Anisotropy in optical transmittance and molecular chain orientation of silver‐ dispersed uniaxially drawn polyimide films

The anisotropy in optical transmittance in the visible and near‐infrared region observed for uniaxially drawn and silver‐dispersed polyimide (PI) films was investigated. The films were prepared in a one‐step operation that consists of thermal curing and simultaneous uniaxial drawing of poly(amic acid) (PAA) films dissolving 5.7 ∼ 20 mol% of silver nitrate. The PAA was converted to PI by heating, and the PI chains were orientated along the drawing direction during curing. Silver nanoparticles were precipitated in the films when they were cured in air and under nitrogen. In particular, silver nanoparticles aggregated along drawing direction and spheroidal nanoparticles (size of longer axis: 10–25 nm, aspect ratio: ca. 1.5) were observed in the films cured in air, and distinct anisotropy in optical transmittance was observed. The maximum optical anisotropy was obtained with a specific holding time at the final curing temperature (320 °C). In addition, the anisotropy can be controlled by polymer chain orientation when films are cured with the optimal holding time. In optimized preparing conditions, anisotropies in transmittance larger than 500 : 1 were obtained at the wavelengths between 700 and 900 nm, and its optical properties were retained after annealing at 150 °C for 1 hr. The PI films thus obtained can be used as thermally stable thin‐film polarizers.