Langmuir 1994,10, 246-251
246
Fabrication of Alternating Diacetylenic Multilayers and the Role of Subphase Conditions Steven P. Walsh and Jerome B. Lando* Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106 Received May 20, 1993. In Final Form: September 21,1993@ Multilayer assemblies were prepared by alternating acid and amine amphipliles of aliphatic and diacetyleniccompounds in an effort to prepare polar molecular assemblies. Divalent cationic stabilization was required for floating diacetylenic acid monolayers. As a consequence of the deposition technique ionization in both acid and amine compounds was thus considered, and the isoelectricdeposition process developed. Spectroscopic analysis showed at the isoelectricpH the alternated acid and amine monolayers to be deposited as fully ionized salts. Structural investigationsshowed the aliphatic assemblies to possess near vertical alignment of the deposited amphiphiles, whereas the diacetylenic compounds were tilted to accommodate the diyne functionality.
Introduction The interest in Langmuir-Blodgett monolayers and multilayers in the past has been directed to the extreme thinness and thickness control afforded by the deposition process. These systems, regarded here as passive, utilize the films as ultrathin dielectrics,l resists,2 and optical coatings.3 The current trend in technology though has been to impart some active or response capability to these assemblies through using a chemical and/or structural approach. Chemical activity can be imparted through a chemical response where a measurable physical property (e.g. conductivity) is altered through some chemical or physicochemical stimuli (e.g. oxidant concentration) resulting in a chemical-type enso or.^ Structural systems impart the activity structurally by utilizing the electrothermal, thermoelastic, electromechanical, and/or electrooptical reversible interaction phenomena of noncentrosymmetric structure^.^ Although for both active device types the response is engineered into the assembly, the approach taken for each is different. Chemical-type sensor response is generally due to the chemical structure of individual molecules in the multilayers, typically relying on some reactive functionality. Systems utilizing a structural approach require control of both chemical and physical structures of the multilayers in order to produce the desired response. The principal direction of this research has been to prepare polymerizable noncentrosymmetric LangmuirBlodgett superlattices based on diacetylenic amphiphiles using an alternating monolayer deposition technique. The asymmetry was chemically designed by preparing separate diacetylenicamphiphileswith carboxylic acid and primary amine polar entities. Acid and amine functionalities were selected in an effort to prepare superlattice assemblies demonstrating a permanent spontaneous external polarization as required for pyroelectric activity. The polymerizability was intended to secure the alternating structure against thermal disordering. 8
Abstract published in Advance ACS Abstracts, December 15,
1993.
H.; Reynolds, S. I. J. Am. Chem. SOC.1939, 6, 1424. (2) Faris, G.; Lando, J. B.; Rickert, S. E. Thin Solid Films 1983, 99, (1) Race, H.
305. (3) Blodgett, K. Phys. Reu. 1939, 55, 391. (4) Moriizumi, T. Thin Solid Film 1988,160, 413. (5) Nye, J. F. In Physical Properties of Crystals; Claredon Press: Oxford, 1985; pp 68-81 and 168-191.
Experimental Section Materials Preparation. Pentacosa-lO,l2-diyoic acid (128AC)and pentacosa-l0,12-diynylamine (12-8AM)were prepared as previously described.s Briefly, 12-8ACwas synthesized using the method of Tieke and RingsdorF from the corresponding iododkyne and alkynoic acid. A portion of the purified acid was converted to a primary amide by the action of ammonium hydroxide on the acid chloride. The amide was reduced to the primary amine using lithium aluminum hydride as described by Brown? Isolation was affected by recrystallization from ethanol verified by FTIR, lH NMR, and TLC. Eicosanoic acid (CPOAC) and octadecylamine (C18AM)were commercially available in high purity and used as received. Monolayer Studies. Preliminaryfloatingmonolayerstudies were performedto determinesuitabledepositionpanunetersusing an automated Langmuir-type Lauda film balance. Subphase water was obtained from the ultrapurification of softened and carbon-filteredmunicipal water using an IonPure/Milli-Qwater system. Spreadingsolutions were nominally 1mg/mL in HPLC grade chloroform. Surface pressure/area isotherms and compressive creep studies were used to obtain subphase conditions and surface pressures required to produce stable condensed monolayers for each amphiphile. Satisfactory conditions were indicated when unity transfer ratios were obtained for the monolayers studied. At all times the subphase temperature was maintained at 20.0 O C . Alternating Depositions. Film deposition onto appropriate substrates was performed using the general constant pressure approach of BlodgettQutilizing a specially designed alternating deposition trough (see Figure 1). The trough consisted of two parallel rectangular 1cm deep spreadingareas (25.5cm X 43 cm) interconnected by a channel deep enough to accommodate the total submersion of the deposition substrate. Prevention of surfacecross-contaminationwas accomplishedwhilestill allowing transport of the submerged substrate by using abutting Teflon leaf-spring gates similar in design to that of Daniel et al.Io Constant surface pressure w a ~maintained manually using a
floatingbarriedpulley design as describedby Sher and Chanley.ll The floating barrier was machined from a 1 mm thick sheet of low density polyethylene and connected to the pulley system using superfine nylon sewing thread. The pulley consisted of a low mass Styrofoam wheel (d = 78 mm) with a glass hub and nichrome wire axle and was capable of distinguishing mass (6) Walsh, S. P.; Lando, J. B. Langmuir 1994,10, 252.
