Selected thermodynamic aspects of the influence of pressure on polymer systems

Abstract

The influence of pressure and temperature on a selection of physical/thermodynamic properties is explored. In the first part, some experimental facts of the thermal behaviour are summarized. The equations of state of melts and glassy amorphous polymers are considered. From this, the pressure dependence of the glass transition temperature is derived. As a first illustration of the non-equilibrium nature of the glassy state, the impact of cooling rate on the glass transition temperature is demonstrated. In a further demonstration of the non-equilibrium character, the inevitable physical ageing is illustrated for simple and combined temperature and pressure jump experiments. Finally, some examples of thermodynamic properties at high frequencies, such as the dynamic compressibility, are discussed. In the second part of this contribution, a model of the dense disordered state, pertinent to chain and small molecule fluids, is discussed. The model is based on a cell model with additional configurational disorder provided by vacancies. In the theory, two parameters define the intermolecular interactions. In polyatomic systems, a third parameter is introduced, quantifying molecular modes of motion which are perturbed by the surroundings. The theory is successfully applied to describe the equation-of-state behaviour of pure constituents. Typically, the experimental data are described within the experimental uncertainty of the measuring technique. In order to facilitate a discussion of the non-equilibrium and high-frequency properties, the equilibrium theory is complemented with a stochastic formalism. This combination allows the influence of formation parameters on the glassy state to be discussed. For instance, the dependence of the glass transition temperature on cooling rate and pressure is predicted. Also the equation of state of the resulting glasses is predicted and compares favourably with the experimental data. Finally, this method is also applied to address the frequency dependence of thermodynamic properties in general. The presented formalism opens the way to discuss the dependence of the ultimate properties of materials, obtained along a processing route, on the non-equilibrium conditions experienced during processing. In particular, the importance of formation history and physical ageing is clarified.