Controlling the luminescence properties of poly(p-phenylene vinylene) entrapped in Langmuir and Langmuir–Blodgett films of stearic acid

Synthetic Metals 161 (2011) 1753–1759

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Controlling the luminescence properties of poly(p-phenylene vinylene) entrapped in Langmuir and Langmuir–Blodgett films of stearic acid Andrei Sakai a , Shu H. Wang b , Laura O. Péres a,∗ , Luciano Caseli a a b

Laboratório de Materiais Híbridos, Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, Diadema, SP, Brazil Laboratório de Macromoléculas, Departamento de Engenharia Metalúrgica e de Materiais, Escola Politécnica, Universidade de São Paulo, São Paulo, SP, Brazil

a r t i c l e

i n f o

Article history: Received 1 March 2011 Received in revised form 12 April 2011 Accepted 10 June 2011 Available online 5 July 2011 Keywords: Langmuir–Blodgett films PPV Conjugated polymer Stearic acid

a b s t r a c t Properties of hybrid films can be enhanced if their molecular architecture is controlled. In this paper, poly (p-phenylene vinylene) was mixed with stearic acid in order to form stable hybrid Langmuir monolayers. Surface properties of these films were investigated with measurements of surface pressure, and also with polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS). The films were transferred from the air–water interface to solid supports through the Langmuir–Blodgett technique, and the viability of the film as optical device was investigated with fluorescence spectroscopy. Comparing the fluorescent spectra for the polymer in solution, as a casting film, and as an LB film, the emission bands for LB films were narrower and appeared at lower wavelengths. The interactions between the film components and the design for the LB film may take advantage of the method to immobilize luminescent polymers in mixed ultrathin films adsorbed in solid matrices. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of the conductivity in polymers in 1970s [1], conducting polymers have been employed in several technological applications, such as large area panel displays [2–4], sensors [5], field-effect transistors (FETs) [6], and light-emitting diodes (LEDs) [7,8]. Particularly, polymeric light-emitting materials (PLEDs) are used to produce large, flexible and multicolor displays, as well as other devices with high quantum efficiency [9,10]. One of the most studied conducting polymer is poly(pphenylene vinylene), PPV, especially because it can be used as active layer in electroluminescent devices with green emission [3]. Particularly, PPV-based copolymer containing aliphatic spacers is an efficient blue-emitting electroluminescent material, whose emission is highly influenced by the interactions among polymer chains and consequently by the thickness of the material [11–14]. For this reason, it is of interest to control the thickness and organization of the material. A way to produce thin films with thickness controlled at the molecular scale is by using the Langmuir–Blodgett (LB) technique [15]. Moreover, since the current density depends on the thickness of the emissive layer, the use of LB films may improve the device properties. The construction of a thin solid film of a luminescent polymer has already been reported as an efficient strategy to enhance its

∗ Corresponding author. Tel.: +55 11 4043 6428; fax: +55 11 40436428. E-mail address: [email protected] (L.O. Péres). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.019

optical properties [15–19]. Particularly, polymers adsorbed on solid matrices have been reported by using the Langmuir–Blodgett (LB) [18–20] and by layer-by-layer (LbL) [21] deposition techniques, which allow precise control of thickness and film architectures [15,22,23]. It is defended that Langmuir–Blodgett (LB) film deposition technique is one of the best among few methods used to manipulate materials at the molecular level [24]. For instance, alkylated polythiophenes at the air–water interface are described to self-assemble into ␲-stacked conjugated chains, forming a very stable monomolecular layer, which presented optimized electrical conductivity [25]. Also, Park et al. [26] showed that polyfluoreneb-polythiophene diblock copolymer adsorbed on solid supports as Langmuir–Blodgett films exhibits spectral shifts owing to changes in the morphology of the material. Obtaining aromatic polymer LB films is usually difficult because some of them are not classical amphiphiles and they do not spread on the air–water interface in order to form Langmuir monolayers [27]. For this reason, some strategies have been developed to overcome this problem, such as alkylation of the polymers [28]. However, the polymer alkylation may modify the original optical and electronic properties. An alternative approach employed since 1990s [29–31] consists in mixing the polymer with an amphiphile able to spread on the air–water interface, as for example, phospholipids and fatty acids. For instance, it has been recently reported that a polymer containing fluorenyl groups spreads well on the air–water interface when mixed with a negatively charged phospholipid [27], and it can be transferred to solid supports by using the LB technique. In this case, the luminescence of the polymer

