Preparation of Photochromic Waterborne Polyurethane Films: Effect of Spiropyran Concentration on Photochromic Properties

Incorporation of photochromic compounds to the polymer through chemical bonding will result in an enhancement of photoactivity and stabilization of optical properties. Here, spiropyran(SP) was introduced into polyurethane(PU) through semi-continuous emulsion copolymerization to get waterborne polyurethane dispersion(SPU). Studies on photochromic properties by UV-vis technique demonstrated the absorption intensity increased with the increasing of SP concentration and the colorless film turned to purple under UV irradiation. The kinetics for ring-opening and ring-closing reaction against SP were both first order, but the rate constants decreased with the increasing of SP concentration. The prepared SPU films showed reasonable photostability and photoreversibility according to the convenient test methods under UV-vis irradiation. Temperature dependence studies demonstrated that the photobleaching rate increased at higher temperatures with the same SP concentration. The chemical combination between SP and isocyanate(NCO) end group of polyurethane was confirmed through swelling experiments. Morphology studies were conducted by scanning electron microscope(SEM) micrographs and showed good compatibility between SP and PU.