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 C4,5-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 C4,5–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 C4,5–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.

Ionic compounds with melting temperatures below the boiling point of water (ILs) are widely used in industry because of their non-volatile, dispersive, and ionic-conductive properties [13][14][15][16][17][18]. A critical analysis of the structures and properties of ionic liquids was performed by Cabrita's group [13] recently. The interactions between cations and anions in ILs include forces such as electrostatic attraction, van der Waals association, and hydrogen-bonding interactions [13][14][15][16][17][19][20][21][22][23]. The interplay of various interactions in ILs may determine the formation of nanoscale supramolecular domains in bulk ILs. Indeed, the effects of cation-anion associations, such as on viscosity, may influence the properties of ILs. Recently, the mechanisms and properties of ILs that allow for their use as lubricants were extensively reviewed by Calandra et al. [24]. ILs can also be suitable plasticizers to form polymer-salt complexes from PVdF [10,11]. As reported in the literature, the >CF 2 moiety in PVdF may interact with the IL cation. For example, >CF 2 groups in the PVdF chain can interact with the imidazolium C-H of ILs [10,11]. Nevertheless, studies on imidazolium equipped with hydroxyl groups are scarce. Ludwig's group [25] found that ILs with cations containing -C 2 H 4 OH group ILs tend to form cationic clusters. Investigations of ILs with cations containing hydroxyl moieties may shed light on cation-anion clusters (caused by electrostatic forces and weak hydrogen bond interactions) and cation-cation associations (attributed to the influence of traditional hydrogen bonds). This is because the ions in ILs are composed of clusters of various sizes owing to various interactions.
Vibrational spectra, such as mid-infrared spectra, may provide information on the environmental changes of functional groups upon blending. Infrared (IR) spectra were used to characterize ion-modified materials in the past. For example, Ivanov et al. [26,27] used a nondestructive infrared technique to inspect ion-modified polymeric materials. Hydroxyl groups are IR-sensitive to changes in local environments, and the formation of O-H hydrogen bonds can be easily detected by IR techniques. Moreover, C-H can act as a proton donor to form a weak hydrogen bond. When associated with proton acceptor X to form weak hydrogen bonds (C-H· · · X), C-H covalent bonds sometimes shorten themselves through a blueshift in frequency [20,[28][29][30][31]. In this work, we investigated the pressure-dependent local structures of ILs with both O-H and imidazolium C-H, which are IR-detectable.
Combining the IR technique and high pressures may result in pressure-enhanced interactions. As reported in previous studies [32][33][34][35][36], the associated structures of ILs were influenced by the addition of PEO [32] and DNA [33] under high pressure. As the pressure was applied, the polymer (PEO or DNA) disturbed the clustering of the ILs, as polymers and ILs interact with specific pressure-enhanced forces. In the current study, two types of ILs (TFSI anion and two types of cations containing hydroxyethyl groups) were used. Upon blending with PVdF, the conformation and clustering change were studied using the IR technique at various pressures.  Figure S1 (Supplementary Materials). The numbering of imidazolium atoms is shown in Figure S1. Mixtures of IL-PVdF containing 10, 20, 30, 40, and 50 wt% of IL were prepared with various weight percentages of IL and PVdF and suitable amounts of DMF were added as the solvent. The solutions were sonicated at room temperature (25 • C) and then stirred at 50 • C under vacuum. The solvent (DMF) was removed under vacuum and the samples were kept under light in air for at least one day to remove the residual solvent. The samples were further dried at 155 • C using a moisture analyzer (MS-70, A&D Company, Tokyo, Japan) before spectral measurements were performed. The removal of DMF was confirmed by checking the disappearance of the DMF absorption in the IR spectra.

