Polyurethanes containing an imidazolium diol-based ionic-liquid chain extender for incorporation of ionic-liquid electrolytes

Novel polyurethane (PU) cationomers are synthesized using an imidazolium diol-based ionic liquid (IL) chain extender. A systematic comparison with a non-ionic PU analogue reveals effect of the imidazolium cation on physical properties, hydrogen bonding, and morphology of segmented PU. Casting resulting PU solutions with various contents of IL generates novel membranes. Thermal study reveals that IL-containing PU membranes exhibit a constant soft segment T_g at -81 °C; however, the T_g of imidazolium hard segments systematically shifts to lower temperatures with increasing IL content. This suggests that IL preferentially locates into the imidazolium ionic hard domains, which is also evident in small-angle X-ray scattering. Moreover, dielectric relaxation spectroscopy demonstrates increased ionic conductivity of PU membranes by 5 orders of magnitude upon incorporation of 30 wt% IL. Imidazolium-containing polyurethane cationomers serve as nanoscale templates for the incorporation of ionic-liquid electrolytes. Controlled charge density and location of the ionic sites, exclusively in the hard segment, directs the electrolyte to specific domains for enhanced ionic conductivity while maintaining membrane ductility.