Synthesis and properties of polymerized ionic liquids

European Polymer Journal 90 (2017) 245–272

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology - Review

Synthesis and properties of polymerized ionic liquids a,b,⁎

Ali Eftekhari

, Tomonori Saito

c

MARK

a

The Engineering Research Institute, Ulster University, Newtownabbey BT37 OQB, United Kingdom b School of Chemistry and Chemical Engineering, Queen's University Belfast, Stranmillis Road, Belfast BT9 5AG, United Kingdom c Soft Materials Group, Chemical Sciences Division, Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, TN 37831-6210, USA

AR TI CLE I NF O

AB S T R A CT

Keywords: Poly(ionic liquid) Polymerized ionic liquid Ionic liquids Ionic polymers Nanoparticles

Polymerization of ionic liquids results in the formation of ionic polymers, which are called poly (ionic liquid)s or polymerized ionic liquids (PIL). This is a brand new form of ionicity in polymer chains with a broad range of applications, though ionic polymers have a long history with the sub-families of polyelectrolytes and ionomers. Although mobility of ions in ionic liquids has named them as the promising candidates for various applications, their applicability is limited in many practical systems because of not having the advantages of neither liquids nor solids, suffering from both leakage issue and high viscosity. PILs perfectly fit with the practical requirements while having almost all features of ionic liquids. This review summarizes some potential applications of PILs. The architecture of PILs can be easily re-designed by both the polymer backbone and outer ion. Not only by post-polymerization but also by in situ ionexchange, the chemical and mechanical properties of PILs can be tuned. Owing to the high chemical activity and flexible architecture, PILs are the promising candidates for sensors and actuators, electroactive binders, solid and gel electrolytes, non-blocking matrix of nanocomposites, etc.

1. From ionic liquids to network of ions Historically, ionic salts in molten form were of particular importance because of the accessibility of mobile ions in a solvent-free medium. Replacing inorganic ions with large organic ions will reduce the ionic interactions, and thus, the melting point can be low enough to be liquid at room temperature. The possibility of forming purely ionic media at room temperature paved the path for a broad range of applications of a new class of materials namely ionic liquids (ILs). In an advertising fashion, ILs were named “solvents of the future” or “designer solvents,” but they are not practical solvents in action. Instead, they can find new series of applications rather than replace the conventional solvents. Despite the rapidly growing interest in ILs, there are a few cases in which ILs can be employed as replacing solvents. In the electrodeposition of some metals, IL environment is the only choice for reducing a cation to metallic state at room temperature; where they replace the high-temperature molten salts. However, ILs were not able to replace the conventional solvents [1]. There are issues, which should be addressed before the practical applications of ILs such as ion pairing; but there are two fundamental problems, which are unlikely to be addressed: high viscosity and cost. This is indeed an active subject of research to reduce the viscosity of ILs with different approaches; but, from a theoretical perspective, it is almost impossible to reach an acceptable viscosity (comparable with those of conventional solvents) while preserving the intrinsic nature of ILs. Mass production of ILs will definitely reduce the synthesis cost, but again, the purification is time-consuming.

⁎

Corresponding author at: The Engineering Research Institute, Ulster University, Newtownabbey BT37 OQB, United Kingdom. E-mail address: [email protected] (A. Eftekhari).

http://dx.doi.org/10.1016/j.eurpolymj.2017.03.033 Received 4 February 2017; Received in revised form 10 March 2017; Accepted 12 March 2017 Available online 14 March 2017 0014-3057/ © 2017 Elsevier Ltd. All rights reserved.


A. Eftekhari, T. Saito

In general, the desire for utilizing ILs as solvents have misled researchers in various disciplines to put efforts in applications with dark horizons. Instead, ILs should be treated as a native source of mobile ions rather than a medium. For this purpose, it is necessary to organize ions in the structure of IL to have an ordered network while preserving the mobility of ions to some extent. A possible approach is ionic liquid crystals (ILCs), which have attracted considerable attention due to their successful applications [2]. A fascinating feature of ILs is that they can be polymerized to form a polymer chain in which ions are somehow fixed but still have ionic features. This is indeed a form of polymers in which the monomer is not a conventional organic molecule. The common terms are poly(IL)s, polymerized ILs, or polymeric ILs, which are all abbreviated as PILs (not to be mistaken by the same abbreviation used for protic ILs). This terminology is not precise, as similar polymers can be synthesized from solid ionic organic salts, and the monomer liquidity has no characteristic influence on the resulting PIL. As a matter of fact, the definition of “ionic liquid,” ionic compounds with a melting point below 100 °C, is not comprehensive, because of the arbitrary threshold. In fact, this class of polymers well falls within the definition of “ionic polymer” because it is a genuine polymer and its characteristic feature is ionicity. This terminology limits the PILs to the common ILs with low melting point and viscosity (as has already been happened); whereas, it is not a requirement for the synthesis of PILs whatsoever. Regardless of the definition controversies, PIL is the commonly accepted term for this class of ionic polymers. The mobility of ions in ILs provides a vast range of potential applications, but in practice, they are not capable of adapting to the practical requirements. In most cases, PILs can provide the characteristic properties of ILs with an excellent adaptability. For instance, absorbing CO2 is of industrial importance from both energy and environmental standpoints. While ILs showed excellent properties for absorbing gaseous CO2, it is tough to make a practical device from ILs; PILs provide both features at the same time [3]. PILs normally have a higher capacity for CO2 uptake than their corresponding IL monomers [4,5], and the sorption/desorption process is faster and completely reversible [4,6,7]. Moreover, PILs have a better selectivity for the separation of CO2 from other gasses [4,5,8–10]. The sensitivity and selectivity of PILs for CO2 have led to the fabrication of most sensitive sensors with detection limit below the level of ppm [11]. Computational studies revealed that the CO2 sorption is mainly influenced by the polycation rather than anions, as they have a stronger interaction with gas molecules [12]. The charge mobility in PILs results in conductivity, which is of utmost importance in the electrochemical systems. This feature of PILs resembles conductive polymers, which have a rich history over the past four decades [13,14]. Conductive polymers are mainly electrically conductive due to the mobility of π-conjugated electrons along the polymer chain. They can also be ionically conductive as a result of doping by counter ions. However, PILs merely have ionic conductivity because they are entirely made of ions. Thus, these two classes of polymers can be put side by side as ionically conductive polymers vs. electrically conductive polymers. This makes PILs potential candidates in a long range of applications of conductive polymers in which the key process was ionic mobility (i.e., a considerable percentage of their applications). Preparation of the PILs is not usually difficult, and it is normally facile to purify them. Polymerization of ILs can be even conducted in an aqueous solution [15–17]. Although the melting points of almost all PILs are well above the room temperature, there are a few exceptions with low glass transition temperatures, which are liquid at ambient temperature [18]. The glass transition temperature of PILs is directly controlled by the polymer structure, particularly the level of crosslinking [19]. In a recent book edited by one of us [20], it has been discussed in detail that the applicability of ILs can be widely extended to new systems by polymerizing the ions to form a controllable polymer chain. The present manuscript aims to review the potentials and features of PILs prepared by the polymerization of ILs and those obtained by post-polymerization introduction of charged groups. This should serve as a general guide for drawing the road map of research for employing PILs in various applications. Therefore, a broad readership of researchers from quite different disciplines is targeted. We summarize the potential possibilities for utilizing PILs in various applications without detailed discussions from specialized perspectives. Since the PIL applications are in quite different disciplines, the structure of the present review is based on the PIL properties to link the works done in various fields under the same Sections.

