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Langmuir 2006, 22, 5328-5333
Comparison of the Adsorption of Cationic Diblock Copolymer Micelles from Aqueous Solution onto Mica and Silica Kenichi Sakai,*,† Emelyn G. Smith,‡ Grant B. Webber,†,| Christophe Schatz,‡,⊥ Erica J. Wanless,‡ Vural Bu¨tu¨n,§ Steven P. Armes,# and Simon Biggs*,† School of Process, EnVironmental and Materials Engineering, UniVersity of Leeds, Leeds LS2 9JT, U.K., School of EnVironmental and Life Sciences, The UniVersity of Newcastle, Callaghan, New South Wales, 2308, Australia, Department of Chemistry, Eskisehir Osmangazi UniVersity, Campus of Meselik, Eskisehir 26040, Turkey, and Department of Chemistry, Dainton Building, The UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. ReceiVed March 10, 2006. In Final Form: April 7, 2006 The similarities and differences in the adsorption behavior of diblock poly(2-(dimethylamino)ethyl methacrylate)b-poly(2-(diethylamino)ethyl methacrylate) (XqPDMA-PDEA, where X refers to a mean degree of quaternization of the PDMA of either 0, 10, 50, or 100 mol%) copolymers at the mica/ and silica/aqueous solution interfaces have been investigated. These diblock copolymers form core-shell micelles with the PDEA chains located in the cores and the more hydrophilic PDMA chains forming the cationic micelle coronas at pH 9. These micelles adsorb strongly onto both mica and silica due to electrostatic interactions. In situ atomic force microscopy (AFM) has demonstrated that the mean spacing and the dimension of the adsorbed micelles depend on both the substrate and the mean degree of quaternization of the PDMA blocks. In particular, the morphology of the adsorbed nonquaternized 0qPDMAPDEA copolymer micelles is clearly influenced by the substrate type: these micelles form a disordered layer on silica, while much more close-packed, highly ordered layers are obtained on mica. The key reasons for this difference are suggested to be the ease of lateral rearrangement for the copolymer micelles attached to the solid substrates and the relative rates of relaxation of the coronal PDMA chains.
Introduction Recent advances in synthetic methodology allow the design of a wide range of novel diblock copolymers that undergo spontaneous self-assembly in aqueous solution in response to stimuli such as solution pH1,2 or temperature.3 Under appropriate conditions, these copolymers form micelles whose physicochemical properties (critical micelle concentration (cmc), micelle size, and zeta potential) may be controlled by adjusting synthesis parameters including block composition and overall copolymer molecular weight. Such stimulus-responsive diblock copolymer micelles are being developed for drug delivery applications,4-7 as nanoreactors for preparing nanosized materials,8 and as building blocks for the fabrication of smart surface coatings.9 * To whom correspondence should be addressed. E-mail:
[email protected] (K.S.);
[email protected] (S.B.). † University of Leeds. ‡ The University of Newcastle. § Eskisehir Osmangazi University. # The University of Sheffield. | Current address: Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Victoria, 3010, Australia. ⊥ Current address: Laboratoire de Chimie des Polyme ` res Organiques, LCPO (UMR5629) - ENSCPB, 16 Avenue Pey Berland, 33607 PESSAC cedex, France. (1) Liu, S.; Armes, S. P. Curr. Opin. Colloid Interface Sci. 2001, 6, 249. (2) Sumerlin, B. S.; Lowe, A. B.; Thomas, D. B.; McCormick, C. L. Macromolecules 2003, 36, 5982. (3) Aoshima, S.; Sugihara, S.; Shibayama, M.; Kanaoka, S. Macromol. Symp. 2004, 215, 151. (4) Ro¨sler, A.; Vandermeulen, G. W. M.; Klok H.-A. AdV. Drug DeliV. ReV. 2001, 53, 95. (5) Rodriguez-Hema´ndez, J.; Lecommandoux, S. J. Am. Chem. Soc. 2005, 127, 2026. (6) Che´cot, F.; Bruˆlet, A.; Oberdisse, J.; Gnanou, Y.; Mondain-Monval, O.; Lecommandoux, S. Langmuir 2005, 21, 4308. (7) Hruby, M.; Koo`a´k, EÅ .; Ulbrich, K. J. Controlled Release 2005, 103, 137. (8) Gohy, J.-F.; Lohmeijer, B. G. G.; Schubert, U. S. Chem. Eur. J. 2003, 9, 3472.
