The grafting reaction of epoxidized natural rubber with carboxyl ionic liquids and the ionic conductivity of solid electrolyte composites

Abstract

A novel hybrid material was prepared via a grafting reaction of 50% epoxidized natural rubber (ENR 50) with 1-carboxymethyl-3-methylimidazoliumbis(trifluoromethylsulfonate)imine ([(HOOC)C₁C₁Im][NTf₂]). The grafting reaction and cross-linked structure of the hybrid material were investigated using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, differential scanning calorimetry (DSC) and the equilibrium swelling method. The analysis results indicated that the carboxyl group of [(HOOC)C₁C₁Im][NTf₂] can react with the epoxy group of ENR 50 to generate an ENR 50–[(HOOC)C₁C₁Im][NTf₂] graft polymer under conditions of 40 °C for 24 h. Furthermore, the grafting of ENR 50 formed ionic clusters and led to ionic crosslinking. In addition, ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiTFSI electrolyte composites were prepared through introducing bis(trifluoromethanesulfon)imide lithium salt (LiTFSI) into the ENR 50/[(HOOC)C₁C₁Im][NTf₂] hybrid system and the ionic conductivity of these electrolyte composites were studied. The results showed that the electrolyte composites have high ionic conductivity and reached a maximum ionic conductivity of 3.01 × 10⁻⁴ S cm⁻¹ (23 °C) in the experimental range.

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1. Introduction

Epoxidized natural rubber (ENR) is a modified natural rubber. ENR has distinctive characteristics such as a low glass transition temperature (T_g), good elasticity and adhesion properties.¹ Furthermore, ENR has a polar group in its epoxy ring that will provide coordination sites for Li⁺ conduction. The epoxy group in ENR can react with other groups like carboxyl and amine groups.

Ionic liquids (ILs) are molten liquids at room temperature with low melting points.² Ionic liquids comprise bulky cations and anions and have been considered to show promise as green electrolytes because of characteristics such as negligible vapour pressure, non-flammability, non-corrosiveness and high thermal stability, which makes them suitable for numerous applications,^3,4 e.g. as solvents in preparative chemistry and electrolytes in lithium batteries. In general, the design and synthesis of functional ionic liquids (FILs) is achieved through adding functional groups onto the IL side chains. Such chemical functionalization usually enhances the versatility of ILs, thereby leading to a large number of diverse FILs with improved properties.^5,6

It has been reported that ILs are used as plasticizers for solid polymer electrolytes (SPEs) or gel polymer electrolytes (GPEs) in the recent literature,^7–9 and most related publications have focused on the physical blend of the ILs and the polymer matrix. However, some of our previous publications were concerned with SPE and GPE preparation by ILs compounded with rubbers:^10,11 the results showed that the ILs easily migrated from the polymer matrix and resulted in the instability of the ionic conductivity and poor mechanical properties due to compatibility problems between the ILs and the polymer matrix upon increasing the IL content. In this work, the carboxyl ionic liquid [(HOOC)C₁C₁Im][NTf₂] was grafted onto 50% epoxidized natural rubber (ENR 50) chains through a reaction between the carboxyl group of [(HOOC)C₁C₁Im][NTf₂] and the epoxy group of ENR 50. In this way, ionic crosslinking of ENR 50 chains occurred and an ENR 50/[(HOOC)C₁C₁Im][NTf₂] hybrid material was prepared. The grafting reaction and cross-linked structure of the hybrid material were investigated using ATR-FTIR, DSC and the equilibrium swelling method. Furthermore, ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiTFSI electrolyte composites were prepared through introducing bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) into the ENR 50/[(HOOC)C₁C₁Im][NTf₂] hybrid system and the ionic conductivity of the electrolyte composites was studied.

2. Experimental

2.1. Materials

The ENR used was epoxidized natural rubber with 50% epoxidation and the trade name ENR 50, produced by the Chinese Academy of Tropical Agricultural Sciences (CATAS, Hainan, China) and its structure is shown in Fig. 1(a). 1-Carboxymethyl-3-methylimidazolium-bis(trifluoromethylsulfonate)imine ([(HOOC)C₁C₁Im][NTf₂]) and bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) were purchased from Lanzhou Institute of Chemical Physics (LICP, Lanzhou, China), the former’s structure is shown in Fig. 1(b). All solvents including tetrahydrofuran (THF), xylene and toluene were supplied by Sinopharm Chemical Reagent Co. Ltd, China.

