step 168) can be performed after each pair of printing steps 164, 166. The heating (step 168) can be substantially continuously as the layer 146, 148 is being printed (e.g., as steps 164 and 166 are repeated). Successive layers can be built (e.g. a third layer on the second, a fourth layer on the third, and so on) until the structure 102 is completed. For example, these steps can be repeated for a third layer 150, a fourth layer, a fifth layer, a sixth layer, and so on until the structure 102 is fully formed. Support structures 126 can be printed along with any layer 146, 148, 150, etc. to provide support for subsequently printed layers. If a single-layer structure 102 is being printed, than steps 164, 166, and 168 need not be repeated to form the single-layer structure 102.

[0055] As described in more detail below, the bisanhydride precursor compound can comprise one or more aromatic bisanhydride precursor compounds, such as one or more bisphenol bisanhydrides, for example bisphenol A bisanhydride. The diamine precursor compound can comprise one or more aromatic diamine precursor compounds, such as metaphenylene diamine. The first and solvents can comprise at least one of water and an aliphatic alcohol, such as at least one of methanol and ethanol. One or both of the first and second polyimide precursor solutions can comprise a secondary or tertiary amine. The secondary or tertiary amine can comprise at least one of dimethylethanolamine and trimethylamine.

[0056] Before printing the precursor droplets 120, 122 (steps 164 and 166), the method 160 can include, at 162, preparing the polyimide precursor solutions that will form the precursor droplets 120 and 122. Preparing the polyimide precursor solution can include selecting the selected ratio of the molar concentration of the bisanhydride precursor compound in the first precursor solution relative to the molar concentration of the diamine precursor compound in the second precursor solution, or a printed volume of the first precursor droplets 120 relative to the printed volume of the second printed droplets 122, or both, to provide for a selected molar concentration ratio of the precursor compounds in the reactive mixture droplets 128 to provide a predetermined physical property of the final structure 102 including polyimide.

[0057] The first and second precursor solutions can be prepared (at step 162) by processes similar to those described above for step 152 in method 150 of preparing a single precursor solution. For example, the first precursor solution of one or more bisanhydride precursor compounds can be prepared by dissolving the bisanhydride precursor compound in a solvent, such as water, an aliphatic alcohol, or a mixture of water and an aliphatic alcohol, and heating the solvent and one or more bisanhydride precursor compounds to a refluxing temperature, e.g., about 100 °C for an alcohol solvent or an alcohol and water mixture or about 140 °C for a water solvent, until dissolution of the bisanhydride precursor compound is complete. Similarly, the second precursor solution of one or more diamine precursor compounds can be prepared by dissolving the diamine precursor compound in a solvent, such as water, an aliphatic alcohol, or a mixture of water and an aliphatic alcohol, and heating the solvent and one or more diamine precursor compounds to a refluxing temperature, e.g., about 100 °C for an alcohol solvent or an alcohol and water mixture or about 140 °C for a water solvent, until dissolution of the diamine precursor compound is complete. A secondary or tertiary amine, such as at least one of dimethylethanolamine and trimethylamine, can be added to the precursor solutions, for example to provide for dissolution of the precursor compounds in water or to convert an ethanol-solvent solution to a water reducible solution. Optionally, a chain- stopping agent, such as pthalic anhydride, can be added to one or both of the precursor solutions.

[0058] The methods described herein, such as method 150 of FIG. 3 or method 160 of FIG. 4 can allow for ink-jet type printing of relatively high-molecular weight polyetherimide polymers. These polymers typically have too high of a molecular weight, and thus too high of a viscosity, to be printed by ink-jet methods when they are polymerized. Polyetherimides also cannot be ink-jet printed when in a liquefied form, e.g., when melted, and also have too high of a molecular weight to be put into a solution and printed. The methods described herein allow for rapid prototyping of polyetherimides using ink-jet printing.

[0059] The systems 10, 100 and the methods 150, 160 described above for reactive inkjet printing of polycarbonate precursors can be combined with other methods of additive

manufacturing, for example to make parts that include materials other than the polyimides described herein. For example, methods of fabricating one or more of other polymer materials, metals, or other dispensable or printable materials can be combined. Examples of other additive manufacturing methods that can be combined with the systems 10, 100 and the method 150, 160 described herein include, but are not limited to, UV cured printing, radical initiated printing, selective laser sintering, material extrusion printing, stereolithography, and the like.

[0060] The printing systems described above with respect to FIGS. 1 and 2 and the methods described above with respect to FIGS. 3 and 4 can be performed using the following printing materials.

[0061] As described above, the printing systems and methods described herein provide for inkjet printing of a polyimide part. The systems and methods can use one or more polyimide precursor compounds with solvents other than harsh organic solvents to solubilize the polyimide. For example, the systems and methods described herein can form high-quality polyimide polymers via inkjet printing without solvents such as tetrahydrofuran, chlorinated solvents, such as methylene chloride, chloroform, and dichlorobenzene, or solvents having a boiling point > 150 °C, such as N-methyl pyrrolidone, dimethyl acetamide, or dimethyl formamide. Solvents such as water and alcohol (e.g., methanol and ethanol) are preferred.

