Several kinds of optical polymer films in liquid crystal displays (LCDs) greatly contribute to image quality. However, contrast ratio of LCDs degrades with an increase in birefringence of polymer films used in the LCDs. The major types of birefringence are orientational birefringence and photoelastic birefringence. Furthermore, it was recently found that the birefringence caused by specified conformation and degree of orientation of polymer chains strongly depends on the temperature. We designed and synthesized a temperature-independent zero-birefringence polymer (TIZBP): poly(methyl methacrylate/benzyl methacrylate/phenyl methacrylate = 45/22/33(wt.%)) that has zero intrinsic birefringence over a wide temperature range in the random copolymerization method. However, the glass transition temperature Tg of this TIZBP is 95°C, and heat-drawn films of this TIZBP shrank at lower temperature than Tg, for example, at about 60°C. Upper limit of operating temperature of LCDs in automobile is higher than 60°C. Therefore, this TIZBP is not proper for practical use in automobile.
In this research, after analysis of heat shrinkage of heat-drawn films, heat-resistant TIZBP (HRTIZBP) that exhibits sufficiently low heat shrinkage at 60°C, zero intrinsic birefringence and zero temperature coefficient of intrinsic birefringence was designed and synthesized. First of all, heat shrinkage ratio of heat-drawn films of poly(methyl methacrylate)，poly(methyl methacrylate/benzyl methacrylate) and TIZBP was investigated. The heat shrinkage of polymer films began at Tg-55°C. Based on the results, because targeting Tg for HRTIZBP was determined to 125°C, higher than 115°C with a sufficient margin, poly(methyl methacrylate/phenyl methacrylate/N-ethyl maleimide = 39/45/16 (wt.%)) was synthesized. This polymer exhibits lower heat shrinkage than the existing polymers, especially the heat shrinkage at 60°C is 0.0%. Therefore, this polymer was confirmed to be HRTIZBP.