Fig. 1 Structures of ENR 50 and [(HOOC)C₁C₁Im][NTf₂].

2.2. Sample preparation

ENR 50 was first dissolved in toluene and precipitated in methanol for purification. According to the predetermined proportions, the purified ENR 50 and [(HOOC)C₁C₁Im][NTf₂] were dissolved in a mixed solvent (the volume ratio of THF and xylene was 3/2) and THF, respectively. Then these two solutions were mixed and stirred efficiently for 2 h at room temperature to achieve a homogenous mixture. The solution was then poured into Teflon® moulds protected from dust which were subsequently dried at room temperature for 12 h to evaporate residual solvent. Further drying of the various solutions was continued in a vacuum oven under 40 °C for 24 h. The samples were stored in a desiccator until further usage.

The extraction procedure of crosslinked rubber was that the samples were first immersed in toluene for several days until they reached an equilibrium weight. During this time, the toluene was changed every day. Then the samples were dried in a vacuum oven under 40 °C until they reached an equilibrium weight again.

A similar approach was used to prepare the ENR solid electrolyte composites with different LiTFSI content.

2.3. Characterization

2.3.1 ATR-FTIR spectroscopy. ATR-FTIR analysis was carried out using a Perkin-Elmer Spectrum 1000 in the wavenumber range of 4000 to 650 cm⁻¹ at room temperature with a scanning resolution of 4 cm⁻¹. In order to remove unreacted [(HOOC)C₁C₁Im][NTf₂], the sample was extracted in toluene at 25 °C for 3 days before ATR-FTIR testing.

2.3.2 DSC characterization. DSC analysis was performed by two methods. DSC characterization was performed using a TA Q2000 instrument (TA Instruments, USA). In the first method, samples of compounds of ENR 50 and [(HOOC)C₁C₁Im][NTf₂] were sealed in hermetic aluminum pans and scanned at a heating rate of 10 °C min⁻¹ under a nitrogen flow at a rate of 50 ml min⁻¹. In each case, the following cycle was used: quenching from room temperature to −80 °C, heating at a rate of 20 °C min⁻¹ to 120 °C, quenching at 10 °C min⁻¹ to −80 °C and heating at the same rate of 10 °C min⁻¹ up to 120 °C. The second method was used to study the kinetics of the modification reaction. The mixed solution of ENR 50 and [(HOOC)C₁C₁Im][NTf₂] first had the solvent removed by pumping. Then samples of approximately 10 mg in weight were sealed in hermetic aluminum pans and scanned at heating rates of 5, 7.5, 10 and 15 °C min⁻¹ under a nitrogen flow of 50 ml min⁻¹. The data were analysed using the Universal Analysis 2000 software provided by TA instruments.

2.3.3 Determination of crosslink density. The crosslink density of the ENR 50/[(HOOC)C₁C₁Im][NTf₂] hybrid materials was determined by the equilibrium swelling method.¹² The samples were first weighed to determine their initial dry weights and then inserted into bottles containing 50 ml toluene at 23 °C for several days until they achieved swelling equilibrium. The samples were removed from the solvent and blotted with filter paper to remove excess solvent from their surfaces, then weighed to determine the equilibrium weight. The samples then were dried in an oven under 80 °C for 24 hours and reweighed to determine the final dry weight. The crosslinking density was calculated from the Flory–Rehner equation:¹³Where υ_e is the crosslinking density (in mole per unit volume), υ is the molar volume of the solvent, υ₂ is the volume fraction of the polymer in the swollen mass and calculated using eqn (2), and χ is the Flory–Huggins polymer–solvent interaction parameter calculated with eqn (3).Where m₁ is the mass of the sample before swelling, m₂ is the swollen mass of the sample having reached swelling equilibrium, m₃ is the mass of sample after final drying in the oven, ρ is the density of the polymer and ρ_s is the density of the solvent.where R is the gas constant, δ₁ is the solubility parameter of solvent and δ₂ is the solubility parameter of polymer.

2.3.4 Mechanical properties. Stress–strain tests were performed with a universal material testing machine (Instron 4465, Instron Corp, USA) with a cross-head speed of 500 mm min⁻¹ according to the standard ASTM D412-06a. To measure the mechanical properties, five different dumbbell-shaped specimens were punched from each rubber sample. The tensile strength and elongation at break were measured at room temperature.