1754

A. Sakai et al. / Synthetic Metals 161 (2011) 1753–1759

increased due to the molecular-level interactions among the components of the mixed film, which were confined in a close-packed arrangement. In this present study, poly(p-phenylene vinylene) was cospread with stearic acid on the air–water interface, and its surface properties were investigated in detail by using tensiometry, surface potential, and polarization-modulation infrared reflection–absorption spectroscopy (PM-IRRAS). The films were transferred to solid supports by the LB technique, obtaining hybrid ultrathin films, whose optical properties were evaluated. We may envisage future applications of these films in optical devices such as active layer of efficient organic light-emitting diodes (OLEDs). 2. Experimental 2.1. Synthesis The synthesis of poly[1,8-octanedioxy-2,6-dimethoxy-1,4phenylene-1,2-ethenylene-1,4-phenylene-1,2-ethenylene-3,5dimethoxy-1,4-phenylene], named here, for simplification, PPV, was adapted from the literature [32,33]. The molar mass and the polydispersity were determined by gel permeation chromatography (GPC) using a Waters HPLC system, comprising two columns in series, PLgel mixed-B and PLgel mixed-C, at 35 ◦ C and with 1 mL/min of tetrahydrofuran (THF) as eluent. GPC measurements were carried out using monodisperse polystyrene samples as standard. The results showed molar mass (Mw) of 5240 Da and 1.43 of polydispersity. In a previous work [32] all characterization of this sample was presented, and the results have shown coherency of the synthesized oligomer with the properties of the polymer reported in Ref. [32]. 2.2. Spreading at the air–water interface Ultrapure water (resistivity ≈ 18.2 M cm, pH ≈ 5.5–6.0), supplied by a Millipore® system, was used as subphase. PPV mentioned above as well as stearic acid (Sigma–Aldrich) were dissolved in chloroform to yield a 0.5 mg mL−1 solution, which was considered as a good solvent for PPV. Aliquots of these solutions with different proportions of stearic acid:PPV were co-spread carefully drop by drop on the surface of the aqueous subphase (see simplified scheme in Fig. 1). After spreading, an interval delay of 10–15 min for solvent evaporation was predetermined, just before starting the film compression with two movable barriers at a rate

of 3.0 A˚ 2 /(molecule min). Longer interval delays before compression did not affected the surface pressure–area isotherms. The surface pressure was measured during compression using a Wilhelmy plate with a KSV mini-trough (System 2, total volume of 220 mL). All monolayers were produced at a constant temperature of 23 ± 1 ◦ C. Polarization-modulation infrared reflection absorption spectroscopy measurements were taken with a KSV PMI 550 instrument (KSV instrument Ltd., Helsinki, Finland). The Langmuir trough is set up so that the light beam reaches the monolayer at a fixed incidence angle of 80◦ . The incoming light is continuously modulated between s- and p-polarization at a high frequency, which allows the simultaneous measurement of the spectra for the two polarizations. The difference between the spectra provides surfacespecific information, and the sum provides the reference spectrum. With the simultaneous measurements, the effect of the water vapor is largely reduced. To visualize the monolayer experiments, it is presented in Fig. 1 a scheme with the structures of the materials employed in this work. The motivation for producing such films consists in the viability to use amphiphiles with known spreading properties in order to facilitate the dispersion of the polymer or oligomer on the surface of the water [29–31]. In this previous paper, it was employed the phospholipid dimyristoyl phosphaticid acid (DMPA), with is amphiphilic, water insoluble [27], and it is easy to transfer to solid supports as LB films. In this present work, stearic acid was employed because it presents similar properties in terms of lipid matrix. 2.3. Deposition onto solid supports Quartz slides were used as substrates for the transfer of the LB films. The quartz substrates were cleaned by treating with a 5% KOH ethanol solution in an ultrasonic bath for 5 min. The LB film transfer was carried out with a dipping rate of 1 mm min−1 and a constant surface pressure of 30.0 ± 0.2 mN/m, with the first layer obtained by raising the substrate from the aqueous subphase. For multilayer Y-type LB films, an interval of 10 min elapsed before the subsequent dipping with the plate at the most upward position for drying. Firstly, the quality of the LB deposition was evaluated by obtaining the transfer ratio in each deposition, and then the LB films were characterized by fluorescence (Varian Eclipse fluorescence spectrophotometer), with excitation at the wavelength of the maximum absorption according to the UV–Vis spectra, and by infrared spectroscopy (PM-IRRAS, KSV instrument Ltd., Helsinki, Finland). To evaluate the morphology of the films, atomic force microscopy (AFM) images were obtained in the tapping mode, employing a resonance frequency of approximately 300 kHz, scan rate of 1.0 Hz, and scanned areas of 2.5 ␮m × 2.5 ␮m. For these measurements, it was employed a Digital AFM-Nanoscope IIIA instrument (hold in “Laboratório de Filmes Finos from IFUSP”). The tip was made from silicon. For comparison purposes, cast films of PPV were fabricated by dropping a 0.5 mg mL−1 of PPV mixed with stearic acid solution on the solid support, and allowed to dry at room atmosphere before analysis. All films were produced at a constant temperature of 23 ± 1 ◦ C. 3. Results and discussion 3.1. Langmuir monolayers

