Viscous Liquids and the Glass Transition. II. Secondary Relaxations in Glasses of Rigid Molecules

TLDR

Glasses of rigid molecules exhibit dielectric behavior remarkably similar to that of amorphous polymers. Dielectric loss and permittivity were measured over 50 Hz–10⁵ Hz from −196 °C to ~30 °C above Tg for a variety of chlorobenzene, dichlorobenzene, chloronaphthalene, pyridine, bromobenzene, toluene, tetrahydrofuran, cis‑decalin, o‑terphenyl, iso‑propylbenzene, propylene carbonate, and fused salt systems, with glass transition temperatures determined by DTA. All examined glasses except propylene carbonate display a secondary relaxation, with main‑relaxation Arrhenius plots being nonlinear (55–70 kcal mol⁻¹) and secondary‑relaxation plots linear (5–12 kcal mol⁻¹), confirming that secondary relaxations are an intrinsic property of the glassy state.

Abstract

The dielectric loss factor and dielectric permittivity of 8–16 mol% solutions of chlorobenzene, o-dichlorobenzene, and 1-chloronaphthalene in cis-decalin; 50–60 mol% mixtures of pyridine with chlorobenzene, bromobenzene, 1-chloronaphthalene, and toluene; 50–60 mol% mixtures of tetrahydrofuran with bromobenzene and 1-chloronaphthalene; the pure liquids cis-decalin, o-terphenyl, iso-propylbenzene, propylene carbonate; and two fused salt systems, 45 mol% Ca(NO3)2–KNO3 mixture and Ca(NO3)2·4H2O have been measured from 50 Hz to 1 × 105 Hz from − 196° in the vitreous state to about 30° above their respective glass transition temperatures. The Tg's of the organic glasses have been measured by DTA. With the exception of propylene carbonate, all glasses show the presence of one secondary relaxation between − 196° and their respective Tg's either as a peak or shoulder in a tanδ–temperature plot at a single frequency, or in the dielectric loss spectrum. Arrhenius plots of the frequency of maximum loss against temperature in the main relaxation region for all systems are nonlinear, with the activation energy at the lowest temperature of our measurements ranging from 55 kcal/mol to 70 kcal/mol. The Arrhenius plots in the secondary relaxation region are linear and have activation energies between 5 and 12 kcal/mol. These glasses, most of which are composed of rigid molecules, show a remarkable similarity in their dielectric behavior to amorphous polymers. The results confirm the prediction made by one of the authors that the occurence of secondary relaxations is an intrinsic property of the glassy state.