Studies on the Molecular Dynamics at High Pressures as a Key to Identify the Sub-Rouse Mode in PMMS

Abstract

In this paper, we have investigated the molecular dynamics of the associating polymer, poly(mercaptopropyl)methylsiloxane (PMMS), at high pressure (up to p ∼ 505 MPa) by means of broadband dielectric spectroscopy. Previous studies revealed that PMMS exhibits two dielectric relaxation processes observed above the glass transition temperature related, most likely, to either the mobility within self-assemblies or the sub-Rouse mode (α′-slower process) and segmental (α-faster process) dynamics, whereas mechanical measurements revealed only the presence of terminal and segmental relaxations [Tarnacka et al. Macromolecules 2020, 53 (22), 10225−10233]. In order to determine the origin of the dielectric α′-process, further high-pressure experiments were performed. It was found that the timescale separation between relaxation times of segmental (α) and α′-processes is invariant to the compression, and activation volume calculated for both kinds of motions is comparable. These dynamical features are characteristic of the chain relaxation (called usually normal mode) found in type-A polymers. However, because mechanical data excluded identification of the slow dielectric relaxation as normal mode, we assigned it as the sub-Rouse process. This assignment is in the line with previous studies on poly(methylphenyl)siloxane. Further density functional theory computations revealed that the detection of the relatively strong sub-Rouse process is most likely possible due to the presence of a highly polar side group (thiol, −SH, moiety) that gives a strong contribution to dipole moment along the main polymer backbone. Additionally, we demonstrated that the pressure coefficient of the glass transition temperature, dTg/dp, in PMMS is one of the smallest among those reported to date for various polymers (dTg/dp = 156 K/GPa). This quite surprising finding was assigned to the specific interactions formed by the thiol group. Finally, it should be emphasized that high-pressure experiments turned out to be the key element to identify the sub- Rouse mode in dielectric spectraa process that might provide important information about the chain dynamics in polysiloxanes. However, to finally prove this hypothesis, further studies are required to discard the eventual possibility that the slow mode is somehow related to the nanoscopic organization in PMMS