2. Ionic polymers This is indeed a large family of polymers having ionic units in their structure, which can be somehow categorized by the level of ionicity. The primary category is ionomers with less than 15% ionic units as side group moieties are covalently bonded to the polymer backbone [21,22]. Polyelectrolytes are polymers composed of polyanions and polycations, which normally have over 80% of ionic groups. PILs are technically polyelectrolytes but somehow revisited due to the structural design to totally focus on the ionicity. The first example of ionomers (i.e., still the most common one) is Nafion, which was commercially introduced by DuPont in the 1960s. Nafion is simply a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether. Because of the incompatible components with hydrophobic and hydrophilic nature, it is a conventional membrane with good ionic conductivity [23,24]. In general, all ionic polymers share common properties making them suitable for similar applications; for example, ionic polymer/metal composites have the excellent responses for various types of sensors and actuators [25–30]. PILs are indeed the ultimate definition of this class of polymers, as they are 100% composed of ions. Although they have been known since the 1970s [31–38], they have attracted a growing interest during the last years due to the popularity of ILs. Polyelectrolyte refers to the same type of ionic polymers made from ionic salts (i.e., the same class of ILs, but the liquidity was not a key feature that time). Although the term polyelectrolyte (because they are made of polyanion and polycation) is scientifically reasonable in polymer chemistry, it was somehow confusing in the practical systems where “electrolyte” is a fundamental component of electrochemical cells (e.g., when saying a battery is composed of a polyelectrolyte as the electrolyte). 246



3. From ionic liquids to polymerized forms PILs can be easily prepared by the radical polymerization of IL monomers. In this approach, it is possible to control the chain length and morphological structure [39,40]. PILs have been successfully synthesized by reversible addition fragmentation transfer polymerization [41–44], and radical polymerization mediated by atom transfer [45–49], cobalt [50–52], copper [53], nitroxide [54,55], methyl methacrylate [56], organometallics [57], etc. The only possible way for the preparation of PILs is not to start from the corresponding IL monomer, as it is also possible to start from uncharged neutral polymers, and then, chemically modify the chain by post-polymerization processing. In this case, the polymer structure will be that of the starting polymer. Moreover, PILs provide excellent flexibility for the post-polymerization modification to obtain a specific reactivity [58]. This approach has been used to design organocatalysts [59–63]. The possibility for anion-exchange can be exploited to alter the adsorption capacity along with corresponding chemical sensitivity and selectivity of PILs [64]. Copolymerization by crosslinking the PIL chain with another polymer can result in a high specific surface area in the range of 1000 m2/g [65], which is very desirable for some applications of the PIL matrixes. In fact, PILs can be used to design the architecture of a target polymer or vice versa [39,66]. Increasing the accessible surface area of PILs as excellent absorbents provides an optimal opportunity for the absorption of different compounds, particularly CO2 [4,67–85]. The copolymerization strategy is not limited to neutral polymers, as two PILs can affect the formation of each other in a block copolymerization [17,57]. Owing to the intrinsic interaction of ions, it is possible to prepare PIL nanoparticles in a template-free synthesis [86,87]. It is not difficult to prepare porous membranes of PILs, and the pore size and distribution can be directly tuned by the counter ions [88]. Self-aggregation of ions in ILs is a serious problem for the IL applications, but it is quite useful in the polymerization process to control the shape of the PIL nanoparticles. Interaction of ions forms micelles in the solution, which are somehow separated from the bulk solution and act as a soft self-template to lead the polymer growth pathway. The interesting feature is that the role of secondary ion interactions can be controlled (e.g., by pH) to change the micelle shape. As a result, it is possible to prepare the PIL nanoparticles with unusual shapes (Fig. 1). With this simple strategy, there is also a possibility for the preparation of well-ordered PIL particles with desirable shapes at the nanoscale [89]. In general, the PIL properties are significantly tunable during the synthesis process [90]. The key parameters such as glass transition temperature (Tg), ionic conductivity, and thermal stability of PILs can be controlled by the size and symmetry of counter

Fig. 1. (top) Schematic representation of self-aggregation of semitelechelic POSS-PVim hybrid into nanostructures of different shapes under different conditions. (bottom) SEM image of aggregated nanostructures obtained from aqueous POSS-PVim1 hybrid (0.25 wt.%) solution at different pHs. Reproduced with permission from Ref. [343].

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Fig. 2. SEM images of poly([MTMA][TFSA]) particles prepared by dispersion polymerizations in various methanol/ethanol media. Ethanol concentration (w/w): (a) 0, (b) 33, (c) 50, and (d) 66. Reproduced with permission from Ref. [93].

ion [91]. In fact, the IL monomer, Tg, and crosslink density are the key factors controlling the PIL ionic conductivity, which is indeed the most important feature of PILs [92]. Like the commercially available polystyrenes, PILs can be synthesized in the form of highly ordered microspheres. A practical approach is to stabilize the spherical shape by poly(vinylpyrrolidone) [93]. The size of microspheres can be simply controlled by changing the solvent composition. Fig. 2 shows how the ratio of methanol/ethanol directly monitors the size of the PIL microspheres. Ionic interaction of PILs provides an exceptional flexibility for designing interfacial reactions. For instance, the role of working electrode in a well-defined redox system at the electrode surface is typically limited to charge transfer at the electrode/electrolyte interface. This means that the electrochemical behaviors of a well-defined redox system like Fe(CN)6–3/–4 at various conventional electrodes are similar. Fig. 3 shows that the electrochemical activity of this famous redox system at a PIL electrode is significantly controlled by the PIL anion, ranging from an ideal electrochemical behavior to almost electrochemically inactive [94]. Since the anion of a PIL can be easily subject to ion-exchange, this means that the electrochemical activity of a PIL can be tuned in action. In

Fig. 3. Lectrochemical characterization of an IP with different counter anions utilized as the working electrodes for the well-known redox of K3[Fe(CN)6]. Reproduced with permission from Ref. [94].

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Fig. 4. Representative cationic PILs: imidazolium (a), quaternary ammonium (b), pyridinium (c), pyrrolidinium (d).

conductive polymers, which are an analog to PILs, no such significant influence of anions can be observed. The electrochemical behavior of conductive polymers can be slightly changed by the size and charge of doping anions [95].

