Fabrication, Structural Characterization, and Applications of Langmuir

Prudente/SP, 19060-080, Brazil, Materials and Surface Science Group, UniVersity .... (12) Bang, C. U.; Shishido, A.; Ikeda, T. Macromol. Rapid Commun...
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Langmuir 2008, 24, 4729-4737

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Fabrication, Structural Characterization, and Applications of Langmuir and Langmuir-Blodgett Films of a Poly(azo)urethane Priscila Alessio,† Daniele M. Ferreira,† Aldo E. Job,† Ricardo F. Aroca,‡ Antonio Riul, Jr.,§ Carlos J. L. Constantino,*,† and Eduardo R. Pe´rez Gonza´lez† Departamento de Fı´sica, Quı´mica e Biologia, Faculdade de Cieˆ ncias e Tecnologia, UNESP, Presidente Prudente/SP, 19060-080, Brazil, Materials and Surface Science Group, UniVersity of Windsor, Windsor/ On, N9B3P4, Canada, and UniVersidade Federal de Sa˜ o Carlos, campus Sorocaba/SP, 18043-970, Brazil ReceiVed October 24, 2007. In Final Form: December 20, 2007 The synthesis of a poly(azo)urethane by fixing CO2 in bis-epoxide followed by a polymerization reaction with an azodiamine is presented. Since isocyanate is not used in the process, it is termed “clean method” and the polymers obtained are named “NIPUs” (non-isocyanate polyurethanes). Langmuir films were formed at the air-water interface and were characterized by surface pressure vs mean molecular area per mer unit (Π-A) isotherms. The Langmuir monolayers were further studied by running stability tests and cycles of compression/expansion (possible hysteresis) and by varying the compression speed of the monolayer formation, the subphase temperature, and the solvents used to prepare the spreading polymer solutions. The Langmuir-Blodgett (LB) technique was used to fabricate ultrathin films of a particular polymer (PAzoU). It is possible to grow homogeneous LB films of up to 15 layers as monitored using UV-vis absorption spectroscopy. Higher number of layers can be deposited when PAzoU is mixed with stearic acid, producing mixed LB films. Fourier transform infrared (FTIR) absorption spectroscopy and Raman scattering showed that the materials do not interact chemically in the mixed LB films. The atomic force microscopy (AFM) and micro-Raman technique (optical microscopy coupled to Raman spectrograph) revealed that mixed LB films present a phase separation distinguishable at micrometer or nanometer scale. Finally, mixed and neat LB films were successfully characterized using impedance spectroscopy at different temperatures, a property that may lead to future application as temperature sensors. Principal component analysis (PCA) was used to correlate the data.

Introduction The interest in polymeric materials for potential applications continues to grow. In particular, polyurethanes (PU) have attracted special interest since 1937, when they were discovered by Otto Bayer and his co-workers at I.G. Ferbenindustri, Germany.1 The attention to PU is due to a wide selection of monomeric materials among a long list of macrodiols and diisocyanates. In addition, the variety of possible groups present among the urethane bonds, which can originate different products, result in great versatility.2 The synthesis of PU usually uses diisocyanates as monomers. However, this class of polymers can be prepared by using CO2containing monomers (e.g., dicarbonates), avoiding the use of the very toxic isocyanates. This clean method yields ‘nonisocyanate polyurethanes’ (NIPUs) using organic carbonates as intermediates.3,4 Another encouraging point for using these materials is the interesting optoelectronic and photonic properties that can be obtained through conjugate polymers as specific chromophores as in the case of azo-benzenic compounds.5-7 The azobenzenic-containing polymers (azopolymers) have more stable thermal and mechanical properties than their respective * Corresponding author. E-mail: [email protected]. † UNESP. ‡ University of Windsor. § Universidade Federal de Sa ˜ o Carlos. (1) Chattopadhyay, D. K.; Raju, K. V. S. N. Prog. Polym. Sci. 2007, 32, 352. (2) Raspoet, G.; Nguyen, M. T.; Mcgarraghy, M.; Hegarty, A. F. J. Org. Chem. 1998, 63, 6878. (3) Tamami, B.; Sohn, S.; Wilkes, G. L. J. Appl. Polym. Sci. 2004, 92, 883. (4) Figovsky, O.; Shapovalov, L.; Buslov, F. Surface Coatings International Part B-Coatings Transactions 2005, 88, 67. (5) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Nature 2007, 446, 778. (6) Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.; Seitz, M.; Gaub, H. E. Science 2002, 296, 1103. (7) Tegeder, P.; Hagen, S.; Leyssner, F.; Peters, M. V.; Hecht, S.; Klamroth, T.; Saalfrank, P.; Wolf, M. Appl. Phys. A 2007, 88, 465.

