Development of structural hierarchy in injection-molded PVDF and PVDF/PMMA blends: part III. Spatial variation of superstructure as detected by small-angle light scattering

The shape and size of the superstructure developed in spatial structural gradients formed in the injection-molded PVDF/atactic-PMMA blends were studied using small-angle laser light scattering technique. The surface layers formed due to extensional flow exhibited an unusual two-point SALS patterns when viewed in Hv mode. At high mold temperature (especially with the 70/30 PVDF/PMMA sample due to even lower nucleation density), the region between skin and shear zone that formed under relatively low stresses exhibit low nucleation density resulting in spherulitic superstructure. The inner portions of the shear layer and the core were found to form prolate spheroidal spherulites with short axis oriented in the flow direction. The aspect ratios of these spherulites are about 1.1–1.14 and this anisotropy in shape was found even at the mid-plane of the samples indicating the persistence of the influence of flow on structure at such depths. The results indicate that the mold temperature has the most influence on the evolution of superstructure in the parts as compared to the injection speed though these two parameters are not completely independent of each other. The increase of PMMA concentration drastically reduces the size of the spherulites in all depths in the molded parts. This behavior was attributed to the reduction in crystallizability particularly to the decrease of chain diffusion with the addition of stiffer non-crystallizable PMMA chains