Synthesis,thermal and solar cell application of novel hyperbranched polyurethanes containing azomethine and aryl-ether connectivities

Bis-{[(4-hydroxybutoxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3-dicarboxylate – a novel A₂ diol monomer – and a series of new blocked isocyanates (B₃ monomers) were synthesized in high yield and their structures were confirmed. Using these monomers, different hyperbranched polyurethanes (HBPUs) containing azomethine and aryl-ether connectivities were prepared via an A₂ + B₃ approach. All possible parameters were varied to study their effect on molecular weight build-up. The effects of concentrations of A₂ and B₃ and the A₂: B₃ monomer ratio were well reflected in the molecular weight of the polymer. The structure of the B₃ monomer was also found to have a significant effect on the molecular weight. The polymers were characterized by Fourier transform infrared, proton nuclear magnetic resonance, gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, wide-angle X-ray diffraction and impedance spectroscopy. All the polymers registered unusual sub-T _g; and they invariably underwent decomposition at three distinct stages with significantly improved thermal stability at initial stage. The ionic conductivity of HBPU increased remarkably upon doping with KI and I₂. The dye-sensitized solar cell fabricated using this polymer as electrolyte yielded an overall conversion efficiency of 3.5% upon illumination with visible light.