monomers, and they have been used to produce optical sensor,8 artificial taste sensor,9 and pH sensors.10 However, their main characteristic is the capacity of reversible cis-trans isomerization with applications as optical information storage,11 light switching devices,12 surface relief gratings,13 holograms, and induction of liquid-crystal alignment.14 An important tool, which could be essential to the study of polymers, is the Langmuir-Blodgett (LB) technique that allows the preparation of thin films.15 This technique was developed by Irving Langmuir and Katherine Blodgett in 1934, and since then it has been used for film fabrication in nanostructured devices for nonlinear optic, thin film conductors, and sensor applications.16 An advantage of this technique is the ability to tune film characteristics, which can be achieved by changing some LB parameters such as temperature, surface pressure, and compression rate. In this work, PAzoU Langmuir films were fabricated and characterized by their surface pressure vs mean molecular area (Π-A) isotherm. LB films were also obtained and characterized using UV-vis and FTIR absorption, micro-Raman scattering, and atomic force microscopy (AFM). Films were tested as (8) Grafe, A.; Haupt, K.; Mohr, G. J. Anal. Chim. Acta 2006, 565, 42. (9) Riul, A., Jr.; dos Santos, D. S., Jr.; Wohnrath, K.; Di, Tommazo, R.; Carvalho, A. C. P. L. F.; Fonseca, F. J.; Oliveira, O. N., Jr.; Taylor, D. M.; Mattoso, L. H. C. Langmuir 2002, 18, 239. (10) Uznanski, P.; Pecherz, J. J. Appl. Polym. Sci. 2002, 86, 1459. (11) Constantino, C. J. L.; Aroca, R. F.; Mendonc¸ a, C. R.; Mello, S. V.; Balogh, D. T.; Zilio, S. C.; Oliveira, O. N., Jr. AdV. Funct. Mater. 2001, 11, 65. (12) Bang, C. U.; Shishido, A.; Ikeda, T. Macromol. Rapid Commun. 2007, 28, 1040. (13) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139. (14) Zhang, Y. Y.; Cheng, Z. P.; Chen, X. R.; Zhang, W.; Wu, J. H.; Zhu, J.; Zhu, X. L. Macromolecules 2007, 40, 4809. (15) Ferreira, M.; Constantino, C. J. L.; Olivati, C. A.; Balogh, D. T.; Aroca, R. F.; Faria, R. M.; Oliveira, O. N., Jr. Polymer 2005, 46, 5140. (16) Petty, M.C. Langmuir-Blodgett Films - an Introduction; Cambridge University Press: Cambridge, 1996.

10.1021/la703328z CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008

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Figure 1. (a) PAzoU mer unit molecular structure; (b) Π-A isotherms recorded at 10 mm/min and 20 °C for PAzoU dissolved in different solvents: CHCl3, THF, and DMF. The inset shows the stability test for the PAzoU dissolved in DMF.

Figure 2. (a) One Π-A isotherm until the film collapse and two cycles of compression/expansion (Π ) 8.0 mN/m) both recorded at 10 mm/min and 20 °C for PAzoU dissolved in DMF. (b) Π-A isotherms recorded at 10, 50, 100, and 180 mm/min and 20 °C for PAzoU dissolved in DMF. (c) UV-vis absorption spectra for CHCl3, THF, and DMF PAzoU solutions. (d) UV-vis absorption spectra for PAzoU cast films produced using CHCl3, THF, and DMF solutions. The absorbance scale in Figures 2c and 2d were altered to make a clear comparison between the spectra.

