2ND AND 3RD VIRIAL-COEFFICIENTS FOR POLYSTYRENE IN CYCLOHEXANE NEAR THE THETA-POINT

Second and third virial coefficients (A2 and A3) for six polystyrene fractions ranging in weight-average molecular weight (M(w)) from 2 x 10(4) to 3 x 10(6) in cyclohexane were determined by light scattering at different temperatures (T) between 27 and 45-degrees-C. The THETA-point where A2 vanishes was 34.5-degrees-C (+/- 0.3-degrees-C) independent of M(w) above 5 x 10(4). The curve of A3 vs T obtained for each fraction was nearly parabolic with a broad, positive minimum around THETA, and the minimum became very shallow as M(w) decreased. The values of A3 at THETA were 6.5 (+/- 2.5) x 10(-4) mol g-3 cm6 depending slightly on M(w) and thus demonstrated the breakdown of the binary-cluster approximation at least to A3 at the THETA-point. It was found that the currently available first-order perturbation theories of A2 and A3 for linear flexible chains with three-body segment interactions fail to give a consistent explanation of the molecular weight independent THETA-(for M(w) above 5 x 10(4)) and the positive A3 values at the THETA-temperature.