Electron beam (EB) modification was carried out on the polylactide (PLA) fibers at the temperature both below and above the glass transition temperature (Tg) under nitrogen atmosphere. The irradiation at elevated temperature was performed by using a novel fiber stretching frame setup to protect fibers from shrinking, in which a permanent constant tension was loaded along fiber bundles to maintain their straightness throughout the irradiation. In contrast to irradiation at room temperature, where PLA predominantly undergoes chain scission, irradiation above Tg can introduce long chain branching (LCB) into the PLA matrix without the need of branching additives. The polymer chain orientation in the amorphous regions of drawn fibers is beneficial for achieving a higher yield of LCB. This improvement is attributed to the shortened intermolecular distance and the disentanglement in the amorphous domains which are caused by melt spinning and post-drawing. This is counteracted by the typically high crystallinity degrees in PLA fibers (measured by differential scanning calorimetry, 31.0 % – 41.2 % for neat fibers), due to the chain scission is mainly induced by EB in the rigid crystalline domains. The chain scission effect in the crystalline regions mitigates the branching effect in the amorphous regions on the material properties, such as tensile strength, even in irradiation above Tg. However, by reducing the PLA fiber crystallinity while applying temperatures above Tg, the amount of EB-induced LCB molecules was significantly increased. Moreover, the branched unit per PLA chain increases when a dose of 50 kGy is applied, as determined by analyzing the branching ratio g’ using multi-detector size exclusion chromatography. In addition, the amount of branched unit also increases with the increasing irradiation dose. The tensile properties of fibers with varying drawing ratios were analyzed before and after irradiation. The resulting tensile properties are primarily determined by the draw ratio and appear to be barely affected by crystal orientation (examined using synchrotron X-ray scattering). However, the crystal orientation increases with higher irradiation doses (above Tg) and is likely driven by the interplay between chain scission and cold crystallization.
Zhang et al. (Wed,) studied this question.