Ionomeric poly(urethane semicarbazides) with azobenzene groups in the main chain - studies on photoswitching behaviour and mechanical properties

Poly(urethane semicarbazide) anionomers containing azobenzene chromophores in the main chain were prepared by chain extending the isocyanate terminated prepolymers with chromophoric dihydrazide. Variations were done with respect to the hard segment content and the nature of the diisocyanate and the effect of the variations on the thermal, mechanical properties and wettability were studied. The polymers in solution showed reversible photoisomerization behaviour evinced by UV/vis spectroscopy in which irradiation of polymer solutions with UV light was observed as a decrease in intensity of absorbance corresponding to trans-form and increase in the absorbance of cis-form. The back relaxation took place in the presence of visible light. Photoswitching or the wettability of the films induced by the reversible cis-trans isomerization of the azobenzene chromophores was experimentally shown from water contact angle measurements. Irradiation of the films with 365 nm light effected a decrease in the water contact angle. X-ray diffraction results indicated dense arrangement of crystallized hard segments in compositions containing higher hard segment content. Thermal stability up to 300 °C was exhibited by the polymers as shown by thermogravimetric analysis and the phase separated morphology was confirmed by dynamic mechanical analysis. Tensile strength measurements showed that the films with increasing hard segment content exhibited increasing tensile strength and modulus but decreasing values of elongation.