Effect of Polymolecularity on Deformation of Butyl Polymers

Abstract

Abstract A large quantity of a Butyl polymer was fractionated in 100-gram portions into nine relatively narrow molecular weight fractions. The respective fractions from each batch fractionation were combined in solution to obtain sufficient working samples for two types of deformation tests. The fractionation was so designed that the middle fraction of the nine was used as a building block for a series of polymer fractional blends representing equal average molecular weight with gradually increasing molecular weight distribution. The separate fractions and blends of varying distribution were studied in deformation tests involving conditions of both constant rate of deformation and constant load. Under a constant deformation, at a given rate of compression, and over a temperature range of 40° to 130° C, it was shown that species of very high molecular weight (500,000 viscosity-average molecular weight) require temperatures in excess of 130° C for a transformation to a highly plastic state from one highly elastic. Species of intermediate and low molecular weight (300,000 to 100,000 viscosity-average molecular weight) require progressively lower temperatures for this transformation. Deformations at constant rate, with polymers of constant viscosity-average molecular weight, show the material of narrower molecular weight spread to be the more thermoplastic. A corollary also exists with elasticity; the narrower distribution yields a greater decrease in elasticity when the temperature is increased. Therefore, it can be postulated from the standpoint of equal processing qualities that a narrower distribution of molecular weight can tolerate a higher average molecular weight. A method has been developed for isolating the elastic and plastic or viscous components of deformation under constant load. Under specific conditions of constant load at 40° C, variations in molecular-weight distribution for a given average molecular weight result in variations in high elastic deformation, while the viscous component remains constant. Thus, increasing the molecular-weight distribution for a given viscosity-average molecular weight tends to yield a softer, more elastic polymer. Since the viscous component of deformation is independent of molecular weight distribution and depends only on average molecular weight, the constant load deformation test described yields a rapid method of estimating average molecular weight. Rate of viscous flow, which is independent of time if precautions and limitations are observed in the testing procedures, has been determined for individual fractions as well as for blends of varying molecular weight distribution. The logarithm of flow rate is linearly related to the square root of the viscosity-average molecular weight, as suggested by Flory's viscosity equation.