The present invention discloses methods for enhancing the wear-resistance of polymers, the resulting polymers, and in vivo implants made from such polymers. One aspect of this invention presents a method whereby a polymer is irradiated, preferably with gamma radiation, then thermally treated, such as by remelting of annealing. The resulting polymeric composition preferably has its most oxidized surface layer removed. Another aspect of the invention presents a general method for optimizing the wear resistance and desirable physical and/or chemical properties of a polymer by crosslinking and thermally treating it. The resulting polymeric compositions is wear-resistant and may be fabricated into an in vivo implant.

Crosslinking of polyethylene for low wear using radiation and thermal treatments

A condensed description is given of the fundamental processes involved in radiation damage and the effects of radiation on the physical and chemical properties of organic materials, particularly polymers It is shown that the radiation doses received by specimens in the electron microscope are extremely high, very much greater than those used in radiation chemistry experiments Because of this, only qualitative predictions of behaviour in the electron microscope can be made A number of authors have described the changes in the image or diffraction pattern of particular specimen types during observation in the electron microscope and their work is reviewed here In general, contrast features in the image may disappear, due to loss of mass or crystallinity, or new features may appear due to distortion of ordered regions The effects of radiation damage on attainable resolution, and possible methods of improving the resolution are then discussed

Radiation damage and electron microscopy of organic polymers

A condensed description is given of the fundamental processes involved in radiation damage and the effects of radiation on the physical and chemical properties of organic materials, particularly polymers. It is shown that the radiation doses received by specimens in the electron microscope are extremely high, very much greater than those used in radiation chemistry experiments. Because of this, only qualitative predictions of behaviour in the electron microscope can be made. A number of authors have described the changes in the image or diffraction pattern of particular specimen types during observation in the electron microscope and their work is reviewed here. In general, contrast features in the image may disappear, due to loss of mass or crystallinity, or new features may appear due to distortion of ordered regions. The effects of radiation damage on attainable resolution, and possible methods of improving the resolution are then discussed. I. Introduction Specimens in the electron microscope are inevitably exposed to irradiation by electrons. A certain fraction of these electrons will interact with orbital electrons in the specimen and transfer energy to them. At normal accelerating voltages the excited states formed in metals and most other inorganic substances dissipate their energy as heat, and no permanent change occurs. In some inorganic salts and all organic substances, however, some fraction of the excited states decay to altered structures, giving permanent changes in the chemical and physical properties of the specimen. In general, some fraction of the mass is lost, the chemical composition changes and any long range order originally present is destroyed. When organic material provides the image contrast, the appearance of the specimen changes considerably during observation in the electron microscope, because of radiation damage. The impermanence of the original appearance limits the possible resolution. After considerable irradiation - which may only take a short time in the microscope - the specimen stabilizes in a new form, resistant to further radiation. All of the initial structure of interest may have been

http://rd.springer.com/content/pdf/10.1007%2FBF00540772.pdf

Review Radiation damage and electron microscopy of organic polymers

The effect of high-energy radiation on n-alkanes and polyethylene (PE) are reviewed. The article includes a description of (a) primary radiation events, formation, spatial distribution and decay of free radicals and radical migration mechanisms, (b) formation of cross-links and other stable products, (c) the effects of crystallinity, crystalline modifications and morphology on radiation cross-linking, and (d) radiation-induced changes in the crystal lattice and the destruction of crystallinity at high irradiation doses. An attempt is made to bring together conflicting views on various unsolved problems of the radiation physics and chemistry of these important and much studied substances. Long-chain paraffins are given considerable attention since their radiation behaviour often provides a link between that of PE and its short-chain analogues.

Radiation effects in polyethylene and n-alkanes

Effect of radiation on the crystals of polyethylene and paraffins: 1. Formation of the hexagonal lattice and the destruction of crystallinity in polyethylene

Samples of Marlex 50 crystallized from dilute solution (yielding single crystals) and in the bulk form were irradiated by high-energy electrons and the solubilities compared. The single-crystal specimens remained fully soluble after a dose of 20 megarads, which rendered the bulk two-thirds to three-quarters insoluble. Since the initial free-radical concentration was identical and the chemical changes induced were not widely different in the two kinds of specimens, it is suggested that intramolecular crosslinks, occurring in a larger number in the single crystals, are responsible for this behavior. At high doses the same solubility limit is approached by both specimens, which is ascribed to the possibility of chain cutting becoming increasingly ineffective in the single crystal. In the light of these findings, the limitations of the conventional treatment of radiation effects are discussed and the possible significance of the present results for morphological problems is pointed out. A number of further effects are reported in connection with melting behavior and spin resonance spectra that are at variance with results in the literature.

A. Keller

Papers

The influence of crystallization conditions on radiation effects in polyethylene — I. Crystallization from dilute solution and from the melt