Characterization of Layer-by-Layer Self-Assembled Multilayer Films of

Dec 4, 2007 - This is attributed to longer-term interdiffusion of the copolymer ... Dan Xu , Chris Hodges , Yulong Ding , Simon Biggs , Anju Brooker ,...
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Langmuir 2008, 24, 116-123

Characterization of Layer-by-Layer Self-Assembled Multilayer Films of Diblock Copolymer Micelles Kenichi Sakai,†,‡ Grant B. Webber,†,§ Cong-Duan Vo,| Erica J. Wanless,⊥ Maria Vamvakaki,# Vural Bu¨tu¨n,@ Steven P. Armes,| and Simon Biggs*,† School of Process, EnVironmental and Materials Engineering, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom, Department of Chemistry, Dainton Building, The UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, United Kingdom, School of EnVironmental and Life Sciences, The UniVersity of Newcastle, Callaghan, New South Wales, 2308, Australia, Department of Materials Science and Technology, UniVersity of Crete, 710 03, Heraklion, Crete, Greece, and Department of Chemistry, Eskisehir Osmangazi UniVersity, Campus of Meselik, Eskisehir 26040, Turkey ReceiVed July 13, 2007. In Final Form: October 11, 2007 The in situ layer-by-layer (LbL) self-assembly of low Tg diblock copolymer micelles onto a flat silica substrate is reported. The copolymers used here were a cationic poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2(diethylamino)ethyl methacrylate) (50qPDMA-PDEA; 50q refers to a mean degree of quaternization of 50 mol % for the PDMA block) and zwitterionic poly(methacrylic acid)-block-poly(2-(diethylamino)ethyl methacrylate) (PMAAPDEA), which has anionic character at pH 9. Alternate deposition of micelles formed by these two copolymers onto a silica substrate at pH 9 was examined. The in situ LbL buildup of the copolymer micelle films was monitored using ζ potential measurements, optical reflectometry, and a quartz crystal microbalance with dissipation monitoring (QCMD). For a six layer deposition, complete charge reversal was observed after the addition of each layer. The OR data indicated clearly an increase in adsorbed mass with each additional micelle layer and suggest that some interdiffusion of copolymer chains between layers and/or an increase in the film roughness, and hence in the effective surface area of the micellar multilayers, must take place as the film is built up. QCM-D data indicated that the self-assembled micellar multilayers on a flat silica substrate undergo structural changes over a prolonged period. This is attributed to longer-term interdiffusion of the copolymer chains between the outer two layers after the initial adsorption of each layer is complete. The QCM-D data further suggest that the outer adsorbed layers adopt a progressively more extended conformation, particularly for the higher numbered layers. The morphology of each successive layer was characterized using in situ soft-contact atomic force microscopy, and micelle-like surface aggregates are clearly observed within each layer of the complex film, suggesting the persistence of aggregate structures throughout the multilayer structure.

Introduction The formation of surface coatings with well-organized nanostructures on a solid substrate provides a key challenge for the preparation of novel functional materials. The simplest technology for the production of ordered molecular films involves the in situ self-assembly of surfactant and/or polymer molecules at a solid/solution interface. In another approach, highly functional molecular films can be built up in a layer-by-layer (LbL) fashion through successive applications of surfactant and/or polymer molecules with different functionalities. In the most typical example, the surface film is produced through the alternate use of oppositely charged linear polyelectrolytes.1 The resulting films have considerable mechanical integrity in all dimensions and can be removed from solution and subsequently rehydrated, allowing the possibility of multiple processing steps and hence more complex films. The initial interest in the LbL technique * To whom correspondence should be addressed. E-mail: s.r.biggs@ leeds.ac.uk. † University of Leeds. ‡ Current address: Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. § Current address: Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Victoria, 3010, Australia. | The University of Sheffield. ⊥ The University of Newcastle. # University of Crete. @ Eskisehir Osmangazi University. (1) Decher, G. Science (Washington, DC, U.S.) 1997, 277, 1232.

