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Origin of the α, β, (βα), and “Slow” Dielectric Processes in Poly(ethyl methacrylate)

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journal contribution
posted on 14.07.2009, 00:00 by K. Mpoukouvalas, G. Floudas, G. Williams
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 × 103 and 1.59 × 104 g/mol) for frequencies from 3 × 10−3 to 106 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α, β, αβ, and “slow”. On lowering the temperature or increasing the applied pressure the αβ process is transformed into α and β processes. The relaxation strength of the β-process decreases markedly with increasing pressure (both above and below the glass temperature, Tg) and, for T > Tg, is accompanied by a complementary increase in relaxation strength of the α-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.