4. Polymerization of ionic liquids Cationic PILs have been widely studied because of their less cumbersome synthesis and versatility. Some typical cationic PILs are shown in Fig. 4. Almost any kind of counter anions can be used for the synthesis of PILs. This versatility provides a significant advantage for the systematic tuning of the polymer properties. All of these polymers contain a nitrogen-center, and a tetrasubstitution allows to build cationic groups. The synthesis of these cationic PILs is similar to that of the corresponding ILs. However, the PILs prepared by the polymerization of cationic monomers possess prefixed substituted group, and determining the molecular weight by gel permeation chromatography (GPC) is a challenge. The GPC characterization of the resulting charged polymers requires a very specific combination of mobile phase with salts, but the charged polymers tend to stick to certain columns. Hence, developing new approaches for measuring the molecular weight of PILs is an active area of research. He et al. subtly proposed a GPC approach for the molecular characterization of PILs to address this issue [47]. However, the polymerization of a non-charged precursor monomer followed by post nucleophilic substitution reaction and subsequently ion-exchange is the preferable reaction scheme in many cases. The post-modification strategy can be applied to all of the polymers listed in Fig. 4. For example, the reaction schemes of imidazolium and quaternary ammonium PILs are shown in Fig. 5. Shaplov et al. investigated the effect of the monomer structure on the free radical polymerization process and subsequently the corresponding PIL properties [96]. They showed that the ionic conductivity of PILs does not necessarily increase by increasing the number of ionic centers in the corresponding monomer. Therefore, large IL monomers with numerous ionic centers are not ideal choices for achieving high conductivity due to the electrostatic repulsion and steric impediments between neighboring centers. They suggested that optimal monomer should contain a flexible side spacer of 3–5 carbon atoms. Poly(1-vinyl imidazole) is readily synthesized via a conventional free radical polymerization. The GPC condition of poly(1-vinyl imidazole) is very specific, e.g., water/ethanol (71.5/28.5, v/v%) with 0.05 M HCl and 0.017 M tris(hydroxyethyl)amine [97]. By utilizing AIBN initiator concentration of 0.05 mol%, the conventional free radical polymerization of 1-vinyl imidazole at 65 °C gave ∼49,000 g/mol with PDI of 1.94 [97]. The molecular weight could vary, but other studies also confirmed that this homopolymer synthesis condition provides monomodal molecular weight distribution with PDI of 1.8–1.9 [98]. The capability to obtain MW of this precursor polymer with monomodal distribution by GPC is advantageous over polymerizing an imidazolium monomer, where almost no GPC condition can accurately measure the resulting charged polymers. Subsequently, the pendant imidazole group can be readily

Fig. 5. Synthesis of poly(1-vinylimidazolium) (a) and poly(quaternary ammonium acrylate) (b) via post-modification strategy.

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Fig. 6. Glass transition temperatures of poly(alkyl-1-vinylimidazolium) polyelectrolytes with varying alkyl chain length and counteranion. Reproduced with permission from Ref. [97].

substituted with a halide-functionalized building block by a nucleophilic substitution reaction. This step allows the incorporation of almost any lengths of alkyl or ethylene glycol groups by using the corresponding 1-bromo functionalized chain. To achieve a quantitative conversion, the reaction temperature should be as high as 80 °C, though the reaction temperature for some cases is limited by the boiling point of the 1-bromo compound. Also, a 1-bromo compound is better than the 1-chloride counterpart in this step owing to the reaction efficiency. The final step of the synthesis involves ion-exchange of bromide anion with a large organic ion such as TFSI. The bromide typically comes off well but exchanging with the corresponding salts twice may be necessary to achieve quantitative ion-exchange. This ion-exchange process alters the PIL solubility, i.e., less soluble in a polar solvent like MeOH or water. Thus, precipitation in water and washing with water typically separate the resulting exchanged polymer. Fig. 6 shows the effect of the alkyl chain length and counteranion on Tg. The trend shown in Fig. 6 is typically applicable to all the cationic PILs. When a different length of pendant groups (alkyl in this case) is compared in the same counteranion, the longer chains give a more flexible structure and lower its Tg. On the other hand, the larger counteranions lower the Tg due to its bulkiness and distributed charges in the order of Br– < BF4– < PF6– < TfO– < TFSI–. Another example of the post-modification strategy includes poly(quaternary ammonium acrylate) as illustrated in Fig. 5b. The first step of the synthesis involves a conventional free radical polymerization or RAFT polymerization of dimethylamino ethylacrylate. The resulting poly(dimethylamino ethylacrylate) is fairly soluble in THF, and the MW characteristics can be readily measured by THF with triethylamine. Similar to poly(1-vinyl imidazolium), the capability to predetermine the precursor molecular weights is advantageous since the molecular weight of the charged polymers is difficult to be measured by GPC. Due to the presence of nucleophilic nitrogen on dimethylamino group (similar to imidazole), the post-functionalization of poly(dimethylamino ethylacrylate) can be performed via a nucleophilic substitution reaction with halide compounds. For example, a dimethyl amine group on poly(dimethylamino ethylacrylate) reacts with alkyl bromide, alkyl ether bromide, etc. to produce quaternary ammonium bromide analogs [99]. Bromide ion is readily exchanged in the same way as imidazolium PILs, e.g., BF4–, PF6–, TfO–, and TFSI–, as described above. Fan et al. synthesized poly(quaternary ammonium acrylate) shown in Fig. 5b with R-group of (A) methyl methyl ether, (B) ethyl methyl ether, (C) 4-methoxy butane, (D) 2-(2-methoxyethoxy) ethane and TFSI was used as an anion (Fig. 7) [99]. Their study showed that Tg values were lowered with longer ether chain substitution from 282 K (A) to 253 K (D). Thus, the ion conductivity at a given temperature followed the trend of Tg. However, more rigid structure (higher fragility) (A) showed more decoupled ion conduction and higher ion conductivity as a function of Tg/T (Fig. 7). The degree of decoupling increases (i.e., more efficient ion conduction) with an increase of the fragility of the polymer segmental relaxation due to a decrease in polymer packing efficiency with an increase in fragility. Thus, the study indicated that balancing fragility and Tg value will be a key to obtain high ion conductivity of the cationic PILs. The same group investigated the effect of molecular weight on the ion transport mechanism [100]. Considering the difficulty to utilize GPC for charged polymers, employing an uncharged precursor of the same polymer, poly(dimethylamino ethylacrylate), synthesized via RAFT polymerization, allowed to precisely measure the molecular weight accurately. R-group for quaternization was butyl group (Fig. 4b), and TFSI was used as a counteranion. They synthesized the monomer, dimer, and trimer in addition to 10, 72, 109, and 333 repeat unit of poly((2-(acryloyloxy) ethyl)-N,N′-dimethyl-N-butyl-ammonium TFSI). The successful preparation of a

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Fig. 7. DC conductivity as a function of Tg/T and decoupling exponent decoupling exponent ε vs the polymer fragility. Reproduced with permission from Ref. [99].

wide range of molecular weights allowed to study various characterizatics as a function of molecular weight which has rarely been accomplished for PILs due to the challenging synthesis methods and the challenging determination of the molecular weights. The molecular weight dependence on the Tg values revealed the trend following the classical Fox-Flory equation (Fig. 8). Their study also clearly elucidated the mechanism of ion conductivity as a function of the molecular weight. When the molecular weight is in the range of monomer to trimer, the ion conductivity was highly coupled with the structural relaxation, similar to typical ILs or salts in solution (Fig. 9). When the repeating unit was even just 10, the ion conductivity was significantly decoupled from the structural relaxation, and the degree of decoupling in the ion conductivity was constant at higher molecular weights of the PIL (72, 109, and 333 repeat units). This is critical information for the design of PILs. The molecular weight of PILs affects the Tg values like regular polymers but does not alter ion conductivity except in the range of oligomer molecular weight. The variety of anionic PILs is much more limited as compared with cationic PILs. Classical ionomers such as sulfonate or carboxylate polymers have been studied extensively. However, those sulfonate or carboxylate ionomers with metal counter cations possess very strong ionic interactions. The only exception is sulfonate or carboxylate with very large organic counter ion such as tetrabutyl phosphonium (Fig. 10a). The anionic PIL of polystyrene sulfonate with tetrabutyl phosphonium ions has been studied, which has a thermoresponsive behavior [101]. Apart from these classic ionomer structures, trifluoromethylsulfoimide (TFSI) PILs (Fig. 10b) is one of the few common anionic PILs with a variety of countercations including metal cations. TFSI monomer is typically synthesized from sodium sulfonate monomer such as sodium sulfonate styrene (Fig. 11). Sodium sulfonate group is converted to sulfonyl chloride using either oxaryl chloride or thionyl chloride [102,103]. Due to the use of such harsh reagent in this step, one needs to carefully monitor the possibility of side reactions. Unlike cationic PILs, post-modification of sulfonated polymers is challenging due to their hygroscopic nature, solubility issue, and possible side reaction in the chlorination reaction. The sulfonyl chloride is further reacted with trifluoromethylsulfonamide (NH2SO2CF3) in the presence of trimethylamine (TEA). The TEA counter cation is exchanged with potassium ion (K+) by neutralizing with potassium carbonate. Thus, further ion exchange should be performed from polystyrene trifluoromethylsulfoimide (PSTFSIK). Long et al. reported the property of the homopolymer PSTFSI with different counter cations [103]. The exchange of metal cations increased the Tg values, and the general trend was similar to the typical behavior of sulfonate polymers such as Nafion. Bouchet et al. reported PSTFSI-Li-b-PEO-b-PSTFSI-Li triblock copolymer synthesized via nitroxide-mediated polymerization from

Fig. 8. Molecular weight dependence of the glass transition temperatures determined by DSC, BDS and rheology. The lines are the fits using the Fox-Flory equation. Reproduced with permission from Ref. [99].

