EP2557570A1

EP2557570A1 - Polyelectrolyte films and their preparation

Info

Publication number: EP2557570A1
Application number: EP11176851A
Authority: EP
Inventors: Vincent Ball
Original assignee: Public Research Centre Henri Tudor
Current assignee: Public Research Centre Henri Tudor
Priority date: 2011-08-08
Filing date: 2011-08-08
Publication date: 2013-02-13

Abstract

Polyelectrolyte films can be prepared using a turbid solution of an admixture of a polyanion and a polycation in a solution of supporting electrolyte, and allowing a complex of the polyelectrolytes to be deposited on a substrate.

Description

The present invention relates to polyelectrolyte films and processes for their preparation.
The functionalisation of surfaces using polyelectrolyte multilayer films (PEMs) has emerged as a versatile method to control the thickness of the obtained deposits far above the level of a monolayer [1]. For most PEM systems, the thickness of the obtained films is proportional to the number of deposition cycles, where one deposition cycle consists in the adsorption of one polycationic layer and one polyanionic layer. The accompanying Fig. A illustrates the principle of layer-by-layer (LBL) deposition in the formation of PEM films. Typically, each adsorption step (1) and (3) is separated from the next by a buffer rinse in order to remove weakly adsorbed polyelectrolytes.
PEMs can be used in many fields. For example, they may be used as: matrix materials for enzymes and proteins in sensor applications; as a matrix for active components in solar cells; as a coating for protecting and controlling the healing process of damaged arteries; as permeable membranes for nanofiltration, gas separation, and fuel cells; in the fabrication of non-linear optical materials, coloured electrochromic electrodes (future display devices), and to tailor the properties of photonic crystals; analyte separation processes (chromatography), and the fabrication of thin-walled hollow micro- and nanocapsules.
More recently, it has been discovered that certain polyelectrolyte combinations lead to supralinear growth of the films [2-5]. In such films, the thickness and the surface concentration of the adsorbed material increase faster than linear. In most of these cases, the growth regime is of an exponential nature, before settling back to linear growth once a critical thickness has been reached. In this supralinear regime, the thickness increment per "layer pair" is much higher than for regular "linear growth".
The fact that PEM films can grow in different regimes is related to the interaction strength of the polycations and the polyanions involved. Depending on the physicochemical conditions of the solution, including pH, in the case of weak polyelectrolytes, ionic strength, and temperature, given combinations of polycation and polyanion can lead to either linear or "exponential" growth. The occurrence of either a linear or "exponential" growth regime can be predicted for many systems on the basis of microcalorimetric measurements [6], and it has been found that linear growth is associated with an exothermic reaction enthalpy between the polycation and the polyanion, whereas an endothermic interaction leads to "exponential" growth [6].
If the interactions between the polycations and polyanions are too weak, for instance where the charge density is too low or the ionic strength of the solution is too high, no film deposition is observed.
The fact that PEM films can grow in a "supralinear" manner has important consequences, not only in the reduction of processing time, but also in allowing the manufacture of films with different properties. These differences have been reviewed recently [7]. Films characterised by linear growth have local stratification reminiscent of the deposition process. These films are electrically neutral, without the incorporation of counteranions from the solution, and have an elastic modulus that is high for purely polymeric materials. On the other hand, those films characterised by exponential growth are not stratified, and the polyelectrolytes are intermixed. Thus, it appears that it is the high mobility of at least one of the counterparts in the bulk of the films which is responsible for the exponential growth regime [8].
Despite automated dipping machines having been commercially available for at least 10 years, the build-up of PEM films by the alternated dipping method is long and time consuming even in supralinear growth systems. Therefore, alternatives to LBL have been sought, and include alternated spin coating [9] and spray coating [10]. It appears that for a given combination of interacting polymers, the deposition method has a marked influence on the properties of the obtained coatings. Using either alternated dip coating or spin coating, even the growth regime of the film can be modified: linear growth in the case of spin coating and exponential growth in the case of dip coating [11]. Simultaneous spray deposition leads to films whose thickness increases proportionally with spraying time [12].
Spin and spray deposition have the advantage of allowing a considerable increase in the deposition speed: a layer can be deposited in a few seconds, whereas several minutes are usually needed to reach steady state in LBL.
The drawback to spin coating and spray coating is that large amounts of polyelectrolyte are lost either through centrifugal or gravitational forces (drainage). Levels of loss are generally unacceptable, especially where coatings are produced from rare or costly ingredients.
