Quench sensitivity to defects and shear banding in nematic polymer film flows

Abstract

A flow-orientation algorithm [S. Heidenreich, S. Hess, M.G. Forest, X. Yang, R. Zhou, Robustness of pulsating jet-like layers in sheared nano-rod dispersions, J. Non-Newtonian Fluid Mech., in press, doi:10.1016/j.physletb.2003.10.071; X. Yang, Z. Cui, M.G. Forest, J. Shen, Q. Wang, Dimensional robustness and instability of sheared, semi-dilute, nano-rod dispersions, SIAM Multiscale Model. Simul. 7 (2008) 622–654] for nematic (liquid crystalline) polymer films is coupled with the homogenized effective conductivity tensor of the rod-matrix composite [H. Zhou, X. Zheng, M.G. Forest, Q. Wang, R. Lipton, Extension-enhanced conductivity of liquid crystalline polymer nano-composites, Macromol. Symp. 28 (2005) 81–85; X. Zheng, M.G. Forest, R. Lipton, R. Zhou, Q. Wang, Exact scaling laws for electrical conductivity properties of nematic polymer nano-composite monodomains, Adv. Funct. Mater. 15 (4) (2005) 627–638; X. Zheng, M.G. Forest, R. Zhou, Q. Wang, R. Lipton, Anisotropy and dynamic ranges in effective properties of sheared nematic polymer nano-composites, Adv. Funct. Mater. 15 (2005) 2029–2035; X. Zheng, M.G. Forest, R. Lipton, R. Zhou, Nematic polymer mechanics: flow-induced anisotropy, Continuum Mech. Thermodyn. 18 (2007) 377–394; X. Zheng, M.G. Forest, R. Vaia, M. Arlen, A strategy for dimensional percolation in sheared nano-rod dispersions, Adv. Mater. 19 (22) (2007) 4038–4043] to simulate a simultaneous thermal quench and arrest of the film flow. The final orientational morphology yields nematic polymer film features that determine film performance: anisotropy and heterogeneity in the thermal conductivity tensor, and residual shear and normal stresses. With this numerical toolkit, we amplify an inherent source of nematic polymer film non-uniformity for identical processing conditions. It is well-known that steady parallel plate driving conditions yield unsteady nematic polymer behavior such as director tumbling and wagging, defects, and shear bands. We show that the timing of initial cooling and arrest of plate motion produces different solid films, even with identical isothermal and quench protocols! Simulations show flow reversal and gradient elasticity bands are created when the quench and plate arrest are applied in phase with isothermal shear bands and defect layers, reminiscent of effects reported for small molar mass liquid crystals [R.S. Akopyan, R.B. Alaverdian, E.A. Santrosian, Y.S. Chilingarian, Thermomechanical effects in the nematic liquid crystals, J. Appl. Phys. 90 (2001) 3371–3376; R.S. Hakobyan, G.L. Yesayan, B.Y. Zeldovich, Thermomechanical oscillations in hybrid nematic liquid crystals, Phys. Rev. E 73 (2006) 061707].