Measurements of Viscoelastic Properties of SWNT/Polymer Composite Films Using Nanoindentation
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Measurements of Viscoelastic Properties of SWNT/Polymer Composite Films Using Nanoindentation Hongbing Lu, Gang Huang, Bo Wang School of Mechanical and Aerospace Engineering Oklahoma State University Stillwater, OK74078 Arif Mamedov, Sachin Gupta Nomadics, Inc. Stillwater, OK 74074
Abstract Methods for measuring the mechanical properties of linear viscoelastic materials in both time and frequency domains have been presented using the nanoindentation technique. In the time domain, the viscoelastic functions of materials were measured through the direct differentiation method using the load-displacement curve or the material parameter extraction method by fitting the loaddisplacement curve. In the frequency domain, the complex creep functions of materials were measured in terms of dynamic load-displacement data under a harmonic loading superimposed upon a ramp loading. As an application, these methods were used to determine the material properties for single-wall carbon nanotube (SWNT)/polyelectrolyte mutilayer films and the neat resin film made of polyelectrolyte under nanoindentation tests. The uniaxial relaxation moduli as a function of time for both SWNT/polymer composite films and the neat resin film have been obtained from quasi-static nanoindentation tests. The complex compliance as a function of frequency for SWNT/polyelectrolyte composite films has been obtained from dynamic nanoindentation tests.
1. Introduction With the discovery of the excellent mechanical properties of single-wall carbon nanotube (SWNT), fabrication of SWNT/polymer composites has received increasing attention. SWNT/polymer composites prepared by typical hybrid process, such as blending, polymerization, extrusion and surface modification, have such problems as poor matrix-SWNT connectivity and phase segregation, leading to the premature mechanical failure. Mamedov et. al.1 used the layer-by-layer assembly (LBL) technique to fabricate SWNT/polymer composites, and successfully mitigated the problems of connectivity and phase segregation. In this study, layer-by-layer assembly is used to prepare SWNT/polymer nanocomposite films. In the material development phase, the amounts of materials obtained from LBL technique are small, this leads to difficulty in measuring the mechanical properties of SWNT/polymer composite films using the conventional testing techniques, such as tensile tests and compression of stacking films. The nanoindentation technique should provide an effective approach to measure the properties of small amounts of materials such as SWNT/polymer composite films. Methods for measuring the Young's modulus have been very well established for time-independent materials. Based on the assumption that unloading in the load-displacement curve induces only elastic recovery, Oliver and
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Pharr2 proposed a method to determine the basic material properties such as Young’s modulus. The method is based on Sneddon's solution3 for the relationship between the load and displacement for an axisymmetric indenter indenting into a
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