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Influence of Charge Density and Distribution on the Internal Structure of Electrostatically Self-assembled Polyelectrolyte Films Marc Koetse,†,‡ Andre´ Laschewsky,*,†,§ Alain M. Jonas,| and W. Wagenknecht§ Department of Chemistry, Universite´ Catholique de Louvain, Place L. Pasteur 1, B-1348 Louvain-la-Neuve, Belgium, Department of Material Science and Chemical Engineering, Universite´ Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium, and Fraunhofer Institut fu¨ r Angewandte Polymerforschung, FhG-IAP, D-14476 Golm, Germany Received August 13, 2001. In Final Form: November 7, 2001 Electrostatically self-assembled (ESA) polyelectrolyte films show in general no internal structure. The use of special polycations, however, namely of lyotropic ionenes, may give rise to highly ordered coatings. In this article, the influence of the charge density of the polyanion, as well as the distribution of the charged groups within this polymer, is examined, using a series of anionic cellulose derivatives. Various techniques were used to study the films’ growth and internal structure. Both showed to be affected in particular by the charge density but also by the substitution pattern.
Introduction Electrostatically self-assembled (ESA) films are, in general, fuzzy materials,1 which is not surprising considering that the assembly mechanism is largely entropy driven.2 The freshly adsorbed chains tend to penetrate into the underlying film, preventing real multilayers to form.1 The lack of organization is not necessarily disadvantageous, but certain applications may require either internal structure or orientation of functional fragments in the films. Only recently, it was discovered that specific polycations, namely certain lyotropic ionenes, can form structured coatings, provided that an appropriate “counterpolyanion” is chosen.3 This behavior has been studied, with special emphasis on the ionene and polyanion structure.4-6 Up to now, ionene 1 (see Chart 1) has been the best candidate to give highly structured films, among the different structures studied.4,6 The bulk polymer is semicrystalline, as found by wide-angle X-ray scattering (WAXS) and optical microscopy.5 Small-angle X-ray scattering (SAXS) showed long-range organization in the bulk polymer, and lyotropic behavior in concentrated aqueous solution was evidenced by the birefringent, mobile phases visible under * Corresponding author. Present address: Fraunhofer Institut fu¨r Angewandte Polymerforschung, Geiselbergstr. 69, D-14476 Golm, Germany. E-mail:
[email protected]. † Department of Chemistry, UCL. ‡ Present address: Dutch Polymer Institute, Eindhoven University of Technology, Eindhoven, The Netherlands. § Fraunhofer Institut FhG-IAP. | Department of Material Science and Chemical Engineering, UCL. (1) Decher, G. Science 1997, 277, 1232-1237. (2) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (3) Arys, X.; Jonas, A.; Laguitton, B.; Legras, R.; Laschewsky, A.; Wischerhoff, E. Prog. Org. Coat. 1998, 34, 108-118. (4) Arys, X. Ph.D. Thesis, Universite´ Catholique de Louvain, Louvainla-Neuve, Belgium, 2000. (5) Arys, X.; Laschewsky, A.; Jonas, A. M. Macromolecules 2001, 34, 3318-3330. (6) Arys, X.; Wischerhoff, E.; Laschewsky, A.; Fischer, P.; Legras, R.; Jonas, A. Manuscript in preparation. Glinel, K.; Jonas, A. M.; Laschewsky, A.; Vuillaume, P. Y. In Thin Films: Polyelectrolyte Multilayers and Related Multicomposites; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, submitted for publication.