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Fig. 9. Relationship of molar conductivity (Λ) to structural relaxation rate (1/τS) for various electrolytes (modified Walden plots): including aprotic ionic liquids [BMIM][PF6], and PPG/LiClO4 (20.4%). Reproduced with permission from Ref. [99].

a PEO precursor [104]. They showed several intriguing properties of the TFSI PILs (Fig. 12). The trend of Li-ion conductivity indicated that there was an optimum balance in the ratio of PSTFSI-Li and PEO. The crystallization of PEO was significantly suppressed by the presence of PTTFSI-Li, and the triblock copolymer showed one mixed glass transition indicating the highly miscible nature of PSTFSI-Li in the PEO segment. Moreover, the triblock copolymer significantly enhanced the mechanical and electrochemical properties as compared with PS-b-PEO-b-PS triblock copolymer.

5. Ionic conductivity of PILs There are two types of PILs depending on the polymer backbone; if the polymer backbone is made of anions, the ionic conductivity is provided by the mobility of cations and vice versa. This provides a unique opportunity for designing selective ionic conductivity by a specific ion since both anion and cations of ILs can be simply chosen. While the key feature of the conductive polymers is the flexibility of π-conjugated electrons facilitating electrical conductivity, the abundant presence of ions across the PIL structure guarantees ionic conductivity. However, it is also possible to form π-conjugated electrons over a PIL to make it electrically conductive [105–107]. This is accompanied by sacrificing the presence of ionic functional groups. By balancing the presence of π-conjugated electrons and ionic functional groups, it is possible to gain both features simultaneously [108,109]. The Ionic conductivity of PILs is naturally less than their IL monomers due to the less mobility of ions fixed to the polymer backbone [110]. However, there are several approaches for improving the ionic conductivity of PILs by strengthening the ionic interaction on the open side via doping or mobile ions provided by IL [111]. Similar to conductive polymers, PILs can be an excellent matrix for electroactive materials in various electrochemical systems [112]. Since PILs are widely used as a matrix for the fabrication of nanocomposites, the second component can also contribute to the ionic conductivity of PILs. The ionic conductivity of PIL nanocomposites can be as high as 0.1 mS cm−1 [113]. An interesting feature of PILs is thermal stability. While commercially available membranes based on ionomers lose their ionic conductivities at high temperature (e.g., Nafion at 90 °C [114]), PILs show strong ionic conductivity at high temperatures (e.g.,

Fig. 10. Chemical Structure of polystyrene sulfonate with tetrabutyl phosphonium (a), and polystyrene trifluorosufoimide.

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Fig. 11. Synthesis of STFSIK monomer and of PSTFSI PILs.

Fig. 12. The structure of PSTFSI-Li-b-PEO-b-PSTFSI-Li triblock copolymer and Li-ion conductivity as a function of temperature. Reproduced with permission from Ref. [104].

90 mS cm–1 at 100 °C [115]). 6. Nanoporous PILs It is obviously desirable to synthesize PILs with a high specific surface area. Porous (particularly mesoporous) structures are the ideal nanostructures for the absorption purpose or even chemical accessibility. Various methods have been introduced for synthesizing PILs with nanostructures. Seed swelling method is a simple approach to form PILs on nanosphere seeds [116]. This method is preferred for the preparation of core/shell design [117]. Another method is to utilize colloidal crystal templates [118]. Complexing the PIL functional groups with polyacids can also control the nucleation and growth mechanism of the polymer to form mesoporous PILs [86,119,120]. In general, complexing the counterion during the polymerization results in the formation of porous membranes [121]. A robust and scalable method is to use an analogous IL as a porogen [122]. In general, the interaction of two polymer chains can be employed to control the pore structures [123]. 7. PIL nanoparticles In addition to the external morphology of PILs, polymerization can result in a highly ordered internal structure due to the ordered cross-linking between the polymer chains [124]. Crosslinking plays a substantial role in the structural integrity of the PIL particles in solutions [125]. Dong et al. demonstrated that PIL nanoparticles exhibit strong electrorheological properties in a dry state, which is different from the classic polyelectrolytes [126]. The PIL electrorheological properties can be directly controlled by the size of the anion and cation. Fig. 13 illustrates the internal structure of a PIL nanoparticle. The ordered layered structure resembles the graphitic layers of multi-wall carbon nanotubes. PILs have also been used as a multi-functional agent for hydrothermal synthesis of carbon nanostructure [127]. The exact role of PILs is not clear yet, but it is believed that they act as a stabilizer, pore-generating agent, and nitrogen source. Reducing the size of the PIL individual particles to a nanoscale might be of particular interest as reaching the size of an individual polymer chain. Zhang et al. subtly manipulated the polymerization of a 1-vinyl-1,2,4-triazolium-type ILs in a one-pot dispersion polymerization to synthesize a series of well-defined sub-50 nm homopolymer nanoparticles with specific shapes [128]. The PIL nanoparticles had highly ordered and complex internal structure within a 5-nm domain spacing. Since the PILs based on 1-vinyl1,2,4-triazolium has a higher loading capacity of transition metal ions as compared with the similar imidazolium-based PILs [129], these PIL nanoparticles with controllable shapes can exhibit practical potentials for various responsive devices. 253



Fig. 13. Representative cryo-EM images of crosslinked PIL nanoparticles in aqueous solution. (A & D) poly(1,2,4-triazolium-C12Br/BVTD); (B & E) poly(1,2,4triazolium-C14Br/BVTD); and (C & F) poly(1,2,4-triazolium-C16Br/BVTD). BVTD stands for 1,4-butanediyl-4,4′-bis(1-vinyl-1,2,4-triazolium) dibromide. Reproduced with permission from Ref. [125].

