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

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 Mn = 340 g/mol). Further physicochemical properties (i.e., glass transition temperature, Tg, and conductivity, σdc) 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 (Treaction = 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 Tg 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 Tg) or oligomeric linear polymers (low Tg) 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.

2 or 1-butoxymethyl-1-methylimidazolium chloride [(MOB)MIm] [Cl], respectively. All samples were identically prepared in a glove box. Immediately after preparation, the samples were measured using BDS and DSC techniques. All reagents were purified and, after that, dried by using commonly known procedures. The same experimental protocols were applied to [(MOB)N111] [Sal] and [(MOB)N111] [Cl] (see Fig. S3 in the Supporting Information (SI) file).

Ionic liquids analysis
The structure and purity of each of the synthesized compounds were confirmed by spectral analysis.
Elemental analyses were carried out for all of the obtained substances using a VARIO EL-III. The 1 H NMR and 13 C NMR spectra were recorded for all of the synthesized quaternary imidazolium salts on a Bruker DRX instrument with tetramethylsilane as standard (at 400 and 75 MHz, respectively). Highresolution mass spectra (HRMS) were recorded on a LCT Premier XE Waters spectrometer on positive ESI+ ionization mode for chloride salt and ESI+ and negative ESI ionization mode for salicylate salt (TOF MS ES+/ TOF MS ES-). The synthesized ionic liquids were kept in Schlenk flasks in a desiccator to minimize contact with air moisture and additionally dried between the individual experiments.

The synthesis of new salicylate imidazolium ionic liquid with methyl and butoxymethyl chains
The new ionic liquid containing imidazolium cation was synthesized by means of a three-step approach presented in Scheme 1. A first step involved chloromethyl butyl ether (1) preparation by chloromethylation of n-butanol. Next, 1-butoxymethyl-1-methylimidazolium chloride (2) [(MOB)MIm][Cl] was obtained by a specific type of Menschutkin reaction, which does not request any energy input and is very effective. Utilized [(MOB)MIm][Cl] (2) was performed with a very high product yield of 98.0%. The third step of the whole process was associated with the metathesis reaction between chloride precursor (2) and sodium salicylate. Metathesis was carried out at room temperature using water as a reaction medium. Synthesized new ionic liquid, 1-butoxymethyl-1methylimidazolium salicylate [(MOB)MIm] [Sal] (3) was obtained, including purification process with 3 very high yield 98.5%. The structure of imidazolium salts (2 and 3) was confirmed by 1 H and 13 C NMR spectroscopy.

1-Butoxymethyl-1-methylimidazolium chloride [(MOB)MIm][Cl]
was synthesized according to a modified literature procedure 1 . Briefly, first chloromethyl butyl ether (1) was prepared by passing HCl through a mixture of formaldehyde and n-butanol, using the methodology described previously 2 .
The method of vacuum distillation was applied to purified the obtained chloromethyl butyl ether (1), and through this methodology, a clear liquid was obtained, which was afterward utilized as a quaternary agent in the Menschutkin reaction. The second step of the process contained quaternization, which was conducted under anhydrous conditions, using 20 mL of dry hexane. The freshly distilled 1methylimidazole (0.04 mol) was introduced to the three-necked flask equipped with a reflux condenser, mechanical stirrer, and thermometer. Next, chloromethyl butyl ether (1) (0.043 mol), which was distilled just before the reaction, was dropwise added to the vigorously stirred mixture of imidazole derivative. Although after combining the substrates, the chloride salt (2) precipitated from the reaction 4 mixture almost immediately, the reaction mixture was continuously stirred at room temperature for the next 2 h. After this period, the separation of the chloride was conducted. The process of obtaining

C1OC4][Sal] (3). The metathesis process.
Synthesized by Menschutkin reaction 1-butoxymethyl-1-methylimidazolium chloride (2) (0.030 mol) were dissolved in distilled water (approx. 35 mL). After the mixture become homogenous, the saturated aqueous solution of sodium salicylate (0.032 mol) was added. The metathesis was 5 conducted at room temperature maintaining vigorous mixing throughout this process. After 24 h the crude ionic liquid with salicylate anion (3) was separated and next washed with distilled water four times. Then, obtained product was dissolved in acetone and such mixture was left in the fridge for about 2 hours to precipitate any residues of the by-product. This procedure was perform until as long as the sediment of NaCl stopped being deposited, what was detected by using AgNO3.

Differential Scanning Calorimetry (DSC)
All the DGEBA/ILs mixtures were prepared as a 1:1, 10:1, and 20:1 molar mixtures by weighed and mixed manually with the use of spatula. DSC measurements were conducted at atmospheric pressure using a Mettler-Toledo DSC apparatus. This DSC is equipped with a liquid nitrogen cooling accessory and an HSS8 ceramic sensor (heat flux sensor with 120 thermocouples). Temperature and enthalpy calibrations were performed by using indium and zinc standards. The sample was prepared in an open aluminum crucible (40 μL) outside the DSC apparatus. Each experiment was performed at isothermal conditions at Treaction = 353−383 K, which was followed by nonisothermal measurements. The dynamic scans were performed in the temperature range T = 233−423 K with a constant heating rate of 10 K/min. Each measurement at a given temperature was repeated three times. For each experiment, a new sample was prepared. Additionally, to determine the thermal decomposition of studied imidazolium-based ionic liquids, the samples were heated to T = 573 K with a constant heating rate of 10 K/min. Obtained DSC thermograms are presented in Figure S1.

Nuclear magnetic resonance (NMR)
Proton nuclear magnetic resonance of polymers ( 1 H NMR and 13 C NMR) spectra were recorded using a Bruker Ascend 600 spectrometer (600 MHz, respectively) or Bruker DRX instrument (400 and 75 MHz, respectively) in CDCl3 as a solvent. TMS was used as an internal standard, and its chemical shift, δ, was set at zero. Standard experimental conditions and standard Bruker program were used.

Broadband Dielectric Spectroscopy (BDS)
Dielectric permittivity ε*(ω) = ε′(ω) − iε″(ω) values at ambient pressure were measured by using the impedance analyzer (Novocontrol Alpha) over a frequency range from 1 × 10−1 to 3 × 106 Hz. The temperature was controlled by a Quatro Cryosystem using a nitrogen gas cryostat, with stability better than 0.1 K. Epoxy resin was mixed with ionic liquid at 1:1 and 10:1 molar ratio and transferred to the 7 top of the lower plate of the capacitor. After 1h curing at 373 K and 5 minutes post-curing at 423 K, a hardened system was covered with the second (upper) plate. Dielectric measurements were carried out at the Tg determined from the DSC method.Dielectric measurements were performed at the Tg determined from the DSC method were. The samples were placed between two stainless-steel electrodes (diameter: 20 mm; gap: 0.14 mm) and mounted inside a cryostat.