J. Phys. Chem. C 2008, 112, 10531–10537
10531
Study of Film Growth Properties of Self-Assembled Polyelectrolyte Films of Higher Thickness: Reflectometric and Focused Ion Beam Analyses J. Dejeu,† F. Membrey,*,† S. Diziain,‡ C. Bainier,‡ M. Spajer,‡ D. Charraut,‡ and A. Foissy† Institut UTINAM, UMR 6213 CNRS-UFC - E´quipe Mate´riaux et Surfaces Structure´s, and Institut FEMTO-ST, UMR 6174 CNRS-UFC-UTBM-ENSMM, De´partement d’Optique, UniVersite´ de Franche-Comte, UFR Sciences et Techniques, 16 route de Gray - 25030 Besanc¸on Cedex - France ReceiVed: March 20, 2008; ReVised Manuscript ReceiVed: April 17, 2008
This work addresses the use of fixed-angle laser reflectometry to follow in situ the deposition of self-assembled polyelectrolyte films in a large domain of thickness. It is shown theoretically and illustrated experimentally that the reflectometric output can be interpreted in terms of refractive index and thickness way above the usual range which covers the earlier deposition steps of the polymers (i.e., 30-50 layers, depending on the film composition). Thick films containing up to 320 deposition steps (160 bilayers) were made in the reflectometric cell with the polymer pair poly(allylamine, hydrochloride) and poly(styrene sulfonate) on a silica substrate. They were imaged with AFM, and their thickness was measured using the focused ion beam (FIB) technique. In the course of construction, the reflectometric data started to alternate more or less regularly between positive and negative values, exhibiting absolute values at the extrema which decreased progressively. This phenomenon was coherent with simulated data, once the diffusion of the incident beam and the change of refractive index were taken into account. The phenomenon gives an opportunity to evaluate the composition and the mean thickness of the film during the growth, which is confirmed by comparison with the FIB measurements. I. Introduction Self-assembled polyelectrolyte films can be tailored for a large number of applications, ranging from the functionalization and the protection of surfaces to the remote or the triggered release of encapsulated molecules.1–4 Naturally, any of these applications requires an appropriate control of the film properties, which calls for appropriate molecules and a good knowledge of the interactions involved in the growth process. The buildup of polyelectrolyte films consists simply in the alternate and repeated adsorption of at least two polymers with opposite charge, up to a desired thickness. Films can be made from a few nanometers to a few micrometers thickness, with different densities and permeation properties.5 Two types of self-assembled film growth have been reported: films with thickness that increases linearly with the number of deposition steps6–8 and films for which the thickness grows exponentially, at least nonlinearly.9–11 A linear growth is expected when the adsorbing polymer binds simply with the last deposited one, as it does on an impenetrable surface, the deposited polymers stack up regularly more or less.12 The exponential growth is attributed to a deposition mechanism involving the diffusion of the adsorbing polymers, at least one of them, throughout the underlying polymer film, which is associated with the release of water and electrolyte counterions.11,13 In many cases, a transition was observed between first an exponential and then a linear growth regime.14,15 The transition was said to take place at the film thickness corresponding to the diffusion length of the polymer.16 * Corresponding author. Telephone: +33 3 81 66 20 46. Fax: 33 3 81 66 20 33. E-mail:
[email protected]. † Institut UTINAM, UMR 6213 CNRS-UFC - E ´ quipe Mate´riaux et Surfaces Structure´s. ‡ Institut FEMTO-ST, UMR 6174 CNRS-UFC-UTBM-ENSMM, De ´partement d’Optique.
