Electron-Induced Alteration of PLA Chain Structure

Cross-linked polylactides (PLAs) were produced by electron beam (EB) irradiation at 80 °C using different preset crystallinities (0 to 13.3%), without using cross-linking-promoting additives. Based on functional group conversion detected by Fourier-transform infrared spectroscopy, molecular build-up mechanisms based on a two-stage chain recombination (from geometric T-type to H-type branching) were proposed for EB-induced cross-linking in additive-free PLA. Furthermore, EB-induced PLA crystallization has been discussed based on the combined evidence from morphology, crystallinity, and crystallization behaviors identified by polarized-light optical microscopy, X-ray diffraction, and differential scanning calorimeter (DSC). Several interpretations of the thermal property of EB-induced cross-linked PLA were presented from the point of view of chain scission and cross-linking in a coexisted system. The high irradiation dose obviously shifted the melting temperature (T_m) and cold crystallization temperature (T_cc) of irradiated PLA to a lower region than those of neat PLA. Moreover, the different preset crystallinities of the PLA starting materials did not lead to an obvious difference among irradiated PLAs with respect to T_g, T_m, T_cc, and cross-link density, according to DSC and equilibrium swelling measurements.