Finely Tuning the Lower Critical Solution Temperature of Ionogels by Regulating the Polarity of Polymer Networks and Ionic Liquids

Introduction

Ionic liquids (ILs) are room temperature molten salts that have drawn great attention due to their appealing properties such as nonvolatility, tunable polarity, wide liquid temperature ranges, and strong solvation ability.^1,2 Due to these well-known properties, particularly the wide electrochemical window and good ionic conductivity, combined with the thermal and chemical stability, ILs have been extensively applied as an ideal electrolyte with good electrochemical performance.³ Ionogels are composed of three-dimensional (3D) polymer networks and ILs are promising systems, as they can be shaped like solids while simultaneously maintaining the macroscopic properties of ILs.^4,5 Recently, the development of ionogels has offered great potential for electrical, optical, and biomedical applications, such as energy storage and conversion devices,^6–8 transistors,^9–11 actuators,^12,13 and drug delivery and luminescent materials.^14,15

Ionogels can be classified into three categories according to the nature of the solid-like 3D network, which are organic, inorganic, and organic–inorganic hybrid, respectively. Organic ionogels can be fabricated using low molecular weight gelators or dissolving polymers in ILs directly.¹⁶ The well-known “bucky” gels and silica-based ionogels are typical inorganic ionogels.^17,18 Organic–inorganic hybrid refers to polymeric ionogels reinforced with inorganic fillers.¹⁹ As a facile strategy to prepare ionogel matrices with desired mechanical resistance, the organic 3D network formed by cross-linked polymers is generally adopted and extensively studied. Further, polymeric ionogels may be separated into physical and chemical gels. For physically cross-linked ionogels, the existence of specific functional groups is a prerequisite. For example, in 1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide/poly(vinylidene fluoride-co-hexa-fluoropropylene) ([EMIM][TFSI]/PVDF-HFP) ionogels systems, the PVDF crystallites work as physical cross-linking sites, which provide the mechanical strength to the ionogel.²⁰ In contrast, chemical cross-linking is a ubiquitous strategy, as ionogels can be directly fabricated by polymerizing target monomers in ILs.

Generally, the properties of ionogels are expected to be derived from both from the ILs and the components forming the 3D network. In reality, the intrinsic character of ionogels relies on the intimate combination of ILs and 3D networks. Recent studies indicate that the nanometer scale assembly may affect the properties of ionogels. For example, in some polymeric ionogels, only one glass transition (lower than the T_g of neat ILs and higher than the T_g of pure polymer) was observed, suggesting a compatible system of ionogels.²¹ Consequently, the miscibility between ILs and polymer network directly determines the properties of organic ionogels.

It is worth mentioning that the mutual miscibility between ILs and polymer is not invariable. In certain systems, the solubility of the polymer in ILs can respond to an external stimulus, such as light illumination, heat, and certain chemicals.^22–25 Similar to poly(N-isopropylacrylamide) (PNIPAm) that exhibits lower critical solution temperature (LCST) or upper critical solution temperature (UCST) phase behavior in aqueous solutions, polymer/ILs systems exhibit more diverse and sophisticated LCST and UCST phase behavior.²⁶ For instance, Poly(benzyl methacrylate)/1-alkyl-3-methylimidazolium bis(trifluoromethanesulfone)imide (PBzMA/[C_nMIM][NTf₂]),²⁷ Poly(n-butyl methacrylate) (PnBMA)/[C_nMIM][NTf₂],^28,29 Polyethylene oxide (PEO)/[C_nMIM]Tetrafluoroborate[BF₄], and polyethers/[C_nMIM][NTf₂] are reported for their typical nature of LCST phase behaviors.^30–32 PNIPAm/[C_nMIM][NTf₂] is a system that possesses UCST phase behavior.³³ However, most relative studies have mainly focused on polymer/ILs solution systems, while the LCST or UCST phase behavior of the ionogel states are rarely investigated. Herein, to fundamentally explore these important thermodynamic phenomena and expand the functionality of the ionogels, polyacrylates/[C_nMIM][NTf₂] ionogels with tunable LCST phase behavior are designed and fabricated based on our previous study.³⁴