In the range of lower film thickness (i.e., less than 10 adsorption steps), there is another kind of transition that was described in recent articles.17,19 It is linked to the influence of the substrate in the binding of the polymers.17,20 In brief, up to a certain adsorption step part of the surface is still bare of polymers and remains active in the adsorption of the polymer with an opposite charge. Once the entire surface is covered, the adsorption relies only on polymer/polymer interactions and on the structural properties of the film. The transition between the two regimes takes place in the early steps of the construction. It depends on the surface, the polymer charge densities, and the ionic strength.17,20 To control the growth and the properties of self-assembled polyeletrolyte films aiming to encapsulation or surface protection, which calls for a large thickness, it is therefore important to follow up the adsorption steps in both the early and the later adsorption stages. Nondestructive or in situ control procedures are scarce, and none is reliable in a large range of thicknesses. We have shown previously that laser reflectometry was powerful as a technique to follow up the growth of polymer films, giving access to the composition and the thickness of films formed, say in the first 20 deposition steps.18,19,21 Our work in the present article aims to show that fixed-angle reflectometry may be used also up to a very high step number (320 in this experimental work) for in situ evaluation of the thickness and the composition of the deposited film. The article addresses successively the fabrication and characterization (atomic force microscopy, AFM, and focused ion beam, FIB) of self-assembled polyelectrolyte films up to 160 deposited bilayers (i.e., consecutive deposition of two polymers of opposite charge), the usual reflectometric analysis to track the polymer deposition in the low thickness range and finally the theoretical route to do the same in the higher thickness range, backed with an experimental assessment. The experimental work is made with the pair of polymers
10.1021/jp802457x CCC: $40.75 2008 American Chemical Society Published on Web 06/21/2008
10532 J. Phys. Chem. C, Vol. 112, No. 28, 2008 poly(allylamine, HCl) (PAH) and poly(styrene sulfonate) (PSS), which were formerly used to investigate the interaction mechanism in the contruction of thin films.17,22,24–25 II. Experimental Procedures II.1. Materials and Chemicals. The sodium salt of PSS with an average molecular weight 70 kDa was purchased from Alfa Aesar. PSS is a strong polyacid that is totally ionized in the whole range of pH used in this study.26 PAH, obtained from Sigma Aldrich, was also 70 kDa. It is a weak polyelectrolyte, the ionization of which was measured by Petrov et al.27 The median ionization pK is 8.5. At pH ) 9, the ionization ratio (fraction of protonated amine groups) is 40%. The ionic strength was adjusted with sodium chloride used as received. The refractive index increments of the solutions were 0.2255 × 10-3 m3 kg-1 and 0.1707 × 10-3 m3 kg-1 for, respectively, PAH and PSS. The deposits were made on silica/silicon wafers (1, 0, 0) purchased from ACM. They were 150 mm in diameter, and the silicon was 625 ( 15 µm in thickness. All experiments were carried out on two substrates of different thickness to measure the successive deposited weights as described formely.21 The silica thickness was measured with an ellipsometer (UVISEL-Jobin Yvon). Wafers were cleaned by immersion in a piranha solution (two parts H2SO4, one part H2O2), rinsed, and finally stored in Milli Q water. Each solution was prepared the day before the experiments and stored in the reflectometer room overnight for equilibration of temperature. The pH of the solutions was controlled (and if necessary adjusted) just before the experiments. II.2. Layer by Layer Deposition on Silica Wafer. The polyelectrolyte films were grown in the reflectometer cell on the oxidized silicon wafer as described previously.28 Practically, the polymer solution (100 mg L-1) was introduced through a capillary set perpendicularly to the substrate (impinging jet cell). The reflectometric analyses were done at the stagnation point, at which the transport of molecules was purely diffusive. The solution flow rate was 1.25 cm3 mn-1. No water rinsing was done between polymer deposits. The deposition time was set at 5 min, although the reflectometric output was constant after 2 min. Because of the negative charge of the silicon oxide surface, PAH was deposited first. After each film construction, the wafer was dried under nitrogen and stored in a closed vessel. In this article, we call “bilayer” two successive deposition steps with the two polymers, ignoring the way the two polymers interact and the final morphology of the film. II.3. Characterization of Polymer Films. Optical