Langmuir 1994,10, 905-911
906
Noncentrosymmetric Langmuir-Blodgett Films Containing Nitrobiphenyl Groups S. H. Ou,?V. Percec,?J. A. Mann,* and J. B. Lando'J Department of Macromolecular Science and Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106 Received May 5, 1993. I n Final Form: December 8, 199P The monolayer formationof two nitrobiphenyl-containingcompoundswas facilitated by mixing molecules that have similar structures. Transfer behavior of the monolayersto microscopeslides made hydrophobic was examined by vertical and horizontal deposition. All the monolayersshowed %type vertical deposition. X-ray diffractionresults show that the multilayer structure is Y-type, irrespective of the deposition type. However an alternating-layermethod of deposition was developed,such that nitrobiphenylcan be located either on the hydrophobicor the hydrophilic chain ends, utilizing ita low hydrophilicity. X-ray diffraction and pyroelectric measurements suggest that a polar film was constructed with nitrobiphenyl oriented in the same direction. The pyroelectric coefficient of the alternating-layer film was measured to be 1.7 X
1. Introduction
A Langmuir monolayer can be transferred onto a substrate by three possible deposition types: X, Y, and Z. Each deposition type can be performed by immersing a substrate either perpendicular to the water surface (vertical deposition) or parallel to the water surface (horizontal deposition). Figure 1 shows the vertical depositiontypes. Only filmsdeposited in the X or Z mode have the natural potential for a noncentrosymmetric structure. A noncentrosymmetric structure is essential for a material to show the pyroelectriceffect, piezoelectric effect, and the x@)nonlinearoptical effect. Unfortunately, Y-type deposition usually occurs. In addition, X or Z type deposition usually leads to a Y-type structure14 because of molecular turn-around.596 So far, only a few materials have been reported to produce genuine X-type or Z-type m u l t i l a y e r ~ . ~The ~ ~ -tendency ~~ for amphiphiles to pack in a Y-type structure is because the interactions between adjacent monolayersare either hydrophobic-hydrophobic or hydrophilic-hydrophilic in that structure. Since it is difficult to construct a polar film from a single material, a different approach, the alternating-layer concept, has been adopted. This approach is to work with the natural tendency of the molecules to associate head groups in adjacent layers. In this approach two kinds of monolayers are used. One monolayer (A) is transferred on the downstrip and the other (B)on the uptrip. Thus a stable ABAB Y-type film is obtained with no plane of t Department
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i Figure 1. (a) Possible deposition types for a hydrophobic substrate A, x, y, and z deposition;B, repeat pattern for y layera. (b) A Y-type ABAB LB multilayer film.
of Macromolecular Science.
t Department of Chemical Engineering. 0 Abstract published in Advance ACS Abstracts, February 15, 1994. (1) Momoee,A.;Hirai,Y.;Waki,I.;Imezeki,S.;Temioka,Y.;Hayakawa, K.;Naito, M. Thin Solid Films 1989,178,519. (2) Tregold, R. H.; Allen, R. A.; Hodge, P. Thin Solid Films 1987,155, 343. (3) Biddle, M. B.Ph.D.Disaertation,Cam Western Reserve University, l9s8, Chapter 5. (4) Fukuda, K.; Shiozawa, T.Thin Solid Films 1980,68, 55. (5) Imgmuir, I. Scrence 1938,87, 493. (6) Honig, E. P. J. Colloid Interface Sci. 1973,43, 66. (7) Daniel, M. F.; Lettington, 0. C.; Small,5.M. Thin Solid Films 198S, 99,61. (8) Blinov, L. M.; Dubinin, N.V.; Mikhnev, L. V.; Yudin, S. G. Thin Solid Film 1984,120,161. (9) Popovitz-Biro, R.; Hill, K.; Sharit, E.; Hung, D. J.; Lahav, M.; Leiserowitz, L.; Sagiv, J.; Hsiung, H.; Meredith, G. R.; Vanhemle, H. J. Am. Chem. Soe. 1990,112,2498. (10) Enblmann, V.; Lando, J. B. J. Polym. Sci., Polym. Chem. Ed. 1977,15, 1843.
symmetry (see Figure lb). Daniel et al. were pioneers in this area.11J2 Since then, there have been many papers dealing with the formation and structure of ABAB film^.'^^ The most studied system is the alternation of aliphatic acid and amine monolayers. Therefore, a con(11) Daniel, M. F.;Smith, G. W. Mol. Cryst. Liq. Cryst. 1984,102,193. (12) Smith, G.W.; Daniel, M. F.; Barton, J. W.; Ratcliffe, N. Thin Solid Films 198S, 132,125. (13) Christie,P.; Roberta, G. G.; Petty, M. C. Appl. Phys. Lett. 1986, 4R. --, 1101 - - - -. (14) Christie,P.; Jones, C. A.; Petty, M. C.;Roberta, G.G. J. Phys. D Appl. Phys. 1986,19, L167. (15) Smith, G.W.;Evans, T. J. Thin Solid Films 1987,11,305. (16) Smith,G.W.; Matcliffe, N.;Rose?,5.J.: Daniel, M. F. Thin Solid Films 1987,151,9. (17) Jones, C. A.;Petty, M. C.; Roberta, G. G. Thin Solid Films 1988, 159, 461. (18) See ref 3, p 222.
