Impact of the graft’ structure on the behavior of PMMS-based brushes. High pressure studies

Abstract

In this paper, we have investigated dielectric response at ambient and elevated pressure of poly(mercaptopropyl)methylsiloxane (PMMS) based oligomeric brushes, where the thiol moiety was substituted by various butyl acrylate isomers (n-butyl acrylate (nBA), iso-butyl acrylate (isoBA), tert-butyl acrylate (tertBA)). It was found that the glass transition temperature, Tg, of PMMS is much lower with respect to the other systems despite its ability to form weak hydrogen (H) bonds. What is more, the length scale of cooperativity was the highest in homopolymer and scaled with the steric hindrance of the graft. We also demonstrated that in the examined grafted copolymers, there is one broad relaxation process in dielectric spectra which is wider than that recorded in mechanical response. That probably means that similarly to PMMS, there are two relaxation processes above Tg in grafted copolymers. However, due to similar timescale, they cannot be separated. Further, high-pressure experiments revealed that pressure coefficient of the glass transition temperature, dTg/dp, as well as the activation volume, ΔV, calculated for the segmental process surprisingly does not scale with either the size of monomer nor steric hindrance. This unexpected result must be related to the specific structure adopted by the studied grafted oligomers. Presented herein data expand our understanding on the impact of topology on the dynamics of polymers at ambient and high pressure.