Polymeric Ionic Liquid Research Articles

Preparation of tough and stiff ionogels via phase separation.

Ionogels have the advantages of thermal stability, non-volatility, ionic conductivity and environmental friendliness, and they can be used in the field of flexible electronics and soft robotics. However, their poor mechanical strength and complex preparation methods limit their practical application. Herein, we propose a simple strategy to improve the performance of ionogels by adjusting their phase separation behavior. In a polymer-ionic liquid (IL) binary system with an upper critical solution temperature (UCST) and Berghmans' point, the phase separation behavior will be frozen below the temperature corresponding to the Berghmans' point, and thus, the degree of phase separation can be adjusted by controlling the cooling rate. We found that a polyacrylamide (PAM)-IL binary system possessed a UCST and Berghmans' point and the resulting ionogels had excellent mechanical properties. Their tensile strength, tensile modulus, compressive strength and compressive modulus reached 31.1 MPa, 319.8 MPa, 122 MPa and 1.7 GPa, respectively, while these properties of the other ionogels were generally less than 10 MPa. Furthermore, they were highly transparent, stretchable, stable and multifunctional.

Synthesis of oxanorbornene-based phosphonium polymeric ionic liquids (PILs) and investigation of their electrical properties

A series of phosphonium polymeric ionic liquids (PILs) was synthesized by ring-opening metathesis polymerization (ROMP). The optimum conductivity obtained was 6 × 10−4 S cm−1.

Polymerized ionic liquid Co-catalysts driving photocatalytic CO2 transformation.

Photocatalytic production of CO from CO2 has the potential for safe and atom-economic production of feedstock chemicals via in situ carbonylation chemistry. We developed novel ionic liquid-based polymeric materials through radical copolymerisation of 1-butyl-3-vinylimidazolium chloride and photocatalytically active Re- and Ru-complexes that serve as the CO2 reduction catalyst and photosensitiser, respectively. The crosslinked polymeric framework allows for the facile immobilisation of molecular organometallic complexes for use as heterogenised catalysts; moreover, the involved imidazolium core units co-catalyze the reduction of CO2 via covalent interaction. The ratio of sensitiser and catalyst was analysed by laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) and set in relation to results from photocatalytic experiments. Ultimately, the heterogenous polymeric framework showed high selectivity for CO formation on photocatalytic CO2 reduction with improved stability to the corresponding homogenous system.

Mechanochromic and ionic conductive cholesteric liquid crystal elastomers for biomechanical monitoring and human-machine interaction.

Cholesteric liquid crystal elastomers (CLCEs) that combine rubbery elasticity with structural colour from self-assembled helical nanostructures are of paramount importance for diverse applications such as biomimetic skins, adaptive optics and soft robotics. Despite great advances, it is challenging to integrate electrical sensing and colour-changing characteristics in a single CLCE system. Here, we report the design and synthesis of an ionic conductive cholesteric liquid crystal elastomer (iCLCE) through in situ Michael addition and free-radical photopolymerization of CLCE precursors on silane-functionalized polymer ionic liquid networks, in which robust covalent chemical bonding was formed at the interface. Thanks to superior mechanochromism and ionic conductivity, the resulting iCLCEs exhibit dynamic colour-changing and electrical sensing functions in a wide range upon mechanical stretching, and can be used for biomechanical monitoring during joint bending. Importantly, a capacitive elastomeric sensor can be constructed through facilely stacking iCLCEs, where the optical and electrical dual-signal reporting performance allows intuitive visual localization of pressure intensity and distribution. Moreover, proof-of-concept application of the iCLCEs has been demonstrated with human-interactive systems. The research disclosed herein can provide new insights into the development of bioinspired somatosensory materials for emerging applications in diverse fields such as human-machine interaction, prostheses and intelligent robots.

