Preparation of Polyimide by Gel Method
Organic-inorganic nanocomposites offer significant advantages in enhancing the heat resistance, mechanical properties, and dimensional stability of materials. Incorporating inorganic materials on a nanometer scale into polyimide (PI) is an effective approach to further improve its heat resistance and high-temperature dimensional stability. Among these materials, SiO2 stands out due to its exceptionally low coefficient of thermal expansion. The research on polyimide/silica (PI-SiO2) hybrid membranes began in the early 1990s, and extensive studies have been conducted on their preparation and properties.
The most commonly used method to prepare this hybrid material is the sol-gel method. This process involves decomposing the precursor of an inorganic silicon source to synthesize a sol under specific conditions, followed by the transformation of the solution or sol into a three-dimensional reticulated inorganic oxide gel after solvent volatilization or heating. Polyimides containing silicon prepared by this method show varying degrees of improvement in their thermal and mechanical properties, as well as changes in their electrical properties. Notably, PI-SiO2 hybrid membranes exhibit significantly higher gas permeability at room temperature compared to bulk polyimide membranes, making them promising candidates for applications in material dehydration and dehumidification processes.
Hybrid materials with a low content of SiO2 (not exceeding 10%) exhibit higher thermal stability, improved mechanical properties, and lower linear expansion coefficients than pure polyimides while maintaining polyimide's photosensitivity. The preparation of hybrid membranes under acidic conditions has a more significant impact on the glass transition temperature compared to alkaline conditions, but both versions show good gas permeability and hydrophilicity. Their separation coefficients at specific pressures exceed the theoretical values of Nussen diffusion. However, achieving satisfactory results for hybrid materials with high SiO2 content has proven challenging.
Researchers are continuously enhancing the sol-gel method to increase SiO2 content and reduce phase separation size. Techniques such as reacting polyamide acid with triethylamine to obtain the salt of triethylamine and then carrying out the sol-gel reaction with methanol as the solvent have significantly improved the morphology and phase separation size of the hybrid membrane, resulting in a transparent film containing 20% SiO2. Furthermore, using functionalized polyamide acid prepared from diamine containing ethoxysilane can form a chemical bond with SiO2, enabling the production of transparent membranes even with a SiO2 content of 70%.
By using phenyldiethylsilane (PTEOS) instead of triethoxysilane (TEOS), researchers have obtained a transparent film with a SiO2 content of 45% and good compatibility. Additionally, the addition of a small amount of a coupling agent during the sol-gel process has proven effective in enhancing the compatibility of the two phases.
In conclusion, the preparation of polyimide/silica hybrid materials through the sol-gel method offers promising advancements in enhancing material properties. Researchers continue to explore innovative approaches to increase SiO2 content and improve compatibility, leading to exciting possibilities in various industrial applications.
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