0743-7463/94/2410-0905$04.50/00 1994 American Chemical Society
906 Langmuir, Vol. 10, No. 3, 1994 Scheme 1. Synthetic Pathway and Structure of VBPhOH H2C&H-( M&OH
structive summation of dipole moments from the two monolayers results. In this research, monolayer formation and transfer behaviorof materials containing nitrobiphenyl groups were examined. Nitrobiphenyl chromophores were used to provide polarity for pyroelectricity. The multilayer structures were identified by X-ray diffraction. A nontraditional type of alternating-layer method was employed. In this approach, the nitrobiphenyl groups provide the major contribution to the dipole moment. Nitrobiphenylgroups were located on either the hydrophobic or the hydrophilic chain ends, in the former case utilizing its low hydrophilicity. A polar alternating-layer film was constructed with nitro groups pointing in the same direction and the pyroelectric coefficient was measured. 2. Experimental Section
Synthesis. Materials. 10-Undecen-1-01 (99 %), p-toluenesulfonyl chloride (99%), 4,4'-dihydroxybiphenyl (97 % 1, 4-hydroxybiphenyl(97 %), benzoyl chloride (99%),fumic nitric acid (90%),dimethylsulfate(99%),and ll-bromo-l-undecanol(98%) were obtained from Aldrich and used as recieved. Techniques. 'H-NMR (200 MHz) spectra were recorded on a Varian XL-200 spectrometer. TMS was used as an internal standard. Melting points were obtained with a Thomas Hoover capillary melting point apparatus. High-pressure liquid chromatogrpahy (HPLC) experiments were performed with a PerkinElmer series LC-10 instrument equipped with an LC-100 column oven, and a Nelson Analytical integrator data station with a 900 series interface, using CHCb as solvent. The measurements were made at 40 "C using a UV detector (254 nm). The synthesis of the compounds used in this study is outlined in Schemes 1 to 3. 4-(lO-Undecen-l-yloxy)-4'-hydroxybiphenyl(VBPhOH) was synthesized according to a literature method.% Purity: >99% (HPLC). *H-NMR (CDCls, TMS, 6, ppm): 1.30 (m, 12H, -(CH&-), 1.80 (m, 2H, -CHz-CHz-O-Ar-), 2.02 (m, 2H, -CHzCH=CHz), 4.00 (t, 2H, -CHZ-O-Ar-), 4.98 (m, 2H, CHFCHCHz-), 6.98-6.99 (m, 4H, Ar-H ortho to -0-),7.42-7.50 (m, 4H, Ar-H meta to -0-). 4-(1l-Hydroxyundecan-l-yloxy)-4'-methoxybiphenyl(CHaBPhOH) was synthesized according to a literature method.%#% Mp: 142-143.5 O C . Purity: >99% (HPLC). 'H-NMR (CDClS,
Ou et al. Scheme 2. Synthetic Pathway and Structure of CHaPhON H O-H
Scheme 3. Synthetic Pathway and Structures of VBPhNOz a n d NOzBPhOH
0
W G i Q I
1
KOH
t
(19) Shih, K. S. Ph.D. Dissertation,Case WestemReserve University, 1988, p 219. (20) Hermann, J. P.; Ducuing, J. J. Appl. Phys. 1974,45, 5100. (21) Daniel, M. F.; Smith, G. W. Mol. Cryst.Lip.Cryst. 1984,102,193. (22) Oirling,I. R.; Kolinsky, P. V.; Cade, N. A,; Earls, J. D.; Peterson, I. R. Opt. Commun. 1985,55, 289. (23) Neal, D. B.; Petty, M. C.; Roberta, G. G.; Ahmad, M. M.; Feast,
TMS, 6, ppm): 1.31 (m, 16H, -(CHz)a-), 1.81 (m, 2H, -C&CHz-O-Ar), 3.63 (t, 2H, -CHz-OH), 3.83 (8, 3H, CHsUAr-), 3.98 (t,2H, -CHz-O-Ar-), 6.96-7.01 (d, 4H, Ar-H ortho t o U ) , 7.48-7.53 (d, 4H, Ar-H meta to -0-). Potassium 4-hydroxy-4'-nitrobiphenyl was synthesized according to a literature methodz7with some modification. The procedures are described in the following. 4-Benzoyloxybiphenyl (I). To a reaction mixture of 30 g (0.1763 mol) of 4-hydroxybiphenyl,31.9 mL (0.2292 mol) of dry triethylamine (distilled from KOH) and 360 mL of dry THF (distilled fromLiAlH4)wasadded 22.5 mL (0.1938mol)of benzoyl chloride at 0 OC and the reaction mixture was stirred for 4 h at room temperature. The reaction mixture was then filtered. The liquid (THF and triethylamine) waa evaporated. The resulting solid was washed with Hz0 and hot methanol and recrystallized from l-butanol to yield 46.7 g (97%) of white crystals. Mp: 149150 "C.
