Polymer + Solvent Systems: Phase Diagrams, Interface Free Energies, and Nucleation

Abstract

Some theoretical concepts on polymer + solvent systems and Monte Carlo simulations of corresponding coarse-grained models are briefly reviewed. While the phase diagram of polymers in bad solvents invoking the incompressibility approximation for the polymer solution has been a standard problem of polymer science for a long time, a more complete understanding of compressible polymer solutions, where liquid-liquid phase separation and liquid-vapor transitions compete, has emerged only recently. After giving a phenomenological introduction, we outline and compare three complementary approaches: self-consistent field theory, thermodynamic perturbation theory and grandcanonical Monte Carlo simulation. In order to give a specific example, we focus on the mixture of hexadecane with carbon dioxide. Attention is paid to correlate the description of the phase diagram with the properties of interfaces and the nucleation barrier that needs to be overcome to form a droplet (or bubble, respectively) of critical size, necessary for the decay of the corresponding super-saturated metastable state. Particular emphasis is given to new techniques used for the computer simulation of such phase diagrams where several order parameters compete, and to systematic difficulties that still hamper the prediction of accurate nucleation barriers from the observation of droplets (or bubbles, respectively) in finite volumes. The extent to which real materials can be modeled will also be examined.