Poly(propylene glycol)-Based Non-Isocyanate Polyurethane Ionenes: Thermal, Morphological and Conductive Properties

The synthesis and characterization of a series of polyurethane ionenes using a non-isocyanate approach is disclosed. Imidazole-capped, urethane-containing prepolymers are prepared by first reacting carbonyl diimidazole (CDI) with several poly(propylene glycol) (PPG) diols with variable molecular weight, followed by subsequent reaction with 3-aminopropylimidazole (API). Polymerization with 1,4-dibromomethylbenzene followed by anion exchange resulted in the desired polyurethane ionenes bearing the [NTf₂] counteranion as a series of viscous liquids. NMR and FTIR spectroscopy are used to characterize the intermediates and final ionenes, including molecular weight determination by end-group analysis. A single glass transition temperature (T_g), as determined by differential scanning calorimetry (DSC), is observed for each ionene (−38 to −64 °C) with the T_g decreasing with increasing PPG molecular weight. Thermogravimetric analysis (TGA) indicated a two-step decomposition for each ionene, with the first being degradation of the PPG segment, followed by the urethane/ionic segment. Microphase separation is observed from x-ray scattering profiles with Bragg distances that increased with increasing PPG molecular weight. Ionic conductivity is found to be inversely dependent upon DSC T_g at lower temperatures (RT and below); however, at higher temperatures, conductivity appears to be more dependent upon the ability of ionic aggregates caused by phase separation to interact.