[0062] As described in more detail below, the polyimide material can be formed from one or more polyimide precursor solutions. In some examples, the polyimide precursor solution can comprise one or more bisanhydride precursor compounds and one or more diamine precursor compounds dissolved in a solvent, or a reaction product of the bisanhydride precursor compound and the diamine precursor compound. An amine can also be added to the polyimide precursor solution, which can allow for effective dissolution of the precursor compounds in mild solvents, such as a C _1-6 alcohol, a mixture of a C _1-6 alcohol and water, or in water. Polyimides formed from the polyimide precursor solution can be formed in the absence of a chain- stopping agent, allowing high molecular weight polyimides to be obtained. However, in some examples, a chain-stopping agent may be used. Other components, such as crosslinkers, particulate fillers, and the like can be present. The method is useful not only for layers and coatings, but also for forming composites. [0063] Each of the bisanhydride precursor compound can be a substituted or unsubstituted C ₄ _ ₄ o bisanhydride. The bisanhydride can have the general formula (1)

wherein V is a substituted or unsubstituted tetravalent C ₄ _ ₄ o hydrocarbon group, for example a substituted or unsubstituted C ₆ -20 aromatic hydrocarbon group, a substituted or unsubstituted, straight or branched chain, saturated or unsaturated C2-20 aliphatic group, or a substituted or unsubstituted C ₄ _ ₈ cycloalkylene group or a halogenated derivative thereof, in particular a substituted or unsubstituted C ₆ -2o aromatic hydrocarbon group. Exemplary aromatic

hydrocarbon groups include, but are not limited to, any of those of the formulas

wherein W is -0-, -S-, -C(O)-, -SO2-, -SO-, -C _y H2 _y -, wherein y is an integer from 1 to 5 or a halogenated derivative thereof (which includes perfluoroalkylene groups), or a group of the formula T as described in formula (2) below.

[0064] The polyimides can include polyetherimides. Polyetherimides are prepared by the reaction of an aromatic bis(ether anh dride) of formula (2)

wherein T is -O- or a group of the formula -0-Z-O- wherein the divalent bonds of the -O- or the -0-Z-O- group are in the 3,3', 3,4', 4,3', or the 4,4' positions. The group Z in -0-Z-O- of formula (2) can also be a substituted or unsubstituted divalent organic group, and can be an aromatic C ₆ - 2 ₄ monocyclic or polycyclic moiety optionally substituted with 1 to 6 Ci_ ₈ alkyl groups, 1 to 8 halogen atoms, or a combination thereof, provided that the valence of Z is not exceeded.

Exemplary groups Z include groups derived from a dihydroxy compound of formula (3)

(3) wherein R ^a and R ^b can be the same or different and are a halogen atom or a monovalent Ci_ ₆ alkyl group, for example; p and q are each independently integers of 0 to 4; c is 0 to 4; and X ^a is a bridging group connecting the hydroxy-substituted aromatic groups, where the bridging group and the hydroxy substituent of each C ₆ arylene group are disposed ortho, meta, or para

(specifically para) to each other on the C ₆ arylene group. The bridging group X ^a can be a single bond, -0-, -S-, -S(O)-, -S0 ₂ -, -C(O)-, or a Ci_ ₁₈ organic bridging group. The C _1-18 organic bridging group can be cyclic or acyclic, aromatic or non-aromatic, and can further comprise heteroatoms such as halogens, oxygen, nitrogen, sulfur, silicon, or phosphorous. The CM _S organic group can be disposed such that the C ₆ arylene groups connected thereto are each connected to a common alkylidene carbon or to different carbons of the C _1-18 organic bridging group. A specific example of a group Z is a divalent group of formula (3a)

^→ O ^Q 0^ (3a) wherein Q is -0-, -S-, -C(O)-, -S0 ₂ -, -SO-, or -C _y H _2y - wherein y is an integer from 1 to 5 or a halogenated derivative thereof (including a perfluoroalkylene group). In a specific embodiment Z is derived from bisphenol A, such that Q in formula (3a) is 2,2-isopropylidene.

[0065] Examples of bis(anhydride)s that can be used as the bisanhydride precursor compound include, but are not limited to, 3,3-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane bisanhydride; 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl ether bisanhydride; 4,4'-bis(3,4- dicarboxyphenoxy)diphenyl sulfide bisanhydride; 4,4'-bis(3,4-dicarboxyphenoxy)benzophenone bisanhydride; 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfone bisanhydride; 2,2-bis[4-(2,3- dicarboxyphenoxy)phenyl]propane bisanhydride; 4,4'-bis(2,3-dicarboxyphenoxy)diphenyl ether bisanhydride; 4,4'-bis(2,3-dicarboxyphenoxy)diphenyl sulfide bisanhydride; 4,4'-bis(2,3- dicarboxyphenoxy)benzophenone bisanhydride; 4,4'-bis(2,3-dicarboxyphenoxy)diphenyl sulfone bisanhydride; 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)diphenyl-2 ,2-propane bisanhydride; 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)diphenyl ether bisanhydride; 4-(2,3-dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)diphenyl sulfide bisanhydride; 4-(2,3- dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)benzophenone bisanhydride; and, 4-(2,3- dicarboxyphenoxy)-4'-(3,4-dicarboxyphenoxy)diphenyl sulfone bisanhydride, as well as various combinations thereof.

[0066] Each of the diamine precursor compound can have general formula (4)

H ₂ N-R-NH ₂ (4) wherein R is a substituted or unsubstituted divalent Ci_ ₂ o hydrocarbon group, such as a substituted or unsubstituted C ₆ - ₂ o aromatic hydrocarbon group or a halogenated derivative thereof, a substituted or unsubstituted, straight or branched chain, saturated or unsaturated C ₂ _ ₂ o alkylene group or a halogenated derivative thereof, a substituted or unsubstituted C3-8 cycloalkylene group or halogenated derivative thereof, in particular one of the divalent groups of formula

wherein Q ¹ is -0-, -S-, -C(O)-, -S0 ₂ -, -SO-, -C _y H _2y - wherein y is an integer from 1 to 5 or a halogenated derivative thereof (which includes perfluoroalkylene groups), or -(C ₆ Hio) _z - wherein z is an integer from 1 to 4. In some examples R is m-phenylene, p-phenylene, or 4,4'- diphenylene sulfone. In some embodiments, no R groups contain sulfone groups. In another embodiment, at least 10 mol.% of the R groups contain sulfone groups, for example 10 to 80 wt.% of the R groups contain sulfone groups, in particular 4,4'-diphenylene sulfone groups.

