Porous Polyimide-Silica Composite: A New Thermal Resistant Flexible Material
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Porous Polyimide-Silica Composite: A New Thermal Resistant Flexible Material Yumeto Fukubayashi1 and Satoshi Yoda2 1
Unitika LTD, 23 Uji Kozakura, Uji, Kyoto 611-0021, Japan Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan 2
ABSTRACT We developed a new highly porous polyimide (PI) -silica composite with high flexibility, mechanical strength, and heat resistance. The composite was prepared by a new process consisting of (1) phase separation of a mixture of PI precursor (polyamic acid), solvent, and silicon alkoxide, induced by high-pressure CO2 (40 ºC, 20 MPa), (2) silicate formation by sol-gel reaction, and (3) supercritical CO2 extraction of the solvent. The composite had a bimodal porous structure with micropores of 10-30 μm and nanopores of ~50 nm. In the PI matrix, silica nanoparticles (< 100 nm in diameter) were highly dispersed. Porosity of the composite was 78%, which is higher than that of conventional porous PI prepared by physical foaming technique. Relative dielectric constant of the material was lower than 1.4 at 1 MHz. The porous PI-silica composite sheet was flexible enough to be folded without cracking. Notably, the Young’s modulus (0.80 GPa) and the onset decomposition temperature (600 ºC) of the PI-silica composite were higher than those of conventional porous PI with similar porosity, respectively. The porous PI-silica composite is promising as a flexible thermal insulator for high-temperature use and as a thermal resistant low-k material.
Figure 1. A flexible, thermal resistant porous polyimide (PI) -silica composite.
INTRODUCTION Polyimide (PI) has high heat resistance up to 500 ºC, excellent mechanical properties, and good chemical stability. Porous PI is expected to be a flexible porous material having many advantages over conventional PI, and is promising as low-k materials, filters, and covering materials. Recently, applications of porous PI to flexible thermal resistant material at high
temperature, which is applicable to a kind of engines, batteries, steam or hot fluid lines and heat devices, have attracted significant interest. A variety of preparation processes for porous PI have been reported, such as (a) foaming by carbon dioxide (CO2) [1], (b) dispersion of solvent droplets in the PI matrix followed by extraction using supercritical CO2 [2], (c) foaming by steam [3], (d) preparation of silicaparticles / PI composite followed by dissolution of silica by hydrofluoric acid [4]. Both highly porous structure and good mechanical strength are required especially for thermal insulating materials. However, these two are generally competitive. We have developed a new fabrication process for porous polymer-silica composite. This method involves (1) a homogeneous mixture of polymer, silicon alkoxide (Si (OR)4), and CO2 under high pressure and (2) multiple phase separations of the mixture induced by a pressure drop [5]. We have succeeded to prepare polymer foam-silica composites w
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