pubs.acs.org/Langmuir © 2009 American Chemical Society
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Ordering of Fe3O4 Nanoparticles in Polyelectrolyte Multilayer Films Marta Kolasinska,*,† Thomas Gutberlet,‡,§ and Rumen Krastev†,
Max-Planck Institute of Colloids and Interfaces, 14476 Potsdam, Germany, and ‡Laboratory for Neutron Scattering, ETH Z€ urich & Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. § Present address: Forschungszentrum J€ ulich GmbH, J€ ulich Centre for Neutron Science, Lichtenbergstrasse 1, 85747 Garching, ubingen, Germany. Present address: NMI Natural and Medical Sciences Institute at the University of T€ Markwiesenstrasse 55, 72770 Reutlingen, Germany )
†
Received March 31, 2009. Revised Manuscript Received April 30, 2009 In our work we have focused on the incorporation of magnetite nanoparticles (NPs) into poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) polyelectrolyte multilayers (PEMs). The main goal of presented studies was to control the two-dimentional ordering of NPs within polyelectrolyte films. The ordering of NPs depended on the treatment of the underlying polyelectrolyte films. The NPs were uniformly distributed in freshly prepared samples leading to an interfacial mixture of polyelectrolytes and particles, while a highly concentrated layer of NP was formed only when the PEMs were exposed to elevated temperature after their preparation. The observed effect was correlated to glass-melt phase transitions of the PEMs. Such ordering of functionalized species in a polymer matrix may enhance the response from the studied nanocomposites.
Introduction The procedure of the thin film formation by sequential adsorption of polycation and polyanion layers, layer-by-layer deposition (LbL), has progressed significantly as being an efficient method for obtaining various materials of well-defined properties.1-5 LbL assembly consists of sequential deposition of polyelectrolyte (PE) monolayers onto oppositely charged PE layers.6 The process is applied in cyclic manner profiting from the surface charge overcompensation, which occurs while macromolecular species are adsorbed at the solid-liquid interface.7 The LbL technique was used successfully for preparation of less than 1 μm thin polyelectrolyte multilayer (PEM) films with different applications,8,9 and it is well described in the literature [ref 1 and references therein]. The structure of PEM can be easily controlled on a molecular level.10 The versatility of the multilayer formation process, with respect to the variety of support materials and the possibility of incorporation of different functional species into multilayers, results in extreme interest in such ultrathin objects.1-3,11,12 Such nanometer thick polymer-based materials with inhomogeneities embedded in the polymer matrix possess a number of specific properties, pertaining to their structure, thermodynamics, and *Corresponding author. E-mail:
[email protected]. (1) Decher, G., Schlenoff, J. B. Multilayer Thin Films; Wiley-VCH: Weinheim, Germany, 2003. (2) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319–348. (3) Sch€onhoff, M. J. Phys.: Condens. Matter 2003, 15, R1781–R1808. (4) Riegler, H.; Essler, F. Langmuir 2002, 18, 6694–6698. (5) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430–442. (6) Decher, G. Science 1997, 277, 1232–1237. (7) Caruso, F.; M€ohwald, H. J. Am. Chem. Soc. 1999, 121, 6039–6046. (8) M€uller, R.; K€ohler, K.; Weinkamer, R.; Sukhorukov, G. B.; Fery, A. Macromolecules 2005, 38, 9766–9771. (9) Ross, E. E.; Rozanski, L. J.; Spratt, T.; Liu, S. C.; O’Brien, D. F.; Saavedra, S. S. Langmuir 2003, 19, 1752–1765. (10) von Klitzing, R. Phys. Chem. Chem. Phys. 2006, 8, 5012–5033. (11) Shi, X.; Shen, M.; M€ohwald, H. Prog. Polym. Sci. 2004, 29, 987–1019. (12) Grigoriev, D.; Gorin, D.; Sukhorukov, G. B.; Yashchenok, A.; Maltseva, E.; M€ohwald, H. Langmuir 2007, 23, 12388–12396. (13) Brown, K. R.; Lyon, L. A.; Fox, A. P. Chem. Mater. 2000, 12, 314–323. (14) Musick, M. D.; Pena, D. J.; Botsko, S. L. Langmuir 1999, 15, 844–850. (15) Hrapovic, S.; Liu, Y.; Enright, G. Langmuir 2003, 19, 3958–3965.