There remains considerable interest in the properties of PEM films, and many groups have investigated how the two polyelectrolytes interact in solution, in order to better understand the deposition process. This is particularly so, as there are many conditions under which a polyelectrolyte layer can be deposited, only for exposure to the subsequent polyelectrolyte solution to erode the previous layer, or strip it away completely.
Investigations have revealed that, in general, there is a strong relationship between the interaction of two oppositely charged polyelectrolytes in solution and the conditions necessary for each to successfully deposit films using the LBL technique [c.f. 13,14]. Turbidity in a solution of a polyelectrolyte pair can be taken as an indicator that, if corresponding parameters are applied to the deposition solutions, then LBL layers can successfully be built up without either of the polyelectrolyte solutions re-dissolving a previous layer.
Porcel and Schlenoff [15] disclose the preparation of PE solids using ultracentrifugation and high salt concentration. This process only works in ultracentrifugation tubes, is inconvenient, and it is difficult to control the product obtained. PEM films have not been prepared in this way.
There remains a need to find a deposition method for polyelectrolytes that allows a high deposition yield. There is also a need to find a simplified deposition method for polyelectrolytes.
We have now surprisingly found that it is possible to substantially simplify the creation of polyelectrolyte films by using a coating mixture comprising both polyelectrolytes. This process enables the production of novel films, and it is possible to carry out the process in a one pot reaction.
Thus, in a first aspect, the present invention provides a process for the preparation of a polyelectrolyte film on a substrate, comprising exposing the substrate to an admixture of a polyanion and a polycation in a solution of supporting electrolyte, wherein the polyanion and polycation are provided in sufficient quantity to render the solution of electrolyte turbid to the naked eye, and allowing precipitation from the admixture to occur on the substrate.
The substrate may be anything upon which it is desired to create a polyelectrolyte film, and may be an item or object on which it is desired to have a PE surface, or may be a substrate that can be dissociated from the film to yield the film in isolation.
The film is not similar to LBL films, as it is not created by individual layers of oppositely charged polyelectrolytes. Instead, the at least two polyelectrolytes are allowed to interact in solution, suspension, or colloidal suspension, and are deposited on the substrate in complexed, or associated, form. Without being bound by theory, it seems that, provided that the associated PE complexes do not agglomerate to any great degree before precipitating or depositing on the substrate, then the precipitated PE complexes can further interact with other complexes already present on the substrate, such that any one polyelectrolyte molecule can associate with two or more polyelectrolytes of opposite charge, owing to the numerous charges present on each. Where the interaction is too strong, PE molecules can agglomerate before precipitating, and the resulting flakes are not able to intimately interact to form a cohesive film. With strong interaction, individual pairs of PE molecules may strongly cohere and not interact with any other molecules, with the result that the solution may not even become turbid, and no deposition is seen.
A suitable method to judge whether the interaction is of sufficient strength for the process of the present invention is to assay the formation of turbidity in increasing strength of support electrolyte. If turbidity is still observed at concentrations of support electrolyte above about 2 M, then the interaction between the selected polyelectrolytes is likely to be too strong. It is likely, that the interaction is too strong if turbidity is observed at greater than 1 M concentration of support electrolyte.
The support electrolyte is preferably an inorganic salt of an alkali, or alkaline earth, metal with a dissociative anion, such as a halogen, or an anion derived from a suitable acid, such as mineral acid. It has been established that films form more slowly with increasing complexity of the support electrolyte, so that NaCl encourages faster film formation than NaClO₄, for example.
Increasing concentration of support electrolyte is also associated with slower film formation, with concentrations in excess of about 0.5 M often inhibiting film formation altogether.
If the solution of support electrolyte containing the admixture does not turn sufficiently turbid to be detected by the naked eye, then deposition of a film will not occur. This does not necessarily mean that the constituents of the admixture are not able to form turbid preparations, but that conditions may need to be altered. Concentration of support electrolyte is preferably between 0.05 M and 0.6 M, preferably between 0.1 M and 0.3 M. It is preferred that the polyelectrolytes are present in stoichiometric amounts by comparison with each other, in that their charges substantially balance out. Deviations are possible, but substantial deviations tend to result in a failure to render the solution turbid.
