State-of-the-art and international context

Reasons for creating the Research Consortium

1 – Ionic Liquids

i – Structure of ILs

Ionic Liquids (ILs) are compounds with which the physical chemistry and organic chemistry communities are very familiar.   These molten salts are liquids at temperatures of below 100 °C and consist exclusively of ions. They generally take the form of a voluminous organic cation of the alkylimidazolium, alkylpyridinium, alkylpyrrolidinium, alkylammonium or alkylphosphonium-types and a mineral counter-ion such as chlorine, iodide, hexafluorophosphate or tetrafluoroborate, or otherwise an organic counter-ion such as a carboxylate, trifluoromethanesulfonylimide or alkylsulfonate. The possibility of combining different anions and cations (up to 1,018 different combinations have been identified) offers a great variety of physical and physico-chemical properties.  A selection of these different combinations is shown in Figure 1.

figure 1
Figure 1. The most frequently used cations and anions for forming ILs [1]

ii – History of ILs

The first ionic liquid – ethylammonium nitrate – was synthesised at the start of the last century, but ionic liquids did not really start to be synthesised for electro-chemical applications in the field of batteries until the 1970s. A new class of ILs – based on imidazolium salts – emerged in the 1980s, but their development was limited due to their association with aluminochlorate anions, which are highly reactive with air and water. It was not until the start of the 1990s, after the synthesis of imidazolium ILs associated with fluorinated anions which are stable in air and water, that ionic liquids began to attract the interest of researchers [2].

iii – Properties of ILs

The complex structures and interactions in ILs result in remarkable chemical and physico-chemical properties and offer a huge potential for applications in processes (synthesis, catalysis and separations) and devices (optics, batteries and lubrication).

The chemical properties of ionic liquids, in addition to their high polarity and physical properties such as high thermal stability, rheology, solvent power and their low vapour pressure above the decomposition temperature (negligible at ambient temperature), mean that their use can be envisaged in diverse fields of application. ILs are solvents with extraordinary properties which lead to increased reactivity and selectivity for a wide range of chemical reactions.

At the molecular level, ILs are characterised by a nanometric structure with the coexistence of non-polar fields (Van der Waals-type interactions) and polar regions (electrostatic interactions). This nanostructuring was revealed by molecular simulation [3] and also by spectrographic analyses, X-ray diffraction and calorimetric measurements [4]. The dissolution of apolar, polar and associative solutes can be explained at the molecular level [5]: apolar solutes are solvated in the areas formed by apolar alkyl chains; polar solutes interact with the apolar regions and the charged tops, and are therefore often totally miscible with the ILs. Associative solutes are capable of establishing specific interactions with ILs (e.g. hydrogen bonds), which lead to structured solutions.

ILs are used in numerous industrial applications, such as the DifasolTM process (1995, IFP-Axens) and the BASILTM process (2003, BASF). These processes have proven that ILs can be used as substitutes for conventional organic solvents, which have been discredited due to their harmfulness to the environment, in many reactions such as those involving catalytic systems, in metal extraction processes, the nuclear sector, the contamination of water [6], etc. In electro-chemistry, ILs are also very widely used as electrolytes for the electro-deposition of metals on conducting surfaces [7].

2 – Ionic Liquids: Multipurpose agents for polymers

i – Ionic liquid: A reaction medium for polymer synthesis and modification

One relatively recent application of ionic liquids is their use as polymerisation solvents as an alternative to conventional organic solvents [8]. However, the number of publications on this subject remains limited. The improvement combined with higher polymerisation rates and molar masses, which has been described for the majority of polymerisation processes studied in ionic liquid media (conventional radical [9] and controlled [10], anionic/cationic [11], and by coordination [12]), shows the efficiency of these IL solvents for polymerisation processes, as shown by Figure 2 relating to the conversion of methyl methacrylate [13], for example.

figure 2
Figure 2. Influence of the solvent on the radical polymerisation conversion rate  of methyl methacrylate with IL = [EMIm][EtHSO4-] and DMF: dimethylformamide [13]
The very high propagation rate coefficients, KP, and the reduction of propagation activation energy, Ea, in these environments may be explained by the existence of hydrogen bonds between the monomers and the growing polymethylmethacrylate chains one the one hand, and the cations and anions of the ILs, on the other.

Several examples of the polymerisation of monomers which are soluble in ILs have been studied, such as methacrylate monomers (hydroxyethylmethacrylate), styrene, acrylate, vinyl acetate, acrylonitrile, etc. in aprotic ionic liquids based on N-dialkylimidazolium salts associated with fluorinated anions of the [PF6-], [BF4-] or [N(SO2CF3)2] type. The ILs used in suspension polymerisation processes also act as surface-active agents for the synthesised polymer particles (polystyrene [14], conducting particles [15]). Because of the chemical nature and size of the cation and anion and due to the choice of its functionalisation (length and concentration of aliphatic chains), the IL allows for the stabilisation of the polymer particle diameter and its reduction to sub-nanometric diameters. ILs are also effective in solubilising natural polymers and allowing for chemical modifications. Acetylation and other esterifications are thus possible for saccharide derivatives such as guar when they are carried out in allyl methylimidazolium chloride.

ii – Ionic liquid: An aid to polymer implementation

The ability of ILs to solubilise numerous substances such as salts, fatty substances, proteins, amino acids, oligo-and polysaccharides make them a solvent of choice due to their strong polarity and ability to accept hydrogen bonds from their anions. The first patent for the dissolution of cellulose by an ammonium IL dates back to 1934. Since then, ILs have been used to solubilise other biopolymers such as wood, silk, keratin or other polysaccharides such as guar [16-17]. IL/IL interactions are counterbalanced in favour of IL/Polymer interactions through the appropriate choice of the IL’s cation/anion combination. The solid-liquid phase diagrams are dictated by the chemical nature of the IL which may modify the solubility of the polymer and influence the solubilisation temperature [18].

