Enhanced Thermal Diffusivity by Vertical Double Percolation Structures in Polyimide Blend Films Containing Silver Nano particles a Daisuke Yorifuji, Shinji Ando* Introduction Polyimides (PIs) have been widely used in electronic applications such as interlayer dielectric films and sub- strates for flexible printed circuit boards because of their high thermal stability, good mechanical properties, and favourable electrical properties. However, the thermal conductivity of PIs are low, as well as that of commonly used polymers. Therefore, polymer composites containing mm-size ceramic fillers, such as Al 2 O 3 , AlN, and BN, have been studied to enhance their thermal conductivity. [1–3] Such composite films or sheets with high thermal conductivity, especially in the direction perpendicular to the film surface, are increasingly important for electronic packaging and IC chips because their heat dissipation ability limits the reliability, performance, and miniaturiza- tion of advanced electronic circuits. In the case of polymer composites containing mm-size fillers, the thermal conduction mechanism, explained by the established percolation theory, has been widely accepted. [4] Nevertheless, the high volume fraction (gen- erally > 30 vol.-%) of fillers, which is necessary to achieve the percolation threshold, presents a number of limitations, in terms of heavy weight, low flexibility, and poor mechanical performance, as a result of the weak poly- mer/filler interfacial adhesion and agglomeration of fillers. To overcome the limitations of polymer composites, studies of polymer nano-hybrids containing nano-particles, such as nm-size SiC and silver nano-particles (Ag-NPs), have been carried out. [5,6] However, the thermal conductivity of such nano-hybrids containing homogeneously dispersed nano- particles is lower than expected due to high thermal Full Paper S. Ando, D. Yorifuji Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan Fax: þ81-3-5734-2889; E-mail: sando@polymer.titech.ac.jp a : Supporting information for this article is available at the bottom of the article’s abstract page, which can be accessed from the journal’s homepage at http://www.mcp-journal.de, or from the author. The thermal diffusivity along the out-of-plane direction (D ? ) in polyimide (PI) blend films containing silver nano particles (Ag-NPs) was investigated. PI blend films composed of a sulfur- and a fluorine-containing PI were prepared via spin-coating and thermal curing of the precursor solutions dissolving silver nitrate. Micro- phase-separated structures with a ‘‘vertical double percolation (VDP)’’ morphology were spontaneously formed in the films, in which two phases are separ- ately aligned along the out-of-plane direction, and Ag-NPs were preferentially precipitated in the sul- fur-containing PI phase. The blend films exhibited higher D ? values than monophase PI films containing homogeneously dispersed Ag-NPs. These results indicate that the VDP structure functions as an effec- tive thermal conductive pathway. 12 14 16 18 20 22 24 0 20 40 60 80 Thermal diffusivity D ⊥ (10 -8 m 2 /s) AgNO 3 content (mol-%) Vertical double percolation Homogeneously dispersed PI blend films Homo-PI films 2118 Macromol. Chem. Phys. 2010, 211, 2118–2124 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com DOI: 10.1002/macp.201000294