1468
Langmuir 1991, 7, 1468-1472
Control of Molecular Tilt Angles in Oriented Monolayers Deposited by a Chemical Adsorption Technique and Application of the Monolayers as an Alignment Film N. Mino,' K. Nakajima, and K. Ogawa Central Research Laboratories, Matsushita Electric Industrial Co., Ltd., 3-15, Yagumonakamachi, Moriguchi, Osaka 570, Japan Received May 31,1990.I n Final Form: November 30,1990 Oriented monolayers have been prepared from some chlorosilyl-typesurfactantsby a chemical adsorption (CA) technique for use as an alignment film in a liquid crystal cell. The monolayers were deposited from the mixture of one of three chlorosilyl-type surfactants having a long hydrocarbon chain (18-nonadecenyltrichlorosilane, nonadecyltrichlorosilane,and (19-(trimethylsilyl)-l&nonadecynyl)trichlorosilane)and the other having a short hydrocarbon chain ((3-bromopropyl)trichlorosilane).The molecular density of the monolayer, which affects the arrangement of the long hydrocarbon chain of the Surfactants, was controlled by changing the composition ratio of the PTS in the mixture or by changing the scale of the outer functional group. The degree of molecular arrangement in the monolayers was investigated by IR spectroscopy, and tilt angles of the surfactants molecules in the monolayers were determined by measuring pretilt angles (alignment angles of liquid crystal molecules relative to the normal to the surface of the substrate) of the liquid crystal molecules injected into the liquid crystal cell, which was structured by using the monolayers as an alighment film. By changing the composition ratio of the mixture in the CA solution or an outer functional group of the surfactants having a long hydrocarbon chain in the molecules, the pretilt angles of the liquid crystal molecules could be controlled from Oo to 15' relative to the normal to the surface of the substrate. In order to interpret the experimental results, possible model structures in the monolayers have been proposed. Performance of the monolayers as an alignment film for a liquid crystal cell was evaluated further by driving the liquid crystal cells having the monolayers as alignment films. As the monolayers showed a fair degree of alignment effects without rubbing, this technique may be useful to prepare the alignment film for a liquid crystal cell. 1. Introduction
Much effort has been made to improve the performance of alignment films for a liquid crystal cell. It is difficult, however, to obtain a high grade alignment film with rubbed polymer films such as polyimide film or poly(viny1alcohol) film a t present. In order to control an alignment angle to a fair degree, it may be necessary that the surface of the alignment film be controlled at the molecular level. Since Langmuir-Blodgett (LB) films deposited on a substrate have a fair degree of orientation to the film transfer direction and they align the molecular long axis of the liquid crystals parallel to the transfer direction without rubbing, the LB technique is considered seriously as a potential candidate for applications of practical interest such as a rubbing-free alignment film for a liquid crystal Although the LB technique is powerful as a tool for handling molecular entities, it has some inherent drawbacks: it is uncontrollable for occurrence of domain structures and difficult to increase building up efficiency. Accordingly, the LB technique has a certain limitation when the technique is applied to prepare an alignment film. On the other hand, a chemical adsorption (CA)technique takes advantages of the possibility of obtaining oriented compact monolayers onto a planar solid substrate, and fundamental studies on the technique were already carried out by Sagiv3and c o - ~ o r k e r s . ~ We - ~have already reported (1) Ikeno, H.;Oh-saki, A,; Nitta, M.; Ozaki, N.; Yokoyama, Y.; Nakayama, K.; Kobayaehi, S . Jpn. J. Appl. Phys. 1988,27, L475. (2) Nishikawa, Y.; Morikawa, A.; Takiguchi, Y.; Kanemoto, A.; Kakimoto, M.; Imai, Y. Jpn. J. Appl. Phys. 1988,27, L1163. (3) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (4) Polymeropoulos, E. E.;Sagiv, J. J. Chem. Phys. 1978, 69, 1836. (5) Netzer, L.;Sagiv, J. J. Am. Chem. SOC.1983, 105, 674. (6) Netzer, L.;Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. (7) Netzer, L.;Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235.
