Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L 334–L 337 Part 2, No. 3B, 15 March 1999 c 1999 Publication Board, Japanese Journal of Applied Physics Effect of Chemical Structures of Polyimides on Unidirectional Liquid Crystal Alignment Produced by a Polarized Ultraviolet-Light Exposure Michinori NISHIKAWA, Tamas KOSA 1 and John L. WEST 1 Tsukuba Research Laboratory, JSR Corporation, 25 Miyukigaoka, Tsukuba, Ibaraki 305-0841, Japan 1 Liquid Crystal Institute, Kent State University, Kent, Ohio, 44242, U.S.A. (Received January 6, 1999; accepted for publication February 5, 1999) Unidirectional liquid crystal (LC) alignment produced by a polarized ultraviolet-light (UV) exposure was examined using various polyimides (PIs) which consist of different tetracarboxylic dianhydrides and diamines. The photosensitivity and an- choring energy of the LC alignment as a function of UV dosage are summarized with respect to the chemical structures of the PIs. The results suggest that the UV absorption efficiencies, molecular conformations, mechanisms of decomposition, and anisotropic van der Waals forces of the PIs largely affect the photoalignment properties of the LC. KEYWORDS: liquid crystal, photoalignment, polyimide, dichroic ratio, anchoring energy L 334 1. Introduction Most electrooptic applications of liquid crystals (LCs) re- quire well-oriented LC molecular conformations. A good ex- ample is the twisted nematic LC displays which are fabricated using mechanically rubbed polyimide (PI) films as LC align- ment films. 1) However, this method has several problems, such as creation of contaminating particles and production of electrostatic charges, which lowers the production yield of LC displays. Therefore, an alternative technique to align LC without rubbing has been required. Langmuir-Blodgett films, 2) stretched polymers, 3) micro- grooves, 4) stamped polymers, 5) and polarized ultraviolet-light (UV) exposure of polymers, 6) have been developed to pro- duce unidirectional LC alignment. LC alignment produced using polarized UV exposure is the most promising nonrub- bing technique, overcoming the problems mentioned above and greatly simplifying production of multidomain displays. 7) Photoinduced isomerization of azo compounds doped in polymers, 6, 8–10) cis-trans isomerization 11) or cross- linking 12–14) of poly(vinyl cinnamate) derivatives, and pho- todecomposition of PIs, 15–19) have been shown to produce the alignment of LC upon polarized UV exposure. Much effort has been concentrated on the development of photoreactive PIs, which are more heat resistant than azo compounds and poly(vinyl cinnamate) derivatives. One of the key issues re- garding development of photoalignment materials is increas- ing their photosensitivity and anchoring energies as well as improving the quality of LC alignment. However, the param- eters of PI materials which affect the photosensitivity and an- choring energy have not been clarified yet. In this paper, we explore the effect of the PI chemical structures on the pho- toalignment properties of the LC. 2. Experimental PI materials used in this experiment are summarized in Fig. 1. The PIs were prepared by heat curing the precursor polyamic acids, which were synthesized from an equimolar reaction between tetracarboxylic dianhydrides and diamines. LC cells were prepared to measure the dichroic ratios and azimuthal anchoring energies of the LCs aligned by polarized-UV-exposed PI films. PI films were deposited by first spin-coating dilute solutions of the respective polyamic acids on indium tin oxide glass substrates and then curing at 250 ◦ C for 1h to achieve imidization. 20) The thickness of the anchoring energies of the LC cells were measured utilizing the Neel wall method reported previously. 21) The fluorescence spectra of PI films were measured in a front-face arrange- ment and the band passes were 1 nm for both the excitation and emission monochromators. The molecular conformations of the diamines used in the PIs were calculated using the MOPAC Ver. 6 program with AM1 parameters. 22) The PI film birefringences were measured using an instrument described elsewhere. 23) 3. Results and Discussion 3.1 Photosensitivity of LC alignment Figure 2 shows the dichroic ratios of the LC cells as a func- tion of polarized UV exposure time. The dichroic ratios of the LC cells initially increase logarithmically with UV dosage, and then approach a constant value of about 7.0, which is comparable to that of the LC cell with rubbed PI alignment films. The photosensitivity of the LC alignment is largely affected by the chemical structures of the PIs used. Further- more, in most cases, PI shows LC alignment perpendicular to the UV polarization. PI-5, however, aligns LC parallel to the UV polarization. The LC cells (PI-3, 4) with a cyclobutane tetracarboxylic dianhydride moiety in PIs result in higher photosensitivity of LC alignment than those with pyromellitic dianhydride (PI-6, 7). The difference in the photosensitivity can be explained by the main mechanism of the decomposition of the PIs: cy- clobutane ring cleavage 24) and imido ring cleavage 25) which PI film was controlled at 50 nm. The PI films were exposed with polarized UV incident nor- mal to the surface. We used a 450W-xenon lamp (Oriel, model 6266) as a UV source, and a surface film polar- izer (Oriel, model 27320) whose effective range is between 230 nm to 770 nm. The intensity of UV after passing through the polarizer was about 1 mW/cm 2 at 254 nm. LC cells were fabricated using two polarized-UV-exposed sub- strates with antiparallel polarization axis. The cell gaps of the LC cells were controlled at 10 μm. Dichroic LC, n- pentylcyanobiphenyl (5CB, EM Industries) and 0.5% M-618 (Mitsuitoatsu, λ max = 550 nm), and LC, 5CB, were filled into the cells in the isotropic state and slowly cooled to room tem- perature for measurement of the dichroic ratios and azimuthal anchoring energies of the LC cells, respectively. The dichroic ratios of the LC cells were measured using one polarizer and a UV-visible spectrometer. 18) The azimuthal