Structure and rheology of model side-chain liquid crystalline polymers with varying mesogen length

Abstract

We probed the structural transitions and rheological properties of norbornene-based polymeric liquid crystals with attached side mesogenic groups of varying length. Whereas the short side chain liquid crystals (C5) formed a nematic mesophase, the long ones (C9) formed a smectic-A solid. The degree of polymerization of the main backbone affected the isotropic-mesophase transition temperatures, but did not influence the change of the mesophase dynamics from the isotropic to the anisotropic state. The temperature dependence of the shift factors obtained from the time-temperature superposition were divided into three regions, namely isotropic, mesophase, and transition, all exhibiting Arrhenius behavior except for the nematic fluid, which followed a WLF dependence. A remarkable strain hardening observed in both nematic and smectic samples was attributed to the dynamic coupling of the main chain and the mesogenic side groups. By applying large amplitude oscillatory shear, macroscopic alignment was achieved; the long recovery times upon flow cessation suggested a very slow structural reorganization, which was much longer in the smectics compared to the nematics, possibly due to the involvement of smaller length scales in the former case.