OF MAKING THE SAME

TECHNICAL FIELD

This invention relates to crosslinked materials, methods of making crosslinked materials, and to their uses.

BACKGROUND

Many synthetic polymers are linear, unbranched, non-crosslinked long chain molecules. They can be semicrystalline like high density polyethylene, ultra-high molecular weight polyethylene, nylon, polyethylene oxide, polyvinyl alcohol and polytetrafluoroethylene among others. Examples of non-crystalline rubbery polymers are polybutadiene, polydimethyl siloxane and ethylene-propylene diene monomer (EPDM) rubber, which are amorphous or non-crystalline at room temperature.

Examples of amorphous glassy polymers are atactic polystyrene, polycarbonate and atactic polymethyl methacrylate which have a glass transition temperature above room temperature. All of the above polymers can be synthesized to be largely linear, non-crosslinked, non-branched long chain molecules. Some examples of non-linear polymers are branched polymers with long side chains, such as low density polyethylene or branched polymers with short side chains such as linear low density polyethylene. Several polymers can be synthesized to have a molecular weight that is so high, often over 1 million g/mole that when they are raised to a temperature higher than the melting temperature of their crystalline phase or, in the case of noncrystalline polymers, above glass transition temperature, they do not flow to a discernible extent on a short time scale since high molecular weight polymers have a high entanglement density. The advantage of synthesizing very high (or ultra-high, as they are often referred to) molecular weight polymers is that their high chain entanglement density provides an increase in their fracture toughness, impact strength, fatigue strength, wear resistance, scratch resistance and a host of other mechanical properties. This is due to the fact that the large number of entanglements associated with high molecular weight chains behave like crosslinks, which govern its ultimate mechanical properties, such as maximum strength and ductility as well as toughness and wear resistance. When polymers are synthesized, there is often a distribution of molecular weights. Therefore, high and ultra-high molecular weight polymers have not just highly entangled long, chain macromolecules they also have short macromolecules with a low entanglement density. The lower molecular weight fractions of high molecular weight polymers assist in compaction and fusion of the synthesized resin powders in the melt state since they are less entangled and are able to flow more easily than highly entangled, high molecular weight fractions. But low molecular weight polymers have a lower resistance to wear and lower toughness compared to their higher molecular weight counterparts. Nevertheless, the low molecular weight fractions are necessary to process synthesized powders into consolidated blocks due to their flow characteristics. Thus, there is a distribution of entanglements in most polymers by virtue of having a distribution of molecular weights.

Both linear and branched polymers are often crosslinked either by using chemical crosslinking agents or by applying ionizing radiation, which can further improve several properties such as scratch resistance, resistance to chemicals due to lower solubility in the solvent, decrease in gas permeation, shape-memory effects and resistance to wear. Crosslinking makes the linear or branched polymers attain a networks structure since there can be several inter-molecular crosslinks along with intra-molecular crosslinks. The benefit is that both entanglements as well as chemical crosslinks formed using crosslinking agents or ionizing radiation assist in increasing several mechanical and tribological properties for the polymer. In fact, unlike the non- crosslinked polymer, several of the entanglements cannot be disentangled by swelling in a solvent or by stretching the polymer since they are trapped within the network structure associated with crosslinking. Crosslinking is especially advantageous to the lower molecular weight fractions of the polymers since they have a smaller number of entanglements compared to the high molecular weight fractions and are associated with less toughness and wear resistance. While crosslinking by ionizing radiation or by incorporation of chemical crosslinking agents uniformly distributed into the polymer creates a relatively uniform distribution of crosslinks throughout the polymer, which is uniform regardless of molecular weight of the macromolecules of the non-crosslinked polymer, the number of trapped entanglements remains nonuniform since the entanglements vary depending on the molecular weight of the macromolecules. Thus, the overall distribution of "effective" crosslinks, which is the sum of chemical crosslinks associated with ionizing radiation or chemical crosslinking agents and the physical entanglements trapped between crosslinks, remains non-uniform in the crosslinked polymer by virtue of its wide molecular weight distribution and consequently wide distribution of the number of

entanglements. It is difficult to make the effective crosslinks more uniform with respect to molecular weight of the polymer chains by radiation since it is generally non-selective with respect to molecular weight and it is also difficult to prepare compositions of chemical crosslinking agents in the polymer so that highly entangled, high molecular weight chains are crosslinked to a less extent than the less entangled, low molecular weight chains, which would then make the effective crosslink density more uniform regardless of molecular weight of the polymer chain. However, methods to alter the entanglements of the chains selectively so that the high molecular weight segments have reduced entanglements while the lower molecular weight chains are at their equilibrium or maximum entanglement density or close to it has great value to maximize toughness and ductility of the polymer and maintaining high resistance to wear.

Articles requiring high toughness, such as medical devices, pier fenders, ski bottoms etc are usually fabricated from ultra high molecular weight polymers using sintering, a process that involves compressing the as-synthesized polymer, melting the compressed preform and then cooling under pressure to recrystallize and solidify the preform. Common methods for processing of ultra-high molecular weight are compression molding and ram extrusion. One of the drawbacks in the fabrication of large articles, such as blocks or bars is that the thermal conductivity of most polymers, such as ultra-high molecular weight polyethylene is very low, requiring the maintaining the polymer in the melt state for a long duration for the polymer to melt all the way through into its interior from the hot surface. Under such conditions, the polymer chains in the surface regions of the article, especially the large chains that require a long time to achieve their maximum entanglement density, entangle to a larger extent than the chains in the interior, which take a longer time to attain the surface temperature. Also, during cooling, which also occurs from the surface, occurs much faster in the surface regions due to the polymer's low thermal conductivity. Thus a skin layer of solid material is formed while the interior is still in the melt state and slowly solidifies as crystallization progresses into the bulk material. This mismatch in the state of the polymer in different regions of the article induces stresses into the material and non uniformity in the crystal orientation between the surface regions of the article and the bulk regions, resulting in a variation in mechanical properties throughout the article. The spatial variation in structure has been discussed in Bellare et al., Biomaterials, 17, 2325-2333 (1996) and the spatial variation in creep properties of extruded bars is discussed in Lee et al., KSME International Journal, 17, 1332- 1338 (2003). A process that minimizes the spatial variation in structure and properties of articles processed from ultra-high molecular weight polymers would be beneficial for their applications, especially in medical devices.

The resistance to wear enables polymeric materials such as, for example, ultra-high molecular weight polyethylene to be used in medical prostheses, such as total hip replacement prostheses and total knee replacement prostheses. Ultra-high molecular weight polyethylene is a highly entangled, semicrystalline polymer with an average molecular weight exceeding one million g/mole and is polydisperse with a wide distribution of molecular weights. The large number of chain entanglements associated with a high molecular weight polymer act as physical crosslinks and provide high wear resistance to the polymer as it is articulated against a metallic or ceramic counterface in the human joint while the low molecular weight chains assist in fusion of the resin to form consolidated sheets or rods by molding or ram extrusion. Further chemical or radiation crosslinking of the highly entangled polymer provides an increase in resistance to wear. Desirable characteristics for such high molecular weight polymeric materials used in joint replacement prostheses include

biocompatibility, a low coefficient of friction, a relatively high surface hardness, and resistance to wear and creep. It is also desirable for such polymers to be readily sterilizable, e.g., by using ionizing radiation, or by utilizing a gas sterilization such as ethylene oxide or gas plasma, prior to implantation in a body, e.g., a human body. Ionizing radiation, e.g., gamma or electron beam radiation is often a preferable method of sterilization for some implants, because in addition to sterilizing the endoprostheses, the high energy ionizing radiation can crosslink the polymeric materials, thereby improving their wear resistance. However, while treatment of polymeric prosthetic components with ionizing radiation can be beneficial, high- energy radiation can also have deleterious effects. For example, treatment of polymeric components with high-energy radiation can result in the generation of long- lived, reactive species within the polymeric matrix, e.g., free radicals, radical cations, or reactive multiple bonds, that over time can react with oxygen, e.g., of the atmosphere or dissolved in biological fluids, to produce oxidative degradation in the polymeric materials. Such degradation can reduce the wear resistance of the polymeric material. Thermal treatment after radiation is often used to decrease the number of reactive species in the polymer. However, both radiation dose and thermal treatment after radiation can decrease the overall ductility, ultimate tensile properties, fracture toughness and resistance to fatigue crack propagation in highly crosslinked, highly entangled polymers, such as ultra-high molecular weight polyethylene.

Radiation sterilization of polymeric materials, crosslinking, and entrapment of long- lived, reactive species, and their relationship to wear, resistance to crack propagation and other mechanical properties are discussed in Kurtz et al., Biomaterials , 20, 1659- 1688 (1999); Kurtz et al., Biomaterials, 27, 24-34 (2006); Tretinnikov et al., Polymer, 39(4), 6115-6120 (1998); Maxwell et al., Polymer, 37(15), 3293-3301(1996); Wang et al., Tribology International, 31(1-3), 17-33 (1998); Oral et al., Biomaterials, 26,

6657-6663 (2005); Oral et al., Biomaterials, 25, 5515-5522 (2004); Muratoglu et al., Biomaterials, 20, 1463-1470 (1999); Hamilton et al., European Patent Application No. 1072276A1 ; Li et al., U.S. Patent No. 5,037,928, NcNulty et al., U.S. Patent No. 6,245,276; and Muratoglu et al., PCT Publication No. WO 2005/074619. Additional references include U.S. Patent Nos. 5,414,049, 6,228,900, 6,547,828, 6,464,926,

6,641,617 and 6,786,933; Baker DA, Bellare A and Pruitt L, "The Effect of Degree of Crosslinking on the Fatigue Crack Propagation Resistance of Orthopedic-Grade Polyethylene," Journal of Biomedical Materials Research (2003) 66A: 146- 154; Oral E, Wannomae KK, Hawkins NE, Harris WH, Muratoglu OK, "Alpha-Tocopherol Doped Irradiated UHMWPE for High Fatigue Resistance and Low Wear,"

Biomaterials (2004) 25(24):5515-5522); and U.S. Published Patent Application Nos. 2005/0043431, 2003/0149125, and 2005/0194722.. SUMMARY

This application relates to crosslinked polymers, methods of making crosslinked polymers with a higher ductility than obtainable with previous methodologies, and to uses of the same.

Described herein is a method to produce a uniform consolidated preform from an ultra-high or high molecular weight polymer resin or a previously consolidated preform with non-uniform crystalline morphology, by heating the non-uniform preform under pressure, maintaining the polymer at an elevated temperature below the melting temperature until the temperature is uniform across the entire preform, then melting the polymer at a constant temperature by releasing the pressure to a value wherein the polymer enters the melt state, maintaining the polymer in the melt state for a specific time period and then recrystallizing the preform by increasing pressure at a constant temperature and then cooling the polymer to room temperature under pressure, and finally releasing the pressure. Such a preform can be used prior to disentangling the polymer using the aforementioned methods described but could also serve as a method to disentangle the polymer if the polymer is maintained in the melt state for a short duration, wherein the polymer does not become fully entangled. The major advantage of using pressure as a method to melt the polymer isothermally is that pressure is uniform throughout the article and therefore the pressure change will melt the entire polymer at the same instance whereas melting at a constant or variable pressure by heating results in the outer surface achieving the melt state first and the interior regions take much longer to melt due to the low inherent thermal conductivity of polymers. Similarly, recrystallizing the article by isothermal pressure- crystallization ensures that all regions of the article will crystallize uniformly and thus solidify at the same time whereas if the article is cooled from the melt state isobarically, the surface region in contact with the cold surface of the mold will crystallize and solidify first, forming a skin layer and then the interior would solidify slowly. Another advantage of using pressure to crystallize the article isothermally is that rapid pressure increase will increase the rate of crystallization, thereby

"quenching" the article from the melt state to a solid state rapidly, whereas large articles cannot be quenched by cooling due to the low thermal conductivity of the polymer. The disentangled materials provided can be effectively and efficiently crosslinked by using ionizing radiation (e.g., generated by a gamma radiation source and/or an electron beam source) or by chemical crosslinking agents. Following the crosslinking of the polymeric material with various heat treatments described herein effectively reduces the concentration of reactive species trapped in the polymer, such as free-radicals or radical cations, resulting in oxidation resistant materials. Thus, the methods provide materials that are stable over extended periods of time and that are resistant to oxidation. In addition, parts formed from the crosslinked polymeric materials have, e.g., high wear resistance, high resistance to creep deformation, along with high ductility, high impact strength and a high level of fatigue and crack propagation resistance. Some of the crosslinked polymeric materials have a low coefficient of friction. The polymeric material may include an antioxidant, radical scavenger or stabilizer to induce oxidation resistance associated with free radicals introduced by radiation, free radicals generated by an extraneous fluid absorbed into the polymer, such as synovial fluid or a joint or by free radicals associated with high temperature treatment of the polymer.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform in the form of resin powder or consolidated block including a substantially non-crosslinked semicrystalline polymer, isothermally pressurizing the preform, isobarically heating the preform to a temperature below the melting temperature at that specific temperature, then decreasing the pressure isothermally so that the preform enters the melt state, maintaining the preform in the melt state for a fixed period of time, applying a pressure so that the polymer recrystallizes, then lowering the temperature isobarically until room temperature and then lowering the pressure isothermally. The isothermal de-pressurization of the preform from the solid state into the melt state and pressurization into the melt state may optionally be repeated several times.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform in the form of resin powder or consolidated block including a substantially non-crosslinked semicrystalline polymer, isobarically increasing the temperature of the preform to a temperature close to but below the melting temperature, increasing the pressure and temperature to an elevated level without allowing the preform to melt, then isothermally decreasing the pressure until the preform is in the melt state and maintaining it in the melt state for a period of time, then isothermally increasing the pressure until the preform

recrystallizes and then decreasing the temperature and pressure in no particular order but in a sequence or simultaneously so as to not allow the preform to enter the melt state again. The pressure or temperature of the preform in the melt state can be varied but the recrystallization and solidification is conducted isothermally by an increase in pressure.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform in the form of resin powder or consolidated block including a substantially non-crosslinked semicrystalline polymer, isothermally increasing the pressure, then heating the polymer until it is in the melt state, lowering the temperature under the applied pressure until it recrystallizes, then decreasing the pressure isothermally to enable the preform to enter the melt state for a period of time, then increasing the pressure to recrystallize the preform and finally decreasing both temperature and pressure in no particular order but in a sequence or simultaneously so as to not allow the preform to enter the melt state again.