591.
(26) Hsu, C. S.; Rodriguez-Parada,J. M.; Percec, V. Makromol. Chem. 1987,188, 1017. (27) Leslie,T.M.;Demartino,R.N.;Choe,E.W.;Khanarian,G.;Hass,
W. J.; Girling,I. R.;Cade, N. A,; Kolinsky, P. V.; Peterson, I. R. Electron. Lett. 1986, 22, 460. (24) Percec, V.; Heck, J. J. Polym. Sci., Polym. Chem. Ed. 1991,29,
(25) Rodriguez-Parada,J. M.; Percec, V. J.Polym. Sci.,Polym. Chem. Ed. 1986, 24, 1363.
D.; Nelson, G.; Stamatoff, J. B.; Stuetz, D. E.; Teng, C.; Yoon, H. Mol. Cryst. Li9. Cryst. 1987, 153, 451.
Langmuir, Vol. 10, No. 3, 1994 907
Noncentrosymmetric L-B Films
4-Benzoyloxy-4f-nitrobiphenyl(II).Twenty grams (0.073 mol) of I was mixed with 160mL of glacial acetic acid and heated to 85 "C. Fuming nitric acid (48mL) was slowly added such that the temperature was kept between 85 and 90OC and the reaction mixture was stirred for another 30 min. The reaction mixture was filtered while hot. The resulting solid was washed with HzO and methanol and recrystallized from glacial acetic acid to yield 10.5 g (45.1%) of light yellow crystals. Mp: 211-214 "C. 'HNMR (CDCla,TMS, 6, ppm): 7.39 (d, 2H, ortho to -OOC), 7.57 (m, 3H, 2H meta to -COO, 1H para to -COO), 7.71 (d, 2H, meta to-OOC),7.76 (d, 2H,metato-NOz),8.24 (d, 2H,orthoto-COO), 8.32 (d, 2H, ortho to -NO& Potassium 4-Hydroxy-4'-nitrobiphenyl(III). Ten grams (31.3 mmol) of I1 was mixed with 60 mL of ethanol and heated to reflux. A solution of 6 g of KOH in 20 mL of water was added dropwise at reflux. The reaction mixture was refluxed for 30 min, then cooled overnight. The resulting purple crystals were filtered and washed with THF to yield 8.3 g (98.6%) of 111.The salt was utilized in subsequent reactions. I-Hydroxy-l'-nitrobiphenyl can be obtained by dissolving the salt in water and adding 50/50 HCl/H20. The yellow solid was filtered, washed with water, and recrystallized from ethhol. Mp: 203-204 OC. Purity: >99% (HPLC). 'H-NMR (acetone-& TMS, 6, ppm): 6.99 (d, 2H, ortho to -OH), 7.67 (d, 2H, meta to -OH), 7.88 (d, 2H, meta to -NOz), 8.29 (d, 2H, ortho to -Nod. 4 41O-Undecen-l-yloxy)-4'-nitrobiphenyl (VBPhN02). A reaction mixture containing 4 g (14.9 mmol) of 111,6.3 g (19.4 mmol) of 10-undecen-1-yltosylate, 100 mL of ethanol, and 18 mL of Hz0 was stirred at 80 OC for 6 h. The solid was filtered and recrystallized from methanol to yield 4.6 g (84.1%) of yellow crystals. Purity: >99% (HPLC). Mp: 69-70 OC. *H-NMR (CDCls, TMS, 6, ppm): 1.32 (m, 12H, -(CHz)s-), 1.81 (m, 2H, -CH~-CHZ-O-AI-), 2.03 (m, 2H, -CHz-CHECHz), 4.01 (t, 2H, -CH&Ar-), 4.94 (m, 2H, CHpCH-), 5.80 (m, lH,-CH=CHZ), 6.99 (d, 2H, Ar-H ortho to alkoxy), 7.63 (d, 2H, Ar-H meta to alkoxy),7.72 (d, 2H, AI-H meta to NOz),8.30 (d, 2H, Ar-Hortho to NOz). 441 l-Hydroxyundecan-l-yloxy)-4'-nitrobiphenyl(NO*BPhOH). A reaction mixture containing 2 g (7.4 m o l ) of 111, 2.2 g (8.8 mmol) of 11-bromo-1-undecanol,55 mL of ethanol, and 10 mL of water was refluxed for 12 h. The solid was fiitered, washed with hot water, and recrystallizedfrom methanol to yield 2.3 g (80.7%)ofyellowcrystals. Mp: 101-103 "C. Punty: >99% (HPLC). 1H-NMR (CDCb, TMS, 6, ppm): 1.32 (m, 16H, -(CHz)r), 1.87 (m, 2H, -CHZCHz-O-Ar), 3.70 (t, 2H, -CH2-
molecular uu (AZ/molecule)
Figure 2. Surface pressure-molecular area isotherm of NOzBPhOH. Note the small coarea.
