Transient and stationary flow behaviour of side chain liquid-crystalline polymers: Evidence of a shear-induced isotropic-to-nematic phase transition
Laboratoire Léon Brillouin (CEA-CNRS),
CE-Saclay - 91191 Gif-sur-Yvette, France
2 Department of Physics & Astronomy and Polymer IRC, University of Leeds Leeds LS2 9JT, UK
Accepted: 30 April 2002
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 iso- tropic-to-nematic (IN) phase transition above a critical shear stress and construct non-equilib- rium 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.
PACS: 36.20.Ey – Conformation (statistics and dynamics) / 64.70.Md – Transitions in liquid crystals / 83.50.Ax – Steady shear flows, viscometric flow
© EDP Sciences, 2002