Protic ion-crosslinked polymer ionic liquid (PIL) based on linear oligomers

Abstract

A method for the synthesis of a protic ion-crosslinked polymeric ionic liquid (PIL) which turns into a liquid state at a temperature below 50 °C using a reaction of basic and acidic linear oligomers was developed. The product of interaction of α,ω-diglycidyl ester of oligoethylene oxide MM 1000 with an excess of 1-(3-aminopropyl)imidazole (PEO–2Im) was used as the basic oligomer. It contains two types of basic centers with a significant difference in basicity (aliphatic secondary amino groups and imidazole heterocycles), as well as secondary hydroxyl groups at the ends of oligoether chain. A linear oligomer with terminal sulfonic acid groups (PEO–2SO3H) was obtained by the interaction of of oligoethylene oxide MM 1000 with 2-sulfobenzoic acid cyclic anhydride. Protic ion-crosslinked PIL was synthesized by completely neutralization of basic centers of oligomer PEO–2Im by acidic oligomer PEO–2SO3H at their molar ratio 1:2 respectively. The structure of the obtained PIL was characterized by FTIR and 1Н NMR- spectroscopy methods. According to DSC, the synthesized ion-crosslinked PIL contains two types of crystalline formations with melting temperatures of 36,3 °C and 45,8 °C formed by fragments of oligoethylene oxide during the transfer of protons between different types of ion centers. Determined by the TGA method the temperature of the onset of decomposition, which corresponds to 5% mass loss, is 265 °C. The proton conductivity of the ion-crosslinked PIL was studied by the DRS method in anhydrous conditions in the temperature range from 40 to 100 °C. At a temperature of 100 °C, the proton conductivity is 3,1·10-4 S/cm. The achieved values of proton conductivity and thermal stability make the obtained compound promising as a proton-conducting medium for various electrochemical devices.