Enhanced Polymerization Rate and Conductivity of Ionic Liquid-Based

Article pubs.acs.org/Macromolecules

Enhanced Polymerization Rate and Conductivity of Ionic LiquidBased Epoxy Resin Paulina Maksym,*,†,‡ Magdalena Tarnacka,†,‡ Andrzej Dzienia,‡,§ Karolina Matuszek,∥ Anna Chrobok,∥ Kamil Kaminski,*,†,‡ and Marian Paluch†,‡ †

Institute of Physics, University of Silesia, ul. Uniwersytecka 4, 40-007 Katowice, Poland Silesian Center of Education and Interdisciplinary Research, University of Silesia, ul. 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland § Institute of Chemistry, University of Silesia, ul. Szkolna 9, 40-007 Katowice, Poland ∥ Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, ul. Krzywoustego 4, 44-100 Gliwice, Poland ‡

ABSTRACT: The kinetics of polymerization of Bisphenol-A diglycidyl ether (DGEBA), a well-known epoxy resin, with two ionic amines 1-(3aminopropyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide ([apbim][NTf2]) and the tetrabutylammonium leucine ([N4444][Leu]) have been studied with the use of differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) at various temperatures. We found many fundamental differences between the progress of this reaction with respect to the classical system (curing of epoxy resin with ordinary nonconducting hardeners). One of the most significant differences is related to the mechanism of polymerization. It is worthwhile to mention that usually the autocatalytic model is used to describe the curing of DGEBA with ordinary amines. However, herein, the kinetic curves followed a clearly exponential shape characteristic of first-order kinetics. We claim that the change in mechanism of polymerization is related to the presence of a conducting amine that acts as both the substrate and the catalyst of this specific chemical conversion. Also, it is presented that the pace of the reaction only weakly depends on temperature, which is reflected in the relatively low activation energy. On the other hand, the degree of monomer conversion stays around 45%−70% as typically reported for the polymerization of DGEBA with nonconducting hardeners. In addition, we measured the time evolution of dc conductivity as the reaction proceeded and observed that a change in this parameter correlates very well with the monomer conversion in contrast to the reaction of nonconducting systems. Finally, ionic conductivity of the resulted cured samples was investigated and found to be quite significant at the glass transition temperature with respect to other polymerized ionic liquids.

1. INTRODUCTION

based curing agents has an effect on the curing speed and workability of the resin; in particular, it improves their compatibility and reduces reactivity to CO2 in the air or toxicity of amines.9 Other hardeners for epoxy resins include polyamide resins, tertiary and secondary amines, imidazoles or metal complexes,10,11 polymercaptan-based agents, anhydrides, and one of the most important classlatent curing agents, such as boron trifluoride−amine complex,12−14 RNH3+BF4− salts,15 dicyandiamide (DICY),16−18 and organic acid hydrazide.19 Latent curing agents play an important role as hardeners due to their stability in storage at room temperature as well as relatively fast curing by heat, pressure, irradiation with UV or microwaves, and others.20 Moreover, some latent hardeners, i.e., DICY, are solids at room temperature, and only an increase in temperature causes

Epoxy resins are one of the most versatile classes of polymers that can be used for a wide range of applications, such as coatings, laminates, adhesives, casting compounds, sealants, etc. This versatility results from the use of various chemical compounds to cure epoxy resin, which enables different epoxy systems with a large range of physicochemical properties.1−3 Curing agents play an important role in controlling the polymerization of epoxy resin; since they influence the reaction rate, curing kinetics, gel time, degree of cure, and viscosity.4−6 The most widely used thermally initiating curing agents are based on amines,7 including aliphatic, aromatic, or their modified forms, such as polyamine epoxy−resin adducts and ketimines. They usually react with epoxy resin via a polyaddition scheme. Aliphatic amines are able to cure the epoxy resins at room temperature. Interestingly, fabricating products have excellent properties with high heat resistances, up to 373 K, whereas the aromatic amines can achieve higher heat and chemical resistances.8 The modification of amine© XXXX American Chemical Society

