Modulus Determination of Polymer Matrix Composites: Comparison of Nanoindentation and Dynamic Mechanical Analysis
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Modulus Determination of Polymer Matrix Composites: Comparison of Nanoindentation and Dynamic Mechanical Analysis Jaime C. Grunlan,1 David Rowenhorst,2 Lorraine F. Francis,3 and William W. Gerberich3 1 Department of Chemical Engineering and Materials Science, University of Minnesota 151 Amundson Hall (Box 69), 421 Washington Ave SE, Minneapolis, MN 55455, U.S.A. 2 Materials Science and Engineering Department, Northwestern University 2225 N. Campus Drive, Evanston, IL 60208, U.S.A. 3 Department of Chemical Engineering and Materials Science, University of Minnesota 151 Amundson Hall, 421 Washington Ave SE, Minneapolis, MN 55455, U.S.A. ABSTRACT The results of modulus measurements, on carbon black-filled poly(N-vinylpyrrolidone), using dynamic mechanical analysis and nanoindentation were compared. It was shown that beyond the critical pigment volume concentration for this composite system (~ 25 vol% carbon black), the storage modulus, obtained with dynamic mechanical analysis, decreased with increasing filler concentration. This dropping modulus was due to porosity that developed in the composite films when the critical pigment volume concentration had been exceeded. Elastic modulus obtained with nanoindentation showed the opposite trend, with modulus increasing with additional carbon black loading. An analysis of the method used to calculate modulus based upon indentation data was performed and a mechanism was proposed to explain the disparity between the moduli obtained using these two different methods of measurement. INTRODUCTION The elastic modulus (E) of polymeric materials can be determined using a variety of different test methods. Nanoindentation techniques have recently become more accepted for obtaining the modulus of polymeric materials. Challenges have been encountered when trying to extrapolate useful information from nanoindentation data due to the viscoelastic nature of the polymer samples [1]. In most cases, the elastic modulus calculated for various polymers based upon nanoindentation data are elevated 20 – 30% relative to values obtained with more standard tests [1, 2]. This larger E is believed to be due to tip-sample adhesion that effectively increases the contact area of the tip during indentation and can be avoided by using a tip material that exhibits weaker adhesion [3]. Further complications are introduced when the polymeric sample to be indented is a particulate-filled composite that is heterogeneous on a microscopic scale. In this case it is imperative to use a tip large enough to probe an area of the sample that is representative of the material as a whole. Another variable to consider is the critical pigment volume concentration (CPVC) that marks the amount of filler at which the polymer can no longer sufficiently bind all of the particulate and fill the interstitial spaces and voids begin to form. These issues are explored in the following analysis of carbon black-filled poly(Nvinylpyrrolidone) (PVP). Dynamic mechanical analysis and nanoindentation were used to obtain elastic modulus valu
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