STUDY OF A CHOLESTERIC LIQUID-CRYSTAL POLYMER IN SOLUTION IN A LOW MOLAR WEIGHT LIQUID-CRYSTAL - PHASE-DIAGRAM AND ELECTRIC-FIELD EFFECT

We have studied the effect of an electric field on a binary mixture be tween a cholesteric liquid crystal polymer and a low molar weight liqu id crystal from results a cholesteric polymer film in a confined geome try. The phase diagram shows, moreover an excellent compatibility betw een the compounds, a very large variation of the cholesteric pitch. Ac cording to the magnitude and the frequency of the applied electric fie ld, various behaviours are observed: stable homeotropic nematic, stabl e birefringent filaments, ''fingerprint'' texture, . . . We have focus ed our study on the effect of an electric field on the filament textur e corresponding to a large cholesteric pitch. The transition of a text ure with filaments to a homeotropic structure is described by the exis tence of two optical thresholds (the appearance of the homeotropic and the disappearance of the filaments) whom the variation is studied acc ording to the frequency. We are describing particular phenomena added to this general behaviour, like the nucleation and the growth under fi eld of quasi perfect spirals issue of the bending of the filaments jus t before the obtaining of the homeotropic state. Besides, we give an e xample where the quenching of the spiral texture is possible thanks to the polymeric compound of the binary mixture. From the configuration of the filament proposed by Lequeux or Oswald et al., it is possible t o imagine a mechanism where, in given conditions, a propagation of the authors' structure is produced.