Polymer Multilayer Films Obtained by Electrochemically Catalyzed click chemistry

Polymer Multilayer Films Obtained by Electrochemically Catalyzed click chemistry

(Parte 3 de 3)

By treating the data with the Voigt model,56 the best fit to the data gives a film thickness, at pH 3.5 and 9, respectively of 5 and 200 nm, leading to a swelling ratio of 3.6 with an elasticity of

4 and 192 kPa and a viscosity of 9.6 and 6.7 mPa3sa t pH 3.5 and 9, respectively. The swelling ratio is defined as the ratio of the film thickness at pH 9 to that at pH 3.5. Two additional experimentswere performedto testthe reproducibilityof the swelling experiments. They gave respectively 62 and 65 nm at pH 3.5 and216and220nmatpH9(withaswellingratioof3.5forboth).

Topography of the PAAAlk/PAAAz Film. The topography of a PEI-(PAAAlk/PAAAz)7 film built by electrochemically catalyzedclickchemistrywasimagedbyAFMinliquidstate.Thefilm was constructed at a reduction potential of -350 mV with a Cu(I) concentration of 0.3 mM at pH 3.5 and the AFM images weretakeninthepresenceof10mMNaNO3atpH3.5andpH9. The film appears homogeneouswith a surface roughnessthatcan beestimatedfroma10μm 10μmAFMimage(rms)tobeofthe order of 6 and 12 nm respectively at pH 3.5 and at pH 9. By scratching the film, the AFM image allows determining the film thickness.

Figure8 shows a typical image of the scratched film (a) and the corresponding profile (b). The substrate appears entirely covered. The thickness of the film is of the order of 40 nm at pH 3.5. This film was then brought in contact with a solution at pH 9. The swelling observed by QCM is confirmed by AFM (Figure 8c,d), giving a thickness of 140 nm with a swelling ratio of the order of 3.5,closetothatfoundbyQCM.Itcanbenoticedthattoperform the scratch we had to build the film with a PEI precursor layer instead of a PEIAz precursor layer. In this latter case the scratch- ing was extremely difficult, indicating again the robustness of these films. With a PEI precursor layer, the scratching was much easier because the first PEI layer interacts with the PAAAlk only through electrostatic interactions.

Discussion

The click reaction takes place only in the presence of Cu(I) which is produced at the electrode by the reduction of Cu(I). It has been shown by the non-buildup of the film while the polyelectrolytes were brought in contact with the film only once the þ600mVpotentialwasapplied.Thefilmbuildupisthusgoverned by the rate at which the polyelectrolytes and the Cu(I) ions reach the film/solution interface where the click reaction takes place. The polyelectrolytes diffuse from the solution toward the interface whereas the Cu(I) ions diffuse from the electrode toward the same interface. As the polyelectrolytes reach the interface, they starttoformcovalentbondswiththefilm.Asinglechainbecomes gradually anchored onto the film through the reaction of its azide or alkyl functions. The characteristic time needed for a single chaintoestablishitsmaximumofcovalentlinksdependsuponthe click reaction kinetics. When the Cu(I) concentration is higher, the click reaction is faster and thus the anchoring time of a single chain is shorter. Ifduring thisanchoring timea new chain diffuses in the vicinity of the one that is in the process of anchoring, it can also establish covalent bonds with the film, thus leading to steric repulsion between chains. In this case, each chain establishes fewer links with the surface compared to the case where a single chain deposits, leading to a more “loopy” conformation of the chainsand then tohigherdepositedmassateachdepositionsteps. Increasing the Cu(I) concentration leads to two opposite effects. On the one hand, a faster reaction induces faster deposition of a single chain which can thus block the incoming of new ones, leading toa reduction ofthedeposited mass.Onthe other hand, a faster reaction also promotes a more rapid anchoring of several chains leading to an increase of the mass deposited. Our experiments show that it is the second effect, namely an increase of the film growth, that dominates. This mechanism also implies that when keeping the Cu(I) concentration fixed, the film thickness shouldincreasewhenincreasingthe polyelectrolyte concentration in the solution, as it was observed experimentally. Indeed, increasing the polyelectrolyte concentration in solution also enhances the probability of a new chain to anchor in the vicinity of an already partially anchored chain leading to an increase of the final deposited polymer mass (Figure S-5 in Supporting Information).

