Excluded-Volume Effects on the Hydrodynamic Radius of Atactic and Isotactic Oligo- 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-) oligo- and poly(methyl methacrylate)s (PMMA) in acetone at 25.0 °C in the range of weight-average molecular weight M w from 1.83 × 10 3 to 9.52 × 10 5 for the former and from 3.04 × 10 3 to 9.78 × 10 5 for the latter. For each PMMA, the values of the (perturbed) hydrodynamic radius R H (defined from D ) in acetone were found to agree with those of the radius R H,ϑ previously (and presently) obtained in the unperturbed (ϑ) state (i.e., in acetonitrile at 44.0 °C for a-PMMA and at 28.0 °C for i-PMMA) in the oligomer region. The hydrodynamic-radius expansion factor α H was then determined correctly from the values of R H and R H,ϑ . The results for α H as a function of the scaled excluded-volume parameter z̃ defined in the Yamakawa−Stockmayer−Shimada theory for the helical wormlike chain with excluded volume are consistent with the previous results for atactic polystyrene, poly(dimethylsiloxane), and polyisobutylene. This leads to the conclusion that the quasi-two-parameter scheme may be valid for α H as well as for the gyration-radius and viscosity-radius expansion factors α S and α η irrespective of the large differences in chain stiffness, local conformation, and solvent condition. It is again found that the Barrett equation overestimates α H . This disagreement between theory and experiment may be explained only semiquantitatively by the Yamakawa−Yoshizaki theory, which takes account of the possible effect of fluctuating hydrodynamic interaction on α H . This indicates that it requires further theoretical investigations. It is also again found that α H coincides with α η within experimental error over the whole range of M w studied.