The Possibility of Obtaining Films by Single Sedimentation of

Article pubs.acs.org/IECR

The Possibility of Obtaining Films by Single Sedimentation of Polyelectrolyte Complexes Vincent Ball,*,†,‡,§,⊥ Marc Michel,† Valérie Toniazzo,† and David Ruch† †

Advanced Materials and Structures, Centre de Recherche Public Henri Tudor, ZAE Robert Steichen 5 rue Bommel L-4940 Hautcharage, Luxembourg ‡ Institut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche 1121, 11 rue Humann, 67085 Strasbourg Cédex, France § Université de Strasbourg, Faculté de Chirurgie Dentaire and ⊥Fédération de Médecine Translationelle de Strasbourg, 1 Place de l’Hôpital, 67000 Strasbourg, France S Supporting Information *

ABSTRACT: Surface coatings obtained by a step-by-step (SBS) deposition process of polymers carrying mutually complementary moieties (like polycations and polyanions) allow for a versatile multifunctionalization of a vast kind of materials. However the SBS deposition process can be extremely time-consuming when it is performed by alternatively dipping the substrates in the solutions carrying the interacting species. There is hence an important need to obtain coatings of similar composition as those obtained in the SBS manner but in a “one pot” manner. Here we show that the sedimentation of stoichiometric polyelectrolyte complexes can lead to a homogeneous coating. This is possible if the polylectrolyte complexes display a high internal mobility. The film growth rate is then in direct relation with the sedimentation rate of its constituting complexes. On the other hand when the interactions between the oppositely charged polyelectrolytes are too strong, the obtained coating is neither continuous nor homogeneous. occurring during the drainage of the liquid film resulting from the adhesion of the droplets spayed on the substrate. It is hence of interest to develop new methodologies allowing the production of films of controllable thickness made of polyelectrolytes and on large area substrates (larger than tens of square centimeters) with minimal loss of polyelectrolytes. This is of particular importance for up-scaling the attainment of polyelectrolyte based films. On the basis of the analogies between polyelectrolyte complexation in solution and the SBS deposition from solutions of the same polyelectrolytes11−13 we address the question if it is possible to obtain films by sedimentation of stoichiometric polyelectrolyte complexes in conditions where phase separation occurs leading to the formation of thin films having mechanical properties markedly dependent on the salt concentration of the solutions used during the preparation of the complexes.14 These so-called “saloplastic complexes” are also compatible with processing steps like extrusion allowing the production of new materials.15 In this article, our main goal is to investigate if homogeneous polyelectrolyte containing films can be obtained on silicon oxide substrates put in contact with solutions containing stoichiometric polycation−polyanion mixtures. In such conditions, phase separation occurs most often16 and we expected that if the sedimented polyelectrolyte complexes would coalesce, an homogeneous coating could be obtained. Otherwise, in the absence of coalescence, one expects

1. INTRODUCTION The deposition of thin films by the so-called layer-by-layer (LBL)1 or step-by-step (SBS) method allows the production of a plethora of functional and stimuli responsive coatings.2 Such deposition methods have many advantages, among which their versatility with respect to the geometry of the substrate and the existence of various processing methods (alternated dipping, alternated spraying,3,4 simultaneous spraying,5,6 spin coating,7 other nonconventional methods8), but one of their major drawbacks is the long processing time when employing the alternated dipping method or the waste of polyelectrolytes when using the alternated spraying method. Spin coating allows the production of very homogeneous coatings with improved properties with respect to the films of similar composition but produced by the alternated dipping method.8 Spin coating has however the drawback to be applicable only on substrates with a small surface area. Therefore, to solve the problem of a long processing time of SBS deposition methods, the highest interest is now to develop “one pot” deposition methods yielding films having a composition and properties as close as possible to those of polyelectrolyte multilayer films. Some interesting attempts have been done to obtain the equivalent of PEM films in a one-pot approach.9 Up to now, these highly elegant approaches require complicated ways of processing and can only be undertaken on conductive substrates. Simultaneous spraying belongs to such one-pot deposition methods.5,6 A reaction, either interpolyelectrolyte complexation or ionic precipitation, occurs in the intermixing droplets and part of the obtained species are deposited on the vertically held substrates. The only major drawback of the simultaneous spraying of interacting species is an important loss of substance © 2013 American Chemical Society

Received: Revised: Accepted: Published: 5691

December 19, 2012 March 29, 2013 April 2, 2013 April 2, 2013 dx.doi.org/10.1021/ie303535s | Ind. Eng. Chem. Res. 2013, 52, 5691−5699