(7) Tieke, B.; Wegner, G.; Naegele, D.; Ringsdorf, H. Angew. Chem., Int. Ed. Engl. 1976,15,764. (8)Brown, W . G. In Organic Reactions; Adams, R., Ed.; J. Wiley and Sons: New York, 1951; Vol. 6, p 488. (9) Blodgett, K. B. J. Am. Chem. SOC.1935,57, 1007. (10) Daniel, M. F.; Dolphin, J. C.; Grant, A. J.; Ken, K. E. N.; Smith, G. W. Thin Solid Films 1985,133, 235. (11)Sher, I. H.; Chanley, J. D. Reu. Sci. Instrum. 1955, 26, 266.
0743-7463/94/2410-0246$04.50/00 1994 American Chemical Society
Polar Molecular Assemblies
Langmuir, Vol. 10, No. 1, 1994 247 1000 coadded scans collected at a resolution of 4 em-' with a 0.5-cm-1 angular aperture resolution. Reflection spectra were obtained from 2000 coadded scansat a resolution of 4 cm-' through a 2.0-cm-1 angular aperture resolution using an 80° fixed angle reflection attachment (Spectra Tech Technologies, FTS-80). Skimmed floating monolayers were studied as KBr pellets. Depositedstructuresfor transmission analysis were prepared on freshly cleaned germanium plates. Reflection studies were performed on assemblies deposited on freshly evaporated aluminum mirrors. In all cases a matching blank substrate was used for the reference spectrum.
Alternating Ionic Depositions
Figure 1. A plan view of the trough base used in the fabrication of the alternating multilayer assemblies showing the side-bysidetrough arrangement,the depositioncanal,and the placement of the leaf spring gate assembly.
c t
1t
P
8
I v)
0.30
Molecular Area
0.35
o.!o
(rq. nmlmolrcule)
Figure 2. Singlepoint stepwiseforce/arearesponse for eicosanoic acid on pure water compared with a conventional continuous force/area isotherm. differencesof 0.02 g. The generalutility of the systemwas verified by comparing a stepwisecompressionisotherm of eicosanoicacid with that obtained from the previously mentioned commercial system (see Figure 2). Deposition was performed by spreading and compressing monolayers of the two amphiphiles to a fixed surface pressure on the desired subphase solution. The surface nature of all substrates was made hydrophobic by first applying a single monolayer of C2OAC or 12-8ACfrom the appropriatesubphase. Monolayer alternation was achieved by translation of the submergedsubstrate beneath one floatingmonolayer to the other, with application of the new amphiphile on the subsequent uptrip. Changes in the floating barrier positions were monitored during the deposition process to ensure complete coverage. In all depositions it was found necessary to apply the amine compound on the down-trip and then to cover with the corresponding acid amphiphile. Performing the reverse inevitably led to the retransfer of the deposited amine monolayer to the subphasesurface on the subsequent down-trip even after drying periods in excess of 12 h. Usual drying times did not exceed 10
The fabrication of alternating monolayer structures from a single deposition system inevitably requires determining suitable subphase conditions such as ionic content and concentration, pH, and temperature that are the same for each floating monolayer. The requirement of the deposition process for the translation of the substrate below the floating monolayers makes it impossible to also afford a unique subphase for each monolayer. In the past, this concern in fabricating alternating structures has been