Effect of Industrial Processing on the Morphology of Crystalline Polymers

Our understanding of crystailinity in polymers was greatly advanced by the discovery of the folded-chain lamella as the unit of structure in crystallization both from solution and the melt. Subsequently, it was found that the organization of lamellae into larger units is dependent on the nature of the nuclei initiating crystallization. Under quiescent conditions, nuclei are randomly distributed throughout the crystallizing volume and growth proceeds independent of neighboring structures. In the presence of shearing stress, either in a melt or in solution, nuclei are not randomly distributed but appear along "rows". However, what seems to be a row of nuclei actually is a special kind of nucleus, a fibril of oriented molecular chains, about 300 A in diameter. The presence of these fibril nuclei determines whether the polymer will crystallize spherulitically or in a "row structure" morphology. The type of morphology, in turn, influences many of the physical properties, particularly the stress-strain relationship. Industrial processes in which "row structures" play a major role by virtue of crystallization under melt shear stress include blown film, injection molding and fiber spinning.