The amount of polyelectrolyte is not critical to the present invention. Preferably, the polyelectrolytes are first prepared as separate solutions, suspensions, or colloids, and are then blended, preferably by gradual introduction of one PE into the other with agitation. Any other constituents may be present in one or both of the preparations, or may be introduced subsequently. It is preferred that the substrate is exposed to the solution containing the admixture as soon as turbidity is observed.
Rather than rely on attraction of one PE to the other to form layers, a single layer is formed under the influence of gravity. The resulting layer is homogeneous, and requires no external power source. By controlling exposure time, the thickness of the resulting film can be controlled. In addition, as it is no longer necessary to dip objects in a variation of the LBL process, there is no limit on the size of object that can be used as substrate.
Thus, it is an advantage of the process of the present invention that no external energy input is required.
It is a further advantage that natural gravitational forces can be used in the deposition of the polyelectrolyte film.
It is a yet further advantage that the process of the present invention permits the coating of substrates having large surface areas, limited only by manufacturing considerations, which is not feasible with conventional LBL coating methods.
The process of the present invention is inherently economical, and is capable of providing high deposition yield, in that the majority of polymers used in the process eventually form a part of a PEM coat. Usage rates of >75% are possible, preferably >90%, and usage rates of >95% are readily achievable.
By using an admixture of polyelectrolytes in turbid solution, the problem of re-dissolving the immediate previous layer is avoided.
It is preferred that the amounts of the polyanion and polycation are in stoichiometric relationship each with the other, such that the charges carried by the charged molecules balance each other out.
Buffers have been found to be advantageous at low concentrations, preferably at levels below 0.01 M. A Tris (Tris(hydroxymethyl) amino methane) buffer of between 0.001M and 0.005 M is preferred.
The process of the present invention does not use the polyelectrolytes separately, but together in admixture. Such admixtures are also referred to herein as "complexes". The terms "complex" and "complexes" as used herein indicate that the two or more polyelectrolytes have an interaction with at least one other polyelectrolyte present on the basis of having opposite charges, but do not necessarily form complexes in the commonly accepted sense.
When the interactions between the two oppositely charged polyelectrolytes are sufficiently strong, phase separation occurs in the blend.
Once sufficient quantity or density of complexes form, then sedimentation occurs, leading to the appearance of two phases, wherein the upper phase, or supernatant, is clear and poor in polyelectrolytes, and the lower phase, or precipitate, is rich in polyelectrolyte complexes.
The precipitate generally takes either the form of flakes, or of a carpet in which no individual particles are visible. Admixtures that are capable of forming homogeneous layers, or carpets, are particularly useful in the present invention, as it has been found that these layers are capable of forming an integral, or cohesive, film on the substrate onto which they precipitate. Such cohesive films may be grown to sufficient thickness to be separated from the substrate. Flakes will not generally coalesce to form an integral film, and it is preferred to avoid the appearance of flakes in the precipitate. The difference generally lies in the rate of precipitation, and admixtures that form flaky precipitates at higher concentrations or temperatures may precipitate homogeneous layers under less conducive conditions, such as reduced temperature, lower concentrations of polyelectrolytes, or higher ionic strength of supporting electrolyte. In general, if the sedimentation rate is low, then the interaction between the particles is such that they coalesce in the condensed phase to form a continuous film of the present invention.
Without being bound by theory, the thickness increase of the obtained film with time appears to be in direct proportion to the sedimentation rate measured in solution. The rate can be changed by various means as will be apparent to those skilled in the art, including modifying temperature, altering concentration of the admixture, changing the ionic strength of the solution, or by modifying the nature of the supporting electrolyte at a given ionic strength.
Thus, a preparation of one of a polycation and polyanion is added to a preparation of the oppositely charged polyelectrolyte, thereby to form a turbid solution in a suitable receptacle. Substrates placed in the bottom of the receptacle will then be coated by sedimentation of polyelectrolyte complexes. Coated substrates can be removed and substituted as desired for as long as sedimentation continues to occur, which can continue for weeks, such as between about 40 and 100 days, although the duration of precipitation will depend on such factors as speed of sedimentation and concentration of the polyelectrolytes. Other factors will be readily apparent to the skilled person.
Although it is possible to encourage deposition by enhancing gravity, such as by centrifuge, it is preferred to allow the PE layer to form by gravity. This allows for controllable growth rates, in the range of a few nm per hours, up to 100 nm per hour or more, depending on the polyelectrolyte combination and the ionic strength used. It is advantageous that such methods require no energy source, and that macroscopic collectors can be used.