simplified since both amphiphiles were suitably stable on pure water.12 By extention of the materials to include diacetylenic compounds, special consideration was required since although 12-8AM shows stable condensed monolayer characteristics on pure water, 12-8AC demonstrates excessive creep a t surface pressures less then 10 dyn/cm and a collapse pressure below 30 dyn/cm. It has long been known that improvement in the floating stability and deposition characteristics of fatty acid type amphiphiles can be achieved by minute (>lO-3 M) concentrations of divalent cations added to the subphase.'3 The dramatic enhancement for 12-8ACwith the addition of M CdC12 to the subphase can be seen (see Figure 3) with the collapse increasing to above 50 dyn/cm with a long-term compressive stability of over 30 dyn/cm. Unfortunately as a consequence of the alternating deposition system, one must now also contend with the effects of the dissolved anions on the floating amine monolayer. The concern over the composition of the monolayers at the gas-water interface stems from the desired utilization of these monolayers in the deposited form as pyroactive media. There is strong evidencelP16 showing that the extent of ionization at the gas-water interface is paralleled in the deposited film. Depicted in Figure 4 are the four extreme scenarios for alternating acid/base assemblies prepared from ionized monolayers. Since the goal is to produce a large permanent polarization, it is clear that assemblies a and d are most desirable, where the polar moments advance in the same direction. For diacetylenic compounds assembly a cannot be considered due to the need for monolayer stabilization in 12-8AC, which leaves the question of how to ensure a structure such as assembly d. To address this problem it is convenient to think of the monolayers not as separate molecules but as a single molecular system. Consideringthe two-monolayer system as an idealized amino-acid allows one to determine the pH value for both the acid and amine functionalities to be fully ionized by using the concept of the isoelectric point.
min.
Superlattice Analysee. Chemical and some structural information waa obtained via a combination of transmissionand glancing angle reflection FTIR spectroscopic techniques on skimmedand depositedmonolayersand multilayersusing a single beam Digilab/Biorad FTS-BO Fourier transform infrared spectrometer equipped with a mercury-cadmium-telluride (MCT) photoelectricdetector. Transmissionspectra were obtained from
(12)Smith, G.W.;Daniel, M. F.; Barton, J. W.; Ratcliffe, N. Thin Solid Films 1985, 132,125. (13)Langmuir, I. Sctence 1936, 84, 479. (14)Langmuir, I.; Schaeffer, V. J. J. Am. Chem. SOC.1936, 58, 284. (15)Aveyard, R.;Binks, B. P.;Cam,N.;Croee, C . W . Thin Solid Films 1990,188, 361. (16)Mabubura,A.; Matuura, R.;Kimizuka, H. Bull. Chem. SOC.Jpn. 1966,38, 369.
Walsh and Lando
248 Langmuir, Vol. 10, No. 1, 1994
EE h
f l 3 I?
a
Molecular Area (sq.nmlmolecule)
ih
C d Figure 4. Schematicizedeffects of ionization upon alternating multilayer assemblies prepared from acid/amine amphiphiles: (a) no dissolved ionic species in the subphase; (b) preferential acid ionization (highpH values);(c)preferentialamine ionization (low pH values); (d) equivalent acid and amin e ionization (isoelectric pH value).
f n I?
