Identification of nonmonotonic behaviors and stick-slip transition in liquid crystal polymers
Résumé
The recent identification of shear-induced phases in the isotropic melts of liquid crystal polymers shows that these materials are expected to display original nonlinear behaviors. We have investigated the flow behavior of a nematic sidechain polymer above its isotropic-nematic transition temperature. Nonlinear rheology and bire-fringence measurements indicate the appearance, above a critical shear rate, of the shear-induced isotropic-nematic phase transition. The rheological behavior of this induced phase is characterized by undamped time-periodic shear stress oscillations. These sustained oscillations are interpreted in terms of a stick-slip mechanism alternating high-friction static state and low-friction kinetic state. PACS number͑s͒: 83.80.Xz, 47.20.Hw, 83.50.Ax, 64.70.Md Polymers are non-Newtonian fluids ͓1͔ whereas liquid crystals do not behave as simple fluids close to phase transitions ͓2͔. When these two complex fluids are coupled to form a melt of sidechain liquid crystal polymers ͑SCLCPs͒, the resulting rheological behavior is expected to be peculiar. The very first flow studies ͓3͔ have indeed indicated that SCLCP melts display strong nonlinear behaviors above the isotropic-nematic transition temperature (T NI). This behavior looks similar to the well-studied shear-induced behavior of giant micelle solutions which display a shear-induced IN transition above T NI ͓4,5͔. The SCLCP shear-induced transition was revealed by flow birefringence and via the existence of a stress plateau in the stress versus shear rate curve. The stress plateau can be explained by entering an unstable flow region; above a critical shear rate, the region is characterized by a decreasing stress with increasing shear rate. The system is then supposed to phase separate into homogeneous bands ͑shear banding͒ to maintain the imposed shear rate ͓5͔. The existence of such nonequilibrium states opens the question of identification of the coupling parameters associated with the critical shear rate. Clearly, the shear induced SCLCP critical times are not associated with the lifetime of the pretransi-tional fluctuations, suggesting a coupling with slower time scales which could be rather consistent with the existence of macroscopic heterogeneities as proposed by Collin et al. ͓6͔. The shear-induced phase conformation of the polymer main chain was also determined using small angle neutron scattering. For a LC polymer characterized in the equilibrium nem-atic phase by a perpendicular main chain/mesogen coupling ͑oblate conformation͒, we observed that the initially perpendicular coupling is inverted in the shear-induced nematic phase to a parallel coupling with the main-chain conforma-tion becoming prolate ͓3͔. This structural rearrangement can be proposed as a working hypothesis to explain the appearance of shear-induced transitions in SCLCP isotropic melts. The purpose of the present paper is to analyze the flow behavior produced above T NI by a SCLCP whose main-chain conformation is already prolate in the equilibrium nematic phase ͓7͔. The experimental techniques used are nonlinear rheology and flow birefringence. A nonequilibrium phase compatible with shear-banding is identified together with the observation of a second nonlinear behavior corresponding to an oscillating regime. The SCLCP chosen, PA 4-CN, is characterized as a prolate nematic polymer ͓7͔. The monomers have been synthesized at the Laboratoire Lé on Brillouin and polymerized by Poly-merExpert via controlled radical polymerization. The polymer described here corresponds to a molecular weight of M W ϭ85 800 and a polydispersity index of Iϭ1.1. This molecular weight corresponds to a nonentangled polymer and no rubbery plateau was found in viscoelastic measurements. This PA 4-CN presents the following succession of me-sophases: Tg-30 °C-N-116 °C-I and corresponds to the formula
Domaines
Matière Molle [cond-mat.soft]
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