FixedAngle Reflectometry Measurement. The polymer films were grown in the reflectometric cell. Reflectometry delivers a signal that depends on the thickness and the refractive index of the adsorbed layer. In short, the relevant output of the reflectometer S, defined as S ) fRp/Rs, is an intensity ratio between the parallel (Rp) and the perpendicular (Rs) components of the polarized light beam that is reflected by the interface. The factor f is an apparatus parameter. Practically, all experimental data are calculated as ∆S/S0 ) (S - S0)/S0 where S is the signal during the deposition experiments and S0 is the signal given by the bare substrate. For monolayer adsorption and at low film thickness, ∆S/S0 is fairly proportional to the deposited polymer weight.29 We showed formerly that the limit of linearity for monolayers of polymers was a few milligrams per square meter.30 In the case of self-assembled films, one may consider that the change of output is caused by a change of the adsorbed amount, but it is not right to assume a linear relationship.31 To
Dejeu et al. measure step by step the deposited weight, we designed a procedure that consists practically in the exact repetition of the film construction on two substrates of different thickness.21 From the two series of data, it is possible to determine the mean refractive index and the mean thickness of the deposited film, therefore to calculate the deposited weight. In the present study, the two series of experiments were made using two silicon wafers with thicknesses of 114 and 298 nm. Atomic Force Microscopy. AFM experiments were performed using a stand-alone SMENA scanning probe microscope (NT-MDT). AFM images were recorded on polymer films built at the estimated stagnation point, in constant force mode. During the scan, the deflection of the cantilever was maintained to a preset value by the feedback loop, and the topographic map of the surface was obtained by the vertical displacement of the scanner. Silicon conical tips on the extremity of AFM cantilevers with a force constant from 0.1 to 0.3 N/m were chosen to probe the samples (NT-MDT). Focused Ion Beam. The polymer film deposited on the wafer was covered with a chromium layer by cathodic sputtering. The aim of this layer was to increase the contrast and reduce the electrostatic charge accumulation. The cathodic sputtering was made with a Plassys apparatus at the pressure 7 × 10-3 mbar, using a current of 1A and a deposition rate 120 nm/min during 75 s. The 30 keV focused Ga+ ions (FIB Orsay Physics Canion 31) were used to etch the polymer films and the Si/SiO2 substrate. After FIB irradiation, scanning ion microscopy (SIM) was used to observe the surface. The wafer was tilted and viewed at an incidence angle of 40° or 35°; the same ion beam, with a current limited to 30 pA, was used as a scanning probe to image the cross section of the film. The different layers in the interface could be clearly identified thanks to the quality of contrast in the ion-induced secondary electron image. The thicknesses of the SiO2 substrate and the polymer film were measured directly on the image of the cross section. III. Results and Discussion III.1. Film Construction and Characterization. AFM Examination. Films containing 10, 15, 20, 40, 80, and 160 bilayers of PAH and PSS were built on two different silica wafers with thicknesses of 114 and 298 nm, respectively. Figure 1 shows representative AFM pictures of films containing between 10 and 160 bilayers. These pictures reveal a rough relief, with a strong increase in heterogeneities with the thickness of the film. Be aware of the different horizontal (µm) and vertical (nm) scales; peaks in the pictures are large bumps in reality. Bumps start forming at the second deposition step from the aggregation of PAH and PSS (see below). The average height between peaks and valleys approximates 200 nm in the early steps (10-15 bilayers) and 1.2 µm for 160 bilayers. The roughness varies between 20 and 180 nm in the same range of thickness. Details of the growth mechanism, the parameters, and the film morphology were given earlier.17,18 All pictures reveal a rough relief. No leveling of the surface was observed, contrary to the study of Buron et al. with the pair MAQUAT/PAA32 where “bumps” started to coalesce after about 40 bilayers. FIB Examination. The FIB pictures of the film’s cross section also showed a strong heterogeneity of the deposits (Figure 2). Thinner films exhibited bumps, and thicker ones showed a granular, cauliflower-like morphology (Figure 2). The question whether holes and cracks in the film could result from
Properties of Polyelectrolyte Films of Higher Thickness
J. Phys. Chem. C, Vol. 112, No. 28, 2008 10533
Figure 1. Topographic image of the polymer film of PAH/PSS on silica substrate: (a) 10, (b) 15, (c) 20, (d) 40, (e) 80, and (f) 160 bilayers.
