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Langmuir and Langmuir–Blodgett films of fluorine-containing side chain liquid crystal copolysiloxanes
Thin Solid Films 372 Ž2000. 240᎐245
Langmuir and Langmuir᎐Blodgett films of fluorine-containing side chain liquid crystal copolysiloxanes Jin Mu, Hiroaki Okamoto, Shunsuke TakenakaU Department of Ad¨ anced Materials Science and Engineering, Faculty of Engineering, Yamaguchi Uni¨ ersity, Ube, Yamaguchi 755-8611, Japan Received 13 January 2000; received in revised form 11 April 2000; accepted 26 May 2000
Abstract The Langmuir film behavior of fluorine-containing side chain liquid crystal copolysiloxanes at the air᎐water interface has been studied over a temperature range of 277.5᎐310 K. The surface pressure᎐area isotherms exhibited a transition from an expanded to a condensed phase. In the expanded phases the main chains of the polymers were anchored at the air᎐water interface. In the condensed phases the extension or partial exclusion of the main chains at the air᎐water interface was mainly controlled by the cross-sectional area of the side chain. The transferred Langmuir᎐Blodgett ŽLB. multilayer did not give any X-ray diffraction peak. With heating, a molecular ordering of the LB film was induced so that the film showed the Bragg diffraction peak. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Liquid crystal copolysiloxanes; Langmuir films; Langmuir᎐Blodgett films; Fluorine substituent effect
1. Introduction In recent years, monolayers and Langmuir᎐Blodgett ŽLB. films of polymers, especially polysiloxanes, have received a great deal of attention owing to higher thermal and mechanical stability w1᎐8x. It is well known that the polysiloxane chain is very flexible and the oxygen atoms of the silicon backbone give an amphiphilic property to the polymer. The hydrogen bond between the polysiloxane chain and water is responsible for the spread of the polymer w3x. Full extension or coiling of the main chain depends on the property of the substituent w4x. Liquid crystal compounds have been shown to yield easily well-controlled organized monolayers w9᎐11x and multilayers w12᎐14x at the air᎐water interface. Their organization at the air᎐water interface depends on the
liquid crystal ŽLC. molecule᎐LC molecule interactions and the LC molecule᎐aqueous subphase interactions as well as the compressing state of the film. Recently, we reported on the effects of alkyl chain length on the monolayer formation of copolysiloxanes having a 4alkoxyphenyl benzoate core in the side chain w15x. The results showed that the surface pressure can induce the tight packing of the liquid crystal core in the side chain and removal of the ŽCH3 .2 SiO segment in the main chain from the water surface. In this paper, we will report some recent results on Langmuir films and Langmuir᎐Blodgett films of fluorine-containing side chain liquid crystal copolysiloxanes. 2. Experimental 2.1. Materials
U
Corresponding author. Tel.: q81-836-35-9964; fax: q81-836-359965. E-mail address: [email protected] ŽS. Takenaka..
The present polymers were prepared according to the scheme in Fig. 1.
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 0 5 5 - 5
J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
241
Fig. 1. Synthetic scheme of copolysiloxanes.
2-Perfluorooctylethanol was purchased from Daikin Kogyo Co., and used without further purification. The siloxane copolymer, TSF-483 ŽToshiba-sirikon Co.. was used without further purification. The purity of the compounds was checked by a 1 H NMR spectroscopy and a differential scanning calorimeter ŽDSC. by using a ‘DSC-PURITY’ program ŽSeiko-denshi Co... 4-Trifluoromethylphenyl, 4-trifluoromethoxyphenyl, and 4-w2-Žperfluoro-octyl.ethoxy-carbonylxphenyl benzoates, M1, M2, and M3, respectively, were prepared by the condensation of the corresponding phenols and 4-aryloxybenzoic acid.
M1: IR ŽKBr disc. 1649 Ž Cs C . and 1728 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 4.64 Ž2H, d, J s 5.3 Hz., 5.34 Ž1H, dd, J s 10.4, 1.5 Hz., 5.44 Ž1H, dd, J s 17.2, 1.5 Hz., 6.04 Ž1H, ddt, J s 17.2, 5.3, 10.4 Hz., 7.00 Ž2H, d, J s 8.9 Hz., 7.33 Ž2H, d, J s 8.6 Hz., 7.68 Ž2H, d, J s 8.6 Hz., 8.14 Ž2H, d, J s 8.9 Hz. ppm. M1 did not exhibit any mesophase even in a rapid cooling process, m.p. 360 K Ž ⌬H s 78 Jrg.. M2: IR ŽKBr disc. 1649 Ž Cs C . and 1734 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 4.63 Ž2H, d, J s 5.3 Hz., 5.34 Ž1H, dd, J s 10.6, 1.3 Hz., 5.44 Ž1H, dd, J s 17.2, 1.3 Hz., 6.04 Ž1H, ddt, J s 17.2, 5.3, 10.6
Fig. 2. Optical micrographs of: Ža. M2 at 350 K; Žb. P2 at 340 K; Žc. M3 at 350 K; and Žd. P3 at 350 K.
