Preparation and Characterization of Polyion-Complexed Langmuir

Preliminary SHG measurements for LB films of two systems, 12Q/CMC−Na and ...... Series 601; American Chemical Society: Washington, D.C., 1995. ...
1 downloads 0 Views 216KB Size
Download PDF
3606

Langmuir 2004, 20, 3606-3615

Preparation and Characterization of Polyion-Complexed Langmuir-Blodgett Films Containing an NLO Chromophore Gangadhar Panambur,† Yubao Zhang,† Ararat Yesayan,‡ Tigran Galstian,‡ C. Geraldine Bazuin,†,§ and Anna M. Ritcey*,† Centre de Recherche en Sciences et Inge´ nie´ rie des Macromole´ cules (CERSIM) and Chemistry Department, Center for Optics, Photonics and Lasers (COPL) and Physics Department, Universite´ Laval, Que´ bec, Canada G1K 7P4 Received July 16, 2003. In Final Form: January 21, 2004 Polyion complexes formed by monolayers of quaternary ammonium amphiphiles containing the 4-nitro4′-alkoxy azobenzene chromophore spread at the surface of aqueous solutions of a number of anionic polyelectrolytes were investigated. In general, Π-A isotherms were found to depend on the nature of the polyion present in the subphase, with monolayers of complexes involving polycarboxylates tending to exhibit larger limiting areas than those formed with polysulfonates or polysulfates. Monolayers of the polyion complexes can be transferred to hydrophilic solid substrates to yield Z-type LB films, although some peeling off for more than 10 layers is an impediment. X-ray reflectivity measurements indicate that relatively smooth and uniform films are obtained up to about 10 layers. Average layer thicknesses are, however, significantly smaller than extended molecular lengths, implying that the amphiphiles are strongly inclined from the surface normal. Polarized FT-IR measurements also indicate poor molecular orientation perpendicular to the surface. Preliminary SHG measurements for LB films of two systems, 12Q/CMC-Na and 12Q/PAA, confirm the presence of noncentrosymmetric out-of-plane chromophore ordering. Stable signals are observed for elevated temperatures up to 130 °C and for a period of 4 months at room temperature. To the best of our knowledge, this represents the first report of stable SHG in LB films of polyion complexes.

Introduction The development of new organic materials exhibiting nonlinear optical (NLO) properties has been the focus of intense research effort for more than a decade.1 Organic materials, particularly polymeric ones, offer several advantages over the inorganic crystals currently employed in NLO devices. The combination of the versatility of organic synthesis with the favorable mechanical properties of polymers allows for the preparation of tailor-made materials that can be readily processed.2,3 Despite these advantages, organic materials will find application in commercial devices only when their NLO efficiencies and thermal stability are demonstrated to be comparable to or to surpass those of current materials. In addition to these requirements, the fabrication of organic devices based on second-order NLO properties presents the additional challenge of preparing stable noncentrosymmetric arrangements of active chromophores. The various strategies employed for the preparation of molecular assembles with large second-order nonlinearities have been recently reviewed by Dalton et al.4 and Marks and * To whom correspondence should be addressed. † CERSIM and Chemistry Department. ‡ Center for Optics, Photonics and Lasers (COPL) and Physics Department. § Current address: De ´ partement de Chimie, Universite´ de Montre´al, Montre´al (QC), H3C 3J7. (1) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers; John Wiley & Sons: New York, 1991. (2) Dalton, L. Adv. Polym. Sci. 2002, 158, 1. (3) Polymers for Second-Order Nonlinear Optics; Lindsay, G. A., Singer, K. S., Eds.; ACS Symposium Series 601; American Chemical Society: Washington, D.C., 1995. (4) Dalton, L. R.; Harper, A. W.; Ghosn, R.; Steier, W. H.; Ziari, M.; Fetterman, H.; Shi, Y.; Mustacich, R. V.; Jen, A. K.-Y.; Shea, K. J. Chem. Mater. 1995, 7, 1060.

Ratner5 and include polymer poling, self-assembly, and use of the Langmuir-Blodgett technique. Of these approaches, polymer poling is the best adapted to eventual large-scale processing but is limited by both the low concentration of active chromophores at which phase separation typically occurs and the inherent metastable nature of the poled state.6,7 The Langmuir-Blodgett technique, on the other hand, leads to films with chromophore number densities and orientational order parameters that greatly exceed those obtainable in poled polymer systems.5 Traditional LB films, prepared from simple amphiphilic molecules, possess a center of symmetry since monolayer transfer occurs during both the up- and downstroke of the dipping process, leading to a head-to-head, tail-to-tail (Ytype) assembly. Noncentrosymmetric Y-type LB films can, however, be fabricated through the use of alternating layers, and second-harmonic generation has been observed from samples prepared in this way.8-10 Noncentrosymmetric order has also be achieved in special cases where monolayer transfer occurs in only one direction (during either the up- or the downstroke) of a dipping cycle. Popovitz-Biro et al.11demonstrated that tail-to-head (Ztype) LB films are obtained when the surface of the (5) Marks, T. J.; Ratner, M. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 155. (6) Stacey Fu, C. Y.; Lackritz, H. S.; Priddy, D. B., Jr.; McGrath, J. E. Macromolecules 1996, 29, 3470. (7) Firestone, M. A.; Park, J.; Minami, N.; Ratner, M. A.; Marks, T. J.; Lin, W.; Wong, G. K. Macromolecules 1995, 28, 2247. (8) Penner, T. L.; Motschmann, H. R.; Armstrong, N. J.; Ezenyilimba, M. C.; Williams, D. J. Nature 1994, 367, 49. (9) Wijekoon, W. M. K. P.; Wijaya, S. K.; Bhawalkar, J. D.; Prasad, P. N.; Penner, T. L.; Armstrong, N. J.; Ezenyilimba, M. C.; Williams, D. J. J. Am. Chem. Soc. 1996, 118, 4480. (10) Ashwell, G. J.; Walker, T. W.; Leeson, P.; Grummt, U.-W.; Lehmann, P. Langmuir 1998, 14, 1525.

