Characterization of pH-induced changes in the morphology of polyelectrolyte multilayers assembled from poly(allylamine) and low molecular weight poly(acrylic acid)

We report characterization of pH-dependent behavior in polyelectrolyte multilayers (PEMs) fabricated from poly(allylamine) (PAH) and low molecular weight poly(acrylic acid) (PAA) synthesized by living/controlled polymerization. Exposure of these films to solutions of low pH (e.g. pH 2.0-3.2) resulted in transformations from films that were smooth and uniform to films with porous morphologies, as characterized by scanning electron microscopy (SEM). We observed large differences in both the extent of this transformation and the sizes of the pores that resulted compared to films fabricated using higher molecular weight PAA used in past studies. Whereas transformations reported in past studies generally lead to pores with sizes in the range of 0.3-2 μm, we observed larger-scale transformations and films with cell-like internal structures comprised of networks of closed pores, interconnected pores, and through-pores with sizes as large as 10-15 μm depending on pH and the manner in which the films were incubated. Films fabricated using fluorescently end-labeled samples of PAA permitted real-time imaging of changes in internal structure using confocal microscopy (LSCM). The results of these studies also revealed large differences in the nature of these transformations when films were placed in contact with surfaces as opposed to when dipped into aqueous solutions. Our results reveal approaches that can be used to fabricate films with large pores (e.g., pores with sizes on the order of 10-15 μm) and suggest methods that could potentially be used to generate PEMs having controlled gradients in pore size.     

Synthesis, multilayer film assembly, and capsule formation of macromolecularly engineered acrylic acid and styrene sulfonate block copolymers

We report the use of copolymers synthesized with specific block ratios of weakly and strongly charged groups for the preparation of stable, pH-responsive multilayers. In this study, we utilized reversible addition-fragmentation chain transfer (RAFT) polymerization in the synthesis of novel pH-sensitive copolymers comprising block domains of acrylic acid (AA) and styrene sulfonate (SS) groups. The PAA x- b-SS y copolymers, containing 37%, 55%, and 73% of AA groups by mass (denoted as PAA 37- b-SS 63, PAA 55- b-SS 45, and PAA 73- b-SS 27, respectively), were utilized to perform stepwise multilayer assembly in alternation with poly(allylamine hydrochloride), PAH. The ratio of AA to SS groups, and the effect of the pH of both anionic and cationic adsorption solutions, on multilayer properties, were investigated using ellipsometry and atomic force microscopy. The presence of SS moieties in the PAA x- b-SS y copolymers, regardless of the precise composition, lead to films with a relatively consistent thickness. Exposure of these multilayers to acidic conditions postassembly revealed that these multilayers do not exhibit the characteristic large swelling that occurs with PAA/PAH films. The film stability was attributed to the presence of strongly charged SS groups. PAA x- b-SS y/PAH films were also formed on particle substrates under various adsorption conditions. Microelectrophoresis measurements revealed that the surface charge and isoelectric point of these core-shell particles are dependent on assembly pH and the proportion of AA groups in PAA x- b-SS y. These core-shell particles can be used as precursors to hollow capsules that incorporate weak polyelectrolyte functionality. The role of AA groups in determining the growth profile of these capsules was also examined. The multilayer films prepared may find applications in areas where pH-responsive films are required but large film swelling is unfavorable.     

Fabry-Perot fringes and their application to study the film growth, chain rearrangement, and erosion of hydrogen-bonded PVPON/PAA films

We observed Fabry-Perot fringes in the absorption spectra of hydrogen-bonded layer-by-layer (LBL) films of poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA), which stem from the interferences between beams transmitted and partially reflected at the highly smooth film-air interface and film-quartz interface. The appearance and disappearance of Fabry-Perot fringes can be used to evaluate the homogeneity of the film. They also provide information about the film thickness. Using this optical phenomenon, with a minimal requirement of instrumentation, we studied the effect of several experimental conditions on the film buildup and structure. The film grows linearly with dipping cycles. Films fabricated from higher molecular weight polymers tend to be thicker. Increasing the concentration of the assembly solutions can also make thicker films. However, films from high molecular weight polymers or high concentration assembly solutions may be heterogeneous and do not display Fabry-Perot fringes in their absorption spectra. The defects in these heterogeneous films can be healed by a postannealing in water or diluted HCl to allow the chain rearrangement to complete. We further found the PVPON/PAA films can be eroded by long-term annealing in water or diluted HCl by monitoring the movement of the Fabry-Perot fringes. In most cases, the erosion rate is constant with annealing time. The erosion rate decreases with a decrease in the pH of the media and an increase in the molecular weight of the polymers.     

