OPTICAL EVIDENCE OF THE LAYERED ARRAY OF GRAIN-BOUNDARIES IN TGB(A) AND TGB(C) MESOPHASES

We report optical pitch measurements on chiral mesophases: cholesteric , twist grain boundary smectic (TGB(A) and TGB(C) and helicoidal smect ic C. The nature of the phase transitions is discussed. A new series o f defect lines parallel to the usual Grandjean-Cano steps is observed in the TGB phases only. They are interpreted as edge dislocations of t he layered array of smectic slabs stacked along the TGB screw axis and thus as a direct visual evidence of the Renn-Lubensky structure. Thei r origin and stability are discussed in an elastic model of strain of a TGB sample. Their core is described as a wall constituted of edge di slocation lines.