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.

Noboru Osaka,Yuichi Minematsu,Masatoshi Tosaka

doi:10.1039/c8ra08514e

Abstract

The thermal behavior of poly(vinylidene fluoride)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide/lithium bis(trifluoromethylsulfonyl)amide (PVDF/[C2mim][TFSA]/LiTFSA) gels, prepared by cooling from the hot solution, was investigated with various concentrations of LiTFSA (CLiTFSA). The peak melting temperature (Tm) of the gels shifted toward higher temperatures with increased CLiTFSA. However, the thickness of lamellar crystal was found to decrease with the increase in CLiTFSA, which meant that the increase in Tm was not caused by the thickening of lamellar crystal. Furthermore, we found the appearance of domains above Tm in the high CLiTFSA 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 CLiTFSA toward the phase separation increased the Tm. 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.

Highlights

Ionic liquids (ILs), which are molten salts with melting points below 100 C,1–3 are attracting great interest in various elds because of their high ionic conductivity and negligible volatility and in ammability
This peak shi ed toward higher wavenumber with the increase in concentrations of LiTFSA (CLiTFSA) in PVDF/[C2mim][TFSA]/LiTFSA solutions (details are shown in Fig. S4(b)).† Lithium ion is well known to form the complex with [TFSA]À anion via a larger binding energy compared with that of the complex of [C2mim]+ cation and [TFSA]À anion
The effects of lithium salt on the thermal behaviors of the PVDF/[C2mim][TFSA]/LiTFSA gels prepared by cooling from the hot solutions were investigated at various CLiTFSA

Summary

Introduction

Ionic liquids (ILs), which are molten salts with melting points below 100 C,1–3 are attracting great interest in various elds because of their high ionic conductivity and negligible volatility and in ammability. Among the abovementioned strategies to prepare the polymer gels in ILs, the simplest way is by cooling the hot solutions of thermoreversible homopolymers in ILs, leading to the formation of crystallites as the physical cross-linking points. Size of the spherulites decreased with the increase in concentrations of ILs and lithium salt, and the membrane became amorphous.[40] Despite these studies, few studies are reported on the PVDF/IL gels prepared by cooling from the hot solutions.[48]. We found that the phase separation competed with the subsequent crystallization, which led to the formation of micrometer-sized pores and nanometersized voids in the spherulites Control over these speci c structures would help improve the transportation of lithium ion for GPEs. Microscopic observations, wide- and small-angle X-ray scattering measurements and Fourier-transform infrared spectroscopy measurements were performed to investigate the thermal behavior from the viewpoints of hierarchical structures. The fractured surfaces of the gels were coated by sputtered Ag for observation using FE-SEM

Results and discussion

Tm0 ðpureÞ

Conclusions