Flow-induced phase transitions are a fundamental (but poorly understood) property of non-equilibrium systems, and are also of practical importance for tuning the processing conditions for plastics, petroleum products, and other related materials. Recognition that polymers may exhibit liquid crystal properties has led to the discovery of the first tailored side-chain liquid crystal polymers (SCLCPs), which are formed by inserting a spacer between the main polymer chain and the lateral mesogen liquid-crystalline graftings. Subsequent research has sought to understand the nature of the coupling between the main polymer chain and the mesogens, with a view to obtaining better control of the properties of these tailored structures. We show here that the parallel or perpendicular orientation of the mesogens with respect to the main chain can be reversed by the application of an external field produced by a shear flow, demonstrating the existence of an isotropic nematic phase transition in SCLCPs. Such a transition, which was theoretically predicted 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 study of these non-equilibrium phenomena.

Abstract. -This letter describes the non-linear rheology of the isotropic phase of a thermotropic side chain liquid-crystal polymer (SCLCP), from which we infer a flow-induced isotropic-to-nematic (IN) phase transition above a critical shear stress and construct non-equilibrium phase diagrams. In contrast to the well-studied wormlike-micellar solutions and predictions for simple liquid-crystalline systems, the critical stress does not vanish as the equilibrium transition temperature is approached from the above. We postulate that this is due to: i) the coupling between mesogens and the polymer backbone, whose equilibrium oblate nematic backbone conformation contrasts with the prolate non-equilibrium conformation; and ii) the peculiar topological constraints in SCLCP melts, which have been previously postulated as leading to long-lived clusters.Introduction. -Shear flow dramatically influences the microstructural order in complex fluids, and can sometimes induce phase transitions. Examples include the shear-induced isotropic-to-nematic (IN) phase transition in liquid-crystalline systems [1, 2] and in colloidal suspensions [3]. The possibility of inducing an isotropic-to-nematic (IN) transition by shear flow is appealing, and has been reported experimentally in semiflexible wormlike micelles [2]. Although this system is well studied, polymeric-like entanglements, the annealed length distribution, and solvent greatly complicate the physics. Mather et al. [4] have recently studied a linear thermotropic polymer and shown that it can undergo a flow-induced IN transition. A similar transition was then discovered in side chain liquid-crystalline polymers (SCLCPs) [5], based on birefringence measurements. In one SCLCP system, Small-Angle Neutron Scattering (SANS) demonstrated that, while the mesogenic side group is perpendicular to the polymeric backbone in the equilibrium nematic phase, flow induces paral lel alignment, with backbone and mesogen distribution functions prolate and oriented generally in the flow direction [5]. The aim of this paper is to continue this work with a study of the non-linear rheology of this system.The constitutive behaviour of a typical flow-induced shear-thinning phase transition is shown in fig. 1. For imposed strain rates in the unstable region, in which the stress decreases