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 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) hydrodynamic radius R(H) (defined from D) in acetone were found to agree with those of the radius R(H,Theta) previously (and presently) obtained in the 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. The hydrodynamic-radius expansion factor alpha(H) was then determined correctly 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 tilde 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 alpha(H) as well as for the gyration-radius and viscosity-radius expansion factors alpha(s) and alpha(eta), irrespective of the large differences in chain stiffness, local conformation, and solvent condition. It is again found that the Barrett equation overestimates alpha(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 alpha(H). This indicates that it requires further theoretical investigations. It is also again found that alpha(H) coincides with alpha(eta), within experimental error over the whole range of M(w) studied.