Decisive Role of the Specific Nanosized Secondary Crystals on the Phase Transition of Poly(vinylidene fluoride) Induced by Melt Memory

Nanosized crystals induced by secondary crystallization are common occurrences in semicrystalline polymers. However, their significance is often overlooked due to their small proportion and the difficulty in distinguishing them from primary crystals (PCs). This study selected poly(vinylidene fluoride) (PVDF) as a model system. We distinguished the thickness of the SCs from that of the PCs based on small-angle X-ray scattering data and further built the relationship between the melt memory of α-PVDF materials and the crystallinity of nanosized α-phase SCs by adjusting the end crystallization temperature. Notably, these nano α-SCs exhibit a remarkable role in transitioning from TGTG′ to T3GT3G′ conformation during their partial melting and subsequent cooling process. The conformational transition stemming from the partial melting of the SCs effectively promotes the transition from the nonpolar α-phase to the polar γ-phase. The observed phase transition can be attributed to the matching between the melting thermal dynamics of α-SCs and the thermal dynamics of the conformational transition within amorphous chains. Our findings underscore the significant effect of moderate nanosized SCs on the phase transformation and introduce a practical and effective methodology for producing polar-phase PVDF materials.