Chart 1. Structures of the Polyelectrolytes Studied
crossed polarizers.5 Bulk polyelectrolyte complexes made of ionene 1 with various strong polyanions, for instance poly(vinyl sulfate) (PVS) or poly(vinylsulfonate) (PVSo), show long-range order in their diffractograms measured by SAXS. However, bulk complexes made e.g. with poly(2-acrylamido-2-methylpropanesulfonate) (PAMPS) or poly(styrenesulfonate) (PSS) do not. In certain cases, the same substructuring was found in ESA films, where the reflectograms of the films made of ionene 1 and of the first two polyanions do show Bragg peaks, whereas reflectograms of films made with the latter two do not.5 It should be noted that the bulk complexes and the films do not necessarily show the same organization.6 The reason for the appearance of Bragg peaks in some cases and their absence in others is not fully understood yet, although a partial explanation has been proposed to be the volume of the monomeric units.4,6 The degrees of substitution (DS) of the above cited polyanions are all 1; i.e., they contain 1 charge/repeat unit. To study the influence of the charge density, different samples of cellulose sulfate (2a-f) and of carboxymethyl cellulose (3a,b; see Chart 1 and Table 1) were employed in ESA films as well as in bulk polyelectrolyte complexes with ionene 1. In fact, polysaccharides have occasionally been used for ESA films,7 mainly aimed at biocompatible surfaces (see ref 2 and references therein). But surprisingly little attention has been given to the fact that polysaccharides form excellent synthons toward polyelectrolytes. (7) Hong, J. D.; Jung, B. D.; Kim, C. H.; Kim, K. Macromolecules 2000, 33, 7905-7911.
10.1021/la011280e CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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Table 1. Characteristic Degrees of Polymerization (DP), Charge Densities (DS), and Substitution Patterns of the Used Cellulose Sulfates (2a-f) and Carboxymethyl Celluloses (3a,b) degree of substituion (DS)a DP
tot.
C2
C3
C6
2a 2b 2c 2d 2e 2f
150 230 200 375 305 200
Cellulose Sulfate 1.0 0.30 0.7 0.2 0.48 0.15 0.43 0 0.40 0.30 0.33 0.09
0 0.15 0.11 0 0 0.04
0.70 0.35 0.22 0.43 0.1 0.2
3a 3b
370
Carboxymethyl Cellulose 1.0 0.4 0.22 1.1 0.65 0.44
0.38 0
a DS of the 2a-f samples was determined by 13C NMR; the values for 3a,b were obtained from 1H NMR.
Most monomeric units contain up to three sites that can be chemically modified. Depending on the chosen chemistry, degree and pattern of substitution can be widely altered.8,9
Figure 1. UV/vis spectra of ionene 1 in aqueous solution (5.5 × 10-5 M) at pH ) 2 and pH ) 8 (dashed lines) and spectra of {1/2f}10, {1/3a}10, and {1/3b}10 films. The absorbance of the {1/3}10 films is multiplied by a factor of 10 for clarity. with water, followed by centrifugation and decantation. Finally, the complexes were freeze-dried. Analytical Methods. UV-vis and linear dichroism spectra were recorded on a SML Aminco DW-2000 spectrometer. For the linear dichroism spectra, Polaroid filters were placed before and after the sample with the polarizing plane horizontally. The samples were measured either under normal incidence, giving an Ax spectrum, or tilted in the polarization plane over an angle ω ) 45° (Aω spectrum). The latter spectra were corrected for the longer path length by,13 giving an Axz spectrum:
Experimental Section Materials. The synthesis of the cellulose sulfate samples was described elsewhere;10 an overview of the properties and degree and pattern of substitution is given in Table 1. The 2,3-bis(carboxymethyl) cellulose sample (3b) was a gift from Prof. Heinze (University of Jena, Jena, Germany); its synthesis was described elsewhere.11 Ionene 1 and potassium poly(sulfopropyl methacrylate) (PSPM) were synthesized as described before.12 The degree of substitution after azo-coupling of ionene 1 used for these experiments was about 80%. Poly(ethyleneimine) (PEI) and carboxymethyl cellulose (3a) were obtained from Aldrich. According to the supplier, 3a has a DS of about 0.7. However, according to 1H NMR, this sample has a DS of 1 with a normal distribution pattern. The water used for the experiments was purified by an Elgastat water purification system and