Structure of the smectic B phase formed by linear and branched perfluoroalkyl-alkanes

The structure of the smectic B phase formed by perfluorodecylalkanes has be en analyzed through the synthesis of three molecules and a careful investig ation of their mesophases. Molecules with C-8, C-10 and a branched C-11 alk yl chain were considered. The structure factor analysis of 001 reflexions i s consistent with the picture of alternating electron rich and electron poo r sublayers. A structural model with segregated perfluoroalkyl chains in th e middle of the layer and disordered alkyl chains on each sides is proposed . This model is discussed in terms of space filling arguments: chain length s, cross section compatibility in the layer and compactness. Infrared dichr oic ratios of the CH2 and CF2 stretching modes confirm that perfluoroalkyl subunits are perpendicular to the S-B layer while alkyl ones, in a quasi mo lten state, fill the remaining layer space.