Fig. 1. Simplified scheme for mixed PPV–stearic acid monolayer produced in this present work. The size and orientation of molecules are only illustrative and out-ofscale.

Fig. 2 shows the surface pressure–area isotherms for stearic acid in several proportions of PPV incorporated. Isotherms for pure stearic acid shows a minimum area of ca. 20 A˚ 2 /molecule (extrapolation from the more condensed phase to zero surface pressure), offset of surface pressure at ca. 33 A˚ 2 /molecule, and transition from

A. Sakai et al. / Synthetic Metals 161 (2011) 1753–1759

A

60

surface pressure (mN/m)

1755

2914

Pure Stearic Acid Stearic acid + PPV (12.5%) Stearic acid + PPV (25.0 %) Stearic acid + PPV (50.0 %)

50

PM-IRRAS signal (a.u.)

40

2848

30 20 10 0 0

10

20

30

40

50

2887

60

Average Molecular Area (Å2) Fig. 2. Surface pressure–area isotherms for PPV–stearic acid mixed monolayers in several PPV aliquots. It was considered the average molecular area, taking into account the repeating unity of PPV as a “molecule”. The percentages of PPV shown calculated consider mol of stearic acid per mol of repeating unit of PPV.

3000

2950

2900

2850

2800

wavenumber (cm-1)

B 35

12

Surface Pressure (mN/m)

6

25 0

20 30

15

First Cycle Second Cycle

10

40

compression

1120

PM-IRRAS signal (a.u.)

compression

30

961 1282

5 0 10

20

30

40

50

Mean Molecular Area (Å2)

1400

1200

1000

800

wavenumber (cm-1)

Fig. 3. Compression–decompression curves for mixed monolayers of PPV (12.5%)–stearic acid.

Fig. 4. PM-IRRAS of PPV–stearic acid mixed monolayers at 30 mN/m.

the liquid-condensed to solid phase at a surface pressure of ca. 27 mN/m, which is in agreement with the literature [33]. With introduction of PPV, taking into consideration only molecular area of stearic acid, the curves are shifted to higher molecular areas because of the surface occupancy of PPV. Considering each repeating unity of the polymer as a “molecule”, and therefore obtaining an average molecular area between stearic acid and the repeating unites, the isotherms, as shown in Fig. 2, are shifted to lower molecular areas for the two lowest percentage of PPV employed. This can be attributed to the possible conformation of PPV folding at the air–water interface and allowing for a higher amount of repeating unites per surface area. With the highest proportion of PPV employed (50%), the isotherm shifts to higher average molecular areas, which can be an indication of polymer unfolding. With higher quantities of PPV employed (75%), or even with pure PPV, the interface visually presented segregated phases and the surface pressure–area isotherms recorded were irreproducible, which is strong evidence that pure PPV solution spread on the air–water interface (or spread in high proportions of PPV:stearic acid) is not able to form a stable Langmuir monolayers. Although PPV presents polar groups in its molecular structures, such as methoxyl and 1,8dioxyl-octane moieties, these groups are not hydrophilic enough to stabilize the liquid film. As it seems more interesting to fabricate mixed films with lower percentage of PPV in relation of stearic

