Thermoresponsive ionogels

Abstract

This paper explores the innovative realm of thermoresponsive ionogels, which leverage the unique properties of ionic liquids, such as high ionic conductivity, robust stability, and minimal volatility. Distinguished from traditional hydrogels and organogels, ionogels overcome common limitations by incorporating an ionic liquid phase, thus broadening their application spectrum. The paper delves into the Lower Critical Solution Temperature (LCST) and Upper Critical Solution Temperature (UCST) types of thermoresponsive polymer and ionic liquid, examining the principle of phase separation. It offers a comprehensive review of the evolution of thermoresponsive ionogels, from their initial discovery to recent advancements, and provides insights into future developments in this field.

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1. Introduction

Polymer gels, distinguished by a complex, three-dimensional polymeric network and substantial solvent holdup, are crafted via a plethora of interactions including metal coordination, hydrogen bonding, host–guest interactions, ionic binding, and hydrophobic interactions. All these binding phenomena collectively forge the structural framework of the gels.^1–3 These gels exist mainly in two classifications, hydrogels and organogels, each employing water and organic solvents respectively during their construction, and are constrained to certain applications due to the inherent characteristics of the solvents.^4,5 Hydrogels are known for their propensity to freeze at low temperatures and evaporate at heightened temperatures, thereby exhibiting a deficiency in durability. On the contrasting side, organogels tend to be unsuitable for a plethora of applications due to the inherent toxicity and volatility of organic solvents.^6–9 Ionogels, a relatively modern development, have captured significant attention as a pioneering gel-type material. They feature a solvent phase which incorporates an ionic liquid (IL), thus offering attributes such as chemical, thermal, and electrochemical stability, non-volatility, ionic conductivity, and non-flammability. This unique inclusion of IL negates the limitations posed by hydrogels and organogels and amplifies the range of gel material applications on a broader canvas.^9–12

Ionogels, first introduced in 2005 with organic or inorganic body networks, have been the focus of substantial research.^13,14 The application spectrum of these materials is impressively broad, covering domains including energy storage, bioelectronics, gas-separating membranes, sensors, and actuators.^15–22 As the materials’ requirements vary based on specific applications, no one-size-fits-all solution exists for ionogels. Their efficacy is contingent upon the specific needs of the application at hand. ILs, unlike the homogenous structure of water, boast diverse and elaborate designs, thereby considerably broadening the prospects for specific applications, such as stimulus-responsive materials.^23–28 Materials that react to stimuli (e.g. temperature, light, pH, force, and humidity) have been garnering formidable attention due to their ability to showcase high degrees of functionality and response to a wide variety of external elements.^29–32 Thermoresponsive ionogels, renowned for their superior mechanical and electrical properties complemented by a wide range of thermoresponsive behaviors, are paving the way for their implementation in a variety of applications including soft robotics,³³ batteries,³⁴ and smart devices.³⁵ Despite the existence of numerous reviews on ionogels,^9,16,36–42 a comprehensive review focusing solely on thermoresponsive ionogels is currently non-available. In this light, this article serves to furnish a thorough overview of existing studies delving into thermoresponsive ionogels, complemented by an intermediary discussion on the thermoresponsive properties of polymers in ILs, and potential future research trajectories in this field.

2. Phase separation principle of thermoresponsive polymers

Recognized as intelligent polymeric materials, thermoresponsive polymers are characterized by their reaction to temperature changes. Crucially, these polymers exhibit a drastic shift in solubility at their phase transition temperature, primarily due to the precise miscibility gap in their binary polymer/solvent phase diagram.⁴³ They are classified into two main categories based on their thermoresponsive attributes: Lower Critical Solution Temperature (LCST) behavior and Upper Critical Solution Temperature (UCST) behavior (Fig. 1A).⁴⁴ LCST behavior implicates that polymers remain soluble in the solvent until the phase transition temperature is achieved. Upon surpassing this temperature, two distinct phases materialize – a diluted polymer phase and a concentrated polymer phase.⁴⁵ In contrast, polymers with UCST behavior undergo phase separation before the phase transition temperature but retain mutual solubility thereafter.⁴⁶ In aquatic environments, a paradigm known as coil-to-sphere transition occurs. This thermodynamically preferred process, arising primarily from polymer–polymer interactions, prompts the dissociation of water molecules from polymer chains and the conversion from expanded coils to collapsed spheres.⁴³ This transition process also applies to thermoresponsive ionogels, and the phase transition process of thermoresponsive ionogels is illustrated in Fig. 1B. It is crucial to distinguish between LCST/UCST and phase transition temperature as these terms are frequently, yet erroneously, considered synonymous. Essentially, LCST characterizes the point of minimum phase diagram for a miscible system, beyond which only a single phase can exist, irrespective of polymer concentration. On the other hand, UCST denotes the state at which the maximum of the bi-nodal curve is reached, past which only one phase can exist (Fig. 1A).⁴³ The phase transition temperature, also referred to as the cloud point (T_cp), signifies the temperature at which phase separation arises in the polymer/solvent mixture. This phenomenon presents itself as a turbid suspension due to changes in refractive index within the concentrated polymer phase. Recent studies have proffered a variety of technologically advanced methods for characterizing the phase transition of thermoresponsive polymers, notably T_cp measurement.⁴⁷ For example, nephelometry is a common method that calculates the scattering of light by polymer-rich droplets formed during phase separation. The polymer/solvent mixture's absorption is evaluated at a certain wavelength during the heating process, producing an S-curve of absorbance or transmittance concerning temperature. The inflection point of this curve denotes T_cp.⁴⁸ Other prevalent approaches include nuclear magnetic resonance analysis (NMR)⁴⁹ and differential scanning calorimetry (DSC).⁵⁰ This section intends to focus predominantly on LCST and UCST polymers, while also briefly elucidating multi-temperature-responsive polymers to highlight variations in temperature-responsive behavior.

Fig. 1 Phase transition diagrams. (A) LCST and UCST behavior of polymer/solvent mixtures. (B) Phase transition processes in thermoresponsive ionogels.

2.1. Thermoresponsive polymers with LCST in water

Among water-soluble thermoresponsive polymers, those displaying LCST are the most common. Their LCST behavior is the product of a fine interaction balance between the hydrophilic and hydrophobic components of the unions, which inferred that the polymer's capacity to form hydrogen bonds has a significant influence on this LCST.⁵¹ An essential aspect to consider is how these monomers alter the phase transition temperature in statistical copolymers. When hydrophilic monomers are added, the phase transition temperature tends to rise due to the increased hydrogen bonding, thus raising the LCST. Conversely, integrating hydrophobic monomers lowers the LCST, given their tendency to reduce the degree of hydrogen bonding.^52,53 The alteration of LCST in nonionic polymers relates to variations in the hydrophobic effect and ΔS_mix, the entropy of mixing. Since the hydrophobic effect outweighs the entropy of combinatorial mixing, and given that the LCST change primarily depends on the hydrophobic effect, it follows that the LCST can be considered entropy-driven. Remarkably, the LCST behavior becomes unaffected by the nature of the second monomer when a well-defined block copolymer highlighting at least one thermoresponsive block is achieved. This can be mostly attributed to the constrained interaction between blocks in the copolymer, as the influence of one block on another's hydrogen bonding is typically negligible or impossible.^54,55 By introducing either hydrophilic or hydrophobic monomers into statistical copolymers, it becomes possible to precisely adjust the LCST and incorporate specific functional groups into the polymer chain, elucidating the LCST's finely-tuned regulation.

Poly(N-isopropylacrylamide) (PNIPAm) (Fig. 2A) serves as a pioneering thermoresponsive polymer, distinctly known for its LCST behavior in water. This LCST is measured at roughly 32 °C, a temperature remarkably akin to human body temperature. Fascinatingly, the phase transition of PNIPAm remains quite stable in the face of varying conditions such as pH variations, changes in concentration, and differing chemical environments, potentially earmarking this polymer for biomaterial applications.⁵⁶ However, the polymer's response to salts and surfactants, specifically those with high charge-to-volume ratios such as SO₄²⁻, results in a dramatic effect on the LCST. These ions compete with the polymer chains for water molecules, triggering dehydration and thus lowering the LCST.⁵⁷ Conversely, the interaction of surfactant micelles with N-alkyl substituents forms a barrier against chain–chain interactions, raising the polymer's LCST.⁵⁸ A significant drawback of PNIPAm involves the lack of cohesion between the heating and cooling behavior. Once the transition temperature is surpassed, the polymer's collapse initiates the formation of intramolecular hydrogen bonds, inhibiting rehydration and dispersion during the subsequent temperature decrease.^59,60 Furthermore, NIPAm monomers exhibit a higher cytotoxic profile compared with their polymer counterparts, which lack in vitro cytotoxicity and acute toxicity. Therefore, purification of the polymers is essential to ensure improved biocompatibility, especially when utilized in biomedical applications.^61,62 Concurrently, the exploration of various other thermoresponsive polymers continues to gain significant attention. Other thermoresponsive polymers being scrutinized include the poly(N-vinylamides), specifically poly(N-vinyl caprolactam) (PVCL, Fig. 2B),^63,64 the poly(2-alkyl-2-oxazoline)s class with LCST dependency on alkyl chain length, paralleling PNIPAm (Fig. 2C),^65,66 the poly(ether)s class (Fig. 2D),^67,68 poly(N,N-(dimethylamino)ethyl methacrylate), which displays both LCST and UCST behaviors (Fig. 2E),^69,70 and the poly(oligo(ethylene glycol)(methyl ether)(meth)acrylate)s (POEGMA, Fig. 2F) polymer class.^71,72

Fig. 2 Chemical structures of LCST/UCST thermoresponsive polymers in water.

