Ionic Liquids Tuning the Performance of Coatings

The design of “smart” coatings and structural materials continue to offer immense promise in many emerging technologies ranging from self-healing coatings to stimuli-responsive drug delivery vehicles. Significant advances demand integration across length scales, ranging from non-covalent interactions and self-assembly at the nanometer dimension to the engineering and fabrication of functional devices at the millimeter scale. Polyurethanes are broadly used in coatings applications due to versatile, modular synthetic strategies and the formation of nanoscale morphologies for tunable thermomechanical performance. Ion-containing polyurethanes also serve as nanoscale templates for the directed introduction of ionic liquid (IL) electrolytes for electro-active coatings and elastomers. Poly(ethylene oxide) (PEO)-based sulfonated polyurethanes bearing either sulfonated soft segments (SSSPU) or sulfonated hard segments (SHSPU) were prepared using the prepolymer method. Comparative studies revealed that SSSPU exhibited superior mechanical properties compared to SHSPU. SSSPU exhibited a well-defined microphase-separated morphology, and in sharp contrast, SHSPU showed phase mixing due to increased compatibility between PEO soft segments and sulfonated ionic hard segments. IL incorporation into SSSPU afforded novel SSSPU/IL composites. DSC and DMA studies indicated that IL systematically reduced glass transition temperatures of sulfonated PEG soft segments and maintained well-defined rubbery plateaus for sulfonated polyurethanes. Atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) revealed the influence of IL on SSSPU morphologies. SSSPU/IL composite membranes showed long, needlelike, microphase-separated hard domains in AFM phase images and enhanced X-ray scattering intensity of microphase separation in SAXS profiles. Dielectric relaxation spectroscopy (DRS) revealed that ionic conductivity of SSSPU/IL composite membranes exhibited Vogel-FulcherTammann (VFT) temperature dependence and systematically increased with IL content. SSSPU/IL composites provide high ionic conductivity and excellent mechanical properties for emerging electromechanical membrane applications.