The first normal stress difference and viscosity in shear of liquid crystalline solutions of hydroxypropylcellulose: new experimental data and theory

The constitutive equations for liquid crystalline polymers recently proposed by one of us [1] are applied here to interpret the behaviour of the shear viscosity η $ \dot \gamma $ and the first normal stress difference N 1 ( $ \dot \gamma $ ) measured for liquid crystalline (LC) solutions of hydroxypropylcellulose in acetic acid. N 1 ( $ \dot \gamma $ ) is observed to change from positive to negative and again to positive, as the shear rate $ \dot \gamma $ increases, at lower concentrations, in the LC phase. The $ \dot \gamma $ ‐values at which N 1 changes sign depend on the molecular mass (degree of polymerization) and on the concentration. η $ \dot \gamma $ shows a small Newtonian plateau at low shear rates and a strong shear‐thinning at higher values of $ \dot \gamma $ . The rate of decrease of η $ \dot \gamma $ in this region shows an “hesitation” similar to one previously observed in LC solutions of poly‐γ‐benzyl‐L‐glutamate PBLG. All these observations can be rationalized within the frame‐work of Martins' theory. The expressions for N 1 ( $ \dot \gamma $ ) and η $ \dot \gamma $ derived from this theory fit very well (quantitatively) to the experimental data and some fundamental viscoelastic parameters of the system under study are thereby obtained for the first time.