PVT Relationships for Liquid and Glassy Poly(vinyl acetate).

Abstract

PVT measurements were made on liquid and glassy poly(vinyl acetate) over ranges of -30 to 100 °C and 0 to 800 bar (gage pressure). The data were obtained by three different thermodynamic histories: (a) variable formation pressure, (b) constant formation pressure at one atmosphere, and (c) constant formation pressure at 800 bar. In all of these the glass was formed by isobaric cooling at 5 °C/h. The salient characteristics resulting from the different histories are the following. History (a) produces a glass of structure varying with formation pressure and, hence, does not necessarily give the proper thermodynamic properties of a "single physical substance." However, the liquid-glass intersection temperature, T _g (P), is an important kinetic, or relaxational, property which approximates an isoviscous state. Accordingly, the values of dT _g /dP are in close agreement with those obtained from dynamic mechanical and dielectric time-temperature-pressure superposition. Constant formation histories (b) and (c) give proper thermodynamic properties of the glasses, but very little information with respect to kinetics. Increasing the pressure at which the glass is formed increases the density of the glass (at the given cooling rate) considerably in contrast to the entropy (from other work), which appears to be essentially independent of formation pressure. A considerable part of the paper is definitional. The results are related to other PVT, dynamic mechanical, dielectric, and thermodynamic measurements. Interpretations are given in terms of both phenomenological and molecular models.