The process of the present invention is particularly suitable for use with polyanions wherein carboxylate is the predominant ionisable function, while suitable polycations preferably have an amine, such as a primary, secondary, tertiary or quaternary amine, as the predominant ionisable function. Polyanions having sulfate or sulfonate groups as the predominant function are less preferred, as these molecules tend to interact strongly with many polycations, and have a greater tendency to form flakes.
It is a particular advantage that the present invention provides means to be able to obtain films of controlled thickness, preferably in a single processing step, such that formation of the totality of the film is a one-step reaction, this being a preferred embodiment.
A particularly preferred polycation is poly(-L-lysine hydrobromide).
A particularly preferred polyanion is sodium hyaluronate.
HA/PLL films obtained through sedimentation and which contain amino groups, such as from PLL, and carboxylic groups, such as from HA, can easily be crosslinked with thermal treatment, such as 120 °C for 12 h, to form amide bonds. Such crosslinked films can then be readily detached from their substrate, such as a glass plate or a silicon wafer, typically by cutting the edges of the film and dissolving the substrate, such as by slow immersion in a 2 % HF solution, thereby to obtain a free standing membrane.
Situations where the sedimented complexes do not coalesce to make a film can occur, such as with complexes made from poly(allylamine hydrochloride) and poly(sodium-4-styrene sulfonate). In this case no film formation could be obtained when the NaCl concentration of the supporting electrolyte was varied from 10 mM up to 5 M. In this instance deposition occurred in flakes. However, if poly(diallyl dimethyl ammonium) chloride is used in place of poly(allylamine hydrochloride), then it is possible to generate a turbid solution that deposits a PE layer in accordance with the present invention. Without being bound by theory, it is believed this is because the ammonium ion is less accessible, and so does not interact as strongly with the sulfonate, thereby allowing a slower deposition to form a film.
Admixtures of the present invention form turbid solutions in the presence of a supporting electrolyte, such as NaCl. A maximum turbidity is reached at a defined concentration of supporting electrolyte before falling off with higher ionic concentration of the supporting electrolyte. In situations where turbidity continues to increase with higher ionic concentration, then interaction between the polyelectrolytes is too strong to be able to successfully deposit a stable PEM film.
A "turbidity diagram", such as detailed in reference [14], may be used by the skilled person, wherein the effect of changing parameters can be measured against film properties. Suitable parameters include different ionic strengths of the chosen electrolyte and "stoichiometric complexes" of the chosen polyelectrolytes, with turbidity being measured immediately after polyelectrolyte mixing to form the complexes. If the "turbidity diagram" displays a maximum at a given ionic strength, followed by a drop off with increasing ionic strength, as is the case for the HA/PLL combination, then the formation of polyelectrolyte films by spontaneous sedimentation is possible. In addition, film growth rate will typically be maximal at the ionic strength where the initial turbidity is maximal.
When the "turbidity diagram" shows no maximum, and turbidity increases in a continuous manner with ionic strength, then there is typically no chance to obtain a polyelectrolyte containing film by sedimentation of the complexes made with such a combination of polyelectrolytes. This is typical of PSS/PAH combinations, because of overly strong interactions between the polyelectrolytes which cannot be diluted or swamped by the supporting electrolyte in any concentration. In admixtures of the present invention, and without being bound by theory, it appears that, above a certain level, the ionic strength increase prevents or reduces intermolecular interaction, thereby preventing the coalescence and sedimentation of the polyelectrolyte complexes, but that for those where increase in ionic concentration has no effect because of the strength of the interaction between the two polyelectrolytes, flakes typically form.
In a preferred embodiment, spontaneous deposition of polyelectrolyte complexes from solutions containing substantially equimolar amounts of both cationic (such as from poly(-L-lysine)) and anionic groups (such as from hyaluronic acid) allows the formation of polyelectrolyte containing films in a one pot manner, without the input of external energy, by relying simply on the spontaneous sedimentation of the polyelectrolyte complexes. Observations suggest that film growth is in direct relationship with the sedimentation rate measured in solution, and that film growth rate can be adjusted by changing the ionic strength and/or the nature of the supporting electrolyte in solution. Observations also suggest that deposition rate depends on the nature of the chosen polycation/polyanion pair.
For untested polycation/polyanion pairs it is possible to predict if sedimentation assisted deposition is possible by establishing a "turbidity diagram", as described above.
The deposition yield of such films can be very high, and this is a major advantage when compared to the deposition of "layer by layer" films by alternated spray or alternated spin deposition.