[0067] Examples of organic diamines that can be used as the diamine precursor compound include, but are not limited to, ethylenediamine, propylenediamine, trimethylenediamine, diethylenetriamine, triethylene tetramine, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, nonamethylenediamine, decamethylenediamine, 1,12-dodecanediamine, 1,18-octadecanediamine, 3-methylheptamethylenediamine, 4,4- dimethylheptamethylenediamine, 4-methylnonamethylenediamine, 5- methylnonamethylenediamine, 2,5-dimethylhexamethylenediamine, 2,5- dimethylheptamethylenediamine, 2,2-dimethylpropylenediamine, N-methyl-bis(3- aminopropyl)amine, 3-methoxyhexamethylenediamine, l,2-bis(3-aminopropoxy)ethane, bis(3- aminopropyl)sulfide, 1,4-cyclohexanediamine, bis-(4-aminocyclohexyl)methane, m- phenylenediamine, p-phenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, m- xylylenediamine, p-xylylenediamine, 2-methyl-4,6-diethyl-l,3-phenylenediamine, 5-methyl-4,6- diethyl-l,3-phenylenediamine, benzidine, 3,3'-dimethylbenzidine, 3,3'-dimethoxybenzidine, 1,5-diaminonaphthalene, bis(4-aminophenyl) methane, bis(2-chloro-4-amino-3,5-diethylphenyl) methane, bis(4-aminophenyl) propane, 2,4-bis(p-amino-t-butyl) toluene, bis(p-amino-t- butylphenyl) ether, bis(p-methyl-o-aminophenyl) benzene, bis(p-methyl-o-aminopentyl) benzene, l,3-diamino-4-isopropylbenzene, bis(4-aminophenyl) sulfide, and bis(4-aminophenyl) ether. Combinations of these compounds can also be used. In some embodiments the organic diamine is m-phenylenediamine, p-phenylenediamine, 4,4'-sulfonyl dianiline, or a combination comprising one or more of the foregoing.

[0068] In some embodiments, the one or more aromatic bisanhydride precursor compounds of formula (1) or (2) can be reacted with a diamine component comprising an organic diamine (4) as described above or a mixture of diamines, and a polysiloxane diamine of formula (6)

wherein each R' is independently a Ci_i ₃ monovalent hydrocarbyl group. For example, each R' can independently be a C M ₃ alkyl group, CM ₃ alkoxy group, C ₂ -i ₃ alkenyl group, C ₂ _i ₃ alkenyloxy group, C ₃ _ ₆ cycloalkyl group, C ₃ _ ₆ cycloalkoxy group, C ₆ -i ₄ aryl group, C ₆ -io aryloxy group, C ₇ -i ₃ arylalkyl group, C ₇ _i ₃ arylalkoxy group, C ₇ _i ₃ alkylaryl group, or C ₇ _i ₃ alkylaryloxy group. The foregoing groups can be fully or partially halogenated with fluorine, chlorine, bromine, or iodine, or a combination comprising at least one of the foregoing. In some examples no halogens are present. Combinations of the foregoing R' groups can be used in the same copolymer. In some examples, the polysiloxane diamine comprises R' groups that have minimal hydrocarbon content, e.g., one or more methyl groups.

[0069] E in formula (6) has an average value of 5 to 100, and each R ⁴ is independently a C ₂ -C ₂ o hydrocarbon, in particular a C ₂ -C ₂ o arylene, alkylene, or arylenealkylene group. In some examples R ⁴ is a C ₂ -C ₂ o alkyl group, specifically a C ₂ -C ₂ o alkyl group such as propylene, and E has an average value of 5 to 100, 5 to 75, 5 to 60, 5 to 15, or 15 to 40. Procedures for making the polysiloxane diamines of formula (6) are well known in the art.

[0070] The diamine component can contain 10 to 90 mole percent (mol%), or 20 to 50 mol%, or 25 to 40 mol% of polysiloxane diamine (5) and 10 to 90 mol%, or 50 to 80 mol%, or 60 to 75 mol% of diamine (4). The diamine components can be physically mixed prior to reaction with the bisanhydride(s), thus forming a substantially random copolymer. Block or alternating copolymers can be formed by selective reaction of (4) and (6) with aromatic bis(ether anhydride)s (1) or (2), to make polyimide blocks that are subsequently reacted together. Thus, the polyimide-siloxane copolymer can be a block, random, or graft copolymer.

[0071] A polyimide precursor solution can be prepared for printing in order to form a polyimide part. For example, the precursor droplets 32 in the example printing system 10 of FIG. 1 and the example method 150 of FIG. 3 can comprise a polyimide precursor solution that is used to print a structure 12 including polyimide. The polyimide precursor solution can comprise a polyimide prepolymer and a solvent, such as a solvent comprising a Ci_ ₆ alcohol. The polyimide precursor solution can also include an amine to effectively solubilize the polyimide prepolymer in the alcohol solvent, in a mixture of an alcohol solvent and water, or in water.