Numerous studies have unveiled diverse polymers that exhibit LCST behavior. For example, poly(vinyl alcohol-co-vinyl acetal), a thermoresponsive random copolymer which is water-soluble, is produced through the partial condensation reaction of poly(vinyl alcohol) (PVA) with acetaldehyde (Fig. 2G). This copolymer reveals a transition temperature range in its aqueous solutions between 17 °C to 41 °C. Notably, variations in the transition temperature are dictated by factors such as the ratio of the introduced acetal functional group and the relative molecular mass of PVA.⁷³ Similarly, the responsiveness of poly(2,3-epoxypropan-1) was transformed through esterification with acetic anhydride, culminating in the formation of a thermoresponsive, water-soluble copolymer known as poly(glycidol-co-glycidol acetate) (Fig. 2H). Intriguingly, the transition temperature of these copolymers can be modulated from 4 °C to 100 °C, dependent on the degree of acetal substitution of the polyglycidol.⁷⁴ Furthermore, both homopolymers and block copolymers of poly(2-hydroxypropyl acrylate) (PHPA, Fig. 2I) have been found to exhibit LCST behavior. Studies suggest a correlation between the polymer's molecular weight and cloud point. Particularly, polymerizations with a degree less than 20 led to the complete dissolution of the polymer at all experimental temperatures, whereas higher molecular weight polymers presented lower cloud points.⁷⁵

Research has spanned across the temperature responsiveness of numerous chemically-modified natural polymers in addition to synthetic polymers, with studies on derivates based on cellulose, chitosan, and starch exhibiting significant outcomes. A study by Li et al. specifically concentrated on the thermogelation behavior of aqueous methyl cellulose solution. They elucidated the solution's role-transforming property, where it metamorphoses into a gel form upon heating and subsequently reverts to its liquid state upon cooling.⁷⁶ Contrastingly, Selmani et al. devised a thermoresponsive hydrogel using an aqueous chitosan solution. After neutralizing this solution with counterionic salts of polyols, the resultant hydrogel exhibited a unique feature of remaining liquid at temperatures below room level but undergoing a phase transition and forming a gel around human body temperature. This characteristic thus earmarks this hydrogel for potential use as an in situ injectable drug carrier.⁷⁷ Furthermore, Zhang et al. spotlighted the synthesis of a thermo-responsive polysaccharide constructed from a starch variant modified with butyl glycidyl ether (Fig. 2J). They were able to manipulate the cloud point of the resultant polymer solution by adjusting the degree of functionalization of the modified hydroxyl group of starch, enabling it to fluctuate between 4.5 °C and 32.5 °C.⁷⁸ Consequently, such scholarly endeavours underscore the significant potential and broad diversity of chemically-modified natural polymers in the field of temperature responsiveness.

2.2. Thermoresponsive polymer with UCST in water

UCST behavior is less commonly evidenced in thermoresponsive polymers relative to LCST behavior, mainly due to their increased sensitivity to environmental factors such as pH, concentration, and ionic strength.⁴⁶ Such materials can be classified based on the interactions that trigger phase separation, typically falling into two categories: hydrogen bonding interactions between the chains or ionic interactions.⁴³ Materials such as poly(N-acryloyl glycinamide) (PNAGA, Fig. 2K), its derivatives, and copolymers,⁷⁹ urea-based derived polymers⁸⁰ (Fig. 2L), and poly(acrylamide-co-acrylonitrile) copolymers⁸¹ (Fig. 2M) demonstrate thermal responsiveness due to hydrogen bonding between polymer side groups, similar to LCST polymers. However, a key differentiation resides in the primary mechanism instigating UCST behavior. Unlike LCST polymers where the phase transition is entropy-driven due to polymer–polymer and solvent–solvent interactions, UCST behavior reflects robust inter-polymer and intra-solvent interactions juxtaposed with weaker polymer–solvent interactions, rendering it predominantly enthalpy-driven.^46,82 UCST behavior presents an intriguing attribute in that it can be strategically modified via a manipulation of polymer composition. For example, the inclusion of hydrophobic monomers or increases in the homopolymer's molecular weight decreases the mixture's total entropy, thereby augmenting the UCST of a statistical copolymer. Conversely, incorporation of hydrophilic monomers diminishes hydrogen bonding, hence lowering the UCST.⁸³ This equilibrium offers a convenient tactic for calibrating the UCST to meet specific requirements, enabling the integration of particular functional units in the polymer chain through statistical copolymerization with either hydrophilic or hydrophobic monomers. Despite their potential, thermoresponsive polymers come with significant challenges, primarily their high susceptibility to minor fluctuations in factors such as pH, polymer concentration, polymer hydrolysis, and the chemical environment, which can significantly affect the transition temperature. For instance, adding a minimal quantity of ionic groups to PNAGA can completely eliminate its UCST.⁸⁴

The material alluded to in the second instance is an ionic polymer, distinguished by its densely charged cationic and anionic groups that typically maintain an overall neutral charge (Fig. 2N). This ionic polymer displays amphiphilic properties, possessing high dipole moments that enable both intra- and intermolecular dipole–dipole interactions. These interactions induce the polymer chains towards self-aggregation, driving the polymer to a super-collapsed, nonconjugated state.⁸⁵ At sufficiently elevated temperatures (above the UCST), the polymer chains become wholly solvated and separated due to the disruption of dipole–dipole interactions. The two discernible conformations of these amphiphilic brushes mainly diverge in their overarching wettability. In the conjugated state, for example, the materials exhibit vigorous electrostatic-induced hydration, typically rendering them hydrophilic.⁸⁶ Conversely, when the polymers exist in a non-conjugated state, they present decreased hydrophilicity as water morphs into an unfavorable solvent, resulting in reduced polymer swelling.⁸⁵ However, it is worth noting that the degree of polymer hydrophilicity remains lower than that seen in their unattached state. The principle of dipole–dipole self-association is postulated for all amphiphilic polymers, but not all manifest UCST behavior. Sulfobetaine (SB) polymers exemplify thermoresponsive behavior, while carboxybetaine (CB) polymers skirt phase separation.⁸⁷ This disparity in behavior is governed by the cationic and anionic groupings within each ampholyte monomer, adhering to Collins’ “law of matching water affinities,” where ion pairings with similar charge densities generally display enhanced binding and reduced water interaction.⁸⁸ The lesser charge difference between the cationic trimethylammonium and anionic sulfonate groups in SB polymers, compared with the same cationic group and the anionic carboxyl group in CB polymers, accounts for the increased binding and minimized water interaction.⁸⁷ Moreover, the incorporation of varied anions and cations into the polymer solution can shatter the dipole–dipole interactions, amplify the polymer's water solubility, and contract its UCST.⁸⁹ Consequently, the characteristics and categories of the cationic and anionic groups play an instrumental role in the behavior of amphiphilic polymers.

2.3. Thermoresponsive polymers with a multi-temperature response in water

Traditional thermoresponsive homopolymers undergo dual property alterations: a transition from hydrophilic to hydrophobic, and the initiation of hydrophobic interactions prompting aggregation or precipitation beyond the critical aggregation concentration. In contrast, block copolymers display a more complex behavior. By binding two thermoresponsive chain segments via covalent bonds at each chain's end, dual thermoresponsive traits arise, signifying conformational shifts.⁹⁰ These properties are deftly engineered through the inclusion of both LCST- and UCST-type segments. Within these dual thermoresponsive block copolymers, a variety of structures can be forged by modifying the properties of every chain segment. Below the critical micelle concentration (CMC), the solution temperature dictates the formation of hydrophilic–hydrophilic, amphiphilic, or hydrophobic–hydrophobic structures. However, beyond the critical micelle concentration (CMC) – a threshold value that is temperature-dependent – the dual thermoresponsive block copolymer undergoes a transformation in its self-assembling behavior, manifesting as dissolution, aggregation, or even precipitation.⁴⁴ This results in more complex states, or structures, relative to homopolymers. Conversely, triple block copolymers showcase triple temperature responsiveness, leading to 12 unique hydrophilic/hydrophobic structures. These distinct structures emerge from varying combinations and arrangements of segments with LCST or UCST. Interestingly, block copolymers showcasing quadruple or higher temperature responsiveness can undergo even more complex structural transformations in aquatic solutions. Over recent years, these thermoresponsive characteristics have been deemed invaluable, leading to their utilization in various polymeric material contexts, including, but not restricted to, gels and nanoparticles.⁹⁰

3. Phase separation in ionic liquids induced by temperature

ILs can be subdivided into two primary categories according to the character of the cation: protonic ILs and non-protonic ILs. Protonic ILs take form through a simple amalgamation of Brønsted acids and Brønsted bases, catalyzing the transfer of protons from the acid to the base.⁹¹ Contrastingly, non-protonic ILs necessitate a lengthier synthetic route. The conventional fabrication of proton-free ILs involves the alkylation of the corresponding amine, phosphine, or sulfide utilizing an alkyl halide (R–X) to yield a halide salt with the desirable cation. Should this halide salt possess a melting point under 100 °C, it can be categorized as an ionic liquid. To exchange the halide with the desired organic anion, an anion swap can be performed through a multifaceted decomposition process. The prevalent cations and anions present in ILs are exhibited in Fig. 3.⁹

Fig. 3 Common cations and anions in ionic liquids, with R=H or alkyl groups.

