Reversible secondary crystallization during cooling of polypropylene

Ultra-violet microscopy has been used to follow the distribution of fluorescent additives during the isothermal crystallization and cooling of polypropylene. During cooling from the crystallization temperature there is a flow of additives away from the spherulite centres and into the spherulite boundaries. This demonstrates a marked densification at the centre of the spherulites with less within the bulk and only a small increase at the boundary. This change reverses when the sample is reheated to the crystallization temperature. Thus spherulites which appear to be quite uniform in crystallinity at high temperatures become non-uniform on cooling. The spherulites also become fibrous in appearance when cool. These effects are explained in terms of non-uniform concentrations of poorly crystallizable polymeric species which retard secondary crystallization.