J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
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Hz., 7.00 Ž2H, d, J s 8.9 Hz., 7.23 Ž2H, d, J s 9.6 Hz., 7.27 Ž2H, d, J s 9.6 Hz., 8.13 Ž2H, d, J s 8.9 Hz. ppm. M2 exhibited a monotropic smectic A phase ŽSm A. having a focal conic fan texture, as shown in Fig. 2a, m.p. 365 K Ž ⌬H s 89 Jrg. and Sm A-isotropic ŽI. 356 K Ž ⌬H s 23 Jrg.. M3: IR ŽKBr disc. 1647 Ž Cs C . and 1726 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 2.62 Ž2H, tt, J s 18.5, 6.4 Hz., 4.64 Ž2H, t, J s 6.4 Hz., 4.65 Ž2H, t, J s 5.3 Hz., 5.35 Ž1H, dd, J s 10.6, 1.3 Hz., 5.44 Ž1H, dd, J s 17.3, 1.3 Hz., 6.05 Ž1H, ddt, J s 17.3, 5.3, 1.3 Hz., 7.00 Ž2H, d, J s 9.1 Hz., 7.31 Ž2H, d, J s 8.9 Hz., 8.12 Ž2H, d, J s 8.9 Hz., 8.14 Ž2H, d, J s 9.1 Hz. ppm. M3 exhibited an enantiotropic Sm A phase having a focal conic fan texture, as shown in Fig. 2c, m.p. 380 K Ž ⌬H s 35 Jrg. and Sm A-I 438 K Ž ⌬H s 11 Jrg.. The X-ray profile for the Sm A phase showed a sharp reflection at 2 s 2.40⬚ Ž3.68 nm. arising from the smectic layer, as well as a broad one at approximately 2 s 20⬚. P1: To a solution of M1 Ž2.44 g, 6.96 mmol. and copolysiloxane ŽTSF-483, 0.8 g. in dry toluene Ž36 ml., dry THF Ž0.1 ml. containing 3 wrw% of H2 PtCl6 ⭈ 6H2 O was added under a nitrogen atmosphere, and the resulting solution was stirred at 110⬚C for 24 h. After the reaction was completed, the reaction mixture was poured into methanol Ž150 ml., and the solid precipitated was collected by decantation. The crude product was purified by repeating reprecipitation with a solvent mixture of toluene and methanol, giving P1 as a colorless gum, 4.34 g Ž56%.. IR ŽKBr disc. 1734 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 0.09 Ž9H., 0.66 Ž2H., 1.85 Ž2H., 3.91 Ž2H., 6.92 Ž2H., 7.21 Ž2H., 7.59 Ž2H., 8.02 Ž2H. ppm. The intensity ratio of the peaks at 0.09 Ž9H. to 8.02 ppm Ž2H. suggested that more than 96% of the hydrogens of copolymer were substituted by the liquid crystalline core. P1 exhibited a nematic Ž N . phase with a shear texture. Tg 273 K and N-I 369 K Ž ⌬H s 6 Jrg.. The X-ray profile for the N phase showed a weak and broad reflection at approximately 2 s 20⬚. P2, and P3 were also obtained by a similar manner. P2: IR ŽKBr disc. 1730 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 0.10 Ž9H., 0.67 Ž2H., 1.85 Ž2H.,
3.92 Ž2H., 6.93 Ž2H., 7.21 Ž4H., 8.03 Ž2H. ppm. The intensity ratio of the peaks at 0.10 Ž9H. to 8.03 ppm Ž2H. indicated that more than 95% of the hydrogens of the copolymer were substituted by the liquid crystalline core. This polymer exhibited an N phase with a shear texture, as shown in Fig. 2b. Tg 259 K and N-I 352 K Ž ⌬H s 5 Jrg.. The X-ray profile for the N phase showed a broad reflection at approximately 2 s 20⬚. P3: IR ŽKBr disc. 1647 Ž Cs O . and 1726 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 0.10 Ž9H., 0.67 Ž2H., 1.85 Ž2H., 2.59 Ž2H., 3.92 Ž2H., 4.60 Ž2H., 6.92 Ž2H., 7.20 Ž2H., 8.20 Ž4H. ppm. The intensity ratio of the peaks at 0.10 Ž9H. to 8.20 ppm Ž2H. indicated that more than 95% of the hydrogens of copolymer were substituted by the liquid crystalline core. P3 exhibited an Sm A phase with a fine focal conic fan texture, as shown in Fig. 2d. Sm A-I 374 K Ž ⌬H s 8 Jrg.. P3 did not show a glassy transformation in DSC thermogram. The X-ray profile for the Sm A phase showed a sharp reflection at 2 s 2.64⬚ Ž3.34 nm. arising from the smectic layer, as well as a broad one at approximately 2 s 20⬚. 2.2. Method Film experiments were performed on a computercontrolled film balance, FSD-110P ŽUSI System Ltd., Japan.. The surface pressure was measured by a Wilhelmy plate with a precision of "0.1 mNrm. Reported temperatures refer to the subphase temperatures with an accuracy of "0.2 K. The copolysiloxanes were dissolved in chloroform Žspectro grade. with a concentration no higher than 0.8 mgrml. The solutions with a precise volume were dropped carefully on the distilled water surface using a microsyringe. After complete evaporation of the solvent Žapprox. 20 min. the surface pressure᎐area isotherms were recorded at a constant compression speed of 0.48 cm2rs. The isotherm reproducibility was within better than 1 mNrm at a given surface area. The glass slides were cleaned and made hydrophobic as shown by Honig et al. w16x. LB multilayers were transferred onto hydrophobic glass slides by the Y-type deposition mode at a constant target pressure and
Table 1 Transition temperatures and characteristic monolayer data Ž296.5 K. of polymersa P
P1 P2 P3
Transition temp. ŽK.
Onset area Žnm2 .
Tg273N369I Tg259N352I Sm A374I
0.487 0.917 1.363
Inflection point A Žnm2 .
ŽmNrm.
0.779 0.966
2.0 2.0
A 0 Žnm2 .
A e Žnm2 .
0.212 0.20 0.389
0.221 0.208 0.438
a Tg is a glossy transformation temperature. N, Sm, A and I indicate nematic, smectic A and isotropic phase, respectively AO is the lining area per repeat unit of the condensed state. Ae is the cross-sectional area of the side chains evaluated by the CS Chem 3D Pro Vision 4.0.
