Highly refractive, heat-resistant BaTiO3 nanocomposite films were fabricated via in situ polymerization to homogeneously disperse barium titanate (BT) nanoparticles into polyimide (PI) matrix. BT nanoparticles surface-modified with O-phosphorylethanol phthalimide (PPHI) were employed to the in situ polymerization in which condensation reactions of a diphthalic anhydride and a diamine were conducted to form the prepolymer of poly(amic acid) (PAA) that was thermally imidized in the following step. The nanoparticles surface-modified were added to PAA solution at different times in the polymerization to examine the effect of PAA molecular weight on the refractive index (RI) of the nanocomposite films, which indicated that relatively low molecular weights (<10,000) of PAA formed at the point of nanoparticle addition was appropriate for enhancement of nanocomposite RI. An additional treatment of chemical imidization using acetic acid anhydride and pyridine, which was followed by the thermal imidization, was performed to examine the effect of polyimide structure on RI of nanocomposite films. The RI of nanocomposite films with excellent thermal stability could be successfully enhanced to n = 1.88 by the chemical imidization.
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