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A facile technique was developed to improve the water barrier properties of transparent polyimide (PI) films. Transparent and organo‐soluble PI films were synthesized from an alicyclic tetracarboxylic dianhydride (bicyclo[2.2.2]oct‐7‐ene‐2,3,5,6‐tetracarboxylic dianhydride) and an aromatic diamine (3,4′‐oxydianiline) in a co‐solvent of dimethylacetamide (DMAc) and γ‐butyrolactone via a one‐step process. Thermally reduced graphene (RG) was then blended with the PI in DMAc solution to fabricate PI/RG nanocomposite films without the addition of coupling agent. With the incorporation of only 0.1 wt% highly exfoliated RG in the PI matrix, the resultant PI/RG‐0.1 nanocomposite exhibited a superior barrier to moisture and retained high transmittance in the visible light region. The surface of PI/RG was more hydrophobic than that of pure PI and simultaneously the water vapor transmission rate was significantly reduced to 13 g m−2 day−1 for the PI/RG‐0.1 nanocomposite compared to 181 g m−2 day−1 for pure PI. Notably, the PI/RG‐0.1 nanocomposite also exhibited favorable thermal stability with a lower coefficient of thermal expansion and a higher thermal degradation temperature compared to pure PI. The easy processing of PI solution and RG nanosheets, the good orientation of RG in PI and the excellent barrier and thermal properties of PI/RG make the transparent PI nanocomposite films potential substrate materials in flexible electronic applications.

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