Accordingly, in an aspect, the present application provides a method of preparing a cross-linked polymer preform (e.g., wherein the substantially non- crosslinked polymer preform is substantially non-crosslinked polymer such as polyethylene, preferably high-molecular weight polyethylene, most preferably ultrahigh molecular weight polyethylene), comprising:

(a) increasing temperature and pressure of a substantially non-crosslinked polymer preform, to a first temperature and first pressure, wherein the substantially non-crosslinked preform is below its melt state;

(b) maintaining the substantially non-crosslinked polymer preform at said first temperature and first pressure for a first period of time; (c) decreasing the pressure of the substantially non-crosslinked polymer preform to a second pressure isothermally at said first temperature, wherein the substantially non-crosslinked polymer preform enters its melt state;

(d) increasing pressure of the substantially non-crosslinked polymer preform to a third pressure isothermally at said first temperature, wherein the substantially non-crosslinked polymer preform recrystallizes from its melt state; and

(e) crosslinking the substantially non-crosslinked polymer preform using ionizing radiation.

In some embodiments, in step (a), the pressure is increased to said first pressure followed by increasing the temperature to said first temperature at constant pressure.

In some embodiments, the first period of time is about 1 minutes to about 48 hours, about 1 minute to about 24 hours, about 1 minute to about 12 hours, about 1 minute to about 6 hours, about 1 minute to about 4 hours, about 1 minute to about 2 hours, or about 1 minute to about 1 hour.

In some embodiments, after step (c), the substantially non-crosslinked polymer preform is maintained in a melt state for a second period of time at said first temperature and said second pressure.

In some embodiments, the second period of time is about 1 second to about 48 hours, about 1 second to about 24 hours, about 1 second to about 12 hours, about 1 second to about 6 hours, about 1 second to about 4 hours, about 1 second to about 2 hours, or about 1 second to about 1 hour.

In some embodiments, further comprising, after step (d), but before step (e):

(i) decreasing the temperature of the substantially non-crosslinked polymer preform, to a second temperature wherein said second temperature is below said first temperature;

(ii) decreasing the pressure of the substantially non-crosslinked polymer preform to a fourth pressure isothermally at said second temperature, wherein the substantially non-crosslinked polymer preform enters its melt state;

(iii) increasing pressure of the substantially non-crosslinked polymer preform to a fifth pressure isothermally at said second temperature, wherein the substantially non-crosslinked polymer preform recrystallizes from its melt state. In some embodiments, steps (a)-(d) are repeated one or more times prior to step (e).

In some embodiments, the substantially non-crosslinked polymer preform is an as-synthesized resin in the form of flakes, powder, pellets or a mixture thereof.

In some embodiments, the substantially non-crosslinked polymer preform is a consolidated block, sheet, extruded rod or a net-shaped part.

In some embodiments, the first pressure is from about 10 to about 1000 MPa.

In some embodiments, the second pressure is from about 1 kPa to about 50

MPa

In some embodiments, the first pressure and the third pressure are the same pressure.

In some embodiments, the first temperature is from about 133°C to about

240°C.

In some embodiments, step (e) is conducted at room temperature and atmospheric pressure.

In some embodiments, the ionizing radiation is gamma radiation or electron beam. In some embodiments, the ionizing radiation is microwave radiation.

In some embodiments, the radiation in step (e) is at a dose of from about 1 Mrad to about 100 Mrad.

In some embodiments, the radiation in step (e) is at a dose of from about 4 to about 100 Mrad.

In some embodiments, the radiation in step (e) is at a dose of from about 5 to about 20 Mrad.

In some embodiments, the substantially non-crosslinked polymer preform further comprises a stabilizer.

In some embodiments, the stabilizer is an antioxidant.

In some embodiments, the antioxidant is a vitamin E.

In some embodiments, the stabilizer is a UV light stabilizer.

In some embodiments, further comprising annealing the crosslinked polymer preform after step (e) at an elevated temperature but below the melting temperature.

In some embodiments, the crosslinked polymer preform has a crosslink density greater than about 0.1 mol/dm ³ . In some embodiments, the crosslinked polymer preform has a degree of crystallinity greater than 55%.

In some embodiments, the crosslinked polymer preform has a lamellar thickness greater than 25 nm.

In some embodiments, the crosslinked polymer preform has a lamellar thickness greater than 30 nm.

In some embodiments, the crosslinked polymer preform has a crosslink density greater than about 0.1 mol/dm ³ , a degree of crystallinity greater than 55%, and a lamellar thickness greater than 25 nm.

In some embodiments, the crosslinked polymer preform has a crosslink density higher than 0.1 mol/dm ³ and a lamellar thickness greater than 30 nm.

In some embodiments, the crosslinked polymer preform has a crosslink density higher than 0.1 mol/dm ³ , a degree of crystallinity greater than 55%, and a lamellar thickness greater than 30 nm.

In some embodiments, the crosslinked polymer preform is suitable for use as a component of a medical device. In some embodiments, the medical device is a joint replacement prosthesis, a tibial insert, an acetabular cup, a glenoid component, a component of an ankle replacement, or a maxillofacial implant.

In another aspect, the present application provides a crosslinked polymer preform made according to any of the methods described herein.

In another aspect, the present application provides a component of a medical device made according to the any of the methods described herein.

In some embodiments, the medical device is a joint replacement prosthesis, a tibial insert, an acetabular cup, a glenoid component, a component of an ankle replacement, or a maxillofacial implant.

In another aspect, the present application provides a polymeric preform comprising crosslinked ultra-high molecular weight polyethylene with a crosslink density greater than 0.100 mol/dm ³ , a degree of crystallinity greater than 55% and a lamellar thickness greater than 25 nm.

In another aspect, the present application provides a polymeric preform comprising crosslinked ultra-high molecular weight polyethylene with a crosslink density higher than 0.1 mol/dm ³ , a degree of crystallinity greater than 55%, and a lamellar thickness greater than 30 nm.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a polymeric rod of diameter Di being extruded through a die of smaller diameter D2.

FIG. 2 is a schematic of a polymeric rod of diameter Di and height Hi being uniaxially compressed to a height H2 with a larger diameter D2.

FIG. 3 is a schematic of an integrated process of formation of a polymeric rod of diameter Di from the polymer powder in the melt state by ram extrusion using a plunger, extrusion through a conical die of diameter D2 and then expansion through a conical interface to a rod of diameter D3. A plunger applies a back stress to enable the melt to expand. Not shown in the schematic is a heating system to control the temperature of the process. This method will enable the rod to be stretched in both the longitudinal direction and radial direction sequentially

FIG. 4 is a schematic of plane strain compression of a molded sheet of a polymer of thickness Hi into a sheet of thickness H2 by rolling.

FIG. 5 is a schematic of a rod undergoing torsional deformation by rotation of each end of the rod in opposite direction with a net angular deformation of γ.

FIG. 6 is a two dimensional schematic of a block undergoing shear deformation through a shear (angular) strain of γ.

FIG. 7 is a schematic of the phase diagram of polyethylene with a y-axis as temperature and x-axis as pressure showing the melt region and solid-state regions where orthorhombic and hexagonal are present.

FIG. 8 is a schematic of the phase diagram of polyethylene depicting a polymer at a temperature Ti and applied pressure Pi being isothermally pressurized to a pressure P2, then isobarically heated to a temperature T2 and then isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted (depicted by black arrows). Thereafter, the preform is pressurized to a pressure P2 and then cooled isobarically to its original temperature Ti and then returned to the original pressure Pi (grey arrows indicate this

recrystallization and cooling process on the phase diagram). In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively.

FIG. 9 is a schematic of the phase diagram of polyethylene depicting a polymer at a temperature Ti and applied pressure Pi being isothermally pressurized to a pressure P ₂ , then isobarically heated to a temperature T2 where it enters the hexagonal phase and then isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted (depicted by black arrows). Thereafter, the preform is pressurized to a pressure P2 where it crystallizes via the hexagonal phase and then is cooled isobarically to its original temperature Ti and finally returned to the original pressure Pi (grey arrows indicate this recrystallization and cooling process on the phase diagram). In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively.

FIG. 10 is a schematic of the phase diagram of polyethylene depicting a polymer at a temperature Ti and applied pressure PI being isothermally pressurized to a pressure P ₂ , then isobarically heated to a temperature T2 and then isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted (depicted by black arrows). Thereafter, the preform is pressurized to a pressure P4 and then cooled isobarically to its original temperature Ti and then isothermally returned to the original pressure Pi (grey arrows indicate this recrystallization and cooling process on the phase diagram). In some embodiments P4 < P ₂ , and in other embodiments, P ₄ > P2 as depicted in this schematic. In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively.

FIG. 11 is a schematic of the phase diagram of polyethylene depicting a polymer at a temperature Ti and applied pressure Pi being isobarically heated to a temperature T ₂ , then isothermally pressurized to a pressure P ₂ , then isobarically heated to a temperature T3 and then isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted (depicted by black arrows). Thereafter, the preform is pressurized to a pressure P ₄ and then cooled isobarically to its original temperature Ti and then returned to the original pressure Pi (grey arrows indicate this recrystallization and cooling process on the phase diagram). In some embodiments P ₄ < P2, in other embodiments, P ₄ > P2 as depicted in this schematic, and in yet other embodiments P ₄ =P2. In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively.

FIG. 12 is a schematic of the phase diagram of polyethylene depicting a polymer at a temperature Ti and applied pressure Pi being isobarically heated to a temperature T2, then isothermally pressurized to a pressure P2, then isobarically heated to a temperature T3 and then isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted. Then in the melt state, the sample is isobarically heated to a temperature T ₄ (this entire process is depicted by black arrows). In some embodiments, T3>T ₄ . Thereafter, the preform is isothermally pressurized to a pressure P ₄ whereupon the sample crystallizes in the hexagonal phase, then cooled isobarically to its original temperature Ti and then returned to the original pressure Pi (grey arrows indicate this recrystallization and cooling process on the phase diagram). In some embodiments P ₄ < P2, in other embodiments, P ₄ > P2 as depicted in this schematic, and in yet other embodiments P ₄ =P2. In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively.

FIG. 13 is a schematic of the phase diagram of polyethylene depicting a polymer at a temperature Ti and applied pressure Pi being isothermally pressurized to a pressure P2, then isobarically heated to a temperature T2 whereupon it enters the melt state for the first time, then isobarically cooled to a temperature T3 during which time it crystallizes via the hexagonal phase and then enters the orthorhombic phase. Then the polyethylene is isothermally reduced in pressure to a pressure P3, whereupon the sample re-enters the melt state (this entire process is depicted by black arrows). Then the sample is isothermally pressurized to a pressure P ₄ whereupon it crystallizes via the orthorhombic phase. Thereafter, the preform is cooled isobarically to its original temperature Ti and then returned to the original pressure Pi (grey arrows indicate this recrystallization via the orthorhombic phase and cooling process on the phase diagram). In some embodiments P ₄ < P2, in other embodiments, P ₄ > P2 as depicted in this schematic, and in yet other embodiments P ₄ =P2. In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively. FIG. 14 is a schematic of the phase diagram of polyethylene depicting a polymer at a temperature Ti and applied pressure Pi being heated and pressure simultaneously in neither isobaric nor isothermal processes to a temperature T2 and a pressure P ₂ , and then isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted (depicted by black arrows). Thereafter, the preform is isothermally pressurized to a pressure P2 and then cooled to its original temperature Ti while simultaneously reduced in pressure to its original pressure Pi in a process that is neither isothermal nor isobaric (grey arrows indicate this recrystallization and cooling process on the phase diagram). In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively.

DETAILED DESCRIPTION

Generally described herein are several methods to disentangle polymers, preferably their high molecular weight fractions, and then to crosslink them in the disentangled state. A method to disentangle a semicrystalline polymer, such as ultra- high molecular weight polyethylene, high density polyethylene, polyethylene oxide and polytetrafluoroethylene, among others, includes mechanical stretching above or below the melting temperature of the semicrystalline polymer followed by relaxing the polymer by unstretching or unloading. In the case of a glassy polymer, such as polystyrene and polycarbonate or rubbery polymer, such as polydimethyl siloxane and polybutadiene, the stretching may be conducted above or below the glass transition temperature. In the case of semicrystalline polymers, another method to disentangle the polymer is to melt the polymer and then slowly crystallize it at low undercooling i.e. at a temperature within a range of 0-20°C below the equilibrium melting temperature of the polymer (which is 145°C for polyethylenes). A third method to disentangle a semicrystalline polymer is to crystallize it at high temperatures under a high applied pressure above 10 MPa, preferably above 100 MPa and most preferably above 250 MPa, which leads to the formation of thick, extended chain crystals and a high degree of crystallinity. The high-pressure crystallized, high crystallinity polymer can then be further melted to form random coils of the disentangled polymer and then recrystallized at a pressure below 250 MPa or no applied pressure to form largely chain- folded crystals from the disentangled melt. The formation of high pressure, extended chain crystals and the re-melting process can be combined into one process where the polymer enters the melt state prior to cooling to room temperature.