Process design, kinetics, simulation, and techno-economic analysis of biodiesel production from Pongamia pinnata seed oil using a magnetically recyclable acidic ionic liquid catalyst

A magnetically recyclable polymeric acidic ionic liquid nanocatalyst was fabricated in response to the demand for economical and eco-friendly one-pot conversion of inexpensive oils to biodiesel. To achieve this, Fe3O4–SiO2 nanocomposites with Fe3O4 as the core and SiO2 as the shell were prepared, and polymeric acidic ionic liquid (PAIL) was grafted over the core–shell nanocomposite (Fe3O4-SiO2) to produce Fe3O4–SiO2–PAIL nanocatalyst. The characterization results demonstrated that Fe3O4–SiO2–PAIL nanocatalyst had a strong saturation magnetization of 19.16 emu/g, a large surface area of 13.067 m2/g, and a high acid loading of 6.1 mmol/g. The Fe3O4–SiO2–PAIL was used as a heterogeneous catalyst to transesterify Pongamia pinnata seed oil (PPO) into fatty acid methyl esters (FAME). The biodiesel yield was 94.89% under optimized reaction conditions of 6.14 % (w/w) catalyst loading, methanol:oil molar ratio of 12:12, 57.37 °C, and 1.67 h. The produced FAME complied with the ASTM specifications for biodiesel. The transesterification reaction exhibited a low activation energy of 28.298 KJ/mol with pseudo-first-order kinetics. The positive values of ΔH° and ΔG° indicated that the transesterification reaction was endothermic, endergonic, and non-spontaneous. The negative value of ΔS° illustrated that the reaction is less disordered. Furthermore, the reusability studies of the heterogeneous nanocatalyst was investigated. The Fe3O4-SiO2-PAIL nanocatalyst was successfully retrieved swiftly by magnetic force, with no discernible mass loss, and employed at least six additional times without significantly losing its catalytic activity. The techno-economic analysis was executed to investigate the technical and economical feasibility of 15 metric tonne/batch biodiesel production from Pongamia pinnata seed oil using the Fe3O4–SiO2–PAIL nanocatalyst. The economic reports demonstrated a positive NPV of 20,736,000 $/year, a gross margin of 9.41%, and a biodiesel minimum selling price of 0.926 $ with a payback period of 1.88 years. This work paves the way for the sustainable use of P. pinnata as a feedstock for biodiesel production using the Fe3O4–SiO2–PAIL nanocatalyst under specified circumstances.

(Best Poster Award - 2nd Place) Energy Harvesting from CO2 Emission Exploiting an Ionic-Liquid Based Electrochemical Capacitor

Global warming is one of the most serious problems that the humanity is facing in his history.Since 1900 to nowadays, the global temperature has risen by 1 °C and, according to the scientificcommunity, the cause is related to the climate-changing emissions due to anthropogenic activities.In order to avoid an environmental disaster, 196 countries have signed the Paris agreement thataims to maintain global warming below 1.5 °C compared with pre-industrial levels.The reduction of greenhouse gas emissions is a key issue in contemporary science, and in thiscontext, the ability of certain ionic liquids (ILs) to capture carbon dioxide has attracted a lot ofinterest. The idea presented in this work is to design an electric double layer supercapacitor whichis able to harvest energy by exploiting the reaction between an ionic liquid and carbon dioxide. Inparticular, the system consists of two carbon-based electrodes and a polymeric separator soakedin ionic liquid. The use of carbon electrodes ensures a high surface area and excellent electricalconductivity while imidazole-based ionic liquid acts and as electrolyte and as capture media.The operating principle of this system is a novel mechanism which exploits the formation of avoltage drop across the device when the electrode-IL interfaces are different; the two differentinterfaces are ensured by the presence of imidazole carbamate in a part of the cell (Im − CO2reaction’s product) and just imidazole in the other part.Unfortunately the high viscosity of ionic liquids affects a lot the carrier transport and consequentlythe device’s performance too; this aspect worsens when imidazole reacts with CO2. Improved ionmobility is achieved by reducing the ion pairing through the use of high dielectric solvents capableto screen the ions’ electric field. Polar aprotic solvents tend to have large dipoles making themsuitable for the solvation of charged species.Several mixtures have been analyzed to detect the best dilution which guarantees high ionic mobilitystill maintaining a large amount of active species.A further performance improvement has been achieved through the engineering of the electrodes,namely the capacitance of the supercapacitor has been enhanced by exploiting the huge specificsurface area of the activated carbon.The results show that all the upgrades mentioned lead to an improvement of 500 times in terms ofharvested energy.Acknowledgement: This result is part of a project that has received funding from the European Research Council(ERC) under the European Union’s ERC Starting Grant Grant agreement No. 949916 Figure 1