2.3.5 Determination of the ionic conductivity. Ionic conductivity determination for the ENR solid electrolyte composites was carried out using an Autolab PGSTA302 in the frequency range of 1 Hz to 1 MHz with a 10 mV amplitude at room temperature. The polymer solid electrolyte was sandwiched between the stainless steel ion-blocking electrodes with a surface contact area of 0.25 cm². The bulk resistance (R_b) was determined from the equivalent circuit analysis. The conductivity values (σ) were calculated from the equation⁹ σ = d × R_b⁻¹ × S⁻¹, where d is the sample thickness and S is the active area of the electrode (cm²).

3. Results and discussion

3.1. ATR-FTIR analysis

The grafting reaction of ENR 50 with [(HOOC)C₁C₁Im][NTf₂] was confirmed by the ATR-FTIR spectra as shown in Fig. 2. Fig. 2(a)–(c) show the ATR-FTIR spectra of ENR 50 gum, [(HOOC)C₁C₁Im][NTf₂] and ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products, respectively. ENR 50 has absorption peaks with the basic characteristics of natural rubber, such as a C–H stretching mode at 2960 and 2920 cm⁻¹,^14,15 a C=C stretching mode at 1651 cm⁻¹ and a CH₂ scissoring mode at 1451 cm⁻¹.

Fig. 2 ATR-FTIR spectra of purified ENR 50, pure [(HOOC)C₁C₁Im][NTf₂] and ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products.

The characteristic absorption peak of ENR 50 is that of C–O–C stretching from the epoxy ring mode at 873 cm⁻¹. The characteristic absorption peaks corresponding to the carboxyl group of [(HOOC)C₁C₁Im][NTf₂] are 3300, 1743 and 974 cm⁻¹, which are attributed to the free carboxyl O–H, C=O and C–O stretching vibrations, respectively. The absorption peaks at 3160 cm⁻¹ and 3170 cm⁻¹ are assigned to the stretching vibration of the C–H bond on the imidazole ring. The absorption peaks at 1576 cm⁻¹ and 1174 cm⁻¹ are assigned to the framework vibration of the imidazole ring and the bending vibration of the C–H bond on the imidazole ring, the absorption peaks at 1347 cm⁻¹ and 1131 cm⁻¹ are assigned to the characteristic absorption of the –CF₃ group and the absorption peaks at 1052 cm⁻¹ are assigned to the characteristic absorption of the –SO₂–N–SO₂– group. As is shown in Fig. 2(c), when the ENR 50 and [(HOOC)C₁C₁Im][NTf₂] were mixed, reacted and extraction processed, the characteristic absorption peaks corresponding to the carboxyl group of [(HOOC)C₁C₁Im][NTf₂] (1743 cm⁻¹ and 974 cm⁻¹) and the epoxy ring group of ENR 50 (873 cm⁻¹) almost disappeared in the ATR-FTIR spectra of the ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products, and new absorption bands are observed around 3401 cm⁻¹ and 1718 cm⁻¹, which are attributed to hydroxyl groups (–OH) and ester C=O bonds, respectively.

Fig. 3 shows the effect of the [(HOOC)C₁C₁Im][NTf₂] content on the ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products and all ATR-FTIR spectra were baseline corrected and normalized using the aliphatic methylene bending peak at 1449 cm⁻¹. Also, the unreacted [(HOOC)C₁C₁Im][NTf₂] in the samples was removed by extraction processing. As shown in Fig. 3, with increasing [(HOOC)C₁C₁Im][NTf₂] content, the absorption peak corresponding to –OH (3401 cm⁻¹) is shifted to a lower wavenumber and divided into two absorption peaks, and the absorption peaks corresponding to the ester C=O bonds are shifted to 1697 cm⁻¹ from 1718 cm⁻¹. This implies that the effect of hydrogen bonding becomes more intense with increasing [(HOOC)C₁C₁Im][NTf₂] content. Furthermore, the increasing [(HOOC)C₁C₁Im][NTf₂] content led to the C–O–C stretching from the epoxy ring mode (873 cm⁻¹) showing a subtle change, that is to say, the conversion rate of the epoxy groups first increases then decreases (as shown in Table 1). This result is mainly due to the aggregation of clusters of the ionic liquid ([(HOOC)C₁C₁Im][NTf₂]), formed with increasing solution concentration in the process of sample preparation, thus decreasing its grafting reaction efficiency.¹⁶ Besides this, the absorption peaks corresponding to the framework vibration and C–H stretching vibration of the imidazole ring emerge and shift to lower wavenumbers, the characteristic absorption of the –CF₃ and –SO₂–N–SO₂– groups have faint enhancements and the carboxyl group peaks (1743 cm⁻¹) are faintly enhanced as the [(HOOC)C₁C₁Im][NTf₂] content increases. This is probably due to aggregation or cluster effects of the ionic liquid, caused by unreacted [(HOOC)C₁C₁Im][NTf2] that cannot be completely removed from the samples.¹⁷

