Plasticization Effects of PEG of Low Molar Fraction and Molar Mass on the Molecular Dynamics and Crystallization of PLA-b-PEG-b-PLA Triblock Copolymers Envisaged for Medical Applications

We prepared and studied a series of triblock copolymers based on poly(ethylene glycol) (PEG) and poly(lactic acid) (PLA). PLA blocks were in situ by ring-opening polymerization (ROP) of lactide (LA) onto the two sites of PEG. While in our recent work on similar copolymers with varying LA/PEG molar ratios and fixed PEG blocks [Bikiaris, N. D. Mater. Today Commun. 2024, 38, 107799], herein, we kept this ratio quite low, at 640/1, and employed different molecular weights, Mn, of the initial PEG at 1, 4, 6, and 8 kg/mol. The triblocks demonstrated high homogeneity, as manifested by the single thermal transition (glass transition, crystallization) with corresponding alternations in a systematic way with the Mn of PEG. With the increase of the latter Mn, accelerated segmental mobility and lowering of Tg by up to 15 K were recorded, accompanied by suppression in the chain fragility (cooperativity). Compared with linear PLAs of various Mns [Klonos, P. A. Polymer 2024, 305, 127177] and other PLA-based copolymers prepared by similar ROPs, with the overall Mn of our copolymers, PEG here sees to play the role of plasticizer on PLA, leading to increased free volume. Due to these effects, in general, the low crystalline fraction of PLA (∼3%) was significantly enhanced in the copolymers (20–26%), and the formed spherulites were mainly enlarged. Contrary to these, nucleation was barely affected; thus, the copolymers exhibited altered semicrystalline morphologies as compared to that in neat PLA. Both aspects of molecular dynamics, free volume and crystallization, were connected to the processability as well as the performance of these systems, considering the envisaged biomedical applications.