drying was considered previously.15 A former study showed that drying did not modify the general morphology of films beyond five bilayers.18 The high roughness may be a consequence of the growth mechanism. When PAH adsorbs on the top of the PAH/PSS aggregates, it induces a coalescence phenomenon that creates large cavities. The average thickness of the polymer film was determined from FIB images as described elsewhere.15 Table 1 summarizes the values for deposits between five and 160 bilayers. The uncertainty in the thickness is evaluated from the discrepancies in repeated measurements. Reflectometry. Representative examples of the reflectometric data collected during the growth of the deposit are presented in Figure 3. As observed in a former study with the polymer pair MAQUAT/PAA,33 the signal shows a complex oscillation profile with an opposite alternation for the two substrates of different thickness (114 and 298 nm). The oscillations result from the variation of the cosinus of the phase term Φ in the adsorbed layer, with Φ reading:29
Φ)
4π · dads · nads · cos θ3 λ
(1)
where dads and nads are the thickness and the refractive index of the polymer layer, respectively, λ is the laser wavelength in vacuum, and θ3 is the incidence angle of the laser in the polymer layer (Figure 4). The opposition of maxima and minima for the two substrate thicknesses results from the phase term of the substrate itself (the silica layer), which reads the same as eq 1, but with the appropriate parameters for the silica layer (dsub, nsub, θ2). The reflectometric response can be modeled and calculated rather easily for the lower series of deposits (i.e., up to about the first extremum).29 These calculations show that the output data develop in the right direction for each substrate, and initial
Figure 2. SIM images of polymer films deposited on a silica wafer and covered by chromium layer with a tilt of 30° to study the cross section: (a) five bilayers and (b) 160 bilayers. All the scales must be divided by sin 35° to give the real thickness.
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TABLE 1: Mean Polymer Film Thickness versus Number of Bilayer Deposits bilayers
5
10
15
20
40
80
160
thickness (nm) uncertainties (nm)
66 15
83 20
107 50
145 100
400 100
1000 150
2350 400
values are positive and go through a maximum for the silica thickness 114 nm and reversely for the thickness 298 nm. III.2. Reflectometric Analysis of Thinner Polymer Deposits. In the earlier steps (up to about the first extremum as we shall see), the reflectometric signal gives easily access to the mean thickness and the mean refractive index of the polymer film.18,19,21 Using the De Feijter relationship,34 we use the mean refractive index to further calculate the average polymer concentration C in the film:
C)
(nads - nsol) dn dc
(2)
where dn/dc is the mean refractive index increment of the polymer in the solution, nsol is the refractive index of the polymer solution injected in the cell (1.33301), and nads is the refractive index of the polymer film. A combination of the polymer concentration and the thickness of the film leads to the calculation of the deposited weight. For example, Figure 5 gives the calculated properties of the polymer film (mean thickness, refractive index, and deposited weight) for the first 25 bilayers using data in Figure 3. As we see, the thickness reaches quickly about 60 nm at the fourth deposition step (i.e., two bilayers), after which a slower
Figure 5. (a) Calculated thickness (9) and refractive index (∆) of the polymer film PAH/PSS using the “thickness” method.21 (b) Total adsorbed amount of polymer calculated by the De Feijter relationship.
TABLE 2: Polymer Concentration (PAH + PSS) and Weight Water Percentage in Polymer Deposits bilayer -3
Cpolymer (kg m ) wtwater % in the deposit
Figure 3. Variation of the reflectometric signal (∆S/S0) collected during the formation of 40 and 160 bilayers on substrates with thicknesses of 298 and 114, respectively.
