Observation of textures of nematic polymers and estimation of the elastic constants

Abstract

The texture of a copolyesteramide, Vectra® B950, has been studied by optical microscopy. At rest, the defects are typical of a biaxial nematic phase, but under the influence of a magnetic field or under shear, the nematic transforms into a uniaxial phase. The Freedericksz geometry yields a K ₁ elastic constant of the order of 10⁻⁸ N. The diffusivities K _III/_n and K _II/_n are obtained by measuring the light intensity transmitted through a sample during shear start-up and relaxation, and from this we estimate K _III ≅ 10 ⁻⁹N and K _II ≅ 10 ⁻¹⁰N. Since the biaxiality is small, these uniaxial constants adequately describe the phase at rest.