Transient and stationary flow behaviour of side chain liquid-crystalline polymers: Evidence of a shear-induced isotropic-to-nematic phase transition

Abstract

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

Richness of Side-Chain Liquid-Crystal Polymers: From Isotropic Phase towards the Identification of Neglected Solid-Like Properties in Liquids

“…The second significant divergence with the prediction is the shape of the flow curve (stationary shear stress versus shear rate curve-for a description of the transient behavior, see [36,37]). The flow-induced phase transition is supposed to exhibit a 3-zone stress-shear rate curve (Figure 4(a)): at low shear rates, the paranematic phase corresponds to a Newtonian behavior, at intermediate strain rates, a stress plateau appears when the phase is induced and "phase-separates" into homogeneous bands flowing at strain rates 1 C  and 2 C  , to maintain the average imposed strain rate.…”

Gradient and vorticity banding

“…In some (diluted and concentrated) worm-like systems, wall slip is reported (Manneville et al 2007;Salmon et al 2003b). Gradient banding is also reported to occur in entangled polymers (Callaghan and Gil 2000;Britton and Callaghan 1997a;Tapadia et al 2006), micellar cubic phases (Eiser et al 2000a, b), supra-molecular polymer solutions (van der Gucht et al 2006), transient networks (Michel et al 2001), and thermotropic side chain liquid crystal polymers (Pujolle-Robic et al 2002). Experiments indicate that gradient banding can also occur in hexagonal phases of surfactant solutions (Ramos et al 2000).…”

Rheology of Wormlike Micelles: Equilibrium Properties and Shear Banding Transitions

“…The systems considered are aqueous solutions of surfactants, but some results are clearly generalized to other self-assembled phases of anisotropic structures [85,86,89,90,92,93,[237][238][239][240]. Our approach has been to show the existence of a common rheological behavior shared by most semi-dilute and concentrated wormlike micellar solutions.…”