2ND AND 3RD VIRIAL-COEFFICIENTS FOR POLYISOBUTYLENE IN THE VICINITY OF THE THETA-POINT

The second virial coefficient A2 and the third virial coefficient A3 have been determined for five polyisobutylene fractions ranging in weight-average molecular weight M(w) from 8 x 10(4) to 1.6 x 10(6) in isoamyl isovalerate at different temperatures T between 21 and 37-degrees-C by light scattering. The theta temperature THETA where A2 Vanishes is found to be 27-degrees-C independent of M(w). The curve of A3 vs. T obtained for each fraction has a broad minimum around THETA, and the minimum becomes very shallow as M(w) decreases. These features are very similar to those observed previously for polystyrene in cyclohexane. The values of A3 at THETA are in the range between 3 x 10(-4) and 7 x 10(-4) mol g-3 cm6, and thus demonstrate the breakdown of the binary cluster approximation to A3 for polyisobutylene near THETA. It is shown that existing theories of A2 and A3 are incapable of explaining consistently the positive A3 at THETA and the molecular weight independence of THETA in the range of M(w) studied, as found to be the case for polystyrene.