Fourthly, a semicrystalline polymer can also be disentangled by gel-crystallization or solution crystallization after dissolving in a solvent. The solvent can be a supercritical fluid, such as supercritical carbon dioxide or supercritical propane, which have the disadvantage of being easily removed from the polymer after processing in a gaseous form. The polymer containing solvent-grown crystals can further be melted and recrystallized from the melt state before the disentangled polymer can re-entangle substantially. Finally, chain disentanglement can also be achieved by synthesizing the polymer from the catalyst so that the active sites on the catalyst are far removed from each other so that the growing polymeric chains cannot entangle with each other. The polymerization temperature can also be adjusted to be below the melting temperature of the semicrystalline polymer so that it immediately crystallizes upon being synthesized. In the case of amorphous, polymers, the temperature can be adjusted to be below the glass transition temperature so that the amorphous polymer becomes glassy when it attains a certain molecular weight. The polymer may be disentangled using one or more of the five methods outlined herein.

Other aspects:

Described herein are novel methods of processing polymeric materials, such as UHMWPE, that can include an antioxidant, such as Vitamin E or alpha-tocopherol. For example, in some instances, as described in more detail herein, polymeric materials, e.g., in the shape of a cylinder, are elongated in a solid state or molten state to disentangle the chains of the polymer. Samples can be re-melted (if the stretching was performed in the solid state) to recover the induced strain and make the material more anisotropic. In order to reduce the likelihood of re-entanglement when melted after stretching, it can be desirable to heat the polymeric material to a temperature only slightly above the melting temperature, such as between about 0.1°C to about 20°C above the melting temperature of the stretched polymeric material, e.g., within 0.1°C and 5°C of the melting temperature. Generally, it is preferable to not maintain the polymeric material at this melting temperature for long time periods, since this can also facilitate re- entanglements. . It can also be desirable to apply pressure while decreasing the pressure after disentanglement to enable the polymer to solidify rapidly before substantial re-entanglement occurs. Upon strain recovery, the polymeric material can be cooled to a temperature below the melting temperature and then irradiated by ionizing radiation, e.g., gamma or electron beam radiation, to a does range of from about 1 kGy to about 1000 kGy Any post-melting cooling or re- crystallization can be done rapidly (quenching) if a low crystallinity product is desired. In the case of an amorphous, non-crystalline polymer, the melting refers to a state where the temperature of the polymeric material is above the glass transition temperature. In the case of semicrystalline polymeric material, they can later be annealed at atmospheric pressure or high-pressures to thicken the crystals and increase overall crystallinity. Also described herein are methods to disentangle semicrystalline polymers by decreasing the rate of crystallization by maintaining the polymer at a fixed or variable crystallization temperature that is 20°C or less below the equilibrium melting temperature. Also described herein are methods to disentangle semicrystalline polymers by melting the polymer, crystallizing the polymer under high-pressures to form a highly disentangled, highly crystalline polymer, re-melting the polymer to form a disentangled melt, and crystallizing the polymer at atmospheric pressure or under moderate pressure less than 50 MPa. Also described herein are methods to disentangle polymers by synthesizing the polymer using catalysts where active sites are too far for the synthesizing polymer to entangle during the polymerization process, which can be further controlled by conducting the synthesis below the melting temperature, and in some instances below the glass transition temperature of the polymer. Also described herein are methods to disentangle semicrystalline polymers by melting and then crystallizing in the presence of a solvent, preferably a supercritical fluid which are easy to extract from the polymer in the gaseous form. Disentangled polymers using these methods or a combination thereof can be effectively crosslinked by ionizing radiation e.g., gamma or electron beam radiation, to a does range of from about 1 kGy to about 1000 kGy or by incorporation of chemical crosslmking agents e.g., dicumyl peroxide.

The disentangled materials provided can be effectively and efficiently crosslinked by using ionizing radiation (e.g., generated by a gamma radiation source and/or an electron beam source) or by chemical crosslinking agents. Following the crosslmking of the polymeric material with various heat treatments described herein effectively reduces the concentration of reactive species trapped in the polymer, such as free -radicals or radical cations, resulting in oxidation resistant materials. Thus, the methods provide materials that are stable over extended periods of time and that are resistant to oxidation. In addition, parts formed from the crosslinked polymeric materials have, e.g., high wear resistance, high resistance to creep deformation, along with high ductility, high impact strength and a high level of fatigue and crack propagation resistance. Some of the crosslinked polymeric materials have a low coefficient of friction. The polymeric material may include an antioxidant, radical scavenger or stabilizer to induce oxidation resistance associated with free radicals introduced by radiation, free radicals generated by an extraneous fluid absorbed into the polymer, such as synovial fluid or a joint or by free radicals associated with high temperature treatment of the polymer.

In one aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked polymeric materials. The methods include selecting a non-crosslinked preform having a first dimension and including a substantially non-crosslinked semicrystalline polymer; elongating the non-crosslinked preform in a first direction at a first temperature that is below the melting point of the substantially non-crosslinked material or above the melting point of the substantially non-crosslinked polymeric material to provide an elongated preform having a second dimension larger than the first dimension and including a first substantially non- crosslinked, disentangled polymeric material; fixing the elongated preform to provide a fixed, elongated preform that includes a second substantially non-crosslinked, disentangled polymeric material; heating the fixed, elongated preform to a second temperature about (e.g., within about 8 °C) or above a melting point of the second substantially non-crosslinked, disentangled polymeric material, to recover the strain induced during elongating and to provide a relaxed preform that includes a third substantially non-crosslinked, relaxed polymeric material; and crosslinking the third substantially non-crosslinked, relaxed polymeric material of the relaxed preform to provide a crosslinked preform that includes a crosslinked polymeric material. Prior to fixing, the elongated preform that includes a non-crosslinked disentangled polymeric material can optionally be held under a fixed or variable load or under a fixed or variable strain or both for a period of time to enable the polymer to continue to disentangle under strain or load or both. Also, the preform having a second dimension larger than the first dimension and including a first substantially non-crosslinked polymer can be unloaded and allowed to recover strain prior to fixing so that heating to a second temperature above melting temperature is not necessary. Finally, fixing of the semicrystalline disentangled polymer melt may be performed either by cooling from the melt at atmospheric pressure or under an applied pressure.

In another aspect, the invention features methods of making highly crosslinked preforms that include one or more highly crosslmked amorphous polymeric materials. The methods include selecting a non-crosslinked preform having a first dimension and comprising a substantially non-crosslinked polymeric material; elongating the first crosslinked preform in a first direction at a first temperature that is below the glass transition temperature of the first crosslinked material or above the glass transition temperature of the first crosslinked polymeric material to provide a crosslinked, elongated preform having a second dimension larger than the first dimension and including a first disentangled, crosslinked polymeric material; optionally maintaining the crosslinked, elongated preform under a fixed or variable load or at a fixed or variable strain for a period of time; releasing the load or strain and cooling the preform to provide a fixed, uncrosslinked elongated preform that includes an uncrosslinked, disentangled polymeric material; optionally heating the fixed, elongated preform to a second temperature about or above the glass transition temperature of the second uncrosslinked, disentangled polymeric material to recover residual strain induced during elongating and to provide a relaxed, uncrosslinked preform that includes a relaxed, uncrosslinked polymeric material; and crosslinking the relaxed, uncrosslinked preform comprising the relaxed,

uncrosslinked, polymeric material to provide a highly crosslinked preform that includes a highly crosslinked polymeric material. The amorphous, disentangled polymer can, after fixing be stored at a temperature below room temperature.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform including a substantially non-crosslinked semicrystalline polymer; heating the non-crosslinked perform to a temperature exceeding the melting temperature; cooling the non-crosslinked perform including a substantially non-crosslinked polymer to a temperature less than 20°C below its equilibrium melting temperature, preferably between 15 to 20°C below its equilibrium melting temperature to slow the rate of crystallization and allow the chains time to disentangle from the melt and be incorporated into the growing crystals; maintaining the perform at a fixed or variable temperature within 20°C below its equilibrium melting temperature for a period of time exceeding 60 seconds to increase the crystallinity of the substantially non-crosslinked polymer; slow cooling or rapidly cooling the non-crosslinked perform including a substantially non-crosslinked polymer to room temperature; and crosslinking the substantially non-crosslinked, relaxed polymeric material of the cooled preform to provide a crosslinked preform that includes a crosslinked polymeric material. Prior to crosslinking, the perform including the disentangled, non-crosslinked polymeric material can optionally be annealed at an elevated temperature below its melting temperature at atmospheric pressure or under an applied pressure exceeding 1 kPa, preferably in a pressure range of 1 kPa- 10 MPa, more preferably in a pressure range of 10- 1000 MPa without entering the melt-state.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform including a substantially non-crosslinked semicrystalline polymer; heating the non-crosslinked preform to a temperature exceeding the melting temperature; cooling the non-crosslinked preform including a substantially non-crosslinked polymer rapidly to a temperature at least 100 degrees lower than the equilibrium melting temperature; heating the non- crosslinked perform including a substantially non-crosslinked polymer to a temperature within 20 degrees of its equilibrium melting temperature; maintaining the perform at a fixed or variable temperature within 20°C below its equilibrium melting temperature for a period of time exceeding 60 seconds to increase the crystallinity of the substantially non-crosslinked polymer; slow cooling or rapidly cooling the non- crosslinked perform including a substantially non-crosslinked polymer to room temperature; and crosslinking the substantially non-crosslinked, relaxed polymeric material of the cooled preform to provide a crosslinked preform that includes a crosslinked polymeric material. Prior to crosslinking, the perform including the disentangled, non-crosslinked polymeric material can optionally be annealed at an elevated temperature below its melting temperature at atmospheric pressure or under an applied pressure exceeding 1 kPa, preferably in a pressure range of 1 kPa- 10 MPa, more preferably in a pressure range of 10-1000 MPa without entering the melt-state.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform including a substantially non-crosslinked semicrystalline polymer; heating the non-crosslinked perform to a temperature exceeding the melting temperature; applying a pressure in a range of 50- 1000 MPa to high-pressure crystallize the polymer for a period of time; decreasing the pressure so that the disentangled, highly crystalline polymer containing high-pressure crystals melt to form a disentangled melt; decreasing the temperature under atmospheric pressure or under moderate pressures of 1 kPa-50 MPa to form a preform including a disentangled, semicrystalline polymer; and crosslinking the substantially non-crosslinked, relaxed polymeric material of the cooled preform to provide a crosslinked preform that includes a crosslinked polymeric material. The initial heating of the polymer can alternatively be conducted under a fixed or variable applied pressure as long as it enters the melt state prior to application of high pressures.

Similarly, the decrease of pressure at elevated temperature of the high-pressure crystallized, disentangled polymer to melt the crystals can be conducted while simultaneously lowering the temperature as long as the polymer enters the melt state prior to cooling down to room temperature at moderate pressure of 1 kPa - 50 MPa. The high pressure crystallized non-crosslinked, disentangled polymeric perform can optionally, after high pressure crystallization, be cooled in such a way that the combination of temperature and pressure during cooling does not allow the polymer to melt. Then the pressure can be released and followed by a separate step of re- melting the polymer at atmospheric pressure or a moderate applied pressure in a range of 1 kPa- 50 MPa to form a disentangled melt, which is then cooled rapidly to room temperature by contact with a cold surface or held at a fixed or variable undercooling temperature, which is within 20 degrees of the equilibrium melting temperature of the polymer at the applied pressure and is then followed by rapid cooling or slow cooling to room temperature prior to the crosslinking step. Alternatively, the disentangled melt can be rapidly crystallized by increasing the applied pressure at a constant temperature or while lowering the temperature.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked polymeric materials. The methods include selecting a substantially non-crosslinked, disentangled polymeric resin prepared using a catalyst with active sites far apart and optionally at a polymerization temperature lower than melting temperature in the case of a semicrystalline polymer or glass transition temperature in the case of a non-crystalline polymer so that the synthesized macromolecules do not entangle as they grow on the catalyst; molding or ram extruding the powder to fuse it at a temperature exceeding the melt temperature for a period of time less than the time taken for the polymer melt to attain its equilibrium entanglement state; decreasing the temperature of the disentangled melt under atmospheric pressure or at an applied pressure to form a preform including a disentangled, semicrystalline polymer; and crosslinking the substantially non- crosslinked, relaxed polymeric material of the cooled preform to provide a crosslinked preform that includes a crosslinked polymeric material. The heating and cooling of the polymer to a temperature exceeding the melt temperature can be conducted under a fixed or variable applied pressure as long as it enters the melt state upon heating prior to cooling.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform including a substantially non-crosslinked semicrystalline polymer, swelling the non-crosslinked perform in a solvent, preferably a supercritical fluid, and at a temperature exceeding the melting temperature; cooling the swollen perform including the substantially non-crosslinked, disentangled semicrystalline polymer and crystallizing it in the solvent; removing the solvent at a temperature not exceeding the melting temperature; and crosslinking the substantially non-crosslinked, dry perform including a semicrystalline, disentangled polymeric material to provide a crosslinked preform that includes a crosslinked polymeric material. Prior to crosslinking, the preform including the disentangled, non- crosslinked polymeric material can optionally be annealed at an elevated temperature above or below its melting temperature at atmospheric pressure or under an applied pressure exceeding 1 kPa, preferably in a pressure range of 1 kPa- 10 MPa, more preferably in a pressure range of 10- 1000 MPa without entering the melt-state.