10.1021/la0302910 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/02/2004

Polyion-Complexed Langmuir-Blodgett Films

deposited LB film is sufficiently polar to be wet by water during both the upstroke and the downstroke of the dipping cycle. Ashwell et al.12 also obtained Z-type films from quinolinium derivatives through the introduction of a second hydrophobic chain onto the positively charged chromophore. Despite these successful demonstrations of second-order nonlinear optical properties, important limitations of LB films have been noted4 and are primarily related to poor temporal stability of chromophore orientation and the decrease in molecular order typically observed with an increasing number of deposited layers. Neither of these phenomena is well understood, and the investigation of LB films with the objective of identifying the factors controlling long-term molecular orientation thus remains highly pertinent. One important source of film instability is interlayer diffusion, which has been demonstrated in small-molecule LB films by both spectroscopic13 and neutron reflectivity14 studies. A number of research groups15,16 have therefore focused on polymeric LB films in which NLO chromophores are incorporated as side chain substituents. An alternative approach to the reduction of molecular mobility in LB films through the introduction of polymers involves polyion complexation. The stabilization of surface layers of charged amphiphilic molecules through their interaction with water-soluble ionic polymers of opposite charge has been reported by a number of researchers; for example, refs 17-25. In general, a polyion complex is formed in situ at the air-water interface, as evidenced by a substantial increase in surface pressure. Surprisingly, very few of the numerous polyion-complexed systems reported in the literature contain chromophores selected for the study of NLO properties.26-28 Bock et al.28 demonstrated that ionic interactions can improve the temporal stability of poled systems but did not report SHG results for the corresponding polyion-complexed LB films containing NLO chromophores. Hickel et al.26 reported χ2 (11) Popovitz-Biro, R.; Hill, K.; Shavit, E.; Hung, D. J.; Lahav, M.; Leiserowitz, L.; Sajiv, J.; Hsiung, H.; Meredith, G. R.; Vanherzeele, H. J. Am. Chem. Soc. 1990, 112, 2498. (12) Ashwell, G. J.; Jackson, P. D.; Crossland, W. A. Nature 1994, 368, 438. (13) Shimomura, M.; Song, K.; Rabolt, J. F. Langmuir 1992, 8, 887. (14) Feigin, L.; Konovalov, O.; Wiesler, D. G.; Majkrzak, C. F.; Berzina, T.; Troitsky, V. Physica B 1996, 221, 185. (15) Teerenstra, M. N.; Klap, R. D.; Bijl, M. J.; Schouten, A. J.; Nolte, R. J. M.; Verbiest, T.; Persoons, A. Macromolecules 1996, 29, 4871 and references therein. (16) Raz´na, J.; Hodge, P.; West, D.; Kucharski, S. J. Mater. Chem. 1999, 9, 1693 and references therein. (17) Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976, 55, 73. (18) Stepina, N. D.; Tal’roze, R. V.; Lebedeva, T. L.; Yanusova, L. G.; Bezborodov, V. S.; L’vov, Y. M.; Plate´, N. A. Polym. Sci. 1993, 35, 538. (19) Stroeve, P.; van Os, M.; Kunz, R.; Rabolt, J. F. Thin Solid Films 1996, 284-285, 200. (20) Shimomura, M.; Kasuga, K.; Tsukada, T. Thin Solid Films 1992, 210-211, 375. (21) Engelking, J.; Menzel, H. Thin Solid Films 1998, 327-329, 90. (22) Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, Y.; Takahara, A. Langmuir 1993, 9, 760. (23) Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1991, 7, 2323. (24) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (25) Seki, T.; Tohnai, A.; Tanigaki, N.; Yase, K.; Tamaki, T.; Kaito, A. Macromolecules 1997, 30, 1768. (26) Hickel, W.; Lupo, D.; Ottenbreit, P.; Prass, W.; Scheunemann, U.; Schneider, J.; Ringsdorf, H. In Organic Materials for Non-Linear Optics II; Hahn, R. A., Bloor, D., Eds.; The Royal Society of Chemistry: Cambridge, 1991; p 75. (27) Aiba, S.; Ohmori, A.; Shimomura, M.; Miyano, K. Chem. Funct. Dyes, Proc. Int. Symp, 2nd; Yoshida, Z., Shirota, Y., Eds.; Mita Press: Tokyo, 1993; p 500. (28) Bock, H.; Advincula, R. C.; Aust, E. F.; Ka¨shammer, J.; Meyer, W. H.; Mittler-Neher, S.; Fiorini, C.; Nunzi, J.-M.; Knoll, W. J. Nonlinear Opt. Phys. Mater. 1998, 7, 385.

Langmuir, Vol. 20, No. 9, 2004 3607

Figure 1. General structure of the amphiphiles investigated. Samples are referred to as nQ, where n represents the number of methylene groups in the hydrocarbon spacer.

values obtained from LB films composed of a cationic hemicyanine amphiphile complexed to poly(acrylic acid). Although film stability was investigated by thermal desorption measurements, no results are presented concerning the temporal stability of the χ2 values. Aiba et al.27 observed SHG from polyion-complexed LB films containing a stilbazolium amphiphile. The secondharmonic intensity, however, was found to increase only slightly with a 5-fold increase in the number of deposited layers, and these authors concluded that although deposited as Z-type films, molecular rearrangement to a symmetric structure occurs during the dipping process. Seki et al.25 and Pen˜acorada et al.29 similarly concluded from X-ray measurements that LB films of polyioncomplexed arachidic acid initially deposited as either Z-type or X-type multilayers invert to Y-type bilayers upon aging. In the present article, we describe the fabrication and characterization of polyion-complexed LB films of quaternary ammonium surfactants containing the 4-nitro4′-alkoxy azobenzene chromophore, as shown in Figure 1. Preliminary SHG measurements indicate that these systems form noncentrosymmetric LB films that maintain molecular order over a period of at least several months. Experimental Section Materials.The synthesis of the amphiphiles, nQ, has been described previously.30 Docosanyl triethylammonium bromide, DS-Q, was obtained by quaternization of 1-bromodocosane (Aldrich), similar to the procedure used for the nQ amphiphiles. Poly(acrylic acid) (PA-acid) (Mw 1 250 000), sodium polyacrylate (PA) (Mw 30 000), sodium polystyrene sulfonate (PSS) (Mw 70 000), sodium polyvinyl sulfonate (PVS), and sodium carboxymethylcellulose (CMC) (reported by the supplier to have a solution viscosity of 3000-6000 cP in 1% aqueous solution and a degree of substitution 0.9 carboxylate/glucose unit) were all obtained from Aldrich. Sodium cellulose sulfate (CS) was obtained from Acros Organics and determined by elemental analysis to have a degree of substitution of about 2.6.31 Pressure-Area (Π-A) Isotherms and Fabrication of LB Films. Π-A isotherms were recorded using a KSV model 3000 balance with symmetrical monolayer compression. Typically, 70 µL of amphiphilic solution (0.5 mg‚mL-1) in chloroform was spread on the polyanion-containing aqueous (Nanopure II, (29) Pen˜acorada, F.; Reiche, J.; Zetzsche, T.; Dietel, R.; Brehmer, L.; de Saja, J. A. Thin Solid Films 1997, 295, 246. (30) Panambur, G.; Robert, C.; Zhang, Y.; Bazuin, C. G.; Ritcey, A. M. Langmuir 2003, 19, 8859. (31) Tibirna, C.-M. Development of New Supramolecular Polymers of Interest for Nonlinear Optical Applications. Ph.D. Thesis, Laval University, Que´bec, Canada, 2003.

