Macrostructure of melt-crystallized vinylidene fluoride/trifluoroethylene copolymers (37/63, 65/35, 73/27 mol%)

Macrostructures of melt-crystallized vinylidene fluoride/trifluoroethylene (37/63), (65/35) and (73/27) mol% copolymers have been studied using a scanning electron microscope (SEM) and small-angle X-ray scattering (SAXS). The SEM images show stacks of lamellar crystals with thicknesses of about 1000 Å. However, experimental SAXS intensities do not show diffraction peaks that are sufficiently distinct to suggest the presence of the lamellar stacks; rather, they decrease almost monotonically as the scattering angle increases. Two structural models are introduced to calculate theoretical SAXS intensity, and the degree of agreement between theoretical and experimental SAXS intensities is examined for each model. The first model is the finite lamellar stack model, which presents a good approximation of the macrostructures shown in the SEM images. However, this model yields poor agreement in higher angular regions, which indicates that the lamellar stack model is not valid for analyzing the experimental SAXS intensities of melt-crystallized copolymers. Thus, a second model is proposed: crystalline blocks cohere and constitute quasi lamellae, which are then stacked. The theoretical SAXS intensity based on the second model gives a satisfactory description of the experimental SAXS intensity.