To understand this phase behavior from a deeper perspective, we systematically investigated the structural effects of ILs and monomers on the LCST phase transition of ionogels. The mechanism of LCST phase behavior is analyzed, and the prerequisites for the LCST phase behavior of the ionogels are proposed. Meanwhile, how hydrogen bonds between polymer and ILs influence the LCST of ionogels are studied. By varying the mixing ratio of two structurally similar ILs in their blends, the LCST of ionogels can be tuned within a wide temperature range. Resemble the effect of molecular weight on LCST in polymer/ILs systems, the LCST of ionogels decreased as increasing the polymerization degree of the monomers. Taking advantage of this property, thermoresponsive ionogels with desired patterns are prepared using a photomask. These nonvolatile ionogels with high optical transparency, tunable LCST, good durability, and wide temperature range mechanical stability show promising applications in fields related to flexible sensors and “smart” optical devices.

Experimental Methods

Preparation of ionogels

Butyl acrylate (monomer), ethyleneglycol dimethacrylate (EGDMA, cross-linker), and diethoxyacetophenone (DEOP, photoinitiator) were dissolved in ILs, and then the mixture was stirred for 10 min until a fully transparent solution was obtained. The polymerization was initiated by UV irradiation, and the solution was cured under UV light (365 nm, 800 mW/cm²) for 10 min.

Mechanical characterization

The mechanical characterization of ionogels was realized using a tensile machine (SUNS, UMT4103, Shenzhen, China). The sample was cut into a dumbbell shape, and both ends of the dumbbell-shaped sample were connected to the clamps with a fixed lower clamp. The upper clamp was pulled by the load cell at a constant velocity of 20 mm/min at room temperature, by which the stress–strain curve was recorded and the experimental data were further analyzed. The tensile strength was obtained from the failure point. We determined the modulus of ionogels by the average slope over 0–10% of strain ratio detected from the stress–strain curve.

LCST characterization of ionogels

The LCST of ionogels was determined by optical transmittance measurements using dynamic light scattering (Litesizer™ 500; Anton Paar, Graz, Austria). A transmittance of 100% indicates ionogel homogeneity, and a decrease in transmittance suggests that the ionogel undergoes LCST phase separation. We define the LCST as the temperatures at which the transmittance drops to 80% ( Supporting Information Figure S4).

Thermal analysis

Thermal analysis was conducted using a thermogravimetric analyzer (TGA) (Q500; TA Instruments, New Castle, DE) and a differential scanning calorimeter (DSC) (Q2000; TA Instruments) at a heating rate of 10 °C/min.

Results and Discussion

Preparation and the LCST phase behavior of ionogels

The ionogels were fabricated by in situ photoinduced polymerization ( Supporting Information Figure S1) of acrylates monomers in ILs using EGDMA and DEOP as cross-linker and photoinitiator (Figures 1b and 1c), respectively. Polyacrylates are typically used to prepare functional elastomer matrices due to their good mechanical resistance and adhesion. The structural formula of acrylate monomers with different alkyl chain lengths is given in Figure 1a. Figures 1d and 1e show the structures of ILs used in this work. The basic physicochemical properties of poly(butyl acrylate) (PBA) ionogels are characterized and given in Supporting Information Figure S2. It is worth noting that different from previous reports stating that [C₂MIM][NTf₂] cannot form transparent and soft ionogels with PBA due to differences in polarity,³⁵ the as-prepared PBA/[C₂MIM][NTf₂] ionogels are transparent substances without any leakage of ILs at room temperature. We supposed that the PBA/[C₂MIM][NTf₂] ionogel exhibits a LCST phase behavior. This hypothesis is subsequently demonstrated by a thermal response test of the PBA/[C₂MIM][NTf₂] ionogels. As shown in Figure 1f, the transparent PBA/[C₂MIM][NTf₂] ionogel disc quickly becomes opaque as the temperature is increased from room temperature (∼25 °C ) to 42 °C . Besides, this thermoresponsive phenomenon of the ionogel is completely reversible. Note that no obvious ILs are leaked from the ionogel during this process, and the volume of ionogel remains unchanged at the macroscale before and after phase separation. As a result, the PBA/[C₂MIM][NTf₂] ionogel undergoes rapid, isochoric, and reversible thermoresponsive phase behavior, which is known as LCST phase behavior.