c
3
to use the monolayer ionization constants as opposed to the bulk values in determining the isoelectric pH. Earlier ~ o r kto this ~ ~endt has ~ shown ~ ~a great variability in the Molecular Area (sq. nmlmolecub) pH at half ionization for both fatty acid and fatty amine Figure 3. Monolayer stabilization of 12-8AC by the addition of monolayers, with a strong dependence on the chemical 10-8 divalent cation: (a) 12-8AC compressed on a pure water nature and concentration of the dissolved ions. subphase; (b) 12-8AC compressed on a 10-8 M CdClz at a pH By use of a method of spreading and skimming monovalue of 7.0-7.5. layers from controlled subphases, FTIR spectroscopic Classically, the isoelectric point of an amphoteric analysis showed 85-90% conversion on 10-3 M CdC12 to molecule is defined as the degree of ionization at which the neutral carboxylate salt a t a bulk subphase pH of 7.0 such a molecule will not undergo electrophoretic migraand total salt formation at a pH value of 7.5. In an tion." The isoelectric point can also be calculated by analogous fashion the pH value for 100% conversion to realizing that for the molecules to have no electric field the amine hydrochloride was found to occur at a pH of 7.0 migration they must possess no net charge, thus with near complete conversion at a pH of 7.5. By selection of a pH for total ionization between the limits found ['NHs-R-COOH] = [NHZ-R-COO-] spectroscopically, pHi value of 7.2 was calculated. For similar compounds other pK,, values have been For an amino acid, the acid-base equilibrium therefore obtained. Glazer and Dolgan2' potentiometrically found becomes the pK, for octadecylamine on a chloride-containing ki ka subphase to be 8.5, and although not explicitly calculated, +NH,-R-COOH +NH,-R-COONH2-R-COOa pKa value of 5.6 can be determined for 12-8AC from the with the two ionization constants being spectral presentation of Tieke et aleBWith these values, an acid/amine isoelectric point on CdCl2 is calculated to kl = ~[H+][+NH3-R-COO-])/~[+NH,-R-COOH]) be 7.1, in good agreement with the value previously 122 = ([H'] [NH~-R-COO-]]/([+NH~-R-COO-]) determined. Combining the ionization constants and evaluating a t the Fabrication of Alternating Assemblies isoelectric point (eq 1)yields With the appropriate subphase conditions obtained, deposition trials yielded the followingresults. Alternating k1k2 = [H+12or [H'I = (klk2)1/2 Thus the defined isoelectric pH value is simply (18)Davies, J. T. R o c . R. SOC.London 1961, A208,224.
-
P H ~= (PK, + &)/2 Due to the presence of an excess hydrogen ion concentration a t the gas-water interface,laJg it is more prudent (17) Greenstein, J. P.; Minitz, M. In Chemistry of Amino Acids; J. Wiley and Sone: New York, 1961; pp 435-533.
(19) Davies, J. T. J. Colloid Sci. 1966, 11, 377. (20) Havinga, E. Red. Trau. Chim. Pays-Ban 1962, 71, 72. (21) Sobotka, H.;Demney, M.;Chanley, J. D. J. Colloid Sci. 1968,13,
___
565.
(22) Spink, J. A. J. Colloid Sci. 1963,18,512. (23) Boggs, J.; Adamson, M. A.; Fichman, M.; Haber, M. D.; Gregor, H.P.J. Am. Chem. SOC.1964,86,2759. (24) Glazer, J.; Dolgan, M. Z. Tram. Faraday SOC.1963,44,448. (26) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980,68,77.
Polar Molecular Assemblies
3100
2700
2300
Langmuir, Vol. 10, No. 1, 1994 249
1900
1500
1100
Wavenumber (cm.1) Figure 6. Reflection FTIR spectrum of a C2OAC/C18AM multilayer assembly deposited on evaporated aluminum under isoelectric conditions.
Wavenumber (cm-1)
Figure 7. Reflection FTIR spectrum of a Cd12-8ACf12-8AMHCl multilayer assembly deposited on evaporated aluminum under isoelectric conditions.
0.0020~
g 0.0030 0.0015
..-, -0.0015 1
3100
2700
23w
1WO
lSW
1100
1800
1700
1BW
1500
1404
1300
1
0
Wavenumber (cm-1) Figure 6. Reflection FTIR spectrum of a CdC20AC/C18AMHC1 multilayer assembly deposited on evaporated aluminum under isoelectric conditions.
Wavenumber (cm-1) Figure 8. Reflection FTIR spectrum of a CdC20ACfC18AMHCl multilayer assembly deposited on evaporated aluminum at a p H value below the isoelectric point (pH = 5.7).
assemblies of C2OAC and C18AM were produced from a bicarbonate-buffered subphase (l0-iM NaHCOdHCl) at a pH of 7.0-7.5. Ion containing assembles of CdC2OAC/ C18AM-HC1 and Cd12-8AC/12-8AM-HCl were prepared from a 109 M solution of CdClz with the pH adjusted to 7.0-7.5 with NaOH. Deposition rates were 5 cm/min a t a surface pressure of 30 dyn/cm with the following exception. Monolayers of 12-8AM-HC1 were deposited a t a surface pressure of 35 dyn/cm as required to form a fully condensed film at the gas-water interface.6 A drying pause of 10 min was used between each round trip.