Study and Characterization of Solid State, Flexible, Wire-Shaped Supercapacitor for Textile Energy Storage Purpose

Flexible and lightweight energy storage devices have become increasingly important for the development of wearable and portable electronics. In this work, we present a novel wire-shape supercapacitor based on carbon nanotube (CNT) yarns and an ionic liquid based polymer electrolyte. The combination of these materials offers high surface area, high conductivity and excellent flexibility, making them a promising candidate for flexible energy storage.The CNT yarns were characterized in a three-electrode configuration by means of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) either in water-based electrolytes and polymer electrolytes. The polymer electrolytes were prepared by mixing polymers with ionic liquid, which was characterized by thermal analysis and electrochemical measurements.The wire-shape supercapacitor was assembled by coating two CNT yarns with the polymer electrolyte and twisting them together. The devices exhibited high specific capacitance for a waterless system, additionally, they maintained their performance even under extreme deformation: the flexibility of the devices were tested by bending at different angles .Overall, this work highlights the advantages of using CNT yarns and ionic liquid polymer electrolytes for the development of flexible and high-performance supercapacitors. The wire-shape design of the device offers additional advantages for integration into wearable or flexible electronics, providing a promising path towards the development of highly efficient and adaptable energy storage systems.

Methods and Mechanical Properties of Polymer Hybrid Composites and Hybrid Polymer Composites: Influence of Ionic Liquid Addition

Polymer hybrid composites and hybrid polymer composites are distinct but interconnected composite classes, each with unique compositions and design philosophies. The mechanical properties of these composites are vital in advanced materials due to their impacts on performance, durability, and suitability for various applications. The addition of ionic liquids into these composites is a promising innovation in advanced materials. In this short review, various polymer matrices (e.g., thermosets, thermoplastics, and biopolymers), fillers (e.g., inorganic, carbon, organic, and metal), and ionic liquids (e.g., imidazolium- and phosphonium-based) used to fabricate polymer hybrid composites and hybrid polymer composites with added ionic liquids are identified. Furthermore, the addition of ionic liquids into these composites through different methods (e.g., magnetic stirring, mechanical stirring, solid grinding, etc.) is discussed. The influence of ionic liquid addition on the mechanical properties, specifically the tensile properties of these composites, is also shortly reviewed. The changes in the tensile properties, such as the tensile strength, tensile modulus, and elongation at break, of these composites are explained as well. The information presented in this review enhances the understanding of the methods applied to add ionic liquids into polymer hybrid composites and hybrid polymer composites, along with their tensile properties. In short, some ionic liquids have the capacity to enhance the tensile properties of hybrid polymer composites, and several ionic liquids can reduce the tensile properties of polymer hybrid composites.

Polymeric ionic liquid sorbent coatings in thin film microextraction: Insight into sorbent selectivity for pesticides and cannabinoids

Polymeric ionic liquid (PIL) sorbent coatings consisting of polymerizable cations and anions were employed as sorbent coatings in thin film microextraction (TFME) for the extraction of pesticides and cannabinoids. The blades consisted of a thin film of PIL sorbents chemically bonded to vinyltrimethoxysilane-functionalized nitinol sheets. The imidazolium- or ammonium-based PIL sorbents contained aromatic benzyl moieties as well as polar hydroxyl groups or aliphatic functional groups within the chemical structure of the IL monomer. The chemical structure of the IL crosslinkers of the PILs were kept constant across each sorbent, except for the anion, which consisted of either bis[(trifluoromethyl)sulfonyl]imide ([NTf2−]), p-styrenesulfonate ([SS−]), or 3-sulfopropyl acrylate ([SPA−]). Temperature, salt content, and methanol content were optimized as extraction conditions to maximize pesticide-cannabinoid selectivity using Doehlert design of experiments (DOE). Effects of these three factors on selectivity and extraction efficiency are discussed. The optimal extraction conditions consisting of sample temperature (31°C), sodium chloride (30% w/v), and methanol content (0.25% v/v) are compared to initial sorbent screening conditions at a sample temperature of 40°C, 15% (w/v) sodium chloride, and 2.5% (v/v) methanol content. PIL sorbent swelling behavior at different salt and methanol content conditions and its effect on extraction efficiency are hypothesized. Selectivity factors for the sorbents indicated that aromatic moieties within the IL monomer may enhance pesticide-cannabinoid selectivity under optimized conditions, but the extraction efficiency of pesticides that are known to coelute with cannabinoids in the chromatographic separation may be enhanced by employing sorbent coatings with [SPA−] anions.

