Polymer Chain Collapse in Supercritical Fluids

Abstract

Recent computer simulations of a polymer chain in a solvent have provided evidence, for the first time, of polymer chain collapse near the lower critical solution temperature (LCST). Motivated by these results, we have studied further this system to understand the effect of solvent and monomer sizes, chain length, and solvent and monomer energetic interactions. By means of extensive Monte Carlo simulations, the mean radius of gyration R _g and end-to-end distance R, are calculated for a single chain in a solvent over a broad range of volume fractions, pressurs and temperatures. Our results indicate that in general, the chain collapses as temperature increases at constant pressure, or as density decreases at constant temperature. A minimum in R _g and R occurs near the LCST and slightly above the coil-to-globule transition temperature (C-GTT), where the chain adopts a quasi-ideal conformation, defined by the balance of binary attractive and repulsive interactions. At temperatures well above the LCST, the chain expands again suggesting an upper critical solution temperature (UCST) phase boundary above the LCST forming a closed-immiscibility loop. However, this observation strongly depends on the solvent-to-monomer size ratio.