controlled by the pulse rates of the D/A outputs on the IBM PC. Speeds varying from 0.01 to 50 mm/min can be achieved. Horizontal deposition will be discussed in the Results and Discussion section. Substrates. Multilayers were usually deposited on Corning microscope slides. The slides were degreased as follows: pretreated in 70 % nitric acid for 12 h and 5 min of ultrasonicagitation in (1)trichloroethylene, (2) acetone, and (3) methanol, respectively. To obtain hydrophobic substrates, the degreased slides were then exposed to hexamethyldisilazane (HMDS) vapor in a glass desiccator for at least 48 h. Contact Angle. Contact angles of water on the samples were measured by a Reme-Hart contact angle goniometer. The uncertaintity of the measurements was about 2 deg. X-Ray Diffraction. X-ray diffraction traces were obtained using a Phillips diffractometerwith Cu Ka radiation. Multilayera deposited on microscope slides were examined. Pyroelectricity. The pyroelectric device consisted of an alternating-layer film sandwiched between two orthogonal rectangular aluminum strips (vacuum deposited); the working area was 1 cmz as defined by the width of the aluminum strip. The electrode patterns were applied onto hydrophilic glasssubstrates using aluminum shadow masks. The substrates with bottom OH),4.06(t,2H,-CH2+A~),7.05(d,2H,Ar-Horthotoalk0~y), electrodes were rinsed with chloroform and then made hydro7.62 (d, 2H, Ar-H meta to alkoxy), 7.73 (d, 2H, AI-H meta to phobic by storing in HMDS vapor for at least 48 h. The thickness -NO*), 8.27 (d, 2H, AI-H ortho to -Nod. of the bottom electrode was 2000 A and that of the top electrode Monolayer Characterizationand Deposition. Monolayer was 500 A. The deposition rate was 500 A/min. The slow manipulations were performed on a commercial Lauda film deposition rate is most important for the top electrode so as to balance that employs the floating barrier method of measuring prevent damage to the organic thin film. surface pressure. The brass trough was coated with Teflon. An Contacts to the electrode surfaces of the device were made IBM PC was interfaced with the film balance for data acquisition using gold wire probes mounted on a Wentworth Lab probing and processing.% All the isotherm collection and deposition station. The entire station was enclosed in an aluminum box to experiments were done in class 10 laminar flow areas inside a reduce air currents and electrical interference. A Keithly Model class 100 clean room. Subphase water was obtained from a 616 digital electrometer was operated in the coulomb mode in Millipore water system. In addition, a "shake test" of subphase order to evaluate the change in charge with temperature. The water in a clean volumetric flask was used to show the complete samples were placed on the heating stage of the probing station absence of any tendency to foam; the surface tension is always which was heated electrically and cooled with circulating water/ within the experimental uncertainty of the literature value for methanol (-10 "C). The measurements were carried out under pure water. Capillary ripple damping of the pure water in also ambient conditionswithan air temperature of 20 OC and arelative checked. humidity of 67 % The spreading solutions had concentrations of 0.5to 1mg/mL in HPLC grade CHCls. Initial spreading areas were greater than 3. Results and Discussion 70 Az/molecule and dwell times of 10 min were used to ensure 3.1. Monolayer Formation and Stability. T h e initial complete evaporation of the spreading solvent. Compression rates were 3.25 cm/min. The water temperature was controlled attempts to form monolayers out of the two nitroby circulating thermostated water underneath the brass trough. containing compounds (VBPhN02 and N02BPhOH) were The temperature of the water in the trough was measured by a not successful; VBPhNOz formed aggregates at the airsurface probe to a precision of i0.1 OC. water interface upon spreading. Apparently the nitro Vertical Deposition. The vertical deposition of monolayers groups are not sufficiently hydrophilic for good spreading. onto substrates was achieved via a computer-controlled stepping While NO2BPhOH could be spread, t h e system formed a motor. The motor has 3.9-pm steps and the dipping speed was three-dimensional structure upon compression, judging from the substantially low molecular area in t h e condensed (28) Shutt, J. D. Ph.D. Dissertation,Case Western Reserve University, phase (see Figure 2). This behavior has been observed Cleveland, OH, 1988.