showed a resistance of 18.2 MΩ cm-1. Substrates were either quartz (Suprasil) or one side polished n-type 〈100〉 silicon wafer (ACM, France). Film Deposition. The cleaning procedures of the substrates, as well as the deposition method, are described ref 12. All substrates were precoated with a “buffer layer” of poly(ethyleneimine) and poly(sulfopropyl methacrylate) to facilitate the subsequent adsorption steps and to smoothen the surfaces. Adsorption of ionene 1 took place from a 5 × 10-3 M aqueous solution (with respect to charges), acidified to pH 3-4 by a few drops of HCl. Deposition took place during 15 min. Under these conditions, ionene 1 is not stable (slow hydrolysis of the ester), so that the solution can only be used for a few days. Cellulose sulfates and carboxymethyl cellulose derivatives were deposited from 10-2 M solutions in pure water (with respect to charges). Deposition took place during 20 min. No cosolutes were present in these solutions. Bulk Polyelectrolyte Complexes. The bulk polyelectrolyte complexes were formed by titrating an equimolar (based on charges) amount of ionene 1 solution to the cellulose sulfate solution under vigorous stirring at room temperature. The same solutions were used as for the deposition. The complexes were separated by centrifugation, decanted, and washed three times (8) Heinze, T. Cellulose Derivatives: Modification, Characterization, and Nanostructures; American Chemical Society: Washington, DC, 1998. (9) Heinze, T. Macromol. Chem. Phys. 1998, 199, 2341-2364. (10) Klemm, D.; Heinze, T.; Wagenknecht, W. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 730-733. (11) Heinze, U.; Heinze, T.; Klemm, D. Macromol. Chem. Phys. 1999, 200, 896-902. (12) Laschewsky, A.; Wischerhoff, E.; Kauranen, M.; Persoons, A. Macromolecules 1997, 30, 8304-8309.
Axz ) Αωx(1 - n-2 sin2 ω) Here n ) 1.61 is the refractive index (as found by ellipsometry for {1/2} films, at 632.8 nm) and is considered to be constant over the spectral width. The Az spectrum (spectrum perpendicular to the surface) could then easily be calculated using the following: 13
Az )
(Axz - Αx sin2 ω) cos2 ω
The ellipsometry, X-ray reflectivity, and SAXS measurements and data analysis are described in ref 5.
Results and Discussion The general structures of the employed polyions are shown in Scheme 1. Their charge densities and distributions are summarized in Table 1. The DS of the cellulose sulfate samples ranges from 0.33 to 1 average charges/ repeat unit. The distribution of the sulfate groups over the repeat unit varies widely and is related to detailed chemistry employed to synthesize the samples.8,10 For instance, 2b,c show a statistical distribution, reflecting that the order of reactivity for the different positions in a glucopyranose unit is typically C6 > C2 > C3. For carboxymethyl cellulose, prepared under standard conditions, this order is somewhat different. For carboxymethylation of cellulose in ethanol or 2-propanol and activation with an aqueous NaOH solution, the order is typically C2 g C6 > C3.10 Selective reactions on precursor polymers (generally, cellulose acetates) provided the nonstatistical cellulose sulfate derivatives. The DS of the two carboxymethyl cellulose (3) samples both are ∼1. 3a shows the typical statistical distribution pattern, whereas 3b bears the carboxymethyl groups at the positions 2 and 3 only. Film Growth, Optical Density, and Thickness. The vis spectra of a {1/3a}10 and {1/3b}10 film are given in Figure 1. The shape of the absorbance bands is in both cases virtually the same; λmax is found at 430 nm and does not differ significantly between the two spectra. This value (13) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VCH Publishers: New York, 1986.
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Figure 3. Growth of a {1/2f}n film as followed by UV/vis spectroscopy (×) and ellipsometry: thicknesses measured after adsorption of ionene 1 (9) and after adsorption of 2f (0). The error bars give the standard error.
Figure 2. (a) Evolution of absorbance (×, +) and thickness (9, 0) of {1/2a-f}10 and {1/3a,b}, respectively, vs the total degree of substitution. (b) λmax of {1/2a-f}10 (×) and {1/3a,b} (+) vs degree of substitution.