[0072] The polyimide prepolymer in the polyimide precursor solution can be a reaction product of the bisanhydride precursor compound and the diamine precursor compound described above, such as a reaction product between a substituted or unsubstituted C- _O bisanhydride and a substituted or unsubstituted divalent Ci_ ₂ o diamine. The polyimide precursor can comprise more than 1, for example 10 to 1000, or 10 to 500, structural units of formula (7)

wherein each V is the same or different, and is as described in formula (1), and each R is the same or different, and is defined as in formula (4). The polyetherimides comprise more than 1, for example 10 to 1000, or 10 to 500, structural units of formula (8)

wherein each T is the same or different, and is as described in formula (2), and each R is the same or different, and is as described in formula (4), preferably m-phenylene or p-phenylene.

[0073] The polyetherimides can optionally further comprises up to 10 mole%, up to 5 mole%, or up to 2 mole% of units of formula (8) wherein T is a linker of the formula (9)

(9)

In some embodiments no units are present wherein R is of these formulas. [0074] In some examples in formula (1), R is m-phenylene or p-phenylene and T is -0-Z-O- wherein Z is a divalent group of formula (3 a). R can be a m-phenylene or p-phenylene and T is -O-Z-0 wherein Z is a divalent group of formula (3a) and Q is 2,2-isopropylidene.

[0075] In some examples, the polyetherimide can be a polyetherimide sulfone. For example, the polyetherimide can comprise the etherimide units wherein at least 10 mole percent, for example 10 to 90 mole percent, 10 to 80 mole percent, 20 to 70 mole percent, or 20 to 60 mole percent of the R groups comprise a sulfone group. For example, R can be 4,4'-diphenylene sulfone, and Z can be 4,4'-diphenylene isopropylidene, providing units of formula (10).

(10)

[0076] In another embodiment the polyetherimide can be a polyetherimide- siloxane block or graft copolymer. Block polyimide- siloxane copolymers comprise imide units and siloxane blocks in the polymer backbone. Block polyetherimide-siloxane copolymers comprise etherimide units and siloxane blocks in the polymer backbone. The imide or etherimide units and the siloxane blocks can be present in random order, as blocks (i.e., AABB), alternating (i.e., ABAB), or a combination thereof. Graft copolymers are non-linear copolymers comprising the siloxane blocks connected to a linear or branched polymer backbone comprising imide or etherimide blocks.

[0077] In some examples, a polyetherimide-siloxane has units of the formula

(I D

wherein R', R ⁴ , and E of the siloxane are as in formula (6), R is as in formula (4), Z is as in formula (2), and n is an integer from 5 to 100. In a specific embodiment, the R of the etherimide is a phenylene, Z is a residue of bisphenol A, R ⁴ is n-propylene, E is 2 to 50, 5, to 30, or 10 to 40, n is 5 to 100, and each R' of the siloxane is methyl. In some examples the polyetherimide- siloxane comprises 10 to 50 weight %, 10 to 40 weight %, or 20 to 35 weight % poly siloxane units, based on the total weight of the polyetherimide-siloxane.

[0078] The polyimide prepolymer can comprise partially reacted units of formulas q and r to fully reacted units of formula s.

(q) (r) (s)

wherein V and R are as defined above. The polyimide prepolymer contains at least one unit (q), 0 or 1 or more units (r), and 0 or 1 or more units (s), for example 1 to 200 or 1 to 100 units q, 0 to 200 or 0 to 100 units (r), or 0 to 200 or 0 to 100 units (s). An imidization value for the polyimide prepolymer can be determined using the relationship

(2s+r)/(2q+2r+2s)

Wherein q, r, and s stand for the number of units (q), (r), and (s), respectively. In some embodiments, the imidization value of the polyimide prepolymer is less than or equal to 0.2, less than or equal to 0.15, or less than or equal to 0.1. In some embodiments, the polyimide prepolymer has an imidization value of greater than 0.2, for example greater than 0.25, greater than 0.3, or greater than 0.5, provided that the desired solubility of the polyimide prepolymer is maintained. The number of units if each type can be determined by spectroscopic methods, for example FT-IR.

[0079] The polyimide precursor solution can further include an amine. The amine can comprise a secondary amine, a tertiary amine, or a combination comprising at least one of the foregoing. In some embodiments, the amine preferably comprises a tertiary amine.

[0080] The amine can be selected such that less than or equal to 0.5 grams of the amine is effective to solubilize 1 gram of the polyimide prepolymer in deionized water.

[0081] In some embodiments, the amine is a secondary or a tertiary amine of the formula (12)

R ^A R ^B R ^C N (12)

A B C

wherein each R , R , and R can be the same or different and are a substituted or unsubstituted

A B C

Ci_i8 hydrocarbyl or hydrogen, provided that no more than one of R , R , and R" are hydrogen.

A B C

In some examples, each R , R , and R is the same or different and are a substituted or unsubstituted Ci_i ₂ alkyl, a substituted or unsubstituted Ci_i ₂ aryl, or hydrogen, provided that no

A B C A B C

more than one of R , R , and R are hydrogen. In some examples, each R , R , and R are the same or different and are an unsubstituted Ci_ ₆ alkyl or a Ci_ ₆ alkyl substituted with 1, 2, or 3

D E

hydroxyl, halogen, nitrile, nitro, cyano, Ci_ ₆ alkoxy, or amino groups of the formula -NR R

D E

wherein each R and R are the same or different and are a Ci_ ₆ alkyl or Ci_ ₆ alkoxy. In some

A B C

examples, each R , R , and R are the same or different and are an unsubstituted Ci^ alkyl or a Ci^ alkyl substituted with one hydroxyl, halogen, nitrile, nitro, cyano, or Ci_ ₃ alkoxy. [0082] In some embodiments, the amine comprises triethylamine, trimethylamine, dimethylethanolamine, diethanolamine, or a combination comprising at least one of the foregoing. For example, the amine comprises triethylamine. For example, the amine comprises dimethylethanolamine. For example, the amine comprises diethanolamine.