.
908 Langmuir, Vol. 10, No. 3, 1994
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50
L3(1 fl
E
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'0
10
20
30
40
50
60
mole~u~ar area (AVmolecde)
-0
10
20
30
40
50
60
molecular uu (AUnoIecule)
Figure 3. Surface pressure-molecular area isotherms of VBPhNOz mixed with VBPhOH with different mixing ratios (NOdOH: a = 3/1,b = 1/1,c = 1/3,and d = O/l).
Figure 4. Surface pressure-molecular area isotherms of NO* BPhOH mixed with CHaPhOH with different mixing ratios (NOdCHa: a = 3/1,b = 1/1,c = 1/3,and d = O/l).
Table 1. Limiting Molecular Areas and Collapse Pressures of Mixed Monolayers VBPhNOz and VBPhOH with Different Mixing Ratios mixing ratio coarea (Ao) collapsepressure NOdOH (A9 (dm/cm) 3/1 14.5 1/1 23.3 46 1/3 28.1 43 41 o/ 1 21.3
Table 2. Limiting Molecular Areas and Collapse Prearures of Mixed Monolayers N O a P h O H and C&BPhOH with Different Mixing Ratios mixing ratio coarea (Ao) collapse pressure NOdOCHa (A? (dys/cm) 1/0 4.8 21.1 311 11 1/1 26.8 55 113 29.6 52 011 29.2 42
with spread monolayers of thermotropic liquid crystal mo1ecules.m We found that a mixed monolayer of VBPhNO2 and NOzBPhOH enhances the spreadability and monolayer formation. The technique was first used by Xu et al.m*30 In this research, VBPhOH was mixed with VBPhNOz and CH3BPhOH was mixed with N02BPhOH. The selection of these partners was based on the consideration of the effect of similar structures on spreading and stabilization of the monolayer. Figure 3 shows the surface pressure-molecular area (TA ) isotherms of VBPhNO2 mixed with VBPhOH with different mixingratios (NOdOH = 3/1,1/1,1/3, and OD). The monolayers were evaluated by three factors: coarea (Ao)which is the molecular area obtained by extrapolating the linear part of the isotherm to T = 0, collapse pressure, and the shape of the T-A isotherm. Table 1lists the values of A0 and collapse pressure. The cross sectional area of planar biphenyl was estimated to be 22.9 Az 31 and will be used as a reference Ao; deviation from 22.9 A2 can be expected due to different packing. If the two molecules did not mix well, one can expect a much smaller A0 than 22.9 A2 since VBPhNO2 molecules would form a threedimensional structure. The cowea of monolayers with 1/3and 1/1 ratios are either larger or comparable to 22.9 A2,which suggestseffectivemixing. There seems to be an optimum mixing ratio close to 1/1,at which molecules can readilyform a more condensed two-dimensional structure, judging from the small A0 (23.3A2) and the relativelysmall transition region. The relatively condensed packing probably resulted from the interaction between hydroxy and nitro groups. At a higher ratio (NOdOH = 3/1),there were not sufficient hydroxybiphenyl groups to interact
with nitrobiphenyl groups and thus a substantial amount of three-dimensional structures resulted from VBPhNO2 molecules. Indeed, a rather small A0 (14.1 A2) was obtained. Figure 4 shows the F A isotherms of NOzBPhOH mixed with CH3BPhOH at different mixing ratios (NOdCHs = 3/1,1/1, 1/3, and O/l). Table 2 lists the values of A0 and collapse pressure. The same trend was observed an optimum ratio close to 1/1 and three-dimensional structures at a higher ratio of 3/1. However, the monolaver of N02BPhOH/CH3BPhOH is more expanded than *PhNOdVBPhOH. Figure 5 shows the creep testa of VBPhOH, VBPhNOd VBPhOH (l/l), CH3BPhOH, and NOzBPhOH/CHsBPhOH (1/1) at 7~ = 25 dyn/cm and T = 20 "C. The monolayers of VBPhOH and VBPhNOdVBPhOH showed comparable stability. The monolayer of N02BPhOH/CHsBPhOH appeared to be more stable than CHaBPhOH. The monolayers with biphenyls attached to hydrophilic head groups are more stable than those with biphenyls at hydrophobic chain ends. This suggeststhat the biphenyl packing needs to be stabilized, which was accomplished through the interaction of the attached hydrophilic head groups with water. 3.2. Transfer Behavior. Vertical Deposition. The transfer behavior was first examined by a single upstroke of vertical deposition at variable surface pressures and at a fixed speed of 5 mm/min, using hydrophilic glass substrates. Table 3 lists the transfer ratios for monolayers VBPhOH, VBPhNOdVBPhOH (l/l), CHsBPhOH, and N02BPhOH/CHsBPhOH (l/l). The transfer ratio can be used to evaluate the quality of transfer and a ratio of 1 will be regarded as an indication of good transfer, although different packings between monolayers at the air-water interface and on the substrate can result in deviation from the ideal 1. From Table 3, it can be seen that the monolayers with biphenyls attached to hydrophilic head groups show a better transfer behavior, which is
~
(29) deMul, M.; Mann, J. A. Submitted, preprints are available from J. Mann. (30) Xu, X.; Ern, M.; Tsutsui, T.;Saito, 5.Thin Solid F i l m 1989,178, 541. (31) Harpeaves, A.; Rizvi, S. H. Acta Cryetallogr. 1962, 15, 366.
