Thermomechanical and Conductive Properties of Thiol–Ene Poly(ionic liquid) Networks Containing Backbone and Pendant Imidazolium Groups

Abstract

A series of covalently cross-linked poly(ionic liquid) networks were prepared using thiol–ene “click” photopolymerization. In these networks, imidazolium groups are placed in the backbone and pendant to the main chain, creating a “hybrid”-type network architecture. The pendant imidazolium groups were incorporated into the networks from monofunctional “ene” monomers that contained either a terminal alkyl group at the imidazolium N-3 position of variable length (R = C1, C4, C8, C12, C16, or C20) or a variable alkyl tether spacer (n = 6 or 10) between the newly formed sulfide and the imidazolium ring. Thermal characterization of these networks indicated a general decrease in Tg as the length of the terminal alkyl chain length increased from C1 to C8, followed by an abrupt increase in Tg up to C20 due to increased van der Waals interactions between longer chains. X-ray scattering data confirmed the presence of chain-extended crystallites within the network cavities for the C16 and C20 systems, leading to the observed increase in Tg and the appearance of a melting transition for both systems. Ionic conductivities of the PIL networks were determined from dielectric relaxation spectroscopy (10–6 to 10–7 S/cm at 30 °C, 30% RH), and a direct correlation with polymer Tg was found.