Received: December 21, 2016 Revised: March 23, 2017

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DOI: 10.1021/acs.macromol.6b02749 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

hardener and epoxies used in polymerizations led to linear, fully polymerized systems characterized by 100% degree of conversion. As a consequence, a significant reduction or even elimination of unreacted substrates in the resulting polymer was achieved. In this study, DSC and BDS techniques were used to investigate the kinetics, time evolution, and final conductivity of the obtained polymers. We discuss the kinetics data which revealed a change in the mechanism of polymerization DGEBA with conducting amines. In addition, properties of the formed polymers including glass transition, monomer conversion, and conductivity have been discussed in detail. The results show that the selected ILs can be considered as convenient hardeners for DGEBA. Additionally, ILs which represent a relatively new class of epoxy curing agents offer a great “green” alternative to conventional amine hardeners, with both the mechanism of polymerization and physicochemical properties of the resulting polymer carried out with ILs being different from those in ordinary amine curing agents.

their melting and dissolution into the resin which allows them to initiate the polymerization. As a result, inhomogeneity of the curing system leads to severe difficulties in the attainment of uniform properties. In this context, an excellent solution would be the use of some miscible liquid initiators that possess a similar range of latent properties, such as ionic liquids (ILs). The unique physical−chemical properties of ILs has resulted in a great deal of interest in different fields of science, especially in green chemistry. These compounds have negligible vapor pressure at near ambient conditions, which means that many of them show no signs of distillation below the thermal decomposition temperature. Thus, it is possible to minimize the risk of atmospheric contamination and ultimately reduces environmental concerns. ILs are also characterized by low melting points ( 440 K was recorded, most likely connected to the thermal decomposition of the studied material. Having the degree of monomer conversion as well as the time evolution of the heat related to the formation on the new covalent bonds determined, the kinetic curves for the DGEBA cured with both examined herein ionic amines have been constructed, as presented in Figure 3. At first glance, some very important and striking differences between the polymerization with conducting and conventional hardeners can be observed. First, the rate of reaction is significantly different. The reaction of DGEBA mixed with ionic liquids proceeds extremely fast. Just to mention that polymerization is finished within 1 or 2 h. On the other hand, for the conventional systems, 10 h or more is required for the reaction to be completed at the same temperatures.62,63 Moreover, the time dependence of monomer conversion of conducting and nonconducting curing systems follows a completely different shape indicating noticeable changes in the mechanism of reaction. It is worthwhile to stress that to describe the progress of epoxy amine polymerization usually second-order kinetics or autocatalytic models are considered. The most popular one are those proposed by Horie or Kamal et al.:64

(1)

where αDSC is the conversion determined from DSC measurements, ΔH(iso) is the enthalpy change as a function of the polymerization time at given temperature (isothermal measurements), and ΔHtotal is the total heat of the reaction. As reported widely in the literature, ΔHtotal = ΔH(iso) + ΔH(noniso) is the sum of enthalpies of the isothermal (ΔH(iso)) and nonisothermal experiments (ΔH(noniso)).59−61 Just to mention that α varies between 60−70% and 45−55% for the reaction with [apbim][NTf2] and [N4444][Leu], respectively. In comparison for nonconducting systems, the α changes in the range 60−90% in the studied range of temperatures.62,63 The monomer conversion plotted versus temperature of reaction is presented as an inset in Figure 1b. As it can be seen, the estimated monomer conversion increases with temperature, typically reported for many reacting systems including the curing of epoxy resin, i.e., DGEBA with various hardeners. Additionally, the nonisothermal DSC measurements revealed the first important difference between DGEBA mixed with conventional and conducting amines. For the former systems, the glass transition temperature, Tg, is located near the temperature of reaction, Treac. This is consistent with literature data showing that the kinetic arrest, related to the freezing of the monomer diffusivity at the Tg, is a limiting step of the polymerization. On the other hand, we found that the glass transition temperature of the conducting systems is significantly E


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Macromolecules dα /dt = (k1 + k 2α m)(1 − α)n

