Excluded-Volume Effects in Star Polymer Solutions. Six-Arm Star Polystyrene in Benzene

Seven narrow-distribution samples of six-arm star polystyrene ranging in weight-average molecular weight Mw from 6.1×104 to 3.4×106 in benzene at 25°C have been studied by light scattering and viscometry to determine their z-average radii of gyration, second and third virial coefficients, and intrinsic viscosities. The ratios of the respective properties to those of linear polystyrene in the same solvent are established for high Mw. Data analysis shows that the relation between the radius expansion factor and the conventional excluded-volume parameter z comes close to the known relations for four-arm star and linear polystyrenes of high molecular weight and is described fairly satisfactorily by the previously proposed interpolation formula. On the other hand, the viscosity expansion factor vs. z curve appears slightly below that for linear polystyrene though almost superimposed on that for the four-arm star polymer. Thus the difference in this expansion factor between the linear and star chains remains to be explained theoretically. The experimental interpenetration function for the six-arm star polymer gradually decreases to about 0.6, a value close to recent Monte Carlo data, with increasing Mw. Its comparison with the previously constructed interpolation expression suggests that, as was the case for linear flexible polymers, the effect of chain stiffness on the second virial coefficient needs to be considered for Mw below 106.