Atomic force microscopy on tree-like crystals in polyvinylidene fluoride blends


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Abstract

Polyvinylidene fluoride (PVDF), in homogeneous blends with an amorphous polymer under specific conditions, forms crystals with a tree-like architecture. Tree-like crystals have a loose fractal structure of crystalline branches which are impregnated with the amorphous polymer. In a previous study, this spectacular morphology was investigated in blends of PVDF and a random copolymer of styrene and methylmethacrylate (SMMA, 13% wt/wt styrene) by light microscopy. The crystal growth was interpreted in terms of a rate competition of crystallization and interdiffusion. The submicroscopic structure of tree-like PVDF spherulites has now been characterized by atomic force microscopy. Surface profiles were prepared by etching PVDF/SMMA blend films with toluene. The tree-like spherulites proved to be less disordered than expected. Their core consists of a compact array of lamellae, the planes of which are periodically twisted, producing a ring pattern. Near the crystallizing front, branches grow, separated from each other, into the amorphous phase. They are internally as compact as the core, but are covered by a loose fur of disordered lamellae. The tree-like appearance of these crystals comes from frequent splitting of the branches into twigs. Crystallographic bifurcation as in dendritic crystals was not observed. At interfaces between two tree-like crystals, the lamellae form patterns of backfolding.

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