Fabrication, structural characterization, and applications of langmuir and langmuir-blodgett films of a poly(azo)urethane.
ABSTRACT The synthesis of a poly(azo)urethane by fixing CO(2) 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 (Pi-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.
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ABSTRACT: This minireview describes the main developments of electronic tongues (e-tongues) and taste sensors in recent years, with a summary of the principles of detection and materials used in the sensing units. E-tongues are sensor arrays capable of distinguishing very similar liquids employing the concept of global selectivity, where the difference in the electrical response of different materials serves as a fingerprint for the analysed sample. They have been widely used for the analysis of wines, fruit juices, coffee, milk and beverages, in addition to the detection of trace amounts of impurities or pollutants in waters. Among the various principles of detection, electrochemical measurements and impedance spectroscopy are the most prominent. With regard to the materials for the sensing units, in most cases use is made of ultrathin films produced in a layer-by-layer fashion to yield higher sensitivity with the advantage of control of the film molecular architecture. The concept of e-tongues has been extended to biosensing by using sensing units capable of molecular recognition, as in films with immobilized antigens or enzymes with specific recognition for clinical diagnosis. Because the identification of samples is basically a classification task, there has been a trend to use artificial intelligence and information visualization methods to enhance the performance of e-tongues.The Analyst 10/2010; 135(10):2481-95. · 4.23 Impact Factor
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 Cie ˆncias 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 CO2in 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
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.
The interest in polymeric materials for potential applications
special interest since 1937, when they were discovered by Otto
among a long list of macrodiols and diisocyanates. In addition,
The synthesis of PU usually uses diisocyanates as monomers.
However, this class of polymers can be prepared by using CO2-
containing monomers (e.g., dicarbonates), avoiding the use of
the very toxic isocyanates. This clean method yields ‘non-
isocyanate polyurethanes’ (NIPUs) using organic carbonates as
intermediates.3,4Another encouraging point for using these
that can be obtained through conjugate polymers as specific
chromophores as in the case of azo-benzenic compounds.5-7
stable thermal and mechanical properties than their respective
monomers, and they have been used to produce optical sensor,8
artificial taste sensor,9and pH sensors.10However, their main
devices,12surface relief gratings,13holograms, and induction of
An important tool, which could be essential to the study of
it has been used for film fabrication in nanostructured devices
An advantage of this technique is the ability to tune film
characteristics, which can be achieved by changing some LB
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
* Corresponding author. E-mail: email@example.com.
‡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,
(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.
(8) Grafe, A.; Haupt, K.; Mohr, G. J. Anal. Chim. Acta 2006, 565, 42.
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,
(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.
Langmuir 2008, 24, 4729-4737
10.1021/la703328z CCC: $40.75© 2008 American Chemical Society
Published on Web 04/10/2008
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
in DMF. (c) UV-vis absorption spectra for CHCl3, THF, and DMF PAzoU solutions. (d) UV-vis absorption spectra for PAzoU cast films
4730 Langmuir, Vol. 24, No. 9, 2008Alessio et al.
component analysis (PCA).
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 co-
polymerization with a diamine-azo dye. Poly(azo)urethane prepara-
bottom flask reactor equipped with reflux condenser. An oil bath
was used for heating. Reactions were carried out in acetonitrile as
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-state13C 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),
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
by the Wilhelmy method, where the plate was placed perpendicular
to the barriers to avoid undesired displacements due to the rigidity
of the monolayers.17The polymer was diluted in different solvents
(N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and chlo-
( 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
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
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,
a linear growth. FTIR measurements were conducted in a Bruker
50/50 LB film and casting films deposited onto a ZnSe substrate.
The FTIR were collected with 64 scans, 4 cm-1spectral resolution,
and using a DTGS detector. The surface morphology of the LB
1999, 70, 3674.
Figure 3. Π-A isotherms recorded at 10 mm/min and 20 °C for
mixed PAzoU/SA dissolved in DMF using different mass %: 25,
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.
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.
