THEORY OF NUCLEATION AND CRYSTAL GROWTH OF POLYMERS IN CONCENTRATED SOLUTIONS

ABSTRACT The process of crystallization in concentrated solutions depends strongly on the rate of crystallization and the rate of long range diffusion of the polymer chains. If the crystallization proceeds slowly compared to diffusion, this type of crystal nucleus will be formed for which the free energy of nucleation is smallest. By taking into account entropy effects which are characteristic for chain molecules one can show that the crystal with smallest free energy of nucleation is a crystal with almost regular chain folds on the surfaces. The influence of supercooling, concentration and molecular weight on the thickness and growth rate of such crystals is discussed. If the crystallization proceeds rapidly compared to the large range diffusion each part of the chain will crystallize as far as possible at the same place where it is lying in the solution. The nucleation process is not governed by thermodynamics. The type of crystal which is formed depends in this case on the concentration and the order of the chains in solution. A single parameter has to be introduced in the theory: the probability p that a new molecule is incorporated into the crystal. If p is large, crystals with loose loops with comparatively large end-to-end distances will be formed. With decreasing p the case of short regular folds with adjacent reentry is approached. In dilute solutions small values of p can be expected. With increasing concentration p increases, provided that the polymer molecules penetrate each other