Phase diagram of poly(methyl-p-tolyl-siloxane): A temperature- and pressure-dependent dielectric spectroscopy investigation

The dynamics of poly(methyl-$p$-tolyl-siloxane) (PMpTS) have been studied as a function of temperature (in the range from 143 to 413 K), pressure (0.1–300 MPa), frequency ${(10\mathrm{}}^{-2}–{10}^6\mathrm{Hz}),$ and molecular weight. Independent pressure-volume-temperature (PVT) measurements (for temperatures in the range from 293 to 393 K and for pressures in the range from 10 to 200 MPa) allowed calculation of the relevant thermodynamic parameters. Two dielectrically active channels of relaxation were found, one in the glassy state reflecting a localized motion of the substituted phenyl ring and one at higher temperatures reflecting the usual segmental (α) relaxation. In PMpTS, there are two dominant control variables; both density and temperature have a strong influence on the segmental dynamics. The PVT results allowed us to follow distinct thermodynamic $\mathrm{}(T,P)\mathrm{}$ paths resulting in states bearing the same density. These isodensity states are characterized by an apparent activation energy ${(Q}_V)$ that is not very different from the corresponding activation energy under isobaric conditions ${(Q}_V{/Q}_P\sim 0.55)$ reflecting the importance of thermal effects. At temperatures above the glass temperature ${(T}_g),$ strong orientation correlations exist above some critical pressure that depends on temperature. This state extends from $T_g$ up to $1.08T_g$ and separates a normal liquid at higher temperatures from an oriented liquid at lower temperatures. Using the “phase diagram” we discuss separately the influence of the temperature and density on the PMpTS dynamics.