In another aspect, the invention features methods of making crosslinked preforms that include one or more crosslinked semicrystalline polymeric materials. The methods include selecting a non-crosslinked preform in the form of resin powder or consolidated block including a substantially non-crosslinked semicrystalline polymer, heating the preform to an elevated temperature below the melting temperature, maintaining the preform at the temperature until a uniform temperature is achieved in the preform, reducing the pressure until the polymer enters the melt state, maintaining the preform in the melt state for a fixed period of time, applying a pressure so that the polymer recrystallizes, then lowering the temperature and pressure without allowing the preform to re-enter the melt state.

Aspects and/or embodiments can have one or more of the following features.

The methods can further include annealing the crosslinked perform, such as by heating the crosslinked preform below the melting point of the crosslinked polymeric material. For example, the annealing cant include heating the crosslinked preform to between about 100°C and about 1°C below the melting point of the crosslinked semicrystalline polymeric material or to between about 100°C and about 1°C below the glass transition temperature of the crosslinked amorphous polymer. The annealing can include applying a pressure of greater than 10 MPa to the crosslinked polymeric material, while heating the crosslinked material to a temperature below a melting point of the crosslinked polymeric material at the applied pressure for a time sufficient to provide an oxidation resistant crosslinked polymeric material. For example, the applied pressure is greater than 350 MPa. The annealing can include heating the crosslinked polymeric material to a temperature that is about 25°C to about 0.5°C below the melting point of the crosslinked polymeric material, and then applying pressure above nominal atmospheric pressure. The annealing can be carried out in the presence of a reactive gas that can quench residual reactive species trapped in the crosslinked polymeric material. For example, the reactive gas can include one or more unsaturated compounds, such as acetylene. The non-crosslinked preform that includes a substantially non-crosslinked polymeric material can further include one or more molecules, each having a permanent dipole moment. The heating can include exposing the fixed preform to microwave radiation. The crosslinked preform that includes the crosslinked polymeric material can further include one or more molecules, each having a permanent dipole moment. The annealing can include exposing the crosslinked preform to microwave radiation. The heating can be performed such that the temperature is not more than about 8°C below the melting point of the second substantially non-crosslinked, disentangled polymeric material, or from about 0.1°C to about 20°C above the melting, such as less than about 0.1°C to about 20°C. This can be performed for a time of not more than about 180 minutes, such as not more than about 5 minutes. Any method can further include cooling the relaxed preform that includes the substantially non-crosslinked, relaxed polymeric material, such as by immersing the relaxed preform in a liquid at a temperature more than 100 °C lower than a melting point of the third substantially non-crosslinked, relaxed polymeric material. The crosslinked preform that includes the crosslinked polymeric material can further include one or more antioxidants, such as one or more phenolic compounds. The one or more phenolic compounds include alpha- tocopherol. The one or more antioxidants can be infused into the preform. The antioxidant can have a melting point above about 50°C. The substantially non- crosslinked preform and/or the crosslinked preform can be in sheet or rod form. The substantially non-crosslinked preform and/or the crosslinked preform can be in the form of a medical device or portion thereof. The substantially non-crosslinked preform can be in rod form having a longitudinal length, and the first dimension can be the longitudinal length of the substantially non-crosslinked preform. The substantially non-crosslinked preform can be in sheet form having a length, a width and a thickness, and the first dimension can be either the width or the length of the preform. During elongating the non-crosslinked preform in the first direction, the preform can also elongated in a second and third direction, such as in directions that is substantially perpendicular to the first direction. The elongation in each direction can be performed sequentially or simultaneously. The substantially non-crosslinked preform in the first direction can be performed by stretching the substantially non- crosslinked preform in the first direction. The elongating of the substantially non- crosslinked preform in the first direction can be performed by compressing the substantially non-crosslinked preform in a direction perpendicular to the first direction. The elongation in the first direction can be performed by extrusion through a die of smaller dimension than the preform. The elongating of the substantially non- crosslinked preform in the first direction can be performed at a temperature of between 140°C to about 180°C, such as between about 142°C to about 160°C. The elongating the substantially non-crosslinked preform can be performed by uniaxial tensile stress, biaxial tensile stress, uniaxial compression, channel-die compression, rolling, extrusion, shear stress, torsion, biaxial compression, biaxial compression followed by elongation, or combinations thereof. Chain elongation can be performed under static or dynamic loads. The second dimension can be between about 0.5 percent and 500 percent larger than the first dimension. The second dimension can be between about 5 percent and 100 percent larger than the first dimension. The second dimension can be between about 10 percent and 50 percent larger than the first dimension. The fixing of the elongated preform can result from stretching the first elongated preform in a manner so as to increase a material melting point above a temperature at which the stretching is performed. During elongation of a

semicrystalline polymer, there can be strain-induced crystallization. The elongation process and optionally the relaxation process can be integrated into the process by which the preform is being fabricated from the polymeric resin powder. The fixing of the elongated preform can include cooling the elongated preform below a material melting point. The fixing can also be performed by applying pressure to the melt. The fixing can be performed while simultaneously decreasing the temperature and increasing the pressure. The crosslinking can be performed with an ionizing radiation, such as with gamma rays or high energy electrons. The ionizing radiation can be applied at a total dose of greater than 1 kGy, such as greater than about 25 kGy or 1000 KGy. The ionizing radiation can be applied at a dose rate greater than 0.1 kGy/hour. Crosslinking can occur below a melting point of the second substantially non-crosslinked, elongated polymeric material. The first and second substantially non-crosslinked, elongated materials can each have a different crystallinity and/or a different melting point or glass transition temperature. The substantially non- crosslinked polymeric material can include a polyethylene, such as an ultra-high molecular weight polyethylene. The substantially non-crosslinked polymeric material can include a melt processible polymer or a blend of melt processible polymers. The crosslinking occurs at about nominal atmospheric pressure. The crosslink density can be greater than about 100 mol/m3, such as greater than about 160, 175, 200 or 250 mol/m3. The crosslinking of the relaxed, crosslinked preform can include exposing the relaxed, crosslinked preform to a radiation dose of greater than about 25 kGy, such as greater than about 50 kGy or greater than about 100 kGy. The crosslinked preform that includes the first crosslinked polymeric material can have a molecular weight between crosslinks of greater than about 7,500 g/mol, such as greater than about 10,000 g/mol, such as greater than about 15,000 g/mol or greater than about 25,000 g/mol. Crosslinking the substantially non-crosslinked polymeric material and/or crosslinking the relaxed, crosslinked preform can be performed in the absence of oxygen, such as in the presence of an inert gas, e.g., nitrogen, helium, argon or mixtures of these gases. The substantially non-crosslinked polymeric material includes one or more antioxidants.

Aspects and/or embodiments can have any one of, or combinations of, the following advantages. The crosslinked materials are stable over extended periods of time and are resistant to oxidation. The semicrystalline crosslinked polymeric materials are highly crystalline, e.g., having a crystallinity of greater than 54 percent, e.g., 57 percent or higher. The polymeric materials have a low degree of chain entanglement, which can improve crosslinking degree, quality, and/or efficiency. The crosslinked polymeric materials are highly crosslinked, e.g., having a high crosslink density, e.g., greater than 100 mol/m3, and/or a relatively low molecular weight between crosslinks, e.g., less than 7500 g/mol. When the crosslinked polymeric material is UHMWPE, it can have a relatively high melting point, e.g., greater than 140°C, in combination with a relatively high degree of crystallinity, e.g., greater than about 52 percent. Parts formed from the crosslinked polymeric material have high wear resistance, enhanced stiffness, as reflected in flexural and tensile moduli, a high ductility, a high level of fatigue and crack propagation resistance, and enhanced creep resistance. Some of the crosslinked polymeric materials have a low coefficient of friction. In addition, the described methods are easy to implement. An "antioxidant" is a material, e.g., a single compound or polymeric material, or a mixture of compounds and/or polymeric materials that reduce the rate of oxidation reactions.

An "oxidation resistant crosslinked polymeric material" is one that loses less than 25 percent of its elongation at break (ASTM D412, Die C, 2 hours, and 23°C) after treatment in a bomb reactor filled with substantially pure oxygen gas to a pressure of 5 atmospheres, heated to 70°C temperature, and held at this temperature for a period of 14 days.

A "substantially non-crosslinked polymeric material" is one that is melt processible, or in the alternative, dissolves in a solvent, whereas a "substantially crosslinked polymeric material" is one that is not melt processible, or in the alternative, one that does not dissolve in any solvent, although it may swell.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

General Methodologies:

Generally, oxidation resistant, crosslinked, polymeric materials that have desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a non-crosslinked preform having a first dimension, such as a length or a width or a diameter and that includes a substantially non-crosslinked polymeric material. The non-crosslinked preform is elongated in a first direction at a first temperature that is below the melting point of the substantially non-crosslinked material (e.g., 20°C below), or above the melting point (e.g., 20°C above) of the substantially non-crosslinked polymeric material to provide an elongated preform having a second dimension larger than the first dimension and including a first substantially non-crosslinked, disentangled polymeric material. The elongated preform is fixed to provide a fixed, elongated preform that includes second substantially non-crosslinked, disentangled polymeric material. The fixed, elongated preform is heated to a second temperature about (e.g., at or slightly below) or above the melting point of the second substantially non- crosslinked, disentangled polymeric material, to recover strain induced during elongating and to provide a relaxed preform that includes a third substantially non- crosslinked, relaxed polymeric material. The third substantially non-crosslinked, relaxed polymeric material of the relaxed preform is crosslinked to provide a crosslinked preform that includes a crosslinked polymeric material. After elongation, the elongated preform is optionally held in the elongated state for a period of time under load or under a geometric constraint prior to fixing. Also, the elongated preform is optionally elongated in a direction or directions perpendicular to the direction of first elongation prior to fixing. The fixing is optionally conducted after release of constraint and/or applied load to relax the polymer prior to fixing and crosslinking.

Any of the preforms, such as a crosslinked preform, may be annealed, is described in more detail below. For example, to anneal the crosslinked preform, the annealing can include heating the crosslinked preform below the melting point of the crosslinked polymeric material. As will be described in greater detail below, any preform may be annealed in the presence of a reactive gas or quenching material, such as acetylene, that can quench residual reactive species, such as radials and radical cations that may be trapped in a polymeric material, such as a crosslinked polymeric material. Generally, the reactive gas can assist in crosslinking of a polymeric material by acting as a bridge between two reactive moieties, and it can also terminate such reactive moieties, which can prevent oxidation over the long-term.

While a few embodiments for elongating the substantially non-crosslinked material, e.g., as a preform, have been shown, other elongating embodiments are possible. More generally, elongation can be achieved, e.g., by uniaxial tensile stress, biaxial tensile stress, uniaxial compression, channel-die compression, shear stress, or combinations thereof.

Generally, in some embodiments, a stretched dimension is, e.g., between about 0.5 percent and 10,000 percent larger than an unstretched dimension, e.g., between about 1.0 percent and 5,000 percent larger, or between about 2.0 percent and 100 percent larger.

In some embodiments, the fixing of the elongated preform results from stretching the first elongated preform in a manner so as to increase a material melting point above a temperature at which the stretching is performed.

In some embodiments, the fixing is accomplished by cooling the preform, e.g., by contacting, e.g., by submerging, the substantially non-crosslinked polymeric material with a fluid having a temperature below about 0°C, e.g., liquid nitrogen with a boiling point of about 77 K. This can allow for rapid cooling rates, especially of skin portions of the substantially non-crosslinked polymeric. In such cases, cooling rates can be, e.g., from about 50°C per minute to about 500°C per minute, e.g., from about 100°C to about 250°C per minute. Rapid cooling rates can result in more nucleation sites, smaller crystallites, and a material having a higher surface area. Cooling can also reduce crystallinity.

Any of the preforms described herein may include one or more antioxidants, such as Vitamin E and/or 2,6-di-fert-butyl-p-cresol, to reduce oxidation of the materials that form the preform during processing and/or use.

Any of the preforms described herein can include one or more molecules that include a permanent dipole moment so that any preform that includes such a molecule can be heated using microwave energy, e.g., during annealing or melting.

Referring now to FIG. 1, a cylindrical preform of diameter Di prior to elongation, after extrusion through a die of smaller diameter forms a cylindrical preform of diameter D2 and its new elongated length increases by a factor of (D2/D1) ² compared to its original length. Generally, the non-crosslinked polymeric material of preform of diameter D2 is less entangled in the longitudinal direction than is the non- crosslinked polymeric material of the preform of diameter Di.

Referring now to FIG. 2, a cylindrical preform of diameter Di and height Hi is uniaxially compressed above or below the melting temperature to elongate it in a radial direction and expand its diameter to D2 while decreasing its height to H2.

Generally, the compressed cylindrical preform of diameter D2 and height H2 is less entangled than is the polymeric material of diameter Di and height Hi. The cylindrical preform of diameter Di and height Hi can be either a preform of highly entangled polymer at its maximum or equilibrium entanglement density or close to it or it can be a preform of a polymer disentangled along its longitudinal direction by a method, such as the method described in Figure 1, including other methods such as uniaxial tension among others.