(2)

where m and n are empirical factors, with m + n known as the overall reaction order, while k1 and k2 are respectively the noncatalytic and the autocatalytic rate constants. According to this approach, the curing process is considered to consist of at least two types of chemical reaction characterized by different constant rates and activation barriers. The first reaction described by k1 is supposed to be related to ring-opening with a ternary transition state of amine. On the other hand, the second one with k2 is believed to be a subsequent reaction connected to the autocatalytic ring-opening that involves amine, epoxide, and hydroxyl group formed upon opening of the first epoxide ring. Interestingly, it was found that the first reaction is more sensitive to temperature changes and is characterized by a higher activation barrier than the second one connected with the catalytic impact of formed OH moieties. We carried out a detailed analysis of the calorimetric data measured during the polymerization of nonconducting systems (DGEBA−aniline and DGEBA−cyclohexylamine) with the use of eq 2. The parameters m, n, k1, and k2 were determined accordingly to the procedures described in the literature.65−68 Furthermore, k1 and k2 were plotted versus temperature, see Figure 4. As observed, the temperature dependence of both constant rates is different, indicating a variation in activation barriers for both reactions. To quantify this observation, the Arrhenius equation has been applied to determine the activation barriers of both steps of the reaction

k = k 0 exp(Ea /RT )

(3)

where k0 is a pre-exponential factor, Ea is the activation barrier, and R is the universal gas constant. Interestingly, it was found that the Ea of the initial and subsequent reaction is 50 and 24 kJ/mol for DGEBA−aniline systems and 89 and 46 kJ/mol for DGEBA polymerized with cyclohexylamine.63 Both results are consistent with the literature data showing a higher activation barrier for the initial reaction with respect to the catalyzed one. On the other hand, in the case of DGEBA−ionic liquid systems, the monomer conversion plotted versus time follows clear exponential character which is characteristic of first-order kinetics. Consequently, the progress of polymerization can be described with the use of a single constant rate, suggesting that the mechanism of polymerization has been changed due to the presence of ionic amine. This supposition is not surprising taking into account data reported in the literature for reactions carried out in various ILs. For the majority of cases, ILs act as a very effective catalyst affecting the pace of the reaction as well as the activation barrier.47,69,70 Therefore, the conducting amines should not only be considered as the substrate of the reaction but also as the catalyst. Note that in the case of nonconductive hardeners (i.e., primary amine as aniline) the curing occurs as follows: the amine group of the curing agent breaks the epoxy ring forming the covalent bond between the amines nitrogen atom and the terminal atom of epoxide molecule and the −OH group. Consequently, the primary amine becomes a secondary one, which reacts further breaking the next epoxy ring and forming two additional covalent bonds. As a result, the nitrogen atom from the curing agent becomes a link between the two epoxide molecules. According to this scheme, the polymer structure grows in −A−B−A−B− sequence.71 Herein, both hardeners have a free amine group, which might react with the resin as described above (as a substrate) leading to the comparable structure as observed for

Figure 4. Temperature dependence of the constant rates obtained from DSC measurements for polymerization performed with four curing agents: aniline (a), cyclohexylamine (b), [apbim][NTf2] (c), and [N4444][Leu] (c). Red lines represent the best fit to eq 3. Data obtained for the curing of DGEBA with aniline and cyclohexylamine were taken from ref 63.

nonconductive agents. The difference is made by the presence ammonium cations, which gives this additional catalytic character. Note that both examined ILs differ by the location of the −NH2 group. For [apbim][NTf2], this group is in the cation, while for the other this group is in the anion leading to two different ionic polymers: polycation or polyanion in the backbone, respectively (see Scheme 1). Kinetic curves constructed for both DGEBA−ionic amine systems and presented in Figure 3 were analyzed further to estimate the constant rate, k, by application of the following equation: α = A exp( −kt )

(4)

where α is the degree of reaction, k is the rate of reaction, t is time, and A is a preexponential factor.72 As shown in Figure 3, the exponential fit describes the time dependence of the monomer conversion very well, enabling a precise estimation of k for both ionic binary mixtures, plotted in Figure 4c. Next, the activation barriers for the polymerization in ionic mixtures have been determined with the use of the Arrhenius approach (eq 3). Surprisingly, the activation barriers of the curing DGEBA F