IL
High pressures (up to~2 GPa) were generated using a diamond anvil cell (DAC) equipped with two type IIa diamonds with a culet size of 0.6 mm. In the laboratory, we utilized a Fourier-transform (FT) spectrophotometer (Spectrum RXI, Perkin-Elmer, Naperville, IL, USA) combined with a beam condenser to obtain the IR spectra. The beam condenser was combined with a spectrometer to enhance the intensity of the IR beam. The absorption spectra of the samples were measured and subtracted from those of the DAC to eliminate absorption by the diamond anvils. A 0.25-mm-thick Inconel gasket with a 0.3 mm diameter hole was prepared as the sample holder. Transparent CaF 2 crystals were placed in the sample holder before the samples were inserted to avoid the saturation of the IR bands. The pressure calibration followed Wong's method [37,38].  Figure 1a show broad O-H absorptions, two main imidazolium C-H bands, and several alkyl C-H peaks at 3300-3700, 3050-3250, and 2800-3000 cm −1 , respectively, at ambient pressure [25,[32][33][34][35][36]. As shown in Figure 1a, the O-H absorption also displays a shoulder peak at ca. 3430 cm −1 . The two imidazolium C-H bands at 3162 and 3123 cm −1 represent the vibrational absorptions of the C 4,5 -H and C 2 -H groups, respectively, whereas the C 2 -H peak of pure [HEMIm][TFSI] reveals one shoulder band at 3109 cm −1 in Figure 1a. The pure PVdF vibrational spectra in Figure 1e show two main C-H absorptions, which are attributed to asymmetric and symmetric PVdF C-H stretching vibrations at 3021 and 2985 cm −1 , respectively [6][7][8].  [39]. In addition, anions prefer to have close contact with the C 2 -H of the imidazolium ring [39]. As PVdF blends into [HEMIm][TFSI], the IL clusters may be divided by the polymer into smaller clusters by the formation of C 4,5 -H· · · PVdF interactions (i.e., breaking the C 4,5 -H· · · anion interaction), leading to the blueshift of C 4,5 -H at low concentrations of [HEMIm][TFSI] for [HEMIm][TFSI]-PVdF mixtures, as shown in Figure 2b. Figure 3 shows the IR spectral features of pure [HEMIm][TFSI] at various pressures. As the pressure increases, the C 2 -H and C 4,5 -H bands show a blueshift accompanied by mildband broadening. In contrast to C-H absorption, the O-H stretching bands show a redshift as the pressure is increased from ambient pressure to 2.5 GPa, as shown in Figure 3. The results in Figure 3 demonstrate that C 2 -H and C 4,5 -H may suffer pressure-induced changes in weak hydrogen-bonding interactions (a blueshift) with anions [32][33][34][35][36]. Conversely, the pressure-induced redshift of the O-H vibrational bands may result from both pressureenhanced strong hydrogen bonding and the formation of larger cationic clusters (lower frequencies accompanied by band broadening) via pressure-induced association [32][33][34][35][36].     [TFSI]-PVdF mixtures demonstrate similar tendencies upon compression; that is, the local environment of C 2 -H may not be easily interfered with by PVdF under high pressures. These observations agree with the fact that C 2 -H has stronger acidity than C 4,5 -H, while C 2 -H· · · anion hydrogen-bonding interactions are more attractive than C 4,5 -H· · · anions. For pure [HEMIm][TFSI], the imidazolium and anions may involve at least three kinds of associations: O-H· · · anions, C 4,5 -H· · · anions, and C 2 -H· · · anions. As the polymer is added to [HEMIm][TFSI] to form [HEMIm][TFSI]-PVdF mixtures, the weaker interactions between imidazolium and the anion may be disturbed by the PVdF molecule. As shown in Figure 5, the stretching frequencies of C 4,5 -H are significantly influenced by PVdF via pressure-enhanced C 4,5 -H· · · PVdF interactions. The C 4,5 -H· · · anion seems to be the weakest interaction when compared to the O-H· · · anion and C 2 -H· · · anion at high pressures.

Results and Discussion
In order to shed more light on the association between cations and anions, choline [TFSI] was studied. Figure 6 shows the IR spectra for pure choline [TFSI], three choline [TFSI]-PVdF mixtures with various concentrations, and pure PVdF obtained at ambient pressure. The spectrum of pure choline [TFSI] in Figure 6a shows broad O-H absorption at ca. 3541 cm −1 and alkyl C-H stretching bands located in the region of 800-3100 cm −1 . The pure PVdF IR spectrum in Figure 6e shows two dominant C-H stretching bands in the region from 2900 to 3150 cm −1 . Unfortunately, the choline C-H absorption overlaps with the PVdF C-H bands in the region from 2800 to 3100 cm −1 ( Figure 6); thus, we focused on O-H stretching in the region from 3200 to 3800 cm −1 for the mixtures. As choline [TFSI] is diluted by the polymer, an additional peak appears at ca. 3630 cm −1 for choline [TFSI]-PVdF mixtures, as is shown in Figure 6b Figure 9, the O-H stretching frequencies reveal mild redshifts from 3541 cm −1 (pure choline [TFSI]) to 3534 cm −1 (10 wt% mixture) under ambient pressure. In contrast, large redshifts from 3548 cm −1 (pure choline [TFSI]) to 3531 cm −1 (10 wt% mixture) were detected at a pressure of 2.5 GPa. We note that such significant redshifts under high pressures in Figure 9 are not observed in the O-H of the [HEMIm][TFSI] systems in Figure 5a. In the absence of weaker interactions (such as C 4,5 -H· · · anion), the O-H group is a favorable site for pressure-enhanced cation-PVdF interactions in choline [TFSI]-PVdF mixtures.

Conclusions
In this study, we demonstrated that the presence of PVdF can influence the clustering structures of [HEMIm][TFSI] and choline [TFSI]. The increase in free -OH absorption at ca. 3634 cm −1 for diluted IL-PVdF mixtures suggest an increase in the number of smaller clusters and isolated cations upon dilution at ambient pressure. As the pressure increased, the C 4,5 -H· · · anion associations were likely further disturbed by pressureenhanced [HEMIm][TFSI]-PVdF interactions. Upon compression, PVdF tended to modify the local structure of the hydrogen bond O-H in choline [TFSI]-PVdF mixtures. The results suggest that high pressure can change the clustering structures and might be successful in modifying the electrochemical properties (such as electric conductivity) of IL-PVdF mixtures. The differences in clustering structures under various pressures may suggest the potential of IL-PVdF mixtures to serve as pressure-dependent electronic switches. Hopefully, the experimental results presented in this study may stimulate further simulation research in the future to rationalize the structural changes of IL-PVdF mixtures in detail under high pressures.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.