was for the formation of relatively simple polymeric multilayers. Recently, multilayer composite films have been constructed by replacing one or both of these polyelectrolytes with other charged species such as proteins,2-5 DNA,6-9 dendrimers,10-13 and colloidal nanoparticles.14-20 Multilayer films formed from micelles21-24 or vesicles25-27 via the LbL self-assembly technique (2) Caruso, F.; Niikura, K.; Furlong, N.; Okahata, Y. Langmuir 1997, 13, 3427. (3) Caruso, F.; Mo¨hwald, H. J. Am. Chem. Soc. 1999, 121, 6039. (4) Houska, M.; Brynda, E.; Bohata´, K. J. Colloid Interface Sci. 2004, 273, 140. (5) Dedinaite, A.; Lundin, M.; Macakova, L.; Auletta, T. Langmuir 2005, 21, 9502. (6) Pei, R.; Cui, X.; Yang, X.; Wang, E. Biomacromolecules 2001, 2, 463. (7) Schu¨ler, C.; Caruso, F. Biomacromolecules 2001, 2, 921. (8) Quinn, J. F.; Yeo, J. C. C.; Caruso, F. Macromolecules 2004, 37, 6537. (9) Vinogradova, O. I.; Lebedeva, O. V.; Vasilev, K.; Gong, H.; Garcia-Turiel, J.; Kim, B.-S. Biomacromolecules 2005, 6, 1495. (10) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1999, 13, 2171. (11) Khopade, A. J.; Caruso, F. Nano Lett. 2002, 2, 415. (12) Khopade, A. J.; Caruso, F. Biomacromolecules 2002, 3, 1154. (13) Kim, B.-S.; Lebedeva, O. V.; Kim, D. H.; Caminade, A. M.; Majoral, J. P.; Knoll, W.; Vinogradova, O. I. Langmuir 2005, 21, 7200. (14) He, J. A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268. (15) Caruso, F.; Spasova, M. AdV. Mater. 2001, 13, 1090. (16) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (17) Sennerfors, T.; Bogdanovic, G.; Tiberg, F. Langmuir 2002, 18, 6410. (18) Schuetz, P.; Caruso, F. Colloids Surf., A 2002, 207, 33. (19) Esumi, K.; Akiyama, S.; Yoshimura, T. Langmuir 2003, 19, 7679. (20) Angelatos, A. S.; Radt, B.; Caruso, F. J. Phys. Chem. B 2005, 109, 3071. (21) Ma, N.; Zhang, H.; Song, B.; Wang, Z.; Zhang, X. Chem. Mater. 2005, 17, 5065. (22) Qi, B.; Tong, X.; Zhao, Y. Macromolecules 2006, 39, 5714.

10.1021/la7021006 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/04/2007

Multilayer Films of Diblock Copolymer Micelles

have also been of great interest in anticipation of their potential use as nanocapsules. Diblock copolymer micelles are an important class of molecular building blocks for the formation of nanostructured surface films. We have investigated the adsorption of micelles of the low Tg copolymer poly(2-(dimethylamino)ethyl methacrylate)-blockpoly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) at solid/aqueous solution interfaces to develop next-generation surface coatings.28-32 This PDMA-PDEA diblock copolymer exhibits stimulus-responsive behavior in aqueous solution. It is molecularly dissolved in acidic solution, while in alkaline solution, it forms core-shell micelles with the PDEA chains being located in the hydrophobic cores and the hydrophilic PDMA chains forming the cationic micelle coronas.33-35 These micelles spontaneously form an adsorbed layer structure at pH 9 on negatively charged surfaces such as mica and silica due to electrostatic interactions.29-31 This adsorption causes a charge reversal of the pristine substrate surfaces;31 therefore, further buildup can be achieved by the adsorption of a second layer of anionic micelles onto the initial layer of cationic PDMA-PDEA micelles, and so on. We have used this approach to produce complex micelle-only films on particles.24 On flat surfaces, most earlier reports concerning copolymer micelles within a multilayer film have used hybrid films of homopolymers interspersed with micelle layers.21 More recently, the LbL assembly of a surface film exclusively using copolymer micelles as the building blocks has been reported for copolymer micelles where the cores are a glassy high Tg material.22,23 In a recent report, we have unequivocally demonstrated the persistence of micelle structures throughout a five-layer example of a film formed exclusively from low Tg block copolymer micelles.36 In this article, we provide more detailed evidence concerning the nature of these micelle-only copolymer multilayers at the silica/aqueous solution interface using a range of complementary techniques. We note that the multilayer is not prepared by the usual dip method employed in LbL multilayer preparation; rather, the multilayer is always hydrated and in contact with the electrolyte solution. In situ monitoring of the multilayer formation was performed using a quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). The ζ potential of the (multi)layer adsorbed on a silica plate was estimated using streaming potential measurements. In situ atomic force microscopy (AFM) images, following the protocol of Smith et al.,36 were used to characterize the morphology of each layer within the film. (23) Cho, J.; Hong, J.; Char, K.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9935. (24) Biggs, S.; Sakai, K.; Addison, T.; Schmid, A.; Armes, S. P.; Vamvakaki, M.; Bu¨tu¨n, V.; Webber, G. B. AdV. Mater. 2007, 19, 247. (25) Michel, M.; Vautier, D.; Voegel, J. C.; Schaaf, P.; Ball, V. Langmuir 2004, 20, 4835. (26) Michel, M.; Izquierdo, A.; Decher, G.; Voegel, J. C.; Schaaf, P.; Ball, V. Langmuir 2005, 21, 7854. (27) Michel, M.; Arntz, Y.; Fleith, G.; Toquant, J.; Haikel, Y.; Voegel, J. C.; Schaaf, P.; Ball, V. Langmuir 2006, 22, 2358. (28) Webber, G. B.; Wanless, E. J.; Bu¨tu¨n, V.; Armes, S. P.; Biggs, S. Nano Lett. 2002, 2, 1307. (29) Webber, G. B.; Wanless, E. J.; Armes, S. P.; Tang, Y.; Li, Y.; Biggs, S. AdV. Mater. 2004, 16, 1794. (30) Webber, G. B.; Wanless, E. J.; Armes, S. P.; Biggs, S. Faraday Discuss. 2005, 128, 193. (31) Sakai, K.; Smith, E. G.; Webber, G. B.; Schatz, C.; Wanless, E. J.; Bu¨tu¨n, V.; Armes, S. P.; Biggs, S. Langmuir 2006, 22, 5328. (32) Sakai, K.; Smith, E. G.; Webber, G. B.; Schatz, C.; Wanless, E. J.; Bu¨tu¨n, V.; Armes, S. P.; Biggs, S. J. Phys. Chem. B 2006, 110, 14744. (33) Bu¨tu¨n, V.; Billingham, N. C.; Armes, S. P. Chem. Commun. 1997, 671. (34) Lee, A. S.; Gast, A. P.; Bu¨tu¨n, V.; Armes, S. P. Macromolecules 1999, 32, 4302. (35) Bu¨tu¨n, V.; Armes, S. P.; Billingham, N. C. Polymer 2001, 42, 5993. (36) Smith, E. G.; Webber, G. B.; Sakai, K.; Biggs, S.; Armes, S. P.; Wanless, E. J. J. Phys. Chem. B 2007, 111, 5536.