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The LCST-type phase separation is a kind of phenomenon where the mixing Gibbs free energy (ΔG_mix) changes from negative to positive with increasing temperature. According to the pioneering work by Lee and Lodge,²⁸ it is believed that the coexistence of structure-forming solvatophobic and solvatophilic groups in the polymer is necessary for LCST phase behavior. Similarly, in PBA/[C₂MIM][NTf₂] ionogel system, the solvatophilic acrylate groups and the solvatophobic n-butyl side chains play important roles in the LCST phase behavior of ionogel (Figure 2a). We speculate that the oriented solvation between the polymers and ILs caused by hydrogen-bonding effects and van der Waals interactions may serve as the driving force for the LCST phase behavior in our system. Since hydrogen-bond interaction is stronger than van der Waals interaction, we supposed that the hydrogen bonds serve as the key factor for the LCST phase behavior in this system, which will be discussed later in this work. The van der Waals interaction here refers to the interactions between the polar domain of ILs and polar acrylate groups of the polymer backbone, as well as the interaction between nonpolar alkyl substituents on cation of ILs and the nonpolar alkyl side chain on polyacrylates network. The formation of polar and nonpolar domains in ILs is ascribed to Coulombic interactions³⁶ because the charged groups selectively solvate the charged groups and the uncharged groups (alkyl side chain of imidazolium cations) are expelled from this region as shown in Figure 2b. Herein, [C₂MIM][NTf₂] is a polar domain-dominated IL due to the short side chain on the imidazolium cation. Consequently, for the PBA/[C₂MIM][NTf₂] ionogel at a temperature below its LCST, it was supposed that ILs selectively solvate the solvatophilic acrylate groups close to the polymer backbone, which results in the formation of a homogeneous and transparent ionogel, as shown in Figure 1g (left). The kinetics of thermoresponsive phase separation of ionogels with increasing temperature is described as follows. The hydrogen-bonds and van der Waals interactions between polymer networks and ILs are gradually weakened by heating. When the electrostatic interactions between ILs overwhelm the hydrogen-bonds and van der Waals interactions between polymer networks and ILs, the macroscopic phase separation occurs in the ionogel as shown in Figure 1g (right).

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To fundamentally understand the mechanisms of this phase behavior, the effects of structures of polymer and ILs on the LCST of ionogels are systematically investigated ( Supporting Information Table S2). The phase diagram in Figure 2c shows the compatibility of ionogels composed of different acrylate monomers and ILs. It is found that the as-prepared ionogels are compatible when the length of the alkyl side chain on polyacrylates is less than three, which is ascribed to the poor solvatophobic effect of the nonpolar domain (short side chain length) on the polymer exerting on the ILs. For certain ILs, miscibility between polymer networks and ILs in ionogels decreased with increasing n-alkyl side chain length (nonpolar domain) in polymer networks (n = 2, as indicated by the vertical dotted line in Figure 2c), which is due to the enhanced solvatophobic effect of the nonpolar n-alkyl side chain of the polymer on the polar ionic group of ILs. Meanwhile, the miscibility between the polymer network and ILs increased as the alkyl chain length increased on the imidazolium cations of ILs (m = 6, as indicated by the horizontal dotted line in Figure 2c), which is due to the increased affinity between nonpolar components in the polymer (n-alkyl side chain) and ILs (alkyl side chain on imidazolium cation) through solvophobic interactions enhancing the ionogel compatibility. To provide a more accurate description of the relationship between molecular structure and phase separation temperature in the current system, the specific phase separation temperature of ionogels in Figure 2c is also summarized and plotted, as shown in Figure 2d. Finally, the prerequisites for the LCST phase behavior of these ionogels are proposed as follows:

1.	The coexistence of solvatophobic and solvatophilic groups in the polymer network, corresponding to n ≥ 3 (alkyl chain length of acrylate) in our system.
2.	Matching between the nonpolar domains (m and n) in polymer network and ILs.