3608

Langmuir, Vol. 20, No. 9, 2004

Barnstead) subphase. After waiting 10 min for the chloroform to evaporate, the monolayer was compressed at a rate of 5 mm/ min. All measurements were done at 15 °C. Molecular areas were calculated with respect to the ammonium amphiphile. Monolayers from the air/water interface were transferred onto various substrates (glass, germanium, and silicon) by the conventional vertical dipping technique. The film transfer efficiency is expressed in terms of the transfer ratio, which is defined as the ratio of the monolayer area lost to the area swept by the substrate during the transfer process. Subphase concentrations of 10 mg‚L-1 were employed for all transfers, unless otherwise specified. Dipping speeds were 200 and 5 mm/min for downward and upward strokes, respectively, unless otherwise specified. Monolayers were stabilized at the transfer pressure for 3 min before dipping. The deposited film was allowed to dry for 15 min between successive dipping cycles. Glass slides were cleaned with an ultrasound bath, being treated for 30 min, successively, in each of following cleaning solutions: a 10% detergent solution (Decon 75, BDH), a mixture of H2O2 (30%)/NH3(25%), and a 10% HCl solution. The slides were then washed several times with Nanopure water and finally ultrasonically washed with chloroform for 30 min before being stored in 2-propanol. Silicon wafers were cleaned in a heated bath of 70%H2SO4/30%H2O2 and then rinsed thoroughly with deionized-water. UV-vis Spectroscopy. UV-vis absorption spectra of multilayers on hydrophilic glass were recorded with either an HP 845A UV-visible or a Cary 500 spectrophotometer. Polarized spectra were recorded with the Cary 500 spectrophotometer equipped with a Harrick Glan-Taylor polarizer or with a spectroscopic ellipsometer. For all transmission measurements, the number of layers given includes the layers deposited on each side of the substrate. IR Spectroscopy. Polarized IR spectra were recorded for samples transferred to Ge ATR crystals with a Nicolet 560 FTIR equipped with a Harrick ATR accessory. Spectra shown are the sum of 1000 scans, accumulated at a resolution of 2 cm-1. X-ray Reflectivity. X-ray reflectivity measurements were performed with a Rigaku X-ray diffractometer [Rigaku Rotaflex RU-200BH rotating anode operated at 55 kV/190 mA, producing Ni-filtered Cu KR radiation (λ ) 1.542 Å)] in a θ-2θ reflection geometry, using a reflectometry stage constructed in our laboratory. If the thin film is homogeneous and has a smooth surface, so-called Kiessig fringes are recorded and the thickness (d) of the thin film can then be determined according to d ) (2n 1)π/qmin, where qmin is the scattering vector (q ) 4π sin(θ)/λ, 2θ is the scattering angle, and λ is the X-ray wavelength) at the minimum reflected intensity and n is the order of the interference minimum. Second-Harmonic Generation. SHG measurements were performed in transmission mode. Incident pulses (1064 nm; repetition rate 10 Hz; pulse duration 6-9 ns) from an actively Q-switched Nd:YAG laser were focused on a glass slide deposited with LB multilayers on both sides. The slide was mounted on a rotation stage with the rotation axis vertical and perpendicular to the propagation direction. Interference fringes were recorded for sample rotation around the axis orthogonal to both the film normal and the dipping direction. The SH signal at 532 nm was detected with a photomultiplier and analyzed with an integrator after appropriate spectral filtering. Measurements were performed with four different positions for the polarizer (placed between the source and the sample) and the analyzer (placed between the sample and the detector).

Results and Discussion Π-A IsothermssEffect of Polyion Structure. Surface pressure-area (Π-A) isotherms recorded for monolayers of 10Q spread on aqueous solutions of PA are shown in Figure 2 as a function of the concentration of the polyion in the subphase. At a polymer concentration of 0.1 mg‚L-1, no significant surface pressure is observed. As the polyion concentration increases to 1.0 mg‚L-1, the recorded isotherm shifts to higher molecular areas and surface pressures reaching about 16 mN‚m-1 are attained. These

Panambur et al.

Figure 2. Π-A isotherms of 10Q spread on aqueous solutions of PA of various concentrations as indicated.

observations suggest the formation of a surface-active polyion complex similar to others reported in the literature, for example, refs 17-24. Figure 2 also indicates that the dependence of the Π-A isotherm on polyion concentration becomes less pronounced above 1.0 mg‚L-1. Although some variation in isotherm shape is still observed, notably in the higher pressure region, the limiting molecular area, as evaluated by extrapolation of the linear portion of the first surface pressure rise to zero, remains approximately constant at a value of about 160 Å2 for polyion concentrations between 1.0 and 10 mg‚L-1. This is illustrated in Figure 3, where the limiting molecular area is plotted as a function of polymer concentration, expressed as the molarity of the polymeric anionic groups. Data for the same amphiphile spread on aqueous solutions of CMC30 are also shown. Despite the structural differences between CMC and PA, the corresponding polyion complexes occupy essentially identical molecular areas at each polyion concentration. It appears that, at least in this case, neither polymer backbone flexibility nor the distance between functional groups along the polymer chain is a determining factor. Polyion complexation was also observed for 10Q spread on aqueous solutions of several other polyelectrolytes. Π-A isotherms recorded at a single concentration of 10 mg‚L-1 are given in Figure 4. All of the isotherms exhibit similar limiting pressures (between 14 and 18 mN‚m-1) at which the onset of a constant pressure transition is observed. However, extrapolated limiting molecular areas, plotted as single points in Figure 3, vary from one polyion to another. The observed limiting area for the complex formed with PA-acid falls directly on the curve obtained for CMC and PA. The three other polyions considered, CS, PSS, and PVS, however, provoke the formation of more condensed monolayers. These results suggest that the nature of the anionic group may have an important influence on the polyion complex. The differences between the isotherms observed for the polyion complexes involving the two cellulose derivatives (CMC and CS) are particularly striking. In the case of CS, no significant increase in surface pressure is observed in the compression isotherm until a mean molecular area of about 80 Å2 is reached, at which point the surface pressure

Polyion-Complexed Langmuir-Blodgett Films

Figure 3. Limiting molecular area, obtained by linear extrapolation of the first surface pressure rise observed in Π-A compression isotherms, as a function of subphase concentration, expressed as the molarity of the polymeric ionic groups, for various polyions. The dashed line has been added only as a guide to the eye.