Fig. 3 ATR-FTIR spectra of pure ENR 50, and ENR 50 with varying content of [(HOOC)C₁C₁Im][NTf₂] dried under 40 °C after extraction.

Table 1 The conversion rate of epoxy groups with varying [(HOOC)C₁C₁Im][NTf₂] content

[(HOOC)C1C1Im][NTf2] (phr)	The peak area of ν (epoxy ring)	The conversion rate of epoxy groups (%)
0	0.325	0
35	0.075	76.80
70	0.061	81.10
140	0.057	82.42
280	0.111	65.89

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The above results are evidence that consumption of epoxy groups is accompanied by the formation of hydroxyl groups and ester groups, which further suggests that ENR 50 and [(HOOC)C₁C₁Im][NTf₂] undergo the grafting reaction shown in Scheme 1.

Scheme 1 Reaction of ENR 50 with [(HOOC)C₁C₁Im][NTf₂].

3.2. DSC characterization

Fig. 4 shows DSC thermographs obtained at a heating rate of 10 °C min⁻¹ for ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products. Two endotherms are shown in Fig. 4(a) while there is only one endotherm in Fig. 4(b) for the sample of identical [(HOOC)C₁C₁Im][NTf₂] content.

Fig. 4 DSC thermographs of pure ENR 50, and ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products with varying [(HOOC)C₁C₁Im][NTf₂] content: (a) without extraction, (b) with extraction.

As Fig. 4(a) shows, two endothermic peaks are observed at −31.89 °C and −41.37 °C, which are assigned to the glass transition temperatures (T_g) of pure ENR 50 and [(HOOC)C₁C₁Im][NTf₂], respectively. The T_g values of the ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products with varying [(HOOC)C₁C₁Im][NTf₂] content were calculated using Fig. 4 and the results are displayed in Table 2: it can be seen that the T_g values of the ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products with varying [(HOOC)C₁C₁Im][NTf₂] content increased by about 25 °C compared to the pure ENR 50, while the T_g shifted to lower temperatures with increasing [(HOOC)C₁C₁Im][NTf₂] content. These results can be explained by noting that the ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products, namely, the hydroxy ester macromolecules with ionic side groups form ionic clusters,^18–21 resulting in ENR 50 crosslinking as in Scheme 2, based on ATR-FTIR analysis. Gan S. N. et al.²² argued that the significant elevation of the T_g associated with dibasic acid vulcanized ENR is most likely predominantly caused by the modified main chain structure and not the crosslinking network. However, with increasing [(HOOC)C₁C₁Im][NTf₂] content, the side group content of the grafted ENR 50 increases and chain regularity is reduced, thereby inhibiting the ionic crosslinking. In addition, the T_g of the samples with extraction were higher than the samples without extraction where the unreacted [(HOOC)C₁C₁Im][NTf₂] acted as a plasticizer.

Table 2 The T_g of samples with varying [(HOOC)C₁C₁Im][NTf₂] content

[(HOOC)C1C1Im][NTf2] (phr)	The Tg values of samples without extraction, °C	The Tg values of samples with extraction, °C
0	−31.89	−31.89
35	3.45	16.61
70	−6.58	15.84
140	−6.89	8.34
280	−10.24	9.13

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Scheme 2 Schematic diagram of the ionic cluster structure.

