Intermediate-Angle Neutron and X-ray Scattering Functions of Poly(methyl methacrylate) Chains

Abstract

The molecular scattering functions of isotactic, syndiotactic, and atactic poly(methyl methacrylate) (PMMA) chains have been calculated according to the three different rotational isomeric state (RIS) models in the literature and compared with experiments. Comparison with the neutron scattering experiments on glassy (atactic) PMMA shows that only the most rigorous six-state model predicts all the features exhibited by the experimental results over the entire scattering vector q (=(4π/λ) sin (θ/2)) to ca. 0.6 Å-1. In particular, the two-state and the three-state models predict the occurrence of a second maximum in the absolute-scale Kratky plot at values of q that are considerably smaller than the experimental results. In contrast, the scattering function calculated in the framework of the six-state model shows the two maxima and the intervening minimum in the same locations of q as those in the experimental curve. The six-state RIS model also predicts the molecular scattering functions in good agreement with the available neutron scattering results on the isotactic and syndiotactic PMMA in the bulk, up to q ≃ 0.25 Å-1 as well as the X-ray scattering experiments on syndiotactic PMMA in solution up to q ≃ 0.7 Å-1. Therefore, our results show that even in the case of a complicated looking polymer like PMMA the chain conformations in the bulk amorphous state correspond very closely to the unperturbed random coils, not only for the overall chain configuration but also for local chain segments constituted by a few units. © 1990, American Chemical Society. All rights reserved.