Shear Flow Induced Transition from Liquid-Crystalline to Polymer Behavior in Side-Chain Liquid Crystal Polymers

International audienceWe determine the structure and conformation of side-chain liquid-crystalline polymers subjected to shear flow in the vicinity of the smectic phase by neutron scattering on the velocity gradient plane. Below the nematic-smectic transition we observe a typical liquid-crystal behavior; the smectic layers slide, leading to a main-chain elongation parallel to the velocity direction. In contrast, a shear applied above the transition induces a tilted main-chain conformation which is typical for polymer behavior. [S0031-9007(96)01984-9] PACS numbers: 61.41. + e, 47.50. + d, 64.70.Md, 83.50.Ax The effect of shear constraint on the behavior of liquid crystals or on the behavior of " ordinary " polymers has been studied for more than two decades both experimentally and theoretically [1]. In contrast, the investigation of liquid crystal polymer behavior under shear flow has only recently begun [2]. Very interesting behavior can be expected for liquid crystal polymers owing to the competition between the internal strain produced by the side-chain mesogens on the main chain and the external strain brought by the shear process. Recently, we have studied the evolution of the main-chain conformation in the shear plane of a liquid crystal polymethacrylate as a function of the shear rate [3]. It is shown that macro-scopic shear is transmitted at a microscopic level by the smectic layers, ensuring an efficient shear of the polymer main chains. The outcome is a macroscopic orientation of the smectic monodomains whose smectic planes are established parallel to the shear plane. The polymer main chains already confined by the mesogenic layers become more elongated along the velocity direction with increasing shear (Fig. 1). These results were obtained from measurements carried out in situ in the shear plane, whereas other planes were observed on a quenched sample after shearing. In this article we consider measurements made in situ in the plane of the velocity gradient (vorticity plane). This plane is particularly interesting since it allows the simultaneous observation of the formation of the smectic phase and of the conforma-tion of the polymer main chain versus shear rate. Such a direct study has never been carried out in bulk, even for nonliquid crystalline polymers. This last experiment [4] confirmed the theory that bulk polymer conformation under shear flow is a tilted elongated shape resulting from a combination of rotational and translational motions. We obtained the same result using our cell with a polystyrene melt. The liquid-crystalline polymer PMA-OC 4 H 9 used here is the same as described in Ref. [2] and corresponds to the structural formula with either X ෇ H or X ෇ D (we used a 1:1 isotopic mixture in order to obtain the central scattering associated with the main-chain conformation). The sample has the following mesophases and transition temperatures: T g 35 ± C –SA1 (smectic)–99 ± C–N (nematic)–10