Crystallinity of Polymers and the Means to Influence the Crystallization Process

Abstract

One of the most important factors determining the properties of a solid polymer is the packing order. Crystallization is a means to control the degree and morphology of packing. There is a great difference between "free” and “induced” crystallization.
The “free” crystallization leads to quasi-isotropic products, which are strong and tough if the crystallites are small. This way of crystallization is rather well understood so that we are able to predict, with a fair amount of accuracy, the rate of crystallization and the average size of the spherulites. In spherulites the molecular chains are normally folded.
It is possible to carry out the crystallization in such a way that the molecular chains are partly extended.
Pressure-induced crystallization leads to extended chains, but the crystallites lie in random directions so that the products are very brittle.
Stress-induced crystallization leads to a completely different type of morphology: the micro-flbrillar structure with a row-nucleated core. Shear forces are far less important than tensile stresses. The rate of nucleation in this type of crystallization is some orders of magnitude higher than in spherulitic crystallization. Extremely high moduli can be obtained by superdrawing and similar techniques. These have no practical importance yet.
An interesting new phenomenon connected with stress-induced crystallization is the hard elastic fibre.