Referring now to FIG. 3 is a process to integrate the elongation process with fabrication of the polymeric preform from a polymeric resin powder. A cylindrical preform of diameter Di is ram extruded from the polymer powder in the melt state by using a plunger, which is then extruded through a die to form a rod of diameter D2. The die is described in FIG. 3 as being a conical contraction of the rod diameter Di into a rod of diameter D2 but can also be a sharp interface with a direct contraction to a smaller diameter D2. After a time of residence in the portion of the extruder with a diameter D2, the cylindrical preform can optionally be expanded in the radial direction either through a gradual conical expansion or a direct expansion radially to a cylindrical of diameter D3. A plunger may be used to apply a stress in an opposite direction to the flow of the preform to enable the melt to expand to a diameter D3. The plunger can be controlled by displacement or by a fixed or variable load. In some cases, the ratio of diameter D3 to D2 is equal to the square of the ratio of the diameter D2 to the diameter Di . The cylindrical rod of diameter D3 is then fixed by cooling at atmospheric pressure or applied pressure and optionally heated to the melt state to relax the polymer and decrease anisotropy. Not shown in the FIG 3 is a heating system to control the temperature of the process. This method will enable the rod to be stretched in both the longitudinal direction and radial direction sequentially.

Generally the non-crosslinked polymer of the cylindrical preform of diameter D3 is less entangled in both longitudinal direction and radial direction than the non- crosslinked polymeric material of the cylindrical preform of diameter Di.

Referring now to FIG. 4, a compression molded sheet of thickness Hi is plane strain compression of a molded sheet of a polymer of thickness Hi into a sheet of thickness H2 by mechanical rolling at an elevated temperature. The length of the rolled sheet will increase by a factor of H2/H1. Generally, the non-crosslinked polymer of the preform of height H2 is less entangled than the non-crosslinked polymer of the preform of height Hi . A sheet may be rolled along three orthogonal directions sequentially to disentangle the polymer along three orthogonal directions. Other plane strain compressions may be performed on the non-crosslinked preform, such as channel-die compression.

Referring now to FIG. 5 is a schematic of a cylindrical preform undergoing torsional deformation by rotation of each end of the rod in opposite directions with a net angular deformation of γ about the longitudinal axis passing through the center of the cylindrical preform. Generally, after torsional deformation, the non-crosslinked polymeric material of the cylindrical preform has less entangled chains compared to the undeformed cylindrical preform. Annealing of the preform after torsional deformation will relax the chains to form a cylindrical preform of disentangled, relaxed polymeric material. The torsional deformation can be conducted in a static mode or dynamic, oscillatory mode. In both cases, the polymeric chains will stretch without a substantial change in the external dimensions of the preform, which may be desirable in some cases.

Now referring to FIG. 6 is a schematic of a block undergoing shear deformation through a shear (angular) strain of γ, which may be conducted in oscillatory mode with fixed or variable amplitude or in a unidirectional mode at constant or variable rate to stretch the polymer chains and elongate the preform. The resulting preform will have a less entangled non-crosslinked polymeric material compared to the non-crosslinked polymeric material of the preform prior to shear deformation.

In some embodiments, there is strain-induced crystallization during elongation in one or more directions.

The polymeric preforms described herein can have their polymeric material elongated by one or more of the methods described.

In many of the several methods described herein, the fixing of an elongated preform is accomplished under conditions so as to prevent re-entanglement of polymer chains during the fixing.

In other embodiments, crosslinked, oxidation resistant polymeric materials (e.g., in a desired shape) that have a desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a non-crosslinked preform having a first dimension and including a substantially non-crosslinked polymeric material. The preform is elongated in a first direction at a first temperature that is below the glass transition temperature of the first crosslinked material or above the glass transition temperature of the first crosslinked polymeric material to provide a crosslinked, elongated preform having a second dimension larger than the first dimension and including a first disentangled, crosslinked polymeric material. The crosslinked, elongated preform is optionally maintained at an elevated temperature under a fixed or variable load or at a fixed or variable strain for a period of time. Then the load or strain or geometric constrained is removed and the preform cooled at atmospheric pressure or under an applied pressure to provide a fixed, uncrosslinked elongated preform that includes a non-crosslinked, disentangled polymeric material. The amorphous, disentangled polymer can optionally be stored at a temperature below room temperature prior to crosslinking.

In still other embodiments, crosslinked, oxidation resistant semicrystalline polymeric materials (e.g., in a desired shape) that have a desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a non-crosslinked preform, heating the preform to a temperature exceeding the melting temperature, such as 20°C above the melting temperature, cooling the non-crosslinked preform to a temperature less than 20°C below equilibrium melting temperature of the polymer, preferably between 15 and 20°C below its equilibrium melting temperature and maintaining the temperature in this temperature range. Generally, the polymer, for example polyethylene, will crystallize at a slow rate providing the time to disentangle the polymeric chains molecules to form crystals. It is also expected that the higher molecular weight chains will participate in the crystallization process selectively over lower molecular weight fractions, which can remain in the melt state. After a period of time between 60 seconds and several weeks, the polymer is then rapidly cooled or slow cooled to room temperature. The temperature range in which crystallization is performed can be adjusted so that the rate of crystallization is lower than the highest rate of crystallization of the polymer by at least a factor of 2, and preferably by a factor of 10 or more. The relaxed, non-crosslinked, disentangled preform is then crosslinked efficiently by ionizing radiation or with the assistance of chemical crosslinking agents, which remain substantially inactive during melting and crystallization but are then activated by external stimuli. For example, silane crosslinking agents are activated by water or humidity. Prior to crosslinking, the perform including the disentangled, non-crosslinked polymeric material can optionally be annealed at an elevated temperature below its melting temperature at atmospheric pressure or under an applied pressure exceeding 1 kPa, preferably in a pressure range of 1 kPa- 10 MPa, more preferably in a pressure range of 10- 1000 MPa without entering the melt-state.

In still other embodiments, crosslinked, oxidation resistant semicrystalline polymeric materials (e.g., in a desired shape) that have a desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a non-crosslinked preform, heating the preform to a temperature exceeding the melting temperature, such as 20°C above the melting temperature, cooling the non-crosslinked preform to a temperature at least 100°C to form a large number of nuclei in the polymer and rapid crystallization. The polymer is then heated to a temperature higher than the melting temperature but close to it, such as 2-5°C above melting temperature. Generally, the non-crosslinked polymer is expected to retain a large number of nuclei if it is not melted to a temperature substantially higher than its melting temperature. Then the polymer is maintained at a temperature less than 20°C below equilibrium melting temperature of the polymer, preferably between 15 and 20°C below its equilibrium melting temperature and maintaining the temperature in this temperature range. Generally, the polymer, for example polyethylene, will crystallize at a slow rate providing the time to disentangle the polymeric chains molecules to form crystals but the large number of nuclei present will make it crystallize faster than a polymer that is directly cooled to this temperature range. It is also expected that the higher molecular weight chains will participate in the crystallization process selectively over lower molecular weight fractions, which can remain in the melt state. After a period of time between 60 seconds and several weeks, the polymer is then rapidly cooled or slow cooled to room temperature. The temperature range in which crystallization is performed at low undercooling can be adjusted so that the rate of crystallization is fastest when the polymer is rapidly cooled and then is lower than the highest rate of crystallization of the polymer by at least a factor of 2, and preferably by a factor of 10 or more, when it is maintained at a low undercooling temperature. The relaxed, non-crosslinked, disentangled preform is then crosslinked efficiently by ionizing radiation or with the assistance of chemical crosslmking agents, which remain substantially inactive during melting and crystallization but are then activated by external stimuli. Prior to crosslmking, the perform including the disentangled, non-crosslinked polymeric material can optionally be annealed at an elevated temperature below its melting temperature at atmospheric pressure or under an applied pressure exceeding 1 kPa, preferably in a pressure range of 1 kPa- 10 MPa, more preferably in a pressure range of 10-1000 MPa without entering the melt-state.

In still other embodiments, crosslinked, oxidation resistant semicrystalline polymeric materials (e.g., in a desired shape) that have a desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a non-crosslinked preform and heating the non-crosslinked perform to a temperature exceeding the melting temperature. Then a hydrostatic pressure a range of 50-1000 MPa is applied to high-pressure crystallize the polymer for a period of time. Generally, high-pressure crystallization at an elevated temperature and elevated pressure can lead to the formation of extended- chain crystals and the formation of a disentangled, highly crystalline polymer with a crystallinity exceeding 50%, preferably exceeding 60% and most preferably exceeding 70%. The disentangled, highly crystalline polymer containing high- pressure crystals is then melted by lowering the pressure at constant temperature or simultaneously lowering the pressure and temperature as long as it enters the melt state to form a disentangled melt. After formation of a disentangled melt, the temperature is decreased under atmospheric pressure or under moderate pressures of 1 kPa-50 MPa to form a preform including a disentangled, semicrystalline polymer. It is expected that under moderate pressure, chain-folded crystals are more likely to form in large numbers without the presence of a substantial number of extended chain crystals. The preform is then crosslinked to provide a crosslinked preform that includes a crosslinked polymeric material. The initial heating of the polymer can alternatively be conducted under a fixed or variable applied pressure as long as it enters the melt state prior to application of high pressures. Similarly, the decrease of pressure at elevated temperature of the high-pressure crystallized, disentangled polymer to melt the crystals can be performed while simultaneously lowering the temperature as long as the polymer enters the melt state prior to cooling down to room temperature at moderate pressure of 1 kPa - 50 MPa. The high pressure crystallized non-crosslinked, disentangled polymeric perform is optionally, after high pressure crystallization, be cooled in such a way that the combination of temperature and pressure during cooling does not allow the polymer to melt. Then the pressure can be released at room temperature and followed by a separate step of re-melting the polymer at atmospheric pressure or a moderate applied pressure in a range of 1 kPa- 50 MPa to form a disentangled melt, which is then cooled rapidly to room

temperature by contact with a cold surface, slow cooled or held at a fixed or variable undercooling temperature, which is within 20°C of the equilibrium melting temperature of the polymer at the applied pressure and is then followed by rapid cooling or slow cooling to room temperature prior to the crosslinking step.

Alternatively, the disentangled melt can be rapidly crystallized by increasing the applied pressure at a constant temperature or while lowering the temperature.

Referring now to FIG 7, which is a schematic of the phase diagram of polyethylene with a y-axis as temperature and x-axis as pressure showing the melt region and solid-state regions where orthorhombic and, hexagonal and monoclinic crystals are present for any combination of temperature and pressure. The non- crosslinked preform may be at, for example at room temperature and 0.1 MPa pressure, which is atmospheric pressure. Then polymeric preform is then heated to the melting temperature in a range of 150-260°C. A pressure of 10- 1 OOOMPa is applied so that the combination of temperature and pressure, makes the polymer enter into the region of hexagonal crystals of the phase diagram. The temperature is then decreased to 150°C where the crystals become orthorhombic of monoclinic. Then the pressure is decreased to atmospheric pressure so that the polymer is in the melt state and all crystals are melted. The polymer is then cooled to room temperature at atmospheric pressure of at an applied pressure less than 50 MPa. This processing route can be adjusted so that the combination of pressure and temperature is increased to values where the polymer is in the melt state, then changed to values where the polymer crystallizes to form hexagonal crystals or orthorhombic crystals at a pressure exceeding 100 MPa. Then the pressure and temperature is lowered so that the combination of temperature and pressure enables the polymer to enter the melt state and completely melt the crystals prior to lowering temperature and pressure less than 50 MPa until the polymer is at room temperature and atmospheric pressure. This disentangled polymer is expected to have chain folded crystals and crystallinity greater than 50%. It is then effectively crosslmked by ionizing radiation or with the use of crosslinking agents.

Referring now to FIG 8, a noncrosslinked polymeric preform of polyethylene in the form of a resin, powder, flakes or consolidated form, is at a temperature Ti and applied pressure Pi and is then isothermally pressurized to a pressure P ₂ , then isobarically heated to a temperature T2 below its melting temperature and maintained in this state until the temperature of the preform is uniform. Then the preform is isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted. After a period of time, the preform is pressurized to a pressure P2 whereupon it crystallizes. The isothermal pressure changes to melt or solidify the preform may optionally be repeated several times. Then the preform is cooled isobarically to its original temperature Ti and then returned to the original pressure Pi. Ti and Pi can be considered to be room temperature and atmospheric respectively. The preform is then effectively crosslinked by ionizing radiation or with the use of crosslinking agents, providing a uniform, crosslinked preform with lowered entanglements and crystallinity exceeding 50%

Referring now to FIG. 9, a noncrosslinked polymeric preform of polyethylene in the form of a resin, powder, flakes or consolidated form, is at a temperature Ti and applied pressure Pi and is isothermally pressurized to a pressure P ₂ , then isobarically heated to a temperature T2 where it enters the hexagonal phase. After maintaining at this temperature until the temperature of the preform is uniform, the preform is then isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted. After a period of time, the preform is pressurized to a pressure P2 where it crystallizes via the hexagonal phase. The isothermal pressure changes to melt or solidify the preform may optionally be repeated several times. The preform is then is cooled isobarically to its original temperature Ti and finally returned to the original pressure Pi . The preform is then effectively crosslinked by ionizing radiation or with the use of crosslinking agents, providing a uniform, crosslinked preform with lowered entanglements and crystallinity exceeding 50% Referring now to FIG. 10, a noncrosslinked polymeric preform of polyethylene in the form of a resin, powder, flakes or consolidated form, is at a temperature Ti and applied pressure Pi and is isothermally pressurized to a pressure P2, then isobarically heated to a temperature T ₂ . After maintaining the preform at this temperature until the temperature is uniform, the preform is then isothermally reduced in pressure to a pressure P ₃ , whereupon the sample enters the melt state and becomes fully melted. Thereafter, the preform is pressurized to a pressure P ₄ and recrystallizes. The isothermal pressure changes to melt or solidify the preform may optionally be repeated several times. The preform is then cooled isobarically to its original temperature Ti and then isothermally returned to the original pressure Pi. The preform is then effectively crosslinked by ionizing radiation or with the use of crosslinking agents, providing a uniform, crosslinked preform with lowered entanglements and crystallinity exceeding 50% In some embodiments P ₄ < P2, in other embodiments, P ₄ > P2, and in yet other embodiments, P ₄ = P2.