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL PART Solutions and Chemicals. All the solutions were made from distilled and deionized water (Milli Q Plus, Millipore, Billerica, MA, USA) with a resistivity of 18.2 MΩ·cm. The used polyanions were hyaluronic acid (HA, viscosimetric molecular weight MWvis = 4.2 × 105 g·mol−1, Lifecore Biomedical, Chaska, MN, USA) and poly(sodium 4-styrene sulfonate) (PSS, Mw = 7.0 × 104 g·mol−1, Aldrich). As polycations, we used poly-L-lysine (PLL, ref P7890, lot 066K5101, MW = 26500 g·mol−1, Sigma-Aldrich, St. Louis, MA, USA), poly(diallyldimethylammonium chloride) (PDADMAC, Mw = 4.36 × 105 g·mol−1, Aldrich, in the form of a 20% w/v solution) and poly(allylamine hydrochloride) (PAH, SigmaAldrich, ref 28.322-3, Mw = 70000 g·mol−1). All salts, including NaCl, NaNO3, NaBr, NaClO4 and KI, were purchased from Sigma-Aldrich (at least 99% in purity) and used without further purification. Substrates. The silicon substrates used, to be coated by the sedimentation of the polyelectrolyte complexes and for their characterization through ellipsometry and scanning electron microscopy, were cleaned with a freshly prepared piranha solution (2 volume fractions of 98% H2SO4 and 1 volume fraction of 30% H2O2), rinsed intensively with Milli Q water, and dried under a gentle stream of nitrogen. The same procedure was used for the quartz slides used as substrates for UV−vis spectroscopy. The substrates were cleaned immediately before the beginning of each experiment. Formation of Polyelectrolyte Complexes and Sedimentation Experiments. All polyelectrolyte solutions were prepared at 10−3 M in monomer units of the polyelectrolytes. The polycation containing solution was admixed with the polyanion solution in a dropwise manner and under strong agitation (300 rpm) in an Erlenmeyer conical flask. The agitation was continued for 1 min after the end of the polycation addition. Note that the size of complexes may depend on the order as well as on the speed of mixing.20 In our experiments we always poured the polycation containing solution (50 mL) into the polyanion containing one (50 mL) at a flow rate of 25 mL·min−1 and under magnetic agitation (300 rpm) in identical cleaned glass beakers (150 mL). All experiments were performed under conditions at pH close to 6, where the ratio between the number of cationic monomers and the number on anionic monomers was equal to 1 (±0.05). At pH 6, all the used polyelectrolytes displayed maximal charge density which is favorable for the formation of interpolyelectrolyte complexes owing to the pKa values of the used polyelectrolytes. For all polyelectrolyte solutions, with the exception of PSS, the pH was close to 6 (±0.2) in the presence of the electrolyte solution alone and without further pH adjustment. The PSS solutions were adjusted to pH 6 with diluted HCl solutions. Such conditions of equal amount of negative and positive charges carried by the polyanion and the polycation generally allow the most favorable phase separation of the polyelectrolyte complexes out of the solution.16,21 The solution containing the mixture of polyelectrolyte complexes was then poured slowly into a cylindrical beaker glass (11 cm in diameter) into which the slides to be coated had been previously introduced. The overall volume of the polyelectrolyte containing solution plays an important role in the final thickness able to be reached by the films. It was expected that, for a given polyelectrolyte concentration, and for

to obtain a snowflake (and useless) deposit on the substrate. These two possibilities are illustrated in Scheme 1. Scheme 1. Schematic Representation of the Sedimentation of Stoichiometric Polyelectrolyte Complexesa

a Sedimentation yields either a homogenous film (A) or an heterogeneous “snowflake” deposit of complexes (B). In situation A, the complexes undergo coalescence and intermix their content when deposited on the surface of the collector. In situation B, the complexes do not undergo coalescence after their deposition.

To test the validity of our assumption, we investigated three polycation−polyanion combinations: PLL−HA, PDADMAC− PSS and PAH−PSS. PLL, PDADMAC, and PAH are the polycations and correspond to poly(L-lysine hydrobromide), to poly(diallyldimethylammonium chloride), and to poly(allylamine hydrochloride), respectively. The used polyanions, HA and PSS, correspond to poly(sodium hyaluronate) and to poly(sodium-4-styrene sulfonate), respectively. These systems correspond to three different situations when considering the alternated adsorption of the polycations and the polyanions: (PLL−HA)n films grow in an exponential manner for ionic strengths larger than 10 mM in NaCl,17 (PDADMAC−PSS)n films undergo a transition from linear to exponential growth above 1.0 M in NaCl,18 whereas this transition is even shifted to higher ionic strengths for (PAH−PSS)n films.19 Herein, we give experimental evidence to strongly suggest that the polyelectrolyte complexes, produced in conditions where their SBS deposition leads to exponential growth of the film thickness, allow for the sedimentation of homogeneous films on flat collectors of macroscopic surface area with an excellent deposition yield (situation A in Scheme 1). We demonstrate that the growth rate of the deposits is directly related to the sedimentation rate of the polyelectrolyte complexes in solution, which is indirectly determined through the change of the solution turbidity at a given height in a quartz cuvette. On the other hand, when the interactions between the polyelectrolytes in their complexes are too strong, that is, in conditions where the SBS deposition yields to linearly growing films, the coating obtained by sedimentation is made from an inhomogeneous snowflake deposit (situation B in Scheme 1). We also show an ion specific effect in the rate of film deposition in the polyelectrolyte combination made from hyaluronic acid and poly(L-lysine). In the case of homogeneous coatings, we show that they are able to incorporate polyelectrolytes from the solution in contact with the films as the films made from the same components but in a regular SBS manner. However, this incorporation is much slower in the case of films obtained by sedimentation than in their SBS counterparts, suggesting that their compacity is higher and that they display a different Donnan potential. 5692

