Eight-arm star polystyrene in methylcyclohexane: Cloud-point curves, critical lines, and coexisting densities and viscosities

We measured the cloud-point curves of eight-arm star polystyrene (sPS) in methylcyclohexane (MCH) for polymer samples of three total molecular masses [weight-average molecular weight (M-w) X 10(-3) = 77, 215, or 268]. We found a downward shift of 5-15 K in the critical temperature (T-c) of the star polymer solutions with respect to linear polystyrene (PS) solutions of the same Mw. The shift in T-c became smaller as M-w increased. The critical volume fraction for eight-arm sPS in MCH was equal within experimental uncertainty (10-40%) to that of linear PS in MCH. For sPS of M-w = 77,000 in MCH, we studied the mass density (rho) as a function of temperature (T) As for linear polymers in solution, the difference in rho between coexisting phases (Deltarho) could be described over t = (T-c - T)/T-c for 1.1 X 10(-4) < t < 4.7 X 10(-3) with the Ising value of the exponent beta in the expression Deltarho = B t(beta). Both rho(T) above T-c and the average value of rho below T-c were linear functions of temperature; no singular corrections were observed. The measurements of the shear viscosity (eta) near T-c for sPS (M-w = 74,000) in MCH indicated a strong critical anomaly in eta, but the data were not precise enough for a quantitative analysis. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 129-145, 2004.