Impact of texture on stress growth in thermotropic liquid crystalline polymers subjected to step-shear

Abstract

The Landau-de Gennes tensor order parameter equations of nemato-dynamics are formulated, solved and used to find the impact of textural transformation on stress growth in thermotropic liquid crystalline polymers subjected to shear start-up flow. The simulated textural transformations include nucleation and annihilation of twist inversion walls. Coarsening processes include wall-wall annihilation, wall pinching and wall-bounding surface reactions. In the absence of defect-related effects, the stress growth is characterized by an early stress plateau, intermediate power law growth, and a late stage stress plateau. As the Deborah number (De) increases, flow-induced textural transformations affect the late stage and then the intermediate stress growth stage. Defects are found to be stress sinks, and so removal of defects increases stress. At lower Deborah numbers, few defects arise and coarsening rates are low, so the main texture effect in this regime is in the late stage plateau region, causing localized step increases. At Deborah numbers close to one, nucleation and coarsening rates increase, and textural effects appear closer and closer to the intermediate stress growth regime. As De increases further, coarsening by pinching processes overcomes nucleation, and all defects disappear in the intermediate stress growth regime, causing the stress growth to exhibit a smooth staircase shape. Strain and amplitude scaling is not observed. Simulated textural transformations show that smooth staircase stress growth is the result of defect annihilation processes. The non-monotonic stress growth is consistent with experimental observations. Simulated textures provide specific knowledge important to the eventual understanding of the rheologies of textured liquid crystal polymers.