MEAN-SQUARE RADIUS OF GYRATION OF OLIGO(METHYL METHACRYLATE)S AND POLY(METHYL METHACRYLATE)S IN DILUTE-SOLUTIONS

The mean-square radius of gyration 〈S2〉 was determined by small-angle X-ray scattering and light scattering for 20 samples of atactic oligo- and poly(methyl methacrylate)s (a-PMMA), each with the fraction of racemic diads fr, = 0.79, in the range of weight-average molecular weight Mw from 4.02 × 102 to 2.83 × 106 in acetonitrile at 44.0 °C (θ). The ratio 〈S2〉/xw as a function of the weight-average degree of polymerization, xw, exhibits unusual behavior; it passes through a maximum at xw ≃ 50 before reaching its asymptotic value for large xw. First, a comparison is made of the experimental data with the theoretical values on the basis of three types of the rotational isomeric state model, and it is shown that none of them can explain the observed maximum. Then, it is shown that the helical wormlike (HW) chain theory may well explain the data with the parameter values λ−1 k0= 4.0, λ−1 τ0 = 1.1, λ−1 = 57.9 Å, and ML = 36.3 Å−1, where k0 and τ0 are the curvature and torsion, respectively, of the characteristic helix taken at the minimum of energy, λ−1 is the stiffness parameter, and Ml is the shift factor. The present a-PMMA chain has a larger λ−1 (stiffness) but smaller Kuhn segment length (average chain dimension for large xw) than an atactic polystyrene chain (fr = 0.59), on which a similar study was previously made. In order to illustrate the situation, a picture is given of representative instantaneous contours of HW Monte Carlo chains for the two polymers. © 1990, American Chemical Society. All rights reserved.