Modulation of the functions of osteoblast-like cells on poly(allylamine hydrochloride) and poly(acrylic acid) multilayer films

Deposition of layer-by-layer polyelectrolyte multilayer (PEM) films has been a widely applied surface modification technique to improve the biocompatibility of biomaterials. The objective of this study was to investigate the impact of the deposition of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) multilayer films on adhesion, growth and differentiation of osteoblasts-like MG63 cells. PAH and PAA were deposited sequentially onto tissue culture polystyrene at either pH 2.0 or pH 6.5 with 4-21 layers. While the MG63 cells attached poorly on the PAH/PAA multilayer films deposited at pH 2.0, while the cells adhered to the PEM films deposited at pH 6.5, depending on layer numbers. Cell adhesion, proliferation and osteogenic activities (alkaline phosphatase activity, expression of osteogenic marker genes and mineralization) were highest on the 4-layer PAH/PAA film and decreased with increasing layer numbers. On the other hand, the behavior of MG63 cells did not show any difference on the adjacent even and odd layers, except PEM4 and PEM5, i.e. the surface charges of the PAH/PAA multilayer films with over ten layers seem indifferent to osteoblastic functions. The results in this study suggested that the mechanical properties of PEM films may play a critical role in modulating the behavior of osteoblasts, providing guidance for application of PEM films to osteopaedic implants.     

Spontaneous wrinkling of layer-by-layer assembled polyelectrolyte films for humidity-responsive superhydrophobicity

The fabrication of smart films with reversible wettability enabled by the stimulus-induced morphology changes has attracted growing interest but remains a challenge. Here we report a smart film that can reversibly changes its wettability between transparent hydrophobicity to translucent superhydrophobicity through the humidity-induced wrinkling/de-wrinkling process. The film was fabricated by depositing hydrophobic SiO₂ nanoparticles (NPs) on poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) films, followed by partially exfoliating the films from the underlying substrates. The partially exfoliated PAA/PAH film can reversibly wrinkle and de-wrinkle when being alternately subjected to humid and dry environments. The deposition of hydrophobic SiO₂ NPs on the wrinkling PAA/PAH film does not hinder the humidity-enabled wrin-kling/de-wrinkling ability of the composite film. The hydrophobic SiO₂ NPs and the underlying humidity-wrinkling PAA/PAH film enable the composite film to spontaneously change from hydrophobic and transparent to superhydrophobic and translucent with the rise of environmental humidity.     

Smooth model surfaces from lignin derivatives. II. Adsorption of polyelectrolytes and PECs monitored by QCM-D

For the first time to the knowledge of the authors, well-defined and stable lignin model surfaces have been utilized as substrates in polyelectrolyte adsorption studies. The adsorption of polyallylamine (PAH), poly(acrylic acid) (PAA), and polyelectrolyte complexes (PECs) was monitored using quartz crystal microgravimetry with dissipation (QCM-D). The PECs were prepared by mixing PAH and PAA at different ratios and sequences, creating both cationic and anionic PECs with different charge levels. The adsorption experiments were performed in 1 and 10 mM sodium chloride solutions at pH 5 and 7.5. The highest adsorption of PAH and cationic PECs was found at pH 7.5, where the slightly negatively charged nature of the lignin substrate is more pronounced, governing electrostatic attraction of oppositely charged polymeric substances. An increase in the adsorption was further found when the electrolyte concentration was increased. In comparison, both PAA and the anionic PEC showed remarkably high adsorption to the lignin model film. The adsorption of PAA was further studied on silica and was found to be relatively low even at high electrolyte concentrations. This indicated that the high PAA adsorption on the lignin films was not induced by a decreased solubility of the anionic polyelectrolyte. The high levels of adsorption on lignin model surfaces found both for PAA and the anionic PAA-PAH polyelectrolyte complex points to the presence of strong nonionic interactions in these systems.     

