Effect of ion-dipole interaction on the formation of polar extended-chain crystals in high pressure-crystallized poly(vinylidene fluoride)

Abstract

The manipulation of crystal superstructure and polymorphism of poly(vinylidene fluoride) (PVDF) are of great technical importance to expand its applications in flexible electronics. In this work, strong ion-dipole interaction via the introduction of cetyltrimethylammonium bromide (CTAB) in PVDF induced zig-zag conformers [i.e., the precursor nuclei of β/γ folded chain crystals (FCCs)]. Meanwhile, favorable conditions (i.e., high temperature and high pressure) for the pseudohexagonal (hex) paraelectric (PE) phase with gauche conformers [i.e., the precursor nuclei of extended-chain crystals (ECCs)] are purposely chosen to study whether there is any synergy between the ion-dipole interaction and ECCs. It is interesting to find that the high pressure/high temperature hex PE phase can be substantially affected by the strong ion-dipole interaction between PVDF and CTAB. For the neat PVDF melt, most hexagonal folded-chain (h-FC) nuclei ultimately lead to major β/γ ECCs. For the PVDF melt with 5 wt% CTAB, the β/γ FC nuclei induced by the strong ion-dipole interaction compete with the h-FC nuclei at the initial nucleation stage. The β/γ FC nuclei are found to be more kinetically favorable in the beginning under a high pressure, leading to major β/γ FCCs and minor β/γ ECCs. Furthermore, compared to neat PVDF, addition of 5 wt% CTAB resulted in ECCs of a thinner lamellar thickness. Finally, this new understanding leads to a modified T-P phase diagram, in which the kinetically favorable area for α/β/γ FC nucleation expands to higher temperatures because of the strong ion-dipole interaction.