Mechanisms of orientational and photoelastic birefringence generation of methacrylates for the design of zero-zero-birefringence polymers

The intrinsic birefringence Δn⁰ and photoelastic coefficient C of poly(methyl methacrylate), poly(2,2,2-trifluroethyl methacrylate), poly(phenyl methacrylate), and poly(2,2,3,3,3-pentafluorophenyl methacrylate) were determined. We categorized these methacrylate polymers into four birefringence-types, even though their molecular structures differed only by the substituents on the side chains. Based on the results of Δn⁰ and C, novel polymers that exhibit neither orientational nor photoelastic birefringence, i.e., zero-zero-birefringence polymers, were designed and synthesized by quaternary copolymerization system. Furthermore, we confirmed that the mechanisms of orientational birefringence and photoelastic birefringence generation were different in these methacrylate polymers. The conformation of the repeat unit of the polymers was nearly constant during the generation of orientational birefringence. In contrast, the conformation of the repeat unit of the polymers changed during the generation of photoelastic birefringence in the glassy state. These findings demonstrated the reasonability of evaluating orientational and photoelastic birefringence separately, as well as the adequacy of the classification of polymers into four birefringence-types. Given these results and the fact that zero-zero-birefringence polymers could be prepared successfully by four-birefringence type monomers, we demonstrated the reasonability of the method for designing the zero-zero-birefringence polymers.