Dual Role of Ionic Liquid in High-Pressure Formation of Extended-Chain Poly(vinylidene fluoride) Crystals: Interplay between Thermodynamics and Kinetics

Exploring milder high-pressure processing conditions is essential for developing heat-resistant electroactive poly(vinylidene fluoride) (PVDF) materials based on polar extended-chain crystals (ECCs). Herein, we introduced a typical imidazole ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate, to manipulate the melt crystallization of PVDF under high pressure. Our findings reveal a dual role of IL in ECCs formation through various structural and morphological characterizations. First, it increases the free energy of growing nuclei by raising the end surface free energy σe, making ECCs formation thermodynamically unfavorable. Second, it facilitates chain sliding diffusion and lowers the diffusion activation energy during nuclei growth, which promotes ECCs generation in kinetics. These two effects compete with each other, influenced by crystallization conditions and IL content. Kinetic promotion prevails at low crystallization temperatures (Tp) and low IL fractions, while thermodynamic inhibition dominates at high Tp and/or high IL fractions. This interplay is illustrated by high-pressure phase diagrams of PVDF/IL blends. A low IL fraction (e.g., 0.2%) significantly expands the ECCs growth region into lower Tp and pressure conditions, while a higher IL fraction has the opposite effect. This knowledge enables the rapid preparation of abundant ECCs with minimal IL addition at milder low temperatures, thereby minimizing sample degradation. Furthermore, the IL-induced ECCs primarily consist of the polar β-phase and maintain a high melting point. This study elucidates the high-pressure crystallization mechanism of PVDF/IL blends and proposes an effective strategy for enhancing the production of polar ECCs in PVDF homopolymer.