J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
Fig. 3. Temperature dependency of surface pressure᎐area isotherms for: Ža. P1; Žb. P2; Žc. P3.
temperature of 293 " 0.2 K. The dipping speed for upstroke and down stroke was all 5 mmrmin. X-Ray diffraction experiments of LB films were performed on a Rigaku-denki RINT 2200 diffractometer, where CuK ␣ which corresponds to a wavelength of 0.1541 nm was used as an X-ray source. 3. Results and discussion 3.1. Surface pressure᎐area isotherms All three copolysiloxanes form stable Langmuir films at the air᎐water interface. Their surface pressure᎐area isotherms as a function of temperature are presented in Fig. 3. Transition temperatures in bulk as well as characteristic areas and surface pressures at 296.5 K are summarized in Table 1 for comparison. From Fig. 3 we can see that the copolysiloxanes studied show the respective characteristic isotherms. The order of their onset areas at 296.5 K is P1 - P2 P3 ŽTable 1.. It is reasonable to assume that before the
243
onset of the pressure the main chains of the copolysiloxanes adopted an extended configuration, with the oxygen close to water, and the rest stretched into the air. On compression, this process was not accompanied by an increase in pressure up to the point of the onset area, where the interaction between adjacent molecules became observable. Therefore, the distinction of the onset areas should apparently attribute to the effects of length Žsee Table 2., steric hindrance, and various possible arrangements of the side chains. The relatively high polarity of the fluoromethyl group directly attached to the mesogenic core may cause a spreading of incompletely extended polymer, which leads to the smallest onset area. In addition, the hydrophilic affinity of the oxygen atom for P2 and carboxyl group for P3 attached to the mesogenic core may play a role in the lateral coherence of the side chains at larger areas per repeat unit. P1 did not exhibit the lower pressure plateaus at higher temperatures Ži.e. 296.5 and 310 K.. In other words, the film could directly be compressed into a condensed state at higher temperatures. When the subphase temperature was decreased only slight indications were seen of inflections of the plateaus. At 277.5 K, extrapolating the steep part of the isotherm to zero pressure leads to a limiting area of 0.221 nm2rrepeat unit, which is just consistent with the cross-sectional area of the side chain evaluated from the CS Chem3D Pro Version 4.0. According to this result we can postulate that in the condensed state the siloxane unit connecting with the side chain was anchored at the water surface. The plateaus correspond to a transition from an expanded to a condensed phase w10x. Before the plateau the main chains of P1 still lay flat on the water surface. As the pressure increased the side chains tilted gradually away from the water. At the inflection the ŽCH3 .2 SiO segment in the main chains began to be squeezed out of the water surface, leading to tight packing of the side chains. The isotherms of P2 showed the lower pressure plateaus at any temperature at the surface pressure of approximately 2.0 mNrm. After passing the plateau the isotherms turned upward and all converged at an area of approximately 0.2 nm2rrepeat unit at four temperatures. After further compression P2 passed through a second transition and then the surface presTable 2 X-Ray diffraction data of LB films a P
d-spacing Žnm.
P1 P2 P3
2.54 3.27 a
l Žnm. 2.00 2.12 3.25
l is the length of the side chains bearing the CH 3 SiO group evaluated by a semi-empirical molecular orbital calculation ŽAM1 method, MOPAC Ver. 6.0..
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J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
sure increased steeply again. It is evident that the second transition corresponds to the collapse of the film. When the subphase temperature was decreased, a small increase in the pressure corresponding to the plateau was observed. The property of the lower pressure plateaus is analogous to P1. The negative temperature dependency of the plateau pressures for P1 and P2 can be explained as an increase in mobility of main chains and side chains at higher temperature favoring both removal of the ŽCH3 .2 SiO groups from the water surface and the closest packing of the side groups w15,17x. It is interesting to note that the plateau pressure of P3 is much higher than that of P1 and P2. A reasonable explanation for this is that the steric hindrance of the long perfluorocarbon group is much stronger than the short trifluoromethyl and trifluoromethoxyl group. P3 needs much higher surface pressure to force the packing of the side chains. In contrast with P1 and P2 the plateau pressure of P3 was hardly altered by subphase temperature Žsee Fig. 3c.. Since twice the value, 0.193 " 0.013 nm2rmonomer for polydimethylsiloxane found by Noll and coworkers w18x is smaller than the crosssectional area Ž0.483 nm2 , see Table 1. of the side chain, it is plausible to postulate that the plateau of P3 only reflects compression-induced tight packing arrangement of the side chains w19x, and in the condensed state the siloxane groups in the main chains are still all anchored at the air᎐water interface. In other words, the skeleton effect of the side chain maintains the extension of the main chain at the water surface. Repeated compression of the Langmuir film and subsequent expansion have no essential effect on the shape of the isotherms for P1᎐P3. The recompression isotherm is nearly identical with the initial one up to the inflection of the transition. If the film was expanded beyond the plateau and then recompressed, the isotherms were shifted to lower areas per repeat unit, indicating that the condensed phase contains the composition with irreversible structural changes. We notice that the limiting areas per repeat unit of three polymers in the condensed state are all slightly smaller than the evaluated cross-sectional areas of corresponding side chains Žsee Table 1.. This may imply a partial multilayer formation or a staggered packing arrangement of the side chains w20x. When the subphase temperature was increased the limiting areas per repeat unit of P1 and P3 in the condensed state were shifted toward smaller values. We can explain this phenomenon from two aspects. First, increasing temperature favors the molecular motion to form multilayer aggregates or a staggered packing arrangement of the side chains. This led to the areas being shifted toward smaller values. Second, it was observed that the film appeared very viscous while being compressed above the plateau, actually moving
the Wilhelmy plate out of its vertical position in the direction of film compression. Consequently, the high viscosity of the film produced a measuring error for a Langmuir trough using the single barrier compression system w21x. By the way, it is related to the high viscosity of the film that there are not apparent collapse pressures in the isotherms of P1 and P3. 3.2. Langmuir᎐Blodgett films When the glass slide was hydrophilic, the first stroke was always an upstroke. Whenever the slide was withdrawn, it appeared wet. We waited for the slide to dry even for 30 min before attempting to transfer the following layer; this still failed as it was only possible to deposit less than 10 layers Žtransfer ratios approx. 0.5. and the films showed inhomogeneity. However, except for P1, the films of P2 and P3 could easily be transferred onto the hydrophobic glass slides at a surface pressure of 10 mNrm for P2 or 20 mNrm for P3, respectively. In such a case, particular care was still taken to ensure complete drying of the emersed film before further deposition. The deposition of P2 was Y-type with a transfer ratio on the down stroke of approximately 1.0 and on the upstroke of approximately 0.8. The deposition of P3 resulted in X-type with a transfer ratio on the down stroke of approximately 1.0 and on the upstroke of less than 0.1. For P1 no transfer could be achieved whether the slide was hydrophilic or hydrophobic. It may be caused by unusually high monolayer viscosity of P1 above the plateau. X-Ray diffraction measurements were performed on the P2 and P3 LB films. The transferred LB films did not show any diffraction peak. This result clearly demonstrates that the natural layer structure normal to the film plane was destroyed during or after the transfer. There are two factors that need to be considered to explain this observation. First, according to the flow orientation model, rearrangement of the molecules and molecular aggregates may take place during monolayer flow due to the transfer from the air᎐water interface onto the solid slide w22x. Second, the condensed film at the air᎐water interface is a metastable state which has a tendency to form spontaneously the most stable phase. However, it is a relatively slow process because of the strong interaction between liquid crystal molecules in the closely-packed state. After storing the film samples at room temperature for 3 months, there was still not any diffraction peak to appear. When the film samples were heated in an oven at temperatures of 338 K for P2 and 353 K for P3 for 3 h, respectively, and then cooled to ambient temperature, the diffraction peaks had been found to correspond to layer spacings of 2.54 nm for P2 and 3.27 nm for P3, respectively, as shown in Fig. 4. Heating thus enhanced the ordering of the LB
J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
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cated Langmuir film results would be tentative. Further studies are continuing. Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research No. 07454189 from the Ministry of Education, Science and Culture of Japan. References Fig. 4. X-Ray diffraction patterns of LB films: Ža. 40-layer P2 deposited at 10 mNrm; and Žb. 34-layer P3 deposited at 20 mNrm.
