PHOTOMECHANICAL EFFECTS IN PHOTOCHROrIC SYSTEMS

Indoleninospirobenzopyrans (ISBP) undergo under ultraviolet irradiation ring opening with production of strongly coloured merocyanines, which can return thermally or photochemically to the colourless spirobenzopyrans. Their decolouration rate in stretched polymer matrices depends on their molar volume, the physical properties of the matrix and on the orientation of the polarized light with respect to the stretch orientation. In polyvinyl alcohol they exhibit reverse photochromism, i.e. the coloured merocyanines are stable and decolourize under visible light irradiation. These merocyanines, dissolved in stretched polyvinyl alcohol, develop strong absorption dichroism, resulting from their molecular orientation. On the basis of these properties the photochemical contraction of ethyl acrylate networks cross-linked with ISBP-bismethacrylate has been further examined. The influence of the bismêthacrylate concentration and the influence of the relative elongation have been studied, as well as the reversibility of the contraction/elongation cycles and their activation energies. Using monochromatic light of different wavelengths, it is possible to bind the photoresponse of the system with the absorption spectrum of the merocyanine in the visible. The influence of the orientation of the merocyanines was analyzed using polarized light. Shrinking is stronger with parallel polarized light than perpendicular with respect to the stretch orientation. The contraction dichroism of stretched samples equals 1.5 (D// / DJ ), while the decolouration rate cichroism amounts to about 1.14.