Enhancement of polymerization rates for rigid rod-like molecules by shearing

Abstract

Rigid, rod-like polymers are of considerable technological importance in the production of high-strength and high-modulus fibres. Polymerization reactions generally require near-parallel orientation of such molecules, and because of the slow rotational diffusion of the reacting species, the rate of polymerization is found to decrease significantly in the later stages of the reaction; this ultimately limits the molecular weight of the polymer. It is known, however, that rod-like molecules can be readily oriented in solution by means of imposed flow fields. Here we examine the effect of a shear flow on the polymerization reaction between p-phenylene diamine and terephthaloyl chloride, where the product, poly(p-phenylene terephthalamide), has a rigid, rod-like structure. We find that once the molecular weight of the reactants exceeds a critical value, shearing of the reaction mixture induces a pronounced orientation of the molecules, accompanied by a significant increase in the polymerization rate. The technological implications are clear: exposure of polymerizing mixtures of rod-like molecules to high shear rates can result in a dramatic enhancement in the molecular weight of the resulting polymer.