ILs are also good substitutes for traditional plastifiers in polymers such as polylactic acid (PLLA), polymethyl methacrylate (PMMA) and polyvinyl chloride (PVC) [19-20].
ILs are also effective compatibilisers for facilitating the dispersion of organic or inorganic charges in a polymer matrix and thus creating nanocomposites with controlled morphology. In nanocomposites with lamellar nanofillers, the cationic exchange carried out with ILs of the pyridinium, imidazolium or phosphonium types improves the thermal stability of the nanofillers and allows for the implementation of polymer/clay nanocomposites [21] in a molten state. ILs also help to stabilise and control the size of metallic nanoparticles [21]. The adsorption of the IL on the nanoparticle is dependent on the function carried out by the LI – ionic vs. silylated or thiol vs. aromatic – which respectively generate ionic interactions on lamellar silicates [23], covalent interactions on hydroxyl groupings of oxides such as silica or on a gold surface [24], or P-P interactions on carbon nanotubes [25].

iii – Ionic liquid: A polymer (nano)structuring agent

The IL may also act as a polymer (nano)structuring agent and lead to the creation of physical or chemical gels which are obtained by the process shown in Figure 3 [22]. The association of ILs with electrolyte polymers, either directly by polymerisation from a reactive IL (monomer) [26] or by the solubilisation of the electrolyte polymer in the IL [27], represents an excellent alternative to electrolytes which have been strongly criticised on grounds of safety as they are often flammable and volatile, such as the liquids used in the manufacture of lithium batteries and supercapacitors [28]. These membranes, whose operating range has been extended thanks to the thermo-stable nature of the ionic liquid and which offer suitable electro-chemical properties if the ionic liquid is properly chosen, may also have excellent potential for use in fuel cells [29]. The great versatility of ILs allows these ionogels to meet the requirements of a broad range of electro-chemical applications by facilitating ionic, protonic or electronic conductivity due to the nature of the IL and its association with the polymer [30].

figure 3
Figure 3. Diagram of the structuring of an electrolyte gel after the insertion of an IL into a macromolecular network [22]
The versatility displayed by ionic liquids due to the nature of the cation, anion and organic ligands makes them functional objects that have variable interactions with the polymer matrix and are capable of generating a structure within the polymer. The phase separations generated in the material between the IL and the matrix lead to different structures of the ionic domains in the polymer matrix from the micrometric scale to the nanometric scale, as shown by the transmission electron microscopy photographs shown in Figure 4 [31]. In this way, co-continuous morphologies in a percolating (spider-web) network may be generated according to the type of anion or cation. Ionic liquids thus behave as a network of cations and anions associated with ionic interactions that give rise to physical nodes in the matrix with similar properties to ionomers, although they do not have the numerous disadvantages that require the synthesis of the latter.

figure 4
Figure 4. Variable structures according to the nature of the counter anion associated  with octadecylphosphonium dispersed in a proportion of 1% by weight in a fluorinated matrix [31]

iv – Ionic liquid: For functional polymer materials

Due to their functionalisation role, ILs can be used to produce many functional materials (photopolymerisable gels [32], latex particles [33], thermoplastic films [34] and thermosetting networks [35], electrolytic membranes [36] and intelligent thermosensitive materials [18]). ILs introduced into polymer materials offer a new way to tackle the recurrent challenge in materials science of designing and creating new polymers with improved physical properties and combinations of behaviours (multipurpose polymers). Initial works have shown the potential for these new multipurpose agents to improve the thermal and mechanical properties of these polymer materials [34]. Due to their high ionic conductivity and the resulting modifications of the matrix in terms of structure and crystallinity, the electric and dielectric properties and ability to act as a barrier to the diffusion of gases may also be affected, which offers new prospects in the fields of energy and fuel cell membranes. Other fields of physical behaviour may also be concerned, such as the great ability of ILs to reduce the friction coefficient and wear of polymers against metals [37].

Ionic liquids thus appear to be versatile compounds which are easy to synthesise and possess very attractive properties that have earned them the description of “magic” compounds. They offer numerous alternatives to volatile organic solvents and conventional electrolytes. Ionic liquids also pave the way towards new types of chemistry (supramolecular chemistry and chemical modifications) conducted in new polymerisation media with greater efficiency and greater purity (ability of ILs to dissolve numerous types of catalytic systems). The capacity of ionic liquids to structure and functionalise polymer materials from the nanometric scale upwards, allows us to envisage the development of new materials (nanomaterials, electrolyte membranes, porous materials, etc.) which offer significant industrial potential for the energy, structural materials and biomaterials sectors. This promising development must motivate (and is already motivating) academic and industrial research to understand the interactions between ILs and polymers and the structure-properties relationships that result from the association of ILs and polymers.


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