A distinct smectic C$_2$ liquid crystal observed in main-chain liquid crystalline polymer

We present experimental evidence for the formation of a smectic C$_2$(S$_{{\rm C}_2}$) phase in main-chain liquid crystalline polymers with the general repeat unit, (see the PDF version). The S$_{{\rm C}_2}$ phase was identified by definitive observation using X-ray and optical methods and found to have a novel layered structure. The mesogenic groups in this phase are titled with respect to the layer normal, but unlike the S$_{\rm C}$ phase the tilt direction is opposite in adjacent layers. This distinct structure is considered to arise from a coupling of polymeric and mesogenic effects in which the spatial orientation of mesogenic groups is controlled by the confined conformation of the polymer chain