Polymer Chain Collapse in Supercritical Fluids

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My means of extensive Monte Carlo simulations the mean radius of gyration and the end-to-end distance are calculated for a single chain in a solvent over a broad range of volume fractions, pressures 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 the radius of gyration and the end-to-end distance occurs near the Lower Critical Solution Temperature (LCST) and slightly above the coil-to-globule transition temperature, 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.