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electronic, spectroscopic, optic, electromagnetic, or chemical features13-16 with potential application in chemistry, (bio)sensing, and material science.11,17 The properties of the obtained materials depend strongly on the inter particle distances in the matrix. The internal structure of aggregates ranges from close-packed clusters to tenuous fractals, depending on the system and preparation, with consequences for the mechanical response and optical or magnetic properties of the samples.18 Nanoparticles (NPs) can be used as such nonhomogeneities. The clustering of filler particles is generally favored by strong attraction, whereas steric and long-range electrostatic forces may stabilize individual beads. Extensive studies in the field of the nanocomposite materials are ongoing. The main challenge of these studies is the enclosure of monodisperse NPs in polymer matrices and tuning the interparticle distances and, respectively, the amount of incorporated NPs. Successful solution of the existing problems needs detailed understanding of the interactions between particles, the dynamics of the polymer matrix, and the layer organization. The aim of the presented studies was to generate PE-NP composite films with specific properties and to organize NPs in two-dimensional (2D) structures. We fabricated and studied nanocomposites that consisted of negatively charged magnetite (Fe3O4) NPs on planar PE multilayers from poly(allylamine hydrochloride) (PAH), and poly (sodium 4-styrenesulfonate) (PSS). The NP deposition and its influence on the thickness, roughness, and the density of obtained nanocomposites dependent on preparation conditions was studied using neutron reflectometry (NR). These experiments were complemented with quartz crystal microbalance (QCM) and atomic force microscopy (AFM) measurements.
Materials and Methods Materials. PEs used were PAH with a mean molecular weight of 70 kDa and branched poly(ethyleneimine) (PEI) with a (16) Rivas, L.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. Langmuir 2001, 17, 574–577. (17) Rifai, S.; Breen, C. A.; Solis, D. J.; Swanger, T. M. Chem. Mater. 2006, 18, 21–25. (18) Oberdisse, J. Soft Matter 2006, 2, 29–36.
Published on Web 06/11/2009
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Kolasinska et al. molecular weight of 750 kDa as polycations, and PSS of 70 kDa molecular weight was used as the polyanion. PAH, PEI, and PSS were purchased from Sigma-Aldrich (Germany). The molecular structures of polyions used are depicted in Figure 1. As NPs, commercially available magnetite NPs were used, in the form of a 3 wt % aqueous suspension of Fe3O4 stabilized with nitrate from PlasmaChem GmbH (Germany). They were negatively charged (zeta potential ca. -30 mV) with a diameter of about 8 nm. NaCl (99.5%) used for PE solutions was obtained from Fluka (Germany). H2SO4 (96%) and H2O2 (32%) used for silicon cleaning procedure were from Aldrich (Germany). Deuterium oxide (D2O; 99.9%) used in some NR experiments was from Aldrich (Germany). Aqueous solutions were prepared using a Milli-Q Plus 185 water generation system with a resistance of over 18 MΩ (Millipore). Silicon blocks were used as support materials for PE deposition studies by NR. They were of dimensions 8 cm 5 cm1.5 cm and orientation Æ1 0 0æ (Siliciumbearbeitung Andrea Holm (Tann/Ndb., Germany)). Standard gold/quartz sensors QSX 301 (Q-Sense AB V€astra Fr€ olunda, Sweden) were used as supports for the PEM/NP for QCM experiments. All substrates;Si blocks and gold/quartz sensors;were cleaned before use with piranha solution, which is a mixture of equivalent volumes of concentrated sulfuric acid and perhydrol. (Precaution! This solution is a very strong oxidizing agent and should be handled carefully.) Substrates were dipped into piranha solution for 30 min and then carefully rinsed with hot Milli-Q water followed by 30 min of being dipping into hot water (ca. 70 C). Samples and Experimental Procedures. Nanosized magnetite