Evaluation of Elastic Modulus of Polymeric Materials Using Nanoindentation

Abstract

The elastic modulus of polymeric materials has significance when its dependence on time, temperature and several environmental conditions is identified. Among them, time-dependency, known as the viscoelasticity, has been widely studied by many researchers. To clarify the time dependency of elastic modulus, evaluation of elastic modulus at various rates is needed. Nanoindentation technique is one of the useful methods to evaluate the viscoelastic properties along with tensile test and dynamic mechanical analysis (DMA). This technique has its advantage in that it is literally able to evaluate the material in small scale. In nanoindentation, the elastic modulus is determined by unloading curve which represents the materials nature to return to its original state. In case of viscoelastic materials, however, the material tends to move toward the loading direction during the unloading process and thus it distorts the shape of load-displacement curve. This is called as nose because it has the convex shape in the unloading part of the curve. As a result, the slope of unloading curve, so called stiffness, changes and so does the elastic modulus. This is caused by viscous behavior of the material which is induced by the applied force during the loading process. Therefore, in order to achieve the goal of obtaining the elastic modulus at various rates, this study presents experimental parameters and conditions in which viscous effect on unloading curve can be effectively reduced at the slow unloading rates. In addition, we investigate the criterion which can be used to discriminate whether the viscous effect on the unloading curve is eliminated or not. Based on this criterion, the modulus of elasticity of the polymeric material will be evaluated at various unloading rates under the criterion that the viscous effect disappeared and compared to the results of the conventional experimental condition. Finally, for verification, the comparison with the elastic modulus determined by tensile test will be carried out.