Pressure-Dependent Clustering in Ionic-Liquid-Poly (Vinylidene Fluoride) Mixtures: An Infrared Spectroscopic Study

The nanostructures of ionic liquids (ILs) have been the focus of considerable research attention in recent years. Nevertheless, the nanoscale structures of ILs in the presence of polymers have not been described in detail at present. In this study, nanostructures of ILs disturbed by poly(vinylidene fluoride) (PVdF) were investigated via high-pressure infrared spectra. For 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([HEMIm][TFSI])-PVdF mixtures, non-monotonic frequency shifts of the C^4,5-H vibrations upon dilution were observed under ambient pressure. The experimental results suggest the presence of microheterogeneity in the [HEMIm][TFSI] systems. Upon compression, PVdF further influenced the local structure of C^4,5-H via pressure-enhanced IL-PVdF interactions; however, the local structures of C²-H and hydrogen-bonded O-H were not affected by PVdF under high pressures. For choline [TFSI]-PVdF mixtures, PVdF may disturb the local structures of hydrogen-bonded O-H. In the absence of the C^4,5-H⋯anion and C²-H⋯anion in choline [TFSI]-PVdF mixtures, the O-H group becomes a favorable moiety for pressure-enhanced IL-PVdF interactions. Our results indicate the potential of high-pressure application for designing pressure-dependent electronic switches based on the possible changes in the microheterogeneity and electrical conductivity in IL-PVdF systems under various pressures.