Molecular Mobility in Supercooled Liquids

Abstract

This paper summarizes the examinations on various factors governing the molecular mobility in the supercooled liquids and the glass transition temperature Tg. The examinations were performed on some thermodynamically characterized samples including a linear polymer (polyvinylchloride: PVC), a crosslinked polymer (chlorinated polyethylene: CPE), and a relatively simple molecular liquid (a mixture of triphenylchloromethane and ortho-terphenyl: TPCM/oTP). Various thermodynamic excess quantities such as the free volume and the configurational entropy and energy were evaluated as possible factors governing Tg and the relaxation time. Theoretical predictions expressed in terms of such excess quantities were compared with the experimental results on pressure dependence of Tg, and isochronal T-P relation obtained from the dielectric dispersion. The main conclusions are as follows: (1) The WLF type free volume is a good factor determining Tg and the relaxation time for TPCM/oTP, but not for the polymers; (2) The Adam-Gibbs parameter is good for the polymers, but not for TPCM/oTP; (3) The configurational entropy and internal energy are good for all of the present systems; (4) The glass transition of TPCM/oTP may be treated as a quasi-equilibrium thermodynamic second-order transition, but those of the polymers cannot; (5) The ratio of isochoric activation energy to isobaric activation energy, H*v/H*p, is 0.7~0.9 for the polymers, while H*v/H*p is less than 0.5 for TPCM/oTP. The difference in intramolecular degrees of freedom may be responsible for this difference in the value of H*v/H*p.