The Influence of Graft Rigidity on the Dynamical Behavior of PMMS-Based Polymer Brushes at Ambient and High Pressures

In this paper, we investigated the molecular dynamics of polymer brushes based on poly(mercaptopropyl)methylsiloxane (PMMS), in which the thiol group was grafted with different homologous flexible acrylates (acrylate-based PMMS copolymers) and more rigid methacrylate (methacrylate-based PMMS copolymers) monomers of varying lengths of alkyl chain under ambient and elevated pressure conditions. It was found that the glass transition temperature, T_g, of PMMS homopolymer is significantly lower compared to the other systems. Moreover, surprisingly, in the methacrylate-grafted copolymers, there are two relaxation processes (α and α′), while in the systems grafted with various acrylates, only a single process is present in the supercooled phase. Complementary rheological investigations indicated that the faster α process comes from the segmental motions, while α′ is not detected in the mechanical response. Further high-pressure experiments showed that there is a superposition between segmental and α′ modes irrespective of applied pressure, p, in methacrylate-based PMMS copolymers. This result suggests that the latter process might be considered as a sub-Rouse mode, or alternatively, it may originate from the dielectric active relaxation of the rigid polar side chain (grafts). Moreover, analysis of the high-pressure data allowed us to estimate the pressure coefficient of the glass transition temperature, dT_g/dp, which was much higher for polymer brushes with respect to the PMMS homopolymer. Interestingly, the values of dT_g/dp for methacrylate-grafted copolymers are slightly higher compared to acrylate-based PMMS copolymers, which may be due to the different flexibility/rigidity of both groups of materials as all examined materials have the same degree of polymerization of homopolymer backbone (N_bb ∼ 12) and side chain (N_sc = 1). However, for both groups of studied systems, dT_g/dp values did not scale with chain length. This unexpected result must be related to the structure of the studied grafted copolymers and the character of grafts, derivatives of acrylates/methacrylates. The data presented here extend our knowledge of the influence of the architecture of different molecules on the dynamics of polymers at ambient and high pressures.