Multiple-heating-rate methods are iso-conversion methods, that is, they assume that the conversion value is constant at the peak exotherm temperature, the extent of reaction at the peak exotherm (a_p) in a DSC analysis, and that it is independent of the heating rate.²³ This makes it equally effective for both the nth order and the autocatalytic reactions. Two such multiple-heating-rate methods that have been shown to be effective are those proposed by Ozawa²⁴ and Flynn and Wall,²⁵ and that proposed earlier by Kissinger.^26,27

From the peak temperature heating rate data, evaluation of the reaction activity and reaction order was provided regardless of the reaction process with the following Kissinger and Flynn–Wall–Ozawa equations. The Kissinger equation is:

where β is the heating rate (Kelvin per minute), T_p is the peak endotherm temperature (Kelvin), and R the universal gas constant (8.314 J mol⁻¹ K). It is simplistic to assume a single reaction occurring during the curing process given the complexity of the reaction. Thus the value of E obtained in eqn (4) is an overall value representing all complex reactions that occur during curing.

The Flynn–Wall–Ozawa method is:

where A is the frequency factor.

The average values of activation energy calculated by the Kissinger and Flynn–Wall–Ozawa methods were introduced into the following Crane²⁸ equation:

where n is the reaction order. When E/nR ≫ 2T_p, the equation simplifies toTherefore, a plot of ln β versus 1/T_p gives E.

The samples with varying [(HOOC)C₁C₁Im][NTf₂] content were tested at four heating rates carried out in DSC, and the curves of ENR 50–[(HOOC)C₁C₁Im][NTf₂] with 70 phr [(HOOC)C₁C₁Im][NTf₂] are shown in Fig. 5. Applying the Kissinger and Flynn–Wall–Ozawa methods to the maximum reaction rate, linear relationships were obtained by plotting ln β/T_p² against 1/T_p and log(β), confirming the validity of the models. The plots for all samples are shown in Fig. 6. Table 3 summarizes the obtained kinetic parameters and coefficients of correlation (R-squared) for all samples. The overall reaction orders, n, are shown in Table 4. As shown in Table 3, the values of E are respectively, 63.79–76.51 and 67.09–79.28 kJ mol⁻¹ using the Kissinger and Flynn–Wall–Ozawa methods, which are in agreement with the values of the earlier literature.²⁹ From Table 3, the values obtained by the Kissinger method are slightly lower though the trend of the change of the values is similar. The activation energies were higher than those obtained by the Flynn–Wall–Ozawa methods, but the activation energy first shows an increase then has a slight decrease with increasing [(HOOC)C₁C₁Im][NTf₂], which means that too much further addition of [(HOOC)C₁C₁Im][NTf₂] will hinder the reaction. This may be because the viscosity affects the curing rate during the curing process of the thermosetting resin.³⁰ The more [(HOOC)C₁C₁Im][NTf₂], the more viscous the system is. As the reaction proceeds, three-dimensional crosslinking occurs, and the systems become dense increasing the viscosity and hindering the reaction.

Fig. 5 Nonisothermal dynamic DSC curves of ENR 50–[(HOOC)C₁C₁Im][NTf₂] with 70 phr [(HOOC)C₁C₁Im][NTf₂].

Fig. 6 Plots of ln(β/T_p²) and log(β) versus 1/T_p of ENR 50–[(HOOC)C₁C₁Im][NTf₂] with varying [(HOOC)C₁C₁Im][NTf₂] content during the dynamic curing processes: (a) 35 phr; (b) 70 phr; (c) 140 phr; (d) 280 phr.

Table 3 Values of activation energy and n for ENR 50–[(HOOC)C₁C₁Im][NTf₂] with varying [(HOOC)C₁C₁Im][NTf₂] content during the dynamic curing processes

(HOOC)C1C1Im][NTf2](phr)	Kissinger			Flynn–Wall–Ozawa
	EKissinger^a (kJ mol^−1)	ln A	R-square	EFlynn^b (kJ mol^−1)	ln A	R-square
a EKissinger: activation energy obtained from the Kissinger method.b EFlynn: activation energy obtained from the Flynn–Wall–Ozawa method.
35	63.79	17.93	0.9981	67.09	19.34	0.9983
70	72.95	19.88	0.9955	76.07	21.10	0.9963
140	76.57	21.74	0.9901	79.28	22.81	0.9912
280	73.84	21.34	0.9224	76.58	22.44	0.9341

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Table 4 Values of the overall reaction order, n obtained from the Crane method

(HOOC)C1C1Im][NTf2] (phr)	nKissinger^a	nFlynn^b
a nKissinger: using the value of E obtained from the Kissinger method.b nFlynn: using the value of E obtained from the Flynn–Wall–Ozawa method.
35	0.90	0.95
70	0.91	0.95
140	0.90	0.94
280	0.85	0.88

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3.3. The crosslink density

The swelling of the crosslinking polymer involves a diffusion process of the liquid in polymer samples. Therefore, the swelling properties are closely related to the crosslinking density of polymer, and the swelling ratio and swelling rate would decrease with increasing crosslinking density.^31–33

The swelling ratio was estimated as³⁴

where W₀ is the weight of the dry sample, and −W_t is the weight of the swollen sample.

