Order parameters, orientational distribution functions and heliconical tilt angles of oligomeric liquid crystals

Abstract

Twist-bend (TB) phases possess a local helical structure with a pitch length of a few nanometers. The TB phase was first observed in low molecular weight dimers, and as such these have been the focus of efforts to understand the properties and structure of these new phases of matter. Recently several oligomeric and polymeric materials have been reported, but there is little information on the properties and structure of the TB phase in these materials. Herein we compare the order parameters, orientational distribution functions (ODF) and heliconical tilt angles of the TB phase exhibited by a liquid-crystalline dimer (CB7CB) to a tetramer (O4₇) and hexamer (O6₇) by SAXS/WAXS. Following the N-TB phase transition we find that all order parameters decrease, and while 〈P₂〉 remains positive 〈P₄〉 becomes negative. For all three materials the order parameter 〈P₆〉 is near zero in both phases. The ODF is sugarloaf-like in the nematic phase and volcano-like in the TB phase, allowing us to estimate the heliconical tilt angle of each material and its thermal evolution. Surprisingly, the heliconical tilt angle appears to be largely independent of the material studied despite the differing number of mesogenic units.