agrees well with the value of λmax of ionene 1 in an acidic solution (434 nm, for a 1.7 × 10-5 M solution at pH 2 compared to 460 nm at pH 8). This may be expected as the films are deposited from a solution of ionene 1 that has a pH of about 3-4, where the pyridine groups will be largely protonated. The spectra are almost symmetrical and show the presence of a small band at about 474 nm, suggesting a minor aggregation of the chromophores. The {1/3a}10 film has a slightly higher optical density, suggesting a somewhat thicker film. This finding is corroborated by ellipsometric measurements, where the thicknesses found are 5.8 and 6.9 nm (including ca. 1.5 nm for the native SiOx layer on the silicon substrate, using a refractive index of 1.52 for the calculations) for the {1/3a}10 and {1/3b}10 film, respectively. X-ray reflection measurements on the {1/3b}10 film revealed a thickness of 4.0 nm This corresponds, within the error margin, to the thickness found by ellipsometry if corrected for the native oxide layer on the silicon substrate (ca.1.5 nm). The found total thickness indicates an average increase of thickness by 0.4-0.5 nm/deposition cycle. For ESA films made from such bulky polyelectrolytes, this value is rather low, indicating that a single adsorption cycle is not sufficient to produce a dense monolayer on the substrate. Especially, considering the aggregation behavior of ionene 1 in solution, one might have expected thicker films (vide infra). Films made from cellulose sulfates and ionene 1 give remarkably different results, exemplifying the importance of the nature of the ionic groups in ESA.2 Figure 2a shows the absorbances (crosses) and thicknesses (squares) of {1/2}10 films for the various cellulose sulfates. The thicknesses of the {1/2} films were calculated with a (real) refractive index of 1.61 and include the oxide layer on the silicon (ca. 1.5 nm). The refractive index of the oxide (1.455) is comparable to that of the film, and the oxide layer is thus included in the thickness found by ellipsometry. Both {1/3}10 films and the {1/2a}10 film show a similar absorbance, but the latter film is much thicker. This indicates that the films made with 2a are organized, having the chromophores better aligned along the surface normal (vide supra). The general trend for the {1/2}10 films is that low degrees of substitution (DS) lead to a marked increase of the thickness and of the optical density of the films. This
observation agrees with reports that the thickness of ESA films increases with decreasing density of the charged groups of the complemetary polyion, when the given polyelectrolyte partner has a reasonably high charge density.14,15 This observation can be rationalized by the idea of charge compensation; i.e., the given amount of charged groups in one partner asks for the same amount of charges in the other one, thus requiring a higher absolute amount of complementary polyion with decreasing ion content of the latter. Mostly, thickness and optical density evolve in parallel, indicating a rather constant composition of these films. However, some discrepancies are noted, e.g. for {1/2a}10 and {1/2b}10. The films made with these cellulose sulfates exhibit a lower absorbance for the {1/2a}10 film than for the {1/2b}10 film. However, the ellipsometric thicknesses behave inversely, {1/2a}10 being thicker than the {1/2b}10 film. This finding implies that the chromophores in the {1/2a}10 film are aligned more parallel to the surface normal and, therefore, interact less with the incoming light. From the linear dichroism measurements (vide supra), it became clear that orientational effects indeed may be the origin of the apparent discrepancy between the thickness and optical densities of samples {1/2a}10 and {1/2b}10. The distribution of the charges over the monomeric units seems to play a role for both the films made with carboxymethyl celluloses {1/3}, as with cellulose sulfates {1/2}. The difference in thickness of the films made with {1/3} is about 20%, the statistically substituted sample 3a giving thicker films than 3b, which is substituted exclusively at the C2 and C3 positions. The influence of the distribution pattern for the films made with 2 is evidenced by the scattering of the points around DS ) 0.45 (samples {1/2c}, {1/2d}, and {1/2e}, Figure 2a). The degrees of substitution (0.48, 0.45, and 0.40, respectively) are close, but the substitution patterns are very different (see Table 1). The film {1/2d}10, with the sulfate groups only at the C6 position, gives a much lower absorbance and thickness than the film {1/2c}10, where the sulfate groups are statistically distributed over the repeat unit. The {1/2e}10 film with substitution mainly on position C2 shows an intermediate absorbance and thickness. Clearly, the distribution pattern has an important influence on the thickness and optical density, but the effects are difficult to rationalize on the basis of our data. Figure 3 shows the growth of a {1/2f}n film as followed by UV/vis spectroscopy (circles, absorbance values taken at λmax) and ellipsometry (squares). It should be noted that the samples shown in Figure 3 were dried in hot air after each deposition cycle for the UV/vis measurements (14) Steitz, R.; Jaeger, W.; v. Klitzing, R. Langmuir 2001, 17, 44714477. (15) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408-1412.
