Molecular mobility and thermal transitions study in renewable PLA-polyols star-like copolymers

The molecular dynamics mapping in poly(lactic acid) (PLA)-based star-like copolymers is performed herein, employing dielectric spectroscopy techniques supplemented by calorimetry and other structure characterization methods. Next to the neat linear PLA, three star-like copolymers, with 3, 4 and 5 branches were synthesized via ring opening polymerization at the presence of, respectively, glycerol, pentaerythritol and sorbitol (initiators). When increasing the branch number, consequently, the overall molar mass drops, from 55 kg/mol (neat PLA) to 17k (PLA@glycerol), 25k (PLA@pentaerythritol), and 14k (PLA@sorbitol) and, thus, the branch length decreases to-6k,-6k, and-3 k g/mol in the stars. The latter leads to faster segmental mobility, documented by the accelerated alpha relaxation and the drop in the calorimetric and dielectric glass transition temperatures. Simultaneously, the cooperativity (chains' fragility) drops. Together these results suggest possibly the increase in free volume, a factor that can potentially favour the physical biodegradability. Next to the segmental dynamics, the local beta relaxation is recorded to have a mild dependence on the type of star (mainly decelerates), while the rare case of Normal Mode relaxation is recorded here only in the stars, providing additional support for the quite short branches. The overall data suggest the co-implementation of two antagonistic effects on mobility, i.e., the chain length changes in parallel to the constraints imposed by the star-like structure. Moreover, the in general low crystallizability of linear PLA further drops in the stars in terms of both suppressed crystalline fraction and nucleation, as well as lower quality of crystals. The materials exhibit single thermal transitions, indicating a homogeneous character, whereas a variety of semicrystalline morphologies can be achieved via both the composition and the thermal treatment. This provides a tool for tuning various properties and, consequently, the manipulation of the macroscopic performance.