Fig. 7 shows the effect of [(HOOC)C₁C₁Im][NTf₂] content on the swelling process and it is observed that the swelling ratio and swelling rate increase with increasing [(HOOC)C₁C₁Im][NTf₂] content. The equilibrium swelling ratios are estimated on the basis of the experimental data as shown in Fig. 8 and the crosslink density of samples is calculated by eqn (1) based on swelling experiments. Table 5 enumerates the equilibrium swelling ratio and the crosslink density of samples with various [(HOOC)C₁C₁Im][NTf₂] content, the results indicate that the crosslinking density of ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products with varying [(HOOC)C₁C₁Im][NTf₂] content decreases with increasing [(HOOC)C₁C₁Im][NTf₂] content. The results are consistent with the varying T_g pattern obtained by DSC characterization.

Fig. 7 The swelling ratio of the ENR 50/[(HOOC)C₁C₁Im][NTf₂] reaction products with varying [(HOOC)C₁C₁Im][NTf₂] content (samples with extraction).

Fig. 8 (A) Swelling ratio; and (B) crosslinking density of ENR 50/[(HOOC)C1C1Im][NTf2] reaction products after extraction.

Table 5 The crosslink density of samples with varying [(HOOC)C₁C₁Im][NTf₂] content

[(HOOC)C1C1Im][NTf2] (phr)	The equilibrium swelling ratio (%)	The crosslink density (mol cm^−3)
0	—	0
35	0.74	0.0055
70	0.73	0.0046
140	1.09	0.0033
280	1.72	0.0017

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3.4. Mechanical properties

The influence of [(HOOC)C₁C₁Im][NTf₂] content on the mechanical properties of ENR 50/[(HOOC)C₁C₁Im][NTf₂] composites is illustrated in Fig. 9 and Table 6. It was expected that the mechanical properties of the composites would be enhanced due to crosslinking. As shown, the tensile strength showed a downward trend with increasing amounts of [(HOOC)C₁C₁Im][NTf₂], while the elongation at the break point first increased then decreased. It reached a maximum value of 1190% with 70 phr ILCOOH. Adding a lot of ILCOOH into ENR 50 caused a dilution effect,³⁵ i.e. the volume fraction of the rubber matrix decreased with increasing ILCOOH loading, which would lead to a decrease in crosslinking of composites and then a decrease of the elongation at break point.³⁶ These results are consistent with both the DSC and crosslinking.

Fig. 9 Strain–stress graph of samples with varying [(HOOC)C₁C₁Im][NTf₂] content.

Table 6 Mechanical properties of samples with varying [(HOOC)C1C1Im][NTf₂] content

[(HOOC)C1C1Im][NTf2] (phr)	Tensile strength (MPa)	Elongation at break (%)
35	3.44	851
70	2.16	1190
140	1.30	1057
DCP	1.61	724

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3.5. The ionic conductivity

Polymer electrolyte ionic conductivity is dependent on the actual concentration of the conducting species and the mobility of these species. The intercept on the real axis of the complex impedance plot of a polymer electrolyte film gives the bulk resistance.³⁷ Fig. 10(b) shows the complex impedance plot of the ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiFTSI solid electrolyte composites with 140 phr [(HOOC)C₁C₁Im][NTf₂] at room temperature. It consists of a high frequency semicircle and a low frequency spike. The semicircle corresponds to bulk resistance while the spike corresponds to the interfacial impedance of the solid electrolyte composites. The semicircle can be shown as a parallel combination of a capacitor, i.e. the immobile polymer chain, and a resistor, i.e. the mobile ions inside the polymer matrix.³⁸ While some of the impedance spectrum of the samples’ semicircle is significantly broadened the electrode spike at the low frequency end is distinctly non-vertical indicating the roughness of the electrode/electrolyte interface.³⁹

Fig. 10 A.C. impedance spectra of the ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiFTSI solid electrolyte composites with (a) 70 phr, (b) 140 phr, and (c) 280 phr [(HOOC)C₁C₁Im][NTf₂] at room temperature.

