The dynamics in poly(ethyl methacrylate) (PEMA) was studied as a function of temperature (in the range from 133.15 to 453.15 K), pressure (0.1 to 300 MPa), and molecular weight (2.0 Â 10 3 and 1.59 Â 10 4 g/mol) for frequencies from 3 Â 10 -3 to 10 6 Hz using dielectric spectroscopy (DS). In addition, rheological studies were made within the temperature range from 323.15 to 383.15 K. The studies reveal four dielectrically active processes;R, β, Rβ, and "slow". On lowering the temperature or increasing the applied pressure the Rβ process is transformed into R and β processes. The relaxation strength of the β-process decreases markedly with increasing pressure (both above and below the glass temperature, T g ) and, for T > T g , is accompanied by a complementary increase in relaxation strength of the R-process. The origins of the four dielectrically active processes are discussed in terms of the (i) apparent activation volume and (ii) values of the ratio of activation energies at constant volume and pressure. The latter allowed discussion of the relative contributions of thermal energy and volume to each of the dynamic processes. As a part of the paper, we derive the canonical set of equations that describe the effects of the thermodynamic variables P, V, T on the average relaxation times and these are employed to aid the interpretations of the origins of the individual relaxation processes.

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.

The poly(methyl methacrylate) (PMMA) segmental dynamics in miscible blends of PMMA with poly(ethylene oxide) (PEO) were studied as a function of pressure by means of dielectric spectroscopy (DS). This facilitates a test of the predictions of the self-concentration model proposed by Lodge and McLeish, at elevated pressures. We find that pressure increases the glass temperature and slows down the segmental dynamics but does not affect the length scale or the self-concentration associated with the dynamic glass transition. This is in agreement with the expectations borne out from the model.