Chapter 1
Linear Optical Anisotropy in Aromatic Polyimide Films and Its Applications in Negative Birefringent Compensators of Liquid-Crystal Displays Fuming Li, Edward P. Savitski, Jyh-Chien Chen, Yeocheol Yoon, Frank W. Harris, and Stephen Z. D. Cheng Downloaded by 80.82.77.83 on May 31, 2018 | https://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch001
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Department of Polymer Science and Maurice Morton Institute, University of Akron, Akron, OH 44325-3909 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 solution– cast 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 1
Corresponding author
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© 1997 American Chemical Society
Jenekhe and Wynne; Photonic and Optoelectronic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
Downloaded by 80.82.77.83 on May 31, 2018 | https://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch001
1. LI ET AL.
3 Linear Optical Anisotropy in Aromatic Polyimide Films
(out-of-plane) to the film surface. This phenomenon has been defined as "in-plane orientation" (1-5). Recently, it has been recognized that such anisotropic structure leads to anisotropic thermal, mechanical, dielectric and optical properties along the in-plane and out-of-plane directions (5). Our interests are particularly focused on the linear optical anisotropy (LOA) in solution-cast polyimide films and their applications. Since aromatic polyimide molecules commonly tend to align parallel to the film surface during the film forming process (1-5), the in-plane refractive index is thus larger than the out-ofplane refractive index. The degree of in-plane orientation and the resultant extent of LOA in the films can be estimated readily using refractive index measurements. Therefore, the LOA can be expressed by the birefringence which is the difference in the refractive indices along the in-plane and out-of-plane directions. In the field of optics, this phenomenon is defined as uniaxial negative birefringence (UNB). One of the applications for the LOA in polyimide films is that they may be utilized to design negative biréfringent compensators for twisted and super-twisted nematic liquid crystal displays (TN- and STN-LCDs). Such films can be used in both active and passive forms to improve LCD viewing angles (6-8). However, it is well known that aromatic polyimides are usually difficult to process since they do not melt flow before decomposition and are insoluble in conventional solvents. The traditional approach is to use a two-step polymerization route to make processing possible through the soluble poly(amic acid) precursors. For example, polyimide films are generally produced by solution-casting or spin-coating and then are either thermally or chemically imidized. The imidization history affects the ultimate structure, morphology and properties of the films (9,10). Conventional aromatic polyimides synthesized via the two-step polymerization often possess strong optical absorption in the low W-visible wavelength region (