The PIL particles can be synthesized in the form of core/shell, in which the PIL shell is grown on a seeding core. In this case, the core material is dispersed within the monomer in the polymerization reactor where the polymerization occurs on its surface. A suitable core material is graphene oxide or its reduced form due to the presence of functional groups [130]. This process practically stabilizes the dispersion of highly charged particles such as reduced graphene oxide [131]. This approach is utilized to stabilize nanoparticles in solutions, as will be described later. 8. Forming PILs on substrate surfaces In addition to the bulk synthesis of PILs, they can be beneficial if grown on a surface to modify its chemical properties. For this purpose, it is possible to initiate the polymerization process at the surface by a grafting approach. The substrate is initially functionalized by small functional groups which can initiate the radical polymerization. This method has been successfully used for the chemical modification of silica and glass [132–135]. PILs can be grafted on natural products such as cotton [136,137] or cellulose [138]. Then, they can be used as biocompatible membranes or precursors for the preparation of carbonaceous (nano)materials. In this process, biomass is transformed into the active material. The critical role of PIL is carbonization and pore generation, which are controllable. Moreover, it is possible to conduct in situ doping of the resulting carbon by the neighboring elements such as nitrogen, as there is a demanding interest in doped carbon. 9. Soft templates for the synthesis of nanomaterials One of the popular approaches for the synthesis of nanomaterials is using soft templates to localize the reaction in a liquid medium. This can be done by surfactants as relatively large ions chemically interacting with the reactive species to interfere with the growth mechanism or by well-dispersed particles such as polystyrene which can do the job physically. ILs have been used for this purpose [139–141], as the abundant presence of ions in a purely ionic medium can interfere with the nanoparticle growth. As a pure medium, there is not much flexibility for controlling the reactions. In the case of surfactants, the concentration and its ratio to the reactant concentrations control the shape and size of resulting nanomaterials. PILs can provide a new opportunity in this context. While the PIL nanoparticles are like polystyrene micro/nanospheres to physically localize the reaction within the solution, their ionically active surface can interact with the reactants to control the nanomaterial synthesis. This method has been successfully employed for the preparation of well-ordered nanoparticles or specific 254



Fig. 14. Photographs of (a) Cot-PIL-TFSI before and after carbonization and (b) carbon “flower pads” derived from carbonization of Cot-PIL-TFSI at 1000 °C. Reproduced with permission from Ref. [136].

morphologies of different materials [118,141–143]. The unique feature of PILs in this process is that their chemically active surfaces can be even the nucleation sites to direct the nanomaterial formation. In this case, the chemical features of the PIL surface can be a powerful control for designing various nanomaterials. PILs have also been utilized to fairly disperse reactants within a solid precursor for the synthesis of carbon nanomaterials [41,136,141,144–147]. This is indeed a subtle approach for the preparation of nanostructured materials, as the PIL serves as a scaffold for the precursor to be fairly dispersed during the high-temperature synthesis. The resulting nanomaterials usually have better properties. Fig. 14 shows how a PIL scaffold preserves the macrostructure of a cotton sample after burning at a high temperature. Utilizing PILs for the fabrication of precursors of carbon nanomaterials has been extensively reviewed by Antonietti and his coworkers [148]. It should be emphasized that due to the interacting nature of the PIL ions, soft templates can be used to synthesize nanostructured PILs [149]. 10. Stabilizing nanoparticles A common problem in the preparation of nanoparticles is the strong agglomeration of nanoparticles resulting in the formation of larger particles, which is not obviously the goal of reducing the particle sizes. To overcome this problem, nanoparticles are dispersed in a matrix to immobilize the nanoparticles, avoiding direct contacts. However, the goal is not to simply keep the nanoparticles separated, but also their surface area should be chemically accessible for the targeted applications. Thus, the challenge is to maintain the matrix chemically accessible to reacting species. PILs can well do this job, as the active ions across the polymer chains can create new pathways for the interaction with reactive species entering the polymer matrix. This approach has been successfully utilized for stabilizing metal nanoparticles [150–163], which are usually active catalysts in heterogeneous reactions (as will be discussed later). Depending on the ionicity and structure of the PIL, the catalytic activity of the catalysts can be tuned [164]. Synthesis and immobilization of catalyst nanoparticles can be conducted at the same time, where forming nanoparticles are distributed in the polymer matrix [165]. This approach is not limited to metallic catalysts, as a broad range of nanoparticles can be fairly stabilized by PILs. PIL-stabilized silica has been utilized for the fabrication of water filters, as a uniform arrangement of filter nanoparticles is possible [166–170]. PIL hydrogels can generate a high osmotic pressure for regular water desalination [171]. This design has also been employed for gas filters [172]. Since PILs are capable of both forming a uniform membrane and stabilizing catalyst nanoparticles, a catalytic membrane was built in a subtle design [173]. The applicability of PILs for stabilizing catalysts is not limited to the nanoscale, as larger particles can be well stabilized for catalytic reactions [174]. This is of industrial importance, as current catalysts are still at the microscale. The key point in forming nanocomposites of this kind is strong chemical interactions between the composite components to achieve an acceptable mechanical stability. In addition to the ionic feature of PILs, there is a possibility of interaction with electron donors in the organic rings of the polymer (e.g., nitrogen in imidazole) [155]. PILs can form a composite by natural products for forming biocompatible materials [175]. 255



PILs can also act as a binder in a composite [176,177]. This is crucial for many electrochemical systems needing conductive polymers to form a solid-state electroactive material. Available binders perfectly work to provide high electrical conductivity and mechanical stability, but they are electrochemically inactive and block the electrochemical reaction in parts of the electroactive material. However, PILs are electrochemically active, as they are composed of ions, which can be fairly mobile. The binding role can be directly played during in situ preparation of nanocomposites too; where the electroactive material is bonded to the PIL, typically during a core/shell transformation [178–180]. Zeolites are known because of the extremely high specific surface area with strong potential for absorbing various gasses. However, it is not easy to fabricate thin films of zeolites for the filtration applications. Since zeolites usually have polar charges on their lattices, they can easily form strong bonds with PILs. This excellent possibility has been used for the fabrication of mixed-matrix membrane composed of PILs and zeolites [181]. PILs can also be employed for stabilizing their sister polymer class of conductive polymers [182,183] or even the dispersion of conductive polymers over graphene structure to improve the electrochemical properties [184,185]. It is known that the dangling atoms at the graphene edges can change the polymerization pathway [186]. The same method has also been exploited for mediating the interaction of conductive polymers and carbon nanotubes [187]. This is, for example, very beneficial for solar cell materials [188–191], as the efficiency of energy conversation can go above 8% [192]. 11. Swelling It is well known that PILs can compatibly trap ILs (usually the remaining IL monomer) [193–195]. This can add to the functionality of PILs by a higher degree of ion mobility across the polymer chain. This is indeed the basis for the preparation of polymer gels [193], which have promising applications in energy systems, as they can fairly replace problematic liquid electrolytes while preserving a high level of ion mobility. However, this feature is not limited to ILs only, as PILs can swallow neutral molecules too [196–199]. As a matter of fact, PILs have excellent capacities in this regard, as it can swell hundreds of times of its volume [200]. Fig. 15 depicts a typical swelling of a PIL by absorbing a solvent. The reason for this swelling phenomenon is the repulsion between the trapped species (solvents) and ion fixed on the polymer backbone. In this process, the solvent-polymer interaction is dominating the polymer-polymer interactions (between chains). By matching the solvent and polymer chain structure, it is possible to control the swelling intensity. By swelling the solvent, it is possible to push the reaction bath deep within the polymer. In other words, the polymer matrix is not within the reactor, but it is the reactor itself. This can lead to designing quasi-homogeneous catalysis systems where the entire reaction is occurring within the polymer matrix with finely dispersed catalysts [201]. 12. Ionic matrix for active materials Like other polymers, an interesting feature of PILs is a soft polymer matrix which can be well mixed with various materials. In the electrochemical systems of heterogeneous catalysis, the common problem is that the active material should be spread over a large area within a porous matrix to make more surface chemically accessible. This necessity led to various carbon-based nanocomposites, as carbon nanomaterials with high specific surface areas are widely available. Carbon only assists spreading the active materials over larger areas while it can also block the access to some parts. PILs can play a particular role in this design; while the active materials are spread over the PIL chains, the ionic nature of PILs can mediate chemical reactions. This method has been widely used for various catalytic reactions [202,203]. The flexibility of PILs for controlling the hydrophobic nature of the matrix can assist in reducing possible contaminations [204]. Light-emitting precursors can well form bonds with the PILs to produce highly efficient luminescence materials with a wide range of optical properties [205–209]. Not only the mixed hydrophobicity and hydrophilicity of PILs can assist in limiting the growth mechanism to form nanoparticles, which are of particular importance for optical materials because of a high active surface area; but also the ion interactions can improve the optical properties [210].