Facile Fabrication Technique of Polymeric Ionic Liquids‐Coated Core–Shell Nanoparticles for Polymer Electrolyte Membranes

AbstractNanoparticles and nanofibers are widely used as components of polymer electrolytes for membranes in fuel cells, and many surface modification methods are reported. However, some fabrication techniques are complicated, and it is necessary to develop a simplified and precise control method. Herein, a facile fabrication method is reported for core–shell nanoparticles hierarchically coated with polymeric ionic liquids (PIL) and hydrophobic polymers as a material for polymer electrolytes. A hierarchical polymer layer on the surface of the SiO2 nanoparticles is easily constructed by repeating the facile polymer‐coating technique based on precipitation polymerization several times. The highest proton conductivity of the core–shell nanoparticles is 1.3 × 10−2 S cm−1 at 80 °C and 95% relative humidity. Although the hydrophobic polymers coated as a protective layer reduce the proton conductivity, the formation of the PIL enhances the proton conductivity in various temperature and humidity environments. Therefore, the proposed method enables the facile fabrication of polymer layers with multiple functions, such as a proton‐conductive PIL layer and hydrophobic polymer layers as protective layers on the surface of the nanoparticles. Furthermore, they are expected to be applied to energy supply and gas separation, including polyelectrolytes, for the realization of a sustainable society.

Synthesis of dimethyl carbonate from CO2 catalyzed by spherical polymeric ionic liquid catalyst

Synthesis of dimethyl carbonate from CO2 catalyzed by spherical polymeric ionic liquid catalyst

Thermally-induced phase fusion and color switching in ionogels for multilevel information encryption

The growing significance of data security has propelled the advancement of information encryption materials. Here, thermochromic poly(4-hydroxybutyl acrylate) (PHBA) ionogels with broad response range and excellent cyclic stability were successfully developed through photopolymerization. Due to the reversible phase diffusion between PHBA polymer networks and ionic liquids (ILs), the PHBA ionogels can switch between opaque and transparent states in response to changes in heating temperature. The enthalpic and entropic properties of the PHBA ionogels were modified by adjusting the crosslinking density (1.0%–2.5%) of polymer networks and concentration (30%-60%) of ILs, which in turn accurately controlled their phase fusion temperature (32 °C to over 68 °C). The size and optical properties of ionogels can be effectively maintained even after exposure to surpass daily ambient temperatures (−10 and 50 °C) or humidity (75%) levels for 12 h, as well as undergoing heating–cooling cycles or operating in ambient environments for 60 days. These findings demonstrate the exceptional stability of these ionogels, rendering them suitable for application as reversible information encryption devices. For the first time, proof-of-concept demonstrations of a thermochromic ionogel system for six-level information encryption and ionogel barcodes are presented here. This work is particularly significant for smart soft materials and information encryption fields.

Excellent lithium storage performance of bipyridinium-based polymerized ionic liquids as anode in lithium-ion batteries

For organic electrode materials, slow Li+ diffusion kinetics is a main obstacle to the realization of high lithium storage capacity through a super-lithiation reaction. Here, bipyridinium-based polymerized ionic liquids (PILs) have been synthesized by the ionothermal trimerization of 1,1-dicyanomethyl bipyridinium bromide ILs in molten ZnCl2 matrix, and are used as an anode material for the first time. In which, the ionic chains are beneficial for fast Li+ diffusion, as well as the p-π resonance structure between methylene and aromatic ring helps the electron conduction. Electrochemical analysis shows PILs-400 can deliver 1201.5 mAh g−1 at 0.1 A g−1 and almost no capacity loss with the capacity of 873.6 mAh g−1 over 500 cycles at 2 A g−1. Theoretical calculation reveals that all functional groups of triazine, methylene, and bipyridinium have taken part in the redox reaction, and this reaction is associated with a three-step insertion process of 19 Li+ for each repetitive unit in forming Li19C14N5, along with the reversible formation of N-Li and C-Li bonds.

