Epitaxial nucleation of crystallization at polymer-filler interfaces

In polymer composites the interaction between polymer matrix and filler particles often results in nucleation of spherulites. The principles of polymer crystal nucleation and spherulite growth are investigated using scanning transmission electron microscopy (STEM) and microdiffraction techniques in combination with polarized light microscopy. Simultaneous diffraction patterns from the interface of the filler and the polymer were obtained. Special precautions for successful recording of the diffraction patterns were used to overcome the rapid loss of polymer crystallinity, resulting from electron beam damage. Analysis of the diffraction patterns has shown that partial epitaxial correlation between the atomic periodicity of the particle surface and the molecular periodicity of polymer chains is always present when spherulites are nucleated. STEM images show that only large particles, with well developed facets (cleavage planes), are nucleating. The nucleating efficiency of the filler is therefore dependent on the size as well as on the crystallographic orientation of the facet. Small particles, or those with no suitable facets, do not affect the crystalline structure of the polymer. It is also shown that anisotropic polymer structures can be formed by inhomogeneous dispersion of nucleating filler particles.