Additionally, when categorizing ILs based on their composition, they are generally segregated into hydrophilic ILs or hydrophobic ILs, pertaining to their miscibility with water. Notably, numerous studies have indicated that the miscibility of certain ILs with water significantly hinges on the temperature. This classification ambiguity surfaced with the discovery of thermoresponsive ILs in the later part of the 1990s.⁹² Contrary to thermoresponsive polymers, a greater proportion of ILs demonstrate UCST behavior compared with LCST behavior. This introduction presents a concise synopsis of thermoresponsive ILs’ characteristics and delves into their thermoresponsive behaviors. Ensuing sections will discuss the two types of thermoresponsive IL – LCST-type and UCST-type – while providing a brief elucidation of the mechanism steering their temperature modulation.

3.1. Mechanisms of temperature regulation in thermoresponsive ILs

Thermoresponsive ILs display thermoregulatory characteristics that arise from the dynamic balance of interaction forces among cations, anions, and solvent molecules. One noteworthy factor influencing the distinctive temperature sensitivity of ILs is the presence of hydrogen bonding, in conjunction with electrostatic forces. Hydrogen bonding, recognized as the weak dynamic interaction between hydrogen atoms and electronegative atoms like O, N, or F, plays an essential role in the dynamics of ILs. This not only facilitates the formation of mechanically robust ionic gel structures but also affects the phase separation demeanor of ILs. In ILs, hydrogen bonding is very different from the commonly known hydrogen bonding, which is defined as “bi-ionic hydrogen bonding” between charged substances, specifically, cations, and anions. The formation of a myriad of different hydrogen bonds in ILs is shaped by the diverse types and quantities of hydrogen bond donor and acceptor sites.^93,94 For example, the 1,3-dialkylimidazole IL is viewed as a hydrogen-bonded polymeric supramolecule, where a single cation is encircled by at least three anions, and the quantity of anions encircling the cation is influenced by the size of the anion and the N-alkyl group present within the cation.⁹⁵ Nonetheless, the strength of hydrogen bonding in ILs fluctuates based on the specific molecular structure. The imidazole ring typically forms strong hydrogen bonds, with H2 being the most acidic among the imidazole cations, succeeded by H4 and H5 in the imidazole radical nucleus, but the appearance of methyl substitutes for H2 in imidazolium-like ions detracts from the hydrogen bonds’ strength.⁹⁶ Some even posit that whilst hydrogen bonding does exist in ILs, its strength is not as pronounced as illustrated in literature and does not govern the structure and dynamics of ILs.⁹⁷ Introducing other molecules into pure ILs breaks the hydrogen bonding network and induces the formation of ion pairs. Upon dissolving in solvents with relatively high dielectric constants, such as acetonitrile and dimethylsulfoxide, larger ionic and neutral aggregates can evolve.⁹⁸

Moreover, hydrogen bonding interactions between the hydrogen atom of the cation and the electron-rich center of the anion are generally weaker in N-alkylpyridinium, N-alkylpiperidinium, alkylammonium-based ILs, and alkylphosphonium salts as compared with imidazolium-based ILs. Paradoxically, when the alkyl groups are swapped with hydrogen atoms, the ILs convert to protonated ILs, resulting in a level-up in hydrogen bonding interactions. The proton transfer incites the availability of proton donor and acceptor sites and the construction of a hydrogen bonding network, distinguishing protonated ILs from other ILs.^99,100 The phase conduct of ILs in the company of water or organic solvents is shaped by several clashing interactions, including long-range coulombic forces and short-range hydrogen bonding. Both the IL and the solvent substantially impact the phase separation phenomena. If the dielectric constant (ε) is high for nonionic solvents, the phase transition is commanded by specific interactions such as hydrogen bonding or hydrophobic interactions, as the Coulomb forces are feeble. Conversely, media with lower dielectric constants rely on Coulomb forces for phase separation. The modest electron density of the anion affects the solubility of ILs in solvents, while the cationic structure predominantly influences the solubility via hydrophobic interactions emanating from the alkyl or aryl rings.¹⁰¹ Predominantly, the notable characteristic of ILs lies in their designability, surpassing that of conventional molecular solvents, enabling a customization of the interaction forces by adjusting functional groups and switching the combination of ionic pairs. The thermoregulatory properties of ILs can be viewed as temperature-bound solvation properties, mainly credited to hydrogen bonding or other potential interactions between ILs and specific solvents.^102,103 Recently, ¹H-NMR studies have indicated that hydrogen bonding between IL cations and anions is appreciably attenuated above the system's critical temperature with UCST. This weakening of intramolecular interactions in ILs boosts the intermolecular interactions between ILs and solvents, facilitating the creation of uniform solutions.¹⁰³ Conversely, in systems with LCST, such as amino acid IL–water mixtures, hydrogen bonding interactions between the anions of these ILs intensify as the temperature ascends. This enhancement debilitates the anion–water electrostatic interactions, ultimately provoking the separation of ILs from water and exhibiting LCST behavior.¹⁰⁴ As such, the temperature sensitivity of ILs emerges due to the disturbance or modification of noncovalent interaction forces by temperature fluctuations, resulting in a crucial equilibrium of interaction forces. Thus, there exists no definitive quantitative explanation for the cause of temperature sensitivity, and the interaction forces among IL molecules with differing structures may fluctuate.

3.2. UCST-type ILs

The physicochemical attributes of most ILs are deeply rooted in the combination of cations and anions. Minute alterations to the IL structure can exert a consequential impact on their harmonization with other solvents, encompassing other ILs.¹⁰⁵ Typically, it is noted that the compatibility of ILs in nonaqueous solutions as a trend increases with the lengthening of the alkyl chain attached to the cation. Nonetheless, sometimes the boiling temperature of the solvent could confine the observation of the ultrafast solvent change-time (USCT), thereby complicating the visual detection of ILs’ solubility in solvents. Hence, the deployment of supplemental techniques is often necessary. These can include conductivity measurements, spectroscopic techniques (FT-Raman, NMR, UV spectroscopy), reversed-phase gas chromatography, or other pertinent methods.¹⁰⁶

ILs–H₂O binary mixtures have been subject to extensive study, with water routinely seen as an “impurity” in ILs due to its interaction with ILs via hydrogen bonding, which can consequently tweak the properties of the ILs. The phase separation temperature is concurrently influenced by both the composition of ILs and the proportion of ILs in relation to separately incorporated H₂O. The solubility of water in ILs relies on the blend of cations and anions, with the anions exerting a more profound impact on the phase behavior.¹⁰⁷ Investigations into imidazolium ILs have manifested that the intercalation between water and the ionic liquid diminishes as the alkyl chain length on the imidazolium cation is elongated, precipitated by an increase in hydrophobicity. Similarly, this intercalation also declines with ramping up the hydrophobicity of the anion, in the successive order of [BF₄]⁻ < [CH₃(C₂H₄O)₂SO₄]⁻ < [C(CN)₃]⁻ < [PF₆]⁻ < [NTf₂]⁻.¹⁰² Within these, the [NTf₂]⁻ anion is favored over [PF₆]⁻ and [BF₄]⁻ anions owing to its superior hydrolytic stability, reduced viscosity, and inability to decompose into minuscule, yet toxic amounts of HF.

