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 T_g. 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 T_g and the relaxation time. Theoretical predictions expressed in terms of such excess quantities were compared with the experimental results on pressure dependence of T_g, 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 T_g 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.