Langmuir-Blodgett Films of a Poly(azo)urethane

Figure 3. Π-A isotherms recorded at 10 mm/min and 20 °C for mixed PAzoU/SA dissolved in DMF using different mass %: 25, 50, and 75 (the SA molecular weight is used as reference). The inset shows the absorbance at 400 nm for the 12-layer mixed PAzoU/SA LB films (25, 50, and 75% in mass).

Figure 4. UV-vis absorption spectra for neat PAzoU LB films with 05, 10, 15, and 21 layers. The inset shows the absorbance at 400 nm vs the number of layers.

temperature sensors using impedance spectroscopy and principal component analysis (PCA). Experimental Section DGEBA-bis-carbonate was prepared by fixation-activation of CO2 by the bis-glycidyl ether bisphenol A (DER 331). The obtained cyclic carbonate is used as a monomer for subsequent preparation of polyazourethane (non-isocyanate polyurethane, NIPU) by copolymerization with a diamine-azo dye. Poly(azo)urethane preparation: the polyazourethane was synthesized using a two-neck roundbottom flask reactor equipped with reflux condenser. An oil bath was used for heating. Reactions were carried out in acetonitrile as solvent. The 4-aminomethyl-3-(4′-nitrophenyl-1′-diazo)benzylamine monomer was added to DGEBA-bis-carbonate (in a 2:1 molar ratio), and the mixture was stirred for 24 h at 80 °C. The stirring was stopped, and the mixture was allowed to sit at room temperature. Then, polyazourethane was filtered off and washed with cool acetonitrile. A red solid was obtained after solvent evaporation. In addition to FTIR and Raman spectroscopic characterization of polyazourethane, the material was studied by solid-state 13C NMR. The following shifts (δ in ppm) were observed: 158.0 (NHC(O)O), 154.0 (Csp2-NO2), 152 (Csp2-O), 148.3 (Csp2-NdN), 72.2 (CH2O),

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Figure 5. UV-vis absorption spectra for mixed 50/50 PAzoU/SA LB films with 05, 11, 17, 23, 31, 36, 41, and 51 layers. The inset shows the absorbance at 400 nm vs the number of layers. 70.1 (CH-OH). Thermal analysis coupled with Fourier transform infrared spectrometry (TG/FTIR) has confirmed that the present polyazourethane is thermally stable at temperatures below 130 °C, and partial degradation of this organic material occurred at higher temperatures. The molecular structure of the PAzoU mer unit is given in Figure 1a. Langmuir Films. The Langmuir films were fabricated in a KSV trough, model 2000, and characterized by the Π-A isotherm recorded by the Wilhelmy method, where the plate was placed perpendicular to the barriers to avoid undesired displacements due to the rigidity of the monolayers.17 The polymer was diluted in different solvents (N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and chloroform (CHCl3)), and the solutions were slowly spread onto ultrapure Millipore water subphase (18.2 MΩ.cm and surface tension of (71.5 ( 0.5) mN/m at 20 °C) kept at 20 °C. After about 20 min, required for the solvent evaporation, the symmetrical compression of the monolayers started with the barrier speed at 10 mm/min. Different compression speeds (10, 50, 100, and 180 mm/min) were applied for the Langmuir film formation using the DMF solution, keeping the subphase at 20 °C. In addition, Langmuir films were formed from the DMF solution with a compression speed of 10 mm/min for different subphase temperatures (10, 20, and 30 °C). Stability tests were carried out using the polymer dissolved in DMF, compressing speed at 10 mm/min, and subphase temperature at 20 °C. Langmuir films were also formed by mixing the PAzoU and stearic acid (SA) with different mass % (PAzoU/SA: 25/75, 50/50, 75/25) in DMF (solutions of 0.2 mg/mL), compression speed at 10 mm/min, and subphase temperature at 20 °C. LB Films. The PAzoU Langmuir films (from DMF solution) were transferred onto different solid substrates at a constant surface pressure at 8 mN/m (condensed phase of the Langmuir film) and using a Z-type deposition. The dipping speed was varied from 5.0 to 20.0 mm/min, reaching a transfer ratio close to 1 during the first five layers. The dipping speed was decreased steadily with the number of layers reaching ca. 0.5 mm/min for the 15th layer. At this number of layers, the transfer ratio is around 0.2 and the film growth is lost according to UV-vis absorption data. The UV-vis spectra were recorded using a Varian spectrophotometer, model Cary 50, for 5, 10, 15, and 21 LB layers deposited onto quartz substrates. However, mixed PAzoU/SA 50/50 LB films fabricated up to 51 layers presented a linear growth. FTIR measurements were conducted in a Bruker spectrometer, model Vector 22, for both mixed 21-layer PAzoU/SA 50/50 LB film and casting films deposited onto a ZnSe substrate. The FTIR were collected with 64 scans, 4 cm-1 spectral resolution, and using a DTGS detector. The surface morphology of the LB (17) Constantino, C. J. L.; Dhanabalan, A.; Oliveira, O. N., Jr. ReV. Sci. Instrum. 1999, 70, 3674.