Fabrication of polyelectrolyte multilayer films comprising nanoblended layers

Polyelectrolyte multilayer thin films were prepared via the alternate deposition of poly(allylamine hydrochloride) (PAH) and a blend of poly(acrylic acid) (PAA) and poly(styrenesulfonate) (PSS). When the pH of the blend solution was 3.5, the presence of PAA in this solution significantly increased the total film thickness. With only 10 wt % PAA in the blend adsorption solution, a large increase in film thickness was observed (92 nm cf. 18 nm). It was also demonstrated that the total amount of PSS adsorbed was enhanced by the presence of PAA in the blend solution, showing that the blend solution composition influenced that of the multilayer films. Thin films prepared with nanoblended layers also showed improved pH stability, because they exhibited reduced film rearrangement upon exposure to acidic conditions (pH = 2.5).     

Highly ion conductive poly(ethylene oxide)-based solid polymer electrolytes from hydrogen bonding layer-by-layer assembly

We report the development of a solid polymer electrolyte film from hydrogen bonding layer-by-layer (LBL) assembly that outperforms previously reported LBL assembled films and approaches battery integration capability. Films were fabricated by alternating deposition of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) layers from aqueous solutions. Film quality benefits from increasing PEO molecular weight even into the 10(6) range due to the intrinsically low PEO/PAA cross-link density. Assembly is disrupted at pH near the PAA ionization onset, and a potential mechanism for modulating PEO:PAA ratio within assembled films by manipulating pH is discussed. Ionic conductivity of 5 x 10(-5) S/cm is achievable after short exposure to 100% relative humidity (RH) for plasticization. Adding free ions by exposing PEO/ PAA films to lithium salt solutions enhanced conductivity to greater than 10(-5) S/cm at only 52% RH and tentatively greater than 10(-4) S/cm at 100% RH. The excellent stability of PEO/PAA films even when exposed to 1.0 M salt solutions led to an exploration of LBL assembly with added electrolyte present in the adsorption step. Fortuitously, the modulation of PEO/PAA assembly by ionic strength is analogous to that of electrostatic LBL assembly and can be attributed to electrolyte interactions with PEO and PAA. Dry ionic conductivity was enhanced in films assembled in the presence of salt as compared to films that were merely exposed to salt after assembly, implying different morphologies. These results reveal clear directions for the evolution of these promising solid polymer electrolytes into elements appropriate for electrochemical power storage and generation applications.     

pH‐Switchable Bioelectrocatalysis Based on Weak Polyelectrolyte Multilayers

Weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) were assembled into {PAH/PAA}_n layer‐by‐layer films on electrodes. The cyclic voltammetry (CV) response of ferrocenecarboxylic acid (Fc(COOH)) at {PAH/PAA}₅ film electrodes assembled under the specific condition showed pH‐sensitive “on‐off” switching property. This property was further used to control the electrocatalytic oxidation of glucose by glucose oxidase (GOD) with Fc(COOH) as the electron transfer mediator, so that the pH‐switchable bioelectrocatalysis could be realized. The mechanism of pH‐sensitive behavior of the system was explored and believed to originate from the pH‐dependent structure change of the films.     

Effect of thickness on the thermal properties of hydrogen-bonded LbL assemblies

Layer-by-layer (LbL) assemblies have attracted much attention for their functional versatility and ease of fabrication. However, characterizing their thermal properties in relation to the film thickness has remained a challenging topic. We have investigated the role of film thickness on the glass transition temperature (T(g)) and coeffecient of thermal expansion for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PEO/PMAA) hydrogen-bonded LbL assemblies in both bulk and ultrathin films using modulated differential scanning calorimetry (modulated DSC) and temperature-controlled ellipsometry. In PEO/PAA LbL films, a single, well-defined T(g) was observed regardless of film thickness. The T(g) increased by 9 °C relative to the bulk T(g) as film thickness decreased to 30 nm because of interactions between the film and its substrate. In contrast, PEO/PMAA LbL films show a single glass transition only after a thermal cross-linking step, which results in anhydride bonds between PMAA groups. The T(g), within error, was unaffected by film thickness, but PEO/PMAA LbL films of thicknesses below ~2.7 μm exhibited a small amount of PEO crystallization and phase separation for the thermally cross-linked films. The coefficients of thermal expansion of both types of film increased with decreasing film thickness.     

