THERMOTROPIC AND LYOTROPIC COLUMNAR HEXAGONAL PHASES - FREEZE-FRACTURE OBSERVATIONS OF DEFECTS AND ANOMALOUS CHARACTER OF THE THERMOTROPIC PHASE

Electron microscopy observations of replicas of freeze-fractured sampl es of two columnar hexagonal phases of different nature (a lyotropic o ne, the inverse AOT in water; a thermotropic one; C8HET) yield very di fferent results: most defects at microscopic scales are screw dislocat ions in the lyotropic phase, longitudinal edge dislocations in the the rmotropic phase. A possible way to interpret these differences is as f ollows: in the lyotropic the Lame coefficients lambda and mu and the b end modulus K-3 would not display any anomaly compared to expected val ues; in the thermotropic the shear modulus I-L would be ten times smal ler than the compressibility modulus lambda, while K-3 would Still be comparable to (but larger than) the bend modulus of a small molecules liquid crystal. We present an elementary theoretical model of the latt er case which could explain the anomalous measurements of Kg and of th e longitudinal compressibility B-parallel to (Ref. [10]) without contr adicting more recent measurements of (Refs. [17,22]). Essentially, the C8HET hexagonal phase would be a phase with defects (longitudinal dis locations) akin to an hexatic phase but with some differences.