Referring now to FIG. 11 , a noncrosslinked polymeric preform of polyethylene in the form of a resin, powder, flakes or consolidated form, is at a temperature Ti and applied pressure Pi and is isobarically heated to a temperature T ₂ , maintained at this temperature until the temperature is uniform, then isothermally pressurized to a pressure P ₂ , then isobarically heated to a temperature T3 and maintained at this temperature until the temperature in the preform is uniform. Then the preform is isothermally reduced in pressure to a pressure P ₃ , whereupon the sample enters the melt state and becomes fully melted. Thereafter, the preform is pressurized to a pressure P ₂ . The isothermal pressure changes to melt or solidify the preform may optionally be repeated several times. The preform is then then cooled isobarically to its original temperature Ti and then returned to the original pressure Pi). The preform is then effectively crosslinked by ionizing radiation or with the use of crosslinking agents, providing a uniform, crosslinked preform with lowered entanglements and crystallinity exceeding 50%. In some embodiments P ₄ < P2, in other embodiments, P ₄ > P2, and in yet other embodiments, P ₄ = P2.

Referring now to FIG. 12, a noncrosslinked polymeric preform of polyethylene in the form of a resin, powder, flakes or consolidated form, is at a temperature Ti and applied pressure Pi and is isobarically heated to a temperature T2 and maintained at this temperature for a period of time until the temperature in the preform is uniform. Then, the preform is isothermally pressurized to a pressure P ₂ , then isobarically heated to a temperature T3 and maintained at this temperature until the temperature of the preform is uniform. Then, the preform is isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted. Then in the melt state, the temperature of the sample is isobarically changed to a temperature T ₄ and maintained at this temperature and pressure for a period of time. In some embodiments, the temperature T3 is higher than T ₄ .

Thereafter, the preform is isothermally pressurized to a pressure P ₄ whereupon the sample crystallizes in the hexagonal phase. The preform may be isothermally pressurized and depressurized repeatedly between P3 and P ₄ . Then the preform is cooled isobarically to its original temperature Ti and then returned to the original pressure Pi. The preform is then effectively crosslinked by ionizing radiation or with the use of crosslinking agents, providing a uniform, crosslinked preform with lowered entanglements and crystallinity exceeding 50%. In some embodiments P ₄ < P2, in other embodiments, P ₄ > P2 as depicted in this schematic, and in yet other embodiments P ₄ =P2.

Referring now to FIG. 13, a noncrosslinked polymeric preform of polyethylene in the form of a resin, powder, flakes or consolidated form, is at a temperature Ti and applied pressure Pi and is isothermally pressurized to a pressure P2, then isobarically heated to a temperature T2 whereupon it enters the melt state for the first time. Then, after the preform is completely melted and has a uniform temperature, the preform is isobarically cooled to a temperature T3 during which time it crystallizes via the hexagonal phase and then enters the orthorhombic phase. Then, after uniform temperature has been attained, the polyethylene is isothermally reduced in pressure to a pressure P3, whereupon the sample re-enters the melt state. Then the sample is isothermally pressurized to a pressure P ₄ whereupon it crystallizes via the orthorhombic phase. The isothermal pressure changes to melt or solidify the preform may optionally be repeated several times. Thereafter, the preform is cooled isobarically to its original temperature TI and then returned to the original pressure

Pi. The preform is then effectively crosslinked by ionizing radiation or with the use of crosslinking agents, providing a uniform, crosslinked preform with lowered entanglements and crystallinity exceeding 50%. In some embodiments P ₄ < P2, in other embodiments, P ₄ > P2 as depicted in this schematic, and in yet other embodiments P ₄ =P2. In most cases, Ti and Pi can be considered to be room temperature and atmospheric respectively.

Referring now to FIG. 14, a noncrosslinked polymeric preform of polyethylene in the form of a resin, powder, flakes or consolidated form, is at a temperature Ti and applied pressure Pi and is heated and pressurized simultaneously may neither isobaric nor isothermal processes to a temperature T2 and a pressure P ₂ , and maintained in this state until a uniform temperature has been achieved. Then, the preform is isothermally reduced in pressure to a pressure P3, whereupon the sample enters the melt state and becomes fully melted. Thereafter, the preform is isothermally pressurized to a pressure P2 in order to recrystallize it under pressure. The isothermal pressure changes to melt or solidify the preform may optionally be repeated several times. The preform is then cooled to its original temperature Ti while simultaneously reduced in pressure to its original pressure Pi in a process that need not be isothermal or isobaric. The preform is then effectively crosslinked by ionizing radiation or with the use of crosslinking agents, providing a uniform, crosslinked preform with lowered entanglements and crystallinity exceeding 50%.

In other embodiments, aspects of the above processing methods may be combined by those skilled in the art so that the melting and solidification steps are peformed isothermally by decrease in pressure from an elevated pressure and then increase in pressure, respectively.

In still other embodiments, crosslinked, oxidation resistant polymeric materials that have a desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a non-crosslinked preform in the form of resin powder, sintered resin or consolidated block including a substantially non-crosslinked semicrystalline polymer, heating the preform to an elevated temperature below the melting temperature, maintaining the preform at the temperature until a uniform temperature is achieved in the preform, reducing the pressure until the polymer enters the melt state, maintaining the preform in the melt state for a fixed period of time, applying a pressure so that the polymer recrystallizes, then lowering the temperature and pressure in no specific order but in a way that the preform does not enter the melt state while lowering the temperature and pressure. The time that the preform spends in the melt state is preferably lower than the time required for the polymer to attain its equilibrium or maximum entanglement state so that the final preform is a disentangled, semicrystalline polymer. The polymer is then crosslinked to provide a crosslinked preform that includes a crosslinked polymeric material.

In still other embodiments, crosslinked, oxidation resistant polymeric materials (e.g., in a desired shape) that have a desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a substantially non-crosslinked, disentangled polymeric resin prepared using a catalyst with active sites far apart and optionally at a polymerization temperature lower than melting temperature in the case of a semicrystalline polymer or lower than the glass transition temperature in the case of a non-crystalline polymer so that the synthesized macromolecules do not entangle as they grow on the catalyst. The non-crosslinked, disentangled polymeric powder is then compression molded or ram extruded the powder to fuse it at a temperature exceeding the melt temperature for a period of time less than the time taken for the polymer melt to attain its equilibrium entanglement state. The temperature is then decreased under atmospheric pressure or at an applied pressure to form a preform including a disentangled, semicrystalline polymer. The polymer is then crosslinked to provide a crosslinked preform that includes a crosslinked polymeric material. The heating and cooling of the polymer to a temperature exceeding the melt temperature can be conducted under a fixed or variable applied pressure as long as it enters the melt state upon heating during the molding or extrusion process prior to cooling.

In still other embodiments, crosslinked, oxidation resistant semicrystalline polymeric materials (e.g., in a desired shape) that have a desirable mechanical properties, such as high wear resistance, high ductility and fatigue and crack propagation resistance, are made by selecting a non-crosslinked preform and heating the non-crosslinked perform to a temperature exceeding the melting temperature in a solvent, preferably a supercritical fluid, such as for example supercritical carbon dioxide or supercritical propane. The preform swollen in the solvent to form a gel or completely dissolved is then crystallized in the solvent. The solvent assisted crystallization enables the polymer to disentangle efficiently. The solvent is then removed at a temperature not exceeding the melting temperature with or without the assistance of vacuum. The polymer is then crosslinked by ionizing radiation or using crosslinking agents, which may be infused into the polymer by dissolution in the solvent. After solution or gel-crystallization, the preform including the disentangled, non-crosslinked polymeric material can optionally be annealed at an elevated temperature above or below its melting temperature at atmospheric pressure or under an applied pressure exceeding 1 kPa, preferably in a pressure range of 1 kPa- 10 MPa, more preferably in a pressure range of 10- 1000 MPa without entering the melt-state.

Polymeric Materials:

The substantially non-crosslinked polymeric material can be, e.g., a polyolefin, e.g., a polyethylene such as UHMWPE, a low density polyethylene (e.g., having a density of between about 0.92 and 0.93 g/cm ³ , as determined by ASTM D792), a linear low density polyethylene, a very-low density polyethylene, an ultra- low density polyethylene (e.g., having a density of between about 0.90 and 0.92 g/cm ³ , as determined by ASTM D792), a high density polyethylene (e.g., having a density of between about 0.95 and 0.97 g/cm ³ , as determined by ASTM D792), or a polypropylene, a polyester such as polyethylene terephthalate,

polytetrafluoroethylene, polyethylene oxide, polyurethanes, a polyamide such as nylon 6, 6/12, or 6/10, a polyethyleneimine, an elastomeric styrenic copolymer such as styrene-ethylene-butylene-styrene copolymer, or a copolymer of styrene and a diene such as butadiene or isoprene, a polyamide elastomer such as a polyether- polyamide copolymer, an ethylene-vinyl acetate copolymer, polydimethyl siloxane or compatible blends of any of these polymers. The non-crosslinked polymeric material can be a glassy polymer like polystyrene, polymethyl methacrylate and polycarbonate. The substantially non-crosslinked polymeric material can be processed in the melt into a desired shape, e.g., using a melt extruder, or an injection molding machine, or it can be pressure processed with or without heat, e.g., using compression molding or ram extrusion. The substantially non-crosslinked polymeric material can be purchased in various forms, e.g., as powder, flakes, particles, pellets, or other shapes such as rod (e.g., cylindrical rod). Powder, flakes, particles, or pellets can be shaped into a preform by extrusion, e.g., ram extrusion, melt extrusion, or by molding, e.g., injection or compression molding. Purchased shapes can be machined, cut, or other worked to provide the desired shape. Polyolefins are available, e.g., from Ticona, Montel, Sunoco, Exxon, and Dow; polyesters are available from BASF and Dupont; polytetrafluoroethylene and nylons are available from Dupont and Atofina, and elastomeric styrenic copolymers are available from the KRATON Polymers Group (formally available from Shell). If desired, the materials may be synthesized by known methods. For example, the polyolefins can be synthesized by employing Ziegler-Natta heterogeneous metal catalysts, or metallocene catalyst systems, and nylons can be prepared by condensation, e.g., using transesterification.

In some embodiments, it is desirable for the substantially non-crosslinked polymeric material to be substantially free of biologically leachable additives that could leach from an implant in a human body or that could interfere with the crosslinking of the substantially non-crosslinked polymeric material.

In particular embodiments, the polyolefin is UHMWPE. For the purposes of this disclosure, an ultrahigh molecular weight polyethylene is a material that consists essentially of substantially linear, non-branched polymeric chains consisting essentially of -CH2CH2- monomer or repeat units. The polyethylene has an average molecular weight in excess of about 500,000, e.g., greater than 1,000,000, 2,500,000, 5,000,000, or even greater than 7,500,000, as determined using a universal calibration curve. In such embodiments, the UHMWPE can have a degree of crystallinity of greater than 50 percent, e.g., greater than 51 percent, 52 percent, 53 percent, 54 percent, or even greater than 55 percent, and can have a melting point of greater than 135°C, e.g., greater than 136, 137, 138, 139 or even greater than 140°C. Degree of crystallinity of the UHMWPE is calculated by knowing the mass of the sample (in grams), the heat absorbed by the sample in melting ( H in J/g), and the heat of melting of polyethylene crystals ( Hf = 293 J/g). Once these quantities are known, degree of crystallinity is then calculated using the formula below:

Degree of Crystallinity = 8H/(sample weight)8Hf

For example, differential scanning calorimetry (DSC) can be used to measure the degree of crystallinity of the UHMWPE sample. To do so, the sample is weighed to a precision of about 0.1 milligrams, and then the sample is placed in an aluminum DSC sample pan. The pan holding the sample is then placed in a differential scanning calorimeter, e.g., a TA Instruments Q-1000 DSC, and the sample and reference are heated at a heating rate of about 10°C/minute from about -20°C to 180°C, cooled to about - 10°C, and then subjected to another heating cycle from about -20°C tol 80°C at 10°C/minute. Heat flow as a function of time and temperature is recorded during each cycle. Degree of crystallinity is determined by integrating the enthalpy peak from 20°C to 160°C, and then normalizing it with the enthalpy of melting of 100 percent crystalline polyethylene (291 J/g). Melting points can also be determined using DSC.

In some embodiments, the substantially non-crosslinked polymeric material is substantially amorphous.

In some embodiments, the substantially non-crosslinked polymeric material includes one or more antioxidants, such as any of the antioxidants described herein.

Crosslinking:

In some embodiments, the crosslinking occurs at a temperature from about - 25°C to above a melting point of the substantially non-crosslinked polymeric material, e.g., from about -10°C to about a melting point of the substantially non-crosslinked polymeric material, e.g., room temperature to about the melting point. Irradiating above a melting point of the substantially non-crosslinked polymeric material can, e.g., increase crosslink density.

In some embodiments, the crosslinking occurs at a temperature from about - 25°C to above the glass transition temperature of the substantially non-crosslinked polymeric material, e.g., from about -10°C to about a glass transition temperature of the substantially non-crosslinked polymeric material, e.g., room temperature to about the glass transition temperature. Irradiating above the glass transition temperature of the substantially non-crosslinked polymeric material can, e.g., increase crosslink density.

