COMPETITION IN LOW ORDERED SMECTICS BETWEEN INCOMMENSURATE PHASES S-IC AND 2-DIMENSIONAL MODULATED ONES FOR DIMESOGENIC COMPOUNDS

Systematic physical chemistry studies are in progress concerning the o ccurrence of incommensurate low ordered smectic phases (S-ic) in non-s ymmetric dimesogens varying molecular parameters from the standard com pound KI-5. In the present study, the selected molecules possess the s ame spacer length and the same cholesteryl unit. By means of X-ray dif fraction on orientated samples, commensurate phases, incommensurate fl uid smectics and two-dimensional ones are clearly evidenced depending both on temperature and molecular parameters. So these dimesogenic com pounds respond to the frustration connected to the competition between two incommensurate lengths with the formation either of an incommensu rate phase Sic or of a two-dimensional modulated phase. A new topology in a phase diagram results from this competition in a binary system c omposed of two homologous dimesogens.