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Figure 6. FTIR spectra for cast films of neat SA, neat PAzoU, and mixed PAzoU/SA at different mass % and for 50/50 mixed PAzoU/SA LB film. All the films were produced using DMF solutions. Table 1. Characteristic FTIR Bands of the Neat PAzoU and Stearic Acid Cast Films PazoU (cm-1)

stearic acid (cm-1)

2960 2955 2921 2917 2853 2850 1700 1684 1602 1542 1508 1469 1307 1243 1184 1105 1032 825

films on a micrometer scale was characterized using the Leica optical microscope and CCD camera of the Renishaw in-Via spectrograph. The Raman spectra were recorded using a 50× objective lens, which collects spectra from areas of ca. 1 µm2, a 785 nm laser line, 1200 gr/mm grating, and CCD detector. Raman mappings were built, placing the LB films onto an XYZ motorized stage with a minimum step of 0.1 µm. Data acquisition and analysis were carried out using the WiRE software for windows. AFM images were collected in tapping mode using a Digital Instrument, model Nanoscope IV, with a tip of silicon nitride and spring constant at 0.12 N/m. The impedance spectroscopy (capacitance and resistance) was carried out using a Solartron analyzer, model 1260A, from 1 Hz to 1 MHz and immersing the sensing units into distilled and ultrapure water kept at different temperatures. The sensing units are Au interdigitated electrodes covered by the five-layer LB films (neat PAzoU, neat SA, and mixed PAzoU/SA with 75/25, 50/50, and 25/75 mass %). The Au interdigitated electrodes contain 50 pair of digits with 10 µm width, 0.5 mm length, and 100 nm thickness each, which are 10 µm spaced from each other. PCA analysis was carried out by a routine developed using the C-Builder software.

assignments CH3 stretching CH3 stretching CH stretching (ring) CH2 stretching antisymmetric CH ring stretching symmetric CH2 stretching symmetric carbonyl stretching carbonyl stretching ring stretch + NH bending OCO stretching antisymmetric ring stretching OCO stretching symmetric CH2 bending + CH-OH bending NH bending + (CdO)-O stretching CH ring bending in plane CH wagging O-CH2 stretching aromatic-NO2 scissors

Results and Discussion Langmuir Films. The fabrication of high quality LB films of controlled thickness and architecture at the molecular level depends on the properties of the Langmuir film formed on the water subphase. Correspondingly, a sequence of experiments was carried out to characterize the Langmuir film through Π-A isotherms. Figure 1b presents the Π-A isotherms recorded for the PAzoU Langmuir films using different solvents: DMF, CHCl3, and THF. It can be observed that the mean molecular area per mer unit decreases following the increase in the solvent polarity. This could be explained considering that more polar solvents may drag the PAzoU molecules to the water subphase. The mean molecular area is obtained by extrapolating to Π ) 0 the isotherm portion that corresponds to the condensed phase of the film (Aext in Figure 1b). However, stability tests revealed that the PAzoU Langmuir films are very stable on the water subphase as shown in the inset in Figure 1b. The stability test consists basically in compressing the Langmuir film until a chosen

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Figure 7. (a) 2D optical images obtained using an objective of 50× for mixed 24-layer PAzoU/SA LB films (25, 50, and 75%). The insets present the 3D images. (b) Raman mappings built using spectra collected point-by-point along an area of 40 µm × 40 µm with step of 2 µm. One of the spectra is shown at the top.

