Molecular dimensions in relation to intrinsic viscosities

Abstract

Abstract Root‐mean‐square end‐to‐end dimensions ( r 2 ) 1/2 and molecular weights M have been determined from light scattering studies carried out on a series of fractions of poly‐(acrylic acid) in dioxane (θ point, 30°C.), on two polyisobutylene fractions in cyclohexane and in ethyl n ‐caprylate (θ point, 22°C.), and on a single poly‐(methyl methacrylate) fraction in acetone. Intrinsic viscosities of the various polymers measured in these same solvents at the same temperature (30°C.) support the relationship [η] = Φ( r 2 ) 3/2 / M , where Φ is a constant independent of molecular weight and solvent type and (according to the data herein reported) has the approximate value 2.2 × 10 21 . The θ points, representing the temperatures at which ideal behavior prevails, were determined for the two poor solvent–polymer systems by extrapolation of critical consolute temperatures to M = ∞. The intrinsic viscosity of poly‐(acrylic acid) in dioxane at 30°C. (the θ point) is given by [η] = 0.85 × 10 −3 M 1/2 ; the value of the constant approximates those for nonpolar polymers. The influence of molecular weight heterogeneity on the fundamental relationship of the intrinsic viscosity to ( r 2 3/2 / M ) is analyzed, and it is shown that the true value of the universal parameter Φ will be greater than the apparent value deduced in the foregoing manner without correction for the heterogeneity of the polymer fraction. For a fraction possessing a relatively sharp distribution as denoted by 〈 M 〉 w /〈 M 〉 n = 1.15. the magnitude of this correction may be as high as 20%.