Electrical and mechanical property transitions in carbon-filled poly(vinylpyrrolidone)
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MATERIALS RESEARCH
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Electrical and mechanical property transitions in carbon-filled poly(vinylpyrrolidone) Jaime C. Grunlan, William W. Gerberich, and Lorraine F. Francis Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 17 February 1999; accepted 27 August 1999)
The effect of carbon black content on the mechanical and electrical properties of carbon-black-filled poly(vinylpyrrolidone) composites was determined. Experimental data show a drop in modulus when the volume of carbon black exceeds 25%, coincident with pore formation documented by scanning electron microscopy. This behavior is consistent with surpassing the critical pigment volume concentration. Electrical conductivity, however, does not show a discontinuous change in behavior at 25 vol% carbon black and continues to increase through a carbon black loading of 35 vol%. A qualitative model of microstructural evolution is presented to explain the observed differences in electrical and mechanical behavior.
Carbon-black-filled polymers are becoming more important in applications requiring the electrical conductivity of metals and the mechanical properties of polymers. For example, antistatic layers,1 electromagnetic interference (EMI) shielding,2 and chemical sensors3 employ carbon-black-filled polymer components. Most matrixfiller systems exhibit a transition in various properties upon reaching an effective critical pigment volume concentration (CPVC). Critical pigment volume concentration is the point at which the polymeric matrix just covers all of the filler particles and fills the interstices between them.4 The ultimate (or true) CPVC is reached when all of the particles are close-packed with no regions of random agglomeration present.5 Since a perfect dispersion of filler would be almost impossible to achieve, CPVC is typically reached on a local scale. Many particulate composites exhibit discontinuous transitions in optical, mechanical, and transport properties upon reaching CPVC.6 These transitions are attributed to the formation of voids when filler concentrations exceed the CPVC.5 This report demonstrates that the electrical conductivity of carbon-black-filled polymers does not exhibit a transition at the CPVC and may in fact be slightly enhanced by the presence of voids. Poly(vinylpyrrolidone) (PVP) from Aldrich was used as the matrix material for these composites. PVP is water-soluble and glassy at room temperature (Tg ∼ 175 °C) and has a density of 1.25 g/ml and a weight average molecular weight (Mw) of 55,000. Conductex 975 Ultra carbon black (provided by Columbian Chemicals Co.) acted as the conductive filler. Conductex carbon black has a 21-nm primary partice size, a 1.89 g/ml density, and a nitrogen surface adsorption (NSA) surface 4132
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 11, Nov 1999 Downloaded: 18 Mar 2015
area of 242 m2/g. Composites were prepared by dispersing Conductex powder into a 20 vol% PVP–water solution using
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