surface pressure within the condensed phase (8.0 mN/m in this case) and recording the displacement of the barriers to keep the surface pressure constant during a certain period of time. Higher

stability is attributed to monolayers presenting a lower decrease in area for a certain period of time. A decrease of the area lower than 6% for a period of 1.5 h was observed, which is more than

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Figure 8. AFM images for mixed PAzoU/SA 24-layer LB films (25, 50, and 75%) in 2D (bottom, phase image) and 3D (top, topographic image). (b) AFM image for mixed PAzoU/SA 50/50 24-layer LB film and its profile along an edge made at the center of the film.

enough time to record a Π-A isotherm in this case. Therefore, the smaller Aext value found for DMF, followed by THF and CHCl3, might be related to different conformations that the PAzoU polymer chains reach when dissolved by those solvents and spread onto the water subphase. In the case of PAzoU in DMF, its smaller Aext suggests that the chains might be more packed when dissolved in DMF. Besides, the higher collapsing pressure presented by the PAzoU in DMF supports the idea that the molecules can reach higher packing for this conformation. The collapsing pressure is defined where the rate ∂Π/∂Α|T decreases (see Figure 1b), indicating a possible superposition of the molecules to form multiple layers onto the water subphase (on top or underneath the Langmuir monolayers). This is observed for other materials with molecular structure more complex than

the fatty acids,18,19 which are standard molecules used as reference in the Langmuir technique. The absence of hysteresis when the Langmuir film is submitted to a cycle of compression/expansion is more evidence of PAzoU stability onto the water subphase. Figure 2a shows a Π-A isotherm recorded until the Langmuir film reaches the collapse and a Π-A isotherm recorded for two cycles of compression/expansion for the PAzoU in DMF. The limit of pressure was fixed at 8.0 mN/m during the compression/expansion cycle to avoid the collapse of the film. Usually, if either the molecules interact during the (18) Dhanabalan, A.; Balogh, D. T.; Mendonc¸ a, C. R.; Riul, A., Jr.; Constantino, C. J. L.; Giacometti, J. A.; Zilio, S. C.; Oliveira, O. N., Jr. Langmuir 1998, 14, 3614. (19) Barros, A. M.; Dhanabalan, A.; Constantino, C. J. L.; Balogh, D. T.; Neto, C. P.; Oliveira, O. N., Jr. Thin Solid Film 1999, 354, 215.

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Figure 9. (a) Capacitance vs frequency data for each sensing unit immersed in distilled water at 20 °C. (b) Capacitance values at 10 kHz derived from the capacitance vs frequency data obtained at 10, 15, 20, 25, and 30 °C. (c) Capacitance values at 1 kHz derived from the capacitance vs frequency data obtained at 5, 10, 15, 20, 25, 30, 40, and 50 °C using ultrapure water (18.2 MΩ‚cm).

compression that forms aggregates (domains) or are dragged into the water subphase, a hysteresis is observed during the expansion.20 In addition, an unstable Langmuir film produces a shift of the Π-A isotherms to smaller areas for consecutive cycles of compression/expansion since molecules are being either more packed or lost to the subphase, which is not the case here when two consecutive cycles were recorded. The high stability of the PAzoU Langmuir films on the water subphase is supported by the reproducibility of the Π-A isotherms when the same film is compressed at different barrier speeds. Figure 2b shows the Π-A isotherms for the PAzoU in DMF being compressed consecutively at 10, 50, 100, and 180 mm/min. In the case of unstable film, a shift of the Π-A isotherms to smaller areas would be observed since the molecules are either constantly rearranging themselves on the water subphase or being dragged to the water subphase under these conditions.21 The UV-vis absorption spectra for PAzoU solutions in CHCl3, THF, and DMF can be seen in Figure 2c. The corresponding UV-vis absorption spectra for PAzoU cast films produced using CHCl3, THF, and DMF are given in Figure 2d where the same maximum absorption is seen for all cast films. The differences observed in the UV-vis absorption spectra (wavelength of the absorption band maxima) for the PAzoU solutions suggest that the solvent plays an important role in the stabilization of a preferred conformation of the PAzoU molecules in solution. However, the UV-vis absorption spectra of cast (Figure 2d) and LB films (Figure 4) on a solid substrate are similar. Therefore, the differences found in the Π-A isotherms (Figure 1b) must be related to conformations of the polymer chains in the spreading solution. (20) Oliveira, O. N., Jr.; Constantino, C. J. L.; Balogh, D. T.; Curvelo, A. A. S. Cellul. Chem. Technol. 1994, 28, 541. (21) Constantino, C.J.L.; Dhanabalan, A.; Curvelo, A.A.S.; Oliveira, O. N., Jr. Thin Solid Films 1998, 327, 47.