Figure 4. Π-A isotherms of 10Q spread on aqueous solutions containing various polyions at a concentration of 10 mg‚L-1.

rises abruptly, indicating the presence of a rigid surface film. The polyion complex formed with CMC, on the other hand, exhibits a highly compressible monolayer with nonzero surface pressures being observed at very high molecular areas. Although the differences in the monolayers of the two polyion complexes can be partly attributed to the nature of the anionic group (carboxylate as opposed to sulfate), it must also be noted that the two cellulose derivatives have very different degrees of substitution.

Langmuir, Vol. 20, No. 9, 2004 3609

Figure 5. Π-A isotherms of DS-Q spread on aqueous solutions containing various polyions at a concentration of 10 mg‚L-1.

Shimomura et al.20 showed a significant reduction in the limiting molecular area of a viologen amphiphile complexed to a highly substituted CMC (d.s. ) 2.8) as compared with that observed for a CMC similar to the one used in the present study. The compression isotherms presented by these authors indicate, however, that complexation to CS results in smaller molecular areas than those observed with CMC, even when CS has the lower degree of substitution.20 Limiting molecular areas therefore depend on both the degree of substitution of the cellulose-based polyion and the nature of the anionic group. We recently demonstrated that the dependence of limiting molecular area on polyion concentration (as illustrated in Figure 3) can be attributed to the incomplete complexation of the spread amphiphile at lower polyion concentrations and the subsequent loss of soluble amphiphile into the subphase.30 At insufficient polyion concentrations, recorded isotherms are therefore shifted to apparent molecular areas that are erroneously low. The direct comparison of isotherms obtained with different polyions is thus difficult if data are not available over an appropriate range of concentrations where the limiting molecular area is constant. Despite this difficulty, it is interesting to note that the Π-A isotherms recorded for DS-Q spread on various polyions, and shown in Figure 5, follow trends similar to those obtained for 10Q. Extrapolated limiting areas for the two systems are collected in Table 1, along with data taken from the literature20,32 for a polyion-complexed viologen amphiphile. The three cases present similarities, with the carboxylated polymers producing consistently higher molecular areas than those found for complexes to either PSS or CS. These results seem to parallel detailed studies by Kwak,33,34 reviewed by Goddard,35 of the binding affinity (32) Shimomura, M.; Kasuga, K.; Tsukada, T. J. Chem. Soc., Chem. Commun. 1991, 845. (33) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (34) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866.

3610

Langmuir, Vol. 20, No. 9, 2004

Panambur et al.

Table 1. Limiting Molecular Areas, As Evaluated by Extrapolation of the Linear Portion of the First Increase in Surface Pressure Observed in the Compression Isotherms, for Three Different Amphiphiles as a Function of the Polyelectrolyte in the Subphase limiting molecular area/Å2 polyion

10Q

DS-Q

viologen amphiphilea

CMC (d.s. ≈ 0.9) PA-acid PA PSS CS (d.s. > 2)

160 160 160 100 82

120 130 110 90 65

115 ∼100 (pH 5.5) 80 53

a

Values estimated from data presented in refs 20 and 32.

of quaternary ammonium surfactants to a number of polyanions. The strength of the binding affinity was found to be PSS > PA > CMC, suggesting that the trapping of soluble amphiphile as a polyion complex should be more efficient for the sulfonates and sulfates than for the carboxylates. With this line of reasoning, one would predict lower apparent molecular areas for the weaker complexes, which is contrary to what is observed. Hayakawa and Kwak,34,35 furthermore, investigated the cooperativity of binding and found that polyelectrolyte-surfactant complexation is much more cooperative in the case of PSS and sodium dextran sulfate than for either CMC or PA. These differences in cooperativity imply that at a given polyion concentration, complexes formed by carboxylates will have a lower density of surfactant molecules along the polymer backbone than those formed by sulfonates or sulfates. This prediction is consistent with the formation of expanded fluid monolayers with relatively large molecular areas by complexes with polycarboxylates and the observation of denser monolayers in the case of polysulfonates and polysulfates. In fact, at the slightly acid pH expected for water equilibrated with air, one would predict a higher fraction of protonated groups for the weaker polycarboxylates than for polysulfonates or polysulfates. The resulting lower charge density along the polymer chain would necessarily lead to a lower density of ion-complexed amphiphiles. It may be noted that complexes involving sulfonate or sulfate groups have frequently been reported in the literature to have 1:1 stoichiometry,21,22,24,36,37 whereas to our knowledge, the stoichiometry of complexes involving carboxylate groups has never been determined. Transfer Characteristics. A variety of LB films were prepared by the transfer of polyion complexes from the air-water interface to solid substrates. Many factors, including the surface pressure, the nature of the substrate, the dipping speeds, and the identity of the polyion, appear to influence the transfer behavior. The quality of monolayer transfer is also affected by a number of uncontrolled variables and is typically somewhat irreproducible. Despite the inherent variability of the transfer process, several general trends in the transfer behavior could be identified. Initially, monolayers of polyion-complexed 12Q with various polyelectrolytes (CMC, CS, PA, PA-acid, PSS, and PVS) and of nQ (n ) 6, 8, 10, 12) with CMC were transferred to hydrophilic glass. Similar transfer conditions were employed in all cases with a polyion concentration of 10 mg‚L-1, a transfer pressure of 10 mN/m, and (35) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 4, part II. (36) Engelking, J.; Ulbrich, D.; Menzel, H. Macromolecules 2000, 33, 9026. (37) Engelking, J.; Ulbrich, D.; Meyer, W. H.; Schenk-Meuser, K.; Duschner, H.; Menzel, H. Mater. Sci. Eng. C 1999, 8-9, 29.