Theoretical analysis and molecular dynamics simulation

One empirical interpretation of the LCST phase behavior is based on Flory–Huggins theory. The effective interaction parameter (${\mathrm{\chi }}_{\mathrm{eff}}$) is assumed to follow the functional form:

The effective interaction can be split into two parts: The first one is the enthalpic part ${\mathrm{\chi }}_{\mathrm{h}}=\alpha /T$ with a negative $\alpha$ characterizing the specific attractive interaction due to the hydrogen bonding and the second one is the entropic part ${\mathrm{\chi }}_{\mathrm{s}}=\mathrm{\beta }$ related to the details of molecular packing. The phase separation occurs when the effective interaction parameter becomes larger than a critical value $({\mathrm{\chi }}_{\mathrm{eff}}={\mathrm{\chi }}_{\mathrm{c}}>0)$, leading to the phase separation temperature: Here, both the numerator and the denominator are positive. When $T<T_{\mathrm{c}}$, ${\mathrm{\chi }}_{\mathrm{eff}}<{\mathrm{\chi }}_{\mathrm{c}}$, the system is homogeneous. When $T>T_{\mathrm{c}}$, ${\mathrm{\chi }}_{\mathrm{eff}}>{\mathrm{\chi }}_{\mathrm{c}}$, the system becomes phase separated. When the cation’s alkyl side chain length (n) is increased, this does not affect the hydrogen bonding ($\alpha$ remains almost the same) because the cation has more freedom to adjust its orientation, but the entropic part is reduced because the cation becomes bulky. This leads to a higher value of the LCST temperature $T_{\mathrm{c}}$. The trend is different when the acrylate’s alkyl side chain length (m) is increased. The solvatophobicity of the long alkyl side chain may contribute to the weakening of the hydrogen bonding (the value of $|\alpha |$ is reduced). Meanwhile, since the acrylate is fixed in a network, the change in the entropic part is small. This leads to a lower value of the LCST temperature $T_{\mathrm{c}}$.

The LCST phase behavior of ionogels can also be qualitatively proved by molecular dynamics (MD) simulation. We perform the MD simulation using the large-scale atomic/molecular massively parallel simulator (LAMMPS) to explore the LCST phase transition of the ionogel. In our simulation, we coarse-grain the polymers and cations of ILs as a sequence of connected beads, and model the anions and the cross-linkers as dispersed beads; please see Figure 3a for the schematic illustration of the model and Supporting Information for the simulation details. Our simulation indicates that at T* = 1.0, the polymer/ILs mixtures form homogeneous ionogels, in which the ionic species enrich the solvatophilic polymer backbone regions. With increasing temperature, the ionic gelation networks collapse and ILs aggregate, which results in macroscopic phase separation, as shown in Figure 3b. We further estimate the phase transitions of the ionogel by analyzing S_i(q) (Figure 3c) and show the calculated LCST* in Figure 3d. Our simulation illustrates that the variations in cation chain length (n) and polymer side chain length (m) can sensitively affect the LCST of ionogels, as shown in Figure 3d, which is consistent with the experimental data.

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The influence of H-bonds on the LCST of ionogels

It is worth noting that H-bonds typically exist in polymer/ILs mixtures, which serves as a dominant factor that results in the LCST phase behavior, as reported by the previous study on PEO/[C_nMIM][BF₄] and polyethers/[C_nMIM][NTf₂] systems.^30–32,37 In addition, in the protein folding process, even if the H-bonds do not directly result in the molecular self-assembly, it can drastically affect the nanostructure of the assembly.³⁸ Therefore, the effects of hydrogen bonds on LCST phase behavior of ionogel system require further clarification.

