高分子物質の粘弾性とガラス転移

Abstract

The prevailing relaxation theory on the nature of the glass transition (the sharp bend occurring in specific volume vs. temperature curve measured under a constant rate of cooling) in high polymers and other glass-forming liquids is re-examined.It is pointed out that the glass transition is essentially a non-linear relaxational phenomenon in which volume relaxation time varies steeply with temperature change, the excitation variable in this case, and that this variation is the real cause for the transition. It is therefore emphasized that elucidation of the mechanism of the transition is essentially equivalent to that of the mechanism of the precipitous change of relaxation time or viscosity near the transition temperature, as described in most cases analytically by WLF Eq. or more ultimately by Doolittle type equation in terms of the concept of free volume.It is also pointed out that the transition manifests itself in many physical properties as sharp change of their temperature coefficients and the time-scale determining the transition temperature thus located is only the rate of cooling in any case. The transition or freezing-in of free volume occurs necessarily through a range of temperature owing to the nature of relaxational phenomenon and both ends of the range are to be determined by means of the manifestation of the transition in some appropriate physical properties. As a special example of such cases relation between viscosity and volume retardation is discussed. By using a simple kinetic equation of the first order reaction type for volume retardation and Doolittle type equation for viscosity, it is easily shown that WLF Eq. referred to some equilibrium state is derived so far as volume follows the equilibrium liquid line and further shown that viscosity begins to deviate from WLF curve in response to the commencement of volume retardation with decrease of temperature. The temperature where the deviation starts is the higher temperature end of the transition range and on the other hand the lower end can be determined as the temperature where relaxation effect becomes remarkable with increase of temperature.The above-mentioned considerations are experimentally verified on five polymeric substances by observing viscoelastic and thermal expansion behaviors.