relatively slow substrate dipping and withdrawal speeds, typically 10 and 5 mm/min, respectively. The transfer mode was generally found to be Z-type with a transfer ratio close to 1, irrespective of the amphiphile spacer length and the type or amount of polyion in the subphase. However, it was also observed that peeling off or monolayer redeposition occurred to varying extents during the insertion steps, especially after the first few cycles and with gradually increasing severity. This loss of material from the substrate to the water surface is characterized by a receding barrier position. The severity of the redeposition was found to vary from one polyion to another with the worst case occurring for 12Q/PSS, for which essentially the complete monolayer transferred during the upstroke is lost back to the subphase surface during the subsequent insertion step, even in the first cycle. Peeling off is, in general, less problematic for complexes of 12Q with PA and CMC, although it does occur with increasing number of deposited layers. Frequently, the peeling off is partial and, more precisely, occurs in the upper portion of the deposited layer, i.e., during the latter part of the insertion step. Fast downstroke dipping speeds, of the order of 200 mm/min, retard the peeling off significantly, although they usually do not eliminate it completely. Increasing the transfer pressure to just below the plateau pressure is also helpful. Better results were also obtained when hydrophilic silicon wafers replaced glass as the substrate. The observation that superior results are obtained for transfer to silicon wafers, which are typically smoother than glass slides, implies that peeling off may be aggravated by surface defects or surface roughness, which lead to increased water retention and poorer surface adhesion. Increased peeling off at higher numbers of layers could then be attributed to the successive accumulation of surface imperfections. Popovitz-Biro et al.11 clearly demonstrated a relationship between transfer type and the contact angle of the substrate. Z-type transfer is obtained when the LB film surface is wet by water during both the upstroke and the downstroke of the dipping cycle (both advancing and receding contact angles < 90°). Nitroaniline terminal groups were shown to be sufficiently hydrophilic to inhibit transfer during the downstroke;11 thus, in our case, the Z-type films can be attributed to the 4-nitroazobenzene moiety of the nQ series of amphiphiles. Somewhat surprisingly, we found that polyion complexes of DS-Q with CMC also lead to Z-type transfer, despite the absence of a polar terminal functionality. In this case, the wettability of the film can most likely be attributed to its high porosity to water.11 Monolayer transfer was carried out at molecular areas greatly exceeding that corresponding to close packing of alkyl chains perpendicular to the film surface. Highly tilted chains and poor interchain packing would lead to inefficient masking of the hydrophilicity of the underlying substrate. Interestingly, high-quality Z-type transfer, with essentially no peeling off, is observed for the deposition of up to 20 layers of DS-Q/CMC on glass. Characterization of the LB films. UV-Vis Spectroscopy. UV-vis spectra are presented in Figure 6 for a series of LB films prepared from 12Q complexed to CMC. A dilute solution spectrum for the amphiphile in chloroform is also shown for comparison. All of the spectra exhibit a strong absorption band centered near 375 nm, which can be attributed to the π-π* transition of the azobenzene chromophore.38 The absorption maximum of the LB films (38) Taniike, K.; Matsumoto, T.; Sato, T.; Ozaki, Y.; Nakashima, K.; Iriyama, K. J. Phys. Chem. 1996, 100, 15508.

Polyion-Complexed Langmuir-Blodgett Films

Figure 6. Transmission UV-vis spectra of LB films of 12Q/ CMC deposited on glass composed of (a) 2, (b) 4, (c) 8, (d) 12, and (e) 16 layers (counting the layers deposited on both sides of the glass). The spectrum of 12Q in dilute chloroform solution is also shown (f).

shows a relatively small (∼4 nm) hypsochromic shift relative to that of the solution spectrum. The absence of a significant spectral shift indicates that the chromophores in the LB films are essentially isolated and do not form aggregates. As a result of the peeling off described above, differences in absorption intensities are observed at different positions in the LB film. For higher numbers of layers, this variation becomes more pronounced and in the case of the 30-layer sample the spectrum plotted in Figure 6 corresponds to the absorbance recorded near the bottom of the glass substrate, where peeling off is minimal. For spectra recorded in this way, the absorption intensity at the maximum is found to vary linearly as a function of the number of transferred layers. UV-vis spectra were also recorded with light polarized parallel (A||) and perpendicular (A⊥) to the dipping direction. In general, the films were found to be dichroic. The evolution of the dichroic ratio (A⊥/A||) with the number of dipping cycles, where each cycle corresponds to the transfer of a single layer to each side of the substrate, is plotted in Figure 7 (filled circles). Although the data from several experiments show considerable scatter, a clear tendency to increasing in-plane anisotropy with increasing number of layers is observed. The first layer consistently shows no dichroism (A⊥/A|| ) 1), whereas dichroic ratios of about 2 are obtained for films containing more than 10 layers. The preferential orientation of rigid polymer molecules along the dipping direction of LB films has been investigated in detail by Schwiegk et al.39 and was attributed to monolayer flow at the air-water interface. This model could explain the variation in molecular orientation as a function of the number of deposited layers illustrated in Figure 7, since the distance over which the monolayer must flow increases with the amount of material removed by the transfer process. To test the applicability of this (39) Schwiegk, S.; Vahlenkamp, T.; Xu , Y.; Wegner, G. Macromolecules 1992, 25, 2513.

Langmuir, Vol. 20, No. 9, 2004 3611

Figure 7. Dichroic ratio (A⊥/A||) evaluated at 375 nm for LB films of 12Q/CMC on glass as a function of the number of dipping cycles (one layer is deposited on each side of the substrate during each cycle). Successive layers were deposited from a singlespread monolayer, either on a single substrate (B) or on a series of separate substrates (O). Error bars represent the standard deviation calculated for a number of repeated experiments.

explanation to the present case, a series of samples were prepared from a single-spread monolayer, with a new substrate being employed for each successive dipping cycle. The dichroic ratios for these samples, each composed of a single monolayer deposited on each side of the glass slide, are plotted as open circles in Figure 7. The results clearly indicate that no in-plane orientation is observed. This result leads to the conclusion that in the present case, the increased dichroism observed for thicker films cannot be attributed to flow-induced orientation at the air-water interface. The higher anisotropy observed for thicker films must therefore be related to the nature of the film already present on the substrate. Although no specific mechanism can be offered at this time to explain these observations, it is possible that the film anisotropy is somehow related to the increased peeling off typically observed as the number of layers increases. It is, furthermore, of interest to note that the greater absorbance intensity is recorded for light polarized perpendicular to the dipping direction rather than parallel to it. Any flowinduced molecular orientation would be expected to result in alignment of the long axis of the azobenzene moiety along the dipping direction, which is contrary to what is observed. Possibly, the stiff polyion backbone is preferentially oriented along the dipping direction and the azobenzene side chain substituents adopt an orientation perpendicular to the polymer chain. ATR-FTIR Spectroscopy. LB films were also characterized by polarized infrared spectroscopy. ATR spectra, recorded with radiation polarized both parallel (Ap) and perpendicular (As) to the plane of incidence, for an LB film composed of two layers of 12Q/CMC transferred to a germanium crystal are shown in Figures 8 and 9. Transmission FTIR spectra for bulk samples of pure 12Q, CMC, and the 12Q/CMC complex, prepared as KBr pellets, are also shown for comparison. Bands arising from 12Q can be clearly identified in the spectra of the LB film, and