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Figure 1. Chemical structures of (a) non-quaternized PDMA-PDEA and (b) PMAA-PDEA, where the subscripts refer to the degree of polymerization of each respective block. Approximately 50 mol % of the PDMA block on PDMA-PDEA was quaternized with methyl iodide.

Experimental Procedures Molecular Characteristics of Copolymers. The diblock copolymers used in this study were a cationic sample of 50qPDMAPDEA (where 50q refers to a mean degree of quaternization of the PDMA block of 50 mol %) and a zwitterionic sample of PMAAPDEA. The chemical structures of these copolymers are shown in Figure 1. Full details of the synthetic protocol for these copolymers can be found elsewhere.35-38 The degrees of polymerization in both cases were determined to be 50qPDMA93-PDEA25 and PMAA50PDEA59, respectively. Micellar Characteristics of Copolymers. The non-quaternized PDMA-PDEA diblock copolymer spontaneously forms cationic micelles at pH 9. Quaternization of the tertiary amine residues imparts a permanent cationic charge on the coronal-forming PDMA blocks, also altering the overall charge on the pH-dependent micelles.30 The hydrodynamic diameter (DH) and the ζ potential of the 50qPDMAPDEA micelles at pH 9 in the presence of 10 mmol dm-3 KNO3 were determined to be 9 nm and +23 mV, respectively.30 The PMAA-PDEA sample also shows pH-responsive micellization behavior in aqueous solution, and its isoelectric point was estimated to be in the range of 7-8 in a 10 mmol dm-3 KNO3 solution.39 At pH 9, it forms core-shell micelles comprised of hydrophobic PDEA cores and ionized PMAA anionic coronas. The mean DH and ζ potential of the micelles were determined to be 33 nm and -40 mV, respectively.39 (37) Bu¨tu¨n, V.; Armes, S. P.; Billingham, N. C. Macromolecules 2001, 34, 1148. (38) Lowe, A. B.; Billingham, N. C.; Armes, S. P. Macromolecules 1998, 31, 5991. (39) Sakai, K.; Vamvakaki, M.; Smith, E. G.; Wanless, E. J.; Armes, S. P.; Biggs, S. J. Colloid Interface Sci. 2008, 317, 383.

