The Optimal Structure-Conductivity Relation in Epoxy-Phthalocyanine Nanocomposites L. J. Huijbregts,* ,†,‡ H. B. Brom, †,‡,§ J. C. M. Brokken-Zijp, †,‡ M. Kemerink, † Z. Chen, †,‡ M. P. de Goeje, ‡,| M. Yuan, †,‡ and M. A. J. Michels †,‡ Technische UniVersiteit EindhoVen, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX EindhoVen, The Netherlands, Kamerlingh Onnes Laboratory, Leiden UniVersity, P.O. Box 9500, 2300 RA Leiden, The Netherlands, and TNO, P.O. Box 6235, 5600 HE EindhoVen, The Netherlands ReceiVed: June 8, 2006; In Final Form: September 6, 2006 Phthalcon-11 (aquocyanophthalocyaninatocobalt (III)) forms semiconducting nanocrystals that can be dispersed in epoxy coatings to obtain a semiconducting material with a low percolation threshold. We investigated the structure-conductivity relation in this composite and the deviation from its optimal realization by combining two techniques. The real parts of the electrical conductivity of a Phthalcon-11/epoxy coating and of Phthalcon- 11 powder were measured by dielectric spectroscopy as a function of frequency and temperature. Conducting atomic force microscopy (C-AFM) was applied to quantify the conductivity through the coating locally along the surface. This combination gives an excellent tool to visualize the particle network. We found that a large fraction of the crystals is organized in conducting channels of fractal building blocks. In this picture, a low percolation threshold automatically leads to a conductivity that is much lower than that of the filler. Since the structure-conductivity relation for the found network is almost optimal, a drastic increase in the conductivity of the coating cannot be achieved by changing the particle network, but only by using a filler with a higher conductivity level. 1. Introduction Insulating polymers can be made semiconducting by adding (semi)conductive filler particles. To maintain the mechanical and processing properties of the matrix, which is preferable for applications, the filler fraction should be as low as possible, while to obtain a (semi)conductive material, the fraction should be above the critical threshold value at which the particles just form a continuous path from one side of the sample to the other. According to percolation theory, the critical filler fraction for spherical particles randomly dispersed in a matrix is 16 vol %, 1 but much lower critical filler fractions (even as low as 0.03 vol %) can and have been obtained for fillers with high aspect ratios and for fillers that form extended fractal aggregates. 2-12 Although intensive research has been done on the electrical conductivity of nanocomposites with low filler fractions, the variety of fillers that has been used is relatively small. The majority of the studies concentrate on carbon black; see, for example, refs 2-7. Other fillers that are becoming increasingly important are carbon fibers, 7,8 carbon nanotubes, 8,9 conjugated polymers, 13,14 inorganic semiconducting nanoparticles, 15 and metal particles. 16 Nanocrystals of aquocyanophthalocyaninatocobalt (III), also called Phthalcon-11, can be used in very low amounts to make insulating thermoplastic and thermoset polymers semiconduct- ing. 11,17 In cured epoxy coatings, the critical filler fraction (φ c ) of Phthalcon-11, measured along the film, decreases with increasing coating thickness, approaching a value of 0.55 vol % for bulk percolation. 12 The conductivity levels of the coatings that have been obtained are a factor g10 5 lower than the intrinsic conductivity of Phthalcon-11. To shed light on the origin of this large difference, a detailed investigation of the relation between conductivity and composite microstructure is needed. The present paper reports on such a study, in which a range of conductivity data is compared and an analysis is made in terms of the microscopic buildup of the conductive filler network. So far, Phthalcon-11/epoxy composites were mainly studied by four-point direct current (DC) measurements at room temperature in the Ohmic regime. 11 Measuring the conductivity as a function of frequency (f ) ω/2π), temperature (T), and electric field (E) gives more information on the morphology of the particle network and the conduction mechanism. We combined these scans with measurements of the local conduc- tion, using conducting atomic force microscopy (C-AFM). C-AFM makes it possible to capture topographic images as well as current-voltage profiles. 18-21 It is often assumed that the large difference between the conductivity of the nanocomposite and that of the filler is caused by insulating layers between filler particles. 22,23 However, we will show that, for the investigated cured 20 wt % (≈12 vol %) Phthalcon-11/epoxy coatings, the filler particles touch and the influence of the matrix is negligible. The difference in the conductivities of the coating and the filler can be rationalized from the structure of the particle network, in which the Phthalcon-11 nanocrystals have aggregated into fractal building blocks with a large fraction organized in conducting channels that percolate the matrix from contact to contact. 2. Experimental Section Phthalcon-11 nanocrystals are very stable, nontoxic, nonir- ritating, and environment-friendly. The synthesis as described * Corresponding author. E-mail: L.J.Huijbregst@tue.nl. Phone: + 31 40 2473059. Fax: + 31 40 2445619. † Technische Universiteit Eindhoven. ‡ Dutch Polymer Institute (DPI). § Leiden University. | TNO. 23115 J. Phys. Chem. B 2006, 110, 23115-23122 10.1021/jp063567w CCC: $33.50 © 2006 American Chemical Society Published on Web 10/28/2006