3612

Langmuir, Vol. 20, No. 9, 2004

Figure 8. ATR FTIR spectra showing the methylene stretching region obtained for an LB film of 12Q/CMC deposited on germanium (two layers on each side of the crystal), recorded with incident radiation polarized (a) perpendicular and (b) parallel to the plane of incidence. The transmission spectra of 12Q/CMC (c) and pure 12Q (d), prepared as KBr pellets, are also shown.

the position and assignments of the major absorptions are summarized in Table 2. Although the majority of the bands appear at the same position in the LB films as in the bulk samples, significant frequency shifts are observed in three cases. Figure 8 illustrates that bands associated with the symmetric and antisymmetric methylene stretching vibrations appear at higher frequencies in the LB films than in the bulk. For pure 12Q in the bulk, the maximum of the two bands is located at 2849 and 2922 cm-1, respectively. These values are close to those characteristic of an all-trans conformation of a hydrocarbon chain40 and are consistent with the crystalline nature of this sample.31 For the bulk complex, which is characterized by a singlelayer smectic A-like phase, the values are shifted slightly (1-3 cm-1) to higher wavenumbers, indicating the introduction of gauche conformers. This shift is more pronounced in the LB films (4-6 cm-1 relative to pure 12Q in the bulk), showing that the methylene groups are less ordered in these films than in the bulk complex. In comparison, displacements of the order of 3.5 cm-1 are observed for the well-known gel to liquid crystal transition of many lipids.40 Figure 9, showing the spectra at lower wavenumbers, reveals a second difference in the region corresponding to the ether C-O stretching vibrations. The band observed at 1261 cm-1 in the bulk spectrum of pure 12Q is shifted to 1257 cm-1 in the spectrum of the bulk complex and to 1254 cm-1 in the spectrum of the LB film complex. We previously reported41 a similar spectral shift for LB films of a tertiary amine functionalized mesogen, which we (40) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (41) Garneau, S.; Rondeau, P.; Bazuin, C. G.; Ritcey, A. M. Macromol. Chem. Phys. 2000, 201, 2535.

Panambur et al.

Figure 9. ATR FTIR spectra recorded for a two-layer LB film of 12Q/CMC deposited on germanium, recorded with incident radiation polarized (a) perpendicular and (b) parallel to the plane of incidence. The transmission spectra of the 12Q/CMC complex (c), pure 12Q (d), and CMC (e), all prepared as KBr pellets, are shown for comparison. Table 2. Vibrational Assignments and Peak Positions of the Principal Bands Observed for 12Q, LB Films of the 12Q/CMC Polyion Complex and a Bulk Sample of the Same Complex. Order Parameters (Sz), As Calculated from ATR Dichroic Ratios, Are Also Provided for the LB Films frequency at maximum/cm-1 attribution νa, CH2 νs, CH2 ring (ether-subst), ν8 ring (nitro-subst), ν8 ring ν19aa νa, NO2a νs, NO2 C-O-phenyl ring, ν9b a

pure 12Q 12Q/CMC 12Q/CMC (bulk) (LB film) (bulk) 2922 2849 1602 1581 1519 1505 1340 1262 1137

2926 2855 1601 1574 1520 1502 1340 1254 1136

2923 2852 1603 1581 1521 1501 1344 1257 1140

Sz (LB film) -0.08 ( 0.05 -0.06 ( 0.04 -0.17 ( 0.09 -0.23 ( 0.01 -0.19 ( 0.09 -0.2 ( 0.1 -0.11 ( 0.02

Following proposed assignment according to ref 48.

attributed to conformational differences between the ether linkage in the pure crystalline state of the bulk and in the LB films. In the case of simple aliphatic ethers, C-O-C vibrations arising from gauche conformations are observed at lower frequencies than those of corresponding trans conformations,42 although typical shifts are larger than that observed in Figure 9. By analogy, the shifts observed in the present case can be taken to suggest that the proportion of gauche conformers present in the ether linkages of the surfactant molecule increase from the pure 12Q in the bulk to the bulk complex to the LB complex. This parallels the trend in the high-wavenumber region discussed above. (42) Perchard, J. P. Spectrochim. Acta 1970, 26A, 707.

