The Influence of Monomer Structure on the Properties of Ionogels Obtained by Thiol–Ene Photopolymerization

The influence of ene and thiol monomer structure on the mechanical and electrochemical properties of thiol–ene polymeric ionogels were investigated. Ionogels were obtained in situ by thiol–ene photopolymerization of 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATT), 2,4,6-triallyloxy-1,3,5-triazine (TAT), diallyl phthalate (DAP), and glyoxal bis(diallyl acetal) (GBDA) used as enes and trimethylolpropane tris(3-mercaptopropionate) (TMPTP), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), and pentaerythritol tetrakis(3-mercaptobutyrate) (PETMB) used as thiols in 70 wt.% of ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImNTf2). The mechanical strength of ionogels was studied by puncture resistance and ionic conductivity by electrochemical impedance spectroscopy. The course of photopolymerization by photo-DSC method (differential scanning calorimetry) as well as characterization of compositions and its components (by IR and UV spectroscopy-Kamlet–Taft parameters) were also studied. The resulting ionogels were opaque, with phase separation, which resulted from the dispersion mechanism of polymerization. The mechanical and conductive properties of the obtained materials were found to be largely dependent on the monomer structure. Ionogels based on triazine monomers TAT and TATT were characterized by higher mechanical strength, while those based on aliphatic GBDA had the highest conductivity. These parameters are strongly related to the structure of the polymer matrix, which is in the form of connected spheres. The conductivity of ionogels was high, in the range of 3.5–5.1 mS∙cm−1.


Introduction
Polymeric ionogels are materials that can essentially be thought of as a three-dimensional polymeric network that percolates throughout ionic liquid (IL) and is responsible for the solid-like behavior of this material. Ionic liquid prevents a matrix from collapsing into compact mass while the polymeric matrix prevents the ionic liquid from flowing out [1]. ILs are organic salts having a melting point less than 100 • C. They consist only of ions and have many interesting properties such as negligible vapor pressure, non-flammability, a wide electrochemical window, high thermal stability, and high ionic conductivity. Due to these unique physicochemical properties, they have attracted remarkable interest by the current demand in advanced electrochemical devices, such as actuators, lithium ionic batteries, electric double-layer capacitors, dye-sensitized solar cells, or fuel cells [2,3].
Ionogels exhibit superior physical and electrochemical properties by combining the mechanical flexibility of a polymer matrix and the characteristic conductivity of ILs. In the literature, several strategies for preparing ionogels can be found. They can be obtained by (i) swelling a polymer in an IL, (ii) mixing the polymer and the IL together with a co-solvent which is subsequently removed, or (iii) polymerization of monomers in an IL, which is used Gels 2021, 7, 214 3 of 17 enes are characterized by an insertion-isomerization-elimination reaction series, which results in trans-ene formation. The reactivity of terminal enes is affected by substituents directly bonded to the ene. These substituents affect both the radical stability and steric hindrance. Moreover, multifunctional thiol-ene photopolymerization proceeds according to the reaction mechanism established for model systems. In paper [10], the authors present the influence of thiol structure on thiol-ene polymerization. They used three difunctional thiols with the cyclohexane ring, benzene ring, and linear structure. A thiol with a cyclohexane ring has the lowest reactivity with an alkene with a high electron density (vinyl ether), electron withdrawing alkene (methacrylate), and intermediated electron density alkene (allyl ether) compared to the planar benzene ring and linear structure. Therefore, it was proposed that the steric hindrance of the cyclohexane ring structure was the cause of the slowing down of the reaction. Of the three types of alkenes used, vinyl ether exhibited the highest reactivity with all thiols, which was attributed to a high electron density of the alkene.
However, in the case of ionogels, the polymerization takes place in an ionic liquid, and may take the form of dispersion polymerization. The ionic liquid will influence the kinetics of the process and the structure of the polymer, and thus the properties of the obtained ionogels. Not only the structure of the polymer matrix but also the interactions between the ionic liquid and the polymer matrix are very important in such materials. Therefore, we decided to conduct a comprehensive investigation of a series of thiol-ene ionogels based on five different enes and both primary and secondary thiols in one ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImNTf 2 ). The photopolymerization of these compositions, and the mechanical and electrochemical properties of the resultant thiol-ene ionogels, have been thoroughly characterized and analyzed.