To develop a smart surface coating, we have investigated two model systems, i.e., we have studied the adsorption of poly(2(dimethylamino)ethyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) diblock copolymer micelles at solid/aqueous solution interfaces.10-12 PDMA-PDEA shows 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.13-15 These micelles spontaneously form an adsorbed layer structure at a mica/aqueous solution interface due to electrostatic interactions. Interestingly, these strongly adsorbed micelles remain intact at a solution pH that causes rapid micelle dissociation in the bulk solution. AFM studies and surface fluorescence studies indicate that the adsorbed micelles observed at high pH ‘open up’ at low pH due to protonation of the PDEA chains in the micelle cores to form localized polymer brushes. Moreover, the original core-shell micelle structure appears to be reformed on returning to high pH, at least for a mica substrate.11 The precise nature of the adsorbed layers formed by these PDMA-PDEA diblock copolymer micelles at the mica/aqueous solution interface is influenced by the mean degree of quaternization of the PDMA chains.12 For example, the mean spacing of the adsorbed micelles at pH 9 was determined to be 30 ( 2 (9) Russell, T. P. Science 2002, 297, 964. (10) Webber, G. B.; Wanless, E. J.; Bu¨tu¨n, V.; Armes, S. P.; Biggs, S. Nano Lett. 2002, 2, 1307. (11) Webber, G. B.; Wanless, E. J.; Armes, S. P.; Tang, Y.; Li, Y.; Biggs, S. AdV. Mater. 2004, 16, 1794. (12) Webber, G. B.; Wanless, E. J.; Armes, S. P.; Biggs, S. Faraday Discuss. 2005, 128, 193. (13) Bu¨tu¨n, V.; Billingham, N. C.; Armes, S. P. Chem. Commun. 1997, 671. (14) Lee, A. S.; Gast, A. P.; Bu¨tu¨n, V.; Armes, S. P. Macromolecules 1999, 32, 4302. (15) Bu¨tu¨n, V.; Armes, S. P.; Billingham, N. C. Polymer 2001, 42, 5993.
10.1021/la060662n CCC: $33.50 © 2006 American Chemical Society Published on Web 05/05/2006
Adsorption of Micelles to Mica and Silica
nm for a nonquaternized PDMA93-PDEA25 diblock copolymer and 40 ( 8 nm for the same copolymer that had a mean degree of quaternization of 50%. In contrast, the hydrodynamic diameter of the former micelles was 23 nm in dilute aqueous solution at the same pH, whereas that of the latter micelles was only around 9.1 nm. On the basis of these observations, we suggested12 that the cationic PDMA coronal chains must relax after initial adsorption of the copolymer micelles onto the mica surface. Higher levels of quaternization should cause the cationic copolymer to interact more strongly with the mica surface, leading to a larger ‘footprint’ per micelle. Consequently, the surface density of adsorbed copolymer micelles with higher degrees of quaternization is significantly reduced compared to nonquaternized PDMA-PDEA micelles. These observations were in excellent agreement with earlier work from our group involving a diblock poly((2-dimethylamino)ethyl methacrylate-b-methyl methacrylate) (PDMA-PMMA) copolymer system;16 in this case, the PMMA blocks are highly insoluble and so the micelles are kinetically frozen. The adsorption again showed a strong effect of the packing density on the charge difference between the corona and the surface. Large differences in charge magnitude and sign led to collapsed micelles with significant spacings between the cores of the micelles. As the charge imbalance decreased, a higher degree of order was seen. Coronal relaxation is driven by the strong electrostatic attraction between the cationic PDMA coronal chains and the anionic mica surface. Thus, one might expect that the structure of the diblock copolymer micelles is perturbed by the substrate. Mica is an ideal substrate for AFM studies since it is atomically flat, but amorphous silica is a much more relevant surface for technological applications. Both mica and silica are highly anionic at pH 9; however, the surface charge densities of the two substrates are different to each other.17 Nevertheless, it has been pointed out that many studies assume similar adsorption characteristics for mica and silica.18 In this paper, we examine the similarities and differences in the adsorption of PDMA-PDEA diblock copolymer micelles at the mica/aqueous solution and the silica/aqueous solution interfaces at pH 9. A single diblock copolymer of varying degrees of quaternization (0, 10, 50, and 100 mol% with respect to the PDMA chains) was employed. The adsorbed micelles were imaged using in situ soft-contact atomic force microscopy (AFM). Extents of adsorption of these copolymers on silica were assessed using optical reflectometry (OR) and compared with the corresponding hydrated masses estimated using a quartz crystal microbalance with dissipation monitoring (QCM-D) in our previous study.12 In addition, zeta potentials for the two micellecoated substrates were determined using streaming potential measurements.
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Figure 1. Chemical structure of the 0qPDMA-PDEA diblock copolymer precursor. Quaternization of the tertiary amine residues with methyl iodide under mild conditions is restricted to the PDMA block and is statistical in nature for substoichometric amounts of methyl iodide.
Materials. The PDMA-PDEA diblock copolymer precursor was synthesized using group transfer polymerization (GTP) as described elsewhere.15 The chemical structure of the PDMA-PDEA diblock is given in Figure 1. The molecular weight and the polydispersity index of this copolymer were determined to be 19 100 g‚mol-1 and 1.13, respectively, as measured by gel permeation chromatography (GPC) with THF eluent at a flow rate of 1.0 cm3‚min-1 using PMMA calibration standards. The PDMA content was 79 mol%, as judged by 1H NMR spectroscopy. On the basis of these data, the mean degrees of polymerization were calculated to be 93 for the PDMA block and 25 for the PDEA block, respectively.