[0083] The amine can be added to the polyimide precursor solution in an amount effective to solubilize the polyimide prepolymer in a Ci_ ₆ alcohol, in a solution of the Ci_ ₆ alcohol and deionized water, or in deionized water. For example, the amine can be present in the polyimide precursor solution in an amount of 5 to 50 wt.%, or 8 to 40 wt.%, or 9 to 35 wt.%, based on the combined weight of the amine and the dry weight of the polyimide prepolymer.

[0084] The amine can be added in an amount effective to solubilize the polyimide prepolymer in the alcohol, the mixture of the alcohol and water, or in water. In some examples, the solution can be heated at a temperature equal to the boiling point of the C _1-6 alcohol at atmospheric pressure, or at a temperature greater than 100°C at a pressure greater than atmospheric pressure.

[0085] The polyimide precursor solution includes a solvent, e.g., for the dissolution of the bisanhydride precursor compound, the diamine precursor compound, and the polyimide prepolymer. The solvent can be a protic organic solvent. Examples of protic organic solvents include, but are not limited to, a Ci_ ₆ alcohol, wherein the Ci_ ₆ alkyl group can be linear or branched. The Ci ₆ alcohol can include methanol, ethanol, n-propanol, isopropanol, n-butanol, t-butanol, sec-butanol, 1-pentanol, 2-pentanol, 3-pentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2- ethyl-l-butanol, 3 -methyl- 1-butanol, 3-methyl-2-butanol, 2-methyl-2-butanol, 2,2-dimethyl-l- propanol, ethylene glycol, diethylene glycol, or a combination comprising at least one of the foregoing. In some embodiments, the Ci_ ₆ alcohol is substantially miscible with water. For example the Ci_ ₆ alcohol can comprise methanol, ethanol, n-propanol, isopropanol, or a combination comprising at least one of the foregoing. In some examples, the solvent comprises methanol, ethanol, or a combination comprising at least one of the foregoing.

[0086] In some embodiments, the solvent further comprises water, for example deionized water. The solvent can include water in a weight ratio of Ci_ ₆ alcohohwater of about 1: 100 to about 100: 1, such as about 1: 10 to about 10: 1, for example about 1:2 to about 2: 1, such as about 1: 1.1 to about 1.1: 1. In other embodiments, however, no water is present. For example, the solvent can comprise less than 1 weight percent (wt.%), or is devoid of water.

[0087] In some examples, the solvent comprises less than 1 wt.%, or is devoid of harsher organic solvents, such as a chlorobenzene, a dichlorobenzene, cresol, dimethyl acetamide, veratrole, pyridine, nitrobenzene, methyl benzoate, benzonitrile, acetophenone, n-butyl acetate, 2-ethoxyethanol, 2-n-butoxyethanol, dimethyl sulfoxide, anisole, cyclopentanone, gamma- butyrolactone, Ν,Ν-dimethyl formamide, N-methyl pyrrolidone, tetrahydrofuran or a combination comprising at least one of the foregoing. In another embodiment, the solvent comprises less than 1 wt.%, or less than 0.1 wt.% of a nonprotic organic solvent, and in some examples the solvent is devoid of a nonprotic organic solvent. In another embodiment, the solvent comprises less than 1 wt.%, or less than 0.1 wt.%, of a halogenated solvent, and preferably the solvent is devoid of a halogenated solvent.

[0088] The polyimide precursor solution can comprise, based on the total weight of the compositions: from about 1 to about 90 wt.% of the polyimide prepolymer, such as from about 5 to about 80 wt.%, for example from about 10 to about 70 wt.% of the polyimide prepolymer; from about 10 to 99 wt.% of the solvent, such as from about 20 to about 95 wt.%, for example from about 30 to about 90 wt.% of the solvent; and from about 0 wt/% or about 0.001 wt.% to about 50 wt.% of the amine, such as from about 0.01 to about 30 wt.%, for example from about 0.01 to about 15 wt.% of the amine.

[0089] As noted above, in some examples, the polyimide precursor solution that is capable of forming polyimide polymer parts via inkjet printing can be formed with solvents other than harsh organic solvents, including tetrahydrofuran, chlorinated solvents, such as methylene chloride, chloroform, and dichlorobenzene, or solvents having a boiling point > 150 °C, such as N-methyl pyrrolidone, dimethyl acetamide, or dimethyl formamide.

[0090] The polyimide precursor solution can further comprise additional components to modify the reactivity or processability of the compositions, or properties of the polyimides and articles formed from the polyimides. For example, the polyimide precursor solution can further comprise a polyimide chain-stopping agent to adjust the molecular weight of the polyimide. Examples of chain-stopping agents include, but are not limited to, monofunctional amines such as aniline and mono-functional anhydrides such as phthalic anhydride, maleic anhydride, or nadic anhydride. The chain- stopping agent can be present in an amount of 0.2 mole percent to 10 mole percent, more preferably 1 mole percent to 5 mole percent based on total moles of one of the bisanhydride precursor compound or the diamine precursor compound. In some examples, the polyimide prepolymer is partially endcapped with a chain- stopping agent. In another embodiment, however, no chain-stopping agent is present in the polyimide precursor solution.