Possible hydrogen bonds formed between ILs and polymer backbones in homogeneous PBA/[C₂MIM][NTf₂] ionogels are illustrated as shown in Figure 4a according to the previous studies.⁸ As the hydrogen bonds formed between C_4,5–H of the imidazolium rings and polymer (O atoms of polyacrylate) are much weaker than those via C₂–H, as suggested by Wang and Voth,³⁹ we have chosen to study hydrogen bonding between C₂–H of imidazolium cation and how the O atoms of PBA influence the LCST of the corresponding ionogel by a controlling variable method. 1-alkyl-2,3-dimethylimidazolium [ C_nMMIM][NTf₂] is structurally similar to [C_nMIM][NTf₂], except that a methyl group replaces the H atom in the C₂ position of the ring, as shown in Figure 4b (left). We performed the LCST measurements on the PBA/[C_nMMIM][NTf₂] ionogels. The results are summarized ( Supporting Information Table S3) and shown in Figure 4c. It is found that this change (H → CH₃) in ILs structure has a significant influence on the LCST phase behavior of ionogels. For PBA ionogels (60 wt % polymer content), there is a pronounced reduction of about 100 °C in the LCST. Replacing the most acidic proton (C₂) of the imidazolium ring with a methyl group lowers its hydrogen bond-forming capability with the PBA side chain. As a result, without this strong interaction, the miscibility between ILs and PBA is weakened, which causes a decrease in LCST. Besides, the influence of hydrogen bonds between F atoms of the anion and the polymer are also investigated by replacing the CF₃ group with a F atom (Figure 4b, right). The LCST of PBA/[C_nMMIM](fluorosulfonyl)imide[FSI] ionogels is measured and plotted, as shown in Figure 4d. We found that the change in the anionic structure of ILs also exhibited a significant influence on the LCST of ionogel. Replacing the CF₃ on anion of ILs with an F atom reduces the number of hydrogen bonds that form between anions with the PBA network, resulting in a reduction of about 60 °C in the LCST of ionogels. These results suggest that the hydrogen bonds between ILs and polymer play an important role in determining the LCST phase behavior of this system.

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Continuous regulation of the LCST of ionogels

Further, for certain acrylate monomers, the effects of ILs structure and the content of monomer on the LCST of ionogels are explored. Importantly, we found that a slight increase in alkyl chain length of imidazolium cation results in a huge increase in the LCST of ionogel. As shown in Figure 5a, for PBA ionogels (60 wt % content of monomer), the LCST increased from 32 to 100 °C as the cation’s alkyl chain length of ILs increased from 2 to 3. With further increases of the alkyl chain to butyl (4), the LCST of the ionogel reached 175 °C. From a thermodynamic point of view, increasing the alkyl chain length may increase the degree of microphase segregation, which is generated from the polar ionic domains and nonpolar alkyl chain domains.^40–42 The extended microphase segregation in turn lowers the entropy of the ILs, which results in an increase in the LCST of ionogel. Briefly, from the viewpoint of molecular polarity, the extended nonpolar alkyl domains in ILs may enhance the interactions between the ILs and nonpolar domain (butyl side chains and/or alkyl main chain) in PBA, and thus lowers the mixing enthalpy and causes an increase in the LCST. Moreover, it was also found that the LCST of the PBA ionogel increased as the weight fraction of PBA increased from 30 to 60 wt % (Figure 5a). The temperature-composition phase diagrams can explain this phenomenon.²⁸ It is known that the temperature-composition phase diagrams of the LCST system are convex downward. In PBA/ILs systems, the weight fraction of PBA from 30 to 60 wt % may correspond to the rising part of the curve (spinodal curve). Though the polymer/ILs and ionogels are different systems, it could be understood in terms of the results. Furthermore, using IL blends as the solvents, we can regulate the LCST of ionogels almost linearly as the weight fraction of [C₃MIM][NTf₂] and [C₄MIM][NTf₂] increases in the blends, as shown in Figure 5b. These results suggest that the interactions between PBA and ILs do not change significantly upon mixing two structurally similar ILs. Therefore, the IL blends in PBA ionogels are much closer to an ideal random mixing state. Based on this result, continuous regulation of the LCST of ionogels without modifying the chemical structure of the polymers can be realized.