II. Materials and Methods.

Chemicals.

All the solutions were made from distilled and deionised water (Milli Q Plus, Millipore, Billerica, MA, USA).

Polyelectrolytes:

Polyanions

Hyaluronic acid (HA, viscosimetric molecular weight MW_vis = 4.2 x 10⁵ g.mol^-1, LifecoreBiomedical, Chaska, MN, USA).
Poly(sodium 4-styrene sulfonate) (PSS, M_w = 7.0 x 10⁴ g/mol, Aldrich) as the polyanions.

Polycations

Poly-L-lysine (PLL, ref. P7890, lot 066K5101, M_w=26500 g.mol^-1, Sigma-Aldrich, St. Louis, MA, USA).
Poly(diallyl dimethyl aminomethane) (PDADMAC, M_w = 9.36 x 10⁵ g/mol, Aldrich).
All salts were purchased from Sigma-Aldrich (at least 99% in purity) and used without further purification.

Substrates

The silicon substrates used in film characterisation, 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. The polycation containing solution was admixed with the polyanion solution in a dropwise manner and under strong agitation (300 rpm) in an Erlenmeyer conical glass, and agitation continued for 1 min. All experiments were performed under conditions wherein the ratio between the number of cationic monomers and the number on anionic monomers was substantially equal to one. While such conditions generally allow the most favourable phase separation [16], it will be appreciated that the invention contemplates other ratios.
The solution containing the mixture of polyelectrolyte complexes was then poured slowly into a cylindrical becher glass (11 cm in diameter) into which the slides onto which the film was to be deposited had previously been 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 a collector, or substrate, of given area, the maximal thickness of the films will be proportional to the volume of the solution.
All experiments were performed at (22 ± 1) °C.

Labelling of PLL with fluorescein isothiocyanate (FITC).