2
5
10
15
42 95.8
166 83.4
385 61.5
563 47.3
and rather regular growth takes place in the subsequent 20 bilayers. Also, the refractive index of the deposit increases regularly up to 17 bilayers, reaches a maximum (n ) 1.44), then apparently decreases. Results in Figure 5 show obviously some inconsistency with regards to the variation of the refractive index and the resulting deposited weight. There is no sense in the decrease of the deposited weight; this phenomenon results from an invalid hypothesis in the calculation, which will be discussed in the next section. Using the refractive index data, we can calculate the percentage of water (wtwater %) in the deposit:
wtwater %eau )
Figure 4. Sketch of the reflective interface used in the reflectometry laser beam.
npoly - nads × 100 npoly - neau
(3)
where npoly and nwater are the refractive index of the polymer mixture (for example, n ) 1.51 taking 15 wt % PAH and 85 wt % PSS) and water (n ) 1.33), respectively. Values in Table 2 show a dramatic increase in the polymer concentration and the correlated decrease in the water ratio in films between two and 15 bilayers. The progressive densification of the polymer film in the course of construction was already reported by Jaber and Schlenoff for much thicker films made with two polymer pairs: PAH-co-PNIPAM [poly(N-isopropylacrimanide)]/PSS-co-
Properties of Polyelectrolyte Films of Higher Thickness
Figure 6. Incremental deposited weights for each polyelectrolyte: 2 PAH and 9 PSS.
Figure 7. Calculated variation of the reflectometric signal versus the thickness of the polymer film for different values of a constant refractive index of the film. Silica thickness: 114 nm (a) and 298 nm (b).
PNIPAM and PAH/PSS.35 They found 23 and 7% of water in films containing 50 and 250 bilayers, respectively. Thus, we may conclude in such systems that one of the polymers at least penetrates the film after deposition and displaces water molecules. This phenomenon was suggested in our analysis of the stepwise charge balance for the buildup of films with the same PAH and PSS molecules.17,18
J. Phys. Chem. C, Vol. 112, No. 28, 2008 10535 Another interesting feature of reflectometry when the simple reflection model applies is the possibility to calculate the stepwise deposited weight for each polymer, as shown in Figure 6 for PAH and PSS in the 15 first bilayers. In the present case, we see interestingly that the two polymers do not deposit the same way. PAH adsorbs roughly the same amount in each step, whereas PSS adsorbs increasingly up to an apparent steady value (5 × 10-6 kg m-2). This result is coherent with the former observation of the polymer concentration in the film, and therefore the refractive index increases. From step to step, we showed indeed that the film thickness increases significantly via the adsorption of the weakly ionized PAH, which forms loops and tails, whereas the film density increases after adsorption of PSS because of the formation of polymer aggregates and the penetration of PSS into the film. The thicker the film, the more PSS can diffuse inside. As said earlier, there is no reason that the refractive index and the deposited weight decrease at some point (Figure 5). We may rather expect that the refractive index takes a constant value when the two polymers reach a steady deposition ratio. The reason for the inconsistent calculations is that they ignore the influence of heterogeneities and roughness in the reflection properties. This point will be considered in the next section. III.3. Reflectometric Properties of Thick Polymer Deposits: A New Approach. If the film is homogeneous with regard to the refractive index (i.e., the composition) and with a negligible rugosity, calculations show that the signal should oscillate regularly with the film thickness. This is shown in Figure 7 for the two silica substrates and three different values of the refractive index. In addition to the regular alternation between positive and negative values, the figure shows two specific influences of the refractive index: (i) a decrease of the refractive index induces an increase of the distance between extrema, and (ii) a decrease of the refractive index does not change ∆S/S0 at the minima (∆S/S0 ) -1), whereas it causes a decrease at the maxima. The value -1 at the minima results from the definition of S which is the ratio between the reflected intensities of the parallel and the perpendicular components (S ) f · Rp/Rs ) Ip/Is; see the Experimental Procedures). Since S is necessarily positive and may take the value 0, the minimum for ∆S/S0 is -1 (∆S/S0 ) S/S0 - 1). In contradiction with the calculations, the experimental output in Figure 3 shows a progressive decrease of the distance between extrema, together with a damping of the signals. The theoretical curves in Figure 7 show that the decrease in the distance between maxima might result from a progressive increase of the refractive index, but this would contradict the correlated decrease of ∆S/ S0 at the maxima. In addition, we expect that the composition and the refractive index of the film reach a steady value after a certain number of steps, which is the case for the polymer ratio after 15 bilayers (Figure 6). Indeed, the refractive index may not exceed that of a pure polymer mixture with a small fraction of water. The principal reason for the damping of the signal is actually that a significant part of the light is diffused by the polymer