118 Langmuir, Vol. 24, No. 1, 2008 All experiments presented hereafter were carried out at pH 9 in the presence of 10 mmol dm-3 KNO3 as a background electrolyte to ensure a consistent electrolyte environment across the pH range investigated and hence a fixed electrostatic decay length. ζ Potential Based on Streaming Potential Measurements. An Anton Paar electrokinetic analyzer was used to estimate the ζ potential of glass cover slips (Menzel-Gla¨ser) coated with the micellar (multi)layer. The setup of our measurement cell and the cleaning procedure for the cover slips have been described in detail in our previous paper.31 Calculation of the ζ potential was performed measuring both the streaming potential and the specific electrical conductivity of the copolymer solution according to the approach reported by Fairbrother and Mastin.40 After assembling the glass cover slips into the cell, the electrolyte solution was injected from a reservoir. Then, the electrolyte solution was replaced with a copolymer solution of 50qPDMA-PDEA at 50 ppm. We have previously confirmed that this concentration is in the plateau region of the adsorption isotherm and that the surface is fully covered by a copolymer film.41 After equilibration for 1 h, the adsorbed 50qPDMA-PDEA copolymer film was rinsed by a fresh electrolyte solution to remove loosely bound copolymers from the adsorbed layer and to maintain the correct pH environment. The ζ potential measurements were carried out both before and after this rinsing. Similarly, the LbL buildup of the micellar multilayer was performed by injecting a new copolymer solution of either 50qPDMAPDEA or PMAA-PDEA (50 ppm). The ζ potential was always measured both before and after rinsing with electrolyte solution, although the recorded data were the same in each case. All data were collected at approximately 25 °C. OR. Copolymer adsorption to an oxidized silicon wafer (Silicon Valley Microelectronics) was measured by OR as described by Dijt and co-workers.42 Our instrumentation was purchased from the Laboratory of Physical Chemistry and Colloid Science at Wageningen University (Wageningen, The Netherlands), and the detail of its physical principles and operation is given elsewhere.43 The cleaning procedure for the oxidized silicon wafer has been described previously.32 After cleaning, the wafer was set in the OR instrument, and an electrolyte solution was injected from a gravity-fed line. After about 30 min of electrolyte flow, a copolymer solution of 50qPDMA-PDEA (200 ppm) was continuously flowed for 40 min, and the change in the output signal (∆S) was monitored at a stagnation point as a function of time. The adsorbed 50qPDMA-PDEA copolymer layer was then flushed with a fresh electrolyte solution for 20 min. In a similar way to this procedure, further buildup of the multilayer was performed using either 50qPDMA-PDEA or PMAA-PDEA (200 ppm). The instrument was housed in an incubator to maintain a constant temperature of 25 °C. Note that conversion of the ∆S signal into an adsorbed mass for each layer requires the refractive index properties of that layer to be well-defined. In examples such as those explored here where the adsorption of multiple layers may also involve some interlayer mixing and penetration, it is difficult to know these features accurately enough. As a result, it was decided here not to convert the raw ∆S data into mass since the uncertainties in this calculation are too large. QCM-D. A Q-Sense QCM-D 300 was used to assess both the cumulative amount of the copolymer micelles adsorbed at the silica/ aqueous solution interface and the viscoelastic nature of the adsorbed layer. The adsorbed mass was calculated from a change in the third overtone of the resonance frequency by applying Sauerbrey’s relationship.44 A single sensor crystal with a silica coating was used for all QCM-D experiments. The cleaning procedure of the sensor crystal and an O-ring that seals the cell/sensor assembly have been described earlier.32 After assembly in the QCM-D instrument, the electrolyte solution was injected into the cell, and the system was (40) Fairbrother, H.; Mastin, H. J. Chem. Soc. 1924, 75, 2318. (41) Sakai, K.; Smith, E. G.; Webber, G. B.; Baker, M.; Wanless, E. J.; Bu¨tu¨n, V.; Armes, S. P.; Biggs, S. J. Colloid Interface Sci. 2007, 314, 381. (42) Dijt, J. C.; Cohen-Stuart, M. A.; Fleer, G. J. AdV. Colloid Interface Sci. 1994, 50, 79. (43) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2000, 16, 9374. (44) Sauerbrey, G. Z. Phys. 1959, 155, 206.

Sakai et al. allowed to equilibrate for at least 1 h. A copolymer solution of 50qPDMA-PDEA was injected after a stable baseline in the electrolyte solution was achieved. Again, the system was allowed to equilibrate for 1 h in the copolymer solution, and then the solution was replaced with a fresh electrolyte solution to rinse the adsorbed layer. This replacement by the electrolyte solution was repeatedly performed two or three times every 10 min. The subsequent LbL self-assembly of oppositely charged copolymer micelles was examined in a similar way to this first layer of deposition. The copolymer concentrations were always fixed at 200 ppm. All measurements were performed at a constant temperature of 25.0 °C. AFM. Imaging of the adsorbed copolymer films on silica was performed with a Veeco Nanoscope IV AFM. Cantilevers with an integral silicon nitride tip (Veeco NanoProbe, NP-S, nominal spring constant of 0.12 N m-1) were used for in situ AFM experiments and were cleaned using UV irradiation prior to use. The oxidized silicon wafers were cleaned in accordance with a procedure described in our earlier paper.31 After preparation of the first (50qPDMA-PDEA) layer in accordance with our earlier report,31 an in situ soft-contact AFM45 observation was made in this copolymer solution (equilibration time of 30 min). Then, the copolymer supernatant was replaced with a background electrolyte solution adjusted to pH 9. The subsequent LbL self-assembly of oppositely charged copolymer micelles was examined in a similar way to this first layer adsorption, following the protocol of Smith et al.36 The copolymer concentrations were always fixed at 200 ppm. During these observations, interaction forces between the cantilever tip and the copolymer films formed at the silica/aqueous solution interface were also monitored. All measurements were performed at a room temperature of approximately 25 °C.

