Excluded-volume effects in star polymer solutions. Six-arm star polystyrene in benzene

Seven narrow-distribution samples of six-arm star polystyrene ranging in we ight-average molecular weight M-w from 6.1 x 10(4) to 3.4 x 10(6) in benzen e at 25 degrees C have been studied by light scattering and viscometry to d etermine their z-average radii of gyration, second and third virial coeffic ients, and intrinsic viscosities. The ratios of the respective properties t o those of linear polystyrene in the same solvent are established for high M-w. Data analysis shows that the relation between the radius expansion fac tor and the conventional excluded-volume parameter z comes close to the kno wn relations for four-arm star and linear polystyrenes of high molecular we ight and is described fairly satisfactorily by the previously proposed inte rpolation formula. On the other hand, the viscosity expansion factor vs. z curve appears slightly below that for linear polystyrene though almost supe rimposed on that for the four-arm star polymer. Thus the difference in this expansion factor between the linear and star chains remains to be explaine d theoretically. The experimental interpenetration function for the six-arm star polymer gradually decreases to about 0.6, a value close to recent Mon te Carlo data, with increasing M-w. Its comparison with the previously cons tructed interpolation expression suggests that, as was the case for linear flexible polymers, the effect of chain stiffness on the second virial coeff icient needs to be considered for M-w below 10(6).