Thermomechanical and conductive properties of thiol-ene poly(ionic liquid) networks containing backbone and pendant imidazolium groups

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 T_g as the length of the terminal alkyl chain length increased from C1 to C8, followed by an abrupt increase in T_g 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 T_g 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 T_g was found.