Results and Discussion Figure 2 shows the change in the ζ potential of the micellar multilayer films as a function of the layer number as measured after rinsing with electrolyte solution. The initially negative ζ potential of the pristine silica surface became positive after adsorption of the cationic 50qPDMA-PDEA micelles. Subsequent addition of the anionic PMAA-PDEA copolymer micelles onto this first 50qPDMA-PDEA layer again led to surface charge reversal, producing a negative ζ potential. Such charge reversal was repeatedly observed at least up to the sixth (PMAA-PDEA) layer. Although the ζ potential change shown in Figure 2 suggests the formation of micelle multilayers at the silica/aqueous solution interface, it is illustrative to examine the cumulative adsorbed amount of the film. OR is one of the most useful techniques to monitor the adsorbed amount on a flat substrate, and the change in the ratio of the s and p polarizations of the reflected beam (output signal, ∆S) is converted to an adsorbed amount (ΓOR) using the following equation:42

ΓOR )

∆S 1 S 0 AS

where S0 is the initial output value and AS is the sensitivity factor, which is dependent on the refractive index increment of each copolymer as a function of the concentration (dn/dc). In the case of the present study, the dn/dc values for the two copolymers are different,46 and the composition variations within the adsorbed layer are not accurately known. Therefore, it is not possible to determine the adsorbed amount precisely. However, a change in ∆S/S0 should nevertheless indicate a change in the adsorbed amount. (45) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (46) The dn/dc values at pH 9 were determined to be 0.141 cm3 g-1 for 50qPDMA-PDEA and 0.165 cm3 g-1 for PMAA-PDEA, respectively.

Multilayer Films of Diblock Copolymer Micelles

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Figure 2. ζ potential values of the adsorbed copolymer films on planar silica as a function of the layer number. The first, third, and fifth layers are comprised of cationic 50qPDMA-PDEA diblock copolymer micelles, whereas the second, fourth, and sixth layers are comprised of anionic PMAA-PDEA diblock copolymer micelles.

Figure 3. In situ OR monitoring of the LbL self-assembly of the polyelectrolytic copolymer micelles on silica. The first, third, and fifth layers are comprised of cationic 50qPDMA-PDEA diblock copolymer micelles, whereas the second, fourth, and sixth layers are comprised of anionic PMAA-PDEA diblock copolymer micelles. The time allowed for the formation of each successive micelle layer was 40 min, followed by rinsing the (multi)layer film with electrolyte solution for 20 min. The arrows indicate injection of electrolyte solution.

In situ LbL self-assembly of diblock copolymer micelles was monitored using OR, and a typical result is shown in Figure 3. The OR output signal increases immediately when a new copolymer solution is introduced into the cell, and the ∆S/S0 value always reaches a plateau region within 40 min of flowing the copolymer solution. The rapid increase in the ∆S/S0 value observed for each layer demonstrates the electrostatic LbL formation of the micellar multilayers. One may notice a slow (and gradual) increase in the ∆S/S0 value seen for the third, fourth, and fifth layer numbers following the initially rapid increase in its value as each new layer is added. As noted earlier, a change in ∆S/S0 results primarily from a change in the amount of adsorbed material, but we should also remember that complete analysis of the OR data requires knowledge of how to model each layer within the film (i.e., the dn/dc value for each layer): this is especially problematic at the boundary of two layers where interpenetration of the copolymers will result in a composite dn/dc value and a gradual transition from the optical properties of one layer to those of the next. Interpretation of the variations in ∆S/S0, if we assume that the adsorbed mass is invariant in this region, suggests structural rearrangements within the multilayer film and hence changing optical properties. This is consistent with earlier investigations of polymer multilayers using similar surface reflectance techniques where initial overshoot peaks were

related to structural changes within the films.47 We focus on this effect in the following sections with the QCM-D and in situ AFM results providing further insight. The subsequent rinsing with electrolyte solution results in a slight decrease in the ∆S/S0 value. This is most likely due to the removal of some loosely bound copolymers, although once again, structural changes of the film cannot be entirely discounted. The ∆S/S0 values recorded after 20 min in the electrolyte flow are summarized in Figure 4. Interestingly, the increment in the ∆S/S0 value increasing with layer number is correlated to the increase in the adsorbed amount with each layer observed for the micellar multilayers on silica particles.24 The possibility of diffusion of copolymer chains within the multilayers should be considered when attempting to interpret the monotonic increase in the ∆S/S0 increment with the micelle layer number. Lavalle and co-workers48 reported an exponential growth of weakly charged polyelectrolyte multilayers composed of cationic PLL [poly(L-lysine)] and anionic PGA [poly(l-glutamic acid)], where diffusion of the polyelectrolytes in and out of the film plays a key role during buildup of the multilayers. In the (47) Kim, B. S.; Lebedeva, O. V.; Koynov, K.; Gong, F, H.; Caminade, A. M.; Majoral, J. P.; Vingradova, O. I. Macromolecules 2006, 39, 5479. (48) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458.