Fabrication of highly refractive BaTiO3 nanocomposite films using heat resistant polymer as matrix

Highly refractive, heat-resistant BaTiO₃ nanocomposite films were fabricated via in situ polymerization to homogeneously disperse barium titanate (BT) nanoparticles into polyimide (PI) matrix. BT nanoparticles surface-modified with O-phosphorylethanol phthalimide (PPHI) were employed to the in situ polymerization in which condensation reactions of a diphthalic anhydride and a diamine were conducted to form the prepolymer of poly(amic acid) (PAA) that was thermally imidized in the following step. The nanoparticles surface-modified were added to PAA solution at different times in the polymerization to examine the effect of PAA molecular weight on the refractive index (RI) of the nanocomposite films, which indicated that relatively low molecular weights (<10,000) of PAA formed at the point of nanoparticle addition was appropriate for enhancement of nanocomposite RI. An additional treatment of chemical imidization using acetic acid anhydride and pyridine, which was followed by the thermal imidization, was performed to examine the effect of polyimide structure on RI of nanocomposite films. The RI of nanocomposite films with excellent thermal stability could be successfully enhanced to n = 1.88 by the chemical imidization.     

Nanoelectrode ensembles based on conductive polyaniline/poly(acrylic acid) using porous sol–gel films as template

Porous sol–gel (PSG) film has been utilized as a template for the electrochemical polymerization of aniline in presence of poly(acrylic acid) (PAA). The presence of electroactive polyaniline (PAn)/PAA within the porous skeleton of the sol–gel films has been confirmed using cyclic voltammetry, UV–vis spectrometry and atomic force microscopic measurements. The densities and the sizes of the nanoelectrodes can be controlled easily using electrochemical methods. The conductive polymer “wires” of PAn/PAA formation in PSG matrix can behave as an ensemble of closely-spaced but isolated nanoelectrodes. Moreover, the nanoelectrode ensembles based on conductive PAn/PAA for glucose biosensing are fabricated by immobilization of glucose oxidase (GOx) and Nafion onto the surface of conductive polymer. Owing to the biocompatibility of PSG and electro-activity of PAn/PAA at neutral pH regions, the glucose biosensor shows excellent characteristics and performance, such as low detection limit and fast response time.     

Study of film structure and adsorption kinetics of polyelectrolyte multilayer films: effect of pH and polymer concentration

The alternate adsorption of polycation poly(allylamine hydrochloride)(PAH) and the sodium salt of the polymeric dye poly(1-[ p-(3'-carboxy-4'-hydroxyphenylazo)benzenesulfonamido]-1,2-ethandiyl)(PCBS) on quartz crystals coated with silica was studied to understand the structural properties and adsorption kinetics of these films using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D), absorbance, and ellipsometry measurements. In-situ deposition of the polycation PAH on QCM crystals was monitored, followed by rinsing with water and then deposition of the polyanion PCBS. The effects of polymer concentration and pH on film structure, composition and adsorption kinetics were probed. The polymers were adsorbed at neutral pH conditions and at elevated pH conditions where PAH was essentially uncharged to obtain much thicker films. The change in the resonant frequency, Deltaf, of the QCM-D showed a linear decrease with the number of bilayers, a finding consistent with absorbance and ellipsometric thickness measurements which showed linear growth of film thickness. By using the Delta f ratios of PCBS to PAH, the molar ratios of repeat units of PCBS to PAH in the bilayer films as determined by QCM-D were approximately 1:1 at polyelectrolyte concentrations 5-10 mM repeat unit, indicating complete dissociation of the ionic groups. The frequency and dissipation data from the QCM-D experiments were analyzed with the Voigt model to estimate the thickness of the hydrated films which were then compared with thicknesses of dry films measured by ellipsometry. This led to estimates of the water content of the films to be approximately 45 wt %. In addition to the QCM-D, some films were also characterized by a QCM which measures only the first harmonic without dissipation monitoring. For the deposition conditions studied, the deposited mass values measured by the QCM's first harmonic were similar to the results obtained using higher harmonics from QCM-D, indicating that the self-assembled polyelectrolyte films were rigid.     

