Temperature Dependence of the Rheological Properties of Acetoxypropylcellulose in the Thermotropic Chiral Nematic Phase
Résumé
The rheological behaviour of two different molecular weights of a thermotropic liquid crystalline cellulose derivative, acetoxypropylcellulose (APC), at T=120°C, has already been reported [1]. In this work we present the temperature dependence of the rheological properties of two molecular weights of APC. The shear viscosity, η, and first normal stress difference, N1, were measured at temperatures T=120, 130 and 140°C, for MW = 94000 g/mole, and T=120 and 140°C for MW = 129000 g/mole, and for shear rates, γ, between 0.01 and 10 s-1. The shear viscosity η(γ) decreases with increasing T, for both samples, showing a strong shear thinning behaviour at all temperatures and over the whole range of γ studied, except for shear rates between about 0.2 and 1 s-1 (depending on MW and T), where a quasi-Newtonian plateau is observed. The first normal stress difference N1 (γ) also decreases with increasing T at a given γ. It increases with shear rate over the whole γ range studied, and shows an inflection at γ values slightly above γ = 1/τ, where τ is the relaxation time of the polymer memory function. The temperature dependence of the shear viscosity was fitted to an Arrhenius law, giving an apparent activation energy (Ea) in the order of 8-15 kcal/mole, depending on molecular weight and shear rate. The activation energy was found to increase with molecular weight (at a given γ). The variation of Ea with γ, for MW = 94000 g/mole, shows a minimum at γ η 2 s-1. Using a continuum theory for nematic polymers, proposed by Martins [2], some fundamental parameters were obtained from the fit of the theory to the experimental data. Using these parameters it was possible to construct a ''master curve” for the viscosity, η(γ), in good agreement with the experimental data.