Electrorheological effect of self-crosslinked polymerized ionic liquids containing different types of ion spacers

In this paper, we synthesized five self-crosslinked polymerized ionic liquids (S-PILs) containing distinct ion spacers but identical mobile counterions. The structural characterization of these S-PILs was carried out using Fourier transform infrared spectroscopy and wide-angle X-ray scattering. The electrorheological (ER) effect was investigated using rheological measurement and broadband dielectric spectroscopy. The results showed that the ER effect of S-PIL with phosphorus-containing ion spacers was the weakest, that of S-PIL with imidazole-containing ion spacers and S-PIL with pyridine-containing ion spacers was increased, and that of S-PIL with amino-containing ion spacers was the strongest. Dielectric spectra analysis showed that the interfacial polarization time of S-PIL with phosphorus-containing ion spacers was the longest, resulting in its weakest ER effect. The interfacial polarization times of S-PIL with imidazole-containing ion spacers and S-PIL with pyridine-containing ion spacers were increased but their conductivities were relatively large, which led to strong electrode polarization and weakened their ER effect. S-PILs with amino-containing ion spacers had moderate conductivity and suitable polarization time, so they had the strongest ER effect. Finally, we discussed the reasons for these different conductivity and dielectric polarization by combining the dissociation degree of counterions calculated by Raman spectroscopy and the activation energy of ion movement calculated by conductivity data. The study provides a molecular-level understanding for the rational design and preparation of S-PILs with high ER performance.

Poly(N‐acryloyl glycinamide) eutectogels

AbstractThe ever‐growing demand for advanced implantable and wearable electronics has stimulated the development of conductive gels with desirable functions. Hydrogels suffer from poor environmental adaptability. The ionogels prepared by dissolving polymers in ionic liquids have to face the problems of high cost of synthesis and biotoxicity concerns. Recently, eutectogels consisting of various polymer networks in eco‐friendly and cost‐effective deep eutectic solvents (DESs) have emerged as attractive alternatives to overcome the limitations associated with conventional hydrogels and ionogels. In this study, a kind of eutectogels was designed and synthesized through one‐step in situ photopolymerization of N‐acryloyl glycinamide (NAGA) without crosslinkers in low‐cost and biosafe DESs composed of choline chloride and ethylene glycol. The obtained poly(N‐acryloyl glycinamide) (PNAGA) eutectogels exhibit concentration‐dependent and adjustable mechanical properties, high electrical conductivity (about 2 mS cm−1), self‐healing ability as well as good anti‐drying and anti‐freezing properties. PNAGA eutectogels were also demonstrated to serve as conductive gels to construct pressure sensors over a relatively large temperature range. Considering the simple and easy‐to‐scale preparation process and commercially available low‐cost raw materials, favorable flexibility, high ionic conductivity, and wide‐climate tolerance, the PNAGA eutectogels as soft conductive materials have great potential for use in the fabrication of flexible electronics.

Ionogels for flexible conductive substrates and their application in biosensing

Ionogels are highly conductive gels made from ionic liquids dispersed in a matrix made of organic or inorganic materials. Ionogels are known for high ionic conductivity, flexibility, high thermal and electrochemical stability. These characteristics make them suitable for sensing and biosensing applications. This review discusses about the two main constituents, ionic liquids and matrix, used to make ionogels and effect of these materials on the characteristics of ionogels. Here, the material properties like mechanical, electrochemical and stability are discussed for both polymer matrix and ionic liquid. We have briefly described about the fabrication methods like 3D printing, sol-gel, blade coating, spin coating, aerosol jet printing etc., used to make films or coating of these ionogels. The advantages and disadvantages of each method are also briefly summarized. Finally, the last section provides a few examples of application of flexible ionogels in areas like wearables, human-machine interface, electronic skin and detection of biological molecules.

Synthesis and Characterization of New Polymeric Ionic Liquids as Corrosion Inhibitors for Carbon Steel in a Corrosive Medium: Experimental, Spectral, and Theoretical Studies.