acid to avoid phase segregation, from this point of the manuscript, we are going to focus on the results for the percentage of 12.5% of PPV. Further analysis of the surface pressure–area curves also reveals that the inclination of the curves changed, mainly at low surface pressures. For instance, at 10 mN/m, the values of in-plane elasticity (defined as – A (d/dA) [34]; where A is the molecular area, and  is the surface pressure) changed from 35 mN/m, for pure stearic acid, to 22 mN/m, for the mixture with 12.5% of PPV. The decay of the compression resistance can be related to the flexibility of the material due to their segmental arrangements that are relatively disordered. This must decrease the rigidity of the structured fatty acid monolayer, facilitating the area compression. Compression–decompression curves for mixed PPV–stearic acid monolayers (Fig. 3) show a negligible hysteresis in the compression–expansion cycle. However, a small difference between two successive cycles of compression–expansion can be observed, suggesting that the compression–decompression cycle enhanced the unfolding of the PPV molecules within the stearic acid monolayer. Further compressions and decompression do not cause any additional changes in the isotherms in comparison to the second cycle. The combined results of Figs. 2 and 3 are a first indication that there is no phase separation, considering the absence of signifi-

1756

A. Sakai et al. / Synthetic Metals 161 (2011) 1753–1759

A PM-IRRAS signal (a.u.)

LB film Casting film

3000

2950

2900

2850

2800

wavenumber (cm-1)

B

PM-IRRAS signal (a.u.)

1112

1045

930 1251

1400

1300

1200

1100

1000

wavenumber

PM-IRRAS signal (a.u.)

C

1600

1242

900

800

(cm-1)

be better clarified when the infrared spectra are presented and discussed. Fig. 4 shows the PM-IRRAS spectra for the mixed monolayers. Panel A shows the bands for CH2 stretches (asymmetric and symmetric at 2914 and 2848 cm−1 , respectively), and a shoulder at 2887 cm−1 , attributed to C–H stretches in CH3 . These bands can be attributed to C–H groups for both components: stearic acid and PPV. Panel B shows absorption at 961 cm−1 (CH deformation in aromatic compounds or in trans-vinylene) and at 1120 and 1283 cm−1 (C–O stretches in alkyl–aryl ether), evidencing the incorporation of PPV in the monolayer. As PM-IRRAS spectroscopy must show only peaks for surfaces or materials adsorbed mono-molecularly at surfaces, these results are strong evidence that PPV is present at the air–water interface forming a hybrid film with stearic acid.

1096

1146 1131 1317

934 1110

1400

1200

1000

Fig. 6. Illustration on the disposition of PPV and stearic acid at the air–water interface and as a LB film considering the intermolecular interaction between polar groups.

800

600

wavenumber (cm-1) Fig. 5. PM-IRRAS for PPV–stearic acid casting film (A and B) and PPV–stearic acid 3-layer LB film (A and C).

cant hysteresis. Probably, the folding of the polymer molecules at the air–water interface results in a possible preferential orientation of the polymer on the water subphase, leading the polar ether groups close to the water surface and the hydrophobic chains far from the surface. The probable orientation of the molecules will

3.2. Langmuir–Blodgett (LB) films LB films were well assembled until the third layer, seeing that transfer ratio is close to one, which proves the uniformity of mixed PPV–stearic acid film when transferred from the air–water interface onto the solid support. Films produced with more than 3 layers did not result in regular depositions and were discarded. Fig. 5 shows the PM-IRRAS spectra for PPV immobilized on solid supports through two methodologies: LB transfer and casting. For the C–H region, the position and the relative intensities of infrared spectra do not differ substantially for both kinds of film. The main bands may be attributed as for those ones in Fig. 4. On