dx.doi.org/10.1021/ie303535s | Ind. Eng. Chem. Res. 2013, 52, 5691−5699


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chosen squares (10 μm × 10 μm) at a scan rate of 2 Hz. Great care was taken not to damage the films during the imaging process: to that aim repetitive scans over the same area were performed and we checked for the stability of the observed surface features. The UV−visible spectra of the films deposited on cleaned quartz slides were acquired with a double beam UV−mc2 spectrophotometer (SAFAS, Monaco) in the wavelength range between 200 and 700 nm with a spectral resolution of 1 nm. The reference slide was a cleaned quartz slide. UV−vis spectroscopy was also used to investigate the sedimentation rate of the polyelectrolyte complexes. To that aim, immediately after the solution containing the polyelectrolyte complexes had been poured into the beaker containing the deposition substrates, 2 mL of the complex-containing solution was loaded into a polystyrene cuvette (1 cm path length, Prolabo, France). The reference cuvette contained 2 mL of the supporting electrolyte (without polyelectrolyte complexes). Change of absorbance with time was then followed at intervals of 1 min and at a wavelength of 500 nm, a wavelength at which none of the used polyelectrolytes absorbs light. Thus, apparent absorbance corresponds to intensity losses due to light scattering by the polyelectrolyte complexes. If absorbance increases, this means that either the amount of complexes has increased and/or that the number of large complexes has increased. If the absorbance decreases, this means that number or the size of the complexes in the path of the light beam has decreased. In the case of sedimentation, the number of complexes along the light beam decreases. The occurrence of sedimentation can be checked easily by visual inspection of the bottom of the cuvette. Absorbance due to the formation or disappearance of polyelectrolyte complexes is referred to as turbidity herein. Finally UV−vis spectroscopy was also used to investigate the interactions of polyelectrolyte films deposited on quartz slides with PLL−FITC molecules. Permeation of Molecules and Chemical Reactions in the PLL−HA Based Films Obtained through Sedimentation. The (PLL−HA)n films prepared through the alternated dipping method allow for the incorporation of fluorescently labeled PLL23 and can be used as templates for the hydrolysis and polycondensation of the hydrosoluble titanium(bisammonium lactato) dihydroxide (TiBisLac) complex to yield a film loaded with poorly crystalline TiO2.24 To get a comparison of the properties of regular LBL films and films made from the sedimentation of stoichiometric PLL−HA complexes, we put the quartz slides coated with sedimented complexes (24 h of sedimentation) in contact with a 1 mg·mL−1 PLL−FITC solution (in the presence of 150 mM NaCl). The slide was regularly removed from the PLL−FITC solution, rinsed with the NaCl solution, and blown dry under a stream of nitrogen. Its UV−visible spectrum was then measured using a similarly coated quartz slide, not put in the presence of PLL−FITC, as a reference. After the spectral measurement, the coated slide was hydrated again in the NaCl solution and put for prolonged incubation time in the PLL−FITC solution. This experiment was continued up to reaching a saturation absorption at 515 nm, characteristic of the FITC dye. A similar protocol was used to monitor the formation of (poorly crystalline) TiO223 upon contact with a 5 mM TiBisLac solution in the presence of 0.10 M NaCl. Characterization of the Polyelectrolyte Complexes. The hydrodynamic diameter of the PLL−HA complexes was

