Theory of flow-induced fibril formation in polymer solutions

Abstract
A treatment of the formation of a basic core fibril (shish) of the type that is generated by flow-induced crystallization of a polymer from solution is given that features the concept of cumulative strain. Multiple nucleation acts by flow-elongated molecules produce an embryonic fibril that is a connected set of bundlelike nuclei. Surface stress resulting from repulsion of the quasi-random coil chains in the amorphous zone between the nuclei or crystallites builds up at the bundle ends as the nuclei mature, leading ultimately to a high end surface free energy, and to volume strain in the crystallites comprising the core fibril. The theory leads to a stable (or metastable) fibril diameter a s and mean characteristic length l s with a fixed axial ratio, and predicts why the diameter does not grow further even in a medium that is supersaturated with polymer. The predicted dependence of a s, l s , and the axial ratio, on undercooling is in approximate agreement with experiment. The lattice expansion in the crystal resulting from volume strain is also in fair accord with experiment. The effect of annealing, including the commonly encountered case where the volume strain relaxes to give normal lattice dimensions, but with a high end surface energy still remaining, is noted. The effect of volume strain and the distribution of core fibril lengths about l s on the melting behavior is calculated. The theory can reproduce crystallinity versus temperature data on polyethylene fibrils. This procedure yields an independent value of l s . The overall treatment implies that the core fibril is a set of concatenated and substantially extended-chain crystallites with bundlelike ends and a somewhat expanded lattice when unannealed and under tension, the molecular connections between the crystallites consisting of short amorphous ciliary bridges. It is suggested that prolonged annealing at high temperatures can remove a substantial number of the amorphous zones.

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