Transparent thermoplastic polymers that exhibit no birefringence are ideal for optical components such as optical films for liquid crystal displays and various lenses. Copolymerization of a positive birefringent monomer with a negative monomer is an effective technique for obtaining low birefringent polymers, especially zero-photoelastic birefringence polymers that exhibit no photoelastic birefringence even during elastic deformation. We prepared four types of trifluoromethyl-substituted polystyrenes. By substituting hydrogens at the ortho or meta positions of the benzene ring of polystyrene, we demonstrated that poly(2-(trifluoromethyl)styrene), poly(3-(trifluoromethyl)styrene), and poly(3,5-bis(trifluoromethyl)styrene) had negative photoelastic coefficients. However, poly(4-(trifluoromethyl)styrene) had a positive photoelastic coefficient similar to that of polystyrene. Based on these results, we synthesized a zero-photoelastic birefringence polymer of poly(2-(trifluoromethyl)styrene-co-4-(trifluoromethyl)styrene) (55/45 wt.) exhibiting no photoelastic birefringence in elastic deformation, in which the positive photoelastic birefringence of the poly(4-(trifluoromethyl)styrene) unit was compensated for by the negative photoelastic birefringence of the poly(2-(trifluoromethyl)styrene) unit. The discovery of polymers having negative photoelastic coefficients is valuable for the design and synthesis of zero-photoelastic birefringence polymers. The four types of trifluoromethyl-substituted polystyrenes are promising optical materials because they have high transparency (transmittance > 89–92% for 27–34-µm thickness films) in the visible and near-infrared regions and a high decomposition temperature of approximately 400°C.
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