particles were deposited on substrates covered with PEM with the following structure: Si/PEI(PSS/PAH)6. It consisted of a PE film of 13 layers terminated with a PAH polycation layer. In some cases, PEMs were exposed to elevated temperature to undergo glass-melt transition19,20 for some further studies. For this high-temperature treatment, samples were heated in water for 1 h at 70 C, and they are indicated as PEMtemp. All samples were prepared by LbL technique.1-3 Deposition of the PEs onto Si or Au (in the case of QCM experiments) substrates was performed from 0.5 M NaCl solutions at PAH and PSS concentrations of 0.5 g/L. PEI solution was prepared in water, without any addition of salt. Each deposition step took 15 min, and rinsing in between was done three times for 2 min in water. The number of PE layers in the samples was chosen for reflectometric studies to obtain reliable data in the ranges accessible by the reflectometric techniques.21 Positively terminated samples were chosen to investigate the deposition of magnetite NPs, which were negatively charged. NPs were adsorbed on PEM or PEMtemp, exposing the respective PEM to a water (H2O) suspension of Fe3O4 NP with a concentration of 1.5 wt % for 30 min, and was followed by rinsing with Milli-Q water. The particle deposition was followed in situ using QCM or NR. The NR in situ deposition measurements were performed always against D2O to ensure good contrast. In this case, the experimental cell was filled with D2O after the H2O washing step, which followed NP deposition. NR experiments were also performed in the dry state. Those samples were only washed with H2O.
Experimental Techniques. Quartz Crystal Microbalance. QCM experiments were carried out using a QCM-D E4 system (Q-Sense AB V€astra Fr€ olunda, Sweden). The setup enables measurements for real-time studies of adsorption/desorption processes at solid/liquid or solid/gas interfaces. The resonance frequency f of the oscillating sensors depends on the total mass, and it (19) K€ohler, K.; Shchukin, D.; Sukhorukov, G.; M€ohwald, H. Macromolecules 2004, 37, 9546–9550. (20) K€ohler, K.; Shchukin, D.; M€ohwald, H.; Sukhorukov, G. B. J. Phys. Chem. B 2005, 109, 18250–18259. (21) Tolan, M. X-ray Scattering from Soft-Matter Thin Films; Springer: Berlin, 1999.
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Figure 1. Structural formulas of polyions applied in the described studies.
changes upon adsorption/desorption of any mass to/from the sensor. Thus, the QCM operates as a highly sensitive balance. The adsorbed mass can be calculated from the Sauerbrey relation:22-24 cΔf ð1Þ Δm ¼ n where Δm is the adsorbed mass, Δf is the change in the frequency, c=17.7 ng Hz-1 cm-2 for sensors used in presented studies, and n is the number of overtone used for the experiment oscillation: n=1, 3, 5, 7.... Neutron Reflectometry. NR experiments were performed at the neutron reflectometer AMOR at the Paul Scherrer Institute, Villigen, Switzerland,25 in time-of-flight (ToF) mode at three angles of incidence (0.4, 0.9, and 1.5), covering the whole necessary wave vector q range. NR experiments were always performed in a geometry which assures that the phase with higher scattering length density (SLD) is used as a lower phase for the incoming neutron beam. This guarantees the observation of a very well-pronounced critical edge in the reflectivity curves. It was D2O in the case of experiments carried out in a solid/liquid experimental cell26 with an SLD value of D2O equal to 6.3610-6 A˚-2 against Si with an SLD of 2.0710-6 A˚-2.27 In the case of experiments in dry N2 performed in a gastight cell,28 the Si supports were used as a lower phase against N2, which is transparent for neutrons. Atomic Force Microscopy. AFM pictures for PEMs and PEMs with adsorbed NPs were obtained with a Nanoscope IIIa (Digital Instruments, Tonawanda, NY). Contact Angle Experiments. The static contact angle was determined by the sessile drop method using a Profile Analysis Tensiometer PAT-1 (Sinterface, Berlin, Germany).
