The role of temperature and density on the glass-transition dynamics of glass formers.

A correlation between the monomeric volume and the dynamic quantity E*(V)/H*, used to provide a quantitative measure of the role of temperature and density on the dynamics, is demonstrated for a series of polymers and glass-forming liquids. We show that monomeric volume and local packing play a key role in controlling the value of this ratio and thus the dynamics associated with the glass temperature.

Pressure-enhanced dynamic heterogeneity in block copolymers of poly(methyl vinyl ether) and poly(isobutyl vinyl ether).

The static structure factor and associated dynamics have been investigated in a series of block copolymers of poly(methyl vinyl ether) (PMVE) and poly(isobutyl vinyl ether) (PiBVE) using x-ray scattering and dielectric spectroscopy (DS). The origin of the dynamic arrest at the glass temperature (T(g)) of PiBVE has been explored by temperature- and pressure-dependent DS and pressure-volume-temperature measurements. Both temperature and volume are responsible for the segmental dynamics but temperature has a stronger effect. The copolymers display a minimal dynamic asymmetry (Delta T(g) approximately 7 K), nevertheless, are spatially and dynamically heterogeneous. Increasing pressure, unlike temperature, enhances the dynamic heterogeneity. This effect originates from the distinctly different pressure sensitivities of the homopolymers and can be traced back to differences in local packing.

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(-2)-10(6) 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 (alpha) 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 (T,P) 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) approximately 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 1.08 T(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.