Table 1. Observed Absorbances for C20AC/Cl8AM, CdC20AC/C18AM-HCl9 and Cd12-8AC/12-8AM-HCl through the 3000-2700 cm-I Region
Spectral Analyses Samples were prepared on germanium windows and aluminum mirrors for spectroscopic analysis. Figures 5, 6, and 7 show the representative spectral features for the three assembliesin reflection. Additionally,a test sample of CBOAC/C18AMwas prepared outside of the isoelectric pH from a CdClz subphase with a pH value of 5.7. The spectral features for this sample are presented in Figure 8. A cataloging of the specific features for each sample is presented in Tables 1-4. As a general caveate, spectroscopic analysis has demonstrated that deposition of (26) Snyder, R. G.; Hsu, S. C.; Wi, S. Spectrochim. Acta 1975,34A, 395. (27) Bellamy, L. J. In The Infrared Spectra of Complex Molecules; J. Wiley and Sons: New York, 1958; Chapter 13, Amino-Acids, Their Hydrochlorides and Salts, and Amido-Acids. (28) Reference 27; Chapter 14, Amines and Imines.
wavenumber (cm-1) 2961 2938 (sh)a
2917 2873 2849 a sh
assimmentm methyl asymmetric stretch parallel to the skeletal plane methyl symmetric stretch in Fermi resonance with methyl deformation methylene asymmetric stretch methyl symmetric stretch in Fermi resonance with methyl deformation methylene symmetric stretch
= shoulder.
carboxylicacid type monolayers onto aluminum results in the formation of aluminum carboxylate salt in the first monolayer irrespective of the presence of dissolved ions.
Discussion of Spectral Results The 1800-1100 cm-l Region. The first and most obvious feature differentiating those samples prepared (29) Wears Scheuing, D. R. In Fourier Transform Infrared Spectroscopy inColloidandZnterfaceScience;Scheuing,D.R.,Ed.; American Chemical Society: Washington, DC, 1990; pp 90-93. (30) Reference 27; Chapter 10, Carboxylic Acids. (31) Snyder, R. G. J. Mol. Spectrosc. 1960,4,411. (32) Little, L. H. In Infrared Spectra of Adsorbed Species; Academic Press: New York, 1966, pp 233-234. (33) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70,642. (34) Swalen, J. D.; Rabolt, J. F. In Fourier Transform Infrared Spectroscopy; Academic Press: New York, 1985, Vol. 4, pp 304-308.
Walsh and Lando
250 Langmuir, Vol. 10, No. 1, 1994 Table 2. Observed Absorbances for C20AC/C18AM through the 1800-1100 cm-l Redon wavenumber (cm-l) assignment
1592 1550 (ah)" 1469 1418 1300-1 100 a
-COO- asymmetric stretchn -NHs+ deformation27*28 methylenic bending (hexagonal or disordered rotator phase)" -COO- symmetric stretchN progression bands31
sh = shoulder.
Table 3. Observed Absorbances for CdC20AC/ClBAM-HCl through the 1800-1100 cm-l Region
wavenumber (cm-l) 1641 (shp 1550 1468 (sh) 1440 1408 (sh) 130C-1100 a
assignment -NH3+ asymmetric (?) deformation or in-plane deformation of H2032 combination of NH3+ symmetric deformation and -COOasymmetric stretch@*% methylenic bending (hexagonal or disordered rotator phase29 -COO- symmetric stretch% methylenic deformation3l,33 progression bands32
ah = shoulder.