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and each half-deposition cycle for ellipsometric measurements, hence the difference in absorbance and thickness, compared to a nondried specimen (Figure 2, DS ) 0.33). The nonlinear growth is typical for all thicker films grown with cellulose sulfates and is probably related to the increasing roughness of the samples, as evidenced by the increasing standard error (error bars) of the ellipsometric thickness. The nonlinear growth starts after approximately 5 deposition cycles. Growth of these dried samples could, therefore, only be followed up to 8 deposition cycles, after which the films became patchy and incoherent. However, films with more deposition cycles could be grown without drying (vide infra). From the thickness trace, it can be seen that adsorption of ionene 1 (closed squares) increases the thickness more importantly than the adsorption of 2f does (open squares). We assume that a rather thick layer of ionene 1 is adsorbed, followed by the penetration of the cellulose sulfate during the next deposition step.5 Noteworthy, the wavelength of the absorbance maximum, λmax, is strongly influenced by DS too, as shown in Figure 2b. In the thicker samples with low DS, the band is deformed and the absorbance maximum is shifted hypsochromically, indicating H-aggregation. In particular, the evolution of the UV/vis spectra of the {1/2f}n films indicates the appearance of a so-called H-band for the thicker samples (see Figure 1). This is another strong indication that the chromophores tend to aggregate in these films. In contrast, cellulose sulfates with higher DS induce bathochromic shifts in the films compared to the solution spectrum of ionene 1 at pH 2 or to the films made with 3 (that, supposedly, do almost not show aggregation; cf. Figure 1). Apparently, the chromophores tend to organize differently according to the charge density on the cellulose sulfates. Linear Dichroism. To obtain an indication of the alignment of the dye fragments in the films, 4 selected films, covering the whole range of degrees of substitution, were subjected to linear dichroism measurements. Since the alignment is expected to be directed largely parallel to the surface normal, the “tilted plate” method13 was used to obtain the information. The samples were measured in the UV/vis spectrometer using normal incidence and oblique incidence light (45°). From the resulting spectra, the order parameter P2 and an average angle 0.5 of the transition moment of the dye fragments to the surface normal were calculated by13,16
P2 )
Az - Ax 1 ) 〈3 cos2 θ - 1〉 Az + 2Ax 2
Figure 4a shows the measured and calculated spectra for the sample {1/2a}10. It is mainly the H-band of the spectrum that is affected by the polarization of the light. As expected, the organization of the dye fragments is thus related to the formation of H-aggregates. This behavior is less present in the spectra of the {1/2f}10 film (Figure 4b). Either the chromophores still aggregate in this sample, but in a less organized manner, or the aggregation is reduced due to different packing induced by this polyanion. The results of the calculations are summarized in Table 2. As found for the thickness and optical density, the average alignment is strongly influenced by the DS of the cellulose sulfates. The {1/2f}10 film shows an average alignment angle (52°) close to the magic angle (56.4°, i.e., isotropic distribution of the transition moments). The (16) He, J. A.; Bian, S.; Li, L.; Kumar, J.; Thripathy, S. K.; Samuelson, L. A. J. Phys. Chem. B 2000, 104, 10513-10521.
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Figure 4. Polarized vis spectra measured at normal incidence (Ax, dotted lines) and oblique incidence (45° in the plane of polarization (Aω), dashed lines) for (a) a {1/2a}10 film and (b) a {1/2f}10 film. The continuous lines give the calculated parallel spectra (Az).
