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

Potyvinylidene fluoride (PVDF), in homogeneous blends with an amorphous pol ymer under specific conditions, forms crystals with a tree-like architectur e. Tree-like crystals have a loose fractal structure of crystalline branche s which are impregnated with the amorphous polymer. In a previous study, th is spectacular morphology was investigated in blends of PVDF and a random c opolymer of styrene and methylmethacrylate (SMMA, 13% wt/wt styrene) by lig ht microscopy. The crystal growth was interpreted in terms of a rate compet ition of crystallization and interdiffusion. The submicroscopic structure o f tree-like PVDF spherulites has now been characterized by atomic force mic roscopy. Surface profiles were prepared by etching PVDF/SMMA blend films wi th toluene. The tree-like spherulites proved to be less disordered than exp ected. Their core consists of a compact array of lamellae, the planes of wh ich are periodically twisted, producing a ring pattern. Near the crystalliz ing front, branches grow, separated from each other, into the amorphous pha se. 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 come s from frequent splitting of the branches into twigs. Crystallographic bifu rcation as in dendritic crystals was not observed. At interfaces between tw o tree-like crystals, the lamellae form patterns of backfolding. (C) 1998 C hapman & Hall.