films w20,21x. The ordering structure, once formed, was stable for at least 3 months at room temperature. The layer spacing of P3 is well consistent with the length of the side chain estimated ŽTable 2.. Considering its X-type deposition feature, we can conclude that the LB film of P3 shows parallel orientation of adjacent layers with an X-type film structure. Daniel and coworkers w23x have confirmed that the liquid crystal 4-cyano-4⬘⬘-n-pentyl-p-terphenyl gives a Z-type multilayer structure. Such X- or Z-type structure has been expected to apply as non-linear optical, piezoelectric or pyroelectric devices w1,2x. The layer spacing of P2 is situated between one side chain length and two side chain lengths, indicating that the liquid crystal side chains may form a so-called ‘interdigitated partially bilayer’ arrangement. Though we have no further evidence on this arrangement, the antiparallel arrangement would act to reduce dipole repulsion and to stabilize the structure of the film w24x. It is of interest to note that the LB method induced the smectic ordering for the polymer P2 which exhibits only a nematic phase in bulk. Finally, it is necessary to point out that the widen peak at 2 of approximately 6⬚ for all the films corresponds to the reflection of the glass slide surface. In summary, the surface behavior of side chain liquid crystal copolysiloxanes bearing fluorine substituents is quite different from each other. The state of extension or coiling of the flexible polysiloxane chains depends on the compression state of the film and the packing of the side chains. It has been demonstrated that after annealing the LB film of the polymer containing a long perfluorocarbon substituent possesses an uncommon X-type film structure, while the polymer containing a trifluoromethoxyl group gives a Y-type film structure. We think that the present elucidation for such compli-
w1x G. Roberts, Langmuir᎐Blodgett Films, Plenum Press, New York, 1990. w2x A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir᎐Blodgett to Self-Assembly, Academic Press Inc, New York, 1991. w3x W. Noll, H. Steinbach, C. Sucker, J. Polym. Sci. Part C 34 Ž1971. 123. w4x M.K. Bernett, W.A. Zisman, Macromolecules 4 Ž1971. 47. w5x S. Granick, S.J. Clarson, T.R. Formoy, J.A. Semlyen, Polymer 26 Ž1985. 925. w6x J. Adams, W. Rettig, R.S. Duran, J. Naciri, R. Shashidhar, J. Phys. Chem. 97 Ž1993. 2021. w7x T.J. Lenk, D.H.T. Lee, J.T. Koberstein, Langmuir 10 Ž1994. 1857. w8x S.I. Belousov, E. Sautter, Y.K. Godovsky, N.I. Makarova, W. Pechhold, Polym. Sci. Ser. A 38 Ž1996. 1008. w9x M. Vandevyver, P. Keller, M. Rouillay, J.P. Bourgoin, A. Barraud, J. Phys. D Appl. Phys. 26 Ž1993. 686. w10x A. El Abed, J. Daillant, P. Peretti, Langmuir 9 Ž1993. 3111. w11x C. Jego, E. Dupart, P.A. Albouy, C. Mingotaud, Thin Solid Films 327r329 Ž1998. 1. w12x J. Xue, C.S. Jung, M.M. Kim, Phys. Rev. Lett. 69 Ž1992. 474. w13x M.N.G. de Mul, J.A. Mann, Langmuir 10 Ž1994. 2311. w14x M. Ibn-Elhaj, M.Z. Cherkaoui, R. Zniber, H. Mohwald, J. Phys. Chem. B 102 Ž1998. 5274. w15x J. Mu, H. Okamoto, V.F. Petrov, S. Takenaka, Mol. Cryst. Liq. Cryst. 337 Ž1999. 121. w16x E.P. Honig, J.H.T. Hengst, D. Den Engelsen, J. Colloid Interface Sci. 45 Ž1973. 92. w17x J.E. Biegajski, D.A. Cadenhead, P.N. Prasad, Langmuir 4 Ž1988. 689. w18x W. Noll, H. Steinbach, C. Sucker, Ber. Bun. Phys. Chem. 67 Ž1963. 407. w19x J. Minones, E. Iribarnegaray, C. Varela et al., Langmuir 8 Ž1992. 2781. w20x R. Jones, R.H. Tredgold, A. Hoorfar, R.A. Allen, P. Hodge, Thin Solid Films 134 Ž1985. 57. w21x W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Thin Solid Films 210r211 Ž1992. 114. w22x N. Minari, K. Ikegami, S. Kuroda, K. Saito, M. Saito, M. Sugi, Solid State Commun. 65 Ž1998. 1259. w23x M.F. Daniel, O.C. Lettington, S.M. Small, Thin Solid Films 99 Ž1983. 61. w24x S. Chandrasekhar, Liquid Crystals, Cambridge University Press, Cambridge, 1992, p. 1.