A novel series of polymeric ionic liquids (ILs) based on benzimidazolium chloride derivatives, namely, 1,3-diheptyl-2-(2-phenyl-propyl)-3H-benzimidazol-1-ium chloride (IL1), 1,3-dioctyl-2-(2-phenyl-propyl)-3H-benzimidazol-1-ium chloride (IL2), and 1,3-Bis-decyl-2-(2-phenyl-propyl)-3H-benzoimidazol-1-ium chloride (IL3), were synthesized and chemically elucidated by Fourier transform infrared spectroscopy, 1H NMR, 13C NMR, and elemental analysis. Their influence as corrosion suppressors were investigated for C-steel corrosion in 1 M HCl, by weight loss (WL), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) methods, revealing that their exclusive addition decreased corrosion with mounting concentrations. These assays demonstrated that novel ILs are efficient inhibitors at relatively low dosages. The efficacy of the synthesized ILs reached 79.7, 92.2 and 96.9%, respectively, at 250 ppm and 303 K. Parameters for activation and adsorption were calculated and are discussed. The Tafel polarization results demonstrated that the investigated ILs support the suppression of both cathodic and anodic reactions, acting as mixed type inhibitors. Langmuir's adsorption isotherm was confirmed as the best fitted isotherm, describing the physical-chemical adsorption capability of used ILs on the C-steel surface with the change in the free energy of adsorption, ΔG°ads = 32.6-37.2 kJ mol-1. The efficacy of the synthesized ILs was improved by increasing the doses, and the temperature reached 86.6, 96.1, and 98.4%, respectively, at 318 K. Surface morphology was proved by Fourier Transform Infrared spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM), and then, changes in test solutions were checked by Ultraviolet-visible spectroscopy. Theoretical modeling (density functional theory and Monte Carlo) revealed the correlation between the IL's molecular chemical structure and its anticorrosive property.

Effect of polymerized ionic liquids on thermal stability of poly(methyl methacrylate)

Effect of polymerized ionic liquids on thermal stability of poly(methyl methacrylate)

FTIR and NMR spectra of polymeric ionic liquids – Products of reaction between hydroxycontaining amidines and carbon dioxide

Two hydroxy-containing amidine derivatives have been synthesized by condensation of amino-alcohols such as monoethanolamine (MEA) and tris(hydroxymethyl)aminomethane (TRIS) with N,N-dimethylformamide dimethyl acetal. Their interaction with carbon dioxide, leading to the formation of polymeric ionic liquids, was studied by FTIR and NMR spectroscopy. Dilute solutions of polymeric ionic liquids are also investigated by measuring the relative viscosity during the reaction. It has been found that during interaction of MEA-based amidine with CO2 within first 60 min of reaction the intensities ratio of absorbance bands at 1644 and 1705 cm−1 dropped from 2.84 to 1.33. The linear relationship between the time of reaction and the mentioned ratio of intensities has been established. In the similar manner the changes in the intensities ratio of absorbance bands at 1696 and 1629 cm−1 can be used for monitoring the reaction between TRIS-based amidine and CO2. Solution of polymeric ionic liquid prepared by reaction of TRIS-based amidine with CO2 in i-propanol has been used for measuring molecular weight (Mw = 637, Mn= 349; Mw/Mn = 1.828) by gel-permeation chromatography.

Stability of Quadruple Hydrogen Bonds in an Ionic Liquid Environment.

Hydrogen bonds (H-bonds) are highly sensitive to the surrounding environments owing to their dipolar nature, with polar solvents kown to significantly weaken H-bonds. Herein, the stability of the H-bonding motif ureidopyrimidinone (UPy) is investigated, embedded into a highly polar polymeric ionic liquid (PIL) consisting of pendant pyrrolidinium bis(trifluoromethylsulfonyl)imide (IL) moieties, to study the influence of such ionic environments on the UPy H-bonds. The content of the surrounding IL is changed by addition of an additional low molecular weight IL to further boost the IL content around the UPy moieties in molar ratios of UPy/IL ranging from 1/4 up to 1/113, thereby promoting the polar microenvironment around the UPy-H-bonds. Variable-temperature solid-state MAS NMR spectroscopy and FT-IR spectroscopy demonstrate that the UPy H-bonds are largely present as (UPy-) dimers, but sensitive to elevated temperatures (>70°C). Subsequent rheology and DSC studies reveal that the ILs only solvate the polymeric chains but do not interfere with the UPy-dimer H-bonds, thus accounting for their high stability and applicability in many material systems.