Fig. 15. Digital photographs of 10 mm diameter pristine poly(IL) sample punched from the bulk (left), and the poly(IL) material fully swollen to ∼89 mm diameter in DMSO (right). Reproduced with permission from Ref. [200].

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In an inverse strategy, PILs can be the guest facilitating ionic transports in another matrix. Formation of PILs on metal-organic frameworks can create nanochannels for diffusion [211]. Since the PIL chains are trapped within the ordered lattice of the metalorganic framework, the resulting nanocomposite has an excellent mechanical stability, while the PIL can contribute as a charge carrier too. Furthermore, the electrochemically accessible matrix of PILs and their ion-rich medium are a great combination for hosting electroactive materials. Thus, PILs have been employed as catalyst support in quite different systems [155,173,179,212–214]. Many of these catalytic systems have been utilized in the realm of analytical chemistry for the detection of various species. In such sensor applications, the role of the catalyst support is very sensitive, as its presence may cause interference with the detection. The interesting point is that the ion-rich architecture of PILs makes them electroactive materials too [215–217]. In this case, the base matrix can contribute to the system. Nevertheless, this contribution can be harmful in the sensor applications. Since the PIL structure is entirely composed of chemically reactive ions, adsorption of reactive species on the PIL surface can be the basis for various catalytic reactions. Therefore, PILs have also been specifically designed to serve as catalysts in the industrial organic syntheses [174]. In general, homogeneous catalysts provides higher catalytic activities due to the spatial accessibility of the reaction centers. Zhang et al. employed PILs as the catalyst supports for homogeneous catalysis [201]. The swelling capability of PILs provides a rare opportunity for trapping homogenous catalysts within the crosslinked polymer matrix. In this case, the advantages of both homogeneous and heterogeneous catalysis can be combined. 13. PIL latex The applicability of PILs is not limited to the internal matrix, as PILs can cover materials in a core/shell architecture [116]. With a suspension polymerization, it is also possible to form hollow PIL particles by eliminating the inner core [218]. The permeability of the outer shell can be tuned to control the trap and release of internal content. An interesting example is liquid marbles, which are an internal liquid droplet surrounded by a shell of a hydrophobic polymer. For the practical applications, they should be responsive, and release their contents by remote signals such as pH, UV light, and magnetic field [219–223]. A simple case is the solubility of the external polymer at different pHs. PILs have been successfully used for the preparation of liquid marbles [224]. As mentioned earlier, dissolution of PILs can be controlled by ion-exchange. Thus, it is not necessary to change the pH (which is not applicable in all environments, such as biomedical applications), and introducing a specific salt can facilitate the dissolution of the outer shell to release the core content. The interesting point is that this process is not an ordinary dissolution because it can be reversibly reversed. The solubility of PILs is due to the transformation of its nature from hydrophobic to hydrophilic, which can be reversed by an inverse anion-exchange [224–228]. In general, the hydrophilic/hydrophobic nature of PILs comes from the intrinsic charge on its surface [229,230]. Hence, PILs can be used in the core/shell design of hydrophobic/hydrophilic to deal with solubility issues of materials [231]. 14. Degradability of PILs As described throughout this manuscript, the chemical reactivity of PILs provides unique opportunities in various roles. Another interesting feature is the controllable possibility for dissolving the PILs. In the original forms, PILs are insoluble in conventional solvents similar to other polymers, but adding ionic salts can dissolve PILs due to an ion-exchange mechanism. Small ions can replace large ions in the polymer structure and break its chain [93,164]. Fig. 16 shows the gradual dissolution of a PIL in the presence of LiBr. This can be of practical interest for a broad range of applications such as drug delivery because the gradual dissolution of a shell can gradually release the active material (e.g., drug) at targeted places. Since no particular solvent is required and this process can be conducted by introducing biocompatible salts, this approach can be appropriately designed for sensitive systems such as biomedical applications. On the other hand, this feature is of utmost important from the environmental science standpoint. 15. Smart materials Polymers with shape memory play a significant role in designing smart materials. When these polymers are mechanically changed, they can keep the change temporary and then self-heal to reach back the original shape. Supramolecular self-assembly can also behave this way through a host-guest interaction. Due to the flexibility in choosing the active ions of PIL, it is possible to design appropriate host-guest systems to generate shape memory polymer. For instance, bis(trifluoromethyl-sulfonyl)imide (TFSI) can form host-guest inclusion complexes with cyclodextrins [232,233]. Fig. 17 shows a mechanical response of the corresponding PIL [234]. In a similar fashion, the viscosity of PILs can be temporarily changed by an ultrasonic wave with a memory of about 10 min [235]. In the realm of smart materials, vitrimers, which are a class of plastics derived from thermosetting polymers but flows like viscoelastic liquids at high temperature, are of particular interest. Their thermoresponsive behavior is controlled by the covalent networks, which can change their topology by thermally activated bond-exchange reactions. However, their applicability has been limited by a severe drawback as they cannot be recycled or reprocessed. The modular nature of PIL can form permanent cross-links behaving as vitrimers [236]. Owing to the flexibility in the design of PILs, a wide range of vitrimers can be specifically tailored. The structure of PILs can be designed for specific applications, as several moieties in the polymer chain can act as hooks and loops to zip/unzip the polymeric layers. This leads to excellent mechanical changes, which are controlled by mechanical, chemical or electrochemical signals [133]. This responsive feature of PILs can be linked with redox systems for developing various types of 257



Fig. 16. Consecutive optical photomicrographs of the poly([MTMA][TFSA]) particles prepared by dispersion polymerization in ethanol after the addition of the 15 wt. % LiBr/ethanol solution. SEM photographs of the poly([MTMA][TFSA]) particles (a) before and (b and c) after the addition of the 5.0 × 10–3 wt.% LiBr ethanol solution. Amount of LiBr/ethanol solution at 5.0 × 10–3 wt.%: (b) 7 mg and (c) 21 mg. Reproduced with permission from Ref. [225].

actuators [237,238]. Many PILs show strong thermoresponsivity, which can be used for various smart designs [239–248]. An emerging possibility is to design a thermoresponsive PIL nanogel by ternary crosslinking copolymerization of IL and crosslinker [243,249,250]. Fig. 18 illustrates the structure of a PIL nanogel and how it works. The interesting point is that by reducing the size of the gel particles to reach the scale of the individual PIL chains, the responsibility can be subtly tuned. Thermoresponsivity of the PIL membranes can be designed based on hydration/dehydration transition behavior upon slight changes in the temperature [251]. The applications of PILs as smart materials have been extensively described in a book devoted to this topic [20]. 16. Functionality 2D materials such as graphene, which are very thin at one dimension, are prone to be agglomerated due to heavy van der Waals interactions. Similar to the case of stabilizing nanoparticles, PILs can be used to cover the surface of graphene to hinder van der Waals forces resulting in horizontal agglomeration [252–254]. In addition to “grafting to” approach in which the PIL forms chemical bonds with the graphene functional groups; PILs can be directly formed on graphene via “grafting from” approach in which the polymerization is initiated by the graphene functional groups. In practice, PIL-stabilized graphene is much better than stabilizing the graphene by the corresponding IL monomer [252]. Functionalizing graphene with PILs results in better electrochemical performance, as the electroactive material is more electrochemically accessible [255] because the flat structure of graphene reduces the electrochemical reactivity of carbon atoms [130,256]. This can improve the electrode/electrolyte interface structure for better compatibility with the electrolyte, and also the electrode wettability, which is a key issue in using IL-based electrolytes. The wettability of PILs can be simply tuned by controlling the surface charge [257]. Graphene oxide (GO) is intrinsically covered by active functional groups, which can create strong bonds with those of PILs. In this 258



