2ND AND 3RD VIRIAL-COEFFICIENTS FOR POLYISOBUTYLENE IN HEPTANE, AN INTERMEDIATE SOLVENT

Light scattering measurements have been made on nine polyisobutylene (PIB) fractions in heptane, an intermediate solvent, at 25 degrees C to determine the second virial coefficient A(2) and the third virial coefficient A(3) as functions of weight-average molecular weight M(w) ranging from 2.7 x 10(4) to 7.8 x 10(6). For six of the fractions, z-average mean-square radii of gyration have also been determined. In the molecular weight range studied, A(2) and A(3) vary as M(w)(-0.21) and M(w)(0.58), respectively, and the factor g defined by A(3)/A(2)(2)M(w) is about 0.33 almost independent of molecular weight. The data of g and Psi (the interpenetration function) as functions of the cube of the radius expansion factor alpha(s), combined with previous data in cyclohexane, a good solvent, show that the two-parameter theory breaks down unless alpha(s)(3) is larger than about 2 for g and about 5 for Psi. It is concluded from the comparison of these g data with the recent theory of Norisuye et al. that the failure of the two-parameter theory for g at small alpha(s)(3) is due primarily to the neglect of three-segment interactions and that the effect of chain stiffness on g in heptane is of minor importance in the M(w) range studied. On the other hand, the stiffness effect on Psi for PIB is found to be remarkable even at large alpha(s)(3). In fact, the different dependences of Psi on alpha(s)(3) observed for heptane and cyclohexane solutions are explained semiquantitatively by the Yamakawa theory which takes account of the stiffness effect within the binary cluster approximation.