Birefringence induced by the orientation of polymer main chains during an injection-molding or extrusion processing restricts the application of polymers to optical devices that require focusing by lenses or maintaining the polarization states of incident light. We have proposed the "random copolymerization method" and the "anisotropic molecule dopant method" to prepare zero-birefringence polymers. Negative orientational birefringence of poly(methylmethacrylate) based type polymers was compensated by both the methods. However, polymers which have high glass transition temperature (Tg), such as polycarbonate, generally exhibit positive orientational birefringence that has not been eliminated yet. In this report, we proposed the "birefringent crystal dopant method" to compensate the birefringence of polymers by homogeneous doping with an inorganic birefringent crystal which has needle-like shape. In this method, the crystal needs to have opposite orientational birefringence to that of polymers. Strontium carbonate (SrCO₃) crystal was selected and synthesized for this method, with a particle length of 50-200nm and an aspect ratio of 2-5. SrCO₃ was doped into poly(methylmethacrylate-co-benzylmethacrylate)=78/22(wt./wt.) films with a thickness of approximately 40 μ m. The copolymer films were uniaxially drawn at 130C and 4mm/min. The positive birefringence of heat-drawn copolymer films at a wavelength of 633nm was compensated by doping with 0.3wt.% of SrCO₃. Transmittances of blank film and SrCO3 doped film at a wavelength of 633 nm were 92.6 % and 92.5 %, respectively. Furthermore, the glass transition temperatures of blank film and SrCO3 doped film were 109.4 C and 109.6 C, respectively. These results confirmed that SrCO₃ compensated the positive birefringence of optical polymers without losing transparency and thermostability.

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