Excluded-Volume Effects on the Hydrodynamic Radius of Oligo- and Polyisobutylenes in Dilute Solution

The translational diffusion coefficient D was determined from dynamic light scattering measurements for oligo- and polyisobutylenes in isoamyl isovalerate (IAIV) at 25.0 °C (ϑ) and in n -heptane at 25.0 °C in the range of weight-average molecular weight M w from 1.01 × 10 3 to 1.76 × 10 6 . The values of the unperturbed and perturbed hydrodynamic radii R H,ϑ and R H defined from D in IAIV and in n -heptane, respectively, were found to agree with each other in the oligomer region, indicating that the values of R H,ϑ may be adopted as those of the unperturbed hydrodynamic radius R H,0 in n -heptane at 25.0 °C. The values of the hydrodynamic-radius expansion factor α H in n -heptane at 25.0 °C are then obtained as the ratio R H / R H,ϑ from those values thus determined. The data for R H,ϑ are analyzed as usual by the use of the corresponding (unperturbed) helical wormlike (HW) chain theory. The results for α H as a function of the scaled excluded-volume parameter z̃ defined in the Yamakawa−Stockmayer−Shimada theory for the HW chain with excluded volume are consistent with the previous results for atactic polystyrene and poly(dimethylsiloxane). The implication is 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 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 qualitatively explained by the Yamakawa−Yoshizaki theory, which takes account of the possible effect of fluctuating hydrodynamic interaction on α H . It is also again found that α H coincides with α η within experimental error over the whole range of M w studied.