Results and Discussion Preparation of PE Matrix: Post Temperature Treatment. All samples were prepared by LbL deposition of polyions from their solutions.1-3 Some of obtained films were subjected to post preparation treatment to undergo glass-to-melt transition.8,20 Directly after their preparation, PEM films form nonequilibrium structures.19 Upon increasing the incubating temperature of the PEM (melting the PEM) it is possible to rearrange the polyion conformation to reach an energetically more favorable state.19 (22) Sauerbrey, G. Z. Phys. 1959, 155, 206–222. (23) H€oo€k, F.; Rodahl, M.; Brzezinski, P.; Kasemo, B. Langmuir 1998, 14, 729– 734. (24) Rodahl, M.; H€oo€k, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Rev. Sci. Instrum. 1995, 66, 3924–3930. (25) http://kur.web.psi.ch/amor/ (26) Delajon, C.; Gutberlet, T.; Steitz, R.; M€ohwald, H.; Krastev, R. Langmuir 2005, 21, 8509–8514. (27) Sears, V. F. Neutron News 1992, 3, 26–37. (28) Krasteva, N.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Langmuir 2003, 19, 7754–7760.
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Figure 2. NR curves versus wave vector, qz (left-hand side) and scattering profiles (right-hand side) for PEM before the glass-melt transition (empty triangles and dashed lines) and PEMtemp heated above the glass-melt transition (circles and solid lines); both samples were measured at room temperature. Table 1. Parameters That Give the Best Fits to the Experimental Data in Figures 4-5 Presented As SLD Profiles in Figures 4-5 thickness /nm
SLD /nm-2x104
roughness /nm
thickness /nm
PEM 26.1 ( 0.5
3.4 ( 0.1
3.0 ( 0.1
0.6 ( 0.6
PEM/NP
26.3 ( 0.4
3.6 ( 0.1
1.8 ( 0.4
PEMtemp/NP 0.3 ( 0.3
NP PEMtemp
The mobility of the polymer chains increases when the PEMs are in a molten state, and, as a result, the rearrangement of the PEM structure occurs. This glass-melt transition is irreversible because the material freezes.8,20 The transition temperature depends on the used PE couple. For example, PEMs containing PAH and PSS undergo a glass transition around 70 C29 in water. The AFM pictures of PAH/PSS multilayers before and after the glass-melt transition are included in the Supporting Information file. Both PAH/PSS multilayers;nontreated and exposed to elevated temperature;were studied by means of NR against D2O. The very same samples were measured before and after heating. The obtained results revealed a decrease in SLD with a calculated value of about 3.4 10-4 nm-2 for the sample before temperature treatment, and about 3.0 10-4 nm-2 for the multilayer that underwent glass-melt transition (see Figure 2). The fitting parameters are summarized in Table 1 (further in the text). As we reported in ref 30, D2O has a great contribution to the SLD of samples measured in liquid D2O (SLD of PEM in the dry state was 2.9 10-4 nm-2, while the SLD of the same sample but in D2O was 5.2 10-4 nm-2). Thus, the decrease in SLD value upon heating the PEM can be caused by a decrease of water content inside the multilayer. Temperature treatment causes a change in the hydrophobicity of the multilayers, making them more hydrophobic (less D2O penetrates the sample after heating, which is observed as a decrease of SLD). This observation was confirmed with contact angle measurements. The value of the contact angle increases from about 60 for the multilayer before temperature treatment to 90 for PEM after the glass-melt transition, PEMtemp. Water exclusion from the (29) Leporatti, S.; Gao, C.; Voigt, A.; Donath, E.; M€ohwald, H. Eur. Phys. J. E 2001, 5, 13–20. (30) Kolasinska, M.; Krastev, R.; Gutberlet, T.; Warszynski, P. Progr. Colloid Polym. Sci. 2008, 134, 30–38.
