Constant Extensional Strain Rate Measurement Technology Of Extensional Rheological Properties Of Dilute Solutions And Its Application

Solution viscosity is a very important parameter in rheology,and the rheological property of the solution is often characterized by the relationship between viscosity and strain rate.At present,commercial rheometers are mainly used to test the shear rheology of fluids,while the extensional rheology of fluids（especially dilute solutions）is very difficult to measure.Therefore,an extensional rheometer was developed to characterize the uniaxial extension viscosity of dilute solutions.The relative measurement error of the rheometer is5%.In this paper,the device is used to test the extension rheological properties of typical solutions with various shear rheological properties,including the Newtonian glycerol,dilatant starch solution,and pseudoplastic PVDF/DMF solution.When the extensional strain rate is in the range of 35 s^-1,the glycerol has constant extension viscosity with increasing strain rate.The measured Trouton ratio defined as the ratio of extension viscosity to shear viscosity is about 3,which is in line with the theoretical value for a Newtonian fluid.Thus,the homemade extensional rheometer is feasible to test the extension viscosity.The extension viscosity of dilatant starch solution increases gradually with increasing extensional strain rate.The extension viscosities of 45%,50%,and 55%of starch solutions have are 4.87×10^-2,1.25×10^-1,and 3.37×10^-1 Pa·s at an extensional strain rate of 2.0 s^-1,and 1,2.12,and 6.60 Pa·s at an extensional strain rate of 34 s^-1,respectively.The Trouton ratios increase from 2.64,2.96,and 3.59 to 19.18,13.32 and 7.61 with increasing extensional strain rate,respectively.The results indicate that the extensional thickening effect of starch solution is greater than the shear thickening effect.The extension viscosity of pseudoplastic PVDF/DMF solutions decreases with increasing extensional strain rate.The extension viscosities of 10%,12%,and 14%PVDF/DMF solutions are 28.38 Pa·s,72.93 Pa·s,and 147.90 Pa·s at a extensional strain rate of 2.0 s^-1,and 4.44 Pa·s,12.68 Pa·s and 26.83 Pa·s,at a extensional strain rate of 34 s^-1,respectively.With increasing extensional strain rate,the Trouton ratios increase from 2.82,2.95,and 2.98 to 4.72,4.99,and 5.16,and then decrease to 2.89,3.08,and 3.30,respectively.The results show that the extensional thinning effect of PVDF/DMF solutions firstly increases and then decreases by comparison with the shear thinning effect.On the other hand,the extensional relaxation time may characterize the viscoelasticity of PVDF/DMF solutions.The extensional relaxation times of 10%,12%,and 14%PVDF/DMF solutions are 80.91,88.18,and 96.90 ms,respectively.A linear relationship between extensional relaxation time and polymer solution concentration is obtained.Further,PVDF/DMF solution is used to fabricate nanofiber membrane by electrospinnibng.The solution produces a uniaxial extension flow in the tangential direction in space under applied high voltage electric field.Among 10%,12%and 14%PVDF/DMF solutions,the 12%PVDF/DMF solution may produce the most uniform nanofibers,whose extension strain goes up to 88.77%,with the tensile strength of 3.4 MPa.Meanwhile,the polarization and stretching effects of high-voltage electrostatic field on PVDF may effectively convert theαcrystalline intoβcrystalline.The 14%PVDF/DMF solution shows the ability to prepare nanofibrous membranes with moreβcrystalline.The results are helpful to promote the commercialization of the extensional rheometer,know the extensional rheological properties of fluids,and understand the extensional flow phenomena in industrial production processes.They may provide a certain theoretical basis for the establishment of constitutive relation.