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In addition, we found that the LCST of ionogels decreased as the molecular weight of the polymer increased. Such a trend is commonly observed in other polymer/ILs systems that exhibit LCST-type phase behavior,^28,30 as well as our previously reported PBA/ILs system.³⁴ Consequently, we studied the LCST of PBA ionogels as a function of UV curing time. It was found that the LCST of PBA ionogels first decreased and then reached a plateau with increasing curing time during the preparation of the ionogel (Figure 6a). This may be due to the lower molecular weight of PBA in the ionogel obtained during a short curing time. As a result, the as-prepared PBA ionogels show a relatively high LCST. Taking advantage of this property, reversible thermoresponsive patterned PBA ionogels are fabricated. Schematic illustration in Figure 6b shows the preparation process of patterned ionogels using a photomask with specific patterns. The precursors are placed between two glass slides separated by rubber spacers (1.5 mm in thickness). The solution is first illuminated by UV light (365 nm) for 3 min using a photomask. Then the photomask was removed and the solution was illuminated for another 60 s. Through this strategy, ionogels with graphics, letters, and quick response code patterns were successfully prepared, as shown in Figures 6c–6e, respectively. By alternate heating and cooling, the patterns in the ionogels can be repeatedly displayed.

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Finally, taking PBA/[C₂MIM][NTf₂] ionogel as an example, we characterized the thermal stability of ionogel using TGA and DSC. The TGA curve in Supporting Information Figure S5a shows that the ionogels are thermally stable up to 340 °C at 5% weight loss, which suggests that the ionogel exhibited high thermal stability. The glass transition temperatures (T_g) of PBA and PBA ionogel were −47.3 and −60.3 °C , respectively ( Supporting Information Figure S5b). The T_g of the PBA ionogel was lower than pure PBA due to the plasticizing effects of the ILs, which is also founded in other ionogel systems.^8,35,43 Besides, the weight of the PBA ionogel remains unchanged even after storing for 30 days in the open air at 25 °C and 50% relative humidity ( Supporting Information Figure S6). Consequently, these nonvolatile ionogels with high optical transparency, tunable LCST, good durability, and wide temperature range mechanical stability show promising applications in fields related to flexible sensors and optical devices such as “smart windows”.⁴⁴

Conclusion

In this work, we fabricated a polyacrylates/[C_nMIM][NTf₂] ionogel with LCST-type phase behavior and elucidated the structural effects of ILs and monomers on the LCST phase transition of ionogels. We found that the LCST of ionogels is primarily determined by the polarity of polymer (alkyl) side chains and the alkyl chain on the cation of ILs. The prerequisites for the LCST phase behavior of ionogels are proposed as follows: (1) The cosolvatophobic and solvatophilic groups in the polymer network, that is, n ≥ 3 (alkyl chain length of acrylate) in our system; (2) Matching between the nonpolar domains in the polymer network and ILs. We speculate that the hydrogen-bonding interactions between polymer and ILs are the main driving force for the LCST phase behavior of ionogels, which show significant influence on the LCST of ionogels. In addition, by varying the mixing ratio of two structurally similar ILs in their blends, the LCST of ionogels can be tuned almost linearly through a wide temperature range without modifying the chemical structure of the polymers. Finally, taking advantage of the effects of curing time on the LCST of ionogels, thermoresponsive ionogels with desired patterns are prepared using a photomask. These nonvolatile ionogels with tunable LCST enrich the functionality of current ionogels and provide an alternative to the fabrication of smart flexible electronic and optical devices.

Supporting Information

Supporting information is available and includes the general materials used in this work and a detailed description of the characterization methods. In addition, the mechanical properties, optical properties, and thermal analysis of the ionogels, as well a detailed description of the MD simulation of the ionogels.

Conflict of Interest

There is no conflict of interest to report.