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Figure 5. Dielectric spectra collected upon the reaction of the DGEBA−[apbim][NTf2] system presented in various representations: permittivity (a), electric modulus (b), and conductivity (c).

frequency range, which directly yields the value of dc conductivity, σdc, and (iii) the power law behavior at high frequencies.73 Analyzing further the time evolution of M″ and σ′ presented in Figures 5b and 5c for the DGEBA−[apbim][NTf2] mixture, one can observe both (i) a shift of the conductivity relaxation process toward lower frequencies and (ii) a decrease of dc conductivity as reaction proceeds. It should be mentioned that it is a typical feature of the epoxy−amine polymerization, observed also in the case of conventional hardeners. Just to mention that the viscosity of the reacting system increases due to the formation of macromolecules of higher molecular weight and longer chains. However, it should be stressed that the variation in conductivity relaxation times, τdc (determined from the frequency of M″ peak maximum, τdc = 1/(2πfmax)), as well as in dc conductivity itself is rather limited during the curing of both examined herein systems (DGEBA−[apbim][NTf2] and DGEBA−[N4444][Leu]). It obeys maximum few decades. In contrast to the “ordinary” epoxy−amine polymerization, where the change in mobility and conductivity is much more significant exceeding more than 10 decades.62,63 In our recent work, we have focused on testing the relationship between the variation of dc conductivity and the monomer conversion, α, measured in the case of the radical polymerization of imidazolium-based monomeric ionic liquids (MILs) at various temperature and geometrical (dimensional) conditions.25,26 Interestingly, we noted a linear correspondence between both quantities. It seems to be not surprising in view of the linear correlation between the molecular weight, Mw, and the monomer conversion found to work well for the radical polymerization. Nevertheless, one has to remember that in the case of step-growth polymerization there is no linear correspondence between Mw of the formed polymers (determining the viscosity of the system) and α. Hence, it is very difficult to study kinetics of step-growth polymerization by monitoring variables, which are strongly viscosity dependent. Mainly due to this reason, the applicability of dielectric spectroscopy to follow such kind of reaction is strongly limited. It should be mentioned that over the years different models based on the variation in dielectric constant or dielectric permittivity measured at high frequencies were proposed and developed to monitor the progress of the curing of DGEBA with different amines.74 However, none of them take into account monitoring the variation of dc conductivity due to

with [apbim][NTf2] and [N4444][Leu] are 19 and 39 kJ/mol, respectively. Interestingly, these values of Ea correspond very well to those determined for the subsequent reaction in ordinary epoxy amine systems (see crossed symbols in Figure 4a,b). Hence, one can suppose that the presence of ionic liquids (amines, in this case) in the reacting mixture affects the mechanism of the polymerization. Consequently, the conductive amine acts as both substrate and catalyst. One can also recall that similar conclusions have been derived by other authors studying polymerization of DGEBA with different ionic hardeners.37−40 As complementary analysis, we also carried out dielectric measurements to monitor changes in the molecular dynamics upon the curing of DGEBA mixed with both examined conductive amines. The dielectric data collected during the reaction of DGEBA−[N4444][Leu] are presented in Figure 5 in various representations. It should be mentioned that for the majority of glass formers, dielectric spectra are usually shown in permittivity, ε*, representation (see Figure 5a). However, in the case of conductive systems, one can observe a strong rise in the imaginary part of permittivity, ε″, at low frequency originating from the significant contribution of dc conductivity, σdc, which is related to the charge transport. Consequently, it is rather difficult to gain information about molecular dynamics using this kind of representation in the case of strongly conducting samples (such as ionic liquids). To overcome this problem, dielectric data are usually analyzed in the conductivity, σ*, or electric modulus, M*, representation, which are related as follows: M *(f ) =

i 2πfε0 1 = ε*(f ) σ *(f )