Polyion-Complexed Langmuir-Blodgett Films

A third difference can be noted for the aromatic vibration, which is located at 1581 cm-1 for both pure 12Q and the complex in the bulk but is shifted to 1574 cm-1 in the LB film spectra. This band has been shown, by selective deuteration, to be associated with the nitrosubstituted phenyl ring.43 No interpretation of this spectral shift can be offered at this time. Vibrational bands arising from the CMC are difficult to discern in the LB film spectra, since characteristic vibrations in the polymer are much broader and weaker. The broadening of the band at 1602 cm-1 in the LB film spectra in comparison to the small molecule spectrum is certainly suggestive of the presence of the polymer, whose spectrum is characterized by an intense band in this region (due to overlapping contributions from the OH deformation and the asymmetric carboxylate stretching vibrations41). A similar broadening is observed for the bulk complex. An ether band from the cellulose backbone can also be identified at about 1060 cm-1. Finally, the presence of a band at 1727 cm-1 in both the LB film spectra and the bulk complex spectrum must be noted. This frequency can be attributed to the carbonyl stretching vibration of a carboxylic acid and suggests that a small fraction of the CMC carboxylate groups have been protonated. This, in fact, is expected (noting that the bulk complex was also prepared in water31): at the subphase pH of 5.5, about 5-10% of the CMC carboxylates are expected to be protonated.44 Polarized ATR spectra can also provide information concerning molecular orientation. In the case of uniaxial orientation about the surface normal, transition moment order parameters (Sz) can be calculated45 from the dichroic ratio (R ) Ap/As) and electric field amplitudes obtained from Harrick’s equations for thin films.46 The polarized UV-vis spectra discussed above indicate that thicker LB films exhibit significant in-plane anisotropy. For this reason, the present orientational analysis is restricted to films composed of only two monolayers, for which the in-plane dichroism can be considered negligible. Results are given in Table 2 for bands sufficiently resolved to permit a reasonable evaluation of the corresponding absorption maximum intensities. Order parameters of near zero are found for the methylene stretching transition moments, indicating a random orientation for the hydrocarbon spacer. Order parameters close to -0.2 are obtained for azobenzene vibrations at 1601, 1574, and 1340 cm-1. Since the transition moments of these three vibrations lie essentially parallel to the long axis of the azobenzene moiety,47,48 the small negative values for the order parameters suggest a large average tilt angle (>57°) for the chromophore with respect to the surface normal. The ATR measurements thus indicate that the LB films of 12Q/CMC polyion complexes are poorly ordered and are consistent with the band shifts described above that indicate a significant proportion of gauche conformers. (43) Fre´chette, F. EÄ tude par spectroscopie infrarouge de l’orientation photoinduite d’un de´rive´ de l’azobenze`ne dans des films minces d’ace´tate de cellulose. M.Sc. Thesis, Laval University, Que´bec, Canada, 2001. (44) Hoogendam, C. W.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H.; Smit, J. A. M.; van Dijk, J. A. P. P.; van der Horst, P. M.; Batelaan, J. G. Macromolecules 1998, 31, 6297. (45) Fringeli, U. P.; Gu¨nthard, H. H. Infrared membrane spectroscopy. In Membrane Spectroscopy; Grell, E., Ed.; Springer-Verlag: New York, 1981; pp 270-332. (46) Harrick, N. J. Internal reflection spectroscopy; Harrick Scientific Corp.: Ossining, NY, 1967; p 1067. (47) Natansohn, A.; Rochon, P.; Pe´zolet, M.; Audet, P.; Brown, D.; To, S. Macromolecules 1994, 27, 2580. (48) Lagugne´ Labarthet, F.; Freiberg, S.; Pellerin, C.; Pe´zolet, M.; Natansohn, A.; Rochon, P. Macromolecules 2000, 33, 6815.

Langmuir, Vol. 20, No. 9, 2004 3613

Figure 10. X-ray reflectivity curves recorded for LB films composed of (a) 3, (b) 6, (c) 9, and (d) 13 layers of 10Q/CMC deposited on silicon. The curves have been shifted horizontally for clarity.

X-ray Reflectivity. X-ray reflectivity curves of LB films prepared from the 10Q/CMC polyion complex are shown in Figure 10 as a function of the number of transferred layers. Strong Kiessig fringes are observed for samples composed of a relatively low number of layers, demonstrating the homogeneity of the film bulk and the relative smoothness of the film surface. The fringes become less well defined as the total thickness increases, indicating a deterioration in film uniformity that may reflect increasing surface imperfections and that may be related to the decreasing transfer ratios and increasing peeling off associated with the transfer of a higher number of layers. The overall film thicknesses, evaluated from the fringe spacings, are plotted as filled circles in Figure 11 as a function of the number of transferred layers. Film thicknesses determined for three samples of the same amphiphile complexed to PVS are also shown (open circles). A single linear relationship is observed with a slope corresponding to an incremental layer thickness of 7.7 Å. This is thin when compared with the length of the fully extended molecule, which is estimated as 32 Å and implies that the molecules are sharply tilted with respect to the surface normal. This result is in agreement with the polarized FT-IR ATR results discussed above as well as consistent with the large molecular area (110 Å2 for the CMC complex) at which monolayer transfer was carried out. X-ray reflectivity measurements were also performed on LB films of DS-Q/CMC complexes, and the resulting film thicknesses are plotted in Figure 11 as filled squares. Again, a linear relationship is obtained, this time with an incremental layer thickness of 18 Å. This value is greater than that determined for 10Q but remains much below the length of the fully extended amphiphile (34 Å). This once again suggests that the molecules are highly tilted with respect to the surface normal, as would be expected from the high molecular area (∼80 Å2) at which monolayer transfer was carried out. Although overall thicknesses are found to vary linearly with the number of layers transferred for the three polyion

3614

Langmuir, Vol. 20, No. 9, 2004

Panambur et al.

Figure 12. X-ray reflectivity curves recorded for LB films composed of six layers of 10Q/CMC transferred to silicon at (a) 12 and (b) 17 mN‚m-1. Figure 11. Total film thickness, as determined by X-ray reflectivity, as a function of the number of layers for LB films of 10Q/CMC, 10Q/PVS, and DS-Q/CMC, transferred at surface pressures of 18, 12, and 30 mN‚m-1, respectively.

complexes considered, neither of the plots extrapolates to the origin. At first view, this seems to imply that the first layer is significantly thicker than the subsequent ones. In fact, the values corresponding to a single layers36 and 42 Å for the 10Q and DS-Q complexes, respectivelys suggest that, in this first layer, the amphiphiles are nearly fully extended and normal to the substrate surface, considering that the polymer layer may account for 5-10 Å of the layer thickness. This interpretation, however, contradicts the conclusion of highly tilted amphiphiles, supported both by the relatively large limiting molecular areas and the order parameters obtained from polarized FT-IR spectroscopy. It is interesting to note that the X-ray reflectivity data plotted in Figure 11 all extrapolate to the same value (about 27 Å) at the origin. This suggests a fixed contribution to the overall film thickness that is common to all three systems. One possibility is the formation of an adsorbed layer of polyion on the silicon surface during immersion of the substrate on the first dipping cycle. The spontaneous adsorption of CMC on negative surfaces has in fact been previously reported.49 X-ray reflectivity curves were also recorded for LB films of 10Q/CMC transferred at two different surface pressures, 12 (∼120 Å2/molecule) and 17 mN‚m-1 (∼110 Å2/molecule), and are shown in Figure 12. Despite the relatively small difference in the molecular areas at which the monolayers were transferred, the two curves differ significantly. The overall film thickness is found to increase by more than 40% for a corresponding decrease in molecular area of less than 10%, during compression in this region of the isotherm. The two curves are also different with respect to the definition of the Kiessig fringes. The LB film prepared at the higher surface pressure appears to be more uniform and has a lower surface roughness, even though very similar transfer ratios (>0.9 for the upstroke and ∼0 for the downstroke) were observed for the two samples. (49) Hoogendam, C. W.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H.; Batelaan, J. G.; van der Horst, P. M. Langmuir 1998, 14, 3825.

Figure 13. Second-harmonic intensity (arbitrary units) recorded as a function of the angle of incidence of the source beam for an LB film containing eight layers (four on each side of the glass substrate) of 12Q/PA fabricated at 10 mN/m with (a) p-p and (b) s-p polarizations.