Despite the high stability of the PAzoU Langmuir films and the well defined Π-A isotherms, the transfer to solid substrates is restricted to ca. 15 layers. Therefore, to fabricate thicker films it was necessary to use mixed Langmuir films where the investigated material is mixed with a fatty acid.18,21 The Π-A isotherms recorded for mixed Langmuir PAzoU/stearic acid (PAzoU/SA) films in different mass % (75/25, 50/50, and 25/75) are shown in Figure 3. The displacement of the Π-A isotherms toward a larger area is due to the higher % of PAzoU in the mixed Langmuir films since the mean molecular area is calculated based on the number of SA molecules spread onto the water subphase. The presence of two slopes in the Π-A isotherms suggests that the PAzoU and SA molecules are forming separated domains. The first slope (larger areas) refers to the condensed phase of the PAzoU molecules while the second one (smaller areas) refers to the condensed phase of the SA molecules. The inset in Figure 3 shows the absorbance at 400 nm for the 12-layer mixed LB films revealing that the mass % of PAzoU in relation to SA (25, 50, and 75%) used to prepare the solutions is kept in the LB film composition, i.e., the % in the mixed Langmuir corresponds to the % found in the LB films. Langmuir-Blodgett (LB) Films. The growth of PAzoU LB films was monitored using the UV-vis absorption spectroscopy. The UV-vis absorption spectra recorded for different numbers of PAzoU layers deposited onto quartz substrate are presented in Figure 4. The inset shows the absorbance at 400 nm vs the number of deposited layers (5, 10, 15, and 21 LB layers), which is assigned to π-π* electronic transition in the azo groups.22 It can be seen that the amount of material transferred from the water surface to the substrate grows with the number of deposited layers; however, it tends to a plateau above 22 layers. (22) Lambert, J.B.; Shurvell, H.F.; Lightner, D.A.; Cooks, R.G. Organic Structural Spectroscopy; Prentice Hall: Toronto, 1998.

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Figure 10. PCA for all sensing units using a capacitance collected at 10 kHz when immersed in distilled water and the temperature program as follows: 20 f 10 f 20 f 15 f 20 f 25 f 20 f 30 f 20 °C.

Multilayer LB films with up to 51 layers were obtained for mixed PAzoU/SA 50/50 LB films as determined by UV-vis absorption spectra recorded and shown in Figure 5. The inset shows a linear increasing of the absorbance at 400 nm vs the number of LB layers. Notably, a value close to one was observed for the transfer ratio during deposition and the cumulative transfer curve, indicative that not only similar amount of material is transferred per deposited layer but also that the material is transferred homogeneously onto the substrate. The cumulative transfer is given by the ratio between the area scanned by the barriers to keep the surface pressure constant during the LB deposition and the covered area of the substrate. This value is shown online at the computer screen during the deposition, which allows one monitoring the film transfer ratio by controlling the up and down speed of the substrate. Similar LB depositions were found for 75/25 and 25/75 mixed PAzoU/SA LB films (figures not shown). The interaction between PAzoU and SA was studied using vibrational FTIR and Raman scattering spectroscopy. The FTIR spectra recorded for cast films of neat SA, neat PAzoU, and mixed PAzoU/SA at different proportions, as well as the FTIR spectra recorded for a 50/50 mixed PAzoU/SA LB film, are given in Figure 6. The center of characteristic vibrational bands in wavenumber (cm-1) observed in the FTIR spectrum of the neat PAzoU11,23 and SA24 cast films is shown in Figure 6, and their assignments are listed in Table 1. The assignments of the observed PAzoU FTIR bands are supported by theoretical calculations using Gaussian 98 (density functional theory, B3LYP 6-31G) for the monomer. The cast films were formed using DMF as a solvent since it was also used to produce the Langmuir and LB films. It is observed that the spectra of the mixed films produced either by casting or LB are composed of a simple superposition of the spectra from the neat materials (PAzoU and SA), which reveals the absence of strong interactions between these materials, in agreement with what was observed for the mixed Langmuir films (Figure 3). Intermolecular interactions could affect the relative intensity or frequency of vibrational bands related to the chemical groups involved in the interaction. In the case of strong chemical interactions, new vibrational band(23) Lin-Vien, D.; Colthup, N.B.; Fateley, W.G.; Grasselli, J.G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: London, 1991. (24) Teixeira, A. C. T.; Fernandes, A. C.; Garcia, A. R.; Ilharco, L. M.; Brogueira, P.; Gonc¸ alves, da Silva, A. M. P. S. Chem. Phys. Lipids 2007, 149, 1.

