Relaxation Phenomena in Vitrifying Polymers and Molecular Liquids

Recent experimental results on the dynamics of glass-forming materials, particularly polymers, are surveyed. The focus is on aspects of the behavior that are connected to or correlated with structural relaxation. These results include the invariance to thermodynamic conditions (temperature, pressure, volume) of a number of properties—breadth of the relaxation dispersion, number of dynamically correlating molecules, Johari−Goldstein secondary relaxation time, onset of the dynamic crossover, and the product of temperature and specific volume with the latter raised to a material constant—provided the structural relaxation time is maintained constant. Additional salient experimental findings include the correlation of various high-frequency processes, usually measured in the glassy state, with properties of the equilibrium material above Tg. These correlations indicate that the glass transition, although conventionally defined by the relaxation time becoming larger than experimental time scales (>100 s), has its beginning many orders of magnitude sooner. Also described herein are effects of spatial confinement on the glass transition; these can be dramatic, yet taken in toto are rather discombobulating. Such generally observed phenomena must be included in a comprehensive theory or model of the glass transition, since properties intimately connected to structural relaxation cannot be derived separately and be expected to exhibit such correlations by coincidence.