Langmuir, Vol. 10, No. 3, 1994 909
Noncentroaymmetric L-B F i l m
Table 4. Deposition Conditions for X-ray Studies l8
"0
o
40
80
120
a
160
200
240
time (min)
Figure 5. Creep behavior of (a) VBPhOH, (b) VBPhNOdOH, (c) CHgBPhOH, and (d) NO&PhOH/C& at r = 25 dyn/cm and T = 20 "C. Table 3. Transfer Ratios of Monolayers VBPhOH, VBPhNO;/OH, CHaBPhOH, and NO;BPhOH/CHa by Vertical Depoeition at Variable Surface Pressures and at a Speed of S mm/min, Using Hydrophilic Glass Substrates r=25 r = 3 0 r 17 ff = 20 s(dyn/cm) dyn/cm dyn/cm dyn/cm dyn/cm monolayer VBPhOH 1 1 1.2 1.3 VBPhNOdOH 1 1 1 1 CH&PhOH unpredictable NOzBPhOH/CH* 1 1 1.3
consistent with the results from creep tests. Using the optimum surface pressure from Table 3, the construction of multilayer films was attempted. It was found that all monolayers showed %type transfer behavior; monolayers were only deposited during the upstroke. During the downstroke, a sufficient surface pressure was required to prevent the deposited layers from peeling off. When the multilayer films were examined visually, it was found that the films appeared opaque and there were patches and striations perpendicular to the dipping direction. More patches and striationswere observed at the upper portion of the film (relative to the water surface) than at the lower portion of the film. These patches and striations were caused by insufficient adhesion between the deposited layers and the substrate. In order to improve the quality of multilayer films and exploit the transfer behavior further, the glass slides were made hydrophobic. At first, degreased hydrophobic Corning microscope slides (contact angles were measured to be 68") were used. Deposition was poor in some cases. The contact angle was increased by treating the slides in 70% nitric acid to generate more hydroxy groups and then making them hydrophobic by exposing to hexamethyldisilazane vapor. The contact angle was 80". Monolayers of VBPhOH and CH3BPhOH still could not be deposited on downtrips, but monolayers of VBPhNOdVBPhOHand N02BPhOH/CH3BPhOH were deposited on the downstroke. However, the transfer was still not satisfactory; the downtrip deposition could only be carried out for the first few layers. Then the transfer ratios on the downtrip decreased abruptly. Indeed, the transfer became Z-type completely. The Z-type deposition we observed was not surprising, since some aromatic-containingmaterials have shown the same behavior.2J**2s"The possible explanation of Z-type (32) Snkuhnra, T.; N n k h a , H.; Fukuda, K. Thin Solid Films 1988, 159, 346.
r (dyn/cm) monolayer horizontala (X-type) verticala&(%type) VBPhOH 25 22 VBPhNOdOH 26 22 CH&Ph(TH 25 NOaPhOH/CHs 26 22 a Degrensed hydrophobic glass elides. t. First layer by horizontal deposition.
