G. Lunabarcenas et al., RELATIONSHIP BETWEEN POLYMER-CHAIN CONFORMATION AND PHASE BOUNDARIES IN A SUPERCRITICAL-FLUID, The Journal of chemical physics, 107(24), 1997, pp. 10782-10792
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.