MSE Seminar: Long-lived polyenyl radicals in irradiated highly crystalline UHMMPE fibers
Friday, February 22, 2019
2110 Chem/Nuc (bldg #90), UMD College Park
Speaker: Amanda L. Forster, Materials Research Engineer, NIST
Title: Long-lived polyenyl radicals in irradiated highly crystalline UHMMPE fibers
To improve properties such as thermal conductivity, low temperature thermal strain, and creep resistance of UHMMPE fibers, researchers have 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 and are very highly oriented during manufacture through a process known as "superdrawing," 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. Experiments have been 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 to minimize the probability of oxidation. In all cases, the tensile strength was greatly reduced by the irradiation. 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. Chain scission is highly catastrophic in a high molecular weight system, and available crosslink sites are saturated at low dose due to the low amorphous content of these fibers. After the crosslink sites are saturated, chain scission becomes dominant in the fiber, greatly reducing its mechanical properties, likely due to preferential scission of the “taut tie molecules” that connect crystalline regions in the fiber. While this work did not achieve the desired result of improving the mechanical properties of the UHMMPE fiber, a surprising result was found. Several authors have previously examined the effect of irradiation on the physical properties of UHMMPE fibers, however no work could be found that examined the free radicals formed in the fibers after irradiation. Accordingly, the EPR spectrum of the UHMMPE fibers was measured shortly after irradiation, and alkyl radicals were detected. The irradiated samples were stored in dark ambient conditions for at least five years, then then reexamined using EPR for free radicals. Surprisingly, the gamma-irradiated samples showed clear evidence of long-lived polyenyl radicals. The alkyl radicals formed immediately after irradiation would be expected to react quickly to form polyenyl radicals. One possible explanation for the presence of these polyenyl radicals so long after irradiation is the high crystallinity of these UHMMPE as compared to typical bulk polyethylene. Typically, these polyenyl radicals would be expected to eventually migrate to the surface of the crystalline domain and be eliminated in the amorphous region. However, we suspect that due to the high crystallinity and large anisotropy of the highly drawn UHMMPE fiber, the polyenyl radicals were unable to eliminate and were trapped in the crystal. An experiment was performed to test this hypothesis, by which a sample of the irradiated fibers were heated to temperatures above the alpha relaxation for polyethylene, and EPR measurements showed that the polyenyl radical signal persisted. Then, the fibers were heated to temperatures above the melting point of the polymer, and EPR signals showed that the polyenyl signal was rapidly eliminated. These experiments support the hypothesis that the long-lived polyenyl radicals were trapped in the crystalline region of the polyethylene fibers.