In some embodiments, the crosslinking occurs at a pressure, e.g., from about nominal atmospheric pressure to about 5000 atmospheres of pressure, e.g., from about nominal atmospheric pressure to about 5 atmospheres of pressure. Crosslinking above atmospheric pressure can, e.g., increase crosslink density. In some embodiments, the crosslinking of is performed at a temperature that substantially prevents re-entanglement of polymer chains.

In some embodiments, an ionizing radiation (e.g., an electron beam, x-ray radiation or gamma radiation) is employed to crosslink the substantially non- crosslinked polymeric material. In specific embodiments, gamma radiation is employed to crosslink the substantially non-crosslinked polymeric material.

In embodiments in which the irradiating is performed with electromagnetic radiation (e.g., as above), the electromagnetic radiation can have energy per photon of greater than 10 ² eV, e.g., greater than 10 ³ , 10 ⁴ , 10 ⁵ , 10 ⁶ , or even greater than 10 ⁷ eV. In some embodiments, the electromagnetic radiation has an energy per photon of between 10 ⁴ and 10 ⁷ , e.g., between 10 ⁵ and 10 ⁶ eV. The electromagnetic radiation can have a frequency of, e.g., greater than 10 ¹⁶ Hz, greater than 10 ¹⁷ Hz, 10 ¹⁸ , 10 ¹⁹ , 10 ²⁰ , or even greater than 10 ²¹ Hz. In some embodiments, the electromagnetic radiation has a frequency of between 10 ¹⁸ and 10 ²² Hz, e.g., between 10 ¹⁹ to 10 ²¹ Hz.

In some embodiments, a beam of electrons is used as the radiation source.

Electron beams can be generated, e.g., by electrostatic generators, cascade generators, transformer generators, low energy accelerators with a scanning system, low energy accelerators with a linear cathode, linear accelerators, and/or pulsed accelerators. Electrons as an ionizing radiation source can be useful to crosslink outer portions of the substantially non-crosslinked polymeric material, e.g., inwardly from an outer surface of less than 0.5 inch, e.g., less than 0.4 inch, 0.3 inch, 0.2 inch, or less than 0.1 inch. In some embodiments, the energy of each electron of the electron beam is from about 0.3 MeV to about 10.0 MeV (million electron volts), e.g., from about 0.5 MeV to about 3.0 MeV, or from about 0.7 MeV to about 1.50 MeV.

In some embodiments, the irradiating (with any radiation source) is performed until the sample receives a dose of at least 0.25 Mrad (2.5 kGy), e.g., at least 1.0 Mrad (10 kGy), at least 2.5 Mrad (25 kGy), at least 5.0 Mrad (50 kGy), or at least 10.0 Mrad (100 kGy). In some embodiments, the irradiating is performed until the sample receives a dose of between 1.0 Mrad and 6.0 Mrad, e.g., between 1.5 Mrad and 4.0 Mrad.

Prior to elongating, it can be desirable to slightly crosslink the polymeric material, e.g., by treating the polymeric material with a relatively low radiation (e.g., ionizing radiation) dose, such as less than about 50 kGy, less than about 35 KGy, less than about 25 kGy, less than about 15 kGy, less than about 10 kGy, less than about 7.5 kGy, less than about 5 kGy, less than about 4 kGy or less than about 3 kGy. Slightly crosslinking a polymeric material can impart shape memory into the polymeric material, which can help relieve stress imparted during elongation when the material is heated at or above a melting point of the polymeric material. In some

embodiments, a dose of between about 2.5 kGy to about 25 kGy is utilized, e.g., from about 4 kGy to about 20, or between about 5.0 and about 15 kGy.

In some embodiments, the irradiating is performed at a dose rate of between 5.0 and 1500.0 kilorads/hour, e.g., between 10.0 and 750.0 kilorads/hour, or between 50.0 and 350.0 kilor ads/hours. Low rates can generally maintain the temperature of the sample, while high dose rates can cause heating of the sample.

In some embodiments, radical sources, e.g., azo materials, e.g., monomeric azo compounds such as 2,2'-azobis(N-cyclohexyl-2-methylpropionamide) (I), or polymeric azo materials such as those schematically represented by (II) in which the linking chains include polyethylene glycol units (N is, e.g., from about 2 to about 50,000), and/or polysiloxane units, peroxides, e.g., benzoyl peroxide, or persulfates, e.g., ammonium persulfate (NH4)2S208, are employed to crosslink the substantially non-cro

(I) (II) Azo materials are available from Wako Chemicals USA, Inc. of Richmond, VA.

Generally, to crosslink the substantially non-crosslinked polymeric material, the material is mixed, e.g., powder or melt mixed, with the radical source, e.g., using a roll mill, e.g., a Banbury® mixer or an extruder, e.g., a twin-screw extruder with counter-rotating screws. An example of a Banbury® mixer is the F-Series Banbury® mixer, manufactured by Farrel. An example of a twin-screw extruder is the WP ZSK 50 MEGAcompounderTM, manufactured by Krupp Werner & Pfleiderer. Generally, the compounding or powder mixing is performed at the lowest possible temperature to prevent premature crosslinking. The sample is then formed into the desired shape, and further heated (optionally with application of pressure) to generate radicals in sufficient quantities to crosslink the sample.

The degree of crosslinking can be controlled by an initial concentration of the radical source. For example, mild crosslinking can be induced by using an initial concentration of the radical source of from about 0.01 percent by weight to about 1 percent by weight, e.g., 0.05 percent by weight to about 0.5 percent by weight. For example, heavy crosslinking can be induced by using an initial concentration of the radical source of from about 2 percent by weight to about 7.5 percent by weight, e.g., 2.5 percent by weight to about 5 percent by weight.

Measuring Crosslink Density and Molecular Weight Between Crosslinks: Crosslink density measurements are performed following the procedure outlined ASTM F2214-03. Briefly, rectangular pieces of the crosslinked UHMWPE are set in dental cement, and sliced into thin sections that are 2 mm thick. Small sections are cut out from these thin sections using a razor blade, giving test samples that are 2 mm thick by 2 mm wide by 2 mm high. A test sample is placed under a quartz probe of a dynamic mechanical analyzer (DMA), and an initial height of the sample is recorded. Then, the probe is immersed in o-xylene, heated to 130°C, and held at this temperature for 45 minutes. The UHMWPE sample is allowed to swell in the hot o-xylene until equilibrium is reached. The swell ratio qs for the sample is calculated using a ratio of a final height Hf to an initial height HO according to formula (1):

q _s = [Hf/Ho] ³ (1).

The crosslink density vd is calculated from qs, the Flory interaction parameter χ, and the molar volume of the solvent φΐ according to formula (2):

vd = lnd-qs ^'1 +q _s - ¹ + yqi ² (2), φι (q _s ^{" 1/3} - q _s -V2)

where χ is 0.33 + 0.55/qs, and φΐ is 136 cmVmol for UHMWPE in o-xylene at 130°C. Molecular weight between crosslinks M _c can be calculated from vd, and the specific volume of the polymer v according to formula (3):

Measurement of swelling, crosslink density and molecular weight between crosslinks is described in Muratoglu et al., Biomaterials, 20, 1463-1470 (1999).

Annealing:

Any material described herein (crosslinked or non-crosslinked) can annealed.

For example, a preform can be annealed below or above a melting point of a material of the preform.

For example, after crosslinking, a pressure of greater than 10 MPa is applied to the crosslinked polymeric material, while heating the crosslinked material below a melting point of the crosslinked polymeric material at the applied pressure for a sufficient time to substantially reduce the reactive species trapped within the crosslinked polymeric material matrix, e.g., free radicals, radical cations, or reactive multiple bonds. Quenching such species produces an oxidation resistant crosslinked polymeric material. The high pressures, and temperatures employed also increase the crystallinity of the crosslinked polymeric material, which can, e.g., improve resistance to fatigue crack propagation.

In some embodiments, the pressure applied is greater than 25 MPa, e.g., greater than 50 MPa, 75 MPa, 100 MPa, 150 MPa, 200 MPa, 250 MPa, 350 MPa, 500 MPa, 750 MPa, 1,000 MPa, or greater than 1,500 MPa. In some embodiments, the pressure is maintained for greater than 30 seconds, e.g., greater than 45 seconds, 60 seconds, 2.5 minutes, 5.0 minutes, 10 minutes, 20 minutes, 30 minutes, 60 minutes, greater than 90 minutes, or even greater than 120 minutes, before release of pressure back to nominal atmospheric pressure.

In some embodiments, prior to the application of any pressure above nominal atmospheric pressure, the crosslinked polymeric material is heated to a temperature that is between about 100°C below to about 1°C below, e.g., about 25°C below to about 0.5°C, below a melting point of the crosslinked polymeric material. This can enhance crystallinity of the crosslinked polymeric material prior to the application of any pressure.

In some embodiments, a pressure of above about 250 MPa is applied at a temperature of between about 100°C to about 1°C below a melting point of the crosslinked polymeric material at the applied pressure, and then the material is further heated above the temperature, but below a melting point of the crosslinked polymeric material at the applied pressure.

In some embodiments, the annealing includes heating a crosslinked polymeric material to a temperature that is about 25°C to about 0.5°C below a melting point of the crosslinked polymeric material, and then applying pressure above nominal atmospheric pressure.

Various other annealing methods are described by Bellare in USSN

11/359,845, filed February 21, 2006.

Annealing of any polymeric material described herein can be performed by applying microwave radiation to the material. In some embodiments, the polymeric material includes a microwave radiation-active material that aids in the heating of the polymeric material.

Manufacture of Preforms:

In particular embodiments, to make a crosslinked UHMWPE cylindrical preform that is resistant to oxidation, a substantially non-crosslinked cylindrical preform is obtained, e.g., by machining rod stock to a desired height and desired diameter. Preform can be made from a substantially non-crosslinked UHMWPE having a melting point of around 138°C, and a degree of crystallinity of about 52.0 percent. This crystallinity is either reduced, e.g., by heating the preform above the melting point of the UHMWPE, and then cooling, or the crystallinity is maintained, but not increased. Preform is then subjected to gamma radiation, e.g., 50 kGy (5 Mrad; 1 Mrad = 10 KGy) of gamma radiation, to crosslink the UHMWPE. After irradiation, the sample is press-fit into a pressure cell, and then the pressure cell is placed into a furnace assembly. Furnace assembly includes an insulated enclosure structure that defines an interior cavity. Insulated enclosure structure houses heating elements and the pressure cell, e.g., that is made stainless steel, and that is positioned between a stationary pedestal and a movable ram.

The crosslinked UHMWPE sample is first heated to a temperature Temp 1 below the melting point of the UHMWPE, e.g., 130°C, without the application of any pressure above nominal atmospheric pressure. After such heating, pressure P, e.g., 500 MPa of pressure, is applied to the sample, while maintaining the temperature Temp 1. Once pressurization has stabilized, the sample is further heated to a temperature Temp2, e.g., 160, 180, 200, 220, or 240°C, while maintaining the pressure P. As noted, pressure is applied along a single axis by movable ram. The preform may alternatively be encapsulated in a flexible barrier film or packaging and surrounded by a gas or liquid or a solid that melts at elevated temperatures, which can apply a hydrostatic pressure to the preform when the pressure is applied to the gas or liquid or melted solid along a single axis by a movable ram. Pressure at the given temperature Temp2 is generally applied for 10 minutes to 1 hour. During any heating, a gas such as an inert gas, e.g., nitrogen or argon, can be delivered to interior cavity of insulated enclosure structure through an inlet that is defined in a wall of the enclosure structure. The gas exits through an outlet that is defined in a wall of the enclosure structure, which maintains a pressure in the cavity of about nominal atmospheric pressure. After heating to Temp2 and maintaining the pressure P, the sample is allowed to cool to room temperature, while maintaining the pressure P, and then the pressure is finally released. The pressure cell is removed from furnace, and then the oxidation resistant UHMWPE is removed from pressure cell.

By starting with an UHMWPE having a melting point of around 138°C, and a degree of crystallinity of about 52.0 percent, and using a temperature of Temp2 of about 240°C, and a pressure P of about 500 MPa, one can obtain an oxidation resistant crosslinked UHMWPE that has a melting point greater than about 141°C, e.g., greater than 142, 143, 144, 145, or even greater than 146 °C, and a degree of crystallinity of greater than about 52 percent, e.g., greater than 53, 54, 55, 56, 57, 58, 59, 60, 65, or even greater than 68 percent. In some embodiments, the crosslinked UHMWPE has a crosslink density of greater than about 100 mol/m3, e.g., greater than 200, 300, 400, 500, 750, or even greater than 1,000 mol/m3, and/or a molecular weight between crosslinks of less than about 9,000 g/mol, e.g., less than 8,000, 7,000, 6,000, 5,000, or even less than about 3,000 g/mol.

Quenching Materials:

A "quenching material" refers to a mixture of gases and/or liquids (at room temperature) that contain gas and/or liquid component(s) that can react with residual free radicals and/or radial cations to assist in the recombination of the residual free radicals and/or radical cations. The gases can be, e.g., acetylene, chloro-trifluoro ethylene (CTFE), ethylene, or other unsaturated compounds. The gases or the mixtures of gases may also contain noble gases such as nitrogen, argon, neon, and the like. Other gases such as carbon dioxide or carbon monoxide may also be present in the mixture. In applications where the surface of a treated material is machined away during the device manufacture, the gas blend could also contain oxidizing gases such as oxygen. The quenching material can be one or more dienes, e.g., each with a different number of carbons, or mixtures of liquids and/or gases thereof. An example of a quenching liquid is octadiene or other dienes, which can be mixed with other quenching liquids and/or non-quenching liquids, such as a hexane or a heptane.

