EXCLUDED-VOLUME EFFECTS ON THE MEAN-SQUARE RADIUS OF GYRATION OF OLIGOSTYRENES AND POLYSTYRENES IN DILUTE-SOLUTIONS

The mean-square radius of gyration [S2] was determined by small-angle X-ray scattering and light scattering for atactic oligo- and polystyrenes (a-PS) in toluene at 15.0-degrees-C in the range of weight-average molecular weight M(W) from 5.78 X 10(2) to 3.84 x 10(6). The solvent and temperature have been chosen so that the unperturbed dimension [S2]0 of the a-PS chain in that good solvent may coincide with that in cyclohexane at 34.5-degrees-C (THETA) taken as a reference standard. The radius expansion factor as determined from [S2] and [S2]0 decreases extremely rapidly for small M(W) with decreasing M(W) and tends to unity at finite M(W) in contrast to the two-parameter theory prediction. The implication is that there are significant effects of chain stiffness on alpha(S). The results may be quantitatively explained by the Yamakawa-Stockmayer-Shimada theory based on the wormlike and helical wormlike (HW) chain models, with an appropriate value of the excluded-volume strength B along with those of the HW model parameters established from dilute solution properties of a-PS in the unperturbed state. The theory predicts that the effect of chain stiffness on alpha(S) remains rather large even at very large M(W) approximately 10(6) for a-PS. It is also shown that the recent renormalization group theory of alpha(S) by Chen and Noolandi does not work well, yielding a large discrepancy from the present experimental results.