in a previous paper that the molecular alignment angles in the monolayers deposited by the CA technique could be determined by using a liquid crystal.8 Since the CA technique was thought to supply the same function as the LB techniques for aligningthe liquid crystal molecules in the cells, we have tried to prepare oriented monolayers having different tilt angles from some chlorosilyl-type surfactants by the CA technique for use as an alignment film in a liquid crystal cell. The degree of molecular arrangement in the monolayers was investigated by IR spectroscopy and the tilt angles of the surfactants molecules in the monolayes were determined by measuring pretilt angles of a liquid crystal molecules injected into the liquid crystal cell, which was structured by using the monolayers as an alignment film. The performance of the CA monolayers as an alignment film for liquid crystals was evaluated further. In this paper, we wish to report the control technique of the molecular alignment angles in the CA monolayers and the evaluation results of the performance of the oriented monolayers deposited by the CA technique as the alignment films for liquid crystal cells. 2. Experimental Section Preparation of Monolayers. Materials. Three chlorosilyl-type surfactants, 18-nonadecenyltrichlorosilane(CHF CH(CH&SiCl3, V-NTS),nonadecyltrichlorosilane (CHa(CH&aSiC13,NTS),and (19-(trimethylsilyl)-l&nonadecynyl)trichroro~ilane ((CH3)sSiC=C(CHz)1,SiCb, SA-NTS),were synthesizedand were characterized by NMR spectroscopy, IR spectroscopy, and mass spectrometry (MS). For V-NTS, NMR spectra in CC4, bands typical of trichlorosilane with long n-alkyl chains and terminal ethylenic double bonds were observed at chemical shifts 6 5.55 (C=CH-), 4.85 2.1.
(8) Ogawa, K.;Mino, N.; Nakajima, K.; Azuma, Y.; Ohmura, T. Langmuir, 1991, 7, 1473.
0743-1463/91/2401-1468$02.50/0 0 1991 American Chemical Society
Molecular Tilt Angles in Oriented Monolayers (H2C==C-), 1.95(=CCHz-),and 1.25ppm (-CH2-). In IRspectra, strong absorption bands were observed a t 2920 (CH2, v-), 2845 (CHZ, y e ) , and 915 cm-l (C=CCH-) and weak bands a t 3080 (H2C=, v) and 1640 cm-l (C=C, v). The molecular weight was confirmed to be 398 by mass spectrometry. Partial migration of the terminal double bond occurred during the final distillation of V-NTS, as indicated by the appearance of non terminal vinyl bands a t 6 5.00 ppm (multiple) in the NMR spectrum and 995 cm-l (weak) in the IR spectrum. The purity of V-NTS as measured by gas chromatography was 89.4 76. For NTS, in the IR spectrum, strong absorption bands were observed a t 2920 (CH2, v-), 2845 (CH2, vJ, and 1470 cm-l (C=CCH2,6). From the mass spectra, the molecular weight was confirmed to be 400. The purity of NTS as measured by gas chromatography was 98.3 % . For SA-NTS, in the NMR spectra in CC4, bands typical of trichlorosilane with long n-alkyl chains and acetylenic triple bonds were observed a t chemical shifts of 6 2.15 (C=CCH2-) and 1.25 (-CH2-), and a band typical of trimethylsilyl groups was observed a t 0.12 ppm (-Si(CH3)3). In IR spectra, strong absorption bands were observed a t 2930 (CH2, v-), 2860 (CH2, ve), 2195 (CEC), 1475 (CH2, a), 1258 ((CH&Si, a), 840 ((CH&Si, v), and 760 cm-l ((CH3)3Si,v). The molecular weight was confirmed to be 468 by mass spectrometry. The purity of SA-NTS as measured by gas chromatography was 94.4 % . The synthetic methods of the three surfactants will be reported elsewhere. The other surfactant having a short hydrocarbon chain, (3bromopropy1)trichlorosilane(Br(CH2)&3iCl3,PTS) was supplied by Chisso Corp. and used without purification. All solvents were purchased from Aldrich Chemical Co. and used without purification. Substrates were four-sided glass plates having a hydrophilic surface, on which surface-sputtered Si02 and A1 had been deposited for Fourier transform infrared (FTIR) measurements (SiO2/Al/glass) or sputtered Si02 and I T 0 (indium tin oxide) electrode pattern for driving the liquid crystal cell had been deposited (Si02/ ITO/glass). Chemical Adsorption Steps. Monolayers were deposited on solid substrates by using the CA technique already recommended by Netzer et al.' a t different composition ratios of the mixture of the surfactants in the CA solutions. To obtain CA monolayers, the substrates were immersed in the CA solution under a nitrogen gas atmosphere produced by vaporization from liquid nitrogen and passage through a 0.2-pm filter. The CA process took place a t 23 "C (room temperature, RT) and the concentrations of the surfactants were 1X mol/L to 3 X in a solvent mixture of 80 wt % n-hexadecane, 12 wt % CC4, and 8 wt % CHCl3. The substrates were washed by chloroform and then by water, which was purified by ultrafiltration of deionized water in order to stabilize the deposited monolayers. The CA times were changed from 1min to 260 h to decide the best time for depositing monolayers for an alignment film. The surface of the substrates covered with chemically adsorbed monolayers was very hydrophobic. 