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 fibres¹. Polymerization reactions generally require near-parallel orientation of such molecules, and because of the slow rotational diffusion of the reacting species², 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 structure¹. 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.