and the other two types of deposition (X and Y types) was given by Bikerman in 1939.95 Bikerman reasoned that the type of deposition is primarily determinedby the shape of the water meniscus during deposition (advancing and receding contact angles). If the surface is strongly hydrophilic, both advancing (4) and receding contact angles (rJ are 90° and the recedingcontactangle Cry) is goo. The contact angles depend on the polarity of exposed surface groups, roughness and heterogeneity of the surface,38-r11 and water penetration into the Based on these considerations, Popvitz-Biro et al. designed a system containing two amide groups along the .~~~ chain to increase the hydrophilicity of the ~ u r f a c eThey were able to fabricate Z-type deposited multilayers with a Z-type structure. The rationale was that the amide groups located along the hydrocarbon chains on the periphery of the crystalline domains can bind water via hydrogen bonding, rendering the surfacemore hydrophilic and thus Z-type deposition occurred. A similar argument is proposed here to explain the Z-type deposition observed. Because of the difference in the cross-sectional area between biphenyl (22.9 A2)and aliphatic chains (20 A2),a less coherent monolayer might form and appear hydrophilic due to the penetration of water into the voids in the structure. This hypothesis was supported by contact angle measurements. The measurements were performed on one-layer f i e deposited on hydrophilic glass slides. Contact angle values of less than 25" were obtained in all cases showing the layer/air interface to be hydrophilic. The two more expanded monolayers VBPhOH and CHsBPhOH (see Tables 2 and 3) probably contained more voids and thus could not be deposited even by the acid-pretreated hydrophobicslides. We concluded that a very hydrophobic surface was required to facilitate the transfer on the dowtrip and the hydrophobicity provided by the deposited monolayers was not sufficient for the transfer. Thus, to construct multilayers, %type deposition should be pursued. In order to (33) Vickers, A. J.; Tredgold, R. H.; Hodge, P.; Khoahdel, E.; Girling, I. Thin Solid F i l m 1986,134,43. (34) Richnrdeon, T.; Roberta,G.G.;Polywkn, M.E. C.; Davies, S.G. Thin Solid Film 1988.160. 231. (35)Bikerman, J. J..ProC. R. SOC.London 1939, A170, 130. (36) Adamson, A. W. Physical Chemistry of Surfaces;John Wiley & Sons: New York, 1990. (37) Jaycock, M. J.; Parfitt, G. D. Chemistry of Interfaces; Ellis Horwood: Chichestar, 1981. (38) Joanny, J. F.; de Gennes, P. G.J. Chem. Phys. 1984,81, 562. (39) Schwartz, L. W.;Gnroff, S . Langmuir 1986,1, 219. (40) Holmes-Farley, S.R.; Renmey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G.M.Langmuir 1986, I, 726. (41) Holems-Fnrley, S. R.; Whitesidea, G.M.Langmuir 1987, 3, 62. (42) Mnoe, R.; Sngiv, J.,Langmuir 1987, 3, 1034. Lahav, M.;Leieerewitz, (43) Popmtz-Buo, R.; HA, K.; Landau,E.M.; L.;Sngiv, J.; Hsiung, H.;Meredith, G.R.; Vanherzeele, H. J. Am. Chem. Soc. 1988,110, 2672.
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2.mm.4 (DECREES)
0.
Figure 6. X-ray diffractiontraces of ten-layerfiis: VBPhOH, (A) X-type, (B)Z-type; CHBPhOH, (C) X-type; VBPhNOd OH, (D)X-type, (E)Z-type; N02BPhOH/CHa, (F)X-type, (G) Z-type. Table 5. The d-Spacing (001) from X-ray Diffraction and Fully Extended Chain Length by SYBYL Modeling do01(A) from X-ray f d y extended chain material horizontal vertical length (A) by SYBYL 45.4 45.1 26.1 VBPhOH 44.0 42.1 243125.1 VBPhNOdOH CHaPhOH NOzBPhOHICHs
47.1
47.5
47.8
27.4 26.6127.4
enhance adhesion, multilayers were constructed on hydrophobic glass slidesin Z-type vertical deposition except for the first downtrip. The first downtrip was carried out by horizontal deposition. The second and subsequent dowtrips were carried out vertically at a reduced surface pressure to prevent the deposited monolayersfrom peeling off and ensure that no monolayers were transferred. On observation of the film by reflected light, the multilayer films constructed this way did not show patches and striations, although the films still appeared opaque. The opaqueness is probably due to the crystallite size and is a scattering effect. This is further evidence that the films were of reasonable quality. Horizontal Deposition. Another way of building multilayer films is the horizontal lifting method (Schaefer’s method), which was introduced by Langmuir and Schaefer in 1938.44 There are some problems with this method. When the substrate is lifted, a meniscus is formed. Further liftingdisrupts this meniscus and as a result the monolayer from the air-water interface in the periphery is sucked in, thus forming a Y-type bilayer or heterogeneous layer.4s Therefore, some modifications were made in this study, based on the work of Day and Lando.4 A hydrophobic substrate was allowed to penetrate through a monolayer in a horizontal orientation. Then the plate was withdrawn vertically to facilitate the draining of the water. In addition, a small tilt angle between the monolayer and substrate was maintained during penetration to prevent air bubbles from being trapped along the surface. For the construction of one-layer films, the remaining spread monolayer was cleaned off before the substrate was withdrawn. For the constructionof alternating-layerfilms, the remaining monolayer was cleaned up, and a different monolayer was spread, compressed, and deposited vertically as the substrate was withdrawn. (44)Langmuir,I.; Schaefer,J. V. J. Am. Chem. SOC.lSB8,57, 1007. A. An Introduction to Ultrathin Organic Film from Langmuir-Blodgett to Self-Assembly;Academic Press, Inc.: New York, (45) Ulman,
1991; p 128.