Characterization of Components and Compositions
Investigations of the intermolecular interactions were carried out using the FTIR method. The interactions were observed for a given thiol-ene pair in two-component thiol+ene and three-component thiol+ene+IL systems. The results of the shifts of the absorption of SH group in thiol and C2-H of imidazolium ring bands in the mixture in relation to the pure compounds are shown in Figure 1 and Table 1. In the case of the thiols used for research, i.e., mercaptopropionates, high polymerization rates are observed [11,13,14]. This is explained by the formation of hydrogen bonds between the thiol group and the carbonyl oxygen, which weakens the S-H bond. Additionally, in the case of TMPTP and PETMP monomers, such interactions lead to the formation of a cyclic intermediate structure of the six-membered ring, which is particularly geometrically advantageous [10,11,14]. The maximum of the absorption band of the SH group of the TMPTP and PETMP monomers occurs at the same wavenumber, i.e., 2569 cm −1 , while in the case of the secondary thiol it is slightly shifted towards lower wavenumbers (2564 cm −1 ). The addition of GBDA ene practically does not affect the position of the absorption band of the SH group in the studied thiols. This proves that the hydrogen Gels 2021, 7, 214 4 of 17 bonds in thiols between the SH and C=O groups are not destroyed after the introduction of ether tetraene (SH groups show little tendency to form hydrogen bonds with ether groups [15]). The addition of the remaining enes shifts the absorption band towards a higher wavenumber, with a stronger influence of the ester monomer DAP than the triazine isomeric monomers TAT and TATT. Therefore, they have a stronger impact on the destruction of hydrogen bonds between the SH and C=O groups in thiols. In three-component systems containing 70 wt.% of IL, shifts are similar in all tested systems regardless of the type of ene used. The results also indicate a slightly stronger interaction of GBDA monomer with the ionic liquid. All systems containing this monomer have a C2-H imidazolium ring bond shift greater than 2 cm −1 . For the other systems, it remains at a similar level below 2 cm −1 .
obtained ionogels. Not only the structure of the polymer matrix but also the interactions between the ionic liquid and the polymer matrix are very important in such materials. Therefore, we decided to conduct a comprehensive investigation of a series of thiol-ene ionogels based on five different enes and both primary and secondary thiols in one ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImNTf2). The photopolymerization of these compositions, and the mechanical and electrochemical properties of the resultant thiol-ene ionogels, have been thoroughly characterized and analyzed.

Characterization of Components and Compositions
Investigations of the intermolecular interactions were carried out using the FTIR method. The interactions were observed for a given thiol-ene pair in two-component thiol+ene and three-component thiol+ene+IL systems. The results of the shifts of the absorption of SH group in thiol and C2-H of imidazolium ring bands in the mixture in relation to the pure compounds are shown in Figure 1 and Table 1. As we showed in our previous work [16], the course of thiol-ene polymerization in the solvent is influenced by its polarity, which can be determined by Kamlet-Taft parameters. As part of this study, we used one type of ionic liquid, so we decided to check whether the polymerization was influenced by the change in the polarity of monomers after the introduction of IL to the composition. We determined Kamlet-Taft parameters for the tested monomers, except for GBDA and TAT monomers, for which it was impossible to determine them (due to the color of the compound or high melting point). The three independent empirical Kamlet-Taft polarity parameters describe the hydrogen bond donating (α), the hydrogen bonding accepting ability (β), and the polarizability/dipolarity (π*) [17].
The obtained results are shown in Table 2. The tested monomers: thiols, enes, and ionic liquid show a similar ability to hydrogen-bonding accepting ability, while they differ in the ability to hydrogen-bond donating (α). The tested thiols are characterized by similar values of alpha and beta Kamlet-Taft parameters. They are additionally close to the Kamlet-Taft parameters of the ionic liquid EMImNTf 2 . On the other hand, enes are characterized by much lower values of the alpha parameter, which in the case of the DAP ene has the smallest value of 0.08. The TATT monomer also has slightly different characteristics, and a Gels 2021, 7, 214 5 of 17 much higher beta value than the other compounds, which is probably due to the presence of three carbonyl oxides in the monomer molecule. Additionally, there are no hydrogen interactions in this monomer, as is the case with thiols.

