Observation of shear-induced nematic-isotropic transition in side-chain liquid crystal polymers

Flow-induced phase transitions are a fundamental (but poorly understood) pr operty of non-equilibrium systems, and are also of practical importance for tuning the processing conditions for plastics, petroleum products, and oth er related materials(21). Recognition that polymers may exhibit liquid crys tal properties has led to the discovery of the first tailored side-chain li quid crystal polymers (SCLCPs), which are formed by inserting a spacer betw een the main polymer chain and the lateral mesogen liquid-crystalline graft ings(22). Subsequent research has sought to understand the nature of the co upling between the main polymer chain and the mesogens, with a view to obta ining better control of the properties of these tailored structures(22). We show here that the parallel or perpendicular orientation of the mesogens w ith respect to the main chain can be reversed by the application of an exte rnal field produced by a shear flow, demonstrating the existence of an isot ropic nematic phase transition in SCLCPs. Such a transition, which was theo retically predicted(1,2) for low-molecular-weight liquid crystals but never observed, is shown to be a general property of SCLCPs too. We expect that these SCLCPs will prove to be good candidate systems for the experimental s tudy of these non-equilibrium phenomena.