The tertiary amine residues on the PDMA chains of the precursor copolymer were selectively quaternized using a (sub)stoichiometric amount of methyl iodide (MeI), as described previously.19 The mean degrees of quaternization of the PDMA block were assessed using 1H NMR spectroscopy as described previously; these were found to be approximately 10, 50, and 100 mol%, respectively. Hereafter, these diblock copolymers are denoted as ‘XqPDMA-PDEA’, where ‘X’ refers to the mean degree of quaternization of the PDMA block. All experiments were conducted well above the cmc for each copolymer in the region of maximum adsorption. Silicon wafers (purchased from Silicon Valley Microelectronics, CA with a predefined oxide layer of 115 nm) were used for the AFM observations and OR measurements. Menzel-Gla¨ser glass cover slips for an optical microscope (Germany) were employed for the streaming potential measurements. All other reagents were of analytical grade, and water was Millipore Milli-Q grade. Methods. I. Preparation of Copolymer Solutions. The XqPDMA-PDEA diblock copolymers at the desired concentration were directly dissolved in 10 mmol‚dm-3 aqueous KNO3 solution at room temperature (approximately 25 °C). Only in the case of the nonquaternized PDMA-PDEA, the solution pH was adjusted to 4 in order to enhance the solubility by using a very small amount of diluted HNO3 aqueous solution. After being stirred overnight, the copolymer solutions were used for each measurement described below. All copolymer solutions were used within 7 days, and the pH was adjusted to 9 immediately prior to use. This pH change results in micellization of the diblock copolymers without use of any organic solvents. II. AFM. In situ imaging of the adsorbed micelle layers on either muscovite mica or silica was performed with a Nanoscope III AFM (Veeco, CA). Cantilevers with an integral silicon nitride tip (NanoProbe, Veeco, CA) were used for all AFM experiments and were cleaned using UV irradiation (approximately 9 mW‚cm-2 at 254 nm) prior to use. The muscovite mica substrates were freshly cleaved before AFM assembly. The silicon wafers were kept in a dust-free environment and subjected to a three-step treatment before use. This involved UV irradiation for 30 min followed by ultrasonication in ethanol for 20 min. The silica was then stored in ethanol, rinsed just prior to use with Milli-Q water, and soaked in 10 wt% aqueous NaOH for 10 min followed by copious rinsing with Milli-Q water to give an hydroxylated silica surface. A fresh piece of silica was used for each experiment and then discarded. The background electrolyte and copolymer solutions were passed through a syringe-mounted 0.20 µm filter (GHP Acrodisc, Pall Gelman Science, MI) as they were injected into the AFM fluid cell. Images were collected using the soft-contact method;20 this uses the minimum force necessary to obtain an image, thereby minimizing
(16) Webber, G. B.; Wanless, E. J.; Armes, S. P.; Baines, F. L.; Biggs, S. Langmuir 2001, 17, 5551. (17) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207. (18) Liu, J.-F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558.
(19) Bu¨tu¨n, V.; Armes, S. P.; Billingham, N. C.Macromolecules 2001, 34, 1148. (20) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409.
Experimental Section
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scanning-induced deformations of the adsorbed layer. All images presented were deflection images and were zero-order flattened. At the beginning of each experiment, Milli-Q water was adjusted to the pH of the copolymer solution under investigation, injected into the cell, and left for approximately 30 min to allow the substrate to reach charge equilibrium. This also allowed the cleanliness of the substrates to be assessed prior to the injection of the copolymer sample. The equilibration time was at least 4 h after injection of the copolymer solution into the AFM fluid cell. In addition, the copolymer solution was subsequently replaced with 10 mmol‚dm-3 aqueous KNO3 solution at pH 9. This replacement usually resulted in an increase in the image clarity and definition due to removal of weakly or nonadsorbed copolymers which interact with the AFM cantilever as it scans across the surface. However, this rinsing did not cause any significant changes in the adsorbed layer morphology itself. All measurements were performed at 25 ( 2 °C. III. Optical Reflectometry (OR). Copolymer adsorption to silica was measured by the OR technique as described by Dijt and coworkers;21 our instrumentation has been described in detail in a previous paper.22 Briefly, the technique involves measuring the change in reflective properties of a substrate following adsorption. A stagnation point flow cell with well-defined hydrodynamics was used, where diffusion is the only transport mechanism to the surface. A typical experiment involves the copolymer being introduced from a gravity-fed line through a two-way valve after the surface has equilibrated to the desired pH and ionic strength and a stable baseline has been recorded. Continuous adsorption measurements were taken from the change in the ratio of the s and p polarizations of the reflected laser beam over long time intervals with temporal resolution of
