Impact of Imidazolium-Based Ionic Liquids on the Curing Kinetics and Physicochemical Properties of Nascent Epoxy Resins

Abstract

We investigated the influence of anion type (salicylate, [(MOB)MIm][Sal], vs chloride, [(MOB)MIm][Cl]) of imidazolium-based ionic liquid (IL) and its content on the curing kinetics of bisphenol A diglicydyl ether (DGEBA of molecular weight M n = 340 g/mol). Further physicochemical properties (i.e., glass transition temperature, T g , and conductivity, σ dc ) of produced polymers were investigated. The polymerization of the studied systems was examined at various molar ratios (1:1, 10:1, and 20:1) at different reaction temperatures ( T reaction = 353–383 K) by using differential scanning calorimetry (DSC). Interestingly, both DGEBA/IL compositions studied herein revealed significantly different reaction kinetics and yielded materials of completely distinct physical properties. Surprisingly, in contrast to [(MOB)MIm][Cl], for the low concentration of [(MOB)MIm][Sal] in the reaction mixture, an additional step in the kinetic curves, likely due to the combined enhanced initiation activity of anion (salicylate)–cation (imidazolium-based), was noted. To thoroughly analyze the kinetics of all studied systems, including the two-step kinetics of DGEBA/[(MOB)MIm][Sal], we applied a new approach that relies on the combination of the two phenomenological Avrami equations. Analysis of the determined constant rates revealed that the reaction occurring in the presence of the salicylate anion is characterized by higher activation energy with respect to those with the chloride. Moreover, DGEBA/[(MOB)MIm][Sal] cured materials have higher T g in comparison to DGEBA polymerized with [(MOB)MIm][Cl] independent of the IL concentration. This fact might indicate that, most likely, the products of hardening are highly cross-linked (high T g ) or oligomeric linear polymers (low T g ) in the former and latter cases, respectively. Such a change in the chemical structure of the polymer is also reflected in the dc conductivity measured at the glass transition temperature, which is much higher for DGEBA cured with [(MOB)MIm][Cl]. Herein, we have clearly demonstrated that the type of anion has a crucial impact on the polymerization mechanism, kinetics, and properties of produced materials.

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“…29 In agreement with the previous literature, 30,31 The cross-linking reaction occurred mainly includes two aspects: the catalysis of tertiary amine groups and the transfer of active hydrogen on secondary amine groups. 32 As the Figure 9 depicted, firstly, the 2position nitrogen opens the epoxy ring, and then TPP-EP homopolymerizes.…”

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“…123,152,153 Recently, it was also found that the activation energy of the curing reaction of several types of resins decreased upon the addition of IL. 152,154,155 Typical examples of IL include dialkylimidazolium solvents like 1-ethyl-3-methylimidazolium acetate. IL are considered green solvents because of their lack of vapor pressure.…”

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“…They determined that the dielectric constant of the epoxy resin improved by 62.5% after the addition of 5 wt% poly(ionic liquid). Dzienia et al studied the influence of anion type and concentration of imidazolium‐based ionic liquid on the curing kinetics, glass transition temperature ( T g ), and conductivity of DGEBA 23 . They showed that chloride‐anion‐containing systems exhibit high conductivities due to the presence of the polar and highly mobile chloride anion and oligomeric cations.…”

“…Dzienia et al studied the influence of anion type and concentration of imidazolium-based ionic liquid on the curing kinetics, glass transition temperature (T g ), and conductivity of DGEBA. 23 They showed that chloride-anion-containing systems exhibit high conductivities due to the presence of the polar and highly mobile chloride anion and oligomeric cations. Wang et al synthesized the ionic liquid 3-decyl-bis(1-vinyl-1H-imidazole-3-ium-bromide) and used it to improve the electrical conductivity of PF resin.…”