Mean-Square Optical Anisotropy of Isotactic Oligo- and Poly(methyl methacrylate)s in Dilute Solution

Abstract

The mean-square optical anisotropy was determined from anisotropic light-scattering measurements for 10 samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMMA), each with the fraction of racemic diads f r ≃ 0.01, in the range of weight-average molecular weight from 3.58 x 10 2 (trimer) to 1.07 x 10 4 , and also for methyl isobutyrate (the monomer of PMMA), in acetonitrile at 28.0 °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 the local polarizability tensor properly assigned to the repeat unit of the chain in the same manner as in the previous study of atactic (a-) PMMA, i.e., evaluated with the same bond polarizabilities. It is shown that the HW theoretical values may reproduce quantitatively the experimental results in the range of weight-average degree of polymerization x w ≥ 6. The disagreement between theory and experiment for x w ≤5 may probably be due to effects of chain ends. A comparison is also made of the present results with the previous ones for a-PMMA, leading to the conclusion that the HW theory may explain the f r dependence of as well as of , the scattering function, and steady-state transport coefficients.