Ionogel Synthesis
The study was focused on the influence of monomer structure on the properties of thiol-ene polymer ionogels, which can be used as gel polymer electrolytes in electrochemical capacitors. The aim of the research was to search for an ionogel with good mechanical properties as well as ionic conductivity. Due to the appropriate mechanical properties during the preparation of the electrochemical capacitor and its operation, there was no fear of damaging the separator. In addition, a material with high mechanical resistance may have a small thickness. Three multifunctional thiols, one three-functional and two four-functional, primary and secondary, were selected for the study. Allyl compounds of different structures were used as enes: aliphatic GBDA, aromatic DAP, triazine TAT, and additionally an isomer of the former, TATT. The thiol-ene polymerization was carried out with equimolar ratios of the SH:CC functional groups. All compositions tested, both with and without ionic liquid, were homogeneous prior to polymerization. As a result of polymerization, transparent thiol-ene polymers and opaque, white ionogels were obtained. Figure 2 shows photos of selected ionogels prepared with PETMB thiol and studied enes, containing 70 wt.% of EMImNTf 2 . They reflect the properties of all obtained ionogels: flexible but not mechanically strong (F max < 0.4 N: mechanical strength +), mechanically strong (0.4 N < F max < 1.0 N: mechanical strength ++ or F max > 1.0 N: mechanical strength +++) but less flexible, and brittle-destroyed when attempted to roll up. As can be seen in Table 3, most of them are quite flexible, which allows them to be rolled up without damaging them. and a much higher beta value than the other compounds, which is probably due to the presence of three carbonyl oxides in the monomer molecule. Additionally, there are no hydrogen interactions in this monomer, as is the case with thiols.

Ionogel Synthesis
The study was focused on the influence of monomer structure on the properties of thiol-ene polymer ionogels, which can be used as gel polymer electrolytes in electrochemical capacitors. The aim of the research was to search for an ionogel with good mechanical properties as well as ionic conductivity. Due to the appropriate mechanical properties during the preparation of the electrochemical capacitor and its operation, there was no fear of damaging the separator. In addition, a material with high mechanical resistance may have a small thickness. Three multifunctional thiols, one three-functional and two four-functional, primary and secondary, were selected for the study. Allyl compounds of different structures were used as enes: aliphatic GBDA, aromatic DAP, triazine TAT, and additionally an isomer of the former, TATT. The thiol-ene polymerization was carried out with equimolar ratios of the SH:CC functional groups. All compositions tested, both with and without ionic liquid, were homogeneous prior to polymerization. As a result of polymerization, transparent thiol-ene polymers and opaque, white ionogels were obtained. Figure 2 shows photos of selected ionogels prepared with PETMB thiol and studied enes, containing 70 wt.% of EMImNTf2. They reflect the properties of all obtained ionogels: flexible but not mechanically strong (Fmax < 0.4 N: mechanical strength +), mechanically strong (0.4 N < Fmax < 1.0 N: mechanical strength ++ or Fmax > 1.0 N: mechanical strength +++) but less flexible, and brittle-destroyed when attempted to roll up. As can be seen in Table 3, most of them are quite flexible, which allows them to be rolled up without damaging them.   The lack of transparency of the materials indicates a phase separation occurring between the ionic liquid and the polymer matrix. Our previous works show that the thiol-ene polymerization process in the ionic liquid EMImNTf 2 proceeds as dispersion polymerization [7]. Dispersion polymerization begins as a homogeneous solution of monomer and the polymer phase separates during the polymerization process.
In the initial stage, the process takes place in the continuous phase, but in the later stages mainly in monomer swollen polymer particles. It is caused by the loss of solubility by the growing polymer chain in the reaction medium due to insufficient polymer-solvent interactions and/or due to the formation of a polymer network. Coagulation of the oligomer chains continues until steric stabilization of the latex particles begins [18]. No stabilization of the resulting colloid often leads to a wide particle size distribution [19,20]. The stabilizer should have an affinity for both the reaction medium and the polymer particles. In conventional organic solvents, poly-N-vinylpyrrolidone (PVP) is the most commonly used stabilizer [21][22][23][24]. The sterically stabilized particles keep growing until nearly complete monomer consumption [18].