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(s) could be seen directly related to new chemical bond(s). The morphology on a micrometer scale of the mixed PAzoU/SA LB films was examined under an optical microscope. Figure 7a shows the two-dimensional images obtained using an objective of 50× for the samples 25, 50, and 75% of PAzoU while the insets present the three-dimensional images. It can be seen that the homogeneity of the surface decreases as the % of PAzoU increases. This investigation was complemented using the microRaman mapping technique, which allows collection of spectra from areas of ca. 1 µm2 by coupling the Raman spectrograph and optical microscopy, combining both chemical and morphological information on a micrometer scale. Figure 7b shows the Raman mappings which were built using spectra collected point-bypoint along an area of 40 µm × 40 µm with a step of 2 µm. One of the spectra is shown at the top in Figure 7b. These Raman mappings show the distribution of the band at 1319 cm-1 assigned to the stretching of the NO2 moiety,11 where brighter spots refer to higher signal intensity. Two mappings were built considering the band with the center at 1319 cm-1. One takes into account the intensity at 1319 cm-1 (named “intensity at point”) and the other the area below the band at 1319 cm-1 with baseline correction (named “signal to baseline”) to check the background scattering interference. The Raman mappings are shown in two and in three dimensions. It can be observed that in certain spots the intensity of the band is very high, revealing the presence of PAzoU domains, while in other spots the intensity is practically absent, revealing the presence of SA domains. It is important to notice that the signal intensity is not affected by the background since the distribution of the bright and dark spots in the “signal to baseline” mapping follows fairly well the distribution in the “intensity at a point” mapping. Figure 8a presents three-dimensional topographic AFM images at the top and two-dimensional phase AFM images at the bottom showing that the PazoU and SA present a phase separation at nanometer scale supporting the absence of chemical interaction between both materials as suggested by FTIR spectra and the phase separation observed by optical microscopy and microRaman results on a micrometer scale. The dark regions might be related to the polymer since they increase as the % of PazoU increases. The average roughness found for the three mixed LB films was ca. 9 nm, independent of the % of PazoU in the LB film. It was of interest to determine the average thickness of the mixed LB film as shown in Figure 8b for the PAzoU/SA 50/50. The three LB films shown in Figure 8a have 24 layers, which would correspond to a thickness of 60 nm if only the SA were deposited, since the SA is 2.5 nm high.21 It was determined that the average thickness was around 60 nm, which is consistent with phase separation. The term “average” thickness has been used here considering that average roughness found is about 15% of the average thickness. The Π-A PAzoU Langmuir isotherms revealed a high sensitivity of the polymer to the subphase temperature (results not shown). To test the potential variations of LB films with temperature (for potential sensor applications), five-layer LB films were deposited onto Au interdigitated electrodes. Previous work has shown the high sensitivity that can be achieved by the combination of impedance spectroscopy and ultrathin films deposited onto Au interdigitated electrodes in liquid analysis.25 A total of six sensing units were fabricated: bare electrodes, neat SA and PAzoU LB films, and three mixed PAzoU/SA LB films (25/75; 50/50; 75/25). The sensing units were dipped into distilled water at different temperatures and distinguished through (25) Ferreira, M.; Riul, A., Jr.; Wohnrath, K.; Fonseca, F. J.; Oliveira, O. N., Jr.; Mattoso, L. H. C. Anal. Chem. 2003, 75, 953.

