Influence of lithium salt-induced phase separation on thermal behaviors of poly(vinylidene fluoride)/ionic liquid gels and pore/void formation by competition with crystallization

The thermal behavior of poly(vinylidene fluoride)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide/lithium bis(trifluoromethylsulfonyl)amide (PVDF/[C(2)mim][TFSA]/LiTFSA) gels, prepared by cooling from the hot solution, was investigated with various concentrations of LiTFSA (C(LiTFSA)). The peak melting temperature (T(m)) of the gels shifted toward higher temperatures with increased C(LiTFSA). However, the thickness of lamellar crystal was found to decrease with the increase in C(LiTFSA), which meant that the increase in T(m) was not caused by the thickening of lamellar crystal. Furthermore, we found the appearance of domains above T(m) in the high C(LiTFSA) region (≥20 wt%), which was a lithium ion-rich phase caused by the phase separation. Therefore, it is considered on the basis of Nishi–Wang equation that an increase in the interaction parameter with increasing C(LiTFSA) toward the phase separation increased the T(m). The phase-separated domains competed with the subsequent crystallization, which resulted in the formation of micrometer-sized pores and nanometer-sized voids in the spherulites. Spectral measurements revealed that PVDF was not specifically solvated in the solution state above the crystallization temperature, while [TFSA](−) anion formed a complex with lithium ion irrespective of the PVDF content. These results led to the consideration that an increase in the interaction parameter might be caused by the strong interaction between lithium ion and [TFSA](−) anion to form the complex, which would also lower the interaction between PVDF and [TFSA](−) anion.