Figure 7. (a)Evolutionofthenormalizedfrequencyshiftand(b)dissipation,measuredat15MHz(ν=3)byEC-QCM,ofaPEIAz-(PAAAlk/

PAAAz)7film,constructedbuiltinthepresenceof0.3mMCuSO4atpH3.5withE=-350mV,incontactwithsolutionsof10mMNaNO3at pH 3.5 and 9 as a function of time. The contact of the different solutions was operated under a flow rate of 1.4 mL/min.

H DOI: 10.1021/la902874k Langmuir X, X(X), X–X

Article Rydzek et al.

Let us now discuss the existence of a leveling off of the film buildup (see Figure S-4 in Supporting Information). This leveling offcanhavetwoorigins.Afteragivennumberofdepositionsteps, the new incoming polyelectrolyte chains might interact with the surface through all their azide or alkyne groups so that no more groups are available for a subsequent reaction. The buildup then automatically stops. The leveling off might also come from a too smallCu(I) concentrationatthefilm/solutioninterfacesothatthe click reaction can no longer take place. In order to check whether the leveling off is due to the absence of azide or alkyne groups on the surface, we performed the following experiment: a film was built at 0.15 mM of CuSO4 with a reduction time of 2 min.

The leveling off sets in after 6 PAAAlk/PAAAz pairs of layers (see Figure S-6 of Supporting Information). We then increased the

Cu(I) concentration in solution and remained the reduction time constant. We observe that the film buildup takes place again, clearly indicating the presence of available azide or alkyne groups on the surface. Finally, we blocked all the alkyne groups on the surface by reacting them through a click reaction with EG3Az (2-(2-(2-azidoethoxy)ethoxy)ethanol) compound prepared accord- ing to Patel et al.57 When this film was further brought in contact with PAAAz chains, no deposition could be observed anymore. This clearly indicates that the film no longer builds up after the reaction of all alkyne groups. Concerning the leveling off observed after the deposition of six pairs of layers, we can thus conclude that it is not due to the absence of available azide or alkyne groups on the surface. This might be due to a too small Cu(I) concentration at the film/solution interface to obtain the click reaction. Indeed, this reaction is known to be of order 2 with respect to the Cu(I) concentration58 and thus very sensitive to the Cu(I) concentration. Cu(I) ions have to diffuse up to the interface to form covalent bonds between polyelectrolytes and the film. If during the reduction time Cu(I) ions cannot reach the interface, the buildup process levels off. The leveling off should take place after a smaller number of pairs of layers for smaller Cu(I) concentration. This is in accordance with the experimental observations(FigureS-4a).Onealsoexpectsthat,asthereduction time is increased, the number of deposited layers required for leveling off also increases, again in accordance with our observations (Figure S-4b). The leveling off sets in roughly for a film thickness X of the order of 30 nm for a reduction time of ΔtE = 5 min. This allows to get a rough estimate of the diffusion coefficient DCu of Cu(I) in the film (X2 =2 DΔtE), which is of the order of 10-14 cm2 s-1. This value is much smaller than the typical value of 10-6 cm2 s-1 for ions in aqueous solutions. It is of the same order of magnitude as that of methylene blue in poly- (styrenesulfonate)/poly(allylamine) multilayers59 found to be of the order of 10-15 cm2 s-1 and at least 3 orders of magnitude higher than that of other dyes in poly(styrenesulfonate)/poly- (vinylbenzyl chloride) quaternized with N,N-dimethylethanolamine multilayers60 found to be of the order 10-17-10-18 cm2 s-1. In this latter case, the dyes are expected to interact strongly through π-π interactions with the styrene rings from the polyelectrolytes in addition to electrostatic interactions. In our case the small value of the diffusion coefficient could be due to the interactions of the diffusing species, Cu(I) and Cu(I), in the film with carboxylic groups. These groups hinder free diffusion of cations in accordance with the ion transport model in polyelectrolyte multilayersdeveloped by Schlenoff.61 Finally, the click reaction is not instantaneous, and one would have to take the reaction kinetics into account. All these factors reduce the diffusion of ions through the film.