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Figure 4. Cumulative change in the OR output signal with increasing micelle layer number. The first, third, and fifth layers are comprised of cationic 50qPDMA-PDEA diblock copolymer micelles, whereas the second, fourth, and sixth layers are comprised of anionic PMAA-PDEA diblock copolymer micelles. These ∆S/S0 values were recorded after rinsing the (multi)layer film with electrolyte solution for 20 min. The error determined from a typical OR experiment was estimated to be 10%.

proposed mechanism, the number of weakly bound mobile chains within the multilayers is, to a first approximation, proportional to the film thickness. Such mobile chains can interact with the oppositely charged polymer chains in solution and form complexes at the film/solution interface. The number of complexes adsorbed on the film must be proportional to the number of mobile chains diffusing out of the film; hence, the cumulative amount (and the total thickness) of the polyelectrolyte multilayers increases exponentially with the layer number. Similarly, some degree of diffusion of individual chains within the micellar multilayers may also occur for the present system, although the micelles themselves will be stabilized by cohesive forces as a result of their hydrophobic cores.24 However, diffusion between micelle layers is expected to be slower due to the restrictions imposed by the hydrophobic PDEA cores. Another possibility when explaining the observed adsorption data is an increase in the film roughness with the layer number. A greater roughness must result in a higher effective surface area for the (multi)layer films and provides an opportunity to enhance the adsorbed amount of successive micelle layers. It is perhaps noteworthy that a greater surface roughness was clearly observed for silica particles coated with micellar multilayers by transmission electron microscopy, using equivalent copolymers to those reported here.24 This point will be discussed further using the AFM images. The LbL self-assembly of the micellar multilayers was also characterized using QCM-D. Figure 5 shows the recorded change in (a) Sauerbrey mass and (b) dissipation as a function of time. Interestingly, neither the Sauerbrey mass nor the dissipation attained stable values within 1 h, except for the first 50qPDMA-PDEA layer. In particular, the dissipation values observed for the higher numbered (>3) layers increased steeply with time. To understand these QCM-D results, it is important to recall that QCM-D does not simply measure the mass of polymer but also includes any entrained solvent within the adsorbed layer.49,50 Hence, it is very sensitive to conformational changes within any adsorbed film as well as to changes in mass. (49) Ho¨o¨k, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796. (50) Notley, S. M.; Biggs, S.; Craig, V. S. J.; Wågberg, L. Phys. Chem. Chem. Phys. 2004, 6, 2379.

The significant difference in the flow condition between OR and QCM-D51 means that it is difficult to compare the kinetic data directly. Nevertheless, the shapes of the kinetic data obtained by OR and QCM-D are very different. The OR data essentially exhibit the expected adsorption profile with a rapid attainment of a plateau value after a few minutes whenever a new layer was introduced. The QCM-D data are more complex: For layers one and three, the data appear to be fairly straightforward with a rapid increase in signal followed by a slower increase with time. These longer-time scale changes suggest some slow structural rearrangements in the layer as discussed by Notley et al.52,53 Layers two and four suggest a considerably different behavior for the anionic micelle components, although we note again that the OR data suggest a simple adsorption process in terms of adsorbed amount. These data suggest therefore a very complex structural rearrangement for these layers after adsorption. Once again, this is consistent with interpenetration and relaxation of the micelles after adsorption, although it is clearly impossible to be definitive about all the changes that are occurring. Finally, we note that layer five appears to show a different behavior from its counterpart layers, one and three. As we shall discuss next, this seems to be consistent with a significant structural effect after five to six layers of micelles are adsorbed that ultimately leads to a loss of structure for the outermost layer. As noted in the Experimental Procedures, the DH value of the PMAA-PDEA micelles is significantly larger than that of 50qPDMA-PDEA. In addition, the ζ potential of the former micelles is greater in magnitude than that of the latter micelles. This suggests a greater hydrophilic character for the PMAAPDEA micelles, allowing for a greater extension of the micelle layer adsorbed on the relatively stiff and more compact cationic micelle layer. Such an extended PMAA-PDEA layer may undergo slow structural rearrangements as a result of interlayer diffusion (although this is restricted by inefficient contact with the underlying layer in the film) as well as swelling to the solution phase. The observed kinetic data shown in Figure 5 support this (51) Both copolymer and electrolyte solutions were continuously flowed in the OR measurements, whereas QCM-D restricted the injection volume of 1 cm3 per 10 min to retain the temperature stability of the inner cell. (52) Notley, S. M.; Eriksson, M.; Wågberg, L. J. Colloid Interface Sci. 2005, 292, 29. (53) Eriksson, M.; Notley, S. M.; Wågberg, L. J. Colloid Interface Sci. 2005, 292, 38.