Photo-cross-linkable polyelectrolyte multilayers for 2-D and 3-D patterning

We report the synthesis of poly(acrylic acid-ran-vinylbenzyl acrylate) (PAArVBA), a photo-cross-linkable weak polyelectrolyte, and its incorporation into polyelectrolyte multilayer (PEM) films. PEM films assembled from PAArVBA and poly(allylamine hydrochloride) (PAH) are found to exhibit similar thickness trends with assembly pH as those previously reported for poly(acrylic acid) (PAA)/PAH multilayers. Swelling properties of the as-built and photo-cross-linked films are studied by in situ ellipsometry. Two-dimensional masking techniques are used to pattern regions of high and low swelling, as confirmed by atomic force microscopy (AFM), and to provide spatial control over the low-pH-induced microporosity transition exhibited by PAH/PAA PEMs. Films containing alternating blocks of PAH/PAArVBA bilayers and PAH/PAA bilayers were assembled, laterally photopatterned, and exposed to low-pH solution to generate nanoporosity leading to patterned Bragg reflectors, thereby demonstrating three-dimensional control over film structure in these weak PEM assemblies.     

Cell adhesion properties of patterned poly(acrylic acid)/poly(allylamine hydrochloride) multilayer films created by room-temperature imprinting technique

Patterned poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) multilayer films with line structures of different lateral size and vertical height were fabricated by a room-temperature imprinting technique, and their cell adhesion properties were investigated. The nonimprinted PAA/PAH multilayer films are cytophilic toward NIH/3T3 fibroblasts and HeLa cells whether PAA or PAH is the outer most layer. In contrast, the PAA/PAH multilayer films with a 6.5-microm-line/3.5-microm-space pattern structure are cytophobic toward NIH/3T3 fibroblasts and HeLa cells when the height of the lines is 1.29 microm. By either increasing the lateral size of the patters to 69-microm-line/43-mum-space or decreasing the height of the imprinted lines to approximately 107 nm, imprinted PAA/PAH multilayer films become cytophilic. This kind of transition of cell adhesion behavior derives from the change of the physical pattern size of the PAA/PAH multilayer films and is independent of the chemical composition of the films. The easy patterning of layer-by-layer assembled polymeric multilayer films with the room-temperature imprinting technique provides a facile way to tailor the cellular behavior of the layered polymeric films by simply changing the pattern dimensions.     

Hydrogen bonded layer-by-layer assembly of poly(2-vinylpyridine) and poly(acrylic acid): Influence of molecular weight on the formation of microporous film by post-base treatment

This article describes the buildup of hydrogen bonded multilayer film of poly(2-vinylpyridine) (P2VP) and poly(acrylic acid) (PAA), and the influence of polymer molecular weight on the formation of microporous film by post-base treatment. The formation of a microporous film involved a two-step mechanism: the release of PAA from P2VP/PAA multilayer, and the reorganization of the remaining P2VP on the substrate. Fourier transform infrared spectroscopy (FT-IR) indicated that the release of PAA from hydrogen bonded multilayer was a rapid process, which was almost independent of the molecular weight of PAA. Furthermore, the molecular weight of P2VP had a great effect on micropore formation by immersing the P2VP/PAA multilayer in basic solution. The rate of micropore formation increased with increasing molecular weight. We anticipate that a comparative study on P2VP/PAA films containing high or low molecular weight polymer provides a way to control the surface morphology, and will be helpful and constructive for the forthcoming discussion about the formation of the microporous film.     

