Identification of nonmonotonic behaviors and stick-slip transition in liquid crystal polymers

International audienceThe 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 formul

Rheological and flow birefringence studies of rod-shaped pigment nanoparticle dispersions

We study rheological and rheo-optical properties of suspensions of anisometric pigment particles in a non-polar fluid. Different rheological regimes from the dilute regime to an orientationally arrested gel state were characterized and compared with existing theoretical models. We demonstrate the intricate flow behaviour in a wide range of volume fractions. A unique combination of the optical properties of the particles results in a giant rheo-optical effect: an unprecedentedly large shear stress-induced birefringence was found in the isotropic range, exhibiting a sharp pre-transitional behaviour

Recent experimental probes of shear banding

Recent experimental techniques used to investigate shear banding are reviewed. After recalling the rheological signature of shear-banded flows, we summarize the various tools for measuring locally the microstructure and the velocity field under shear. Local velocity measurements using dynamic light scattering and ultrasound are emphasized. A few results are extracted from current works to illustrate open questions and directions for future research.Comment: Review paper, 23 pages, 11 figures, 204 reference

Identification et étude de la transition isotrope-nématique induite sous cisaillement dans les polymères cristaux liquides en peigne

Dans ce travail de thèse, une transition isotrope-nématique induite par cisaillement dans les fondus de polymères cristaux liquides en peigne a été pour la première fois mise en évidence. Cette transition de phase hors équilibre est caractérisée principalement par l'apparition, au delà d'un taux de cisaillement critique, d'une phase fortement biréfringente en phase isotrope (T > T_NI) et par un plateau de contrainte dans les courbes d'écoulement de contrainte en fonction du taux de cisaillement. Ces résultats sont interprétés comme révélateurs d'une transition de phase hors équilibre analogue à une transition du premier ordre, donnant lieu à une région du diagramme de phase hors équilibre (contrainte en fonction de la température) où la phase isotrope et la phase nématique induite coexistent. Deux bandes de biréfringences différentes sont directement visualisées dans le plan de cisaillement (vitesse, gradient de vitesse). Enfin, le mécanisme envisagé donnant lieu à cette transition isotrope-nématique hors équilibre est la déformation de chaînes connectées dans des amas élastiques. Des expériences de diffusion des neutrons aux petits angles, réalisées in situ, dans une cellule de Couette spécialement conçue, permettent l'étude de la conformation des chaînes en phase nématique induite. On a ainsi pu démontrer que la conformation des chaînes subit une réorientation de perpendiculaire au directeur de la phase nématique à l'équilibre à parallèle dans la phase nématique induite. Enfin, dans le cas d'un polymère caractérise par une conformation de chaîne de type parallèle au directeur, en phase nématique induite ainsi qu'à l'équilibre, nous avons relevé l'apparition d'oscillations temporelles auto-entretenues de la contrainte en réponse à un taux de cisaillement imposé en phase nématique induite. Ce phénomène oscillant est interprété en termes de déformation élastique suivie d'une relaxation du milieu.In this work, a shear induced isotropic to nematic phase transition in melts of side chain liquid crystal polymers is pointed out for the first time. This non-equilibrium transition has been revealed through rheo-optical measurements: a high birefringence emerges from the isotropic phase when sheared above a critical shear rate. Furthermore, a stress plateau in the flow curves of stress versus shear rate is observed and interpreted as relevant of a non-equilibrium transition achieved through shear banding. Two bands of different birefringence have been observed in the shear plane (velocity, velocity gradient). The mechanism considered for the formation of the shear induced nematic phase is the deformation of connected chains forming elastic clusters. Small angle neutron scattering experiments, performed in situ, in an especially designed Couette shear cell, allow the determination of the main-chain conformation in the shear induced nematic phase. The oblate conformation observed in the equilibrium nematic phase (at T < T_NI) is changed into a prolate one characterizing the shear induced nematic phase. Lastly, in the case of a side chain liquid crystal polymer characterized by a prolate main chain conformation, in the induced nematic phase as well as in the equilibrium one, we pointed out the appearance of undamped stress oscillations in response to an applied shear rate in the shear induced nematic phase. This oscillating phenomenon is interpreted in terms of an elastic deformation followed by a relaxation of the sample.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

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

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