Über die Druckabhängigkeit der Glasigen Erstarrung von Hochpolymeren

Vitreous solidification is often considered to be a second-order thermodynamic transition and a more or less satisfactory agreement is found between the thermodynamic parameters and the Ehrenfest equations (dP/dT)I = Δα/Δx; (dP/dT)II = ΔP/T · Δα. Measurements of volume retardation carried out by Goldbach and Rehage show that vitreous solidification is not a true transition. The question is if vitreous solidification at least formally can be treated as a thermodynamic transition. Measurements of specific isothermal compressibility and specific volume of amorphous polystyrene in the pressure range up to 10 kb and at temperatures of 100–250°C. carried out by the authors and calorimetric measurements of Koplin on the same polymer show that both Ehrenfest equations differ from experimental data by ca. 50%. The deviations are explained by considerable variation of specific volume with the path of the transformation in the glassy state which also had been measured. This variation with the path of transformation is due to the change of order within the material with increasing pressure along the freezing curve. The Ehrenfest equations are formally valid for a freezing process with the same inner order obtained at all pressures at the corresponding freezing temperature. The Ehrenfest equations can be combined by eliminating dP/dT but the relation obtained does not allow discrimination between a second order transition and the vitreous solidification. In this paper it has been shown that the vitreous solidification distinctly differs in both characteristics and formal treatment from a thermodynamic transition.