Tailoring Electroactive β- and γ-Phases via Synthesis in the Nascent Poly(vinylidene fluoride) Homopolymers

Poly(vinylidene fluoride) (PVDF) has received much attention for its electroactive properties arising from β- and γ-crystallographic phases. Until now, investigation deals with postpolymerization processing methods to secure β- and γ-polymorphs, although a deeper understanding of the promotion of β- and γ-phases during the polymerization of vinylidene fluoride (VDF) is scarcely reported. This study scrutinizes the polymerization conditions and demonstrates how polymerization temperatures, times, and molar masses of the “nascent PVDF powder” affect the formation of the crystalline phases (α, β, and γ). It is presented that specific molar masses produced at various polymerization temperatures strongly control PVDF polymorphs and play a pivotal role in electroactive phase formation. The molar masses ranging from 100,000 to 350,000 g·mol^-1 and polymerization temperatures ≤60 °C result in β- and γ-phases, while the molar mass ≥450,000 g·mol^-1 at 70 °C polymerization temperature promotes α- and β-phases of PVDF. Additionally, PVDF chain defects also influence the electroactive phase formations; the lower the chain defects, the higher the β-phase content in PVDF. The polymorphs of PVDF homopolymers are identified by complementary spectroscopy and diffraction techniques, whereas the morphological changes influenced by the polymerization conditions are followed by electron microscopy. The morphological variation, from facet to spherical morphology, is correlated with the polymerization conditions. Finally, the preliminary results on the processing method demonstrate that the nascent PVDF homopolymers can be processed in the solid state, below their melting temperature, without influencing the polymorph perceived during polymerization. The nascent polymer having the β-polymorph retains its crystallographic and conformational structure prior to melting. The transformation from α to β and γ PVDF is accomplished through solid-state processing (compression-rolling-stretching) below the melting temperature of the nascent PVDF homopolymers. Compared to melt and solution processing, the solid-state processing is of advantage as it circumvents the work required against entropy to secure the oriented chains and the use of carcinogenic solvents. Moreover, environmentally friendly solid-state processing avoids the decomposition of the sample above the melting point that often causes the release of the hazardous gases in fluorinated polymers.