[0091] In another embodiment, the polyimide precursor solution can further comprise a polyimide crosslinking agent. Such crosslinking agents are known, and include, compounds containing an amino group or an anhydride group and crosslinkable functionality, for example ethylenic unsaturation. Examples include, but are not limited to, maleic anhydride and benzophenone tetracarboxylic acid anhydride. The crosslinking agents can be present in an amount of 0.2 mole percent to 10 mole percent, more preferably 1 mole percent to 5 mole percent based on total moles of one of the bisanhydride or diamine precursor compounds.

[0092] The polyimide precursor solution can further comprise a branching agent, for example a polyfunctional organic compound having at least three functional groups which can be, for example, amine, carboxylic acid, carboxylic acid halide, carboxylic anhydride, and mixtures thereof. A branching agent can be a substituted or unsubstituted polyfunctional Ci_20 hydrocarbon group having at least three of any one or more of the aforementioned functional groups. Exemplary branching agents can include a C2-20 alkyltriamine, a C2-20 alkyltetramine, a Ce-20 aryltriamine, an oxyalkyltriamine (e.g., JEFF AMINE T-403™ available from Texaco Company), trimellitic acid, trimellitic anhydride, trimellitic trichloride, and the like, and combinations comprising at least one of the foregoing. When present, the amount of branching agent can be 0.5 to 10 weight percent based on the weight of the polyimide prepolymer.

[0093] The polyimide precursor solution can further comprise a particulate polymer dispersible in the solvent, for example dispersible in the Ci_ ₆ alcohol, in a solution of the C _1-6 alcohol and water, or in water. In some examples, the particulate polymers are preferably dispersible in water. Imidization of the polyimide prepolymer in the presence of the particulate polymer can provide an intimate blend of the polymer and the polyimide. The dispersible polymers can have an average particle diameter from 0.01 to 250 micrometers. Aqueous-dispersible polymers include, but are not limited to, fluoropolymers, (e.g., polytetrafluoroethylene,

tetrafluoroethylene-perfluoroalkylvinylether copolymer, tetrafluoroethylene- hexafluoropropylene copolymer, polychlorotrifluoroethylene, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride), (meth)acrylic and (meth)acrylate polymers (e.g., poly(methyl (meth)acrylate), poly(ethyl (meth)acrylate), poly(n-butyl (meth)acrylate), poly(2- ethyl hexyl (meth)acrylate), copolymers thereof, and the like), styrenic polymers (e.g., polystyrene, and copolymers of styrene-butadiene, styrene-isoprene, styrene-acrylate esters, and styrene-acrylonitrile), vinyl ester polymers (e.g., poly(vinyl acetate), poly(vinyl acetate- ethylene) copolymers, poly(vinyl proprionate), poly(vinyl versatate) and the like), vinyl chloride polymers, polyolefins (e.g., polyethylenes, polyproplyenes, polybutadienes, copolymers thereof, and the like), polyurethanes, polyesters (e.g., poly(ethylene terephthalate), poly(butylene terephthalate), poly(caprolactone), copolymers thereof, and the like), polyamides, natural polymers such as polysaccharides, or a combination comprising at least one of the foregoing. [0094] When present, the dispersible polymers can be present in an amount of 0.1 to 50 wt.%, preferably 1 to 30 wt.%, more preferably from 5 to 20 wt.%, each based on the total weight of the precursor compounds in the composition.

[0095] The polyimide precursor solution can further comprise additives for polyimide compositions known in the art, with the proviso that the additive(s) are selected so as to not significantly adversely affect the desired properties of the compositions, in particular formation of the polyimide. Such additives include a particulate filler (such as glass, carbon, mineral, or metal), antioxidant, heat stabilizer, light stabilizer, ultraviolet (UV) light stabilizer, UV absorbing additive, plasticizer, lubricant, release agent (such as a mold release agent), antistatic agent, anti-fog agent, antimicrobial agent, colorant (e.g., a dye or pigment), surface effect additive, radiation stabilizer, flame retardant, anti-drip agent (e.g., a PTFE-encapsulated styrene- acrylonitrile copolymer (TS AN)), or a combination comprising one or more of the foregoing. In general, the additives are used in the amounts generally known to be effective. For example, the total amount of the additive composition (other than any filler) can be 0.001 to 10.0 wt.%, or 0.01 to 5 wt.%, each based on the total weight of the precursor compounds in the composition.

[0096] For example, a combination of a heat stabilizer, mold release agent, and ultraviolet light stabilizer can be used. Pigments, surface effect agents, and nanosized fillers are also specifically contemplated, as such materials can be readily co-dispersed with precursor compounds, or pre- combined with the precursor compounds. When present, the nanosized fillers can be present in an amount of 0.1 to 50 wt.%, preferably 1 to 30 wt.%, more preferably from 2 to 10 wt.%, each based on the total weight of the precursor compounds in the composition.

[0097] The polyimide precursor solution can be used in the formation of a polyimide part, for example by printing with the printing systems 10 or the method 150. Each precursor compound (e.g., the bisanhydride precursor compound and the diamine precursor compound) can be dissolved into separate solutions and printed separately so that the precursor solutions mix together to form the polyimide precursor solution.

[0098] The printed polyimide precursor solution can be converted to a polyimide part by heating the part at a temperature and for a period of time effective to imidize the polyimide prepolymer and form the polyimide. Suitable temperatures are greater than or equal to about 250°C, such as from about 250 to about 500°C, for example from about 300 to about 450°C. The polyimide precursor solution can be heated for a time from 10 minutes to 3 hours, such as from 15 minutes to 1 hour. The imidization can be conducted under an inert gas during the heating. Examples of inert gasses that can be used include, but are not limited to, dry nitrogen, helium, argon and the like. Dry nitrogen is generally preferred. In an advantageous feature, such blanketing is not required. The imidization is generally conducted at atmospheric pressure.

