Preparation and characterization of colorless and transparent semi‐alicyclic fluoro‐containing polyimide nanocomposite films with enhanced high‐temperature dimensional stability via incorporation of colloidal silica nanofillers

Abstract Colorless and transparent semi‐alicyclic fluoro‐containing polyimide (PI) nanocomposite films with enhanced high‐temperature dimensional stability and sustained optical transparency were designed and prepared via incorporation of nano‐sized colloidal silica (SiO 2 ) fillers into the PI matrix derived from hydrogenated pyromellitic dianhydride (HPMDA) and 2,2′‐bis[(4‐aminophenoxy)phenyl]hexafluoropropane (BDAF). The derived PI composite films, abbreviated as 6FCPI‐X in which X stands for the weight percent of the SiO 2 in the composite films, exhibited superior thermal and comparable optical properties to the pristine 6FCPI‐0 (6FDA‐BDAF) matrix film. For example, as for the thermal properties, 6FCPI‐35 composite film showed a glass transition temperature ( T g ) of 279.1°C and a residual weight ratio at 750°C ( R w750 ) of 66.1%, which were apparently higher than those of 6FCPI‐0 matrix ( T g = 272.2°C; R w750 = 47.1%). More importantly, the 6FCPI/SiO 2 nanocomposite films showed obviously lower linear coefficients of thermal expansion (CTE) values as compared with that of the 6FCPI‐0 matrix. 6FCPI‐35 composite film showed a CTE value of 36.1 × 10 −6 /K in the temperature range of 50–250°C, which was lower than half of that of the 6FCPI‐0 matrix (CTE = 77.9 × 10 −6 /K) in the same range of temperature. In addition, incorporation of nano‐sized colloidal SiO 2 particles basically maintained the intrinsically good optical properties of the pristine 6FCPI‐0 matrix film. 6FCPI‐35 composite film showed the optical transmittances of 81.7%, 85.3%, and 87.2% at the wavelength of 400 nm ( T 400 ), 450 nm ( T 450 ), and 500 nm ( T 500 ), respectively, which were in the same level with those of the 6FCPI‐0 film ( T 400 = 84.0%; T 450 = 86.3%; T 500 = 87.6%). The 6FCPI series of nanocomposite films also exhibited similar CIE Lab color parameters, including the yellow indices ( b *) lower than 2.0 and haze values lower than 1.0%.