Fig. 17. Schematic illustration of PDVA gels immersed in water and aqueous β-CD solution, respectively. A hydrophobic-to-hydrophilic transition of PDVA gel took place induced by host-guest interactions with β-CD. PDVA rod gels are heated and twisted at 85 °C (above Tg: 78 °C), and their the relaxation progresses in water and aqueous β-CD at 15 °C, respectively. CD stands for Cyclodextrin. Reproduced with permission from Ref. [234].

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Fig. 18. Chemical structures of IL-based monomers and as-prepared nanogels (a) and schematical illustration of the preparation of PIL-based nanogels and their thermoresponsive behaviors (b). Reproduced with permission from Ref. [250].

case, the interaction is much stronger. Owing to the swelling effect of PILs described above, GO/PIL can create excellent chemical actuators with considerable memory effect [258]. Fig. 19 demonstrates how a GO/PIL actuator work. This approach has also been employed for carbon nanotubes [146,259]. This can wrap the carbon nanotubes to build nanocomposites by stabilizing the outer van der Waals forces [260,261]. This is not limited to the graphene basal plane, as PILs can actively functionalize the nanotube ends [262]. Most of these PIL-CNT nanocomposites have been made by the “grafting from” approach [263,264], but the “grafting to” approach has also been attempted [265]. While the former approach results in stronger bonds between the PIL and CNT, the latter can guarantee complete polymerization. Since PILs are purely made of ions and these ions can form active redox systems, preparation of functional colloids by gaining such redox systems has been investigated [266]. This is indeed a new generation of electroactive materials in the realm of electrochemistry, as the redox systems can be subtly designed. This attempt is quite new, but preliminary studies revealed excellent electrochemical behaviors, which can be used in various systems [217]. Fig. 20 display a set of typical cyclic voltammograms for a

Fig. 19. Adaptive movement of a GO-PIL/filter paper bilayer membrane (20 mm × 1 mm × 107 μm) placed in acetone vapor (20 °C) and then back in the air. (b) The plot of curvature versus time for the membrane actuator in acetone vapor and then back in the air. (c) The reversible bending/unbending deformation of a GO-PIL/ filter paper bilayer membrane actuator driven by periodic contact with acetone vapor (20 °C). Reproduced with permission from Ref. [258].

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Fig. 20. Cyclic voltammograms of poly(FcIL)modified electrode in acetonitrile containing Bu4NBF4 0.1 M. (a) Scan rate 0.1, 0.2, 0.4, 0.6, 0.8, and 1 V s–1. (b) Scan rate 2, 5, 10, 30, 60, and 100 V s–1. Reproduced with permission from Ref. [217].

native redox system implemented in a PIL. Surprisingly, the rate capability of the redox system is incredible. The antibacterial applications of PILs have attracted considerable attention, as PILs perfectly play this role. Since the PIL functionality can be designed according to a specific application, it can be a strong absorbent for some bacterias. In fact, PIL membranes act as a trap for bacteria [267,268]. Owing to the chemical reactivity of ions across the polymer chain, PILs can be easily functionalized to control their properties further [269]. Due to the flexibility of PILs for connecting with other materials, it can be used for designing pharmaceutical drugs [270]. An interesting feature of this flexibility is tuning the selectivity of sensors [11,189,271–277] and biomedical applications [278]. This can also be the basis of designing highly selective PIL membranes [279]. PILs can be formed on the laminar structure of liquid crystals to build well-ordered photosensitive materials [280]. Fig. 21 shows how the optical properties of PIL-based liquid crystals can be simply tuned by changing the anion. The combination of this optical property with the swelling feature of PILs can lead to a new class of liquid crystals [281]. The arrangement of PILs can create magnetic properties in a pure polymer material [282]. In a similar way, since the PIL functional groups can be photosensitive, it is possible to optically pattern the PIL surface for microelectronic applications [283,284].

Fig. 21. Representative polarized optical micrographs of (a) poly-[AcrC10mim ][Cl–], (b) poly[AcrC10mim ][Acr–], and (c) poly[AcrC10mim]-inter-poly[Acr–]. Reproduced with permission from Ref. [281].

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The high density of dipoles because of abundant anions and cations can result in strong microwave absorption by PILs [285]. This feature has also been used for a uniform distribution of the PIL nanoparticles during the synthesis to prepare highly ordered structures [286,287]. 17. Switchable hydrophobicity and hydrophilicity Since water, at least in the form of moisture, exists in most working environments, there is a growing interest in hydrophobic materials to keep a specific environment dry. Owing to the significant hydrophobicity of a class of PILs, they have been named as the promising hydro repellent materials [288]. However, the interesting feature of PILs in this context is far beyond common hydrophobic materials as described above, and PIL hydrophobicity can be reversibly switched to hydrophilicity. The hydrophobicity/ hydrophilicity of PILs is mainly controlled by the anion [289], and thus, can be simply switched by anion-exchange. The combination of hydrophobicity and hydrophilicity is of particular interest for the practical applications. Recently, biomimetic approaches for water harvesting have been attracted considerable attention [290–292]. In dry climates, collecting and preserving small amounts of water is of vital importance, and biomimetic approaches attempt to follow the water collecting mechanisms used by indigenous plants and animals. In most cases, collecting and taping water in living species is via a tricky mechanism involved in a combination of hydrophilic and hydrophobic surfaces. The native capability of PILs for switching between hydrophobicity and hydrophilicity provides a rare opportunity for collecting and reserving water in dry climates [169]. The presence of hydrophobic/ hydrophilic segments in PILs can be directly controlled by anion exchange [293]. PILs can also be used for the construction of charged membranes, which are usually more efficient than conventional membranes for filtration of specific species. Switching between hydrophilicity and hydrophobicity of PIL coated on a conventional membrane can significantly improve the filtering performance of the base membrane [294]. Amphiphilic nature of PILs has been used to prepare promising flocculants for water purification [170]. Moreover, simultaneous hydrophobicity and hydrophilicity of PILs can create local biphasic regions to create a virtual liquid/liquid interface, which is of interest for purification and separation systems [295]. With a similar concept, Janus nanosheets are made of PILs to interact with two different phases at the liquid/liquid interface, which is indeed a vital process in various purification and separation systems [296]. This is indeed a rare opportunity for fabricating such actuators by a single material responding to wetting/de-wetting [238]. 18. Casting A key requirement for advanced materials is the possibility of casting them for specific applications. IL are neither ideal liquids to be used as a liquid in the practical systems nor solid to be cast in appropriate forms. PILs are suitable for various casting methods. For instance, electrospinning is a common method for the preparation of micro- and nano-structured materials. On the other hand, the chemical reactivity of PILs provides an opportunity for designing different materials. For instance, nitrogen-doped carbon fiber prepared from a PIL precursor had a conductivity of 200 S cm–1 with a high nitrogen content of 8% [297]. 19. PIL membranes Similar to other types of ionic polymers, PILs are suitable candidates for the fabrication of ion conducting membranes [88,278,298]. This covers a broad range of applications from filtration to electrochemical energy storage and conversion. The practical candidates for ionic membranes are still the commercial ionic polymers such as Nafion, but their applicability is limited by high cost. Hence, PILs can provide new opportunities for new applications in which the cost is a limiting factor. For example, Ghilane et al. fabricated a reference electrode by utilizing a PIL ion-conducting membrane [299]. A membrane/separator allowing the passage of specific ions for electrochemical energy storage and conversion is a vital requirement to design the practical cells. A major problem of commercial ionic polymers is the lack of flexibility in design for specific applications. The applications of PILs in lithium-ion batteries have been recently reviewed [1]. These applications can be extended to other electrochemical systems such as supercapacitors since the role of PILs is to form solid electrolytes with high ionic conductivity and wide stable electrochemical window [300]. For the electrochemical energy storage, PILs can serve as a solid electrolyte [301], which is of practical interest to avoid the leakage safety risks. Alternatively, gel polymer electrolytes, in which a liquid electrolyte is trapped within a polymer matrix, are new promising candidates in various electrochemical systems [302]. While ILs are among the common candidates for the liquid part, the polymer matrix can also be built by PILs [303–308]. An emerging architecture for the gel polymer electrolytes is based on PILs and plastic crystals [309]. Succinonitrile is a non-ionic plastic crystal with a high capability for dissolving ionic salts owing to its plasticity and high polarity [310]. The ionicity of succinonitrile has been enhanced by the presence of ILs in the non-ionic polymer matrix [311,312]. However, utilizing a PIL matrix builds a more straightforward architecture. The interesting feature is that the PIL-based electrolytes can be synthesized via an in situ process to maximize the system integrity [211,313]. In the case of solid electrolytes, the interface formed between the electrode and electrolyte is of utmost importance, as the rigid solid-state architecture does not allow easy rearrangement as it is possible in a liquid electrolyte. Therefore, the electrode/ electrolyte interface undergoes irreversible structural changes. Due to the PIL functionality, the interface properties can be tuned; from the type of interactive ions to hydrophobicity/hydrophilicity. In some electrochemical applications such as flow redox batteries, the key feature of the membrane separator is its selectivity to allow the passage of certain ions. In such systems, a specific ion serves as the charge carrier, and other ions should remain on the same 262