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Figure 5. In situ QCM-D monitoring of the LbL self-assembly of the copolymer micelles on silica. Panel a indicates a change in the Sauerbrey mass, while panel b shows a change in the corresponding dissipation. The first, third, and fifth layers are comprised of cationic 50qPDMAPDEA diblock copolymer micelles, whereas the second, fourth, and sixth layers are comprised of anionic PMAA-PDEA diblock copolymer micelles. The time allowed for the formation of each successive micelle layer was 1 h, and then the copolymer supernatant was replaced with electrolyte solution two or three times every 10 min. The arrows represent the first injection of electrolyte solution for each step.

hypothesis (i.e., each anionic PMAA-PDEA layer exhibits a significant delay in the secondary structural rearrangement). Despite the evidence so far presented, the presence and persistence of micelle structures within the film is not definitive. In situ imaging with a scanning probe microscope using the soft-contact technique provides a viable approach to analyze the structure of such films.36 We used in situ soft-contact AFM observations to observe each successive deposited layer, and the results are shown in Figure 6. One can clearly see the micellelike surface aggregates in these images even for the higher numbered layers. However, the image resolution of the higher numbered layers is not as good as that of the lower numbered layers. It is likely that an electrosteric polymer surface layer interacts with the cantilever and progressively hinders scanning with high resolution. The lower resolution observed for the higher numbered layers may also indicate, therefore, an increase in the diffuse nature of the films with increasing layer number as suggested from the QCM-D data. In our previous studies of copolymer micelles adsorbed onto oppositely charged surfaces,30,31 we suggested that the coronalforming chains must relax after initial micelle adsorption. This relaxation is driven by the strong electrostatic attraction between the charged chains and the surface. As a result, the micelles become somewhat flattened at the interface. Indeed, we have previously shown using in situ AFM that the 50qPDMA-PDEA

copolymer forms an adsorbed micellar film through a process of electrostatic adsorption and subsequent hydrophobic (unimeric) adsorption.31 Adsorption of the anionic PMAA-PDEA micelles onto the cationic 50qPDMA-PDEA micelles (and vice versa) is also expected to occur via electrostatic attraction between the oppositely charged coronal chains. However, subsequent relaxation of the copolymer micelles after adsorption may be retarded somewhat due to the adhesive nature of the developing film, and the adsorbed micelles might be expected to stick where they first interact with the underlying layer in the film. Interestingly, the size of the adsorbed micelles observed in Figure 6 is generally larger than the DH values of the corresponding micelles in solution. For example, the diameter of the adsorbed micelles seen in the third (50qPDMA-PDEA) copolymer layer is determined to be approximately 60 nm, which is significantly larger than the reported DH value of 9 nm.30 When discussing micelle aggregate sizes measured with soft-contact AFM, special care is needed since (i) the size observed is always larger than the real size as a result of tip convolution and (ii) the apparent size can also be affected by the applied forces used in the scanning. These effects are more significant for scans on a steric and viscoelastic film, especially if the tip itself becomes coated in copolymer material as expected here. However, some of the increased size may be attributed to structural changes as a result of adsorption. The adsorption of micelles from the second layer

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Figure 6. In situ soft-contact AFM images of micelle multilayers formed on silica. All images presented are deflection images collected at a constant applied force and have been zero-order flattened (the scan size is 2 µm × 2 µm). The first, third, and fifth layers are comprised of cationic 50qPDMA-PDEA diblock copolymer micelles, whereas the second, fourth, and sixth layers are comprised of anionic PMAA-PDEA diblock copolymer micelles.

onward proceeds onto surface adsorbed micelle aggregates; the adsorption of the outer layer is likely to include some penetration into the layer below as the substrate is no longer rigid and impermeable. The dimensions of the outermost layer of aggregates may increase, therefore, as a result of micelle-micelle binding and the interpenetration of the coronal chains with those of the layer below. The adsorption is driven by electrostatic attraction between the oppositely charged copolymers, and we may expect some limited spreading of any adsorbing micelle across and into the oppositely charged layer below. This will result in an increase in the apparent size of the micelles. In fact, an increase in the aggregate size is visually observed up to the third (50qPDMAPDEA) layer, although the increase is not obvious from the fourth (PMAA-PDEA) layer due to the lower image resolution. Finally, surface micellization (in other words, a complete or partial reformation of the micelle-like surface aggregates from unimers) may take place on the multilayer film, in a similar way to that suggested for the first layer formation.31 While it is difficult to propose the precise mechanism only on the basis of the present data, we emphasize the clear presence of micelle-like surface aggregates in each layer of the multilayer film as a result of this in situ LbL deposition. The persistence of micelles in layers below the uppermost has been proven previously by careful scraping the top layer using an AFM stylus probe and subsequent imaging.36 A similar approach was used here to confirm the persistence of the micelles through the film (data not shown). We also note that the persistence of structure throughout a film for essentially the same micelle system on particles was also recently shown using colorimetry.24 The compressive force-distance data recorded for each successive layer in the copolymer supernatant are given in Figure 7. Since the decay length of the force curves recorded in the copolymer supernatant is always significantly larger than the decay length expected for the background electrolyte solution (3 nm),54 the observed repulsion is considered to be electrosteric in nature. Indeed, the apparent separation, over which the repulsive (54) Decay lengths of the inward force curves shown in Figure 7 were calculated to be 13, 13, 22, 15, 36, and 20 nm from the first layer to the sixth layer, respectively.

