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

Abstract

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 Mw from 1.01 × 103 to 1.76 × 106. The values of the unperturbed and perturbed hydrodynamic radii RH,ϑ and RH 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 RH,ϑ may be adopted as those of the unperturbed hydrodynamic radius RH,0 in n-heptane at 25.0 °C. The values of the hydrodynamic-radius expansion factor αH in n-heptane at 25.0 °C are then obtained as the ratio RH/RH,ϑ from those values thus determined. The data for RH,ϑ are analyzed as usual by the use of the corresponding (unperturbed) helical wormlike (HW) chain theory. The results for αH 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 αH as well as for the gyration-radius and viscosity-radius expansion factors αS and αη irrespective of the differences in chain stiffness, local conformation, and solvent condition. It is again found that the Barrett equation overestimates αH. 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 αH. It is also again found that αH coincides with αη within experimental error over the whole range of Mw studied.