Glass transition and molecular dynamics in PHPMA-b-POEGMA block copolymers

Abstract

In the present work we study the glass transition and molecular dynamics in new amphiphilic block copolymers based on hydrophilic poly(oligo ethylene glycol methacrylate) (POEGMA) and water-insoluble poly(hydroxyl propyl methacrylate) (PHPMA) prepared by RAFT polymerization. To that aim, we employ here differential scanning calorimetry (DSC) and dielectric spectroscopy (BDS). At dry conditions and up to ~50 wt% PHPMA, the POEGMA phase dominates on the glass transition (Tg ~ −60 °C) and on local dynamics of the copolymers, while for larger PHPMA contents (80 wt%) the behaviour is that of the strongly plasticized PHPMA phase (neat PHPMA exhibits remarkably high calorimetric glass transition temperature, Tg ~80 °C). Interestingly, a single glass transition step was observed for all compositions. For the copolymer rich in PHPMA (80 wt% PHPMA) the endothermic calorimetric step related to glass transition is much broader than in other copolymers, indicating higher spatial heterogeneity in the copolymer. Molecular dynamics by BDS, namely the secondary-local γ and β relaxations and segmental α relaxations (related with the glass transition) were mapped, especially for neat PHPMA for the first time. The presence of polar hydroxyl group in the side group of PHPMA made possible the dielectric study of the molecular mobility within the nano-domains formed by the side chains, like in n-alkyl methacrylates with long alkyl side chains (PE-like mobility). On the other hand, the dynamics in the brush-like structuring POEGMA is recorded quite similar to that in linear poly(ethylene glycol) chains. In the copolymers our results reveal strong intermolecular interactions at the atomic level among the side chains/groups of the two components and a pronounced nano-structured morphology.