Polymer Chain Collapse in Supercritical Fluids. 1. Molecular Simulation Results

Abstract

AbstractA few years ago we reported the first observation, by computer simulations, of polymer chain collapse near the lower critical solution temperature (LCST).1 In the present work, we extended the above study to understand the underlying physics of a single polymer chain collapse near LCST and its relationship to phase boundaries in the T‐x plane. Effects of solvent and monomer sizes, and solvent and monomer energetic interactions are studied. Using Monte Carlo simulations, the mean end‐to‐end distance (R) and gyration radius (Rg) are calculated for a single chain in a supercritical fluid solvent over a broad range of densities, pressures and temperatures. In general, the chain collapses as temperature increases at constant pressure. Upon a further temperature increase, the chain expands again to approach the athermal limit provided that the temperature is sufficiently high. The collapse is related to an LCST phase boundary while the expansion represents the signature of an upper‐critical solution temperature (UCST) suggesting the existence of a closed‐immiscibility loop. By manipulating the strength of the energetic interactions as well as the solvent‐to‐monomer size ratio, the size of the size of the immiscibility loop can be fine‐tuned. The relationship among size and the segment‐solvent energetic interaction are correlated by a conformational parameter (Ψ) for the first time. By monitoring the Ψ behavior, it is possible to predict solution's phase behavior, transition zone from LCST‐UCST in a closed‐loop miscibility behavior. The above relationship between chain conformation to phase boundaries may be useful in understanding phase stability in compressible polymer‐solvent mixtures.