Structure Microscopique d'un Melange Binaire de Cristaux Liquides Etude par Resonance Paramagnetique Electronique

Abstract

The microscopic structure of thermotropic liquid crystals is experimentally investigated. The behavior of aromatic cores and aliphatic chains are analysed when defects are introduced in the mesophase as follows: to a liquid crystal having long aliphatic chains (40.8) is added one having shorter chains (40.2). An E.S.R. study is performed using two nitroxide probes: in smectic phases the first one is localized close to the aromatic cores while the other remains in the vicinity of aliphatic chains. In the nematic phases, when concentration in 40.2 varies, the orientational potential is found to be homogeneous because of the molecular translation diffusion. For high ordered smectic phases (S_B) the potential does not depend on the concentration of 40.2 while introducing defects close to the aliphatic chains make the corresponding potential vary in a significant manner. In the S_B phase die effect observed by E.S.R. (decrease of the order parameter of probe II) is more significant than the splitting variation previously observed by D.M.R. These results are consistent with a “partial rigidity” of aliphatic chains (interpenetrating structure of layers, intrinsic rigidity, etc.), limiting the new possible conformations in the mixture and with a localization of the probes corresponding to a low orientational potential (the “aliphatic density defects”).