2.2. IR Measurements. The FTIR spectrophotometer (FTIR-4000; Shimadzu Co., Ltd., 2 cm-l resolution) was used to evaluate the molecular arrangement or density of the monolayers. The reflection IR spectra of the monolayers were taken on SiOz/Al/glass substrates with a multiple external reflection (MER) attachment. The angle of incidence was 73O, and the number of external reflections was seven for MER measurements. Seven thousand interferograms had to be accumulated to obtain an IR spectrum of the monolayers. The structure of the cells for the measurements has been reported in detail.g All spectra reported here were the result of subtraction of the curves measured with clean SiO2/Al/glass plates from those measured after reflecting the respective films on the same plates, and assignments of the absorption bands were made in reference to the reports by Hayashi et al.1° and Kimura et al.11 (9) Ogawa, K.; Ueda, K.; Tamura, H.; Hatada, M.; Ishihara, T. Longmuir 1989,5, 1326. (10) Hayashi, S.; Umemura, J. J. Chem. Phys. 1975,63, 1732. (11) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96.
Langmuir, Vol. 7,No. 7,1991 1469 CnH2n-I
CN
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Figure 1. Molecular structure of the liquid crystal (ZLI-1565). Magnetic Field (1Tesld
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Figure 2. Schematic diagram for an explanation of the measurement technique of the pretilt angles of liquid crystal molecules in a liquid crystal cell. 2.3. Measurement of Tilt Angles of Molecules in the Monolayer with a Liquid Crystal. In order to determine the tilt angle of the surfactant molecules in the monolayer deposited by the CA technique, pretilt angles (which was measured relative to the normal to the surface of the substrate) of a nematic lqiuid crystal (ZLI-1565; Merck Co., Ltd., molecular structure shown in Figure 1)injected into the cell having monolayers as alignment films were investigated by measuring the capacitance changes of the liquid crystal cells in a rotating magnetic field of 1 T. The measurement technique is shown schematically in Figure 2. The liquid crystal cells were constructed with a 16-pm gap between two glass plates, inner surfaces of which were covered with the monolayers of the surfactants deposited on Si02/ITO/ glass by the CA technique, as shown in Figure 2. The area of the I T 0 electrode was 1 cm2. In Figure 2, as the effect of the magnetic field is applicable only for the liquid crystal having a dipole moment, the liquid crystal aligns along the magnetic field when the magnetic field is applied. Thus, by measuring the capacitance of the liquid crystal cell without the magnetic field and with a rotating the magnetic field, i.e., by finding the angle of the magnetic field when the capacitance measured with rotating the magnetic field is equal to that without the magnetic field, the pretilt angle of the liquid crystal, which is assumed to correspond to the tilt angles of the surfactant molecules in the CA monolayer, can be determined. 2.4. Evaluation of Performance of the Monolayers as an Alignment Film for a Liquid Crystal. In order to evaluate the performance of the monolayer, the liquid crystal cells were driven with crossed polarizers by applying a voltage of 16 V. The applied voltage was generated by a Hewlett-Packard 3314A arbitary waveform generator. The schematic diagrams for explanation are shown in Figure 3. In Figure 3, the liquid crystal is sandwiched by two CA monolayers. At the portion with the IT0 electrodes, the liquid crystal molecules are aligned along the electric field and the transparency becomes low, when the driving voltage is applied, as shown on the right-hand side of Figure 3. On the other hand, a t the portion without the IT0 electrodes, the liquid crystal molecules are aligned along the surfactant molecules deposited as alignment films in the liquid crystal cells and the light can get through that
Mino et al.
