A STUDY OF HYSTERESIS AND BISTABILITY IN A POLYMER STABILIZED NEMATICLIQUID-CRYSTAL USING PARAMAGNETIC-RESONANCE AND ELECTROOPTICAL STUDIES

We observed hysteresis and bistability in a nematic liquid crystal mix ture (E7) stabilised with an ultraviolet (UV) curable resin (Masterbon d UV 10) in a weight ratio of 9:1. When the mixture is photocured at 9 0 degrees C, for times between 0 and 60 min, the nematic clearing poin t is found to increase From 50 degrees C to the value observed for pur e E7. The degree of polymerization increases with longer cure time, le aving less GV10 input to depress the clearing point. Using electron pa ramagnetic resonance (EPR) spectroscopy, we observed a two phase syste m in the nematic spectra which is much more distinct when the cure is performed well below the clearing temperature, indicating decreased co mpatibility. The spectra also indicate little long range alignment as the sample is heated through the nematic range, but the spectra do ind icate order as the sample is then cooled, yielding a bi-stable system. One possible explanation for this behaviour is that surface energy is overcome by thermal and magnetic energy as the sample is heated, allo wing the magnetic field to align the sample. Typically, the anchoring energy of a liquid crystal to a polymer surface approaches zero as the temperature increases to near, but below, the clearing point. The EPR observations are verified by optical transmission studies.