In order to confirm whether the polymerization in the tested systems also proceeds according to the dispersion polymerization mechanism, SEM pictures of the obtained ionogels were taken ( Figure 3). As can be seen, most of the polymeric ionogels matrices have the character of connected microspheres (with diameter in the range 130-430 nm). Only in the case of ionogels obtained on the basis of the DAP ene, the matrix in the form of a continuous mass with slightly marked microspheres (with diameter in the range 650-750 nm) is visible. Thus, the ionic liquid is a good solvent for monomers and a poor solvent for the polymer, causing the polymer to precipitate rapidly in the form of microspheres from the reaction mixture. At the same time, it seems that the ionic liquid has a stabilizing effect on colloidal polymer particles (e.g., high efficiency of the ionic stabilizer, the fourth-order ammonium surfactant as a dispersion polymerization stabilizer has been demonstrated [25]) thanks to which a polymer in the form of connected microspheres with a fairly uniform size was obtained. This stabilizing effect of EMImNTf 2 in investigated thiol-ene systems is related to forming solvation shells as a result of the formation of hydrogen bonds by proton-donors and proton-acceptors, as well as through electrostatic interactions attracting counterions. Polymer particles, due to thiol structures, possess carbonyl groups on their surface which can hydrogen bond to imidazolium ionic liquid cation. This type of H-bonding will compete with cation-anion interactions and will contribute to polymer particle stabilization in IL. The weakest stabilization of the spheres occurs in the case of the polymer based on the DAP ene, which is characterized by a very low value of the Kamlet-Taft α parameter, which affects the proton-donor and protonacceptor interactions in the system. DAP does not have the ability to give off hydrogen and therefore does not compete with the ionic liquid, which can therefore interact more strongly with the carbonyl groups of the polymer. This leads to the coagulated matrix shown in Figure 3j-l. However, the photos also show the largest spheres. As is known, the size of the microspheres depends on the solubility of the polymer in the reaction medium and on the functionality of the monomers [23,24]. The increased solubility of the polymer (greater compatibility with IL) requires a greater critical chain length or a greater degree of cross-linking for polymer nucleation to occur. The formed nuclei of the separated polymer react with the growing polymer chains and uniformly increase their size. As the functionality of the monomer increases, the solubility of the polymer decreases, nucleation begins with lower conversions, and more nuclei are formed. This in turn, for the same concentration of monomer achievable, leads to smaller microspheres. Therefore, the spheres with the largest diameter are visible in the photos of ionogels based on the DAP ene. This is related, on the one hand, to the higher compatibility of polymerizing medium with ionic liquid. On the other hand, the DAP ene monomer has the lowest functionality (F = 2) of all monomers used, so nucleation during polymerization with this monomer begins at higher conversion and thus fewer spheres are created but they grow to a larger size. In the case of using GBDA, TATT, and TAT cross-linking enes, matrices having the character of connected microspheres were obtained, and matrices with an ene GBDA which has the highest functionality (F = 4) were characterized by spheres with larger dimensions. This may indicate a slightly better compatibility of these polymer matrices with the ionic liquid than in the case of polymer matrices based on the TAT or TATT enes. An additional factor may be the greater flexibility of the network with this aliphatic monomer than with the TAT or TATT monomers with a rigid triazine core. The images of the polymer matrices obtained on the basis of the TMPTP and PETMP thiol monomer for individual enes are very similar, which may result from the analogous structure of these thiols, differing in structure with only one thiol group and the presence of the -CH 3 group in the case of the TMPTP thiol. However, due to the fact that the equimolar ratio of the SH:CC functional groups, is used, the composition is almost identical. Some differences are evident in the case of PETMB secondary thiol, which could be due to hindered rotation of thiol-ether linkages (-S-) afforded by the additional α-methyl group of PETMB.

