Effect of irradiation and detection of long-lived polyenyl radicals in highly crystalline UHMMPE fibers
Amanda L. Forster, Zois Tsinas, Mohamad Al-Sheikhly
To improve properties such as thermal conductivity, low temperature thermal strain, and creep resistance of 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 % crystalline) and are very highly oriented during manufacture 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. 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 significant result was found. The electron pair 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. (truncated)
, Tsinas, Z.
and Al-Sheikhly, M.
Effect of irradiation and detection of long-lived polyenyl radicals in highly crystalline UHMMPE fibers, Polymers, [online], https://doi.org/10.3390/polym11050924
(Accessed December 10, 2023)