Mechanism of UV Modification of Polyimide Film and Its Application to Two-Domain Twisted Nematic LCDs

The mechanism of modifying the liquid crystal (LC) pretilt angle in the UV-type two-domain twisted nematic approach for the wide-viewing angle liquid crystal displays (LCDs) was discovered. The LC pretilt angle (Thetap) could be controlled by the alignment layer (AL) polymer structures and (or) the LCD processes. Thetap was increased by introducing alkyl side chains and (or) other non-polar groups to the AL polymer. On another hand, Thetap was decreased by modifying the AL polymer surface with UV exposure of O(sub2) plasma. The electronic interaction between LC molecules and the AL surface would decrease Thetap while the steric repulsion by the side chains would increase Thetap. Thus the AL polymer and its process can be designed so as to obtain a desired pretilt angle. The fundamental chemistry in molecular level could provide the mechanism of controlling LC pretilt angles. Three commercially available polyimides (Nissan RN1024, SE7210, and SE7792) were employed. The polyimides before and after various treatments were analyzed by contact angle measurements, x-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and electron spin resonance (ESR). Upon UV exposure on these PI films, aromatic (or conjugated) radicals with a long life time (greater than 2 weeks) were generated, and these PI surfaces subsequently became morepolar since polar functional groups such as OH were introduced. The pretilt angles of ZLI-5080 LC on the UV-exposed PI films decreased by 4.5degrees-8.8degrees in comparison with those on the plain films.

By: Kang-Wook Lee, Alan Lien, James H. Stathis and Sang-Hyon Paek (Columbia Univ.)

Published in: Japanese Journal of Applied Physics. Part I. Regular Papers and Short Notes, volume 36, (no 6A), pages 3591-7 in 1997

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