Formation of spherulites during polymerization of lactams

Abstract

Abstract The mechanism of the crystal growth of nylon in the course of alkaline polymerization of lactams was studied in terms of kinetics and morphologies of the resultant crystals. It was found that in the bulk polymerization of α‐pyrrolidone, the resultant polymer precipitated in the form of spherulites. Spherulites formed in the early stage of polymerization were swollen, soft balls in which thin lamellar crystallites were radiating along the spherulitic radius. It is considered that the growing chain molecules, when they attained a critical length beyond which they lost the solubility in the system, will precipitate into the form of lamellae in which molecules are aligned perpendicularly to the lamellar surface. Such spherulites matured as the polymerization proceeded. In the early stage of the polymerization, the mutual coupling of the swollen spherulites occurred. In the subsequent stage, the vacant space was occupied by the product of further polymerization, so that the coupled spherulites grew into a sphere again. On the other hand, the propagation of the chains in the individual spherulites made the spherulites compact. It is postulated that the chains grown from the surface of the lamellar nuclei, precipitate in the interstices between the lamellae and a part of these chains will form molecular ties between the lamellae giving rise to compact spherulites. The decrease in the rate of polymerization observed may be attributable to the occlusion of the growing chain ends into the precipitates. The impurity in the system, i.e. , water, seems to affect the rate of the monomer addition, but does not the morphology. Spherulites were also grown in the case of nylon 6, the polymerization of ϵ‐caprolactam.