In the arena of IL–alcohol systems, it is observed that the solubility of ILs in alcohols tend to be inferior, while conversely, the solubility of alcohols within the realm of ILs tends to be superior. The influence of the alkyl chain length on the cation and the choice of IL anions on the mutual solvency of ILs with alcohols depicts a variation significantly differing from its impact vis-à-vis the mutual solubility with water. In comparison with the [PF₆]⁻ anion, the [BF₄]⁻ anion notably curtails the UCST. The comparative compatibility of alcohols with differing anions follows the sequence: [N(CN)₂]⁻ > [CF₃SO₃]⁻ > [NTf₂]⁻ > [BF₄]⁻ > [PF₆]⁻.¹⁰⁸ The solubility of alcohols in ILs is observed to be contingent on the type of alcohol, with tertiary alcohols boasting a superior solubility when compared with secondary alcohols, and primary alcohols showcasing the least solubility. This trend is inherently bound to the basicity of the alcohols, i.e., their capacity to act as a hydrogen bonding acceptor. The interaction of alcohols with ILs weakens as the alkyl chain of the alcohol lengthens, due to the dwindling of hydrogen bonding, dipole, and coulombic forces, provoking an elevation in UCST.¹⁰⁹ For imidazole ILs, magnification in the length of the alkyl chain on the cation prompts a decline in UCST. The substitution of the most acidic hydrogen at the C2 position of the ring with a methyl group propels the UCST due to a reduction in alcohol–cation hydrogen bonding.

Miscibility studies conducted using the [C_nmim][BF₄] and dihydroxyalcohol system disclose the same behavior associated with the UCST. The length of the alkyl chain present in the alcohols also holds sway over the miscibility, even though this effect reveals a more complex nature compared with monohydroxy alcohols. Specifically, when the chain length at the N-3 location of the [C_nmim] cation lengthens, the temperature of the UCST rises for systems incorporating 1,2-ethylene glycol. Conversely, an inverse pattern prevails for systems infused with monohydroxy alcohols – the UCST temperature decreases as the chain length augments. The effect of the imidazolium cation side chain is contingent on the structure of the diol. The compatibility of [C₂mim][BF₄] diminishes with the elongation of the alkyl chains in 1,2-dihydroxyalcohols, but an opposite effect comes into play for imidazolium salts composed of phosphonium cations. In the context of alkylimidazolium ILs and dihydroxy alcohols, the miscibility follows the order of [BF₄]⁻ > [NTf₂]⁻ > [PF₆]⁻ and is firmly linked with the hydrogen bonding basicity of the anion. Moreover, the relative positioning of the hydroxyl groups within the alcohol molecule also affects miscibility, with adjacent placements promoting miscibility.^110,111

While considering pyridine-based ILs, it is observed that they incite a more predominant degree of immiscibility spanning over an extended range of temperatures compared with their imidazole-based cation counterparts. Nonetheless, in terms of practical applications relative to multiphase and fully ionized systems, the disparities between the two categories of ILs appear to be rather trivial. Imidazole-based ILs, armed with their more amenable lower melting points, might hold the edge for applications conducted at room temperature.^112,113 Phosphonium-based ILs, on the other hand, predominantly display immiscibility with other ILs at ambient temperature. The degree of miscibility demonstrated by these ILs is contingent on temperature and typically follows the UCST behavior.

3.3. LCST-type ILs

In the case of ILs, the LCST behavior, steered predominantly by entropy, is a relatively seldom observed phenomenon when juxtaposed with the UCST behavior. Scrutinizing various amalgamations of cations and anions, it is observed that the majority of reported ILs exhibiting LCST-type behavior hinge largely on the modified hydrophilicity of phosphorus cations, such as tetrabutyl- or tributyl-hexylphosphorus cations ([P_4,4,4,4]⁺ and [P_4,4,4,6]⁺). Contrarily, [CF₃COO]⁻ and benzenesulfonic acid derivatives persist as the most commonly adopted anions for the synthesis of LCST ILs.¹¹⁴ Li et al. delved into the phase separation behavior of poly(ethylene oxide) (PEO) in [C₂mim][BF₄], observing an LCST phase separation.¹¹⁵ During LCST-type phase evolutions, a homogeneous concoction undergoes phase separation on being heated; however, it reverts to a homogeneous state upon cooling down.

In a bid to dissect the parameters that impact the LCST-type phase separation between ILs and water, Ohno et al. laid out a set of ILs exhibiting varying degrees of hydrophobicity by combining either phosphorus or ammonium cations with a variety of anions.¹¹⁶ Their observation concluded that the LCST values escalated in IL–H₂O mixtures as the hydrophilicity of the constituent ions amplified. This insight implies that LCST-type IL–H₂O concoctions could potentially serve as a mechanism to evaluate the hydrophilicity of specific ions, by determining the miscibility temperature (CST) values subsequent to the addition of respective ionic substances.¹¹⁷ Notably, [P_4,4,4,4][CF₃COO] molecules are capable of forming enduring aggregates in aqueous solutions under certain conditions prior to phase separation, in the absence of surfactants. These aggregates manifest unique microemulsion-like properties.¹¹⁸ Moreover, with the incorporation of an apt amount of surfactant (Triton X-100), [P_4,4,4,4][CF₃COO] aqueous solution is able to establish microemulsions under elevated temperatures; underneath the LCST, Triton X-100 molecules amalgamate into micelles while [P_4,4,4,4][CF₃COO] disperses within the encircling aqueous phase. This reversible phase transition oscillating between micelles and microemulsions can be maneuvered by employing temperature as an activator.¹¹⁹ ILs, comprising of tetrapentylphosphine cations and elongated alkyl carboxylate anions with distinct hydrophobic and highly polar characteristics, exhibit LCST-type phase transitions upon the introduction of water. This conduct is deemed fit for cellulose solubilization, marking it as the inaugural report concerning the solubilization of cellulose in a hydrophobic IL.¹²⁰

The study further accentuates the vital role of both the polarity and density of ILs in the blueprint intended for cellulose solubilization. Other ILs anchored on phosphonium styrenesulfonate, which are descendants of [P_4,4,4,4][SS] (tetrabutylphosphonium 4-styrenesulfonate), have evinced an LCST-type phase transition when amalgamated with water. This characteristic prevails even post the polymerization of the [P_4,4,4,4][SS] ILs. However, the incorporation of the more hydrophobic [P_4,4,4,6][SS] ILs considerably decreases the transition temperature of the water concoctions.¹²¹ By drafting the cations and anions independently, polarized ILs endowed with distinctive features can be procured. For instance, alkylphosphine-based ILs such as the ethylphosphonate anion [(EtO)HPO₂]⁻, paired with tetra-n-hexylphosphonium ([P_6,6,6,6]⁺) and tri-n-hexyl-n-octylphosphonium ([P_6,6,6,8]⁺) cations, depict LCST behaviour when interfused with water. Furthermore, tetra-n-octylphosphonium ethylphosphonate([P_8,8,8,8][(PEO)HPO₂]) and [P_4,4,4,4][(PEO)HPO₂] ILs uphold stable phase separation and robust hydrogen bonding with each other, even when incorporated with water. Besides, phosphonium ILs, infusing bis(2-ethylhexyl) phosphate ([DEHP]) anions, such as [P_4,4,4E₃][DEHP], broadcast LCST phase behavior. This characteristic can be employed for the temperature-driven extraction of metal ions within the homogeneous phase, and the LCST can be calibrated by altering the IL structure, whereby elongated oligoethylene chains escalate the LCST.¹²²

The phase demeanor of ILs when interfused with water is impacted significantly by shifts in the hydrophobicity/hydrophilicity effects orchestrated by cations and anions. With the aid of ranking the phase demeanor of IL–water interactions on the scale of the hydrophobicity of the constituting ions, one can spot LCST-type phase transitions prevailing in the intermediate region nestled between hydrophobic and hydrophilic ILs, signifying that ILs with moderate hydrophilicity are more likely to display LCST behavior.^123,124 As a general rule of thumb, orchestrating thermally responsive IL/water interactions entails selecting ion components that secure an optimal equilibrium between hydrophobicity and hydrophilicity, thereby enabling the surfacing of LCST behavior. This equilibrium between hydrophobicity and hydrophilicity is deemed the most consequential parameter in steering the LCST phase demeanor. Alternatively, a fruitful approach to materialize LCST behavior within IL/water systems is by resorting to the use of mixed ILs. In cases wherein the target IL is exceedingly hydrophilic to instigate an LCST-type phase transition, the desired thermal response can nevertheless be fulfilled by amalgamating a more hydrophobic IL.¹²⁵ For instance, the blend of [P_4,4,4,8][CH₃SO₃]/[P_4,4,4,8][CF₃COO] is an ensemble of hydrophilic and hydrophobic ILs. Even though [P_4,4,4,8][CH₃SO₃] amalgamates with water and [P_4,4,4,8][CF₃COO] is immiscible with water, the resultant mixture showcases LCST behavior.¹¹⁶

4. Thermoresponsive ionogels

By unifying the attributes of ionogels and thermoresponsive materials, thermoresponsive ionogels put forth the prospect of additional functionality, alongside a wide spectrum of applications, including temperature-dependent ionic conductivity. This opens avenues towards the advent of smart materials which can be manipulated through temperature. Generally, the construction of thermoresponsive ionogels necessitates the incorporation of thermoresponsive polymers into the polymer network of an ionogel. This is accomplished through a variety of techniques like in situ polymerization or the physical fusion of the said components.