PLL was conjugated with FITC. Briefly, a PLL solution at 1 mg.mL^-1 in Tris buffer (50 mM, pH = 8.5) was contacted with FITC dissolved in a small volume of dimethyl sulfoxide (SdS, Peypin, France), for 1 hour at ambient temperature, and in the dark. For the labelling reaction, pH 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 dialysed 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 dialysate. This was checked by UV-vis spectroscopy at a wavelength of 494 nm. The dialysis steps were performed at ambient temperature and in the dark. We prepared labelled PLL in order to investigate its diffusion rate in deposited polyelectrolyte films.

Ellipsometry.

The growth rate of the polyelectrolyte films was measured by removing silicon slides deposited at the bottom of a becher glass after different sedimentation times.
The silicon slides were gently removed from the becher in order to decrease the influence of the resulting convection on the sedimentation process. They were rinsed under a gentle flow of distilled water and dried under a stream of nitrogen.
The film thickness was then determined by measuring the polarisation change of a He-Ne laser beam on at 5 regularly spaced positions along the main axis of the rectangular silicon slides.
We established that a refractive index of 1.465 was representative of polyelectrolyte films which were considered homogeneous and isotropic.
The experiments were performed at a constant angle of incidence (70°) using a PZ2000 ellipsometer (Jobin Yvon, Horiba, France).

Scanning electron microscopy.

Film morphologies were investigated by SEM using a Quanta FEG 200 ESEM from FEI-Quanta. For top view imaging of the surface, samples were observed directly without any conductive coating. For cross section observations, samples were simply broken and observed in the direction parallel to the plan of the silicon substrate.

UV-vis spectroscopy.

The 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 cm^-1. 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 becher containing the deposition substrates, 2mL of the complex-containing solution was loaded into a polystyrene cuvette (1cm 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 polyelectrolytes used absorbs. 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 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.
UV-vis spectroscopy was also used to investigate the interactions of polyelectrolyte films deposited on quartz slides with PLL-FITC molecules.