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roughness /nm
PEM+NP
PEMtemp 25.6 ( 0.5
SLD /nm-2 x104
6.1 ( 0.7 26.0 ( 0.8
6.6 ( 0.1 3.0 ( 0.1
2.1 ( 0.5 0.4 ( 0.2
PEM interior can be accomplished also by the film densification, which leads to the decrease in the thickness of the PEM.19 The cited results were obtained for free-standing PE capsules. In our case, the PEMs were bound to the substrate, which reduced the freedom of chain reconformation. This might explain the very minor change in the PEM thickness (see Table 1) upon heating. Deposition of Fe3O4 NPs on/into PEMs. The deposition of Fe3O4 NPs on positively charged polycation-terminated PEMs or PEMtemp’s was studied in situ using the QCM technique. The results are presented in Figure 3. They have shown that negatively charged Fe3O4 NPs can be subsequently assembled on the PAH-terminated PEM or PEMtemp. When the PEM is exposed to the suspension of Fe3O4 NPs (marked with an arrow) the frequency normalized to the number of the overtone used decreases on average to 66 Hz. This decrease is equal to a mass adsorption of 1.2010-6 g/cm2. There was no dependence of the frequency, Δf on the number of the overtone, thus the sample behaves like a rigid layer, and the Sauerbray equation could be applied to estimate the adsorbed mass. The sample was washed with H2O after the deposition step. Further small decrease of the frequency was observed, which shows that the Fe3O4 NPs are strongly bounded to the PEM and cannot be desorbed in pure H2O. The small decrease in frequency might be a result of the change of the medium viscosity or some reorganization of the already deposited NP. When the PEMtemp is exposed to the suspension of Fe3O4 NPs (marked with an arrow) the frequency normalized to the number of the overtone used decreases on average to 24 Hz. The medium value of the frequency corresponds to the adsorbed amount of NPs of about 0.4310-6g/cm2. From the AFM pictures of bare PEMs and PEMs with an adsorbed NP layer (see Figure 3) one can see the difference in the samples’ morphology with increase in roughness after NP deposition. AFM pictures of PEMtemp and PEMtemp/NP (with Langmuir 2009, 25(17), 10292–10297
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Figure 3. QCM frequency change, which shows in situ observed process of Fe3O4 NP deposition on PEI/(PSS/PAH)6 (polycation terminated) PEM or PEM after temperature treatment (PEMtemp) and AFM pictures of bare PEM, PEMtemp, and both films with deposited NPs.
Figure 4. NR curves versus wave vector, qz (left-hand side) and SLD profiles (right-hand side) of polycation-terminated PEM (triangles, dashed line) and PEM/NP (circles, full line) after deposition of Fe3O4 NPs.