[0099] The solvent to be removed from the printed polyimide precursor solution during the imidization, or the solvent can be removed from the printed polyimide precursor solution before the imidization, for example by heating to a temperature below the imidization temperature. The solvent can be partially removed, or can be fully removed.

[0100] If a crosslinker is present in the polyimide precursor solution, crosslinking can occur before the imidization, during the imidization, or after the imidization. For example, when the crosslinker comprises ethylenically unsaturated groups, the printed polyimide precursor solution can be crosslinked by exposure to ultraviolet (UV) light, electron beam radiation or the like, to stabilize the printed polyimide precursor solution. The polyimide can be post-crosslinked to provide additional strength or other properties to the polyimide.

[0101] Depending on the precursor compounds and other materials used in the polyimide precursor solution, the polyimides can have a melt index of 0.1 to 10 grams per minute (g/min), as measured by American Society for Testing Materials (ASTM) D1238 at 340 to 370 °C, using a 6.7 kilogram (kg) weight. In some embodiments, the polyimide has a weight average molecular weight (MW) of greater than 1,000 grams/mole (Daltons), or greater than 5,000 Daltons, or greater than 10,000 Daltons, or greater than 50,000 Daltons, or greater than 100,000 Daltons as measured by gel permeation chromatography, using polystyrene standards. For example, the polyimide can have a weight average molecular weight (MW) of 1,000 to 150,000 Daltons. In some embodiments the polyimide has a MW of 10,000 to 80,000 Daltons, specifically greater than 10,000 Daltons or greater than 60,000 Daltons, up to 100,000 or 150,000 Daltons. In some embodiments, the polyimide has a molecular weight that is no more than 10% lower than the molecular weight of the same polyimide formed in the absence of the amine. The polyimides can further have a polydispersity index of 2.0 to 3.0, or 2.3 to 3.0.

[0102] The polyimides can further be characterized by the presence of less than 1 wt.%, or less than 0.1 wt.% of a nonprotic organic solvent. In some examples, it is preferred that the polyimide is devoid of a nonprotic organic solvent. Similarly, the polyimide has less than 1 wt.%, or less than 0.1 wt.% of a halogenated solvent, and preferably the polyimide is devoid of a halogenated solvent. Such properties are particularly useful in layers or conformal coatings having a thickness from 0.1 to 1500 micrometers, specifically 1 to 500 micrometers, more specifically 5 to 100 micrometers, and even more specifically 10 to 50 micrometers.

[0103] The methods of manufacturing polyimides and articles comprising the polyimides described herein do not rely on organic solvents, and allows for very small droplets (e.g., build materials droplets 32 or precursor solution droplets 120, 122), which can allow for thin layers of the polyimide to be obtained. The method is useful not only for layers and coatings, but also for forming composites. Therefore, a substantial improvement in methods of manufacturing polyimides and articles prepared therefrom is provided.

[0104] Set forth below are some embodiments of the systems and methods disclosed herein.

[0105] Embodiment 1: A method of fabricating an article, the method comprising: printing a plurality of first droplets of a polyimide precursor solution as a first printed structure on a substrate, the polyimide precursor solution comprising a polyimide precursor compound (preferably at least two polyimide precursor compounds) in a solvent; and heating the first printed structure of polyimide precursor solution to initiate polymerization of the polyimide precursor compound into a first structure including polyimide.

[0106] Embodiment 2: The method according to Embodiment 1, wherein the polyimide precursor compound comprise at least one of a bisanhydride precursor compound, a diamine precursor compound, and a reaction product of a bisanhydride precursor compound and a diamine precursor compound.

[0107] Embodiment 3: The method according to Embodiment 2, wherein the polyimide precursor solution is formed by a process comprising one of: dissolving the bisanhydride precursor compound and the diamine precursor compound in water in the presence of a secondary or tertiary amine to provide the polyimide precursor solution; dissolving the bisanhydride precursor compound and the diamine precursor compound in an aliphatic alcohol to provide an alcohol-based polyimide precursor solution and optionally adding a secondary or tertiary amine to the alcohol-based polyimide precursor solution to provide the polyimide precursor solution; or dissolving a bisanhydride precursor compound and a diamine precursor compound in a mixture of water and an aliphatic alcohol to provide the polyimide precursor solution.

[0108] Embodiment 4: The method according to Embodiment 3, wherein the bisanhydride precursor compound and the diamine precursor compound are dissolved in an equimolar or substantially equimolar ratio.

[0109] Embodiment 5: The method according to any one of Embodiments 1-4, wherein the solvent comprises at least one of water and an aliphatic alcohol.

[0110] Embodiment 6: The method according to any one of Embodiments 1-5, further comprising: printing a plurality of second droplets of the polyimide precursor solution as a second printed structure on the first structure including polyimide; and heating the second printed structure to evaporate solvent from the solution of the polyimide precursor solution to initiate polymerization of the polyimide precursor compound into a second structure including polyimide.

[0111] Embodiment 7: A method of fabricating an article, the method comprising: printing a plurality of first droplets of a first polyimide precursor solution at a plurality of target locations on a substrate, the first polyimide precursor solution comprising a first concentration of a bisanhydride precursor compound in a first solvent; printing a plurality of second droplets of a second polyimide precursor solution at the plurality of target location on the substrate, the second polyimide precursor solution comprising a second concentration of a diamine precursor compound in a second solvent; wherein the first droplets and the second droplets combine to from a reactive mixture droplet at each of the target locations to form a first printed structure on the substrate; and heating the first printed structure to initiate polymerization of the

bisanhyhdride precursor compound and the diamine precursor compound to provide at least a portion of a first structure including polyimide.