Langmuir and Langmuir᎐Blodgett films of fluorine-containing side chain liquid crystal copolysiloxanes Jin Mu, Hiroaki Okamoto, Shunsuke TakenakaU Department of Ad¨ anced Materials Science and Engineering, Faculty of Engineering, Yamaguchi Uni¨ ersity, Ube, Yamaguchi 755-8611, Japan Received 13 January 2000; received in revised form 11 April 2000; accepted 26 May 2000
Abstract The Langmuir film behavior of fluorine-containing side chain liquid crystal copolysiloxanes at the air᎐water interface has been studied over a temperature range of 277.5᎐310 K. The surface pressure᎐area isotherms exhibited a transition from an expanded to a condensed phase. In the expanded phases the main chains of the polymers were anchored at the air᎐water interface. In the condensed phases the extension or partial exclusion of the main chains at the air᎐water interface was mainly controlled by the cross-sectional area of the side chain. The transferred Langmuir᎐Blodgett ŽLB. multilayer did not give any X-ray diffraction peak. With heating, a molecular ordering of the LB film was induced so that the film showed the Bragg diffraction peak. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Liquid crystal copolysiloxanes; Langmuir films; Langmuir᎐Blodgett films; Fluorine substituent effect
1. Introduction In recent years, monolayers and Langmuir᎐Blodgett ŽLB. films of polymers, especially polysiloxanes, have received a great deal of attention owing to higher thermal and mechanical stability w1᎐8x. It is well known that the polysiloxane chain is very flexible and the oxygen atoms of the silicon backbone give an amphiphilic property to the polymer. The hydrogen bond between the polysiloxane chain and water is responsible for the spread of the polymer w3x. Full extension or coiling of the main chain depends on the property of the substituent w4x. Liquid crystal compounds have been shown to yield easily well-controlled organized monolayers w9᎐11x and multilayers w12᎐14x at the air᎐water interface. Their organization at the air᎐water interface depends on the
liquid crystal ŽLC. molecule᎐LC molecule interactions and the LC molecule᎐aqueous subphase interactions as well as the compressing state of the film. Recently, we reported on the effects of alkyl chain length on the monolayer formation of copolysiloxanes having a 4alkoxyphenyl benzoate core in the side chain w15x. The results showed that the surface pressure can induce the tight packing of the liquid crystal core in the side chain and removal of the ŽCH3 .2 SiO segment in the main chain from the water surface. In this paper, we will report some recent results on Langmuir films and Langmuir᎐Blodgett films of fluorine-containing side chain liquid crystal copolysiloxanes. 2. Experimental 2.1. Materials
U
Corresponding author. Tel.: q81-836-35-9964; fax: q81-836-359965. E-mail address: [email protected] ŽS. Takenaka..
The present polymers were prepared according to the scheme in Fig. 1.
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 0 5 5 - 5
J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
241
Fig. 1. Synthetic scheme of copolysiloxanes.
2-Perfluorooctylethanol was purchased from Daikin Kogyo Co., and used without further purification. The siloxane copolymer, TSF-483 ŽToshiba-sirikon Co.. was used without further purification. The purity of the compounds was checked by a 1 H NMR spectroscopy and a differential scanning calorimeter ŽDSC. by using a ‘DSC-PURITY’ program ŽSeiko-denshi Co... 4-Trifluoromethylphenyl, 4-trifluoromethoxyphenyl, and 4-w2-Žperfluoro-octyl.ethoxy-carbonylxphenyl benzoates, M1, M2, and M3, respectively, were prepared by the condensation of the corresponding phenols and 4-aryloxybenzoic acid.
M1: IR ŽKBr disc. 1649 Ž Cs C . and 1728 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 4.64 Ž2H, d, J s 5.3 Hz., 5.34 Ž1H, dd, J s 10.4, 1.5 Hz., 5.44 Ž1H, dd, J s 17.2, 1.5 Hz., 6.04 Ž1H, ddt, J s 17.2, 5.3, 10.4 Hz., 7.00 Ž2H, d, J s 8.9 Hz., 7.33 Ž2H, d, J s 8.6 Hz., 7.68 Ž2H, d, J s 8.6 Hz., 8.14 Ž2H, d, J s 8.9 Hz. ppm. M1 did not exhibit any mesophase even in a rapid cooling process, m.p. 360 K Ž ⌬H s 78 Jrg.. M2: IR ŽKBr disc. 1649 Ž Cs C . and 1734 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 4.63 Ž2H, d, J s 5.3 Hz., 5.34 Ž1H, dd, J s 10.6, 1.3 Hz., 5.44 Ž1H, dd, J s 17.2, 1.3 Hz., 6.04 Ž1H, ddt, J s 17.2, 5.3, 10.6
Fig. 2. Optical micrographs of: Ža. M2 at 350 K; Žb. P2 at 340 K; Žc. M3 at 350 K; and Žd. P3 at 350 K.