Langmuir-Blodgett Films of a Poly(azo)urethaneLangmuir, Vol. 24, No. 9, 2008 4731
microscope and CCD camera of the Renishaw in-Via spectrograph.
collects spectra from areas of ca. 1 µm2, a 785 nm laser line, 1200
gr/mm grating, and CCD detector. Raman mappings were built,
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
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.
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 (Aextin 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
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
stearic acid (cm-1)
ring stretch + NH bending
OCO stretching antisymmetric
OCO stretching symmetric
CH2bending + CH-OH bending
NH bending + (CdO)-O stretching
CH ring bending in plane
4732 Langmuir, Vol. 24, No. 9, 2008Alessio et al.
surface pressure within the condensed phase (8.0 mN/m in this
case) and recording the displacement of the barriers to keep the
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
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.
Langmuir-Blodgett Films of a Poly(azo)urethaneLangmuir, Vol. 24, No. 9, 2008 4733
enough time to record a Π-A isotherm in this case. Therefore,
the smaller Aextvalue found for DMF, followed by THF and
onto the water subphase. In the case of PAzoU in DMF, its
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|Tdecreases
(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
in the Langmuir technique.
isotherm recorded for two cycles of compression/expansion for
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,
C. P.; Oliveira, O. N., Jr. Thin Solid Film 1999, 354, 215.
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.
4734 Langmuir, Vol. 24, No. 9, 2008 Alessio et al.
compression that forms aggregates (domains) or are dragged
into the water subphase, a hysteresis is observed during the
expansion.20In addition, an unstable Langmuir film produces a
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
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
on the water subphase or being dragged to the water subphase
under these conditions.21The UV-vis absorption spectra for
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
in the stabilization of a preferred conformation of the PAzoU
of cast (Figure 2d) and LB films (Figure 4) on a solid substrate
(Figure 1b) must be related to conformations of the polymer
chains in the spreading solution.
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.18,21The Π-A
isotherms recorded for mixed Langmuir PAzoU/stearic acid
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
based on the number of SA molecules spread onto the water
subphase. The presence of two slopes in the Π-A isotherms
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
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
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.22It
can be seen that the amount of material transferred from the
layers; however, it tends to a plateau above 22 layers.
(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.
(22) Lambert, J.B.; Shurvell, H.F.; Lightner, D.A.; Cooks, R.G. Organic
Structural Spectroscopy; Prentice Hall: Toronto, 1998.
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).
Langmuir-Blodgett Films of a Poly(azo)urethaneLangmuir, Vol. 24, No. 9, 2008 4735
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
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
allows one monitoring the film transfer ratio by controlling the
found for 75/25 and 25/75 mixed PAzoU/SA LB films (figures
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 PAzoU11,23and SA24cast 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
6-31G) for the monomer. The cast films were formed using
and LB films. It is observed that the spectra of the mixed films
produced either by casting or LB are composed of a simple
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.
(s) could be seen directly related to new chemical bond(s). The
morphology on a micrometer scale of the mixed PAzoU/SA LB
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
Raman mapping technique, which allows collection of spectra
information on a micrometer scale. Figure 7b shows the Raman
mappings which were built using spectra collected point-by-
point 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
to the stretching of the NO2moiety,11where 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-1with baseline
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.
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 micro-
Raman 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.21It 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
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
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,
(25) Ferreira, M.; Riul, A., Jr.; Wohnrath, K.; Fonseca, F. J.; Oliveira, O. N.,
Jr.; Mattoso, L. H. C. Anal. Chem. 2003, 75, 953.
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.
4736 Langmuir, Vol. 24, No. 9, 2008Alessio et al.
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
response is dominated by double-layer effects, in the 102to 104
is dominant, and at higher frequencies the measured signal is
ruled by the geometric capacitance. Figure 9b presents a linear
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.
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 dimen-
sionality 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
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
such as gas diffusion from the environment, it is also worth to
note in PC2 a temporal displacement of the data at 20 °C.
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
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
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
of the Langmuir monolayers from water subphase onto solid
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
good correlation with step variations at different water temper-
atures (5, 10, 15, 20, 25, 30, 40, and 50 °C).
(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
Langmuir-Blodgett Films of a Poly(azo)urethaneLangmuir, Vol. 24, No. 9, 2008 4737