Table 4. Observed Absorbances of Cd12-8AC/12-8AM-HCl through the 1800-1100 cm-1 Region
wavenumber (cm-1) 1540 1469 1440 1422 1300-1100
assignment combination of -NH3+ symmetric deformation and -COOasymmetric stretch28JO methylenic bending31 -COO- asymmetric stretch3Q31 observed in Cd12-8AC and Cd13-8AC multilayers and unassignedM progression bands31
under the isoelectricconditions with that of the test sample is the clear presence of a strong absorbance at 1720 cm-l indicating the incomplete salt formation of the carboxylic acid in the latter case. Such a structure, as depicted in Figure 4c, is representative of such structures formed at a pH value below the isoelectric point, as would be expected with a bulk subphase pH of 5.7 when the isoelectric pH value is 7.2. The lack of any absorbances at 1600 cm-l where the NH2 bending band is expected indicates in all cases complete amine salt formation. Samples deposited under the isoelectric conditions show little differences in the 1800-1100 cm-l (region indicative of the similar chemistries within the head groups. The spectral information is consistent with the complete salt formation for both amphiphiles as was predicted from the isoelectric development. It is important from a pyromechanistic viewpoint that structures formed in the absence of dissolved ionic species also demonstrate complete salt formation indicating the protonation of the amine by the carboxylic acid creating an ammonium carboxylate salt. The shifting in the relative absorbance maxima between C2OAC/C18AM and CdC20AC/C18AM-HCl is expected to be due to the difference in the binding energy, effective ionic masses, and chemical environments of the two ionic structures. Comparison between ionized amino acids and carboxylic acid salts shows the carboxylate absorption (asymmetric stretch) to appear at the usual 1550 cm-l for simple metal salts.30 In ionized amino acids (as the zwitterion) the band is shifted higher to 1600 cm-'. On the other hand, the absorbances for the ionized amines (i.e. ammonium carboxylates and ammonium hydrochlorides) are fairly invariant with a broad weak absorbance
at about 1640 cm-1 and a usually more intense absorbance at 1540 cm-1.28 Thus it is clear that within the narrow pH region of the isoelectric value, alternating acid and amine multilayers will deposit as their corresponding salts. In the absence of solvated counterions the multilayers will deposit as the ammonium carboxylate. The effect is not entirely unexpected since when mixed in a nonpolar solvent, fatty acids and fatty amines readily form their conjugate salts. Thus during the application of the alternating acid/amine monolayers, with the polar heads brought together in the aqueous environment of the deposition process, ionic transfer should easily occur even if the bulk pH is well outside of the isoelectric value. The 300tF2700cm-l Region. In the aliphaticstretching region it is possible to obtain structural as well as chemical information regarding the average orientation of the methylenic chains by comparing the magnitude of the absorbance maxima for the asymmetric and symmetric methylenic stretching modes.36~36 Simplistically, due to the overall planar organization of the multilayers, samples observed in reflection will reveal information regarding only the vibrational modes that are parallel to the plane of reflection (perpendicular to the substrate surface) due to, at large reflection angles, the near total cancellation of the perpendicular electric vector. In transmission, the passing radiation will only constructively interact with vibrational modes perpendicular to the incident radiation (i.e. parallel to the substrate surface). By considering an all trans methylenic chain oriented near normal to the substrate surface, it is expected that the symmetric and asymmetric methylene stretches will contribute strongly in transmission and much weaker in reflection. As the chain becomes more tilted relative to the surface normal, an increase in the vibrational components out of the surface plane results in an increase in the observed relative absorbance in reflection. A comparison of the transmission and reflection spectralfeatures for CdC20AC/C18AM-HCl and Cd12-8AC/12-8AM-HCl can be found in Figures 9 and 10. I t should be noted that the features of C20AC/C18AM are indistinguishablefrom CdC20AC/C18AM-HCl and thus will not be discussed separately. A cursory examination of the two spectral sets shows dramatically different molecular orientations for the two multilayer assemblies. The magnitudes of the transmitted to reflected absorbance ratios for CdC20AC/C18AM-HCl are 0.029/0.005, or 5.8, for the asymmetric methylene stretch (2919 cm-l) and 0.019/0.003, or 6.3, for the symmetric methylene stretch (2849 cm-I). These ratio values are a qualitative indication of the degree of inclination, and agree well with those observed by this author for cadmium eicosanoate, CdC2OAC (asym = 3.9 and sym = 4.0),and Umemura et al.36for multilayers of cadmium octadecanoate, CdC18AC (asym = 3.5 and s y m = 4.0). The correlation suggests that as in multilayers of both CdC18AC and CdC2OAC the aliphatic portions of the molecules in the alternating assemblyare nearlynormal to the substrate surface. Similar consideration for the diacetylenic alternating assemblies shows these ratio values to be asym = 0.9 (from 0.010/0.011) andsym = 3.5 (from0.007/0.002). The values for the alternating diacetylenic assemblies are very much in line with those observed for homogeneous multilayers of Cd12-8AC (asym = 1.1 and sym = 3.6). Previous ~~
~
~~
(35) Allara, D. L.; Nuzzo, R. G. Langmuir 1985,1,62.
