Image analysis of shear-induced textures in liquid-crystalline polymers

Abstract

Textures of several liquid-crystalline solutions of poly(benzylglutamate) and a solution of hydroxypropylcellulose were videorecorded during and after shear and the images were Fourier transformed. We find for all the solutions that bands perpendicular to the flow direction form after cessation of prolonged shearing if the Deborah number De exceeds a critical value De_c≈0·1. Here De = τ, where is the shear rate and τ is the characteristic molecular relaxation time. The bands are characterized by an initially broad spectrum of wavelengths; for the poly(benzylglutamate) solutions this spectrum narrows and shifts towards longer wavelengths as time progresses after cessation of shear, leading to an increase in the characteristic band spacing b(t) with time t. The dependences of the band spacing on the shear rate and on the solution viscosity n in poly(benzylglutamate) are very weak, but the time to form bands after shearing ceases is roughly inversely proportional to ₀. Our results suggest that both molecular elasticity and texture elasticity influence band formation, and that the texture elasticity is not described by small gradient (Frank) theory.