(46)Day, D.; Lando, J. B. Macromolecules 1980,13, 1478.
n n n Figure 7. Schematic showing the construction of nitrobiphenyl dipole momenta by an untraditional alternating-layermethod. 3.3. X-ray Diffraction. As mentioned in the introduction, the structure of multilayers is not determined by the deposition type. Therefore, the structure needs to be characterized. Since the molecules are highly oriented in a direction normal to the substrate, an X-ray diffraction trace of the multilayers does provide information about the periodicity of the structure in that direction. By comparing dml with the calculated fully-extended chain length, one can determine if the molecular repeat is one molecule (noncentrosymmetric) or two molecules (centrosymmetric). Ten-multilayer fiis from Z-type vertical deposition and X-type horizontal deposition were examined. The deposition conditionsare listed in Table 4. Figure 6shows the X-ray diffraction traces of multilayers VBPhOH, VBPhNOdOH, CHaPhOH, and NOZBPhOWCHs. The d-spacings can be calculated according to the following equation h = 2d sin 8
where h = 1.5418 A. Table 5 lists the values of dm1 and fully extended chain length. The SYBYL program was used to model the fully extended chains, which have the following characteristics: (1)planar zigzag for aliphatic chains; (2) planar biphenyl structure; (3) torsional angles close to 90° between alkoxy and biphenyl. From Table 5, it can be seen that dm1 is substantially larger than the fully extended chain length, indicating a Y-type structure irrespective of deposition types. At this point, it became obvious that the molecules rearrange into their lowest energy packing (with hydrophilic head groups associated with one another) given any opportunity. One possible method to actually capitalize on this tendency is to employ the alternating-layermethod. Since the two mixed-monolayers have nitrobiphenyl groups directed oppositely,the combinationof the two monolayers
Noncentrosymmetric L-B Films
where p is the pyroelectric coefficient. Since we cannot easily measure P,,we measure the charge that develops on the surface perpendicular to the polar axis. Since the multilayer film can be considered an insulator, the compensationchargeswill be too slow to follow the induced change in P,. A pyroelectric coefficient can be calculated from
3000.
1500.
2000.
E
A Q = p13T
1500.
The pyroelectric coefficient of the alternating layer sample was determined to be 1.7 X 1 0 - l O C cm-2 K-l between 10 and 20 OC. The value is somewhat higher but of the same order of magnitude as acid-amine systems reported in the literature.12-17
1000.
500,
n.
Figure 8. X-raydiffractiontraces of ten alternating-layers (five bilayers) of (A) VBPhNOdOH + NOaBPhOH/CHa and (B)
VBPhOH
Langmuir, Vol. 10, No. 3, 1994 911
+ CHgBPhOH.
should lead to a build-up of nitrobiphenyl dipole momenta. This scheme is shown in Figure 7. The multilayer8 were constructed by depositing NO&PhOH/CHsBPhOH horizontally and VBPhNOdVBPhOHvertically. In this way one can prevent molecular turn-around. For comparison, VBPhOH and CHaPhOH alternating layers were constructed in the same way. Figure 8 shows the X-ray diffraction traces of the alternating layers. The d-spacing corresponding to the 002 peak was calculated to be 22.5 and 21.6 A for the two films, respectively. The 001 peak, which was not observed, can be expected to be weak because of the similar structure between the two monolayers. The result supports a structure as proposed in Figure 7. 3.4. Pyroelectricity. In order to verify that a polar structure has indeed been produced, pyroelectric measurementswere performed on an alternating layer sample, using a static method similar to that described by Smith et al.12 If this altemating-layerfilm is noncentrosymmetric as expected, it should contain a spontaneouspolarization, P8. If P, is a function of temperature, a pyroelectric effect will be obtained. The pyroelectric coefficient is defined as p = dP@/dT
4. Conclusions The two nitrobiphenyl-containingcompounds (VBPhNO2 and N02BPhOH) did not form stable monolayers. This was attributed to the low hydrophilicity of the nitro groups. The monolayer formation of these compounds was facilitated by mixing with materials bearing similar structures (VBPhOH and CHsBPhOH). An optimum mixing ratio of 1:lwas found. Monolayers with biphenyl attached to the hydrophilic head groups are more stable than those with biphenyl located at the hydrophobic chain ends. Transfer behavior of the monolayers was examined by vertical and horizontal deposition. All the monolayers studied tended to be deposited on the uptrip only (%type vertical deposition). X-ray diffraction resulta show that the multilayer structure remains Y-type, irrespective of the deposition type. A new alternating-layer method was developed, using nitrobiphenyl as the primary source of polarity. Nitrobiphenyl can be located on either the hydrophobicor the hydrophilic chain ends because of its low hydrophilicity. X-ray diffraction and pyroelectric measurements prove that a polar film was constructed with nitrobiphenyl moieties oriented in the same direction. The pyroelectric coefficient of the alternating-layer film was measured to be 1.7 X 10-l0 C cm-2 K-l.
Acknowledgment. This work was supported by the National Science Foundation under the Science and Technology Center ALCOM DMR89-20147.
