Relaxation in liquids, polymers and plastic crystals - strong/fragile patterns and problems

An overview of relaxational phenomenology is given in a manner intended to highlight a number of the important problems which, notwithstanding much recent sophisticated investigation, continue to confront the field. The rapidly lengthening timescale for diffusional and/or reorientational motion, which provokes the glass transition, is examined within the framework of the 'strong' and 'fragile' classification of both liquids and plastic crystals. The behavior patterns observed are related to the topological features of the potential energy hypersurfaces which may characterize each extreme. In view of the implication that the observed glass transition is the kinetically obscured reflection of an underlying higher order thermodynamic transition which could be associated with a diverging length scale (at least for fragile systems), the problem of the basic diffusional length scale at the glass transition, using a probe molecule approach, is considered. Then, details of the kinetics of relaxation under isothermal conditions are reviewed to decide on the range of deviations from Debye behavior which may be encountered. A correlation with fragility is strongly indicated. The phenomena of serial decoupling of relaxational modes from the main structural relaxation as T_g is approached is outlined and, finally, the additional phenomena that may be encountered in experiments that explore the state-dependence (or non-linearity) of relaxation are briefly examined.