Using Superposition Technique to Study Effects of Stress History on Viscoelastic Behaviour of Borate Cross-Linked Guar Solutions(ABSTRACT)

摘要

Effects of stress history on viscoelastic behavior of a borate cross-linked guar solution have been investigated using the technique of superposition of oscillations upon steady shear flow.This technique has been recently used to probe the structure of associative polymers during flow and after the cessation flow and to examine the validity of constitutive equations.In this work,the superposition experiments are performed by applying a steady shear stress onto the cross-linked guar solutions.After the steady shear condition is reached,a small amplitude oscillatory measurement is imposed on the sample (0.3 wt% aqueous crosslinked guar solutions).By superimposing small amplitude oscillations on the shear flow at constant stress,the shear-induced network structure of the polymer gel is unperturbed and linear viscoelastic properties of the gel under the applied stress conditions can be obtained.At applied stress of 10 Pa and above,both the storage (G′) and loss modulus (G″) of the gel sample are greatly reduced at low frequencies.Based on Maxwell relaxation time analogy (λ_(max) ),an estimate of the relaxation time of the cross-linked polymer at Max different initial stress conditions can be made.The results show that the relaxation time is reduced by up to 2 orders of magnitude as the imposed stress is increased from 10 to 70 Pa.