adsorbed NPs) are also depicted in Figure 3. One can see the difference in the samples’ morphology with increase in roughness after NP deposition. QCM studies indicate the difference between the deposited amount of magnetite NPs on PEM and PEMtemp (after temperature treatment). The mass adsorbed on PEM (1.20 10-6g/cm2; see Figure 2) was almost 3 times higher than the mass of Fe3O4 deposited on PEMtemp (0.43 10-6 g/cm2). From QCM data or AFM pictures one cannot distinguish whether the particles form 2D layers or whether they diffuse into the PE film. NR experiments can resolve this problem. If the system studied consisted of two separate parts (PEM or PEMtemp cushion and Fe3O4 layer), there should exist a visible difference in the SLDs of the polymer film and the magnetite layer. There should be also a difference in the thickness between bare PEM (PEMtemp) and the matrix with embedded NPs. The NR curves for PEM and PEM with adsorbed NP are shown in Figure 4. Additionally, the respective SLD profiles obtained after data fitting are depicted. Reflectometric curves with well-resolved Kiessig fringes prove that both samples possess structure with reasonably low roughness. The fitting parameters are summarized in Table 1. From experiments and theoretical calculations there is a meaningful difference in SLD of polymer film (less than 410-4 nm-2) (31) http://www.ncnr.nist.gov/resources/sldcalc.html
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(see Table 1) and a layer of magnetite (ca. 710-4 nm-2).31 The SLD of magnetite was estimated for a continuous Fe3O4 layer with a bulk density of 5.3 g/cm3. We expect the surface density of deposited magnetite NPs to be lower than estimated, as they are not tightly packed. However, if they formed a separate layer, it should be distinguished by means of NR as the nonmixed layer with specific SLD. In the present experiments, fitting with one box model was sufficient and introducing more complicated models did not improve the quality of the fits. Thus, NPs do not form separate layer on the top of PE cushion. One can observe a higher value of SLD for PEMs with adsorbed/absorbed NPs (3.6 10-4 nm-2) compared to pure PE film (3.410-4 nm-2). It means that the NPs (with higher SLD) are distributed in the PEM (with lower SLD), increasing the SLD of the whole film. The thickness of the bare PEM was calculated to be about 26.1 nm, while the thickness of the sample after NP deposition is about 26.3 nm. The increase in the thickness is insignificant compared to the NP diameter (8 nm). Thus, we conclude that magnetite NPs diffuse into the PE film, and the reflectometric experiment registered an interfacial “mixture” of PEs and NPs. NR studies were carried out also to find the internal structure of PEMtemp/NP composites. The NR curves measured against D2O and the respective SLD profiles obtained after data fitting are shown in Figure 5. The careful analysis of reflectometric DOI: 10.1021/la9011185
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Figure 5. NR curves versus wave vector qz (left-hand side) and SLD profiles (right-hand side) for Fe3O4 NP deposition on PEMtemp; experiment carried out against D2O. Triangles and dashed lines refer to a PEM after temperature treatment above the glass-melt transition (PEMtemp); circles and solid lines refer to PEMtemp with adsorbed NPs.
Figure 6. NR curve and SLD profiles of PEMtemp/NP nanocomposite; experiment carried out against N2.
curves gives us scattering density profiles (see Figure 5) and quantitative data collected in Table 1. The change in the shape of the reflectometric curves upon the NP adsorption is more visible on the SLD profiles. While analyzing data of PEMtemp with an adsorbed layer of NPs one can distinguish between two different parts of the nanocomposite possessing specific SLD values. The first one is similar to bare PEMtemp film, and it refers to the PEMtemp cushion. In addition to having the same SLD value (3.010-4nm-2), it has also similar thickness (ca. 26 nm) as bare PEMtemp. The second part (closer to the water phase) refers to an additional, separate layer of thickness around 6 nm and SLD about 6.010-4nm-2. The thickness of this layer (6 nm) is close to the NP diameter (8 nm), and its SLD value is close to the theoretically calculated SLD of Fe3O4 (7.010-4nm-2).31 Thus, we can conclude that NPs deposited onto PEMtemp do not penetrate the interior of the film but they form an additional, separate layer on the top of PEMtemp. From the SLD value, the mass density of the interfacial layer of interest can be estimated.21 For this purpose, NR experiments against dry nitrogen were carried out on the PEMtemp/NP nanocomposites. Such setup allowed us to determine the SLD of the NP layer precisely, as it was not mixed with any other nuclei that could scatter neutrons, e.g., D2O. The NR curve done against N2 and the respective SLD profile obtained after data fitting are shown in Figure 6. The fitting parameters are summarized in Table 2. The data analysis revealed the structure of the studied nanocomposite. PEMtemp/NP material consists of two separate parts, which 10296 DOI: 10.1021/la9011185