[0112] Embodiment 8: The method according to Embodiment 7, further comprising at least one of: selecting a ratio of the volume of the first droplets relative to the second droplets to provide for a selected molar ratio of the bisanhydride precursor compound relative to the diamine precursor compound in the reactive mixture droplets; and selecting a ratio of the first concentration relative to the second concentration to provide for a selected molar ratio of the bisanhydride precursor compounds relative to the diamine precursor compounds in the reactive mixture droplets.

[0113] Embodiment 9: The method according to either one of Embodiments 7 or 8, wherein the first and second solvents each comprise at least one of water and an aliphatic alcohol.

[0114] Embodiment 10: The method according to any one of Embodiments 7-9, wherein one or both of the first and second polyimide precursor solutions further comprises a secondary or tertiary amine.

[0115] Embodiment 11: The method according to any one of Embodiments 7-10, wherein the bisanhydride precursor compounds and the diamine precursor compounds are printed in a substantially equimolar ratio in the reactive mixture droplets.

[0116] Embodiment 12: A system for fabricating an article, the system comprising: a print head configured to print droplets of a polyimide precursor solution comprising a polyimide precursor compound in a solvent onto each of a plurality of target locations on a substrate within a build area; a print head actuator coupled to the print head to move the print head; a control system coupled to the print head actuator to control the print head actuator to move the print head and aim the print head at a target location; and an environmental system configured to accommodate the target location during fabrication of the article, the environmental system configured to expose the build area, including the target location, to a temperature selected to initiate polymerization of the polyimide precursor compound into a structure including polyimide.

[0117] Embodiment 13: The system according to Embodiment 12, wherein the polyimide precursor compound comprise at least one of a bisanhydride precursor compound, a diamine precursor compound, and a reaction product of a bisanhydride precursor compound and a diamine precursor compound.

[0118] Embodiment 14: The system according to Embodiment 13, wherein the polyimide precursor solution is formed by a process comprising one of: dissolving the bisanhydride precursor compound and the diamine precursor compound in water in the presence of a secondary or tertiary amine to provide the polyimide precursor solution; dissolving the bisanhydride precursor compound and the diamine precursor compound in an aliphatic alcohol to provide an alcohol-based polyimide precursor and optionally adding a secondary or tertiary amine to the alcohol-based polyimide precursor to provide the polyimide precursor solution; or dissolving the bisanhydride precursor compound and the diamine precursor compound in a mixture of water and an aliphatic alcohol to provide the polyimide precursor solution.

[0119] Embodiment 15: The system according to either one of Embodiments 13 or 14, wherein the bisanhydride precursor compound and the diamine precursor compound are in an equimolar or substantially equimolar ratio in the polyimide precursor solution.

[0120] Embodiment 16: The system according to any one of Embodiments 12-15, wherein the solvent comprises at least one of water and an aliphatic alcohol.

[0121] Embodiment 17: A system for fabricating a part, the system comprising: a first print head configured to print first droplets of a first polyimide precursor solution comprising a first concentration of a bisanhydride precursor compound in a first solvent onto each of a plurality of target locations on a substrate within a build area; a second print head configured to print second droplets of a second polyimide precursor solution comprising a second concentration of a diamine precursor compound in a second solvent onto each of the target locations on the substrate; a control system configured to selectively aim each of the first and second print heads so that a first droplet (preferably at least two first droplets) of the first polyimide precursor solution and a second droplet (preferably at least two second droplets) of the second polyimide precursor solution combine to form a reactive mixture droplet at each of the target locations; and an environmental system configured to accommodate the target locations during fabrication of the article, the environmental system configured to expose the build area, including the target locations, to a temperature selected to initiate polymerization of the bisanhydride precursor compound and the diamine precursor compound into a structure including polyimide.

[0122] Embodiment 18: The system according to Embodiment 17, wherein the first and second solvents each comprise at least one of water and an aliphatic alcohol.

[0123] Embodiment 19: The system according to either one of Embodiments 17 or 18, wherein one or both of the first and second polyimide precursor solution further comprises a secondary or tertiary amine.

[0124] Embodiment 20: The system according to any one of Embodiments 17-19, wherein the bisanhydride precursor compound and the diamine precursor compound are printed in an equimolar or substantially equimolar ratio in the reactive mixture droplets.

[0125] The above Detailed Description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more elements thereof) can be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. Also, various features or elements can be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Inventive subject matter can lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

[0126] In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls.

[0127] In this document, the terms "a" or "an" are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of "at least one" or "one or more." In this document, the term "or" is used to refer to a nonexclusive or, such that "A or B" includes "A but not B," "B but not A," and "A and B," unless otherwise indicated. In this document, the terms "including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Also, in the following claims, the terms "including" and "comprising" are open-ended, that is, a molding system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms "first," "second," and "third," etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. [0128] This application claims priority to U.S. Provisional Application No. 62/170,413, filed on June 3, 2015, the entire disclosure of which is incorporated herein by reference. U.S. Provisional Application No. 62/170,418 and the U.S. Provisional Application No. 62/170,423, are also incorporated by reference as if reproduced herein in their entireties.

[0129] Method examples described herein can be machine or computer-implemented, at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods or method steps as described in the above examples. An implementation of such methods or method steps can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer- readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.

[0130] The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

[0131] Although the invention has been described with reference to exemplary embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.