MEAN-SQUARE OPTICAL ANISOTROPY OF OLIGO(METHYL METHACRYLATE) AND POLY(METHYL METHACRYLATE)S IN DILUTE-SOLUTIONS

The mean-square optical anisotropy [GAMMA2] was determined for 10 samples of atactic oligo- and poly(methyl methacrylate)s (a-PMMA), each with the fraction of racemic diads f(r) = 0.79, in the range of weight-average molecular weight M(w) from 3.02 X 10(2) to 2.95 X 10(3) in acetonitrile at 44.0-degrees-C (theta) and also for methyl isobutyrate (MIB) (the monomer of PMMA) in acetonitrile at 44.0-degrees-C and in carbon tetrachloride at 25.0-degrees-C, from anisotropic light scattering measurements with a photometer equipped with a Fabry-Perot interferometer. The determination was also made for three samples of atactic oligo- and polystyrenes (a-PS) used in the previous study of [GAMMA2] along with cumene in carbon tetrachloride at 25.0-degrees-C in order to compare the present procedure for determining [GAMMA2] with the previous one. A comparison is made of the present data with the helical wormlike (HW) chain theory with the values of the model parameters determined previously from the mean-square radius of gyration [S2] and with the local polarizability tensor alpha0 properly assigned on the basis of the group polarizability tensor for the ester group of the a-PMMA chain evaluated from the polarizability of methyl acetate or MIB. The HW theoretical value of the ratio ([GAMMA2]/x(w))infinity in the limit of weight-average degree of polymerization x(w) --> infinity does not completely agree with the experimental one if such alpha0 is used. This is probably due to the fact that the group polarizability of the ester group may be somewhat altered in the a-PMMA chain. The dependence on x(w) of the observed ratio ([GAMMA2]/x(w))/([GAMMA2]/X(w))infinity may Well be explained by the HW theory, thus leading to the conclusion that the HW model may give a consistent explanation of the behavior of [GAMMA2] and [S2] of a-PMMA. A comparison is also made of the present results for a-PMMA with the previous ones for a-PS; as x(w) is decreased to 3, the above ratio decreases much more steeply for the former.