are distinguishable by neutrons due to their specific SLD values. To this point, NR studies on PEMtemp/NP against dry nitro gen only confirmed the previous set of data shown in Figure 5 and Table 1. The main reason for the presented experiments was to determine the mass density of the NP layer outside of its SLD value. The mass density estimation was based on the relation of SLD with the molecular volume Vm.21 n P bci SLD ¼ i ð2Þ Vm where SLD is the scattering length density, SLD=0.3410-4 nm-2 for an Fe3O4 NP layer (see Table 2), bc is the bond coherent scattering P length of the ith of the n atoms in a molecule. For Fe3O4, ni bci =53.03210 -6 nm-2.27 Thus, molecular volume is equal to Vm=1.560 nm3. The mass density calculated out of the estimated volume was based on the relation M ð3Þ F¼ Vm NA where M=232 g/mol, the molar mass of Fe3O4; NA=6.0221023, Avogadro’s constant; and Vm is the molecular volume. Thus, the density of the adsorbed magnetite layer equals F=0.247 g/cm3. Mass adsorbed can be estimated on the basis of the values of density (F = 0.247 g/cm3) and the thickness d = 9.1 nm of the adsorbed magnetite layer, and it is equal to m=0.2310-6 g/cm2. The estimated mass is in quite good agreement with the value calculated from QCM experiments: m=0.4310-6 g/cm2. Langmuir 2009, 25(17), 10292–10297
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Table 2. PEMtemp/NP Nanocomposite in Dry N2a PEMtemp/NP thickness /nm
SLD /nm-2104
roughness/nm
NP 9.1 ( 1.2 0.34 ( 0.1 3.1 ( 0.9 23.9 ( 1.3 1.73 ( 0.1 0.6 ( 0.4 PEMtemp a Parameters that give the best fits to the experimental data in Figure 6 and are presented as SLD profiles in Figure 6.
Figure 7. Scheme of PEM/NP and PEMtemp/NP composites adsorbed on a silicon block and corresponding SLD profiles.
From the adsorbed mass of particles and their diameter, one can calculate the number of adsorbed particles per unit area, which, in our case, is equal to 1.631011 particles/cm2. Comparing the amount of adsorbed particles with theoretically calculated maximal coverage of the surface by similar particles (1.471012 particles/cm2), one can calculate the coverage of the surface, which is equal to ca. 11 %. Details of those calculations are included in the Supporting Information. From the presented studies one can conclude that modification of the PEM with elevated temperature causing its glassmelt transition prevents adsorbing particles from diffusion inside the multilayer. Thus, particles are ordered into a 2D layer at the PEM surface. In contrast, NPs deposited on nontreated PEMs
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can diffuse inside the multilayer. Both proposed structures of composites studied are schematically depicted in Figure 7.
Conclusions NP/PEM nanocomposites were the subject of the presented studies. We have demonstrated that charged magnetite NPs can be deposited on oppositely charged PEMs. When NPs are adsorbed on PEMs, they diffuse inside the film instead of forming a 2D layer on the top of the PEM. As a result, we observed an interfacial “mixture” of PEs and NPs. In order to realize the 2D ordered NP layers embedded in a polymer matrix, we tuned the properties of the polymer cushion. Up to now one strategy was chosen for this purpose. It was based on changing the properties of the PEM upon a high-temperature glass-melt transition of the multilayer. The films that had undergone a glass-melt transition, PEMtemp, were more hydrophobic. As a result of the modification of the polymer matrix, NPs were organized at the surface into a 2D layer. Such ordering of functionalized species in the polymer matrix may enhance the response from the studied nanocomposites. Acknowledgment. NR studies were performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. The financial support by the European Commission under the sixth Framework Programme through the KeyAction: Strengthening the European Research Area, Research Infrastructures, Contract No. RII3-CT-2003-505925 (NMI3) is highly acknowledged. M.K. acknowledges the Alexander von Humboldt Foundation for the fellowship. Supporting Information Available: AFM pictures of PEM before glass-melt transition and PEMtemp after glass-melt transition, and calculation of the coverage of the surface by particles. This material is available free of charge via the Internet at http://pubs.acs.org.
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