Pan et al. recently summarised the temperature-response mechanisms of ionogels, encapsulating several main mechanisms.³⁶ (1) The temperature-influenced ion-pair uncoupling–coupling effect. At room temperature, cations and anions predominantly exist as neutral ion pairs, with a minute fraction disassociating and free roving. Yet, with a temperature surge, the electrostatic rapport between anions and cations gets perturbed, leading to the bifurcation of neutral ion pairs into free-moving anions and cations (Fig. 4A). Consequently, both the ion concentration and ionic conductivity of the ionogels experience an upsurge. This mechanism is frequently observed in most ionic temperature sensors.¹²⁶ (2) The sol–gel transition mechanism occurs in certain polymer/IL blends, where the polymer forms micelles when dissolved in the IL, and upon heating, these micelles self-assemble to form a gel. Fig. 4B illustrates the capability of ionogels derived from a thermally responsive ABC-triblock copolymer (consisting of poly(benzyl methacrylate) (PBnMA), poly(methyl methacrylate) (PMMA), and poly(2-phenylethyl methacrylate) (PPhEtMA)) in conjunction with ILs to undergo a sol–gel transition at distinct temperatures, showcasing their tunable thermal behavior.¹²⁷ (3) Ionogels may exhibit critical temperature effects, such as LCST or UCST behavior (Fig. 1A). (4) Another key mechanism involves dynamic covalent bonding, which, when incorporated into ionogels, can achieve a temperature response by enhancing the mechanical properties of the gel through cross-linking, and weakening its strength by breaking the bonds. In a study by Fan and colleagues,¹²⁸ they chemically linked furan and maleimide moieties to an ionogel matrix, enabling dynamic covalent crosslinking within the gel structure (Fig. 4C). At 70 °C, the crosslinks were formed, resulting in enhanced mechanical properties of the ionogel. Conversely, when the temperature rose to 110 °C, the dynamic covalent bonds were cleaved, causing a decline in the gel's mechanical strength. (5) Leveraging the advantages of the ionic structure and ILs, with a supramolecular response mechanism holding vital importance in ionogel preparation, given that temperature alterations can trigger the destruction or reconstruction of supramolecular interactions, thereby impacting the modulus of the ionogels. For instance, supramolecular interactions can form between [TFSI]⁻ ions and CD molecules (Fig. 4D), and their dynamics are responsive to temperature variations.¹²⁹ (6) The Soret effect in ionogels relates to the thermal dispersion of ionic components from a high-temperature region to a low-temperature region under a temperature gradient (Fig. 4E). This phenomenon can produce a voltage fluctuating with temperature and can be harnessed to discern the ambient temperature range.¹³⁰ Noteworthy is that for a particular category of thermoresponsive ionogels, the temperature response might need to account for numerous mechanisms. This exploration thus provides an exhaustive synopsis of ionogels that display UCST/LCST behavior.

Fig. 4 Some main temperature response mechanisms of ionogels. (A) Mechanism of the influence of temperature on the ion pair decoupling–coupling. Adapted with permission: Copyright 2020, American Chemical Society.¹²⁶ (B) Scheme of the sol–gel process. Adapted with permission: Copyright 2016, American Chemical Society.¹²⁷ (C) Mechanism of the influence of temperature on the dynamic covalent bonds. Adapted with permission: Copyright 2018, American Chemical Society.¹²⁸ (D) Temperature response of the supramolecular ionogels. Adapted with permission: Copyright 2014, American Chemical Society.¹²⁹ (E) Illustration of the Soret effect. Adapted with permission: Copyright 2022, American Chemical Society.¹³⁰

4.1. UCST type ionogels

In 2006, the UCST-type phase transition of ionogels was first reported. PNIPAm, a widely researched thermoresponsive polymer, exhibits an LCST phase transition in aqueous solutions near body temperature. Conversely, its behavior in ILs exhibits an entirely distinct transition, contrasting with its aqueous behavior. This reversal in phase transition is attributed to the thermodynamic differences in NIPAm's interaction with its solvents. In water, the hydrophilic NIPAm forms hydrogen bonds with water, which are disrupted upon heating, resulting in LCST behavior. In ILs, however, NIPAm does not preferentially form hydrogen bonds with the hydrophobic solvent; instead, it forms stable polymer–polymer bonds. Heating leads to the disruption of these intramolecular interactions, causing the polymer to become solvated by the ILs at a critical temperature, thus manifesting UCST behavior.¹³¹ Watanabe and Ueki unearthed the UCST behavior of PNIPAm in a prototypical hydrophobic IL called 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]).¹³² It was revealed that the phase separation temperature of the system fluctuates in line with the molecular weight or concentration of PNIPAm, wherein a more substantial molecular weight or concentration correlates to a higher phase separation temperature. Proceeding with this observation, they dove into the preparation of PNIPAm polymer gels and converted them into PNIPAm ionogels by substituting the ethanol solvent in the gel particles with [C₂mim][NTf₂]. In a remarkable observation, the gel spheres were detected to constrict at low temperatures and expand in elevated temperatures in [C₂mim][NTf₂]. This revelation of PNIPAm showcasing UCST behavior in ILs laid the foundational stone for subsequent analyses on UCST-type ionogels. Later, Lodge et al. synthesized an ABA triblock copolymer with terminal A blocks of PNIPAm and a central B block of PEO (Fig. 5A), which assembled to a thermoresponsive ionogel in [C₂mim][NTf₂]. This ionogel exhibited significant conductivity and mechanical fortitude even under substantial strains, yet its critical gel temperature (T_gel) languished beneath room temperature, posing a limit on its practical applications.¹³³ To circumvent this predicament, Lodge et al. incorporated solvent-hydrophobic block polystyrene (PS) into the copolymers, leading to the creation of PNIPAm-PS-PEO-PS-PNIPAm pentablock copolymers (Fig. 5B) and broadening the temperature bracket within which the ionogels could be ustilized.¹³⁴

Fig. 5 Schematic representation of the chemical structure and gel transition of copolymers based on PNIPAm showing UCST behavior. (A) Chemical structure and gel transition of PNIPAm-PEO-PNIPAm. (B) Chemical structure and gel transition of PNIPAm-PS-PEO-PS-PNIPAm. Adapted with permission: Copyright 2008, Macromolecules.¹³⁴ (C) Chemical structure of P(AzoMA-r-NIPAm). (D) Chemical structure of PEO-b-P(AzoMA-r-NIPAm). (E) Chemical structure of P(NIPAm-co-NDEAm). (F) Chemical structure and gel transition of P(AzoMA-r-NIPAm)-b-PEO-b-P(AzoMA-r-NIPAm). Adapted with permission: Copyright 2015, Angewandte Chemie International Edition.¹³⁹ (G) Chemical structure of PS-b-PDMAm-b-P(AzoMA-r-NIPAm). (H) Chemical structure of PE-alt-PP-b-PEO-b-PNIPAm.

Subsequently, Watanabe and Lodge conducted further experiments, where they ushered in an azobenzene moiety and fabricated the random copolymer P(AzoMA-r-NIPAm) through the reversible addition–fragmentation chain transfer (RAFT) polymerization of NIPAm and 1-phenylazophenyl methacrylate (AzoMA) (Fig. 5C).¹³⁵ Once dissolved in [C₂mim][NTf₂], this copolymer exhibited UCST behavior and underwent separation through temperature/light dual-initiated phase segregation behavior. Then, they inducted a PEO block with a high solubility quota in ILs, by polymerizing it with P(AzoMA-r-NIPAm) to fabricate the diblock copolymer PEO-b-P(AzoMA-r-NIPAm) (Fig. 5D). Their investigative pursuits noted UCST behavior in 1-butyl-3-methylimidazole hexafluorophosphate ([C₄mim][PF₆]). Notably, the highly solvated PEO block curtails the critical micellar temperature to below 5 °C, marking the first recorded instance where the monomer/micellar transition was observed in ILs under light irradiation at suitable wavelengths.¹³⁶ Additionally, Yoon et al. prepared a diblock copolymer premised on PNIPAm and N,N-diethylacrylamide (NDEAm) (Fig. 5E), which coalesces with ILs to produce ionogels exhibiting UCST behavior, which accompanied by volume alterations in tandem with temperature fluctuations.^137,138

