EXCLUDED-VOLUME EFFECTS ON THE HYDRODYNAMIC RADIUS OF ATACTIC AND ISOTACTIC OLIGO(METHYL METHACRYLATE)S AND POLY(METHYL METHACRYLATE)S IN DILUTE-SOLUTION

The translational diffusion coefficient D was determined from dynamic light scattering measurements for atactic (a-) and isotactic (i-) olig o- and poly(methyl methacrylate)s (PMMA) in acetone at 25.0 degrees C in the range of weight-average molecular weight M(w) from 1.83 x 10(3) to 9.52 x 10(5) for the former and from 3.04 x 10(3) to 9.78 x 10(5) for the latter. For each PMMA, the values of the (perturbed) hydrodyna mic radius R(H) (defined from D) in acetone were found to agree with t hose of the radius R(H,Theta) previously (and presently) obtained in t he unperturbed (Theta) state (i.e., in acetonitrile at 44.0 degrees C for a-PMMA and at 28.0 degrees C for i-PMMA) in the oligomer region. T he hydrodynamic-radius expansion factor alpha(H) was then determined c orrectly from the values of R(H) and R(H,Theta). The results for alpha (H) as a function of the scaled excluded-volume parameter (z) over til de defined in the Yamakawa-Stockmayer-Shimada theory for the helical w ormlike chain with excluded volume are consistent with the previous re sults for atactic polystyrene, poly(dimethylsiloxane), and polyisobuty lene. This leads to the conclusion that the quasi-two-parameter scheme may be valid for alpha(H) as well as for the gyration-radius and visc osity-radius expansion factors alpha(s) and alpha(eta), irrespective o f the large differences in chain stiffness, local conformation, and so lvent condition. It is again found that the Barrett equation overestim ates alpha(H). This disagreement between theory and experiment may be explained only semiquantitatively by the Yamakawa-Yoshizaki theory, wh ich takes account of the possible effect of fluctuating hydrodynamic i nteraction on alpha(H). This indicates that it requires further theore tical investigations. It is also again found that alpha(H) coincides w ith alpha(eta), within experimental error over the whole range of M(w) studied.