The influence of polymer molecular weight on the first normal-stress difference and shear-viscosity of LC solutions of hydroxypropylcellulose

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 M_w =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 M_w. 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 [2]. All these observations are rationalized within the framework of the constitutive equations for liquid crystalline polymers recently proposed by one of us [1]. 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.