a collector, or substrate, of given area, the maximal thickness of the films will be proportional to the volume of the solution for a given sedimentation time. All sedimentation experiments were performed at (22 ± 1) °C. Labeling of PLL with Fluorescein Isothiocyanate (FITC). PLL was conjugated with FITC. Briefly, a PLL solution at 1 mg·mL−1 in sodium carbonate buffer (50 mM, pH = 8.5) was contacted with FITC dissolved in a small volume of dimethyl sulfoxide (SdS, Peypin, France), for 1 h at ambient temperature, and in the dark. For the labeling reaction, the pH of the solution was basic in order to allow FITC to bind to unprotonated amino groups. The initial ratio between the number of FITC molecules and the number of PLL monomers was lower than 0.05. The PLL−FITC/free FITC mixture was then dialyzed against a sodium chloride solution at 0.15 M using a dialysis bag made of cellulose ester with a molecular weight cut off of 104 g·mol−1 (Spectra/Por, Spectrum Laboratories, Rancho Dominguez, CA, USA). This dialysis step was repeated at least 2 times and was stopped when no FITC could be detected anymore in the solution outside the dialysis bag. This was checked by means of UV−vis spectroscopy at a wavelength of 515 nm. The dialysis steps were performed at ambient temperature and in the dark. We prepared labeled PLL to investigate its diffusion rate in films made through the sedimentation of stoichiometric PLL−HA complexes. Film Characterization. The growth rate of the polyelectrolyte films was measured by ellipsometry for thin films (less than 200 nm in thickness) and by scanning electron microscopy for thicker coatings (more than 200 nm in thickness). In all cases, the silicon slides deposited at the bottom of a beaker glass were gently removed from the bath after different sedimentation times. Great care was taken not to create too much convection in the solution during the removal of the silicon slides in order not to resuspend the already sedimented complexes. The coated slides were rinsed under a gentle flow of distilled water and dried under a stream of nitrogen. The time of contact with distilled water was very short (about 1−2 s) in order to minimize structural changes in the structure and morphology of the films. The film thickness was then determined by measuring the polarization change of a He−Ne laser beam on 5 regularly spaced positions along the main axis of the rectangular silicon slides. The measurements were performed with a monochromatic PZ200 elipsometer (Horiba, France) and at a constant incidence angle of 70°. We established that a refractive index of 1.46522 was representative of polyelectrolyte films which were considered homogeneous and isotropic. Scanning electron microscopy (SEM) analyses were performed with an environmental microscope (FEI-Quanta 200 type). The distance between the sample and the detector used for the SEM measurement was 10 mm corresponding to a takeoff angle of 35°. In the environmental SEM mode, the samples were observed directly without making any conductive coating. The silicon slides coated with the sedimented complexes were broken with a diamond knife, the dust was removed with a nitrogen stream and the cross sections were observed by means of SEM to determine the average film thickness. The obtained values correspond to the average (±1 standard deviation) of a least three thickness determinations. The surface morphology of the dried films was investigated by means of atomic force microscopy (AFM) in contact mode (Nanoscope IV, Veeco). The films were imaged over randomly 5693



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calculated from dynamic light scattering experiments performed with a Nanosizer Nano ZS from Malvern Instruments. The freshly prepared PLL−HA complexes, in the presence of filtered electrolyte solution (Millex GV membranes with a pore size of 0.22 μm) were poured in an ethanol cleaned polystyrene container and at least 10 autocorrelation functions were acquired. The scattering angle was of 173° and the measuring chamber was conditioned at 25 °C. The hydrodynamic diameters were calculated from the calculated diffusion coefficients using the Stokes−Einstein equation.

3. RESULTS AND DISCUSSION The stoichiometric PLL−HA complexes prepared by slowly dropping a 2 × 10−3 M PLL solution (in monomer units) into a vigorously shaken HA solution (at 2 × 10−3 M in monomer units) undergo a slow sedimentation process as seen by eye and by following the absorption change at 500 nm in a quartz cell with 1 cm optical path. The same holds true for the stoichiometric PDADMAC−PSS and PAH−PSS complexes at all the investigated salt concentrations. In the particular case of PLL−HA complexes, the absorbance at 500 nm, called turbidity, because it is due to light scattering and not to specific absorption of the used polyelectrolytes, first increases rapidly, reaches a plateau value before decreasing again in a much slower manner (Figure 1). The regime corresponding to a decrease of the absorbance versus time (regime 3 in Figure 1A) is indeed an exponential decrease (Figure 1B). This allows the extraction of a characteristic time from slopes as those shown in Figure 1B. Such a behavior was found for all experiments performed on the PLL−HA system, at different ionic strengths in NaCl and in the presence of different salts (see Supporting Information, Figures SI1 and SI2; however, at 300 mM in NaCl concentration, the fit is of somewhat lower quality with r2 = 0.964). The fact that the absorbance decreases, related to the observation of a sediment at the bottom of the quartz container, is due to a decrease of the concentration of PLL− HA aggregates in the cylindrical region probed by the light beam of the spectrometer. Such a decrease in concentration is related to sedimentation. If no sedimentation was observed, the decrease in absorbance could also be due to a reduction in the size of the PLL−HA complexes. The slope of curves like those displayed in Figure 1B and Supporting Information, Figures SI1 and SI2, give hence an indirect hint on the sedimentation rate of the polyelectrolyte complexes. When similar experiments are performed in beakers of 5.5 cm in radius into which silicon slides have been deposited before the addition of the solution containing stoichiometric PLL−HA complexes, one observes a linear increase in the coating thickness with time (Figure 2). The same holds true even for coatings exceeding 200 nm in thickness as observed by means of scanning electron microscopy (Figure 3). In addition, the obtained coatings seem to be homogeneous. This was further confirmed by means of AFM measurements in the contact mode: the root mean squared (rms) roughness over a 10 μm × 10 μm square was found to be equal to 25 nm for a film obtained after 24 h of sedimentation from a 0.15 M NaCl solution (Figure SI4 of the Supporting Information). Such a film had an average thickness of 1.5 μm. The obtained rms roughness of these films is nevertheless somewhat larger than the 2.1 nm obtained on sprayed PEI−(PLL−HA−PLL)8 films25 having about the same thickness as those of the sedimented films. This difference in

