Kinetics study of phase separation in polyacrylic acid /nematic LC system by optical technique

Thin polymer layers containing liquid crystal LC of non-linear optical properties were obtained from polyacrylic acid. Samples were produced by phase separation as a consequence of chemical polymerisation (PIPS method) resulting in precipitation of liquid crystals in the form of droplets in a polymer matrix being formed. Films were produced in variable conditions of polymerization time and temperature and different content of an initiator (1-3 wt%). They contained of 10-40 wt% of LC. The cell thickness was constant and equal to 20 mum. Systems obtained in this way were subjected to thermooptical and electrooptical studies, morphological structure investigation and DSC analysis. The polymerization time has a significant influence on the size and number of LC droplets. The presence of benzoyl peroxide can cause partial destruction of LC properties which is reflected by a decrease of isotropisation temperature T-i of the liquid crystal. An increase of the initiator amount shortens the time of polymerisation (t(i)), while process enthalpy (DeltaH) increases. Hence, the liquid crystal separation time decreases as well. Films prepared by this method present good electrooptical properties. Rise and decay times of orientation are short and equal to 2-17 ms (depending on driving voltage applied) and about 80 ms, respectively. Threshold voltage for the best sample is equal to 5 V. Optimal conditions are found: LC content = 20%, w(t)(i) = 1%, polymerisation temperature T-P = 100-110degreesC.