A. Sakai et al. / Synthetic Metals 161 (2011) 1753–1759

1757

Fig. 7. Atomic force micrographs for LB films deposited on mica on 2 (micrographs A1, B1, C1) or 3 dimensions (micrographs A2, B2, C2). A: 1-layer stearic acid; B: 1-layer mixed PPV–stearic acid; C: 3-layer mixed PPV–stearic acid.

the other hand, when the films are transferred from aqueous interface to solid supports, we observed a shift of the bands from 961 to 934 cm−1 ; from 1120 to 1096 cm−1 ; and from 1282 to 1251 cm−1 . These shifts are attributed to the new surroundings imposed to the polymer chains entrapped between layers of stearic acid, which dif-

fers substantially from that at the air–water interface, when only one molecular layer is present and PPV is in contact with water. For the casting films, the PPV film should be described as a set of interpenetrated random coils, changing the spectrum considerably. The attributions of the bands for PPV in all cases (Langmuir

A. Sakai et al. / Synthetic Metals 161 (2011) 1753–1759

monolayer, LB and casting) are similar to those already reported elsewhere [32,35], and the differences in terms of wavenumbers and relative intensities are owing to the different kinds of structure and methodology employed. Observing the differences between the spectrum of the casting and the LB film, we emphasize the significant change in the relative intensity of the band intensities in ∼1045–1100 cm−1 (peak A) and ∼930 cm−1 (peak B), ascribed respectively to C–O stretch, and to CH deformation in aromatic compounds or in trans-vinylene. The A/B ratio, considering the area under the bands, is 1.84 for the casting film, and 20.3 for the LB film. This may be related to orientation of these groups in the solid film, as well as the folding of the polymer. The C–O group, being more hydrophilic, may be disposed in such a way that interacts with the carboxyl groups of stearic acid. This interaction may provide a regular orientation of the vibration dipole. The ether group, presenting a higher anisotropy, boosts the absorption signal in the PM-IRRAS spectrum. This fact therefore attests the regular orientation of the chemical groups of PPV when immobilized as a mixed film with stearic acid, in contrast with the casting film, whose orientation of the chemical groups are expected to be more random. The isotropy, associated with the random orientation of the transition moments induces a lower intensity of the ether stretch absorption peak. An illustration considering the interaction of ether with polar groups of stearic acid is portrayed in Fig. 6, where the disposition of the chemical groups is shown for both kinds of film: Langmuir monolayers and LB films. The morphology of LB films is illustrated on the AFM images of Fig. 7. Firstly, it is important to say that as Brewster Angle Microscopy for monolayers at the air–water interface (not shown) did not reveal separate phases in the scale of the equipment (500 ␮m × 500 ␮m, approximately), showing homogenous patterns for the images. For this reason, atomic force microscopy for the deposited films was employed, which could reveal the morphology of the structures at a lower scale. In this sense, uniform LB films were obtained with pure stearic acid, with a roughness of 0.21 nm for a scanned area of 2.5 ␮m × 2.5 ␮m, which is consistent with the literature [36]. A different behavior was observed for the 1-layer mixed PPV–stearic acid films, which were heterogenous, presenting holes of 4–5 nm depth, confirming the presence of a new material in the stearic acid LB film. If such holes are considered in the roughness calculations, a value of 3.41 nm is obtained for a scanned area. AFM images taken in other regions of the film lead to similar results. The 3-layer mixed PPV–stearic acid, being a thicker thin, presents larger depressions, 12–14 nm depth, and a roughness of 4.05 nm. The AFM images confirm the heterogeneity of the films caused by PPV when compared to the homogenous structure formed by pure stearic acid LB film. It is relevant to observe that for 3-layer mixed PPV–stearic acid, the structure in layers can be visually observed (pointed by the arrows in the panels C1 and C2 for Fig. 6), which is related to the layer-bylayer structure expected for LB films. Moreover, PPV casting films, due to its high heterogeneity and roughness, could not have AFM images obtained by tapping mode, damaging the tip. Whereas the monolayer thickness of a stearic acid monolayer is calculated to be 2.5 nm [37], we obtained a depth of 12–14 nm, with a roughness of 4.05 nm. This value indicated that 4 to 5 ± 1.5 layers are deposited on the substrate. Fig. 8 shows the photoluminescence (PL) spectra of PPV in CHCl3 , as a casting film and as a LB film with 3 layers, excited at 370 nm (maximum of absorption) [32]. The absorption and emission of light by the polymers are determined by their chromophoric part [14]. In these spectra, it is possible to observe the shift of the emission maximum from 435 nm, for PPV in solution, to 430 and 450 nm for LB film and casting films, respectively. In the literature, the PL spectrum of this material onto glass plates, with excitation at 390 nm, showed a characteristic blue emission at 453 nm [37].

