FLUCTUATIONS ABOVE THE SMECTIC-A-ISOTROPIC TRANSITION IN LIQUID-CRYSTALLINE ELASTOMERS UNDER EXTERNAL STRESS

Using birefringence measurements, we observe for the first time nemati c as well as smectic fluctuations above the smectic-A-isotropic transi tion in liquid crystalline elastomers under an external mechanical str ess. From a mean-field analysis we find that there are temperatures (f ar above the isotropic-ii transition) for which nematic fluctuations d ominate as well as a range of temperatures (above and close to this ph ase transition) where smectic fluctuations are more important. These t wo regimes are connected by a crossover regime where the coupling betw een the two types of fluctuations becomes important. We also discuss t he possibility of a triple point in liquid crystalline elastomers, whe re an isotropic, a smectic and a nematic phase meet.