Article pubs.acs.org/Macromolecules
Cross-Linked Liquid Crystalline Polyimides with Siloxane Units: Their Morphology and Thermal Diffusivity Yu Shoji,† Ryohei Ishige,‡ Tomoya Higashihara,†,§ Junko Morikawa,† Toshimasa Hashimoto,† Atsushi Takahara,‡ Junji Watanabe,† and Mitsuru Ueda†,* †
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 H-120, O-okayama, Meguro-Ku, Tokyo 152-8550, Japan ‡ Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § PRESTO, Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: Cross-linked liquid crystalline (LC) polyimides with siloxane units have been developed as high thermally conductive materials. The relationship between their thermal diffusivities and film morphologies was investigated. The crosslinking reaction of the LC polyimides was monitored by Fourier transform infrared spectroscopy and differential scanning calorimetry. It was found that the cross-linked LC polyimides successfully maintained their LC structures at room temperature as confirmed by polarized optical microscopy and wide-angle X-ray diffraction (WAXD). The thermal diffusivity of the films in the thickness direction measured by a temperature wave analysis method increased gradually from 0.116 to 0.185 mm2 s−1 with the increasing extent of the crosslinking. In the detailed WAXD study, the obtained cross-linked LC polyimide films showed that the polymer chains vertically aligned in the thickness direction of the films, and the increase in the extent of the cross-linking expanded the areas of chain alignment. Such a chain alignment plays an important role in the phonon conduction in the thickness direction of the films.
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INTRODUCTION The development of electronics is currently accelerating toward integration, miniaturization, and functionalization in which both the conductors and insulating materials in the integrated circuits have been packaged as densely as possible. However, if the insulating materials possess a low thermal conductivity, a substantial amount of generated heat would be accumulated inside the devices, which gives rise to electric connection failure and reduces the lifetime of these devices. Therefore, the high thermally conductive insulating materials have recently become attractive materials for effectively releasing the generated heat from these densely packaged electronics devices. There are several reports about composite insulating materials with a high thermal conductivity by filling the organic polymer matrices with high thermally conductive inorganic fillers to improve the thermal conductivity of the insulating materials.1−7 However, a large amount of fillers leads to a low processability of the composites due to increasing the melt or solution viscosity. In addition, the adhesion ability between the composite materials and substrates becomes poor, and a material surface roughness is induced with increasing weight ratio of the fillers. Therefore, the amount of the fillers should be decreased to as low as possible to maintain such processing properties, without sacrificing the thermal conductivity. To predict the thermal conductivity of the © 2013 American Chemical Society
composites, there are many established models for the composite systems.8,9 According to Bruggeman’s theoretical model,8,9 filling more than 70 vol % fillers with the thermal conductivity of 30 W m−1 K−1 in polymer matrices with one of 0.2 W m−1 K−1 produces a composite with the thermal conductivity of ca. 5 W m−1 K−1, while the thermal conductivity of the composite prepared from less than 50 vol % fillers (30 W m−1 K−1) with polymer matrices (1 W m−1 K−1) would also be ca. 5 W m−1 K−1. Therefore, the development of high thermally conductive polymers is crucial in order to achieve the high thermally conductive composites containing a small amount of fillers (
