Measurement And Characterization Of Extensional Viscosity For Electrospinning Solutions

Electrospinning technology may effectively prepare nanofibers,which are widely used in microelectronic devices such as filters,sensors,and solar cells.During the electrospinning process,the solution has a significant stretching phenomena in space and varies from an initial millimeter-scale hanging drop to deposited micro-nano-fibers.However,it is difficult to investigate the effect of solution extension on electrospinning due to the lack of commercial extensional rheometer to test the extensional properties of solutions.Present test methods of extension viscosity have much obvious defects and cannot meet the requirements of commercial applications.In this paper,an extensional rheometer is developed by ourselves on the base of the principle of uniaxial extensional flow.It is helpful for further study of electrospinning theory in the future.The extensional rheology of both polyethylene oxide（PEO）solution and polyvinylidene fluoride（PVDF）solution,which are commonly used as electrospinning solutions,is measured and characterized by using the extensional rheometer.Experimental data turn out that the self-made extensional rheometer may effectively characterize the extensional viscosity of commonly used electrospinning solutions,such as PEO and PVDF solutions.The measurement range goes beyond the value of 6～450 Pa·s,and the relative error of measurement is about 5%.The Trouton ratio of both PEO and PVDF solution approximates 3 at very low tensile strain rate.This value is in line with the theoretical value,which verifies the correctness of the extensional rheometer.On the other hand,both PEO and PVDF solutions have the characteristics of extension-thinning and shear-thinning.Their Trouton-ratio-strain-rate curves are unimodal curves.The maximum Trouton ratios are 5.5 and 5.9 at critical strain rates of 8 and 10 s^-1for 14%and 20%PEO solutions,and 4.3 and 4.7 at critical strain rates of 7 and 8 s^-1 for10%and 14%PVDF solutions,respectively.The Trouton-ratio-strain-rate curves increase monotonously below the critical strain rate,where the effect of extensional viscosity is more than the one of shear viscosity on continuous flow.Contrarily,the effect of shear viscosity on flow enhances gradually as the Trouton-ratio-strain-rate curves decrease monotonously beyond the critical strain rate.As a result,the experiments show that the self-made extensional rheometer is effective and may measure the extensional viscosity of commonly-used polymer solutions.The research results provide a theoretical basis and practical reference to study the extensional rheology and further electrospinning theory,especially the stretching effect of electrospinning solution and the motion law of electrospinning jet.