Decisive Role\nof the Specific Nanosized Secondary\nCrystals on the Phase Transition of Poly(vinylidene fluoride) Induced\nby Melt Memory

Nanosized crystals induced by secondary crystallization\nare common\noccurrences in semicrystalline polymers. However, their significance\nis often overlooked due to their small proportion and the difficulty\nin distinguishing them from primary crystals (PCs). This study selected\npoly(vinylidene fluoride) (PVDF) as a model system. We distinguished\nthe thickness of the SCs from that of the PCs based on small-angle\nX-ray scattering data and further built the relationship between the\nmelt memory of α-PVDF materials and the crystallinity of nanosized\nα-phase SCs by adjusting the end crystallization temperature.\nNotably, these nano α-SCs exhibit a remarkable role in transitioning\nfrom TGTG′ to T<sub>3</sub>GT<sub>3</sub>G′ conformation\nduring their partial melting and subsequent cooling process. The conformational\ntransition stemming from the partial melting of the SCs effectively\npromotes the transition from the nonpolar α-phase to the polar\nγ-phase. The observed phase transition can be attributed to\nthe matching between the melting thermal dynamics of α-SCs and\nthe thermal dynamics of the conformational transition within amorphous\nchains. Our findings underscore the significant effect of moderate\nnanosized SCs on the phase transformation and introduce a practical\nand effective methodology for producing polar-phase PVDF materials.