Crystallinity and the effect of ionizing radiation in polyethylene. II. Crosslinking in chain-folded single crystals

Effect of Irradiation and Detection of Long-Lived Polyenyl Radicals in Highly Crystalline Ultra-High Molar Mass Polyethylene (UHMMPE) Fibers

To improve properties such as thermal conductivity, low temperature thermal strain, and creep resistance of ultra-high molar mass polyethylene (UHMMPE) fibers, several researchers have previously undertaken efforts to crosslink these fibers using radiation. Ionizing radiation is commonly used to crosslink bulk UHMMPE in other applications, such as artificial joints. However, UHMMPE fibers differ from bulk UHMMPE in that they have a higher crystallinity (approximately 85% to 90%) and are very highly oriented during manufacturing in which the fibers are stretched 50 to 100 times their original length. Thus, the amorphous fraction of the UHMMPE fibers is also highly ordered. Several experiments were conducted to crosslink the UHMMPE fibers using both low dose rate (gamma) and high dose rate (electron beam) irradiation, all in the absence of oxygen. In all cases, the tensile strength of the fiber was greatly reduced by the irradiation. The oxidation index was also measured for the irradiated samples, and oxidation was not found to play a major role in the reduction of tensile strength in the fibers after irradiation. While this work did not achieve the desired result of improving the mechanical properties of the UHMMPE fiber, a significant result was found. The electron paramagnetic resonance (EPR) spectrum of the UHMMPE fibers was measured shortly after irradiation, and a mixture of allyl and alkyl radicals were detected. The irradiated samples were stored in dark ambient conditions for at least six years, then reexamined using EPR for free radical characterization. Surprisingly, the gamma-irradiated samples showed clear evidence of long-lived polyenyl radicals present in the material. Free radicals are very reactive species that will typically migrate to the surface of the crystalline domain and decay in a relatively short time through various reactions in the amorphous regions. It is hypothesized herein that due to the high crystallinity and large anisotropy of the highly drawn UHMMPE fiber, the polyenyl radicals were trapped in the crystal phase and were unable to migrate and decay. An experiment was performed to test this hypothesis, by which samples of the irradiated fibers were heated to temperatures above first the alpha relaxation and then melting point of polyethylene, and EPR measurements were taken. Results showed that the polyenyl radical signal persisted below the T_m, but was rapidly eliminated upon melting of the crystals. These experiments support the hypothesis that the long-lived polyenyl radicals are trapped in the crystalline region of the polyethylene fibers.

Stroboscopic image capture: Reducing the dose per frame by a factor of 30 does not prevent beam-induced specimen movement in paraffin

Beam-induced specimen movement may be the major factor that limits the quality of high-resolution images of organic specimens. One of the possible measures to improve the situation that was proposed by Henderson and Glaeser [Ultramicroscopy 16 (1985) 139-150], which we refer to here as "stroboscopic image capture", is to divide the normal exposure into many successive frames, thus reducing the amount of electron exposure--and possibly the amount of beam-induced movement--per frame. The frames would then be aligned and summed. We have performed preliminary experiments on stroboscopic imaging using a 200-kV electron microscope that was equipped with a high dynamic range Charge-coupled device (CCD) camera for image recording and a liquid N2-cooled cryoholder. Single-layer paraffin crystals on carbon film were used as a test specimen. The ratio F(g)/F(0) of paraffin reflections, calculated from the images, serves as our criterion for the image quality. In the series that were evaluated, no significant improvement of the F(image)(g)/F(image)(0) ratio was found, even though the electron exposure per frame was reduced by a factor of 30. A frame-to-frame analysis of image distortions showed that considerable beam-induced movement had still occurred during each frame. In addition, the paraffin crystal lattice was observed to move relative to the supporting carbon film, a fact that cannot be explained as being an electron-optical effect caused by specimen charging. We conclude that a significant further reduction of the dose per frame (than was possible with this CCD detector) will be needed in order to test whether the frame-to-frame changes ultimately become small enough for stroboscopic image capture to show its potential.

Properties of crosslinked ultra-high-molecular-weight polyethylene

Substantially reducing the rate of generation of wear particles at the surfaces of ultra-high-molecular-weight polyethylene (UHMWPE) orthopedic implant bearing components, in vivo, is widely regarded as one of the most formidable challenges in modern arthroplasty. In the light of this, much research attention has been paid to the myriad of endogenous and exogenous factors that have been postulated to affect this wear rate, one such factor being the polymer itself. In recent years, there has been a resurgence of interest in crosslinking the polymer as a way of improving its properties that are considered relevant to its use for fabricating bearing components. Such properties include wear resistance, fatigue life, and fatigue crack propagation rate. Although a large volume of literature exists on the topic on the impact of crosslinking on the properties of UHMWPE, no critical appraisal of this literature has been published. This is one of the goals of the present article, which emphasizes three aspects. The first is the trade-off between improvement in wear resistance and depreciation in other mechanical and physical properties. The second aspect is the presentation of a method of estimating the optimal value of a crosslinking process variable (such as dose in radiation-induced crosslinking) that takes into account this trade-off. The third aspect is the description of a collection of under- and unexplored research areas in the field of crosslinked UHMWPE, such as the role of starting resin on the properties of the crosslinked polymer, and the in vitro evaluation of the wear rate of crosslinked tibial inserts and other bearing components that, in vivo, are subjected to nearly unidirectional motion.

Influence of sterilization by gamma irradiation and of thermal annealing on creep of hydroxyapatite-reinforced polyethylene composites

Sterilization of medical devices by gamma (gamma)-irradiation is common. The effect of irradiation on a bone replacement material, HAPEX (hydroxyapatite-reinforced polyethylene composite), was investigated. Unfilled and hydroxyapatite-filled polyethylene at 0.20 and 0.40 filler volume fractions were gamma-irradiated at 2.5 Mrad, and the modified properties were studied by differential scanning calorimetery, isochronous experiments, and creep tests. The effect of thermal annealing of the samples from 140 degrees C also was examined. The results suggest that both irradiation and annealing increase creep resistance of the materials. These are associated with the formation of crosslinks and an increase in crystallinity, respectively.

The "quasi-thermal" mechanism for electron beam damage of n-paraffins

Electron diffraction patterns from epitaxially grown microcrystals of n-hexatriacontane, which are slightly damaged by the electron beam, strongly resemble those from the same material when it is warmed just below the pre-melt hexagonal phase. The identity of these diffraction patterns, which display a marked attenuation of lamellar 001 reflections but much less alteration of the strongest reflections, implies that both processes occur via the induction of chain defects which, in turn, generate chain-end voids in the crystal packing. Such defects, however, need not be identical for the two events. With warming they are probably the gtg-1 kink and the gauche chain conformers identified by infrared spectroscopy. The creation of trans vinylene groups during radiation damage will also increase chain flexibility, and perhaps induce the production of gtg-1 kinks.