Universal Concentration Scaling And Computational Simulation On Rheological Properties Of Polymer Solution

Polydisperse and high molecular weight polymer solutions are widely used in mechanical engineering,especially in mass transfer,heat transfer and transmission.However,mechanical engineering problems are solved through empirical trial-and-error methods,resulting in the increase of the number of experiments,and it is difficult to form engineering scientific methods.Therefore,using rheological characterization and computational simulation methods to study the dynamic properties of polydisperse and high molecular weight polymer solutions,using scientific methods instead of empirical methods,is conducive to the formation of scientific theories.In this paper,the linear and nonlinear viscoelastic properties of polydisperse and high molecular weight polyacrylamide aqueous solution were systematically studied.The results show that the characteristic relaxation time and zero-shear viscosity in the semi-diluted concentration range show a universal concentration scaling,and the concentration interval at8<c/c*<10 is the transition region from semi-dilute unentangled region to semi-dilute entangled region.It is found that the linear viscoelastic property of semi-dilute entangled polymer solutions measured under small-amplitude oscillatory shear flow can be superimposed into a main curve,indicating the applicability of the time-concentration superposition principle.The steady-state shear viscosity and shear stress of 18M PAAm aqueous solutions under linear flow conditions can be superimposed into the main curve,whereas the steady shear viscosity and shear stress of lower concentration(c/c*<35)polymer solutions exhibit a universal power law index of 0.73±0.03 and 0.27±0.03,respectively.However,for higher concentrations of polyacrylamide aqueous solution(c/c*?48c*),the power-law index is concentration-dependent.With increasing concentration,the steady-state shear viscosity decreases from 0.73 to 0.57 at lower shear rates and the index increase from 0.57 to 0.91 with increasing shear rate.When the shear rate is further increased,the shear thinning index reproduces the power-law index of the lower concentration(c/c*<35),with a steady-state shear viscosity of 0.73±0.08 and a steady-state shear stress of 0.27±0.08.The results show that the kinetic mechanism of"molecular individualism"of polydisperse polymer solutions under strong flow fields dominates and the influence of their rheological properties is decreased.It is well known that the steady-state shear stress of highly entangled flexible chain molecular solutions hardly increases with the applied shear rate,which is a universal feature of shear banding phenomenon.With the increasing concentration of polyacrylamide solution,the curve of steady-state shear stress vs shear rate showed universal characteristics,which similar to the property of shear banding.In this paper,the computational simulation of the shear banding is conducted based on the Rolie-Poly model,which is chain entanglement molecular configuration model.The calculation results found that when the steady-state shear stress vs shear rate curve is weakly dependent,transient shear banding phenomenon can be occurred.The larger the(2₄)number,the longer the presence of the transient shear banding.