The ionic conductivity of the ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiTFSI electrolyte composites are listed in Table 4. As shown in Table 7, the ionic conductivity depends on the concentration of [(HOOC)C₁C₁Im][NTf₂] and LiTFSI, and the highest conductivity (3.01 × 10⁻⁴ S cm⁻¹) was obtained for the sample with 190 phr LiTFSI and 280 phr [(HOOC)C₁C₁Im][NTf₂]. As for samples 4, 7 and 9, the ionic conductivity of the sample containing [(HOOC)C₁C₁Im][NTf₂] increased by 2–3 orders of magnitude when compared to the samples without [(HOOC)C₁C₁Im][NTf₂]. As mentioned above, the ILs without chemical reactivity are used as plasticizers for solid polymer electrolytes (SPEs) or gel polymer electrolytes (GPEs) to improve the dispersion of LiTFSI in the polymer matrix and to decrease the viscosity of polymer electrolyte composites, and to then increase ionic mobility which consequently leads to increased conductivity.^40,41

Table 7 The ionic conductivity of the ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiTFSI solid electrolyte composites

No.	ENR 50 (phr)	[(HOOC)C1C1Im][NTf2] (phr)	LiTFSI (phr)	σ (S cm^−1)
1	100	0	48	7.60 × 10^−8
2	100	70	24	1.66 × 10^−6
3	100	70	48	1.64 × 10^−6
4	100	70	96	1.06 × 10^−6
5	100	140	24	3.61 × 10^−6
6	100	140	48	3.95 × 10^−6
7	100	140	96	3.53 × 10^−6
8	100	280	48	1.23 × 10^−5
9	100	280	96	1.31 × 10^−5
10	100	280	190	3.01 × 10^−4

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Nevertheless, [(HOOC)C₁C₁Im][NTf₂] with chemical reactivity is not only a plasticizer but also plays a vital role as a crosslinking agent in the ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiTFSI electrolyte composites, and leads to the formation of ionic clusters as shown in Scheme 2 in the electrolyte composites. The ionic clusters promote LiTFSI dissociation and provide effective paths for migration of the lithium ions (Li⁺). The probable mechanism of the ionic conduction of the ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiTFSI electrolyte composites is drawn in Scheme 3 and the Li⁺ possibly moves between ionic clusters under the influence of the thermodynamic movement of the ENR 50–[(HOOC)C₁C₁Im][NTf₂] graft polymer chains.

Scheme 3 The probable mechanism of Li⁺ conduction.

4. Conclusions

The ENR 50–[(HOOC)C₁C₁Im][NTf₂] hybrid material was prepared via a grafting reaction of ENR 50 with [(HOOC)C₁C₁Im][NTf₂]. The grafting reaction and cross-linked structure of the hybrid material were investigated using ATR-FTIR, DSC and the equilibrium swelling method. The results indicated that the carboxyl group of [(HOOC)C₁C₁Im][NTf₂] can react with the epoxy group of ENR 50 to generate the ENR 50-[(HOOC)C₁C₁Im][NTf₂] graft polymer under conditions of 40 °C for 24 h. Furthermore, the grafting polymer formed ionic clusters and led to ionic crosslinking, leading to a large increase of T_g and mechanical properties. Using the kinetic analysis, the apparent activation energy (E) and the Arrhenius frequency factor (A) calculated from the multiple-heating-rate models showed a dependence on the [(HOOC)C₁C₁Im][NTf₂] content and the heating rate β. However, with increasing [(HOOC)C₁C₁Im][NTf₂] content, the amount of side groups in the grafted ENR 50 increased and the chain regularity reduced, thereby inhibiting ionic crosslinking. In addition, ENR 50/[(HOOC)C₁C₁Im][NTf₂]/LiTFSI electrolyte composites were prepared through introducing LiTFSI into the ENR 50/[(HOOC)C₁C₁Im][NTf₂] hybrid system and the ionic conductivity of the electrolyte composites were studied. The results showed that the electrolyte composites have high ionic conductivity and reached a maximum ionic conductivity of 3.01 × 10⁻⁴ S cm⁻¹ (23 °C).

Acknowledgements

The authors would like to extend their sincere appreciation to the National Nature Science Foundation of China (Grant No.: 51373095).

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