Subsequent to their explorations, Watanabe and Lodge initially employed their findings in the fabrication of ABA-type triblock copolymers, with the A block being P(AzoMA-r-NIPAm) and the B block identified as PEO (Fig. 5F). These copolymers were then amalgamated with [C₄mim][PF₆] to yield UCST-type ionogels. The ABA triblock copolymer (at 20 wt%) underwent self-assembly upon phototriggering within [C₄mim][PF₆], paving the way for the formation of a reversible polymer network, and exhibiting a sol–gel transition at the bistable temperature (53 °C) due to the photoisomerization of the azobenzene unit.¹³⁹ It was further discerned that the polymer–IL composites experienced notable alteration in fluidity under light irradiation, leading on to pronounced photorepairing ability in damaged ABA ionogels. Specifically, UV light triggered the dissolution of the damaged portion and made way for filling up of the gaps, which then hardened under the incidence of visible light, effectively restoring the damaged region. Tensile test results reconfirmed the remarkable recovery efficiency of the photorepaired ABA ionogels, registering up to 81% of the fracture energy compared with the untouched samples. These discoveries bear consequential implications for the progression of soft materials encompassing plastic electronics, actuators, and electrochromic gels.¹⁴⁰ However, due to the azobenzene units present in both end segments of ABA triblock copolymers, it came to light that the UV transmittance was somewhat impacted. Taking stock of this, Watanabe et al. subsequently ventured into the design and synthesis of ABC-type triblock copolymers, consisting of a hydrophobic ionic liquid A block, a pro-ionic liquid B block, and a photo/thermal-responsive C block (Fig. 5G). Similar to the ABA structure, these copolymers were blended with an ionic liquid to prepare UCST-type ionogels. By confining the azobenzene fraction solely to the C-terminal, larger quantities of azobenzene were introduced without jeopardizing the UV transmittance. The ABC structure, vis-à-vis the ABA structure, eliminated the cyclic chains that deterred the elasticity of the polymer network. As a result, a lesser polymer concentration was needed to achieve a heightened elastic modulus. This material showcases potential suitability for usage in photoresponsive soft actuators and photoremediation soft materials.¹⁴¹ Moreover, Lodge et al. developed an ABC-type triblock copolymer (PON) constructed of A-block poly(ethylene–propylene) (PEP), B-block PEO, and C-block PNIPAm (Fig. 5H). PON exhibited potential for constituting UCST-type ionogels in [C₂mim][NTf₂].¹⁴² Prior investigations by the same team propounded that PON was capable of forming LCST-type hydrogels in an aqueous solution through a stepwise micelle–gel mechanism.¹⁴³

Building on the cornerstone discovery by Watanabe and Lodge, Liu et al., along with other research cohorts, have contributed notable advancements toward the understanding of thermoresponsive ionogels. Liu's team, for instance, employed [C₁mim][NTf₂] as their chosen IL to synthesize UCST-type ionogels (Fig. 6A), delving into the impact of ILs on UCST, and extended the concept of Berghmans’ point in the phase diagram to their studies (Fig. 6B). In Fig. 6C, the ionogel exhibits a significant stiffness variation, ranging from 0.66 kPa to 62.5 MPa, across different temperatures, which is not affected by the polymer solid content. The resulting ionogels can withstand mechanical loads exceeding 100 times their own weight at low temperatures, while they have good shape adaptation and adhesion at high temperatures (Fig. 6D). The researchers succeeded in manipulating the Bergmann's point through the strategic selection of multi-component ILs, thereby proving that the UCST and rigidity of the ionogels can be calibrated for a plethora of applications encompassing fields such as soft robotics, adhesives, and aerospace. These progressive findings underline the potential applicability of their fundamental concepts to other thermoreversible polymer gels, thereby establishing ionogels as highly adaptable, multifaceted materials for a diverse array of applications.³³

Fig. 6 A UCST ionogel exhibiting significant changes in stiffness. (A) Synthesis of PNIPAm-based ionogels and phase separation process. (B) Schematic diagram of Berghmans’ point. (C) Mechanical properties of PNIPAm-based ionogels. (D) Demonstration of the potential applications of PNIPAm-based ionogels. Adapted with permission: Copyright 2022, Nature Communications.³³

The creation of thermoresponsive ionogels in a range of ionic liquids can be realized by altering the structure of N-isopropyl acrylamide. Demonstrating this, Kim et al. showcased the fabrication of UCST-specific ionogels apt for smart window interlayers through the incorporation of 3-(2-(isopropylamino)-2-oxoethyl)-1-vinyl-1-imidazole-3-bis(trifluoromethanesulfonyl)imide ([VNIm][NTf₂]) IL, serving as a polymer network with 1-butyl-4,4′-bis(trifluoromethanesulfonyl)imide (MBV[NTf₂]) IL and ferrocene (Fig. 7A). This ionogel is characterized by a dual nature of electroluminescence and thermochromic properties. While the presence of water enables a variation in the transparency of the ionogels with temperature fluctuations in aquatic scenarios, in the absence of water it exudes noteworthy electrochromic qualities. The application of a 1.8 V voltage at temperatures beneath the UCST translates into a reversible window color transformation from colorless to violet, and at temperatures surpassing UCST, the color interchanges from purple to blue, due to the collaborative effects of electrochromism and thermochromism. This dual-functioning aspect offers promising potential for its incorporation into future-generation multi-functional smart windows.¹⁴⁴ Apart from N-isopropyl acrylamide and similar entitles, alternative choices are available for the polymeric network of UCST-type ionogels. Song et al. detailed the synthesis of a UCST-type ionogel through a one-step photopolymerization of 2-hydroxyethyl acrylate (HEA) in [C₄mim][BF₄] (Fig. 7B). This ionogel has temperature and strain reactivity and can be shaped into various forms for smart devices and 3D printing. At temperatures below the phase transition temperature, the ionogel sports an opaque white appearance due to the dominating interaction between the IL and the hydroxyl group (–OH) of HEA, which boosts phase segregation to form a construct wherein the ester-rich region serves as the nucleus, encircled by the external hydroxyl groups and IL. An increment in temperature disrupts this balance of interactions, ushering in uniformity and high light transmittance. Rapid phase transmutation of these ionogels is boosted by their low viscosity and minuscule, dispersed phase segregation domain. In addition, the ionogels exhibit excellent tensile properties with an elongation at break of 1150% and a tensile strength of 2.25 MPa (Fig. 7C). The precursor solutions of these ionogels demonstrate their potential for the development of flexible and wearable temperature sensors and smart windows, owing to their ease of spray-application and moldability.¹⁴⁵

Fig. 7 Examples of UCST-type ionogels. (A) Preparation of PIL-based ionogels, transparent/opaque transition of a water-containing device and color transition of a water-free device. Adapted with permission: Copyright 2021, Solar Energy Materials and Solar Cells.¹⁴⁴ (B) Structure and transparent/opaque transition of an HEA-based ionogel. (C) Mechanical properties of HEA-based ionogels. Adapted with permission: Copyright 2022, ACS Applied Materials & Interfaces.¹⁴⁵

4.2. LCST-type ionogels

In 2007, Watanabe and Ueki conducted the initial observation of LCST behavior in ionic liquids.¹⁴⁶ They unveiled that the integration of the benzyl group into a poly(methyl methacrylate) (PMMA) structure can incite LCST behavior in the polymer when mixed in [C₂mim][NTf₂]. This displayed LCST-type phase behavior mirrored the tendencies exhibited by PNIPAm in water. Leveraging this understanding, they embarked on the fabrication of LCST-type ionogels of poly(benzyl methacrylate) (PBnMA) and [C₂mim][NTf₂], witnessing the phase transition of the nanoparticle gel, which expanded at low temperatures and contracted at high temperatures within [C₂mim][NTf₂]. In addition, they probed into the impacts of different component structures on polymer/IL miscibility and phase separation conduct. Distinct derivatives of PBnMA carrying diverse side-chain substituents were synthesized (Fig. 8A) and subsequently amalgamated with different ILs (Fig. 8B). The investigation revealed that inside a scrutinized temperature range (30 °C–250 °C), PBnMA derivatives featuring sparsely soluble substituents on the phenyl m-position proved insoluble in [C₂mim][NTf₂], whereas PBnMA derivatives harboring methoxy and fluoro substituents on the phenyl ring exhibited LCST phase conduct. Therefore, PBnMA can be construed as an amphiphilic polymer in relation to [C₂mim][NTf₂], and also its ester group reveals solvent attraction, while the phenyl group and the main chain evince solvent aversion. Chemical amendments of the phenyl groups on the polymer side chains substantially alter the dissolution characteristics of PBnMA, signifying the critical role of the phenyl group in determining the polymer's critical temperature (T_c) in an IL solution. Moreover, inflating the alkyl chain length between the phenyl group and the ester group decreases T_c, while substituting the imidazolium cation with an elongated alkyl chain boosts T_c. When sustaining the same anion, the solvent compatibility of the polymer/IL system is chiefly steered by the length of the alkyl chain in the cation, and the T_c may also be adjusted by titrating the proportions of two aptly mixed ILs.¹⁴⁷

Fig. 8 Chemical structures of polymers that show LCST behavior in ionic liquids. (A) Chemical structure of PBnMA derivatives; (B) chemical structures of several ILs; (C and D) chemical structure of copolymers based on PBnMA.

