Volume and temperature as control parameters for the dielectric α relaxation of polymers and molecular glass formers

Abstract

Methods of analysing the dielectric α relaxation and equation-of-state data for supercooled glass-forming materials, to determine the relative contribution of volume and temperature, are illustrated using measurements on poly[(o-cresyl glycidyl ether)–co-formaldehyde] (PCGE). The ratio E _V /E _P = 0.66 of apparent activation energies and the ratio |α _τ |/α _P = 1.7, of the thermal expansivities, evaluated at τ = 1 s and P = 0.1 MPa respectively, reveal that for PCGE the volume and temperature both exert a substantial influence on the relaxation times. In this regard, PCGE is similar to other polymers and non-associated molecular liquids for which such analyses have been carried out. This means that the underlying bases of both free-volume and thermal activation models are not correct. The exceptional behaviour is that of hydrogen-bonded liquids, such as polyalcohols, for which competing effects minimize the sensitivity to pressure, whereby temperature becomes the dominant control variable.

Acknowledgements

The work at the Naval Research Laboratory was supported by the Office of Naval Research. M.P. thanks the Komitet Badan Naukowych, Poland (grant 2PO3B 022 20), for financial support.