Excluded-Volume Effects on the Mean-Square Radius of Gyration of Oligo- and Polyisobutylenes in Dilute Solution

The mean-square radius of gyration 〈 S 2 〉 was determined from small-angle X-ray scattering and light scattering (LS) measurements for oligo- and polyisobutylenes (PIB) in n -heptane at 25.0 °C in the range of the weight-average degree of polymerization x w from 1.14 × 10 to 2.48 × 10 3 . The model parameters of the helical wormlike (HW) chain for PIB are then determined by analyzing simultaneously the present data for 〈 S 2 〉/ x w and the previous data obtained from LS measurements for samples with large x w in the same solvent condition and also in isoamyl isovalerate at 25.0 °C (ϑ), by the use of the HW theories with and without the excluded-volume effect. With the parameter values so determined for PIB, the plots of the viscosity- and hydrodynamic-radius expansion factors α η and α H against the scaled excluded-volume parameter z̃ for PIB along with those for atactic polystyrene and atactic and isotactic poly(methyl methacrylate)s are shown to form a single-composite curve better than before in each case, confirming the validity of the quasi-two-parameter scheme. The improvement of the consistency of the results for PIB with those for the other polymers arises from the re-evaluation of z̃ with the present values of the model parameters directly determined from 〈 S 2 〉.