TABLE 3: Location of Maxima and Calculated Mean Thickness (nm) of the Polymer Film Using Data in Figure 2 (Silica Thickness 114 nm) number of bilayers at maxima or minima thickness (nm) n ) 1.44 thickness (nm) n ) 1.47 thickness (nm) n ) 1.51
15 max
36 min
51 max
67 min
80 max
92 min
102 max
115 min
122 110 79
317 300 278
547 525 474
762 719 649
993 917 828
1208 1130 1020
1438 1324 1195
1653 1525 1377
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film, the more so the thicker the film. The diffusion phenomenon results from heterogeneities in the film (interfaces between areas of different refractive index, including the rugosity) with a size close to that of the wavelength of the laser beam. Practically, the increase of the diffusion phenomenon during the film construction was also revealed visually in the reflectometric cell by the progressive increase of the diffused laser light intensity at the solution-film interface. Although the output ∆S/S0 at the maxima is affected by the diffusion phenomena, it is reasonable to assume that their position is not. Thus, considering a homogeneous film with a given refractive index, the distance between two maxima should correspond to a phase difference π. Consequently, the increase of film thickness between two maxima can be calculated from:29
∆d )
λ 4nads cos θ3
(4)
where nads is the refractive index of the film, λ is the wavelength of light in vaccum, and θ3 is the incidence angle of the laser beam (Figure 4). This calculation requires a choice of refractive index. As explained earlier, the refractive index calculated from the reflectometric output at the first maximum (15 bilayers) was n ) 1.44, which revealed a significant amount of water (37 wt %). The refractive index of a pure and dense organic film, containing an equal amount of PAH and PSS, would be n ) 1.53. Thus, calculations of the film thickness were attempted with three constant values of the refractive index (1.44, 1.47, and 1.51), as if the film composition was the same during the whole buildup. However, the progressive decrease of the distance between extrema (Figure 3) demonstrated a progressive increase of the refractive index. It was about 1.44 (water content 37%) after 15 bilayers and increased with the number of bilayers up to a value between 1.47 (water content 22%) and 1.51 (water content 0 wt %). In comparison, Jaber and Schlenoff found 23 and 7% of water in films (PAH/PSS) containing 50 and 250 bilayers, respectively.35 We postulated 1.49 as a reasonable maximum value of the refractive index in the later stage of the growth (water content 11 wt %), which led to the thickness 1452 ( 75 nm for 115 bilayers. Table 3 gives an idea of the influence of the choice of nads in the calculated thickness. The approach and the calculations above extend interestingly the range of applicability of reflectometry to characterize the fabrication of interfacial polymer films. Practically, we see that the evolution of the refractive index, the film composition (polymer ratio, water content), and the film thickness may be fairly deduced from the detailed analysis of the experimental data. As a confirmation, we may now compare the FIB and the reflectometric measurements. III.4. Comparison of the Polymer Film Thickness Found by FIB and Reflectometry. As seen in Figure 8, the FIB and the reflectometric data for the film thickness compare well. In the case of reflectometry, the error bars take into account the measurements made with two silica substrates with different thicknesses (114 nm for a 160 bilayers film and 298 nm for a 80 bilayers film). For clarity, the error bars for the FIB data (of the order of 15%) are not shown. A change of growth rate is observed at about 40 bilayers, which corresponds to an approximate thickness of 200 nm. The change of growth regime, sometimes designated as a transition between an exponential and a linear increase of thickness with respect to the number of adsorption steps, has been linked to the penetration depth of the polymers, at least one of the two.36
Figure 8. Thickness of the polymer film. Measured by FIB (1), calculated from reflectometric data using the regular procedure for thin films (9), the present method for the 80 bilayers buildup (b), and the 160 bilayers buildup (2).
