Cessation Of Steady-state Flow Research Articles

Transient rheological behavior of lyotropic (acetyl)(ethyl)cellulose/m-cresol solutions

The chiroptical properties and transient rheological behavior of (acetyl)(ethyl)cellulose (AEC) m-cresol liquid crystalline solutions have been investigated. Chiroptical properties were manipulated through (i) increasing degree of acetylation of ethyl cellulose (EC), and (ii) blending AEC with EC. At the same average degree of acetylation (DA), the chiroptical properties of pure AEC were different from those of the EC/AEC mixtures. However, at the same DA, the AEC and mixed AEC/EC solutions showed similar steady state flow and oscillatory behaviors, but the transient behaviors were different. At high flow rates the mixed AEC/EC solutions exhibited double recoil after cessation of steady-state flow, whereas the AEC solutions showed double recoil only in the high DA AEC solutions. All solutions, pure and mixed, had the same stress relaxation behavior. Both pitch and handedness affected the transient behavior. After cessation of high shear rate flow, the rate of modulus evolution decreased with increasing pitch, and was faster in right-handed mesophases than in left-handed ones at a similar pitch.

Apparatus to Measure Small-Angle Light Scattering Profiles of Polymers under Shear Flow

The apparatus and the principle to measure flow small-angle light scattering (“flow-SALS”) are presented as one of the rheo-optical techniques. The flow-SALS technique presented here enables one to investigate small-angle light scattering profiles and rheological properties of polymer solutions, liquids, and mixtures, as well as polymer liquid crystals under various types of shearing, i.e., steady-state shear flow, oscillatory shear flow, and shear recovery after the cessation of steady-state flow. This technique may be useful to investigate the flow-induced formation, dissolution, and deformation of supermolecular structure with spatial correlation length of a few microns, and also their effect on rheological properties. In order to demonstrate the prospect of its use, some preliminary results on flow-SALS from the lyotropic polymer liquid crystal [20 wt% poly(γ-methyl-d-glutamate) in m-cresol] are presented.

Stress Relaxation of Viscoelastic Material

The stress relaxation of viscoelastic material after cessation of steady-state flow has been investigated. A method is proposed that allows fairly accurate prediction of the shear stress component, P12(t), as well as the first normal stress component, P11–P22(t), for a relatively wide range of time and shear rates. The main advantage of the method is in its simplicity, which could make it suitable for engineering calculations.

Shear-dependent relaxation spectrum: Conversion from viscosity to stress relaxation after cessation of steady-state flow for polyethylene melts

Abstract Interrelations were established, in a limited range of shear rate γ, among the non-Newtonian shear stress relaxation following cessation of steady-state flow σ(γ, t), the steady-flow viscosity η(γ), and the dynamic viscosity η(ω) or the storage modulus G′(ω) for three linear polyethylene melts (190°C). The data for these melts were obtained using the Weissenberg rheogoniometer and the Instron rheometer. Agreement between the experimental σ(γ, t) and those calculated from the relaxation spectrum H(ln τ) was good and involved no coordinate shift. The calculated values were obtained through the relation where θ = γτ/2, τ is the relaxation time, ω is the radian frequency, h(θ) = (2/π)[cot−1 θ + θ(1 - θ2)/(1 + θ2)2], andg(θ) = (2/π) [cot−1 θ + θ)/(1 + θ2)]. The relaxation spectrum was obtained either from the linear viscoelastic data [η(ω)] using the iterative method of Roesler and Twyman or from the nonlinear steady-flow viscosity using the iterative procedure based on the equation

Shear rate dependent relaxation spectrum for polyethylene melts

AbstractMany of the familiar relations in the theory of linear viscoelasticity which relate the relaxation spectrum H(τ) to the experimentally measured quantities were extended to embrace the concept of shear dependent relaxation spectrum H(τ, \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}). The concept is based on the relation H(τ, \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}) = H(τ)h(θ)g(θ)3/2, where θ = c \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document} τ and c = 0.5. Here \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document} is the shear rate and τ is the relaxation time. The replacement of H(τ) by H(τ',\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}) in the integral relations of the linear theory produced the corresponding nonlinear parameters, e.g., the shear rate dependent viscosity η(\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}), the shear stress relaxation following cessation of steady‐state flow σ(\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document},t) and the dynamic parameters under parallel superimposed rotation η'(ω) and G'(ω, \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}). The concept was used successfully for several high‐density polyethylene melts in interconversion between η(\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}), c(\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document},t) and the dynamic viscosity η'(ω) without involving coordinate shifts or empirical parameters. The failure of this approach for interconversion between η(ω) and η(\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}) for the highly branched polyethylenes was ascribed to the level of long‐chain branching. For the high‐density resins, the agreement between measured η'(ω,\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}) and G'(ω,\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}) and those calculated using H(τ,\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document}) was not quantitative since the parallel superimposed experiments require an additional term in ∂H(τ,\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ $\end{document})/∂ \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \gamma \limits^ ^2 $\end{document} inside the integral.

Second approximation methods for determining the relaxation time spectrum of a viscoelastic material

Second-approximation methods are given for deriving the distribution function of relaxation times of a viscoelastic material from experimental measurements of stress relaxation after sudden strain, stress relaxation after cessation of steady-state flow, the real and imaginary parts of the complex dynamic rigidity, and the real part of the complex dynamic viscosity. The numerical coefficients required for the calculations are tabulated, and the methods are illustrated by application to experimental data on solutions of polyvinyl acetate.