A study of hysteresis and bistability in a polymer stabilised nematic liquid crystal using paramagnetic resonance and electro-optical studies

We observed hysteresis and bistability in a nematic liquid crystal mixture (E7) stabilised with an ultraviolet (UV) curable resin (Masterbond UV 10) in a weight ratio of 9:1. When the mixture is photocured at 90°C, for times between 0 and 60 min, the nematic clearing point is found to increase from 50°C to the value observed for pure E7. The degree of polymerization increases with longer cure time, leaving less GV10 input to depress the clearing point. Using electron paramagnetic resonance (EPR) spectroscopy, we observed a two phase system in the nematic spectra which is much more distinct when the cure is performed well below the clearing temperature, indicating decreased compatibility. The spectra also indicate little long range alignment as the sample is heated through the nematic range, but the spectra do indicate 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, allowing 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.