430

emission (a.u.)

1758

435

450

LB Film Casting CHCl3

350

400

450

500

550

600

λ (nm) Fig. 8. Photoluminescence (PL) spectra of copolymer in CHCl3 , casting and LB film (3 layers).

Interesting, the emission band for the cast film is broader when compared to the other two spectra. Also, the shape is quite similar and the main difference is due to the shift of the maximum. The bandwidth and maximum of the spectra can be explained in terms of the aggregation state in each kind of film. The deposition of the cast film does not present control in terms of structure and thickness, resulting in a thicker film. In solution, solvent (chloroform) molecules may interact with some hydrophilic ether groups at the surface of the polymer backbone, which may provide a spectrum narrower and with maximum emission at higher energies. For the same reason, the polymer chains in the LB film may be disposed in such a conformation that the ether groups may interact with the polar groups of stearic acid, causing therefore narrower and more uniform bands. As a result, the emission properties of from PPV–stearic acid LB films indicate that they are suitable for optical applications, and the interactions with stearic acid promote bands that are more uniform and narrower. Finally, the advantage in using LB films in relation to the chloroform solution, which presents similar properties in the luminescence spectra, is the fact that the Langmuir–Blodgett technique enables the construction of optical and electronic devices in the solid state, being easier to manipulate, less toxic, and less susceptible to the volatility of the solvent.

4. Conclusions In the present work, it was proven that stearic acid serves as a matrix to form stable Langmuir films with PPV, with molecularlevel interactions between the components leading to changes in the film properties. The LB films transferred from mixed monolayers exhibited optical (photoluminescence) properties that could be evaluated. The most important aspect of this paper is to use an alternative method to immobilize PPV in solid films, through the Langmuir–Blodgett technique, by mixing it with fatty acids, and consequently forming a mixture able to spread on the air–water interface. The results presented here contain implications for the interaction between fatty acids and conducting polymers, since it leads to more uniform and narrower bands in the luminescence spectra. We hope these films can be employed in the future as lightemitting devices, whose luminescent properties can be determined by controlling the molecular architecture of the solid LB film.

A. Sakai et al. / Synthetic Metals 161 (2011) 1753–1759

Acknowledgements We thank Dr. Debora T. Balogh to the GPC analysis. This work was supported by FAPESP, CNPq and INCT-INEO (Brazil). References [1] H. Shirakawa, E.J. Louis, A.A.G. MacDiarmid, C.K. Chiang, A.J. Heeger, J. Chem. Soc. Chem. Commun. 2 (1977) 578–580. [2] C. Qu, Z. Xu, F. Teng, X.R. Xu, Chin. Phys. Lett. 20 (2003) 1144–1147. [3] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539–541. [4] D.D.C. Bradley, Polym. Int. 26 (1991) 3–16. [5] L.O. Péres, J. Gruber, Mater. Sci. Eng. C 27 (2007) 67–69. [6] R.J.O.M. Hoofman, M.P. de Hass, D.A. Siebbeles, J.M. Warman, Nature 392 (1998) 54–56. [7] Z.H. Huang, J.H. Yang, L.C. Chen, X.J. Wang, W.L. Li, Y.Q. Quiu, J.T. Zhang, R.S. Wang, Synth. Met. 91 (1997) 315–316. [8] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. dos Santos, J.L. Bredas, J.L.M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121–128. [9] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. 37 (1998) 402–428. [10] P.L. Burn, A.B. Holmes, A. Kraft, D.D.C. Bradely, A.R. Brown, R.H. Friend, R.W. Gymer, Nature 356 (1992) 47–49. [11] B. Hu, F.E. Karasz, Synth. Met. 92 (1998) 157–160. [12] Z. Yang, B. Hu, E.J. Karasz, J. Macromol. Sci. Pure Appl. Chem. A 35 (1998) 233–247. [13] P.L. Burn, A.B. Holmes, A. Kraft, A.R. Brown, D.D.C. Bradley, R.H. Friend, Mater. Res. Soc. Symp. Proc. 247 (1992) 647–654. [14] B. Hu, D. Karasz, D.C. Morton, I. Sokolik, Z. Yang, J. Lumin. 60 (1994) 919–922. [15] K. Blogett, J. Am. Chem. Soc. 57 (1935) 1007–1022. [16] K.A. Marx, L. Samuelson, M. Kamath, J.O. Lim, S. Sengupta, D. Kaplan, J. Kumar, S.K. Tripathy, Mol. Biomol. Electron. 240 (1990) 395–412.

