Synthesis of a Zero-Birefringence Optical Polymer by the Birefringent Crystal DopantMethod
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Synthesis of a Zero-Birefringence Optical Polymer by the Birefringent Crystal Dopant Method Hisanori Ohkita1, 2, Masaki Mukoh1, Akihiro Tagaya2 and Yasuhiro Koike1, 2 1 Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. 2 ERATO Koike Photonics Polymer Project, Japan Science and Technology, K2 Town Campus E-building, 144-8, Ogura, Saiwai-ku, Kawasaki 212-0054, Japan. ABSTRACT Birefringence induced by the orientation of polymer main chains during an injectionmolding or extrusion processing restricts the application of optical polymers to optical devices that require maintaining the polarization state of incident light. We propose the “birefringent crystal dopant method” to compensate the birefringence of polymers by homogeneous doping with an opposite birefringent needle-like crystal. Strontium carbonate (SrCO3) was selected for this method and synthesized, with particle lengths of 50-200nm and aspect ratios of 2-5. SrCO3 was doped into poly(MMA/BzMA= 78/22(wt./wt.)) film. The film was uniaxially drawn at 130°C and 4mm/min. For the first time, the positive birefringence of the drawn copolymer film at a wavelength of 633nm was compensated by doping with 0.3wt.% of SrCO3 without losing transparency and thermostability. INTRODUCTION Optical devices that can transmit and store mass information are indispensable to the recent highly information-oriented society. Many optical devices based on polymer materials have been widely studied for the recent optical technology, for example, polymer optical fibers [1, 2], optical films for liquid-crystal displays, optical disks [3], lenses [4] and optical waveguides5. Polymers have the advantage of easy processing, easy handling, light weight, and low cost as compared with glasses. However, polymers have one serious drawback: they are birefringent. Birefringence degrades the performance of optical devices that require focusing by lenses or maintaining the polarization state of incident light. Birefringence does not occur if polymer main chain orientations are perfectly random without any stress. One of the main sources of birefringence development is the orientation of polymer main chains during a melt flow in injection-molding, extrusion or drawing processing. During the cooling stage in such processing, the orientation of polymer chains does not completely relax and causes orientational birefringence. In order to prepare zero-birefringence polymers, we proposed two methodsthe random copolymerization method [6, 7] and the anisotropic molecule dopant method [8, 9]. Two types of monomers that compose positive and negative birefringent polymers, respectively, were randomly copolymerized. A rod-like molecule that has a polarizability anisotropy was chosen and doped into polymers in the anisotropic molecule dopant method. The negative birefringence of poly(methyl methacrylate) (PMMA) based polymers was compensated by both the methods. Compensating the birefringence of conventional polymers that have high glass transition tem
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