Thermodynamics of the glassy state of polymers

After a short discussion of the nature of the glass transition, a concept of ordering parameters is given which allows a thermodynamic treatment of the glass transition and the glassy state. PVT-measurements on atactic polystyrene show that the Ehrenfest equations which describe a thermodynamic transformation of the second order are not applicable to the glass transition. The introduction of one ordering parameter ζ in addition to the conventional variables T and P is sufficient to describe the behavior of PS in the glassy state nearly quantitatively. The Ehrenfest equations must be replaced by other equations which in the simplest case contain only one ζ parameter (freezing-in model). It will be shown that the nonvalidity of the Ehrenfest equations results in a path dependence of the thermodynamic properties in the glassy state. Recent PVT measurements on a polymer liquid crystal show that the physical properties in the nematic phase do not depend on the path and are not time-dependent. At lower temperatures a glass transition of the nematic phase will be observed which can only be described by the assumption of at least two ζ parameters according to two different states of order in the polymer liquid crystal. The pressure dependence of the clearing temperature obeys the Clausius-Clapeyron equation.