Dielectric Study on the Well-Resolved Sub-Rouse and JG β‑Relaxations of Poly(methylphenylsiloxane) at Ambient and Elevated Pressures

The dielectric behavior of poly­(methylphenylsiloxane) (PMPS) was revisited to investigate the well-resolved α′- and β-relaxations found on the low- and high-frequency flanks of the segmental (α) relaxation, respectively. Comparative studies of the α′-, α-, and β-relaxations in two PMPS samples of widely different molecular weights and a phenylmethylsiloxane–dimethylsiloxane (PMPS–PDMS) copolymer were carried out with a focus on the dynamics of the α′-, α-, and β-relaxations, including the frequency dispersions and relaxation times, τ_α′, τ_α, and τ_β. At temperatures above T_g, τ_α, and τ_α′ are molecular-weight-dependent and τ_α′ has a weaker temperature dependence than τ_α. The α′-mode is identified as the sub-Rouse relaxation by its time and length scales that are intermediate between the segmental motions and the Rouse modes. There are several novel findings in the α′-, α-, and β-relaxations. For the two PMPS homopolymers and the PMPS–PDMS copolymer, τ_β(T) are practically the same in the glassy states and in agreement with τ_α(T) reported for PMPS confined in nanometer spaces. Both the separations between τ_α′ and τ_α and between τ_α and τ_β increase with the molecular weight of PMPS. By applying pressure, we find that the β-relaxation is sensitive to pressure, and the ratios τ_α(T,P)/τ_β(T,P) as well as the frequency dispersion of the α-relaxation at constant τ_α(T,P) are effectually invariant to pressure and temperature changes.