1470 Langmuir, Vol. 7, No. 7, 1991 I
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Figure 5. MER IR spectra of the monolayers of different mixtures of surfactants: (A) pure NTS; (B) NTS:PTS = 1:l;(C) NTS:PTS = 1:2. Table I. Absorption Intensity Ratio of u"(CH2) to v,(CH2) on the MER IR Spectra of the Monolayers Deposited from the Mixture of the Surfactants intensity ratio mixture ratio V-NTS:PTS NTS:PTS SA-NTS:PTS l:o 1.85 1.65 1.77 1:l 1.90 1.71 1.86 1:2 2.10 1.82 1.88
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Figure 4. Relative intensity change a t the typical absorption peaks of the SA-NTS monolayers deposited a t R T as a function of deposition times. portion independent of the applied voltage, as shown on the lefthand side of Figure 3.
3. Results and Discussion 3.1. Determination of the CA Time. In order to decide the best CA time, IR spectra were measured on the chemically adsorbed monolayers as a function of the CA time, as shown in Figure 4. Intensity changesa t the typical absorption band peaks of the SA-NTS due to the CH2 stretching (v,(CH2) 2930 and v8(CH2) 2860 cm-l) and scissoring vibration (CH2, 1450 cm-l), due to the Si-0 stretching vibration (v(Si-0) 1080 cm-l), and due to the C+ stretching vibration (v( C M ) 2200 cm-l) were plotted as a function of the CA time, as shown in Figure 4. The intensity of all the bands leveled off a t about 1 h; the deposition was completed after 1h. Thus the deposition time was decided to be 1h for the following experiments. 3.2. IR Spectra Change of Monolayers When the CompositionRatio of the Surfactants WereChanged.
MER IR spectra of the monolayer made from different mixtures of NTS and PTS are shown in Figure 5. For the monolayer deposited from the mixture of NTS and PTS, two strong absorption bands due to the CH2 stretching vibrations (vW(CH2)2930 and v,(CH2) 2860 cm-l) and a weak band due to the CH2 scissoring vibration (6(CH2) 1450 cm-l) were observed on spectrum A in Figure 5 and a weak band due to the -CH2Br scissoring vibration (G(CH2Br) 1300 cm-l) and a weak band due to the CH3 stretching vibrations (vae(CH3)2960 and v,(CH3) 2900 cm-l) were observed on the spectra B and C in Figure 5. The absorption bands due to the CH2 stretching vibrations (CH2, vu, and CH2, v,) on the CA monolayer split clearly for all spectra, indicating that the surfactant molecules oriented along the surface of the substrates and the bands due to the CH2 stretching vibrations (CH2, vW, and CH2, v8) decreased, and that due to the -CH2Br scissoring vibration (CH2Br, 6) increased with increasing the PTS, as shown in Figure 5. As the spectrum between 1100and 1250cm-l is affected by the sputtered Si02 on the substrates, bands could not be assigned in this region. Accordingly, although the tilt angles of the molecules relative to the normal to the surface of the substrates in the monolayers could not be determined only by these data, the molecular tilt angles of the surfactants having a long hydrocarbon chain should increase with increasing PTS. 3.3. Molecular Alignment Angle Evaluated with Liquid Crystal. Capacitance curves for the monolayers deposited from the mixture of V-NTS and PTS, NTS and PTS, and SA-NTS and PTS were plotted as a function of the angle of the magnetic field applied to the liquid crystal cells having the monolayers deposited a t different composition ratios as an alignment film, as shown in Figure 6. The capacitance curves of the cells did not show any hysteresis a t all on the cells structured with the mono-
Molecular Tilt Angles in Oriented Monolayers
Langmuir, Vol. 7,No. 7,1991 1471 0"
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