Room-temperature imprinting poly(acrylic acid)/poly(allylamine hydrochloride) multilayer films by using polymer molds

Layer-by-layer assembled polyelectrolyte multilayer films of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) have been successfully patterned by room-temperature imprinting using a Norland Optical Adhesives (NOA 63) polymer mold. The proper amount of water in the PAA/PAH multilayer film can decrease the viscosity of the film and facilitate the imprinting. Many factors, such as imprinting pressure, length of imprinting time, and the structure and size of the patterns in the polymer mold, can produce an influence on the final imprinted pattern structures on multilayer films. A high imprinting pressure of 100 bar and elongated imprinting time of several hours is needed to achieve a patterned PAA/PAH multilayer film with a feature size of several tens of micrometers. With a twice imprinting, grid structures can be successfully produced when a NOA 63 mold having line structures is used. Room-temperature imprinting by using polymer NOA 63 mold provides a facile way to fabricate layered polymeric films with various kinds of pattern structures.     

Modulation of hemocompatibility of polysulfone by polyelectrolyte multilayer films

Polyelectrolyte multilayer (PEM) films have been recently applied to surface modification of biomaterials. Cellular interactions with PEM films consisted of weak polyelectrolytes are greatly affected by the conditions of polyelectrolyte deposition, such as pH of polyelectrolyte solution. Previous studies indicated that the adhesion of several types of mammalian cells to PAH/PAA multilayer films was hindered by low pH and high layer numbers. The objective of this study is to evaluate whether the hemocompatibility of polysulfone can be modulated by deposition of poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) multilayer films. PAH/PAA multilayer films with different layer numbers were assembled onto polysulfone at either pH 2.0 or pH 6.5. The number of platelet adhesion and the morphology of adherent platelets were determined to evaluate hemocompatibility of modified substrates. Compared to non-treat polysulfone, the PEM films developed at pH 2.0 decreased platelet adhesion, while those built at pH 6.5 enhanced platelet deposition. Platelet adhesion was found positively correlated to polyclonal antibodies binding to surface-bound fibrinogen. The extent of platelet spreading was increased with layer numbers of PEM films, suggesting that the adherent platelets on thick PEM films were prone to activation. In conclusion, PAH/PAA films with few layers developed at pH 2.0 possessed better hemocompatibility compared to other substrates.     

Preparation of Highly Loaded PAA/PAH Layer-by-layer Films by Combining Acid Transformation and Templating Methods

Enhancing the molecular loading capability of layer-by-layer(LbL)method holds high importance in environmental and biomedical application.Here,we reported a strategy to prepare highly loaded poly(acrylic acid)(PAA)/poly(allylamine hydrochloride)(PAH)LbL films by combining the particulate templating strategy and acid treatment film transformation and realized tlae efficient loading of hydrophilic small molecules.The loaded molecules can be released in a pH-controlled manner.A slow release speed was observed in the acidic solutions with pH value of 3.Abrupt releases were observed at higher pH values(5 or 7).     

Influence of the polyelectrolyte molecular weight on exponentially growing multilayer films in the linear regime

Alternated deposition of polyanions and polycations on a charged solid substrate leads to the buildup of polyelectrolyte multilayer (PEM) films. Two types of PEM films were reported in the literature: films whose thickness increases linearly and films whose thickness increases exponentially with the number of deposition steps. However, it was recently found that, for exponentially growing films, the exponential increase of the film thickness takes place only during the initially deposited pairs of layers and is then followed by a linear increase. In this study, we investigate the growth process of hyaluronic acid/poly(L-lysine) (HA/PLL) and poly(L-glutamic acid)/poly(allylamine) (PGA/PAH) films, two films whose growth is initially exponential, when the growth process enters the linear regime. We focus, in particular, on the influence of the molecular weight (Mw) of the polyelectrolytes. For both systems, we find that the film thickness increment per polyanion/polycation deposition step in the linear growth regime is fairly independent of the molecular weights of the polyelectrolytes. We also find that when the (HA/PLL)n films are constructed with low molecular weight PLL, these chains can diffuse into the entire film during each buildup cycle, even for very thick films, whereas the PLL diffusion of high molecular weight chains is restricted to the upper part of the film. Our results lead to refinement of the buildup mechanism model, introduced previously for the exponentially growing films, which is based on the existence of three zones over the entire film thickness. The mechanism no longer needs all the "in" and "out" diffusing polyanions or polycations to be involved in the buildup process to explain the linear growth regime but merely relies on the interaction between the polyelectrolytes with an upper zone of the film. This zone is constituted of polyanion/polycation complexes which are "loosely bound" and rich in the polyelectrolyte deposited during the former deposition step.