Langmuir-Blodgett Films of a Poly(azo)urethane

impedance spectroscopy measurements taken in the frequency range 1 Hz and 1 MHz. Figure 9a shows capacitances recorded for all sensing units at 20 °C. The same trend was observed for other temperatures (10, 15, 25, and 30 °C, results not shown). Subtle changes in the liquid system are strongly captured in the electric response of the LB films, in close agreement with the model proposed by Taylor and MacDonald.26 Briefly, at low frequencies the electrical response is dominated by double-layer effects, in the 102 to 104 Hz region the effect of an ultrathin film covering a metal electrode is dominant, and at higher frequencies the measured signal is ruled by the geometric capacitance. Figure 9b presents a linear dependence of the measured capacitance on temperature variation, recorded at 10 kHz. A similar trend was observed for capacitance values recorded at 5, 10, 15, 20, 25, 30, 40, and 50 °C using ultrapure Millipore water (18.2 MΩ‚cm) as shown in Figure 9c. This is an important result since reproducibility in ultrapure water is a more difficult task compared to distilled water. The frequency is selected considering values where the capacitance variation is maximized as the temperature of water increases. Principal component analysis (PCA) was used to statistically correlate our samples. It is a mathematical method ordinarily employed to find patterns in data, highlighting their similarities and differences. It accounts for the variability of the data, trying to identify new meaningful variables and reducing the dimensionality of the data set, with minimum loss of information. In this work the PCA was used to correlate temperature step variations that followed the sequence 20 °C f 15 °C f 20 °C f 10 °C f 20 °C f 25 °C f 20 °C f 30 °C f 20 °C, as shown in Figure 10. The right-hand shift of the data with increasing temperature variations correlates PC1 with temperature. The correlation can be observed projecting the data onto the PC1 axis. There is a rightward trend on that (PC1) as the temperature is increased. Despite minor, uncontrolled experimental parameters such as gas diffusion from the environment, it is also worth to note in PC2 a temporal displacement of the data at 20 °C. (26) Taylor, D. M.; MacDonald, A. G. J. Phys. D: Appl. Phys. 1987, 20, 1277.

Langmuir, Vol. 24, No. 9, 2008 4737

Conclusions Langmuir and Langmuir-Blodgett (LB) films of a poly(azo)urethane have been obtained and characterized. Langmuir films were formed on the water subphase and characterized by Π-A isotherms, stability tests, cycles of compression/expansion, varying compression speeds (10, 50, 100, and 180 mm/min), subphase temperature (10, 20, and 30 °C), and solvents used to prepare the polymer solutions (DMF, THF, and CHCl3). The best results in terms of stability and reproducibility were achieved using DMF as a solvent and a subphase at 20 °C. Notably, the Π-A isotherms appear to be independent of the compression speed, within the range investigated, and hysteresis was not observed during the compression/expansion cycles. The transfer of the Langmuir monolayers from water subphase onto solid substrates to form LB films was monitored by UV-vis absorption spectroscopy following the maximum of the π-π* absorption band at 400 nm. The neat LB films grow (absorbance against deposited layers) up to 15 layers. Thicker, homogeneous films were only obtained by mixing the PAzoU with stearic acid. The information on the intermolecular interaction between the materials was extracted from the Π-A isotherms of Langmuir films and from vibrational spectroscopy data (FTIR absorption and Raman scattering) of LB films. The surface morphology of the mixed LB films was investigated on a micrometer scale combining both micro-Raman and optical microscopy and on a nanometer scale using AFM. Both microscopy and vibrational spectroscopy revealed that the mixed LB films present a phase separation between PAzoU and stearic acid. Finally, mixed and neat LB films deposited onto Au interdigitated electrodes showed good correlation with step variations at different water temperatures (5, 10, 15, 20, 25, 30, 40, and 50 °C). Acknowledgment. The authors are grateful to FAPESP, CNPq (IMMP and CIAM), and CAPES from Brazil and NSERC from Canada for financial support and LNLS (Brazil) for the Au interdigitated electrodes used. The authors also thank Professor Eduardo R. de Azevedo from IFSC/USP for the NMR study of the polyurethane. LA703328Z