The first normal-stress difference N1(γ̇) and the shear viscosity η(γ̇) have been measured for liquid crystalline solutions of HPC in acetic acid (AA) as functions of the shear rate 7 and the molecular mass of HPC. The measurements were done over four decades in 7, for two samples of HPC with Mw =60,000 and 100,000 (Klucel E and L, respectively) and solution concentration c = 37% (c > c*). N1 (γ̇) is observed to change from positive to negative and again to positive, as the shear rate γ̇ increases. The γ̇ values at which N1 changes sign depend on Mw. The viscosity η(γ̇) shows a small Newtonian plateau at low shear rates and a strong shear-thinning at higher values of γ̇, including an "hesitation" similar to one previously observed in LC solutions of PBLO . All these observations are rationalized within the framework of the constitutive equations for liquid crystalline polymers recently proposed by one of us . Expressions for η(γ̇) and N1 (γ̇) derived from this theory fit very well (quantitatively) to the experimental data and some fundamental viscoelastic parameters of the system are thereby obtained for the first time.
|Number of pages||8|
|Journal||Molecular Crystals and Liquid Crystals Science and Technology Section A: Molecular Crystals and Liquid Crystals|
|Publication status||Published - 2001|