Figure 1. (A) Evolution of the absorbance at λ = 500 nm as a function of time for stoichiometric PLL−HA complexes in the presence of 0.15 M NaCl. Regions 1, 2, and 3 correspond to the increasing part, the region of maximal absorbance and the decreasing part of the curve, respectively. (B) Logarithm of the absorbance at λ = 500 nm as a function of time for regime 3 of part A. The blue line corresponds to a linear regression to the experimental data and has a slope of 2.1 × 10−5 s−1.

roughness might well be the result of the presence of traces of PLL−HA complexes in the final film morphology. For greater sedimentation times, the slope of the thickness increase versus time has to be measured by SEM because of the periodicity of the ψ(d) and Δ(d) functions in ellipsometry. For films having a refractive index of 1.456 with an angle of incidence of 70° and a wavelength of 632.8 nm, ellipsometry is not able to distinguish changes in film thickness of integer multiples of 280 nm. The SEM data yield a thickness increase of 61 nm·h−1, a value consistent with the 42 nm·h−1 obtained by ellipsometry for films of small thickness (Figure 2). As an interesting observation, a crust of more dense film about 100 nm in thickness is apparent on the upper part of films made from PLL−HA complexes after longer sedimentation times (Figure 3). This may well be due to a progressive reorganization of the film and is worth being considered in future investigations. During the sedimentation process, the upper part of the solutions becomes more transparent (Supporting Information, Figure SI3). At the end of the sedimentation process, the solution above the substrates is transparent and the bottom of the beaker is covered by a coating in the regions not already covered by the silicon substrates. In the case of the deposition of PLL−HA films from NaCl solutions at 0.15 M, we find that 5694



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sediment spontaneously have been removed from solution. The solution may still contain a small amount of very tiny polyelectrolyte complexes whose sedimentation rate is close to zero. After 42 h of sedimentation on a disk of 5.5 cm in radius, we found a film thickness of 3.0 ± 0.1 μm (Figure 3). We make the assumption that the thickness of the sedimented coating is the same on the silicon slides lying flat on the bottom of the beaker as well as on the uncovered part of the beaker. The volume of the film is hence of 0.028 cm3, whereas the volume of the polyelectrolyte complexes present in solution was of 0.054 cm 3 . For this last calculation we assume the polyelectrolyte complexes to have a mass density of 1.2 g·cm−3, which is a consistent value for a material containing a polypeptide and a polysaccharide. This means that an underestimation of the deposition yield, the ratio between the volume of the film, and the volume of the polyelectrolyte complexes initially present in solution, is of 52%. This calculation relies on the fact that the porosity of the film is the same as that in the polyelectrolyte complexes, which may be only a very crude approximation. Indeed we cannot exclude that the film undergoes a kind of densification during its deposition due to structural changes of the polyelectrolyte chains or due to a possible coalescence of the polyelectrolyte complexes. Such a coalescence process would lead to the disappearance of some pores inside of the film. Gravitational effects have to be totally excluded to explain such a film densification: a film volume of 1 μm3 would apply a

Figure 2. Thickness versus time for coatings obtained by sedimentation of stoichiometric PLL−HA complexes (10−3 M in monomer units for each polyelectrolyte) in the presence of 0.15 M NaCl. Each point corresponds to an individual silicon slide put in contact with 200 mL of a PLL−HA complex solution. The straight line corresponds to a fit to the experimental data with a slope of 42 nm·h−1 and the dotted lines to the limits of the 95% confidence interval.

the solution turbidity is close to 0 after 42 h of sedimentation meaning that almost all polyelectrolyte complexes that

Figure 3. Scanning electron microscopy cross sections of coatings obtained by sedimentation of stoichiometric PLL−HA complexes in the presence of 0.15 M NaCl. Note that on the thickest films a kind of crust (marked with red dots) around 100 nm thick is apparent. Each image corresponds to an individual silicon slide put in contact with 200 mL of a PLL−HA complex solution. The arrows on the film obtained after 24 h of sedimentation correspond to a thickness of 1.50, 1.33, and 1.70 μm from the left to the right. The lower part corresponds to the thickness change versus sedimentation time. The linear regression (line) was performed for the data points marked with (○), whereas the film growth stopped after about 48 h (●). 5695



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gravitational force of about 1.2 × 10−14 N on its lower part (assuming again a film density of 1.2 g·cm−3) which is negligible in comparison to the random forces due to Brownian motion (∼10−12 N). The high value for the deposition yield is remarkable because it shows that an important fraction of the polyelectrolytes initially present in the solution can be deposited on the surface, however, at the cost of a long sedimentation time of 42 h. Note however that the sedimentation process corresponds to a onestep deposition method. For all the investigated salts, the deposition rate of the films is directly related to the sedimentation rate (Figure 4) of the complexes as determined by turbidimetry measurements (Figure 1).

