SELECTED THERMODYNAMIC ASPECTS OF THE INFLUENCE OF PRESSURE ON POLYMER SYSTEMS

The influence of pressure and temperature on a selection of physical/t hermodynamic properties is explored. In the first part, some experimen tal facts of the thermal behaviour are summarized. The equations of st ate 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 cha racter, the inevitable physical ageing is illustrated for simple and c ombined temperature and pressure jump experiments. Finally, some examp les of thermodynamic properties at high frequencies, such as the dynam ic compressibility, are discussed. In the second part of this contribu tion, a model of the dense disordered state, pertinent to chain and sm all 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 pol yatomic systems, a third parameter is introduced, quantifying molecula r 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 wit hin the experimental uncertainty of the measuring technique. In order to facilitate a discussion of the non-equilibrium and high-frequency p roperties, the equilibrium theory is complemented with a stochastic fo rmalism. 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 predi cted. Also the equation of state of the resulting glasses is predicted and compares favourably with the experimental data. Finally, this met hod is also applied to address the frequency dependence of thermodynam ic properties in general. The presented formalism opens the way to dis cuss the dependence of the ultimate properties of materials, obtained along a processing route, on the non-equilibrium conditions experience d during processing. In particular, the importance of formation histor y and physical ageing is clarified.