Effect of aerosil dispersions on the photoinduced nematic–isotropic transition

We report differential scanning calorimetric (DSC) and dielectric measurements on the nematic-isotropic transition in the bulk and aerosil composites of a liquid-crystal mixture having a photoactive guest azobenzene compound in a non-photoactive host, 4-n-heptyl cyanobiphenyl (7CB). The DSC scans taken at different cooling rates show that, at slower rates, the bulk displays a single peak across the transition, whereas the composites in the soft gel regime exhibit a double-peak profile. Such a double-peak profile, although seen in high-resolution ac calorimetric studies, has been observed for the first time in DSC experiments. The temperature range of the region between the two peaks is comparable to that seen in ac calorimetric experiments and has similar features. This observation is significant since the appearance of the low-temperature peak in ac calorimetric data has been explained to be due to a crossover from the random-dilution to the random-field limits. This work also constitutes the first experiments on the photoisomerization driven isothermal phase transitions in liquid-crystal-aerosil composites. The studies carried out in the absence and presence of a low-magnitude UV radiation not only bring out the standard features now established for such photostimulated phase transitions, but display a few surprises. Notable among them are that (i) the photoinduced shift in the transition temperature is a non-monotonic function of the aerosil composition and appears qualitatively similar to the dependence of the transition temperature itself, and (ii) the thermal anomaly mentioned above characterizing the crossover is also seen in the temperature-dependent as well as the temporal variation of the sample capacitance for a composite in the soft gel regime. We have also evaluated, using the temporal variation of the capacitance, the different response times associated with the UV-on photochemical process as well as the UV-off thermal back-relaxation process; the response times appear to have a similar dependence on the aerosil concentration as the transition temperature.