Table 1. Hydrodynamic Diameters of the PLL−HA and PDADMAC−PSS Stoichiometric Complexes Prepared at Different Ionic Strengths and by Changing the Nature of the Supporting Electrolyte

PLL−HA PLL−HA PLL−HA PLL−HA PLL−HA PLL−HA PDADMAC− PSS

salt concentration and nature of the salt

hydrodynamic diameter (nm)

sedimentation rate of the complexes (10−5 s−1) as measured by UV−vis spectroscopy

NaCl 0.10 M NaCl 0.15 M NaCl 0.2 M NaCl 0.3 M NaClO4 0.1 M NaBr 0.1 M NaCl 0.1 M

1800 ± 150 1600 ± 200 1900 + 150 2400 ± 400 180 ± 50 260 ± 60 640 ± 60

2.35 2.10 2.40 7.3 0.05 0.59 0.37

The obtained films allow for the diffusion of PLL−FITC (Figure 5) as found by means of UV−visible spectroscopy. But

Figure 4. Relationship between the film deposition rate, as measured by SEM cross sections (see Figure 3) and the sedimentation rate of the corresponding complexes, as measured by UV−vis spectroscopy (see Figure 1) for PLL−HA complexes at 0.1 M in ionic strength (○), and 0.15 M in ionic strength (■). The red line is aimed to guide the eye but does not correspond to a fit.

Figure 5. UV−visible spectra of films obtained through the sedimentation of stoichiometric PLL−HA complexes during 24 h in the presence of NaCl 0.1 M and subsequently put in the presence of a PLL−FITC solution (1 mg·mL−1) in the presence of NaCl 0.1 M for various time durations, 1, 2, 40, and 48 h, as indicated by the vertical arrow.

As an interesting finding, the sedimentation and the film growth rates seem to be lower when the experiments are performed in the presence of chaotropic salts (NaClO4, NaBr) along the Hofmeister series. This goes in the opposite direction to the thickness evolution of polyelectrolyte multilayer films for which an increase in the chaotropic character of the anion in the electrolyte solution translates in higher film thickness.26,27 The reason why the sedimentation as well as the film deposition rates are smaller in the case of experiments performed in the presence of 0.10 M NaClO4 with respect to 0.10 M NaCl can be easily explained by the size of the stoichiometric PLL−HA complexes: their mean hydrodynamic radius is of (900 ± 120) nm and (1600 ± 200) nm in the presence of NaClO4 and NaCl, respectively (see Table 1). A change in size should be reflected in a change in the sedimentation rate and hence in the film growth. We make the assumption that ions play a different role at the film solution interface than for polyelectrolytes in solution. It is likely that more counterions are released upon complex formation in solution than upon interaction of a polyelectrolyte in solution with an already adsorbed one due to conformational restrictions imposed by adsorbed chains. This remains to be investigated in future investigations.

contrarily to (PLL−HA)n films obtained through alternated immersions in the PLL and HA solutions, the process was slow, needing at least 40 h to reach a steady state. On the contrary, for the (PLL−HA)n films of similar thickness (1−2 μm), the diffusion process of PLL−FITC through the whole thickness of the film is achieved in less than 2−3 min.22 Hence our experiments suggest that the films obtained through the sedimentation of PLL−HA complexes are either more dense or less porous than their (PLL−HA)n counterparts. In addition we cannot exclude that the mobility of the PLL chains in the films obtained through the sedimentation of PLL−HA complexes is much lower than that in the films obtained through the alternated deposition of PLL and HA, in which a high chain mobility has been measured by means of fluorescence recovery after photobleaching.28 The presence of a dense crust on the upper part of the sedimented films PLL−HA films (Figure 3) may also slow down the penetration of PLL−FITC in those films. Owing to the slow increase in the absorption band at 515 nm with time, we cannot exclude that PLL−FITC penetrates in the film through an exchange process29 rather than by diffusion. The same kinds of films made by sedimentation of PLL−HA complexes remain stable when put in the presence of a 5 mM 5696



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TibisLac solution in the presence of NaCl at 0.1 M. A progressive increase in absorbance between 200 and 365 nm was found (Figure 6). The edge in this absorption band

Figure 7. Thickness versus time for coatings obtained by sedimentation of stoichiometric PDADMAC−PSS complexes in the presence of 2.0 M NaCl. Each point corresponds to an individual silicon slide put in contact with 200 mL of a PDADMAC−PSS complex solution. The straight line corresponds to a fit to the experimental data with a slope of 6.5 nm·h−1 and the dotted lines to the limits of the 95% confidence interval.