No Bragg reflection is observed for any of the samples. This could be due to either insufficient contrast in electron density along the normal to the layer plane or to the absence of a discrete layered structure. Second-Harmonic Generation. Preliminary SHG measurements were carried out for LB films containing 12Q complexed to either CMC or PA. Measurements were performed with four different combinations of polarizer and analyzer positions, referred to as p-p, p-s, s-p, and s-s, where the first letter corresponds to the incident polarization and the second to the position of the analyzer. The experimental second-harmonic intensity generated by a typical sample with p-p and s-p polarizations is plotted in Figure 13 as a function of the angle of incidence of the Nd:YAG laser beam. Characteristic Maker fringes are observed, as anticipated for the generation of the second-harmonic signal by the LB multilayers on both sides of the glass slide. The near-zero intensity minima indicate that the quality of the monolayer on either side

Polyion-Complexed Langmuir-Blodgett Films

Figure 14. Distribution of the nonlinear dipoles with respect to the substrate surface. Eω and E2ω represent the electric fields of the incident and SH radiation, respectively. Dipoles 1 and 2 lie in the (x, y) plane, whereas dipoles 3 and 4 lie in the (y, z) plane. The x-axis corresponds to the dipping direction.

of the glass substrate is nearly identical and uniform. No second-harmonic signal was observed for either the p-s or the s-s polarizer positions. Figure 13 clearly illustrates that the most intense second-harmonic signal is observed in the p-p polarization, that is when both the polarization of the pump and the polarization of the signal are aligned along the dipping direction. Significantly, the signal is negligible at normal incidence and increases in intensity at higher angles of incidence. This observation indicates that the noncentrosymmetric component of the dipole responsible for SHG does not lie within the plane of the film. This is supported by the additional observation that the SH intensity is independent of rotation in the plane of the sample. The intensity of the s-polarized component (s-p) was found to be an order of magnitude smaller than that of the p-polarized component (p-p).The observed polarization dependence of the second-harmonic intensities (Ipp > Isp . Ips > Iss) can be explained by assuming a conic distribution of molecular nonlinear dipoles about the surface normal, as illustrated in Figure 14. We restrict ourselves here to a qualitative explanation. (A detailed quantitative description is given in ref 50.) Let us consider four different nonlinear dipoles, labeled as 1-4 in Figure 14. The induced nonlinear polarization of each dipole depends on the projection of the incident electric field E B ω on the dipole direction. The p-polarized incident electric field has projections along all of the 1-4 directions, and hence, this polarization will excite all four dipoles. The s-polarized radiation, however, will excite only dipoles 3 and 4 since its electric field has no projections on the dipole directions 1 and 2. As a result, the SH signal generated by p-polarized light will be stronger than that observed for s-polarized irradiation. The intensity of p- or s-components of generated SH will depend on the projection of induced nonlinear dipole polarization on the direction of pˆ or sˆ unit vectors.50 In the case of p-polarized irradiation, all four excited dipoles (14) contribute to the p-component of the generated SH and the intensity, Ipp, of the corresponding signal will be high. At the same time, only dipoles 3 and 4 contribute to the (50) Yesayan, A.; Roberge, M.-M.; Galstian, T. V.; Cottin, P.; Lachance, L.; Pigeon, M.; Ritcey, A. M.; Rahem, T. J. Nonlinear Opt., submitted for publication.

Langmuir, Vol. 20, No. 9, 2004 3615

s-component of SH signal since the projections of dipoles 1 and 2 on the sˆ-direction (y-axis) are zero. As these contributions are opposite, the resulting SH intensity Ips will be very small. The same explanation is valid for the case of s-polarized incident radiation, and the s-component of generated SH, Iss, will also be very small. The p-component of the SH will be smaller for s-polarized irradiation (Isp) than for p-polarized irradiation (Ipp) since only dipoles 3 and 4 contribute to Isp while all dipoles 1-4 contribute to Ipp (according to the explanation given above). To check the stability, daily measurements were performed on 12-layer LB films of 12Q/PA and 12Q/CMC over a 1-week period. Constant second-harmonic intensities were obtained. The stability of the samples was also studied as a function of temperature. Second-harmonic generation remains stable up to at least 120 °C but decreases to about 10% of the original value by 140 °C. This high stability to elevated temperatures may perhaps be correlated with the very high viscosities shown by the bulk complexes up to high temperatures (despite a glasstransition temperature near ambient) and which can be associated with the extensive ionic interactions in the system.31 In another test, a different 6-layer 12Q/CMC film containing six layers was kept in the dark for about 4 months at RT; I2ω was found to be the same for the freshly prepared and aged film. Conclusions Monolayers of ammonium amphiphiles containing the 4-nitro-4′-alkoxy azobenzene chromophore form ionic complexes when spread at the surface of aqueous solutions of a number of anionic polyelectrolytes. For a given amphiphile, Π-A isotherms are found to depend on the nature of the polyion present in the subphase, with monolayers of complexes involving polycarboxylates tending to exhibit larger limiting areas than those formed with polysulfonates or polysulfates. This observation can be successfully correlated with previously reported differences in the cooperativity of surfactant-polyelectrolyte binding.34,35 Monolayers of the polyion complexes are transferred to hydrophilic solid substrates only during substrate withdrawal to yield Z-type LB films. The preparation of LB films composed of more than 10 layers is, however, impeded by the peeling off of transferred layers and their return to the water surface during substrate immersion. LB films are also found to become increasingly dichroic as the number of deposited layers increases, with the more strongly absorbing axis lying perpendicular to the dipping direction. X-ray reflectivity measurements indicate that relatively smooth and uniform films are obtained up to about 10 layers. Average layer thicknesses are, however, significantly smaller than extended molecular lengths, implying that the amphiphiles are strongly inclined from the surface normal. Polarized FT-IR measurements also indicate poor molecular orientation perpendicular to the surface. Despite the low degree of order indicated by FT-IR results, preliminary SHG measurements for LB films of two systems, 12Q/CMC and 12Q/PA, confirm the presence of noncentrosymmetric out-of-plane chromophore ordering. Stable signals are observed for elevated temperatures up to 120 °C and for a period of at least 4 months at room temperature. Acknowledgment. The authors thank C. Tibirna for providing the infrared spectra of the bulk complexes. Financial support from NSERC (Canada) and FCAR (Que´bec) is gratefully acknowledged. LA0302910