Quenching material can be applied to any polymeric material utilized in any step described herein.

Antioxidants and Stabilizers:

Generally, because many of the materials will be used in medical devices, some even for permanent implantation, useful antioxidants are typically either Generally Recognized as Safe direct food additives (GRAS) in Section 21 of the Code of Federal Regulations or are EAFUS-listed, i.e., included on the Food and Drug

Administration's list of "everything added to food in the United States." Other useful antioxidants can also be those that could be so listed, or those that are classified as suitable for direct or indirect food contact. Examples of antioxidants which can be used in any of the methods described herein include alpha- and delta-tocopherol; propyl, octyl, or dodecyl gallates; lactic, citric, and tartaric acids and salts thereof; as well as orthophosphates and sulfur based antioxidants. In some instances, a preferable antioxidant is vitamin E. Still other antioxidants are available form Eastman under the trade name TENOX. For example other antioxidants include tertiary-butyl hydroquinone (TBHQ), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), or mixtures of any of these or the prior-mentioned antioxidants. Antioxidants include Grape seed extracts, procyanidolic oligomers, ferulic acid, 4,4' Butylidene-bis(6-tert-butyl-m-cresol, N-acetyl cysteinamide, 1- ergothioneine, l,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)s-triazine -2,4,6- (lH,3H,5H)-trione, 4,4'-methylene-bis(2,6-di-tert-butylphenol), l, l,3-tris(2-methyl-4- hydroxy-5-tert-butylphenyl)butane, 4,4'-isopropylidene-diphenol, 2,6-di-tert-butyl-4- ethylphenol, 2,6-di-tert-butyl-p-cresol, 1 ,3,5-trimethyl-2,4,6-tris(3,5-di-di-tert-butyl- 4-hydroxybenzyl)benzene, 1 , 1 ,3-tris(2-methyl-4-hydroxy-5-tert-butyl-phenyl)butane, 4,4-thio-bis(2-tert-butyl-m-cresol), tetrakis {methylene-3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate} methane, l,3,5-(4-tert-butyl-3-hydroxy-2,6- dimethylbenzyl)- l,3,5-triazine-2,4,6-(l- H,3H,5G)-trione, bis-[3,3-bis-(4'-hydroxy-3'- tert-butyl-phenyl-butanoic acid]-glycol ester, 4,4'-thio-bis(6-tert-butyl-m-cresol), 2,2'- methylene-bis(4-methyl-6-tert-butylphenol), 4,4'-butylidene-bis(2-tert-butyl-m- cresol), 2,6-di-tert-butyl-4-sec-butylphenol, 1 ,6-hexamethylene-bis(3,5-di-tert-butyl- 4-hydroxyhydrocinnamate), 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid trimester, 2,2'-methylene-bis(4-ethyl-6-tert-butylphenol), l,3,5-tris(3,5-di-tert-butyl-4- hydroxybenzyl) isocyanurate, octadecyl-3-(3'5-di-tert-butyl-4- hydroxyphenyl)propionate, l,3,5-tris(2-hydroxyethyl)-s-triazine-2,4,6- (lH,3H,5H)trione, ,2-Bis (3, 5-di-t-butyl-4-hydroxyhydrocinnamoyl) hydrazine, 4,4'- Bis ( -dimethylbenzyl) diphenylamine, 3,5-Di-t-butyl-4- hydroxyhydrocinnamic acid, 1,3,5-tris (2-hydroxyethyl)-s-triazine-2,4,6-(lH,3H,5H)- trione trimester, Dilauryl thiodipropionate, Distearyl pentaerythrityl diphosphite, Distearyl thiodipropionate, Ν,Ν'-Hexamethylene bis (3,5-di-t-butyl-4- hydroxyhydrocinnamamide), Irganox(® 1098; Lankromark ® LE 109; Naugard® 445; Nickel dibutyldithiocarbamate; Nickel dimethyldithiocarbamate; Pentaerythityl tetrakis [3-(3',5'-di-t-butyl-4-hydroxyphenyl) propionate]; Phenol, styrenated;

Tetramethylthiuram disulfide; Tris (2,4-di-t-butylphenyl) phosphate; 1, 1,3-Tris (2- methyl-4-hydroxy-5-t-butylphenyl) butane; Tris (nonylphenyl) phosphate; Vanlube® PCX, Ethoxyquin, l,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline, Rosemary extract, carnosol, rosmanol, and epirosmanol, phenolic diterpenes or mixtures thereof. A UV light stabilizer may also be used The UV light absorber may be thermally stable up to a temperature of about 300° C. Suitable stabilizers may be Hostavin, Chimassorb 2020, Chimassorb 944, Tinuvin 622, Tinuvin 791, SONGNOX 1010 and Tinuvin 770 available from Clariant Corporation, Charlotte, NC. The UV absorber may comprise one or more benzophenone compounds such as 2-hydroxy-4- methoxybenzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4- octoxybenzophenone, 2-hydroxy-4-benzyloxybenzophenone, 2-hydroxy-4-methoxy- 5-sulfoxytrihydride benzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2- hydroxy-4-methoxy-5-sulfoxybenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxy-5- sodiumsulfoxybenzophenone, bis(5-benzoyl-4-hydroxy-2-methoxyphenyl)methane, 2-hydroxy-4-n-dodecyloxybenzophenone or 2-hydroxy-4-methoxy-2'- carboxybenzophenone. The UV absorber may comprise benzotriazole compounds such as 2-(2-hydroxy-5-tert-octylphenyl)benzotriazole, 2-(2-hydroxy-5- methylphenyl)benzotriazole, , 2,2'-methylenebis[4-(l, l,3,3-tetramethylbutyl)-6-(2N- benzotriazol-2-yl)phenol], 2-(2-hydroxy-3,5-dicumylphenyl) phenylbenzotriazole, 2- (2-hydroxy-3-tert-butyl-5-methylphenyl)-5-chlorobenzotriazol e2-(2-hydroxy-3,5-di- tert-butylphenyl)benzotriazole, 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5- chlorobenzotriazole, 2-(2-hydroxy-5-tert-octylphenyl)benzotriazole, 2-(2-hydroxy-5- tert-butylphenyl)benzotriazole, 2-(2-hydroxy-3,5-di-tert-amylphenyl)benzotriazole, 2- (2-hydroxy-4-octoxyphenyl)benzotriazole, 2,2'-methylenebis(4-cumyl-6- benzotriazolephenyl), 2-[2-hydroxy-3-(3,4,5,6-tetrahydrophthalimidomethyl)-5- methylphenyljbenzo- triazole, 2,2'-p-phenylenebis(l,3-benzooxazin-4-one) or a combination thereof. The concentration of the UV light absorber in the polymer composition may be in the range from about 0.01 to about 1% by weight based on the total weight of the polymer.

In some embodiments, the antioxidant has a melting point above about 50°C, e.g., above about 100°C, above about 150°C, or above about 175°C. Since in the solid state the antioxidants can have less mobility in a polymeric matrix, controlling the melting point of the antioxidant can be a way of controlling the activity of the antioxidant. Microwave Radiation-Active Materials

Any of the polymeric materials described herein can include microwave radiation-active materials, which can aid in the heating of the polymeric materials in any step described herein. Generally, microwave radiation-active materials are those that include a permanent dipole. For example, the microwave radiation-active materials can be inorganic materials, such as ceramics (e.g., carbides, borides, nitrides), metals and metal alloys, quantum dots, or organic materials, such as edible oils or solids, e.g., sunflower oils, corn oils, wheat germ oils, vitamin E, fatty acids, or mixtures of any of these.

Applications

The oxidation resistant crosslinked polymeric materials can be used in any application for which oxidation resistance, long-term stability, high wear resistance, low coefficient of friction, chemical/biological resistance, fatigue and crack propagation resistance, and/or enhanced creep resistance are desirable. For example, the oxidation resistant crosslinked polymeric materials are well suited for medical devices. For example, the oxidation resistant crosslinked polymeric material can be used as an acetabular liner, a finger joint component, an ankle joint component, an elbow joint component, a wrist joint component, a toe joint component, a hip replacement component, a tibial knee insert, an intervertebral disc, a heart valve, breast implant, a stent, medical tubing or part of a vascular graft.

In particular embodiments, the oxidation resistant crosslinked polymeric material is used as a liner in hip replacement prostheses, tibial inserts in knee replacement prostheses, glenoid components of total shoulder replacements or components of other joint replacements, such as ankle replacement or elbow replacement. The oxidation crosslinked polymeric material can also be used in other medical devices, such as for example, intervertebral disc replacement or nucleus pulposus replacement.

EXAMPLES

The invention is further described in the following examples, which do not limit the scope of the invention described in the claims. EXAMPLE 1

Compression molded sheets (MediTech Medical Polymers, Fort Wayne, IN) of GUR 1020 (Ticona, Oberhausen, Germany) containing 0.1% Vitamin E served as control PE. Cylinders of 25.4 mm diameter and 31.2 mm height were machined from the sheets and compressed to a strain of 4.75 (ratio of final diameter to initial diameter of cylinder) at 160°C in a Carver hydraulic press and maintained for 16 hours to allow the macromolecules to disentangle. Thereafter, the load was released and the resulting discs were allowed to relax prior to cooling to enable the chains to relax and lose their preferred orientation. These samples (C-XPE) and a set of sheets of 2 mm thickness, which were heated to 160°C and cooled without any deformation (XPE) were irradiated to a dose of 100 kGy using electron beam radiation at room temperature. Equilibrium swelling experiments were performed on pre-weighed cubic samples [n=6] of approximately 20 mg weight by immersing into xylene maintained at 135°C using a silicone oil bath for a period of 3 hours. The solvent swollen samples were sealed into pre-weighed glass vials and re-weighed. Swell ratio (qeq) crosslink density (v) and molecular weight between crosslinks (Mc) were calculated using equations 1 , 2 and 3 as shown below:

q _e q = Volume of absorbed xylene + Initial volume of sample (1)

Initial volume of sample

v (mol/cm ³ ) = _ lnd-qea ^"1 ) + Pea ^"1 + Xqea ^"2 (2)

(2)

Viqeq- ¹ ' ³

where Vi = 136 cmVmol , X = 0.33 + 0.55/q _eq [2] and v _d = 920 g/dm ³ [8] Equilibrium swelling experiments showed that XPE had a 7.7% lower swell ratio, 18% higher effective crosslink density and 15% lower molecular weight between crosslinks compared to C-XPE, as shown in Table 1 (ANOVA, Fisher's protected least significant difference post-hoc test, p<0.05) even though both C-XPE and XPE were irradiated to identical radiation dose under identical conditions, which can be attributed to lower entanglements in C-XPE, which underwent stretching and relaxation in the melt state prior to irradiation. TABLE 1. Swell ratio, Crosslink density and Molecular weight between crosslinks for various ultra-high molecular weight polyethylenes.

EXAMPLE 2

Compression molded sheets (MediTech Medical Polymers, Fort Wayne, IN) of GUR 1020 (Ticona, Oberhausen, Germany) containing 0.1% Vitamin E served as control PE. A second set of sheets were sectioned into 2 mm thick sections and annealed in a convection oven at a constant temperature of 128°C for a period of 48 hours to perform slow crystallization at low undercooling to slowly disentangle the melt as the crystals form over a period of 48 hours. ASTM D638 standard tensile tests were conducted on Type V specimens punched from sheets of each group (n=6) using an AD MET universal tensile tester operating at a crosshead speed of 1 Omm/min. The ultimate tensile stress of control polyethylene was 43.9 ± 5.1 MPa while that of the 128°C crystallized polyethylene was 60.9 ± 6.2 MPa (p<0.05, ANOVA). The maximum strain or ductility of control polyethylene was 8.2 ± 0.7 while that of the 128°C crystallized polyethylene was 13.0 ± 1.9 (p<0.05, ANOVA). The higher ductility, higher ultimate stress polyethylene is then irradiated to form a crosslmked polyethylene with a high ductility compared to untreated crosslmked polyethylene.

EXAMPLE 3

Compression molded sheets (MediTech Medical Polymers, Fort Wayne, IN) of GUR 1020 (Ticona, Oberhausen, Germany) containing 0.1% Vitamin E served as control PE. Cylinders of 3" height and ½" diameter were machined from the molded sheets and place snugly into a custom built mold, subjected to a pressure of 400 MPa and then heated to a temperature of 200°C. After 1 hour, the cylinders were cooled to a temperature of 155°C and then the pressure was decreased isothermally to atmospheric pressure for a period of 10 minutes. Thereafter, the pressure was isothermally increased to 400 MPa. The temperature was then isobarically decreased to room temperature and then the pressure released. Five cylinders and a portion of the untreated controls were subjected to 9 Mrad dose of gamma radiation to crosslink them. The samples were then machined into rectangular bars, double notched and subjected to Izod impact testing. After impact testing the fracture surfaces were examined and samples that showed any surface defects were not included in further analysis. The untreated, unirradiated control had an impact strength of 145.4 ± 2.6 kJ/m ² (mean ± standard deviation). The untreated radiation crosslinked control had an impact strength of 84.8 ± 2.6 kJ/m ² , which was nearly a 42% decrease in impact strength. The impact strength of the pressure treated, crosslinked PE was 97.2 ± 3.7 kJ/m ² , which was 33% lower than that of the uncrosslinked, untreated control but 14.6% higher than that of the untreated, crosslinked PE. Statistical analysis showed that all results were significantly different from each other.

This application claims the benefit of priority of USSN 61/918,924, filed December 20, 2014, which is incorporated herein by reference in its entirety.

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.