Figure 5. X-ray reflectograms of {1/2}10 films and Patterson functions calculated thereof. The traces have been shifted along the y axis for clarity with increasing charge density from top to bottom. Table 2. Order Parameter (P2) and Average Angle (θ) of the Transition Moment to the Surface normal of Selected {1/2}n and {1/3}n Films film
DStot
P2
〈θ2〉0.5 (deg)
{1/2a} {1/2b} {1/2c} {1/2f} {1/3a} {1/3b}
1.0 0.7 0.48 0.33 1 1
0.49 0.30 0.24 0.050 0.079 0.15
35 42 45 52 52 49
{1/2a}10 film shows an average angle of about 35° with respect to the surface normal. Both the films {1/2b}10 and {1/2c}10 give comparable average angles (43 and 46°, respectively) lying between the 2 extremes of films {1/2a}10 and {1/2f}10. In summary, the order parameter P2 seems to increase with increasing degree of substitution of the employed cellulose sulfates. With the thickness of the films containing carboxymethyl celluloses 3, it is not expected to find an average orientation of the chromophores along the surface normal. Indeed, the LD measurements show an order parameter that is close to an isotropic orientation. Polymer 3b seems to induce a slightly better organization in the film, which is corroborated by the somewhat increased thickness of this film compared to {1/3a}10. Internal Structure. There is no Bragg peak present in the reflectogram of the {1/3b}10 film, indicating a lack of internal organization. XRR measurements on a {1/3a}10 film, with a different charge distribution in the 3 repeat units, gave comparable results. Films made with cellulose sulfates again give a very different result, as is seen from the reflectograms of the {1/2}10 films given in Figure 5a. In contrast to the {1/3}10 films, all {1/2}10 films show one Bragg peak in their reflectogram, indicating that all films have a, more or less pronounced, lamellar substructure. No higher order Bragg peaks were found, except for {1/2a}10 films for which a small second order reflection can be detected. Moreover,
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Table 3. Results of the Analysis of the Patterson Functions of the {1/2}n Films film/bulk complex
DStot
Bragg dist (nm) XRRa SAXSb
{1/2a} {1/2b} {1/2c} {1/2d} {1/2e} {1/2f}
1.0 0.7 0.48 0.43 0.40 0.33
2.51 2.22 2.54 2.53 2.30 2.46
a
2.57 2.57 ndc nd nd 3.02
coherence length (nm) 60 16 20 25 7.4 18.5
In film. b In bulk polyelectrolyte complex. c Not determined.
Figure 6. X-ray reflectograms and Patterson functions of {1/ 2b}n films for n ) 1, 3, 5, and 10 deposition cycles.
from the sharpness of the Bragg peaks, it becomes clear that the charge density influences the quality of the internal structure. The general behavior becomes clearer if the reflectograms are analyzed with the Patterson function, from which the repeat distances and coherence lengths (defined as the last oscillation in the Patterson function) of the lamellar stacking can be easily determined. The resulting traces of this analysis are also shown in Figure 5b. Table 3 resumes the distances between the lamellae and the coherence lengths for all the films. The best internal organization and, inherently, the highest coherence length (i.e. the distance over which there is correlation between the positions of the lamellae, ca. 60 nm) are obtained in films made with the cellulose sulfate having the highest charge density, 2a. For a comparison of the other samples with each other, finding a general trend with DS (as found for the thickness, optical density, and linear dichroism) is less obvious. For instance, a {1/2c}10 film (DS ) 0.48) is better organized as a {1/2b}10 film (DS ) 0.70); the coherence lengths are respectively ca. 20 and 16 nm. The same is true for the samples {1/2e}10 and {1/2f}10 (DS ) 0.4 and 0.33, respectively). The found distances (Table 3) between the lamellae in the film compare well with the distance found by SAXS in the corresponding bulk polyelectrolyte complexe for the pair {1/2a}. This indicates that this bulk complex and the self-assembled film contain comparable internal structures. Here again, the sharpness of the Bragg peak depends strongly on the charge density of the 2 used, in much the same way as found for the corresponding films. However, the {1/2b} and {1/2f} pairs show small but significant difference in the repeat unit between the film and the bulk complex. The correlation distance between the lamellae in the bulk complex (2.57 and 3.02 nm, respectively) is comparable to the other complexes; however, the films show somewhat smaller repeat units (2.22 and 2.46 nm), which may indicate a slightly different packing. Film growth was followed by XRR, too. Figure 6 shows the reflectograms and Patterson functions of four {1/2b}n films, where n is equal to 1, 3, 5, or 10 deposition cycles. Up to 5 deposition cycles, it seems that only 1 lamella, with an average interdistance of ca. 2.2 nm, is added each cycle. Thicker films start to lose coherence and become
