Effect of constrained annealing on the structural and dielectric properties of melt cast PVDF/PMMA films

In this study, the relationship between structural hierarchy in PVDF/PMMA blends as altered by melt casting and annealing and electrical properties was investigated. PVDF was blended with PMMA in three crystallizable compositions: 50/50, 60/40 and 70/30 of PVDF/PMMA using twin screw extrusion followed by film casting. The films were characterized structurally through offline birefringence measurements as well as WAXS, SAXS, IR Dichroism and DSC to understand the processing induced structural changes and their effect on electrical properties. The addition of PMMA to PVDF suppressed crystalization during casting of films. This opened a low temperature film deformation window between T-g and cold crystallization temperatures allowing for development of high preferential chain orientation in the films. Crystallinity in cast films was shown to correlate directly with breakdown strength. Systemic annealing experiments were carried out to enhance crystallinity and the effect of annealing induced structural changes on the dielectric properties were studied. It was found that upon annealing, the amorphous PMMA tends to undergo relaxation that is detrimental to the breakdown properties. As annealing progresses, the mismatch in the kinetics between crystallization/recrystallization and the amorphous relaxation causes density gradients within the bulk of the films and dictate the overall orientation levels in the films, as well as have implications on the dielectric properties.