III. Results

We first studied the deposition rate of polyelectrolyte complexes on silicon slides deposited at the bottom of a collector (a becher glass of 11 cm in diameter). Simultaneously with film growth, measured at short time intervals of up to few hours using ellipsometry, we also estimated the sedimentation rate using UV-vis spectroscopy (supra). Results are shown in Figure 1, showing film growth from HA/PLL complexes. The test solution contained 10^-3 M monomer units of each polyelectrolyte in 0.10M NaCl.
Figure 2a shows the change in turbidity as a function of time for the same system. Turbidity change is shown, as quantified by absorbance at λ= 500 nm, versus time for HA/PLL polyelectrolytes in 0.1 M NaCl, each polyelectrolyte being dissolved at 10^-3 M monomer units.
In all cases where film formation is observed, the turbidity first increases, then passes through a maximum, before decreasing and falling to zero at infinite time, corresponding to a clear solution in which all polyelectrolyte complexes have been sedimented. The curve representing reduction of turbidity with time can be matched with a single exponential decay, as shown in the semi log plot of Figure 2b, in which the decreasing part of Figure 2a is plotted as logA versus time. The solid line corresponds to a linear fit to the data. The value of the rate constant of the exponential decrease is given in the inset. The rate constant of such exponential decay is taken herein as an indication of the sedimentation rate of the polyelectrolyte complexes of the present invention.
When the film thickness exceeds about 280 nm, with the refractive index being held constant and equal to 1.465, single wavelength ellipsometry is no longer capable of providing an objective estimate of the film thickness. Hence, we cut the silicon slides, covered with a polyelectrolyte film, with a diamond knife, and observed them in SEM by cross-sectional analysis (Figure 3). Film growth remained linear with time, up to 24h, but levels off with longer deposition times, when all the complexes were sedimented (data not shown).
The upper part of Figure 3 shows cross sections obtained by SEM of HA/PLL films obtained by sedimentation after 3, 6 and 24 H in the presence of NaCl at 0.1 M, with each polyelectrolyte being dissolved at 10^-3 M in monomer units.
The lower part of Figure 3 shows increase of the average film thickness versus time as obtained from the SEM cross-sectional analysis.
In addition, cross-sectional SEM shows that the films are highly homogeneous in the direction parallel to the silicon substrate. After 24 h we obtained a film thickness of 1.6 + 0.1 µm, corresponding to a high deposition yield of 30 %. The films also appear porous. This is not unexpected, as the expected formation mechanism is based on the coalescence of sedimenting polyelectrolyte complexes.
For HA/PLL films from NaCl solutions at 0.15 M, we found that the solution turbidity was close to 0 after 42 h of sedimentation, from which it was deduced that all polyelectrolyte complexes capable of spontaneous sedimentation had been precipitated. The solution may still contain a small amount of very tiny polyelectrolyte complexes whose sedimentation rate is close to zero. After 42h of sedimentation on a disk of 5.5 cm in radius, we found a film thickness of 4.0 ± 0.1 µm. The volume of the film is hence of 0.037 cm³ whereas the available volume of the polyelectrolyte complexes was of 0.054 cm³. For this last calculation we assume the polyelectrolyte complexes to have a density of 1.2 g.cm^-3, calculated based on a material containing a polypeptide and a polysaccharide. This means that an overestimation 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 68%. This calculation assumes that the porosity of the film is the same as that in the polyelectrolyte complexes. Scanning electron micrographs as those shown in Figure 3 suggest nevertheless that the films obtained through sedimentation of polyelectrolyte complexes are rather porous. An adjustment may be necessary to account for differing porosity.
Film growth rate was mapped against deposition rate for the HA/PLL model, and the result appears to be a master curve, with the data qualitatively following a linear trend as shown in Figure 4. Changing the salt, or the salt concentration, seems to map along the master curve, with higher concentrations and greater chaotropic character of the salt, such as KI, NaClO₄, equating with lower deposition and growth rates.
Figure 4 shows film deposition rate from HA/PLL complexes (as obtained from curves in Figure 1, the film thickness being measured by ellipsometry) versus the sedimentation rate as obtained from curves similar to those of Figure 2. The solid line does not correspond to a fit but is to guide the eye. The influence of salt concentration for NaCl and of the nature of the supporting electrolyte were also investigated, with open circles representing 0.1 M, and the solid square representing 0.15 M.
HA/PLL films obtained in accordance with the present invention were immersed in a bath containing PLL-FITC. The content of PLL-FITC in the film increased slowly with time and reached a steady state after about 2 days, as shown in Figure 5.
Figure 5 shows UV-vis spectra of HA/PLL films obtained by sedimentation in the presence of NaCl 0.15 M (over 24 h) in the presence of a PLL-FITC solution (1 mg/mL in the presence of NaCl 0.15 M). The spectra were acquired after different incubation times (see inset), buffer rinse and drying under a nitrogen stream. The spectra were acquired against a quartz slide coated with an identical HA/PLL film.

IV. The effect of different salts on the "one pot" deposition of PLL/HA complexes

HA and PLL were dissolved at 10^-3 M in monomer units and in the presence of 0.15 M supporting electrolyte, which was either KI, NaCl, NaNO₃, NaBr, or NaClO₄.
The complexes were prepared by injecting the PLL solution in the HA solution under vigorous stirring (300 rpm) during 1 min. The suspension was then poured on top of the cleaned silicon slides used as substrates for the film deposition.

V. Other combinations of polycations/polyanions able to provide continuous deposits of PEM films

1. poly(sodium-4-styrene sulfonate) and poly(diallyl dimethyl ammonium) chloride.

Figure 6 shows the thickness increase with time of PSS/PDADMAC films obtained from sedimentation of PSS/PDADMAC complexes. 10^-3 M monomer units for both polyelectrolytes, with supporting electrolyte NaCl 2M.

2. Hyaluronic acid and poly(allylamine)

The polyelectrolytes were solubilised at a concentration of 10^-3 M in monomer units in the presence of NaCl at 0.5 M.
Scanning electron microscopy showed that the deposits had a thickness of 985 (± 100) nm after 41 h of sedimentation, whereas the film thickness reached a maximal value of 1780 (± 170) nm after 12 days (data not shown).