(5)

where ε0 is the vacuum permittivity. Using eq 5, dielectric data were recalculated from ε″( f) to either M″( f) or σ′( f) representations as shown in Figure 5. The electric modulus representation revealed the presence of a pronounced relaxation peak (identified as a conductivity relaxation process), which reflects the ion mobility changing with viscosity (see Figure 5b). On the other hand, dielectric spectra presented in conductivity representation are depicted in Figure 5c. As observed, there are three regions in the spectra that can be related to (i) the electrode polarization effect at lower frequencies, due to the accumulation of the ions at the sample−electrode surface, (ii) a plateau, on the intermediate G


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Figure 6. Conversion obtained from the variation of dc conductivity of the examined systems as a function of αDSC obtained from calorimetric measurements for DGEBA−[apbim][NTf2] (a) and DGEBA−[N4444][Leu] (b).

some fundamental reasons. As discussed by Tombari et al.,75 the drop in σdc upon curing is related to either (i) the change in viscosity of the system, (ii) the decrease in number of ions related to the reduction of static permittivity accordingly to ion−ion pair equilibria,76 or (iii) the change in population of H bonds affecting proton conductivity. All these factors affect the evolution of dc conductivity upon the curing of DGEBA with nonconducting amines in a significant way. Taking into account these considerations, we decided to check whether the same situation holds for the curing of DGEBA with conducting amines. For this purpose, the variation of dc conductivity measured by BDS technique upon the polymerization of DGEBA with both examined herein amines have been plotted versus the monomer conversion calculated from DSC measurements, as presented in Figure 6. It should be stressed that for this analysis we used the dielectric data measured at constant frequency, f = 104 Hz, corresponding to the plateau observed in σ′ in Figure 5c (for details please see refs 25 and 26). Interestingly as observed in Figure 6, there is a pronounced linear relationship between both parameters with the slope oscillating around 1 for both studied systems. However, it is worthwhile to mention that in the DGEBA+ [N444][Leu] system some small deviation can be seen at high conversion. Nonetheless, it is an interesting result, which might be basically related to two different phenomenon (i) a change in the mechanism of reaction and (ii) a much weaker contribution of proton hopping to the overall conductivity together with a smaller impact of the dissociation constant of small ions present in the investigated system. Consequently, similarly as in the case of radical polymerization,25,26 it seems that σdc can be a suitable parameter used to monitor the progress of step-growth polymerization in strongly conducting samples. As a final point of our investigations, we determined the conductivities of the resulting polymers at Tg to be equal to 5 × 10−8 and 3 × 10−8 S/cm for DGEBA polymerized in the presence of [apbim][NTf2] and [N4444][Leu], respectively. As observed, both parameters are very comparable to each other, although the temperature evolution of dc conductivity of fully polymerized DGEBA−[apbim][NTf 2 ] and DGEBA− [N4444][Leu] is different. Nevertheless, it is also worthwhile to mention that the ionic conductivity of the resulting polymers is very small, making them unsuitable for the current

application as solid polymer electrolytes. However, it should be noted that the dc conductivity was measured at the glass transition for highly viscous polymers. Interestingly, σdc of the polymers synthesized herein are higher with respect to those reported in the literature for both imidazolium (i.e., poly([HSO3−bvim][OTf]), σdc ∼ 4.37 × 10−11 S/cm at the Tg77) and acrylate-based PILs (poly(dimethylaminoethyl acrylate), σdc ∼ 10−9−10−12 S/cm at the Tg78). One should also mention the work by Matsumoto et al.30 The authors produced poly(ethylene glycol)-based networked polymers that had lithium sulfonate salt structures in the network. They showed that the conductivity of these cross-linked polymers was pretty high and varied in the range (3−7) × 10−6 S/cm at room temperature. In addition, it was also observed that as the lithium salt’s concentration increases the conductivity of the system increases as well. However, it must be stressed that this conductivity was obtained for polymers of very low viscosity since their glass transition temperatures were around 213 K. Hence, the applicability of these materials was very low due to their weak mechanical properties. Leclère et al.49 demonstrated that the use of various ILs with quaternary phosphonium cations combined with different anions such as phosphinate [TMP], trifluoromethanesulfonyl imide [TFSI], and dicyanamide [DCA] provides epoxy/amine networks exhibiting good thermomechanical properties in relatively high content of ILs (20−30 wt %). In that case, the networked system also showed ability to swell with 70 wt % of ILs content and equimolar ratio between amine and epoxy.49 In another example, high performance gel electrolytes based on 1,2,3-triazolium epoxy−amine networks were characterized by low glass transition temperatures (from 229 to 208 K) and remarkably high value of conductivity (2 × 10 −7 S/cm), which after modification with 10 wt % of lithium bis(trifluoromethylsulfonyl)imide increased to 10−6 S/cm.50 In this context, it seems that polymers prepared according to the procedure described in this paper reveal some interesting properties such as relatively high glass transition temperatures (located 25−30 K above the room temperature) and conductivity higher to those presented in the literature for other PILs using hardeners to produce non-cross-linked systems. Of course, more work must be done to increase conductivity of ionic liquid-based epoxy resin, at least by a few H