Figure 6. Absorption spectra of a PLL−HA film made by sedimentation (24 h in the presence of 0.1 M NaCl containing stoichiometric complexes) and put in contact with a 5 mM TiBisLac solution also in the presence of 0.1 M NaCl.

the slope of the turbidity change versus time in regime 3 (see Figure 1) is only of 3.7 × 10−6 s−1, whereas it was of 2.1 × 10−5 s−1 in the case of PLL−HA at 0.15 M in NaCl. The smaller apparent sedimentation rate of the PDADMAC−PSS complexes compared to the PLL−HA complexes is due to their smaller size: about 600−700 nm and a small fraction of micrometer-sized aggregates, whereas the PLL−HA complexes are (1600 ± 200) nm in hydrodynamic diameter. The relative sedimentation rate of spherical particles having the same mass density in a fluid of constant viscosity, should scale as the ratio of their squared radii. The radius of HA-PLL complexes is about 2−2.5 times higher than the radius of the PDADMAC−PSS complexes. One would hence expect the former complexes to sediment about 4 to 6.25 times faster than the latter. The ratio of the experimental apparent sedimentation rates is of 21/3.7, namely 5.7, which is in the range of the expected value. Considering the very crude approximation made here (mostly that the complexes are spherical) our result is quite satisfactory and suggests once more that the sedimentation rate of the complexes, and hence the growth rate of the deposited film, is in direct relationship with the size of the complexes. Coming back to the PLL/HA complexes, it appears that for all the used electrolytes and all the salt concentrations investigated (experimental values from Table 1), there is good correlation between the relative sedimentation rate and the relative size of the complexes (Figure SI6 of the Supporting Information). Nevertheless, more quantitative measurement need to be performed to obtain quantitative sedimentation rates of polyelectrolyte complexes using analytical ultracentrifigation.30 There are polyelectrolyte combinations, as the PAH−PSS one, which undergo sedimentation but do not form homogeneous coatings but rather snowflake-like deposits (Figure 8) even when the stoichiometric complexes are prepared from solutions of high salt concentration (above 2.0 M in NaCl). This seems to be related to the absence of coalescence between the sedimented complexes.

corresponds to the band gap of TiO2 (3.2 eV). The process seems to reach saturation after about 1 h. Further evidence for the formation of TiO2 was obtained by means of ATR-FTIR spectroscopy in a previous paper.23 This result means that some amino groups of PLL, which activate the hydrolysis and polycondensation of TiBisLac remain accessible to this anion, as in (PLL−HA)n films obtained by the SBS deposition method.23 Taken together, the results shown in Figures 5 and 6 show that the PLL−HA films obtained through sedimentation of complexes differ from the (PLL−HA)n films by the fact that PLL−FITC penetrates in the former films more slowly than in the later ones. However the diffusion and reactivity of smaller species like TiBisLac seems not to be altered by the film preparation method. A close look at the SEM micrographs of Figure 3 suggests that the PLL−HA film obtained through sedimentation is made from fused aggregates. These aggregates correspond most probably to the shape of the complexes participating to the sedimentation process. Such a morphology is not observed for (PLL−HA)n films prepared in an SBS manner (see ref 25 for example). This finding suggests that coalescence of the complexes seem to play an important role to obtain an almost homogeneous coating as those observed in Figure 3. But this assertion remains to be demonstrated in further studies. This concept can be extended to other polyelectrolyte combinations, for instance to the PDADMAC−PSS complexes, provided the salt concentration in NaCl is higher than 1.0 M. Figure 7 shows the thickness evolution of a PDADMAC−PSS film prepared by sedimentation from a solution containing stoichiometric amounts in monomer units of each polyelectrolyte (10−3 M) in the presence of 2.0 M NaCl. It appears that the film growth rate, 6.5 nm·h−1 is much smaller than for the PLL−HA system (42−60 nm·h−1 in the presence of NaCl 0.15 M). This seems to be related to the much smaller apparent sedimentation rate of the PDADMPACPSS complexes (Supporting Information, Figure SI5). Indeed 5697



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made from the sedimentation of stoichiometric PLL−HA complexes by using fluorescence recovery after photobleaching. Even if the film preparation method presented in this study is limited to flat substrates, it may find industrial applications when coupled to roll-to-roll deposition processes.

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ASSOCIATED CONTENT

S Supporting Information *

Evolution of the absorbance at λ = 500 nm as a function of time for stoichiometric PLL−HA complexes in the presence of 0.2 M NaCl and 0.3 M; optical pictures of erlenmeyer flasks containing a stoichiometric mixture of PLL and HA (at 10−3 M each) in the presence of NaCl at 0.15 M after 6 and 12 h; AFM topographies of PLL−HA films obtained after 24 h of sedimentation of stoichiometric complexes prepared in the presence of 0.15 M NaCl; evolution of the absorbance at λ = 500 nm as a function of time for stoichiometric PDADMAC− PSS complexes in the presence of 2.0 M NaCl. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Optical picture of the snowflake-like sediment obtained at the bottom of the erlenmeyer flask at the end of the sedimentation process of stoichiometric PAH−PSS complexes. The concentration of each polyelectrolyte was of 10−3 M in monomer units in the presence of 3 M NaCl. The sedimentation time was of 24 h.

