Abstract
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 Tm, 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.
Highlights
The results of this study have demonstrated that highly crystalline ultra-high molar mass polyethylene (UHMMPE) fiber is very sensitive
The results of this study have demonstrated that highly crystalline UHMMPE fiber is very to irradiation
The primary effects of irradiating the fiber is a reduction in tensile strength
Summary
IntroductionTo improve properties such as thermal conductivity [1], low temperature thermal strain [2], and creep resistance of ultra-high molar polyethylene (UHMMPE)fibers,thermal many strain researchersTo improve improve properties such such as mass thermal conductivity [1],[1], low temperature temperature thermal strain [2], and andhaveTo properties as thermal conductivity low [2], creep resistance resistance ofcrosslink ultra-highthese molarfibers mass using polyethylene fibers, manyto researchers have undertaken efforts to radiation [1,3,4,5,6,7,8,9,10,11].fibers, Exposure ionizing radiation creep of ultra-high molar mass polyethylene many researchers have undertaken efforts to crosslink crosslink these fibersthe using radiationand [1,3,4,5,6,7,8,9,10,11].Exposure to ionizing ionizing radiation produces alkyl free radicals (Figurethese1) in both crystalline amorphous regions of the radiation polyethylene, undertaken efforts to fibers using radiationExposure to produces alkyl free free radicals (Figure1) in in both both theincrystalline crystalline and amorphous amorphous regions of the the are produces alkyl radicals 1). To improve properties such as thermal conductivity [1], low temperature thermal strain [2], and creep resistance of ultra-high molar polyethylene (UHMMPE). To properties as thermal conductivity low [2], creep resistance resistance ofcrosslink ultra-highthese molarfibers mass using polyethylene fibers, manyto researchers have undertaken efforts to radiation [1,3,4,5,6,7,8,9,10,11].fibers, Exposure ionizing radiation creep of ultra-high molar mass polyethylene many researchers have undertaken efforts to crosslink crosslink these fibersthe using radiationand [1,3,4,5,6,7,8,9,10,11].
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