RELATIONSHIP BETWEEN POLYMER-CHAIN CONFORMATION AND PHASE BOUNDARIES IN A SUPERCRITICAL-FLUID

We investigate the solvent density driven changes in polymer conformat ion and phase behavior that occur in a supercritical fluid, with a par ticular emphasis on conditions near the lower critical solution temper ature (LCST) phase boundary. Using continuous space Monte Carlo simula tions, the mean square end-to-end distance (R) and radius of gyration (R-g) are calculated for a single chain with 20 Lennard-Jones segments in a monomeric solvent over a broad range of densities and temperatur es. The chains collapse as temperature increases at constant pressure, or as density decreases at constant temperature. A minimum in R and R -g occurs at a temperature slightly above the coil-to-globule transiti on temperature (C-GTT), when the chain adopts a quasi-ideal conformati on, defined by the balance of binary attractive and repulsive interact ions. Expanded ensemble simulations of finite-concentration polymer-so lvent mixtures reveal that the LCST phase boundary correlates well wit h the single chain C-GTT. At temperatures well above the LCST, the cha in expands again suggesting an upper critical solution temperature (UC ST) phase boundary above the LCST. (C) 1997 American Institute of Phys ics.