Obviously, if one polymer can diffuse into the predeposited film, a change of the growth regime may be expected when the film thickness reaches the diffusion path of that polymer. Below the critical value, the thickness increases exponentially, since the thicker the film the more it may adsorb a polymer. Oppositely, once the film thickness is higher than the diffusion distance, the growth rate is constant (linear growth regime).14,16 This process was proposed by Hubsch et al.36 and more recently by Salomaki et al.37 for, respectively, (PGAx - PSS1-x/PAH) and (PDADMAC/PSS). This transition was reported around 12 bilayers with the polymer pair poly(L-lysine) (PLL)/hyaluronic acid (HA) by Porcel et al.14 where only the polyelectrolyte HA diffused into the predeposited film. More recently, the same authors showed that the transition took place in the thickness range 150-250 nm,16 in agreement with our present result (200 nm). In our case, the linear regime starts about 40 bilayers and the growth rate is 15 ( 2 nm/bilayer, which gives a thickness of 2.3 ( 0.4 µm for 160 bilayers, identical to the experimental FIB evaluation: 2.3 µm (Table 1). In the literature, we found also the same thickness variation for PAH/PSS22 and HA/PLL.16 Layers in the precursor stage are less compact than the layers in the true multilayer regime (20 nm/layer pair in the precursor regime, 16 nm/layer pair between 6 and 9).16 Porcel et al.16 showed that thickness variation is a function of the molecular weight of the polymer. Indeed, we found also a thickness variation between 16 and 19 nm for HA, molecular weight 400 kDa, with different molecular weight of PLL. IV. Conclusions This article addresses the characterization of self-assembled polyelectrolyte films made with poly(styrene sulfonate) and poly(allyl amine hydrochloride). The characterization involved AFM, FIB, and in situ laser reflectometry. The focus of the article was to show that the stepwise growth of polymer films can be monitored in situ using reflectometry in a large range of polymer film thicknesses (i.e., up to 160 bilayers (320 deposition steps) in the present study). Several films were made by the successive and alternate adsorption of the two polymers up to 320 steps (i.e., 160 bilayers) on two silica substrates with different thicknesses (114 and 298 nm). We showed that the thickness and the average
Properties of Polyelectrolyte Films of Higher Thickness refractive index could be calculated from the reflectometric data, with the usual procedure21 up to about 15 bilayers. In higher stage of the buildup, the reflectometric signal exhibited a complex figure, with a damped alternation of maxima and minima, which was rationalized theoretically by taking into account the progressive increase of the refractive index and the diffusion of the laser beam caused by the heterogeneities and rugosity of the film. Calculations of the thickness based on the reflectometric data were backed by the FIB analysis of the film’s cross section. Overall, the uncertainty in thickness was about 10%. In addition, the results were coherent with other studies, revealing a transition about 30 bilayers (near 200 nm) between an exponential and a linear growth rate for PAH/PSS films. The major interest in the use of reflectometry as described in this article is the possibility to follow in situ during the film construction the evolution of the refractive index and the thickness, from which the polymer content and the water ratio may be easily calculated. Acknowledgment. This work was supported by the EU under Contract NMP4-CT-2003-001428, Nanocapsules for Targeted Controlled Delivery of Chemicals. References and Notes (1) Lvov, Y. M. Thin film nanofabrication by alternate adsorption of polyions, nanoparticles, and proteins. In Handbook of Surfaces and Interfaces of Materials; Nahwa, H., Ed.; Academic Press: New York, 2001; Volume 3, Chapter 4, pp 169-188. (2) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985. (3) Wu, A.; Yoo, D.; Lee, J. K.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 4883. (4) Gupta, N.; Patel, A. A.; Nassar, R.; Lvov, Y. M.; McShane, M. J.; Palmer, J. D. Colloids Surf., A 2004, 245, 137. (5) Multilayer Thin Films: A Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley VCH: Weinheim, Germany, 2003. (6) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (7) Radeva, T.; Milkova, V.; Petkanchin, I. J. Colloid Interface Sci. 2003, 266, 141. (8) Lvov, Y.; Decher, G. Crystallogr. Rep. 1994, 39, 696. (9) Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 5355. (10) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C.; Picart, C. Macromolecules 2002, 35, 4458.
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