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Hz., 7.00 Ž2H, d, J s 8.9 Hz., 7.23 Ž2H, d, J s 9.6 Hz., 7.27 Ž2H, d, J s 9.6 Hz., 8.13 Ž2H, d, J s 8.9 Hz. ppm. M2 exhibited a monotropic smectic A phase ŽSm A. having a focal conic fan texture, as shown in Fig. 2a, m.p. 365 K Ž ⌬H s 89 Jrg. and Sm A-isotropic ŽI. 356 K Ž ⌬H s 23 Jrg.. M3: IR ŽKBr disc. 1647 Ž Cs C . and 1726 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 2.62 Ž2H, tt, J s 18.5, 6.4 Hz., 4.64 Ž2H, t, J s 6.4 Hz., 4.65 Ž2H, t, J s 5.3 Hz., 5.35 Ž1H, dd, J s 10.6, 1.3 Hz., 5.44 Ž1H, dd, J s 17.3, 1.3 Hz., 6.05 Ž1H, ddt, J s 17.3, 5.3, 1.3 Hz., 7.00 Ž2H, d, J s 9.1 Hz., 7.31 Ž2H, d, J s 8.9 Hz., 8.12 Ž2H, d, J s 8.9 Hz., 8.14 Ž2H, d, J s 9.1 Hz. ppm. M3 exhibited an enantiotropic Sm A phase having a focal conic fan texture, as shown in Fig. 2c, m.p. 380 K Ž ⌬H s 35 Jrg. and Sm A-I 438 K Ž ⌬H s 11 Jrg.. The X-ray profile for the Sm A phase showed a sharp reflection at 2 s 2.40⬚ Ž3.68 nm. arising from the smectic layer, as well as a broad one at approximately 2 s 20⬚. P1: To a solution of M1 Ž2.44 g, 6.96 mmol. and copolysiloxane ŽTSF-483, 0.8 g. in dry toluene Ž36 ml., dry THF Ž0.1 ml. containing 3 wrw% of H2 PtCl6 ⭈ 6H2 O was added under a nitrogen atmosphere, and the resulting solution was stirred at 110⬚C for 24 h. After the reaction was completed, the reaction mixture was poured into methanol Ž150 ml., and the solid precipitated was collected by decantation. The crude product was purified by repeating reprecipitation with a solvent mixture of toluene and methanol, giving P1 as a colorless gum, 4.34 g Ž56%.. IR ŽKBr disc. 1734 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 0.09 Ž9H., 0.66 Ž2H., 1.85 Ž2H., 3.91 Ž2H., 6.92 Ž2H., 7.21 Ž2H., 7.59 Ž2H., 8.02 Ž2H. ppm. The intensity ratio of the peaks at 0.09 Ž9H. to 8.02 ppm Ž2H. suggested that more than 96% of the hydrogens of copolymer were substituted by the liquid crystalline core. P1 exhibited a nematic Ž N . phase with a shear texture. Tg 273 K and N-I 369 K Ž ⌬H s 6 Jrg.. The X-ray profile for the N phase showed a weak and broad reflection at approximately 2 s 20⬚. P2, and P3 were also obtained by a similar manner. P2: IR ŽKBr disc. 1730 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 0.10 Ž9H., 0.67 Ž2H., 1.85 Ž2H.,
3.92 Ž2H., 6.93 Ž2H., 7.21 Ž4H., 8.03 Ž2H. ppm. The intensity ratio of the peaks at 0.10 Ž9H. to 8.03 ppm Ž2H. indicated that more than 95% of the hydrogens of the copolymer were substituted by the liquid crystalline core. This polymer exhibited an N phase with a shear texture, as shown in Fig. 2b. Tg 259 K and N-I 352 K Ž ⌬H s 5 Jrg.. The X-ray profile for the N phase showed a broad reflection at approximately 2 s 20⬚. P3: IR ŽKBr disc. 1647 Ž Cs O . and 1726 cmy1 Ž Cs O .. 1 H NMR ŽCDCl3 , 270 MHz. ␦ 0.10 Ž9H., 0.67 Ž2H., 1.85 Ž2H., 2.59 Ž2H., 3.92 Ž2H., 4.60 Ž2H., 6.92 Ž2H., 7.20 Ž2H., 8.20 Ž4H. ppm. The intensity ratio of the peaks at 0.10 Ž9H. to 8.20 ppm Ž2H. indicated that more than 95% of the hydrogens of copolymer were substituted by the liquid crystalline core. P3 exhibited an Sm A phase with a fine focal conic fan texture, as shown in Fig. 2d. Sm A-I 374 K Ž ⌬H s 8 Jrg.. P3 did not show a glassy transformation in DSC thermogram. The X-ray profile for the Sm A phase showed a sharp reflection at 2 s 2.64⬚ Ž3.34 nm. arising from the smectic layer, as well as a broad one at approximately 2 s 20⬚. 2.2. Method Film experiments were performed on a computercontrolled film balance, FSD-110P ŽUSI System Ltd., Japan.. The surface pressure was measured by a Wilhelmy plate with a precision of "0.1 mNrm. Reported temperatures refer to the subphase temperatures with an accuracy of "0.2 K. The copolysiloxanes were dissolved in chloroform Žspectro grade. with a concentration no higher than 0.8 mgrml. The solutions with a precise volume were dropped carefully on the distilled water surface using a microsyringe. After complete evaporation of the solvent Žapprox. 20 min. the surface pressure᎐area isotherms were recorded at a constant compression speed of 0.48 cm2rs. The isotherm reproducibility was within better than 1 mNrm at a given surface area. The glass slides were cleaned and made hydrophobic as shown by Honig et al. w16x. LB multilayers were transferred onto hydrophobic glass slides by the Y-type deposition mode at a constant target pressure and
Table 1 Transition temperatures and characteristic monolayer data Ž296.5 K. of polymersa P
P1 P2 P3
Transition temp. ŽK.
Onset area Žnm2 .
Tg273N369I Tg259N352I Sm A374I
0.487 0.917 1.363
Inflection point A Žnm2 .
ŽmNrm.
0.779 0.966
2.0 2.0
A 0 Žnm2 .
A e Žnm2 .
0.212 0.20 0.389
0.221 0.208 0.438
a Tg is a glossy transformation temperature. N, Sm, A and I indicate nematic, smectic A and isotropic phase, respectively AO is the lining area per repeat unit of the condensed state. Ae is the cross-sectional area of the side chains evaluated by the CS Chem 3D Pro Vision 4.0.