Conclusion

To summarize, we have shown that multilayers with polyelectrolytesofsamechargemodifiedbygraftingeitheralkyneorazide

Figure 8. Typical AFM height 3D images (10 μm 10 μm), obtained in contact mode, of PEI-(PAAAlk/PAAAz)7 in liquid state built in the presence of 0.3 mM CuSO4 with E = -350 mV. Scratch of the film and cross-section profiles imaged at (a, b) pH 3.5 and (c, d) pH 9.

(57) Patel, K.; Angelos, S.; Dichtel, W. R.; Coskun, A.; Yang, Y. W.; Zink, J. I.;

Stoddart, J. F. J. Am. Chem. Soc. 2008, 130, 2382. (58) Rodionov, V. O.; Fokin, V. V.; Finn, M. G. Angew. Chem., Int. Ed. 2005, 4, 2210.

DOI: 10.1021/la902874k ILangmuir X, X(X), X–X

Rydzek et al. Article groups can be constructed through a rapid and efficient electrochemically triggered Sharpless reaction. This new procedure allows the buildup of multilayer films in the absence of chemical reducing agent or Cu(I)-organic ligands and allows the construction of multilayers with components that do not interact via electrostatic attraction or hydrogen bonding. These films can be constructed on electrodes whose potentials are more negative than a critical value that lies between -70 and -150 mV vs Ag/ AgCl (KCl sat.) reference electrode. The film thickness increment per pair of layers appears independent of the applied voltage as long as it is more negative than a critical value but depends upon Cu(I) and polyelectrolyte concentrations in solution and upon the reduction time of Cu(I). An increase of any of the mentioned parameters leads to an increase of the thickness per bilayer. For given buildup conditions, the construction levels off after a given number of deposition steps which increases with the Cu(I) concentration in solution and with the reduction time. A model based on the diffusion of Cu(I) and Cu(I) through the film is proposed to explain these experimental observations. This study was intended to make the proof of principle of the buildup procedure and to get first indications of the parameters affecting the multilayer buildup. Further studies are however needed to precisely understand the role of each parameter and to be able to model the buildup process. Such studies are currently under way.

Acknowledgment. We thank David Martel and Hoan Cong

Nguyen (UMR 7177, CNRS/UdS Strasbourg, France) for fruitful discussions. Financial support from ANR ClickMultilayer ANR-07-BLAN-0169, ANR E-DETACHPEM BLAN08- 1_315174, and Centre National de la Recherche Scientifique are also acknowledged. N.B.A. is indebted to the Universit ed e

Strasbourg for her fellowship. The contributions of the different authors are as follows: P.S. initiated the project, F.B. came up with the concept, and G.R. performed the experiments of QCM. J.-S.T. and NBA performed the synthesis of the polymers, B.Fsupervisedthesynthesizes,P.S.andF.B.supervisedthework, C.C.andA.E.H.performedtheAFMimages,A.P.performedthe XPS experiments, G.R., L.J., P.M., J.-C.V., B.S., B.F., P.S., and F.B. participated actively in the discussions about the results, and F.B. and P.S. wrote the paper.

Supporting Information Available: Detailed synthesis of

PEIAz, PAAAlk, and PAAAz; evolution of the normalized frequency shift measured by EC-QCM as a function of time during two successive cycles of -350 and þ600 mV on a

PEIAz-(PAAAlk/PAAAz)7 film; XPS and X-ray-induced Auger spectroscopy of PEIAz-(PAAAlk/PAAAz)14 film; evolution of the normalized frequency shift for PAAAlk/PAAAlk and PAA/PAA systems on PEIAz in electrochemically cata- lyzed click chemistry conditions and for PAAAlk/PAAAz system with injection of polyelectrolytes at þ600 mV; build- up of (PAAAlk/PAAAz)14 at different CuSO4 concentrations in electrochemically catalyzed click chemistry conditions; evolution of the normalized frequency shift of PEIAz-

(PAAAlk/PAAAz)7 film as a function of the adsorbed polyelectrolyte layers built at different polyelectrolyte concentra- tion; evolution of the normalized frequency shift as a function of the adsorbed polyelectrolyte layers for the

PAAAlk/PAAAz system built in the presence of CuSO4 at 0.15 mM followed by a buildup at 0.3 mM. This mate- rial is available free of charge via the Internet at http:// pubs.acs.org.

(Parte 3 de 3)

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