Effect of aerosil dispersions on the photoinduced nematic–isotropic transition

Abstract

We report differential scanning calorimetric (DSC) and dielectric measurements on thenematic–isotropic transition in the bulk and aerosil composites of a liquid-crystal mixturehaving a photoactive guest azobenzene compound in a non-photoactive host, 4-n-heptylcyanobiphenyl (7CB). The DSC scans taken at different cooling rates show that, at slowerrates, the bulk displays a single peak across the transition, whereas the composites in thesoft gel regime exhibit a double-peak profile. Such a double-peak profile, although seen inhigh-resolution ac calorimetric studies, has been observed for the first time inDSC experiments. The temperature range of the region between the two peaks iscomparable to that seen in ac calorimetric experiments and has similar features.This observation is significant since the appearance of the low-temperature peakin ac calorimetric data has been explained to be due to a crossover from therandom-dilution to the random-field limits. This work also constitutes the firstexperiments on the photoisomerization driven isothermal phase transitions inliquid-crystal–aerosil composites. The studies carried out in the absence and presence of alow-magnitude UV radiation not only bring out the standard features now establishedfor such photostimulated phase transitions, but display a few surprises. Notableamong them are that (i) the photoinduced shift in the transition temperature is anon-monotonic function of the aerosil composition and appears qualitatively similar to thedependence of the transition temperature itself, and (ii) the thermal anomaly mentionedabove characterizing the crossover is also seen in the temperature-dependentas well as the temporal variation of the sample capacitance for a composite inthe soft gel regime. We have also evaluated, using the temporal variation of thecapacitance, the different response times associated with the UV-on photochemicalprocess as well as the UV-off thermal back-relaxation process; the response timesappear to have a similar dependence on the aerosil concentration as the transitiontemperature.