Photopolymerization Kinetics
The polymerization kinetics of each of the thiols TMPTP, PETMP, PETMB mixed with three different types of allyl ethers, DAP, TAT, GDBA, and triazine isomer TATT, was followed by photo-DSC method. For all four types of ene monomers, as presented in Figure 4, the polymerization rate of the samples with secondary thiol, PETMB, is apparently slower: the initial and maximum polymerization rate, R p max , is lower, and also the time for reaching maximum polymerization rate, t max , is longer (see also Table 4 where values of R p max and t max are presented). The reaction rates in other two primary thiols are faster than for the secondary thiol, and the maximum polymerization rates in TMPTP and PETMP are similar. Ionic liquid EMImNTf 2 accelerates the thiol-ene polymerization for all three thiols when cross-linking enes GBDA, TATT, and TAT are used. Thus, in the case of dispersion polymerization, leading to the formation of a matrix in the form of connected spheres, we observe an acceleration of the polymerization reaction in IL. It is related to the influence of the ionic liquid on the thiol-ene reaction, i.e., additional stabilization of the transition state during the chain-transfer step [6]. On the other hand, thiol-ene polymerization slows down when the linear ene DAP is used. Moreover, the kinetic curves of the DAP polymerization, especially in the polymerization with TMPTP and PETMP, show some acceleration at the end of the reaction. As can be seen from the SEM images, the thiol-DAP polymerization is only partially dispersive and partially solvent-based. It seems that the acceleration of the reaction in its final stages is related to the greater proportion of dispersion polymerization, which is manifested by more prominent spheres in the SEM images for TMPTP and PETMP enes. polymerization slows down when the linear ene DAP is used. Moreover, the kinetic curves of the DAP polymerization, especially in the polymerization with TMPTP and PETMP, show some acceleration at the end of the reaction. As can be seen from the SEM images, the thiol-DAP polymerization is only partially dispersive and partially solventbased. It seems that the acceleration of the reaction in its final stages is related to the greater proportion of dispersion polymerization, which is manifested by more prominent spheres in the SEM images for TMPTP and PETMP enes.     The highest polymerization rate, both in bulk and in ionic liquid, is achieved for reaction with enes having a rigid core structure, TATT and TAT ( Figure 5). The poly(PETMP-TAT) and poly(PETMP-TATT) show glass transition T g ( Figure 6) above room temperature (32 • C and 37 • C, respectively, Table 5), which causes additional complexity of radical trapping associated with vitrification, and thus incomplete conversion, which is about 50-60%. These high glass transition temperatures are consistent with the literature, where we can find even higher values for such polymers [12,26]. Polymerization in ionic liquid goes to almost 70% of conversion for a system with TAT and 60% of conversion for a system with TATT, which is related to the increase in the mobility of the polymer networks due to the dilution of the system. The ene with higher functionality, GBDA, polymerizes with PETMP with a lower polymerization rate, but conversion is almost the same, as in the former system. Ionic liquid accelerates the polymerization rate of this system, but the conversion is not higher, so the mobility of the network does not rise enough to increase conversion. Unfortunately, it was not possible to determine the glass transition temperature of the polymer matrix in the ionogel. On the other hand, the T g of bulk polymer is 6 • C. Thus, below room temperature, as well as the earlier-mentioned T g of the polymers based on TAT and TATT monomers. Thus, vitrification does not affect the polymerization process. The thiol-ene conversion of the composition with difunctional aromatic ene DAP is almost 90%, and only a slight increase is observed in the polymerization in the ionic liquid, which indicates that the increase in network mobility as a result of dilution is insignificant. The highest polymerization rate, both in bulk and in ionic liquid, is achieved for reaction with enes having a rigid core structure, TATT and TAT ( Figure 5). The poly(PETMP-TAT) and poly(PETMP-TATT) show glass transition Tg ( Figure 6) above room temperature (32 °C and 37 °C, respectively, Table 5), which causes additional complexity of radical trapping associated with vitrification, and thus incomplete conversion, which is about 50-60%. These high glass transition temperatures are consistent with the literature, where we can find even higher values for such polymers [12,26]. Polymerization in ionic liquid goes to almost 70% of conversion for a system with TAT and 60% of conversion for a system with TATT, which is related to the increase in the mobility of the polymer networks due to the dilution of the system. The ene with higher functionality, GBDA, polymerizes with PETMP with a lower polymerization rate, but conversion is almost the same, as in the former system. Ionic liquid accelerates the polymerization rate of this system, but the conversion is not higher, so the mobility of the network does not rise enough to increase conversion. Unfortunately, it was not possible to determine the glass transition temperature of the polymer matrix in the ionogel. On the other hand, the Tg of bulk polymer is 6 °C. Thus, below room temperature, as well as the earlier-mentioned Tg of the polymers based on TAT and TATT monomers. Thus, vitrification does not affect the polymerization process. The thiol-ene conversion of the composition with difunctional aromatic ene DAP is almost 90%, and only a slight increase is observed in the polymerization in the ionic liquid, which indicates that the increase in network mobility as a result of dilution is insignificant.