side of the cell. The release of these ions from one side to the other (e.g., from the cathode to anode) is called shuttle effect, which is a harmful process resulting in severe capacity fading. This is again a limitation of commercial ionic polymers, as their chemical structure cannot be tuned. The selectivity of PIL membranes for various diffusing species in different systems has been successfully tuned for the practical performance [314]. On the other hand, various types of nanoscale modifiers can be subtly incorporated into the architecture of the PIL membranes to adjust their structure and performance [315,316]. Also, in the electrochemical energy conversion systems such as fuel cells, the membrane selectivity is the key requirement. Various PIL-based proton exchange membranes have been specifically designed for the fuel cell applications and showed excellent properties in terms of proton conductivity and thermal stability [317–319]. In a similar fashion, the membrane permselectivity can be tuned for the separation of gasses. By incorporating the polymers such as polyvinyl acetate into crosslinked PILs, it is possible to control the gas transport properties of the corresponding membranes to improve their gas permeability and the capability for the separation of gasses such as CO2/N2 and CO2/CH4 mixtures [172,181,320–323]. In addition to the ion passage through the membrane, the ionic functionality of each side of the membrane can be useful for specific designs. For example, the performance of a microbial fuel cell was enhanced by one order of magnitude by coating the anode with a PIL membrane [324]. In this case, the ionic functionality of the PIL membrane enhanced the bacterial loading capacity while facilitating the electron transfer in the electrochemical redox system. This adsorbing feature of the PIL membrane has been utilized for the extraction [325–332], removal [333], and detection of specific species [334,335]. The possibilities for designing new membranes are as broad as the range of choices in creating the PIL structure. Rueda et al. reported the fabrication of ion-conducting membranes by the polymerization of wedge-shaped ILCs [336]. Since this membrane can be designed by various counterions such as Li, Na, and K; the design can be customized for specific applications, e.g., the corresponding alkali-metal-ion batteries. The ordered architecture of the ILCs also provides rare opportunities for creating the appropriate channels for the diffusion of electroactive species, which is indeed the prime requirement in the design of electrochemical cells. This flexible architecture can be easily tuned for the specific applications [337]. Dye-sensitized solar cells (DSSCs) are indeed photoelectrochemical cells with the practical requirements similar to other electrochemical systems mentioned before. In the electrochemical systems, the diffusion of electroactive species is the ratedetermining step and defines the rate capability; but there is an additional issue in DSSCs. If the electrochemical reaction at the electrode/electrolyte interface is not fast enough the charges photogenerated will be recombined [338]. Therefore, the role of the electrolyte is critically important. In addition to the PIL gel polymer electrolytes, which were described above as an emerging type of electrolytes in the electrochemical systems, PILs have even been used as charge carrier within the electrolyte [339]. Wang et al. employed a solid-state PIL electrolyte based on poly(1-ethyl-3-(acryloyloxy)hexylimidazolium iodide) in which the π-π stacking of the imidazolium side chain facilitated the hole transport from the photoanode to the counter electrode. Various quasi-solid-state PIL electrolytes, which are usually based on copolymers, have been examined to inspect how the redox couple can be matched with the PIL ionic structure [340–342]. 20. Final remarks The present manuscript summarized the practical potentials of the emerging family of PILs, which has been significantly extended during the last few years because of the enormous possibility of employing IL monomers for designing new PILs. Although the current terminology of poly(IL) or polymerized IL fairly describe this new class of polymers, it has somehow limited the range of polymers to a series of popular ILs. The point is that the applicability of the resulting polymer is not necessarily aligned with the IL monomer. Hence, the ILs, which are categorized as unsuitable for common applications because of inappropriate liquidity and viscosity, might be good monomers for the preparation of PILs. The key feature of ILs is the presence of mobile ions, but this mobility should be under control for specific applications. Polymerization just places the ions through a polymer architecture in an ordered fashion. Here, we simply narrated the common features and properties of PILs, which might be useful for a wide range of applications. The next step is to design the PILs for specific applications. For this purpose, it is necessary to consider that the nature of PILs is the ionbased architecture. Therefore, regardless of the starting monomer, the PIL architecture can be designed by the appropriate ions. The strategy in designing the PILs is somehow different from other polymers. Owing to the ionic structure of PILs, the prime goal is to preserve the ionic mobility of the building blocks. Therefore, instead of having macromolecules composed of long polymer chains, the PIL architecture should be subtly designed to fix the IL monomers in the form of a polymer. In this case, one ion is sacrificed to form the polymer backbone while the other can contribute to the ionic mobility. This is just a simplified picture, as the practical PILs are based on crosslinking and block copolymers. However, in all cases, it is still necessary to preserve the ionicity of the original IL. In fact, forming a polymer architecture of the IL monomer creates rare opportunities for new applications as reviewed here. In this directions, the possibilities are way beyond the available cases. Combining different properties of the PIL can open new gates. For instance, the PIL ionicity is targeted in the electrochemical systems while the mechanical responsivity in the design of smart materials. By tuning the PIL properties, it should be possible to fabricate new devices with different capabilities. 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