Zero-birefringence optical polymers by nano-birefringent crystals for liquid crystal displays

We report a method for compensating the birefringence of optical polymers by doping them with nanometer-size inorganic birefringent crystals. In this method, an inorganic birefringent crystal is chosen that has the opposite birefringence to the polymer and a rod like shape which is oriented when the polymer chains are oriented. As a result, the birefringence of the polymer is compensated by the opposing birefringence of the crystal. Positive orientational birefringence of poly(methylmethacrylate (MMA)-co-benzylmethacrylate (BzMA)) = 78/22 (wt/wt) was compensated by doping with 0.3 wt.% of the smaller strontium carbonate (SrCO₃) crystals with a length of about 200 nm and a width of about 20 nm. The birefringence of the copolymer containing SrCO₃ was almost zero with any draw ratio between 1.0 and 2.2. The polarization state was almost maintained when it passed through the film. On the other hand, we concluded that the size of the larger crystals (about 3.0 μm × about 300 nm) is too large to form an optically isotropic medium with the polymer. In spite of doping with 0.3 wt.% of the smaller SrCO₃ crystal, the transmittance of the doped film with a thickness of 30 μm was almost the same as the undoped one in the visible region. The increase in haze by doping with 0.3 wt.% of the smaller SrCO₃ crystal was 0.1%. Furthermore, the negative birefringence of PMMA was enhanced by the SrCO₃ crystal.