Mechanical and Electrochemical Properties
Good mechanical strength of synthesized materials is a key factor for creating real application of gel polymer electrolytes. To investigate the mechanical strength of obtained ionogels, puncture-resistance tests have been performed by varying the thiol and ene monomer structures. The effect of these factors is well observed in Figure 7. The structure of ene monomers has a great influence on the mechanical strengths of ionogels. Polymeric gels with aromatic, difunctional DAP have the weakest resistance to puncture. This can be connected rather with the structure of the ionogel matrix, i.e., microspheres embedded in a continuous polymeric matrix, than polymeric network. On the other hand, due to the presence of a continuous mass of polymer, the flexibility of these ionogels is high. The other ionogels have a very similar structure of connected spheres, but a more careful analysis shows differences in the size of the spheres and the way they are connected. A rigid triazine core in two isomeric monomers contributes to slightly better mechanical strength of ionogels obtained with these monomers than the aliphatic GBDA monomer. Moreover, ionogels with primary thiols and TATT, due to carbonyl-group presence, are more resistant to puncture. However, ionogel with secondary thiol PETMB is brittle, and it breaks upon attempting to roll it up. It seems that it is correlated with very weakly connected microspheres of this ionogel, which look like glass pearls. In contrast, ionogel with the same thiol and triazine isomeric monomer TAT is characterized by the highest puncture resistance. However, the matrix morphology of this ionogel is quite different; they are connected spheres, with a significant area connecting them.  Ionic conductivity of investigated ionogels (Figure 8) is in the range 3.5-5.1 mS•cm −1 , which accounts for 36-56% of the pure electrolyte conductivity, with the highest values obtained for ionogels synthesized on the GBDA ene. Typically, the relative conductivity for membranes with 70% electrolyte content is a maximum of 50%. Thus, gel polymer electrolytes with good conductive properties were obtained. The ionic conductivity, on the one hand, depends on the porosity of the membrane, and on the other hand, on the interactions between the electrolyte and the polymer matrix. Since all ionogels (except ionogels on the DAP basis) have porous matrices, the differences in conductivity result from interactions between the polymer and the electrolyte. Slightly larger shifts in the FTIR spectra for the C2-H band of the imidazolium ring are visible for systems with the Ionic conductivity of investigated ionogels (Figure 8) is in the range 3.5-5.1 mS·cm −1 , which accounts for 36-56% of the pure electrolyte conductivity, with the highest values obtained for ionogels synthesized on the GBDA ene. Typically, the relative conductivity for membranes with 70% electrolyte content is a maximum of 50%. Thus, gel polymer electrolytes with good conductive properties were obtained. The ionic conductivity, on the one hand, depends on the porosity of the membrane, and on the other hand, on the interactions between the electrolyte and the polymer matrix. Since all ionogels (except ionogels on the DAP basis) have porous matrices, the differences in conductivity result from interactions between the polymer and the electrolyte. Slightly larger shifts in the FTIR spectra for the C2-H band of the imidazolium ring are visible for systems with the GBDA monomer, and therefore it interacts most strongly with the ionic liquid, which may indicate a weakening of ionic interactions between ions, and thus faster diffusion of ions in the ionogel. The structure of the matrix in the case of ionogels based on the DAP ene may result in slightly lower ionic conductivity of these materials.
Ionic conductivity of investigated ionogels (Figure 8) is in the range 3.5-5.1 mS•cm −1 , which accounts for 36-56% of the pure electrolyte conductivity, with the highest values obtained for ionogels synthesized on the GBDA ene. Typically, the relative conductivity for membranes with 70% electrolyte content is a maximum of 50%. Thus, gel polymer electrolytes with good conductive properties were obtained. The ionic conductivity, on the one hand, depends on the porosity of the membrane, and on the other hand, on the interactions between the electrolyte and the polymer matrix. Since all ionogels (except ionogels on the DAP basis) have porous matrices, the differences in conductivity result from interactions between the polymer and the electrolyte. Slightly larger shifts in the FTIR spectra for the C2-H band of the imidazolium ring are visible for systems with the GBDA monomer, and therefore it interacts most strongly with the ionic liquid, which may indicate a weakening of ionic interactions between ions, and thus faster diffusion of ions in the ionogel. The structure of the matrix in the case of ionogels based on the DAP ene may result in slightly lower ionic conductivity of these materials.