more and more rough. Additionally, the thickness of the film requires that more than one lamella is adsorbed/ deposition cycle. For the film made of 10 deposition cycles, no peak indicating the film/ambient interface could be found indicating a high roughness, and only some faint coherence is found for any two lamellae separated by five or six other lamellae. Implications of the Results on the Film Growth Mechanism and Structure Formation. As stated in the Introduction, ionene 1 is known to form ordered ESA films with some highly charged polyelectrolytes such as poly(vinyl sulfate) (PVS) and poly(vinylsulfonate) (PVSo).4,5 Detailed analysis of the studies on the {1/PVS}n system revealed that three mechanisms are involved in the film growth and the formation of internal structure, namely, (1) adsorption of the ionene, forming a preorganized layer due to its lyotropic behavior, (2) diffusion of the polyanion in the underlaying film, and (3) polyelectrolyte complex formation in the underlaying film.5 During this last step, destruction or enhancement of the preordering may arise, depending on the specific matching between polycation and polyanion. Important to note is the fact that more than one lamellae may appear per dipping cycle, depending on the amount of ionene initially adsorbed.5 The growth of the films made with cellulose sulfates 2 (independent of the charge density) is somewhat comparable with that of films made with PVS. For {1/2} films, a linear regime is found, up to ca. 5 deposition cycles (see Figures 3 and 6), while linear growth was observed for up to 50 dipping cycles for {1/PVS}. Thereafter, thickness increases nonlinear and roughness increases considerably, too. In fact, roughness and thickness increase are directly related: since a rough surface has a larger area,4 more polyelectrolyte can be adsorbed. The films containing carboxymethyl celluloses 3, on the contrary, stay very smooth, and are therefore much thinner. Similar results were found using poly(2-acrylamido-2-methylpropylsulfonate), PAMPS, as the polyanion. (Note that the reason of not finding substructures, however, may be totally different: the type of ionic group in the case of 3 and the bulkiness of the pendent group in the case of PAMPS; vide infra). Again, as for the {1/PVS} system, more than one lamellae may appear per dipping cycle. This is obviously the case for {1/2a} films: Figure 5 shows that the correlations between the positions of lamellae extend over about 20 successive lamellae for 10 dipping cycles, indicating that at least two lamellae appear on the average per dipping cycle. This fully supports the hypothesis according to which the structuring of the film results from prestructuring of the “lyotropic” ionene in the swollen adsorbed layer, followed by restructuring upon polyanion deposition and complexation. This is less obvious for other polyanions (Figure 5), for which the coherence of the lamellar stacking extends over much smaller distances. For the {1/2b} system for instance (Figure 6), the number of lamellae appearing per deposition cycle initially equals the number of dipping cycles. This underlines the importance of the polyanion in the control of film structure. Whether or not a substructure is formed depends mainly on the ability of the polyelectrolyte pair to form an organized complex. Polyelectrolyte pairs that are able to form structured bulk complexes typically form structured complexes in the films, too. Notably, the repeat distances between the lamellae are not necessarily the same in the bulk complex or in the films (see Table 3). For films made of ionene 1 with different polyelectrolytes, a profound influence of the type of polyanion used was found.4,6 For
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Conclusions
Figure 7. Coherence lengths found for the cellulose sulfate films vs the total charge density (0) and the charge density at the C6 positions (×).