J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
Fig. 3. Temperature dependency of surface pressure᎐area isotherms for: Ža. P1; Žb. P2; Žc. P3.
temperature of 293 " 0.2 K. The dipping speed for upstroke and down stroke was all 5 mmrmin. X-Ray diffraction experiments of LB films were performed on a Rigaku-denki RINT 2200 diffractometer, where CuK ␣ which corresponds to a wavelength of 0.1541 nm was used as an X-ray source. 3. Results and discussion 3.1. Surface pressure᎐area isotherms All three copolysiloxanes form stable Langmuir films at the air᎐water interface. Their surface pressure᎐area isotherms as a function of temperature are presented in Fig. 3. Transition temperatures in bulk as well as characteristic areas and surface pressures at 296.5 K are summarized in Table 1 for comparison. From Fig. 3 we can see that the copolysiloxanes studied show the respective characteristic isotherms. The order of their onset areas at 296.5 K is P1 - P2 P3 ŽTable 1.. It is reasonable to assume that before the
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onset of the pressure the main chains of the copolysiloxanes adopted an extended configuration, with the oxygen close to water, and the rest stretched into the air. On compression, this process was not accompanied by an increase in pressure up to the point of the onset area, where the interaction between adjacent molecules became observable. Therefore, the distinction of the onset areas should apparently attribute to the effects of length Žsee Table 2., steric hindrance, and various possible arrangements of the side chains. The relatively high polarity of the fluoromethyl group directly attached to the mesogenic core may cause a spreading of incompletely extended polymer, which leads to the smallest onset area. In addition, the hydrophilic affinity of the oxygen atom for P2 and carboxyl group for P3 attached to the mesogenic core may play a role in the lateral coherence of the side chains at larger areas per repeat unit. P1 did not exhibit the lower pressure plateaus at higher temperatures Ži.e. 296.5 and 310 K.. In other words, the film could directly be compressed into a condensed state at higher temperatures. When the subphase temperature was decreased only slight indications were seen of inflections of the plateaus. At 277.5 K, extrapolating the steep part of the isotherm to zero pressure leads to a limiting area of 0.221 nm2rrepeat unit, which is just consistent with the cross-sectional area of the side chain evaluated from the CS Chem3D Pro Version 4.0. According to this result we can postulate that in the condensed state the siloxane unit connecting with the side chain was anchored at the water surface. The plateaus correspond to a transition from an expanded to a condensed phase w10x. Before the plateau the main chains of P1 still lay flat on the water surface. As the pressure increased the side chains tilted gradually away from the water. At the inflection the ŽCH3 .2 SiO segment in the main chains began to be squeezed out of the water surface, leading to tight packing of the side chains. The isotherms of P2 showed the lower pressure plateaus at any temperature at the surface pressure of approximately 2.0 mNrm. After passing the plateau the isotherms turned upward and all converged at an area of approximately 0.2 nm2rrepeat unit at four temperatures. After further compression P2 passed through a second transition and then the surface presTable 2 X-Ray diffraction data of LB films a P
d-spacing Žnm.
P1 P2 P3
2.54 3.27 a
l Žnm. 2.00 2.12 3.25
l is the length of the side chains bearing the CH 3 SiO group evaluated by a semi-empirical molecular orbital calculation ŽAM1 method, MOPAC Ver. 6.0..
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sure increased steeply again. It is evident that the second transition corresponds to the collapse of the film. When the subphase temperature was decreased, a small increase in the pressure corresponding to the plateau was observed. The property of the lower pressure plateaus is analogous to P1. The negative temperature dependency of the plateau pressures for P1 and P2 can be explained as an increase in mobility of main chains and side chains at higher temperature favoring both removal of the ŽCH3 .2 SiO groups from the water surface and the closest packing of the side groups w15,17x. It is interesting to note that the plateau pressure of P3 is much higher than that of P1 and P2. A reasonable explanation for this is that the steric hindrance of the long perfluorocarbon group is much stronger than the short trifluoromethyl and trifluoromethoxyl group. P3 needs much higher surface pressure to force the packing of the side chains. In contrast with P1 and P2 the plateau pressure of P3 was hardly altered by subphase temperature Žsee Fig. 3c.. Since twice the value, 0.193 " 0.013 nm2rmonomer for polydimethylsiloxane found by Noll and coworkers w18x is smaller than the crosssectional area Ž0.483 nm2 , see Table 1. of the side chain, it is plausible to postulate that the plateau of P3 only reflects compression-induced tight packing arrangement of the side chains w19x, and in the condensed state the siloxane groups in the main chains are still all anchored at the air᎐water interface. In other words, the skeleton effect of the side chain maintains the extension of the main chain at the water surface. Repeated compression of the Langmuir film and subsequent expansion have no essential effect on the shape of the isotherms for P1᎐P3. The recompression isotherm is nearly identical with the initial one up to the inflection of the transition. If the film was expanded beyond the plateau and then recompressed, the isotherms were shifted to lower areas per repeat unit, indicating that the condensed phase contains the composition with irreversible structural changes. We notice that the limiting areas per repeat unit of three polymers in the condensed state are all slightly smaller than the evaluated cross-sectional areas of corresponding side chains Žsee Table 1.. This may imply a partial multilayer formation or a staggered packing arrangement of the side chains w20x. When the subphase temperature was increased the limiting areas per repeat unit of P1 and P3 in the condensed state were shifted toward smaller values. We can explain this phenomenon from two aspects. First, increasing temperature favors the molecular motion to form multilayer aggregates or a staggered packing arrangement of the side chains. This led to the areas being shifted toward smaller values. Second, it was observed that the film appeared very viscous while being compressed above the plateau, actually moving
the Wilhelmy plate out of its vertical position in the direction of film compression. Consequently, the high viscosity of the film produced a measuring error for a Langmuir trough using the single barrier compression system w21x. By the way, it is related to the high viscosity of the film that there are not apparent collapse pressures in the isotherms of P1 and P3. 3.2. Langmuir᎐Blodgett films When the glass slide was hydrophilic, the first stroke was always an upstroke. Whenever the slide was withdrawn, it appeared wet. We waited for the slide to dry even for 30 min before attempting to transfer the following layer; this still failed as it was only possible to deposit less than 10 layers Žtransfer ratios approx. 0.5. and the films showed inhomogeneity. However, except for P1, the films of P2 and P3 could easily be transferred onto the hydrophobic glass slides at a surface pressure of 10 mNrm for P2 or 20 mNrm for P3, respectively. In such a case, particular care was still taken to ensure complete drying of the emersed film before further deposition. The deposition of P2 was Y-type with a transfer ratio on the down stroke of approximately 1.0 and on the upstroke of approximately 0.8. The deposition of P3 resulted in X-type with a transfer ratio on the down stroke of approximately 1.0 and on the upstroke of less than 0.1. For P1 no transfer could be achieved whether the slide was hydrophilic or hydrophobic. It may be caused by unusually high monolayer viscosity of P1 above the plateau. X-Ray diffraction measurements were performed on the P2 and P3 LB films. The transferred LB films did not show any diffraction peak. This result clearly demonstrates that the natural layer structure normal to the film plane was destroyed during or after the transfer. There are two factors that need to be considered to explain this observation. First, according to the flow orientation model, rearrangement of the molecules and molecular aggregates may take place during monolayer flow due to the transfer from the air᎐water interface onto the solid slide w22x. Second, the condensed film at the air᎐water interface is a metastable state which has a tendency to form spontaneously the most stable phase. However, it is a relatively slow process because of the strong interaction between liquid crystal molecules in the closely-packed state. After storing the film samples at room temperature for 3 months, there was still not any diffraction peak to appear. When the film samples were heated in an oven at temperatures of 338 K for P2 and 353 K for P3 for 3 h, respectively, and then cooled to ambient temperature, the diffraction peaks had been found to correspond to layer spacings of 2.54 nm for P2 and 3.27 nm for P3, respectively, as shown in Fig. 4. Heating thus enhanced the ordering of the LB
J. Mu et al. r Thin Solid Films 372 (2000) 240᎐245
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cated Langmuir film results would be tentative. Further studies are continuing. Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research No. 07454189 from the Ministry of Education, Science and Culture of Japan. References Fig. 4. X-Ray diffraction patterns of LB films: Ža. 40-layer P2 deposited at 10 mNrm; and Žb. 34-layer P3 deposited at 20 mNrm.