Conclusions
The structure of enes (TAT, TATT, GBDA, DAP) and thiols (primary TMPTP, PETMP, and secondary PETMB) affects the mechanical and conductive properties of ionogels obtained by photopolymerization of thiol-ene in the presence of an ionic liquid EMImNTf 2 . The polymerization process in the case of TAT, TATT, and GBDA enes follows the dispersion polymerization mechanism, which leads to obtaining matrices with phase separation in the form of interconnected microspheres. Their size and connection type depend on the monomer structure and affect the material properties. The larger area of connection of the spheres increases the puncture resistance of ionogels. On the other hand, the simultaneous course of the polymerization according to the dispersion and solvent mechanism, which takes place in the case of the DAP ene, leads to a mechanical weakening of the materials. These ionogels are also characterized by a slightly lower ionic conductivity. Monomer structure and functionality also influence photopolymerization kinetics. The polymerization reaction is faster in compositions with triazine isomeric monomers TAT or TATT.

Solvatochromic Solvent Parameters
Solvatochromic parameters: Reichardt's empirical ET(30) polarity parameter, normalized polarity parameter E T N , empirical Kamlet-Taft polarity parameters: α (hydrogen bond donating ability), β (hydrogen bond accepting ability), and π* (dipolarity/polarizability) were determined using 4-nitroaniline, purity > 99%, N,N-diethyl-4-nitroaniline, purity 98%, and Reichardt's dye, purity 90%. All dyes were supplied by Sigma-Aldrich (St. Louis, MO, USA). Anhydrous methanolic solutions of each dye were prepared at a concentration of 5 × 10 −3 M. The dye solution was added to the IL and monomers, then the mixture was homogenized and the methanol was removed at 40 • C under reduced pressure. The dye concentration in each of the monomers and ionic liquid was sufficient to allow an absorbance band in the range 0.4 to 0.5. The absorption spectrum for each dye was measured by spectrophotometer Jasco UV-530 (Tokyo, Japan). All the spectroscopic measurements were carried out in the measuring range 200-800 nm in a quartz cuvette with a light path length of 1 mm at room temperature. The wavelength corresponding to the absorption maximum (λ max ) was read from each obtained spectrum, and then the solvatochromic parameters were calculated corresponding to Equations (1)-(5) and the methods described in the articles [27][28][29][30]:

Ionogels Samples Synthesis
The ionogel preparation by one-pot reactions of thiols and enes with different chemical structures in the presence of an ionic liquid is shown in Figure 9. The samples were prepared in a glove box under a pure argon atmosphere. The concentration of ionic liquid in the photocurable composition was 70 wt.% calculated on the total amount of the composition. The mixture of thiol+ene monomers was used in stoichiometric ratios of ene functional groups to thiol functional groups (1:1, C=C:SH). The photoinitiator DMPA was used in concentration 0.2 wt.% calculated on the whole composition. The composition consisted of ionic liquid and a mixture of monomers, and the photoinitiator was homogenized in an orbital shaker. Obtained homogeneous photocurable composition was then poured into a glass mold with a thickness of 0.3 mm. Subsequently, the prepared composition was irradiated with UV for a proper time on two sides of the mold with ASN-36W UV lamp (λmax = 365 nm, light intensity 6 mW·cm −2 ). For composition containing thiol TMPTP or PETMP, the duration of irradiation on one side was 5 min, and for composition with secondary thiol PETMB 10 min. Then, test samples of appropriate dimensions were cut from the obtained ionogel sheets. sition was irradiated with UV for a proper time on two sides of the mold with ASN-UV lamp (λmax = 365 nm, light intensity 6 mW•cm −2 ). For composition containing TMPTP or PETMP, the duration of irradiation on one side was 5 min, and for compos with secondary thiol PETMB 10 min. Then, test samples of appropriate dimensions cut from the obtained ionogel sheets.