Manipulation Coupling Fields of Shear Flow, Pressure, and Temperature for Polymorphism Structuring in Poly(vinylidene fluoride) Films

The crystallization behavior is inevitably influenced by complex and coupled processing fields, particularly shear flow, pressure, and temperature fields. However, the combined effects of these fields on the crystalline structure and electrical properties of poly(vinylidene fluoride) (PVDF) remain unclear. In this work, a modified pressuring and shearing device (MPSD) is developed to systematically investigate PVDF crystallization under well-controlled pressure, shear flow, and temperature fields and to correlate its hierarchical structure with dielectric performance. Our results reveal that pressure and shear fields synergistically promote oriented β/γ phases, in contrast to the static condition where only nonpolar α phase is typically formed. This enhancement is attributed to shear-induced molecular orientation coupled with pressure-suppressed relaxation of the aligned chains, which together stabilize locally ordered trans (T) conformations and ultimately facilitate the formation of oriented polar phases. Under a shear temperature of 190 °C and a pressure of 100 MPa, increasing the shear rate up to 131 s –1 results in a maximum β/γ phase content (up to 91%), along with increased lamellar thickness and higher melting temperatures (∼181 °C), indicating significantly improved crystalline perfection and thermal stability. Notably, the formation of shear-induced β and γ phases strongly depends on the degree of molecular orientation, which is governed by the shear rate. Higher shear rates facilitate longer trans-conformational sequences, which favor the nucleation and growth of the β phase. At a shear rate of 131 s –1, the β phase content reaches 68%. The presence of the oriented β phase enhances the dielectric properties of PVDF films, increasing the dielectric constant and reducing dielectric loss. This study provides valuable insights into the processing–structure–property relationship of PVDF and offers a practical pathway for tailoring its performance through controlled processing.