instance, PVS and PVSo give well-structured bulk polyelectrolytes and films, PVS giving the highest amount of order of the two. In contrast, PAMPS does not show structure in both the bulk complex and the films. It was supposed that this effect originates in the more bulky ionic group of PAMPS. Looking in more detail at the current results may provide an additional explanation for this problem. The amount of orientation may be expressed in the coherence length found from the Patterson functions of the different samples (Figure 5). A strong correlation between the total degree of substitution of the cellulose sulfate samples and the coherence length was not found, as illustrated in Figure 7. However, a better correlation is found when plotting the coherence length against the degree of substitution on the C6 position of the repeat units. Noteworthy, no correlation was found with respect to substitution at the other positions (C2 and C3). It seems that substitution of the C6 position enhances the formation of well-organized complexes in the films. Since the sulfate groups at this position are relatively flexible compared to the ones at the C2 and C3 positions, the increased flexibility of these sulfate groups possibly allows for improved packing of ionene 1 in the complex. The more of those groups that are present on the cellulose sulfate, the better is the organization in either the bulk complex or the films. Both the linear dichroism and XRR measurements evidence this. These considerations may also explain the better structure formation for {1/PVS}n compared to {1/PVSo}n films,6 as the sulfate group is somewhat more flexible as the sulfonate group. Although the amount of adsorbed material and the organization in the films originate from 2 different steps in the film growth mechanism (respectively step 1 and step 3 in the model), they are closely related. In fact, a smooth surface will adsorb less material, giving smooth surfaces. These layers will be organized if the complex allows it. Rough surfaces, on the contrary, will adsorb more material that still may organize in lamellar substructures, although only locally. This causes even rougher surfaces, due to the nonideal packing of the complex, and induces even bigger growth in the next deposition cycle, hence, the nonlinear growth observed for the {1/2}n films. The reason that the films made with 3 are not structured may be found in the type of ionic group (carboxylate vs sulfate or sulfonate). The carboxylate groups are, probably, also engaged in H-bonding with the protonated pyridine moieties. Eventually, this may be sterically highly demanding and not allow for a lamellar substructure. For sure, the problem of the complexation step, and film growth on the whole, are far more complicated. Therefore, the discussion above may explain only some aspects of film growth.
The use of cellulose sulfates with different charge densities and distributions in ESA films with a lyotropic ionene has a profound influence on a number of film properties. The polyelectrolyte films obtained at low ionic strength are rather thick. The film thickness and the optical density increase with decreasing charge density of the employed cellulose sulfate, as a general trend. The charge distribution seems to be of importance for the film growth as well, but no plain explication can be given at present. The polyelectrolyte films made with carboxymethyl cellulose, on the contrary, behave as normal ESA films, demonstrating the enormous importance of the type of polyions used in ESA. Different behavior of solvatochromism with respect to the charge density is found, too. Whereas the films made with the carboxymethyl celluloses 3a,b show a value of λmax that is comparable to that of the solution spectra of ionene 1 (at pH 2), the value of λmax of the films made with cellulose sulfates 2a-f varies with the charge density. The values of λmax of films made with cellulose sulfates having a high degree of substitution are shifted bathochromically with respect to that of the solution spectrum. The λmax of films made with low DS cellulose sulfates are shifted hypsochromically compared to that of the solution spectra. Apparently, this behavior is related to the formation of H-aggregates in the low DS films, as evidenced by the form of the spectra and linear dichroism measurements. The latter experiments showed that the average angle of alignment of the chromophores shifts from almost isotropic to highly anisotropic with increasing charge density of the cellulose sulfate. Internal organization was found only in the films made with cellulose sulfates, as studied by X-ray reflectometry. A lamellar substructure was found in the films, as well as in a few selected bulk polyelectrolyte complexes (by SAXS). Actually, the amount of structure is related to the charge density, especially to the charge density on the C6 position of the employed cellulose sulfate. More detailed XRR measurements indicate the complexity of the mechanisms controlling structure formation in the films and the sensitivity of order on subtle details of the chemical structure of the polyanions. In a comparison of the present findings with the results obtained recently for the ionene 1/poly(vinyl sulfate) pair, the mechanism proposed for this pair (adsorption, interpenetration, and complexation) is applicable to the ionene 1/cellulose sulfates pairs, too. But it seems that not only the bulkiness of the pendent ionic groups is of importance (as proposed in the original model) but also their flexibility and their chemical nature. Acknowledgment. We thank X. Arys for his help with the ellipsometry and X-ray reflectometry experiments. T. Heinze (University of Jena, Jena, Germany) kindly provided us with the 2,3-bis(carboxymethyl) cellulose sample (3b) and analyzed the commercial sample (3a). F. Loth (Fraunhofer Institut fu¨r Angewandte Polymerforschung, Golm, Germany) is acknowledged for helpful discussions on cellulose derivatives. Financial support was provided by the Belgian National Fund for Scientific Research and by the DG Recherche Scientifique of the French Community of Belgium (Action de Recherche Concerte´e, convention 00/05-261). LA011280E