VI. The fundamental difference/analogy with conventional LBL films.

Preferred polyelectrolyte complexes used to prepare films of the present invention are stoichiometric, such that the amount of positive charges from the polycation substantially matches the amount of negative charges from the polyanion.
With films made by alternated adsorption from polyelectrolyte solutions, even when the concentration (in monomer units) are identical, the stoichiometry of the resultant films is not necessarily 1 : 1. There are many LBL films that display an extrinsic charge compensation, meaning that the concentration of cationic groups does not exactly match that of the anionic groups. These films are characterised by a Donnan potential and can be used as ion exchange membranes [17].
When films are made in accordance with the present invention, such as with 10^-3 M in monomer units for each polyelectrolyte, supporting electrolyte : 0.1 M NaCl, duration of sedimentation: 7.5 h, and put in contact with a 2 mM solution of titanium (IV) bisammonium lactato dihydroxyde (TiBisLac), they displayed a progressive increase in absorbance below 370 nm. This is a typical signature of TiBisLac incorporation in the film and its subsequent polycondensation into TiO₂ [18]. This shows that films made in accordance with the present invention interact with TiBisLac in a manner similar to (PLL-HA)_n films prepared by the conventional layer-by-layer deposition.
Thus, contrary to the expectation based on the fact that the complexes were prepared from stoichiometric complexes, films of the present invention are capable of displaying a positive Donnan potential allowing for the incorporation of the negatively charged TiBisLac.
Figure 7 shows UV-vis spectra of PLL-HA films obtained through sedimentation of PLL-HA complexes (10^-3 M monomer units of each polyelectrolyte in the presence of 0.1 M NaCl, 7.5h of sedimentation) and put in the presence of TiBisLac at 2 mM for different durations of time.

References

[1] G. Decher, Science 1997, 277, 1232-1237.
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Claims

A process for the preparation of a polyelectrolyte film on a substrate, comprising exposing the substrate to an admixture of a polyanion and a polycation in a solution of supporting electrolyte, wherein the polyanion and polycation are provided in sufficient quantity to render the solution of electrolyte turbid to the naked eye, and allowing precipitation from the admixture to occur on the substrate.
A process according to claim 1, wherein the support electrolyte is an inorganic salt of an alkali, or alkaline earth, metal with a dissociative anion, such as a halogen, or an anion derived from a suitable acid, such as mineral acid.
A process according to claim 2, wherein the support electrolyte is NaCl.
A process according to any preceding claim, wherein the concentration of support electrolyte is preferably between 0.05 M and 0.6 M, preferably between 0.1 M and 0.3 M.
A process according to any preceding claim, wherein the polyelectrolytes are present in stoichiometric amounts by comparison with each other.
A process according to any preceding claim, wherein thickness of the layer deposited on the substrate is determined by the length of time that the substrate is exposed to the admixture.
A process according to any preceding claim, wherein the desired thickness of the film is prepared on the substrate in a one step reaction.
A process according to any preceding claim, wherein wastage of polyelectrolyte is less than 20% w/w, preferably less than 10% w/w, and more preferably less than 5% w/w.
A process according to any preceding claim, wherein a buffer at a level below 0.01 M is used, preferably at a concentration between 0.001M and 0.005 M.
A process according to any preceding claim, wherein the polyanion has carboxylate as the predominant ionisable function.
A process according to any preceding claim, wherein the polycation has an amine, such as a primary, secondary, tertiary or quaternary amine, as the predominant ionisable function.
A process according to any preceding claim, wherein the polycation is poly(-L-lysine hydrobromide).
A process according to any preceding claim, wherein the polyanion is sodium hyaluronate.
A process according to any preceding claim, wherein the film that is formed comprises both amino groups and carboxylic groups, and wherein the process further comprises crosslinking these groups to form amide bonds, such as by thermal treatment.
A film obtainable by a process according to any preceding claim.
A film obtained by a process according to any of claims 1 to 14.