Optically transparent and thermo‐mechanically stable nanocomposite films of polyethersulfone (PESU) and core–shell nanoparticles are prepared by a solvent casting and heat pressing method. As a filler, reactive ZnS‐SiO2 core–shell nanoparticles are prepared by a modified sol–gel process, of which reactive aromatic amine groups are chemically bonded with the polymer matrix. For optical transparency, the shell thickness is adjusted and the core diameter of the core–shell nanoparticles is adjusted to match the refractive index of PESU (nPESU = 1.65). For the composite film with more than 15 wt% of core–shell nanoparticles in PESU, the coefficient of thermal expansion is significantly reduced down to 12 ppm °C−1 in the temperature range of 30–90 °C, which is 80% less than the value of the bare PESU film (≈65 ppm °C−1). Furthermore, their mechanical property is also significantly improved. For 15 and 30 wt%, PESU nanocomposite films show 64 and 74 MPa of yield strength and 1.36 and 2.0 GPa of Young's modulus, respectively, while pure PESU films show only 44 MPa of the yield strength and 0.64 GPa of Young's modulus. Finally, the organic light emitting diode device is demonstrated on the PESU nanocomposite substrate.

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