1759

[17] A. Marletta, D. Goncalves, O.N. Oliveira Jr., R.M. Faria, F.E.G. Guimaraes, Macromolecules 33 (2000) 5886–5890. [18] F.J. Pavinatto, J.Y. Barletta, R.C. Sanfelice, M.R. Cardoso, D.T. Balogh, C.R. Mendonc¸a, O.N. Oliveira, Polymer 50 (2009) 491–498. [19] L.F. Ceridório, L. Caseli, M.R. Cardoso, T. Viitala, O.N. Oliveira, J. Colloid Interface Sci. 346 (2010) 87–95. [20] L.A. Tsarkova, P.V. Protsenko, J. Klein, Colloid J. 66 (2004) 84–94. [21] N. Zhang, R. Schweiss, W. Knoll, J. Solid State Electrochem. 11 (2007) 451–456. [22] I. Langmuir, J. Am. Chem. Soc. 39 (1917) 1848–1906. [23] G. Decher, J.D. Hong, J. Schmitt, Thin Solid Films 210/211 (1993) 831–835. [24] S.A. Hussain, D. Bhattacharjee, Mod. Phys. Lett. 24 (2009) 3427–3451. [25] T. Bjørnholm, D.R. Greve, N. Reitzel, T. Hassenkam, K. Kjaer, P.B. Howes, N.B. Larsen, J. Bøgelund, M. Jayaraman, P.C. Ewbank, R.D. McCullough, J. Am. Chem. Soc. 120 (1998) 7643–7644. [26] J.Y. Park, N. Koenen, M. Forster, R. Ponnapati, U. Scherf, R. Advincula, Macromolecules 41 (2008) 6169–6175. [27] T.C.F. Santos, L.O. Péres, S.H. Wang, O.N. Oliveira, L. Caseli, Langmuir 26 (2010) 5869–5875. [28] V.G. Babak, F. Baros, O. Boulanouar, F. Boury, M. Fromm, N.R. Kildeeva, N. Ubrich, P. Maincent, Colloids Surf. B 59 (2007) 194–207. [29] J.H. Cheung, M. Rubner, Thin Solid Films 244 (1994) 990–994. [30] A. Paul, T.N. Misra, Mol. Cryst. Liq. Cryst. 289 (1996) 265–274. [31] A. Paul, D. Sarkar, T.N. Misra, J. Phys. D 14 (1995) 899–905. [32] L.O. Péres, M.R. Fernandes, J.R. Garcia, S.H. Wang, F.C. Nart, Synth. Met. 156 (2006) 529–536. [33] S. Kundu, D. Langevin, Colloids Surf. A 325 (2008) 81–85. [34] G.L. Gaines, Insoluble Monolayers at Liquid–Gas Interface, Intersciences Publishers, 1966, pp. 162–167. [35] M.R. Pinto, B. Hu, F.E. Karasz, L. Akcelrud, Polymer 41 (2000) 2603–2611. [36] E. Gyorvary, J. Peltonen, M. Lindh, J.B. Rosenholm, Thin Solid Films 284 (1996) 368–372. [37] A. Ulman, Langmuir–Blodgett Films. An Introduction to Ultrathin Organic Films from Langmuir–Blodgett to Self-Assembly, Academic Press Inc., San Diego, 1991, p. 112.