Polar phase formation and competition in the melt crystallization of poly (vinylidene fluoride) containing an ionic liquid

•Polar β- and γ-forms are induced by an ionic liquid in melt crystallization of PVDF.•There exists a competition between the β- and γ-form.•The γ-form is rapidly and directly produced from melt.

Melt crystallization of poly (vinylidene fluoride) (PVDF) usually produces nonpolar α-form, and the generation of polar β- and γ-forms is very rare. This study demonstrates that the presence of an ionic liquid in the PVDF significantly suppresses the α-form but promotes the β- and/or γ-form upon melt crystallization. Moreover, there exists a competition between the β- and γ-form, depending on the ionic liquid loading and cooling rate (crystallization temperature). Increasing the ionic liquid loading or cooling rate benefits the β-form with TTTT conformation but reduces the γ-form with TTTGTTTG′ conformation, and vice versa. It is highly expected that trans sequences induced by the ionic liquid as well as conformational rearrangements in the melt should be responsible for it. In addition, conformational ordering in the melt should account for rapid and direct formation of the γ-form at relatively low crystallization temperatures.