Based on the study by Watanabe and Ueki, Lodge et al. crafted a diblock copolymer utilizing PnBMA and PEO, where the diblock copolymer demonstrated a free chain/micelle transition in an IL when the temperature escalated above the lower critical micellization temperature (LCMT).¹⁴⁸ Building on this, Watanabe and Lodge formulated an ABA triblock copolymer (poly(BnMA-b-MMA-b-BnMA), BMB) (Fig. 8C), where the A block is PBnMA, and the B block is PMMA, which boasts high solubility in [C₂mim][NTf₂]. BMB was found capable of forming LCST-type ionogels in [C₂mim][NTf₂]. The gel underwent a phase transition from a low-temperature sol to a high-temperature gel, with the transition point at roughly 100 °C. Mirroring the sol–gel transition of block copolymers in water, the phase transition of BMB in IL commanded a high polymer concentration. At a polymer concentration of 10 wt% and at 100 °C, gelation failed to occur, despite PBnMA's aggregation, possibly because of the elevated number of molecules in each aggregate that led to an inadequate total number of micelles to form crosslink points, or because the established crosslink points retained mobility due to the low T_g of PBnMA. Moreover, upon reaching the requisite polymer concentration for gel formation (20 wt%), the LCST phase transition temperature diminished with the advancing polymer concentration within a specific range.³⁴ A substantial allotment of non-beneficial cyclic chains in the B block of the ABA triblock copolymers demonstrated a need for enhancing the quantity of polymer for gelation. Furthering their research, Watanabe and Lodge developed an ABC triblock copolymer, where the A block was a PBnMA derivative, poly(2-phenylethyl methacrylate) (PPhEtMA), the C block was PBnMA, and the B block was a solvent-compatible PMMA (Fig. 8D). Due to the dramatically reduced T_c of PPhEtMA (42 °C) in comparison with PBnMA (105 °C), the ABC triblock copolymer showcased a distinct sol–micelle–gel transition with the augmentation of temperature.¹²⁷

Other researchers are venturing into new composites of polymer and IL. For instance, Hu et al. crafted ionogels by chemically crosslinking polyurethane (PU) with polypropylene oxide (PPO), followed by its integration with IL mixtures ([C₄mim][NTf₂] and [C₄dmim][NTf₂]) exhibiting LCST behavior. The material selection in this study was guided by numerous factors: the existence of thermoresponsive interactions, such as hydrogen bonding between the IL and the polymer; the availability of hydrogen-bonding acceptor sites in PPO/PU; and the precise interaction between the IL and PPO – particularly the imperative “active” interactions in the imidazolium cation, which can form a hydrogen bond with the oxygen in the PPO segment. Interplay-based decisions influenced the selection of cations, and the choice of anions was equally pivotal, as different anions with diverse hydrogen bonding tendencies would disrupt or reinforce the hydrogen bonding between the cation and PPO, consequently influencing the LCST. Additionally, integrating an appropriate Lewis base allowed PPO to compete with [NTf₂]⁻ (a Lewis base) in attracting imidazole cations (Lewis acids) and attaining a suitable LCST. The resulting ionogels showcased high homogeneity and transparency at room temperature. As the temperature escalated, the amplified molecular repulsion between the IL and PU network incited phase separation, leading to the development of IL-rich structural domains, culminating in reduced light scattering and transmittance. In contrast, reducing the temperature amplified the affinity between the IL and PU networks, ushering in complete phase mixing and high light transmittance (Fig. 9A). These gels exhibit excellent stability, maintaining high optical performance even after undergoing more than 5000 cycles of cooling and heating (Fig. 9B). In addition, due to the excellent compatibility between ILs and organic dyes, ionogels can be easily dyed in various colors without compromising their thermochromic properties (Fig. 9C). Furthermore, the integration of plasma nanoparticles (antimony tin oxide (ATO)) into the ionogel facilitated a dual optical/thermal response. Fig. 9D demonstrates the quantitative evaluation of the autonomous solar energy blocking performance of ionogels under real conditions by building a sample house, and the results show that the smart ionogels can effectively reduce the indoor temperature compared with the normal gels. This innovative polyurethane-based ionogel presents potential applications in flexible optical switching materials, inclusive of smart windows, wearable electronics, and optical sensorss.³⁵

Fig. 9 Polyurethane-based ionogel showing LCST behavior for use as smart windows. (A) Chemical structures of polyurethane-based ionogel components and schematic diagrams of transparent/opaque phase transition. (B) Polyurethane-based ionogels show a bending cycle performance of >5000 cycles with temperature change. (C) Polyurethane-based ionogels can be dyed in a variety of colors. (D) Schematic diagram of the sample room setup used to test the gel's performance in blocking the sun's rays. Adapted with permission: Copyright 2017, Chemistry of Materials.³⁵

Liu et al. delved into the phase behavior of a solution containing poly(butyl acrylate) (PBA) and [C_nmim][NTf₂] IL, taking note that escalating the molecular weight or minimizing the concentration of PBA led to a decrease in the critical temperature, echoing the behavior of PILs in water. Amplifying their scope, they added a crosslinker in the pursuit of preparing ionogels (Fig. 10A) from the same composite and pinpointed the primary influencers of the LCST of ionogels as the polarity of the polymer side chains and the alkyl chain on cations of ILs. Remarkably, the researchers unveiled that by opting for a blend of structurally parallel ILs and calibrating their mixing ratios, they could modulate the LCST temperatures of the ionogels linearly over a vast temperature range. Furthermore, they examined the impact of the polymer/IL structure on the LCST behavior of the ionogels by formulating a series of acrylate monomers and ILs varying in alkyl chain lengths (Fig. 10B). They discerned that the ionogels’ compatibility decreased with the augmentation of the alkyl side chain length in the polymer network, due to the enhanced solvent-sparing effect. In contrast, the miscibility between the polymer network and ILs surged with the elongation of the alkyl chain on the imidazole cation of the IL, leading to increased compatibility (Fig. 10C). Drawing from their findings, the researchers proposed two conditions essential for the formation of LCST-type ionogels: the presence of both solvent-sparing and solvent-friendly groups in the polymer network, and a matchup between nonpolar structural domains of the polymer network and the IL. These findings provide valuable insights for the future development of new LCST-type ionogel systems and their potential applications in wearable devices, soft robots, and diving sensors. In a notable development, the same team discovered this thermoresponsive ionogel displayed adaptive adhesion properties in both air and water environments. The researchers primarily attributed this adaptive adhesion to the LCST phase separation behavior of the gels owing to a phase separation-triggered collapse of the polymer network and the subsequent venting of ILs on the ionogel surfaces. The hydrophobic PBA network and ILs are recognized for imparting on the ionogels a superior water-resistance ability, rendering them suitable for applications in aqueous environments, as well as in air. Fig. 10D demonstrates the selective capture of objects underwater by the ionogel through switchable adhesion, where the ionogel easily adheres and lifts the object at 25 °C, while it fails to lift the object at 45 °C. Moreover, various ionogels with patterns or codes can be prepared, and the patterns in the ionogels can be reversibly displayed through alternate heating and cooling processes (Fig. 10E). This unique property expands their potential uses, particularly in areas related to wearable devices, soft robots, and interchangeable sensors.^149–151

Fig. 10 PBA ionogel showing LCST behavior. (A) Chemical structure and phase separation process of BA-based ionogel components. Adapted with permission: Copyright 2022, Soft Matter.¹⁵⁰ (B) Different structural acrylate monomers and IL. (C) Different structural acrylate monomers and IL compatibility. Adapted with permission: Copyright 2021, CCS Chemistry.¹⁵¹ (D) Demonstration of adhesion properties of ionogels under water at different temperatures. Adapted with permission: Copyright 2022, Soft Matter.¹⁵⁰ (E) Demonstration of ionogels with different patterns and codes. Adapted with permission: Copyright 2021, CCS Chemistry.¹⁵¹

Modifying the copolymer composition can bring about changes in properties, thereby widening the functionality spectrum of thermoresponsive ionogels. Delving deeper into this, Moon et al. synthesized the random copolymer poly(styrene-r-tert-butyl methacrylate), P(S-r-nBMA), through RAFT polymerization. The copolymer was unified with [C₄mim][NTf₂] to engineer P(S-r-nBMA)-based ionogels of varying compositions, as depicted in Fig. 11A. The prepared ionogels displayed LCST behavior. Upon the surpassing of the LCST temperature, the PnBMA component of P(S-r-nBMA) started to exhibit IL-phobic characteristics, driving its separation from the IL and inciting the formulation of large-sized IL-depleted domains. Therefore, the gel's turbidity grew, and the light transmittance descended. An intriguing discovery was that by manipulating the PS content and the overall polymer content of the gel, the gel's LCST could be adjusted, thus extending the application ambit of the thermal response platform, for instance as a multimodal wearable ion skin (Fig. 11B). Owing to its reliable and high-performance temperature and strain-sensing capabilities, with optical and electrical properties that can be modulated without signal interference, it holds promise as a versatile and deformable sensing platform. Specifically, strain applications only result in resistance changes, without any optical alterations within the gel (mode i in Fig. 11B). Conversely, the gel is expected to become visually opaque when heated above its LCST (mode ii in Fig. 11B).¹⁵²

