Incommensurate and Commensurate Smectic a Phases with Pressure Induced Nematic Phases

Abstract

Our previous resolution of the discrepancy between the molecular length, L _s, and the observed smectic A lattice parameter, d (incommensurate smectic A phase, d ≠ L _s) of some nitro esters now seems unnecessary. A new interpretation is presented. It turns on the appreciation that an x-ray diffraction experiment on intercalated smectic A layers, composed of different sorts of molecules: N molecules of length L _p and M, of length, L _s, would sense an average spacing: d = (NL _p + ML _s)/(N + M). For the nitro esters, L _p is the length of an anti-parallel associated pair (similar to the one proposed for the cyanobiphenyls) and L _s, the (unassociated) molecular length. N is estimated for the previously studied nitro-esters. In contrast, the weaker dipole of fluorinated Schiff bases cannot stabilize such pair-wise associations at one atmosphere resulting in a commensurate smectic A phases: d ~ L _s; N ~0. A pressure induced nematic phase is observed in the decyloxy member of this series, as it was for the nitro-esters, and indicates that pressure stimulates pair production (N>0) and thus incommensurability which in turn stabilizes the nematic phase but at the expense of the smectic A phase. This is consistent with the recent Landau theory of the nematic smectic A transition in which the nematic and smectic A order parameters are coupled.