films w20,21x. The ordering structure, once formed, was stable for at least 3 months at room temperature. The layer spacing of P3 is well consistent with the length of the side chain estimated ŽTable 2.. Considering its X-type deposition feature, we can conclude that the LB film of P3 shows parallel orientation of adjacent layers with an X-type film structure. Daniel and coworkers w23x have confirmed that the liquid crystal 4-cyano-4⬘⬘-n-pentyl-p-terphenyl gives a Z-type multilayer structure. Such X- or Z-type structure has been expected to apply as non-linear optical, piezoelectric or pyroelectric devices w1,2x. The layer spacing of P2 is situated between one side chain length and two side chain lengths, indicating that the liquid crystal side chains may form a so-called ‘interdigitated partially bilayer’ arrangement. Though we have no further evidence on this arrangement, the antiparallel arrangement would act to reduce dipole repulsion and to stabilize the structure of the film w24x. It is of interest to note that the LB method induced the smectic ordering for the polymer P2 which exhibits only a nematic phase in bulk. Finally, it is necessary to point out that the widen peak at 2 of approximately 6⬚ for all the films corresponds to the reflection of the glass slide surface. In summary, the surface behavior of side chain liquid crystal copolysiloxanes bearing fluorine substituents is quite different from each other. The state of extension or coiling of the flexible polysiloxane chains depends on the compression state of the film and the packing of the side chains. It has been demonstrated that after annealing the LB film of the polymer containing a long perfluorocarbon substituent possesses an uncommon X-type film structure, while the polymer containing a trifluoromethoxyl group gives a Y-type film structure. We think that the present elucidation for such compli-
w1x G. Roberts, Langmuir᎐Blodgett Films, Plenum Press, New York, 1990. w2x A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir᎐Blodgett to Self-Assembly, Academic Press Inc, New York, 1991. w3x W. Noll, H. Steinbach, C. Sucker, J. Polym. Sci. Part C 34 Ž1971. 123. w4x M.K. Bernett, W.A. Zisman, Macromolecules 4 Ž1971. 47. w5x S. Granick, S.J. Clarson, T.R. Formoy, J.A. Semlyen, Polymer 26 Ž1985. 925. w6x J. Adams, W. Rettig, R.S. Duran, J. Naciri, R. Shashidhar, J. Phys. Chem. 97 Ž1993. 2021. w7x T.J. Lenk, D.H.T. Lee, J.T. Koberstein, Langmuir 10 Ž1994. 1857. w8x S.I. Belousov, E. Sautter, Y.K. Godovsky, N.I. Makarova, W. Pechhold, Polym. Sci. Ser. A 38 Ž1996. 1008. w9x M. Vandevyver, P. Keller, M. Rouillay, J.P. Bourgoin, A. Barraud, J. Phys. D Appl. Phys. 26 Ž1993. 686. w10x A. El Abed, J. Daillant, P. Peretti, Langmuir 9 Ž1993. 3111. w11x C. Jego, E. Dupart, P.A. Albouy, C. Mingotaud, Thin Solid Films 327r329 Ž1998. 1. w12x J. Xue, C.S. Jung, M.M. Kim, Phys. Rev. Lett. 69 Ž1992. 474. w13x M.N.G. de Mul, J.A. Mann, Langmuir 10 Ž1994. 2311. w14x M. Ibn-Elhaj, M.Z. Cherkaoui, R. Zniber, H. Mohwald, J. Phys. Chem. B 102 Ž1998. 5274. w15x J. Mu, H. Okamoto, V.F. Petrov, S. Takenaka, Mol. Cryst. Liq. Cryst. 337 Ž1999. 121. w16x E.P. Honig, J.H.T. Hengst, D. Den Engelsen, J. Colloid Interface Sci. 45 Ž1973. 92. w17x J.E. Biegajski, D.A. Cadenhead, P.N. Prasad, Langmuir 4 Ž1988. 689. w18x W. Noll, H. Steinbach, C. Sucker, Ber. Bun. Phys. Chem. 67 Ž1963. 407. w19x J. Minones, E. Iribarnegaray, C. Varela et al., Langmuir 8 Ž1992. 2781. w20x R. Jones, R.H. Tredgold, A. Hoorfar, R.A. Allen, P. Hodge, Thin Solid Films 134 Ž1985. 57. w21x W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Thin Solid Films 210r211 Ž1992. 114. w22x N. Minari, K. Ikegami, S. Kuroda, K. Saito, M. Saito, M. Sugi, Solid State Commun. 65 Ž1998. 1259. w23x M.F. Daniel, O.C. Lettington, S.M. Small, Thin Solid Films 99 Ž1983. 61. w24x S. Chandrasekhar, Liquid Crystals, Cambridge University Press, Cambridge, 1992, p. 1.