REENTRANT UNIAXIAL BIAXIAL TRANSITION IN POLY-ETHYLENE-GLYCOL DOPED LYOTROPIC LIQUID-CRYSTALS

Interferometric measurements of the effect of a polymer gradativelly a dded in a lyotropic liquid crystal is reported here. Studies of the ph ase diagram, birefringence and critical exponents as a function of the polymer concentration are presented. Measurements of the optical bire fringence as a function of temperature are performed with the micellar system potassium laurate, l-decanol, and water doped with variable qu antities of the polymer poly-ethylene-glycol (PEG). The transitions fr om the calamitic nematic phase to a reentrant discotic-biaxial-discoti c phase were observed when increasing the PEG molar concentration. Our results confirm the critical properties of the uniaxial-to-biaxial ne matic transition. The critical exponent for the order parameter (beta = 0.38) obtained for the sample without PEG is in good agreement with the value calculated for the XY model. Some results with higher PEG do ping concentration indicate that beta is higher than 0.38 leading to t he mean-field value.