Linear Optical Anisotropy in Aromatic Polyimide Films and Its Applications in Negative Birefringent Compensators of Liquid-Crystal Displays

Abstract

Samples of soluble aromatic polyimides of varying chemical structure and molecular weight were synthesized in refluxing m-cresol at elevated temperatures through a one-step polymerization route. Modifications of the dianhydride and diamine monomers were designed to prepare aromatic polyimides having new architectures based on the requirements of liquid crystal display (LCD) applications. The solutioncast films exhibit linear optical anisotropy (LOA), which is called uniaxial negative birefringence (UNB) and is characterized by the presence of a larger refractive index along the in-plane direction than in the out-of-plane direction. It is found that the UNB is critically associated with the backbone linearity and rigidity as well as the intrinsic polarizability of the polyimides. A specific polyimide synthesized from 2,2'-bis(3,4-dicarboxyphenyl)-hexafluoropropane and 2,2'-bis(trifluoro-memyl)-4,4'-diaminobiphenyl was used as an example to study the molecular weight effect on the LOA. For films having a fixed molecular weight, the refractive indices are constant for film thicknesses below 15 μm. They gradually change with further increase of the film thickness. On the other hand, the refractive index along the out-of-plane direction decreases while the in-plane refractive index increases when the polyimide molecular weight increases. The LOA is closely associated with the anisotropy of other second order parameters which are second derivatives with respect to the energy term. These parameters include the coefficient of thermal expansion, modulus, dielectric constant and refractive index. Films with this UNB can be used as negative birefringent compensators in twisted and super-twisted nematic LCDs to improve display viewing angles.It has been recognized since the 1960s that aromatic polyimide films exhibit structural anisotropy in the directions parallel (in-plane) and perpendicular

Disulfide‐Bridged Dynamic Covalent Triazine Polymer Thin Films by Interface Polymerization: High Refractive Index with Excellent Optical Transparency

“…It is reported that the uniaxial negative birefringence is critically associated with the backbone linearity and rigidity as well as intrinsic polarizability of the polymers. [33] In contrast to the disulfide polymer films reported in this work (with refractive indices in the range of 1.47-1.76 and with high uniaxial negative birefringence values in the range of 0.05-0.25), the analog disulfide polymer examples in the literature show significantly smaller birefringence values. For instance, for polyamide with thioether linkages n = 1.7701 and ∆n = < 0.0076, [34] for triazine-based polyphenylene sulfides n = 1.6902-1.7169 and ∆n = 0.0015-0.0042, [35] for poly(sulfur-codovinyl benzene) n = 1.97 and ∆n = 0.0010, [36] for poly(2-hydroxy-1,4-phenylene sulfide) n = 1.80 and ∆n = 0.003.…”

“…It is reported that the uniaxial negative birefringence is critically associated with the backbone linearity and rigidity as well as intrinsic polarizability of the polymers. [ 33 ]…”