~~~
(36) Umemura,J.;Kamata,T.;Kawai,T.;Takenaka,T.J.Phys.Chem. 1990, 94, 62.
Polar Molecular Assemblies
0.048
0.012
Langmuir, Vol. 10, No. 1, 1994 251
n
J
1
1
3100
n
0.0180
I O0.oQo '
3000
2900
2800
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3100
-
L
3000
Wavenumber (cm-1)
3000
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ZOQO
''
2800
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Wavenumber (cm-1)
b.
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'
2800
I
2700
Wavenumber (cm-1)
0.012
1
3100
b.
3000
2900
28Qo
2700
Wavenumber (cm-1)
Figure 9. FTIR spectra from multilayer samples of CdC20AC/ C18AM-HC1 through the 3000-2700cm-1 region deposited under isoelectric conditions: (a) transmission, 7.5 bilayer sample deposited on germanium windows; (b) reflection, 7.5 bilayer sample deposited on aluminum mirrors.
Figure 10. FTIR spectra from a multilayer samples of Cd128AC/12-8AM-HCl through the 3000-2700cm-1 region deposited under isoelectricconditions: (a) transmission,5.5 bilayer sample deposited on germanium windows; (b) reflection, 7.5 bilayer sample deposited on evaporated aluminum mirrors.
diffraction measurements have shown the molecules of diacetylenic amphiphiles, as the deposited salt, to be inclined about 30' away from the surface normal in order to properly align the diacetylenemoieties, and allow solidstate p o l y m e r i ~ a b i l i t y .Since ~ ~ ~ ~the ~ alternating diacetylenic structures were seen to exhibit solid-state polymerizability in the deposited form, similarity to a general diacetylenic structure would be expected. A final point of interest regarding the diacetylenic structures is the clear differences in the orientations of the asymmetric and symmetric methylene stretches suggesting a monoclinic packing of the hydrocarbon chains. Based on the orientations of the orthogonal dipole momenta associated with the methylene, these data suggest that the tilting of the methylene chains is not axially random but is a result of tilting about the skeletal plane through the methylene segments. In doing so the moment of the symmetric stretch remains parallel to the substrate surface whereas the asymmetric stretch acquires an increasing component parallel to the surface normal. The orientation of the planar zigzag chain thus observed corresponds exactly with that seen via diffraction techniques for nonacosa-10,12-diynoic acid (16-8AC) multilayers.87 Assuming that within reason the hydrophobic portions of tha even-numbered long chain diacetylenic amphiphiles are homologous, it should not be unreasonable to find similar global features in 12-8AC,12-8AC/12-8AM, and 16-8AC multilayer assemblies.
Conclusions
(37) Day, D.;Lando, J. B.Macromolecules 1980, 13, 1483.
In the broadest sense, it has been demonstrated that utilization of an alternating deposition adaptation of the Langmuir-Blodgett technique can produce multilayer assemblies with controlled molecular architectures. Specifically, it has been shown that consideration of the subphase conditions for each monolayer system must be taken independently, as well as together, when physical and chemical structures must be controlled. The development of the isoelectric deposition process has made it possible to prepare alternating acidlamine type assemblies with known chemical and physical structures. The result being an acentric multilayer assembly with a chemically produced permanent and spontaneous polarization as required for pyroelectric activity. The approach should be generally applicable to the production of noncentrosymmetric structures sought to exploit the various mechanicaVthermaVelectricaVopticalinteraction phenomenal in active Langmuir-Blodgett devices. From a global structure stand point, the alternating assemblies adopt multilayer structures with molecular orientations very similar to their parent compounds. C2OACIClBAM and CdC20AC/ClBAM-HCl are aligned nearly normal to the substrate surface, as has been demonstrated on numerous occasions for CdCBOAC. Alternating structures of Cd12-8AC and 12-8AM-HC1 adopt a tilt of about 30° off axis as seen by Cd12-8AC multilayers.
