Mean-Square Optical Anisotropy of Oligo- and Poly(α-methylstyrene)s in Dilute Solution

Abstract

The mean-square optical anisotropy 〈Γ2〉 was determined from anisotropic light scattering measurements for 10 samples of atactic oligo- and poly(α-methylstyrene)s (a-PαMS) with the fraction of racemic diads fr = 0.72 in the range of weight-average degree of polymerization xw from 2 to 67.1 in cyclohexane at 30.5 °C (ϑ). A comparison is made of the present data with the helical wormlike (HW) chain theory with the values of the model parameters previously determined from the mean-square radius of gyration and with those of the local polarizability tensor α0 per repeat unit properly assigned on the basis of the bond and group polarizabilities. The HW theory value of 〈Γ2〉/xw in the limit of xw → ∞ is found to be somewhat larger than the experimental one. It is then shown that if α0 is replaced by 0.90α0, the HW theory may reproduce quantitatively the experimental results for all the samples except those with xw ≤ 4 for which effects of chain ends become appreciable. A comparison is also made of the present results for a-PαMS with previous ones for atactic polystyrene (fr = 0.59), atactic poly(methyl methacrylate) (a-PMMA) (fr = 0.79), isotactic PMMA (fr ≃ 0.01), and poly(n-hexyl isocyanate), and the behavior of 〈Γ2〉 is shown to depend remarkably on chain stiffness and local chain conformation.