Fig. 11 Examples of LCST-type ionogels. (A) Thermal responsive phase transition of the P(S-r-nBMA) ionogel. (B) Schematic diagram of a wearable ion skin. Adapted with permission: Copyright 2023, Small.¹⁵² (C) BA-AzoMA ionogel synthesis process. (D) BA-AzoMA ionogel temperature and thermal response processes. (E) Preparation of BA-AzoMA ionogel into a photoinduced actuator. Adapted with permission: Copyright 2020, European Polymer Journal.¹⁵³

By embedding new functional groups into the copolymer structure, the synthesis of multi-functional materials can be easily achieved. Luo et al. chose BA and fabricated a random copolymer with UV-responsive 4-phenylazophenylmethacrylate (AzoMA) via RAFT polymerization. The ensuing copolymer demonstrated LCST-type phase separation in [C₄mim][NTf₂] and was subsequently crosslinked using ethylene glycol dimethacrylate (EGDMA) to create LCST-type BA-AzoMA ionogels (Fig. 11C). These ionogels manifested dual responsiveness to both temperature and light. They showcased high-temperature contraction and low-temperature swelling, with the contraction temperature hinging upon the photoisomerization state of the azobenzene groups within the polymer. Furthermore, the swelling-contraction behavior of the BA-AzoMA gel can be controlled by alternately exposing it to ultraviolet and visible light at the bistable temperature (Fig. 11D). Operating BA-AzoMA ionogels at the optimum temperature enabled reversible bending through alternating visible and UV light irradiation (Fig. 11E). The BA-AzoMA ionogels’ inherent characteristics can be harnessed to develop a photoinduced actuator.¹⁵³ In a similar vein, Viguerie et al. paired PBA with PNIPAm and prepared two diblock copolymers (PBA-b-PNIPAm and PBA-b-P(NIPAm-stat-NtBuAM)) through reversible addition–fragmentation chain transfer/macromolecular design by an interchange of xanthate (RAFT/MADIX) polymerisation. Owing to the LCST behavior of PBA and the UCST behavior of PNIPAm in an ionic liquid, these copolymers exhibited both LCST and UCST behaviors in [C₂mim][NTf₂].¹⁵⁴

The researchers pinpointed that the IL selectively solvated the acrylate groups in proximity to the polymer backbone, culminating in a homogeneous and transparent ionogel below the phase separation temperature. Subsequently, as the temperature heightened, the interactions between the polymer network and IL dampened, triggering macroscopic phase separation in the ionogels when the electrostatic interactions between ILs overpowered the hydrogen bonding and van der Waals interactions. A key discovery was that by opting for a blend of structurally analogous ILs and calibrating their mixing proportions, they could direct the LCST temperatures of the ionogels nearly linearly over an expansive temperature range. The researchers speculated that these ionogels could have prospective applications in wearable devices, soft robots, and diving sensors. Additionally, they delved into the impact of the polymer/IL structure on the LCST behavior of the ionogels. For this, they designed a series of acrylate monomers and ILs varying in alkyl chain lengths (Fig. 10B). They noticed that when the alkyl side chain length of the polyacrylate was less than 3, the ionogels exhibited compatibility due to the downgraded solvent-phobicity of the nonpolar domains of the polymer. In contrast, the miscibility between the polymer network and ILs dwindled with the escalation of the alkyl side chain length in the polymer network, ascribed to the enhanced solvent-sparing effect. However, the miscibility between the polymer network and IL surged with the lengthening of the alkyl chain on the imidazole cation of IL, due to the enhanced affinity between the nonpolar component of the polymer and IL, leading to increased compatibility (Fig. 10C). Drawing on their findings, the researchers proposed two conditions for the formation of LCST-type ionogels: the presence of both solvent-sparing and solvent-friendly groups in the polymer network, and a harmony between the nonpolar structural domains of the polymer network and the IL (m and n).¹⁵¹ These outcomes offer significant insights for the future development of new LCST-type ionogel systems.

5. Conclusions and future directions

Ionogels tap into the remarkable attributes of ionic liquids, encompassing high ionic conductivity, broad electrochemical windows, stability, non-flammability, and robust solubility. This makes them well-suited for the creation of wearable devices, solid-state batteries, and memristors. Unlike water, ionic liquids manifest a plethora of chemical structures, allowing substantial flexibility in the mode and force of interaction between the polymer frame and the ionic liquid medium within the gel. This adaptability endows ionogel materials with an expansive range of applications across various fields.

Thermoresponsive ionogels harness the temperature-dependent liaison between polymers and ionic liquid molecules, leading to microphase separation and significant transformations in gel transparency, conductivity, and mechanical strength. The magnitude of the change and phase separation temperature can be directed by factors such as the molecular structure of the polymer, type of ionic liquid, preparation technique, and extrinsic chemicals like salts, polymers, and solvents. Hitherto, a handful of ionogel components demonstrating UCST or LCST behavior have been unveiled, with PNIPAm and PBnMA being the most prevalent due to their synthesis simplicity and versatility.

The development of novel thermoresponsive ionogels can be facilitated by drawing inspiration from prior research on thermoresponsive ionogels. From the thermodynamic viewpoint, UCST ionogels exhibit enthalpy-driven phase transitions, where upon cooling, the polymer chains complex with each other by exothermic interaction. In contrast, LCST ionogels are entropy-driven, as they release bound ILs upon heating, thereby enhancing entropy. To engineer the desired temperature sensitivity, judicious combinations of polymers and ILs are crucial. For instance, when selecting a polymer, LCST ionogels necessitate a balance of solvent-sparing and solvent-friendly groups within the network, along with coordinated nonpolar domains between the polymer and ILs, promoting the formation of hydrogen bonds between the polymer and ILs. In UCST ionogels, in addition to the coexistence of both solvent-sparing and solvent-friendly groups, there is the presence of complementary hydrogen bond donor and acceptor sites. This enables the polymers to predominantly form stable intermolecular hydrogen bonds within the IL matrix. In terms of ILs, cations are commonly imidazolium-based due to their reactive hydrogen, which can form interactions with the polymer, while anions should possess an appropriate Lewis basicity to ensure a suitable transition temperature. Further research is integral to discovering more thermoresponsive ionogel polymer–IL pairings and establishing generalized phase separation rules through molecular design, thereby expanding the gel's properties and functionality. Moreover, unraveling synthesis methods, functionalization, and new applications is crucial in propelling the field forward.

Regarding the synthetic methodologies of thermoresponsive ionogels, they encompass in situ free radical polymerization, post-crosslinking, and self-assembly methodologies, each of which significantly influences the properties of the ionogels by modulating factors such as molecular weight, chain distribution, entanglement, and microscopic aggregation. Looking forward, there is potential to explore more synthetic methods that utilize the unique characteristics of ionic liquids. For instance, these could include combining polymerization-induced self-assembly to produce hierarchical, multiphase structured thermoresponsive complex ionogels or using the conductive characteristics of ionic liquids to synthesize thermoresponsive ionogels via electropolymerization, thereby avoiding the monomer precipitation caused by heating.

The functional expansion of thermoresponsive ionogels involves integrating other operative units (e.g., light response, electrical response, redox response) via copolymerization or grafting onto the polymer skeleton. This enables the production of ionogels with multiple responsive behaviors for applications in photolithography, thermoelectric dual-responsive smart glass, and beyond.

Identifying new applications of thermoresponsive ionogels represents a major challenge and a potential bottleneck in advancing this field. To investigate these novel applications, precision control over transition temperature, thermal history, and transition velocity is fundamentally crucial. However, overcoming these challenges requires a more detailed understanding and identification of the intra-chain, inter-chain, and fine interactions between polymer chains and ionic liquid molecules, which might demand the assistance of artificial intelligence and simulation. Overcoming these obstacles holds the promise of instigating significant developments in fields such as battery electrolytes and sensor technologies, among others.

Data availability

All data used in this study are reasonably accessible. We are committed to ensuring the integrity and accuracy of the data to provide a solid foundation for our research.

The data primarily come from publicly accessible databases and officially released statistical information. For some specific data, we obtained them through strict data sharing agreements in collaboration with relevant institutions. We respect and protect all data sources, ensuring that this information is used within legal and ethical frameworks.

To ensure the transparency and reproducibility of our research, we are willing to share partially anonymized datasets with interested researchers, subject to compliance with relevant laws and regulations. For data sharing inquiries, please contact us for detailed information and data access guidelines.

We would like to express our gratitude to all the institutions and individuals who provided the data. Their contributions have been crucial to the success of this study. We pledge to continue adhering to the principles of openness and transparency in future research, providing the scientific community with more high-quality data resources.

Author contributions

Q. Z. and Y. M. wrote and revised the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We thank the support from the National Key R&D Program of China (2023YFB3811100), Shanghai Pujiang Program (23PJ1400400), DHU startup grant, Fundamental Research Funds for the Central Universities, and DHU Distinguished Young Professor Program.

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