Dilute Solution Theory of Polymer Crystal Growth: Fractionation Effects

A nonequilibrium (kinetic) theory of polymer molecular weight (MW) fractionation is formulated and applied to binary and multicomponent polyethylenes crystallized from unstirred xylene solutions. High MW components crystallize more readily than do low MW components. This fractionation effect is enhanced as the crystallization temperature is raised. At low crystallization temperatures (high undercoolings) the polymer tends to fractionate according to the volume fraction distribution of its MW components and thus, the number average MW of the crystal that is formed is approximately equal to the weight average MW of the polymer in solution. It is shown that MW fractionation does not depend on the details of nucleation. but rather on the rates for post-nucleation crystal growth. The effects of MW polydispersity on crystalline properties is considered and in particular it is shown that polydispersity tends to mask the intrinsic dependence of crystal thickness on MW. The variables which govern and influence fractionation are also discussed.