Temperature Dependence of the Rheological Properties of Acetoxypropylcellulose in the Thermotropic Chiral Nematic Phase

Abstract

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 M_W = 94000 g/mole, and T=120 and 140°C for M_W = 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 M_W 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 (E_a) 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 E_a with γ, for M_W = 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.

Key Words:

• liquid crystalline polymers
• rheology
• shear viscosity
• first normal stress difference