Owing to the results obtained for the PLL−HA complexes and the knowledge that PLL has a high diffusion coefficient in (PLL−HA)n films, we make the assumption that intermixing between the sedimented complexes is required to obtain a homogeneous coating during the sedimentation process. This intermixing should be directly related to chain mobility.31 We make hence the assumption that only polyelectrolytes leading to an exponential growth process during their step-by-step deposition can be used to obtain homogeneous films by sedimentation of their stoichiometric complexes. This assumption has to be checked in future investigations. Fortunately, the attainment of exponential growth seems to be more related to experimental parameters like ionic strength, pH, and temperature than to the intrinsic chemistry of the monomer units.32

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the ″MacroLBL″ project from the CRP Henri Tudor.

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REFERENCES

(1) Decher, G. Fuzzy nanoassemblies: Toward layered polymeric multicomposites. Science 1997, 277, 1232. (2) Lavalle, P.; Voegel, J.-C.; Vautier, D.; Senger, B.; Schaaf, P.; Ball, V. Dynamic aspects of films prepared by a sequential deposition of species: Perspectives for smart and responsive materials. Adv. Mater. 2011, 23, 1191. (3) Schlenoff, J. B.; Dubas, S. T.; Fahrat, T. Sprayed polyelectrolyte multilayers. Langmuir 2000, 16, 9968. (4) Izquierdo, A.; Ono, S. S.; Voegel, J.-C.; Schaaf, P.; Decher, G. Dipping versus spraying: Exploring the deposition conditions for speeding up layer-by-layer assembly. Langmuir 2005, 21, 7558. (5) Porcel, C. H.; Izquierdo, A.; Ball, V.; Decher, G.; Voegel, J.-C.; Schaaf, P. Ultrathin coatings and (poly(glutamic) acid/poly(allylamine)) films deposited by continuous and simultaneous spraying. Langmuir 2005, 21, 802. (6) Lefort, M.; Popa, G.; Seyrek, E.; Szamocki, R.; Felix, O.; Hemmerlé, J.; Vidal, L.; Voegel, J.-C.; Boulmedais, F.; Decher, G.; Schaaf, P. Spray on organic/inorganic films: A general method for the formation of functional nano to microscale coatings. Angew. Chem., Int. Ed. 2010, 49, 10110. (7) Chiarelli, P. A.; Johal, M. S.; Holmes, D. J.; Casson, J. L.; Robinson, J. M.; Wang, H. L. Polyelectrolyte spin-assembly. Langmuir 2002, 18, 168. (8) Zhang, X..; Chen, H.; Zhang, H. Layer-by-layer assembly: From conventional to unconventional methods. Chem. Commun. 2007, 1395. (9) Seo, J.; Lutkenhaus, J. L.; Kim, J.; Hammond, P. T.; Char, K. Effect of the layer-by-layer (LbL) deposition method on the surface morphology and wetting behavior of hydrophobically modified PEO and PAA LbL films. Langmuir 2008, 24, 7995. (10) Rydzek, G.; Jierry, L.; Parat, A.; Thomann, J. S.; Voegel, J.-C.; Senger, B.; Hemmerlé, J.; Ponche, A.; Frisch, B.; Schaaf, P.; Boulmedais, F. Electrochemically triggered assembly of films: A onepot morphogen-driven buildup. Angew. Chem., Int. Ed. 2011, 50, 4374.

CONCLUSIONS We demonstrate that it is possible to obtain continuous and homogeneous films from the sedimentation of stoichiometric polyelectrolyte complexes provided the sedimented complexes undergo possible coalescence; otherwise, the coating is snowflake-like. In all cases the growth rate of the deposit correlates well with the sedimentation rate of the polyelectrolyte complexes which is determined by their size (providing their density is constant). This parameter in turn can be determined by the physicochemical parameters (for instance the ionic strength or the nature of the used electrolyte) of the solution in which the polyelectrolyte complexes are prepared. In the case of the PLL−HA complexes a high yield of film deposition can be obtained, however, at the prize of a long sedimentation time. These coatings allow for the incorporation of fluorescently labeled PLL and can be used as templates for the polycondensation of TiO2 from a solution of Ti(IV) (bisammonium lactato) dihydroxyde, as in the (PLL−HA)n SBS films. The films obtained by sedimentation of the polyelectrolyte complexes can hence be an interesting alternative to the films prepared from the same polyelectrolytes but in a step-by-step (SBS) deposition sequence. However the films obtained by sedimentation will never present a remaining stratification as those obtained by an SBS manner. In the future, it will be mandatory to speed up such a kind of “one-step” deposition method. We are currently investigating the film growth from stoichiometric polyelectrolyte complexes on porous membranes by a single filtration process. We plan also to investigate the mobility of PLL−FITC chains in films 5698



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The Possibility of Obtaining Films by Single Sedimentation of

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