Rheo-Optics of an Augned Thermotropic Liquid Crystalline Polymer
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RHEO-OPTICS OF AN AUGNED THERMOTROPIC UQUID CRYSTALLINE POLYMER MOHAN SRINIVASARAO, RAUL O. GARAY, H.HENNING WINTER AND RICHARD S. STEIN Department of Polymer Science and Engineering University of Massachusetts, Amherst, MA 01003. ABSTRACT Rheo-optical studies on aligned and unaligned melts of a main chain thermotropic polyester with flexible spacers are discussed in terms of the predictions of the Ericksen-Leslie equations. It is shown, using conoscopy, that the director is flowaligning at temperatures close to TNI and nonflow-aligning close to a smectic to nematic (TSN) phase transition. The implications of these results are discussed in terms of the change in sign of one of the Leslie viscosity coefficients and interpreted in the framework of the Ericksen-Leslie Theory.
INTRODUCTION In the past decade, in part, due to the technological importance of liquid crystalline polymers (LCP), rheological characterization of these materials have become an important area of research. It is well documented that LCPs exhibit rather unusual transient flow properties [1]. They, for instance exhibit negative first normal stress difference in shear flow, increase in viscosity with decreasing shear rate at low rates of shear, large recoverable strains [2,3]. This has led to a fundamental level In order to state of alignment information about
flurry of recent efforts aimed at the understanding of these phenomena at a [4,5]. understand the flow properties of nematic liquid crystals, one must know the of the local optical axis during flow [6]. In many of the experimental studies, the local optic axis is nonexistent, thus making the data unsuitable for
quantative analysis. The availability of a stable thermotropic LCP and the successful formation of a monodomain has made it possible to study the flow properties of this material. The Ericksen-Leslie theory accounts quite well for the dynamical properties of nematics [6], and introduces six viscosity coefficients (al through a6), five of which remain independent due to Parodi's relation [7]. In the case of shear flow, the response of a nematic is primarily dependent on two director takes on a and remains in the such that at steady
of the viscosity coefficients, a2 and a3. When the product ac2 (3 > 0, the stable, steady alignment to the flow direction, given by the Leslie angle 0o, plane containing the velocity gradient (X-Y plane). The angle 00 is attained state there is no hydrodynamic torque on the director and is given by [6]
tan 2 00 = cE3/a2
( 1)
However, when the product a3a2 < 0, there always exist hydrodynamic torques which cause the
Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Materials Research Society
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director to rotate in the shear plane. In this case, instabilities are expected in steady shearing deformation [8]. In this paper the flow properties of a thermotropic polyester, with a stable nematic phase, are studied using unaligned and aligned (monodomain) samples. We demonstrate that the material studied exhibits two shear-flow regimes, flow-aligning close to
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