Aging Effects on the Rheology of LC Solutions of Hydroxypropylcellulose

Abstract

We apply the recently developed continuum theory of liquid crystalline polymers¹ to interpret the behavior of the shear viscosity η(γ.) and the first normal-stress difference N₁(γ.) measured for liquid crystalline solutions (c = 37%, c > c*) of HPC (Aldrich, M_w = 100,000) in acetic acid (AA) with different ages: 1 day, 1 month and 2 years. η(γ.) and N₁(γ.) were measured over four decades in γ. N₁(γ.) is observed to change from positive to negative and again to positive, as the shear rate γ. increases. η(γ.) shows a small newtonian plateau at low shear rates and a strong shear-thinning at higher values of γ. The rate of decrease of η(γ.) in this region is not monotonous, as usual, but shows an “hesitation” similar to one previously observed in a different system². The aging effect promotes a “depolymerization”³ of HPC. This, in turn, should have a strong influence on the behavior of N_l(γ.) and η(γ.) which is indeed observed⁴. All these observations can be rationalized within the framework of the theory¹.