Figure 7. Inward force curve data for each successive layer. The first, third, and fifth layers are comprised of cationic 50qPDMAPDEA diblock copolymer micelles, whereas the second, fourth, and sixth layers are comprised of anionic PMAA-PDEA diblock copolymer micelles.

interaction is detected, increases with layer number up to the fifth (50qPDMA-PDEA) layer. Since the zero of the apparent separation is defined as a region of constant compliance on the force curve data, it does not correspond to the physical contact of the cantilever with the pristine silica substrate. However, from at least a semiquantitative standpoint, the increase in the repulsive force range suggests an increase in the film thickness with the layer number. The decrease in the repulsive forces observed for the sixth (PMAA-PDEA) layer at a given apparent separation may suggest a partial collapse of the adsorbed micelle multilayer, although we note that the OR mass increases up to the sixth layer. The corresponding decompression force-distance data, shown in Figure 8, also provide insight into the nature of the outer parts of the multilayer film. It is clear from these data that the even numbered layers produce a stronger and longer-range adhesive interaction with the tip as the cantilever retracts from the surface layer. An indication as to why the anionic outer layers provide a stronger and longer range interaction is suggested by the QCM-D

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coated silica particles).24 This suggests that the micelle overlayer has a controllable capacity for the delivery of actives. Moreover, in principle, two or more actives could be introduced at different spatial locations within the micelle film. Thus, this method potentially allows fine control over the rate of release of incorporated actives. The subject of our ongoing work is to study the precise conditions to develop such functional smart surface coatings.

Conclusion Figure 8. Outward force curve data for each successive layer. The first, third, and fifth layers are comprised of cationic 50qPDMAPDEA diblock copolymer micelles, whereas the second, fourth, and sixth layers are comprised of anionic PMAA-PDEA diblock copolymer micelles. Successful formation of multilayer films on a flat silica plate by alternate self-assembly of diblock copolymer micelles has been presented on the basis of ζ potential, optical reflectometry, QCM-D, and in situ AFM data.

data (see Figure 5), where a greater normal extension of these PMAA-PDEA adsorbed micellar layers as compared to the relatively stiff 50qPDMA-PDEA layer was inferred. This increased normal extension suggests a more diffuse surface layer and hence would allow a greater interpenetration of the two interacting films on the tip and surface. This provides an increased opportunity for adhesion between oppositely charged segments in the films as well as more opportunity for the micelle cores on the two surfaces to interact. The longer range of this interaction as well as the increased magnitude also supports the premise of a more extended (normal to the surface) and diffuse steric layer for the PMAA-PDEA layers since long-range adhesive interactions require relaxation and extension of the copolymer under the applied force; this will be facilitated by a less rigid and collapsed polymer layer. A similar result was also seen previously by Notley and co-workers for a standard homo-polyelectrolyte multilayer when analyzing AFM colloid probe interaction data.52 Finally, we note that given the retention of micelle-like surface aggregates in the multilayer film, these coatings offer certain advantages over simple homopolyelectrolyte multilayer coatings, namely, the presence of separate hydrophobic microdomains for potential incorporation of hydrophobic actives. We have recently demonstrated such a possibility using a hydrophobic dye compound (i.e., increasing the number of the dye-induced micelle layers produces an increase in the color intensity for the LbL

Successful formation of multilayer films on a planar silica substrate by alternate self-assembly of cationic 50qPDMA-PDEA and anionic PMAA-PDEA diblock copolymer micelles has been presented on the basis of ζ (streaming) potential, OR, QCM-D, and in situ AFM data. In situ LbL deposition of each successive layer results in (i) complete and reproducible charge reversal of the adsorbed multilayer film, (ii) a monotonic increase in the cumulative adsorbed amount of the copolymer film, (iii) formation of the micelle-like surface aggregates (even for the higher numbered layers), and (iv) greater electrosteric repulsion (and hence, an increase in the apparent separation over which the repulsive interaction is detected) up to the fifth (cationic) layer. On the basis of the QCM-D data, a structural transformation of the adsorbed aggregates is suggested to occur over a prolonged period, as a result of swelling of the outermost layer toward the solution phase as well as interlayer diffusion of the copolymer chains. This latter effect is also suggested by the OR and in situ AFM results. When comparing the odd (cationic) and even (anionic) numbered outermost layers, the anionic layers undergo slower but greater structural rearrangements than the cationic layers. In addition, the anionic outermost layers are more adhesive than the cationic ones. These results suggest that the anionic micelle layers are normally extended more significantly as a result of the greater hydrophilic character. Acknowledgment. The EPSRC is thanked for the linked Research Grants GR/S60419 and GR/S60402 awarded to S.P.A. and S.B., respectively. The ARC is thanked for Research Grant DP0343783. S.P.A. is the recipient of a 5 year Royal SocietyWolfson Research Merit Award. Dr. N. Ishida is thanked for useful discussions regarding the force curve data, and E. Smith is thanked for her useful discussions. LA7021006