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Macromolecules orders of magnitude, to make them suitable for industrial application as solid state electrolytes. However, it should be pointed out that there is always competition between two different factors, a high glass transition temperature and high conductivity, acting conflictingly to each other. A huge demand for polymers of high mechanical stability, secured by the high glass transition temperature of a given material, at the same time limits the ion mobility and furthermore conductivities of the recovered macromolecules. Additionally, in the simplest approach one can increase the concentration of ionic liquids to increase the conductivity of the polymer. However, as discussed above, usually a significant drop in the glass transition temperature is observed. Consequently, the mechanical properties of the produced macromolecules worsen. In this context, it is worth remembering an interesting way to enhance the conductivity of the epoxy resin-based PILS which is the modification of the structure of ionic amines and epoxy resin to make them more mobile or flexible. What is more, one can also introduce additional hydroxyl moieties to increase the contribution from proton hopping along the H-bonded networks to the overall conductivity. Therefore, further studies are required to find the optimal conditions and compositions to produce materials of satisfying mechanical and conductivity properties.

Anna Chrobok: 0000-0001-7176-7100 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.K, A.D., and M.T. are thankful for the financial support from the Polish National Science Centre within SONATA BIS 5 project (Dec 2015/18/E/ST4/00320). The authors thank Eadaoin McCourt for the language assistance during the manuscript preparation.

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4. CONCLUSIONS The polymerization of DGEBA mixed with two conductive amines [apbim][NTf2] and [N4444][Leu] have been studied by DSC and BDS techniques. It was found that the presence of an ionic “green chemistry” substrate instead of the ordinary aminebased hardeners affects the rate, the activation barrier, and the mechanism of reaction. One can observe a clear change from the autocatalytic reaction, observed usually in the case of conventional curing DGEBA with amine, to first-order kinetics for conductive systems. This experimental finding was strictly related to the unique properties of the hardener acting as both a substrate and a catalyst for polymerization. Furthermore, it was shown that the glass transition temperatures of the resulting polymers were located around Tg = 325−330 K. What is more, the obtained polymers have relatively high conductivities σdc = 5 × 10−8 and 3 × 10−8 S/cm at Tg for DGEBA−[apbim][NTf2] and DGEBA−[N4444][Leu], respectively. Of course, this parameter is still too small to make these materials suitable for the industrial application as solid state electrolytes. However, it is quite significant taking into account that it was measured for the highly viscous materials, where ion mobility is highly restricted. One can emphasize that the conductivity of the obtained macromolecules can be enhanced by an increase of ionic hardener concentration. However, for such conditions, the glass transition temperature will drop simultaneously, leading to a deterioration in mechanical properties of the produced materials. Thus, further work is required to find a relationship between the concentration of ionic hardener and Tg as well as the conductivity of the recovered macromolecules.

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*E-mail [email protected]; Tel +48323497610. *E-mail [email protected]; Tel +48323497610. ORCID

Paulina Maksym: 0000-0002-8506-7102 Magdalena Tarnacka: 0000-0002-9444-3114 I


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