MOLECULAR-MOTION IN GLASS-FORMING SYSTEMS

The relaxation behavior of non-polymeric and polymeric glass-forming systems may be studied in the time domain or frequency domain using such techniques as dielectric relaxation, mechanical relaxation, volume relaxation and nuclear magnetic resonance. For relaxation in the region of the dynamic glass transition it is found for systems of very different chemical structure that structural relaxation may be represented approximately by a stretched exponential function phi-(t) = exp -(t/tau)-beta where 0 < beta less-than-or-equal-to 1 and where the effective relaxation time follows the Vogel equation tau = A exp B/(T - T0). It is possible to relate experimentally observed properties, such as the dielectric relaxation spectra or two-dimensional H-2-NMR spectra, to time-correlation functions for the motions of whole molecules, chain segments or individual bond axes. Correlation functions obtained from different experiments may be inter-related, thus providing constraints on the application of different models for motion to experimental data. The relaxation behaviour of several molecular systems is described, giving emphasis to dielectric relaxation properties, and the application of different molecular models for motion to these data is discussed.