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Packing Density Control in P(NIPAM-co-AAc) Microgel Monolayers: Effect of Surface Charge, pH, and Preparation Technique Stephan Schmidt,†,‡ Thomas Hellweg,§ and Regine von Klitzing*,⊥ Max-Planck-Institut fu¨r Kolloid- and Grenzfla¨chenforschung, 14424 Potsdam, UniVersita¨t Bayreuth, Physikalische Chemie I, UniVersita¨tsstrasse 30, D-95447 Bayreuth, and Stranski-Laboratorium fu¨r Physikalische and Theoretische Chemie, Institut fu¨r Chemie, Technische UniVersita¨t Berlin, Strasse des 17. Juni 124, D-10623 Berlin ReceiVed June 10, 2008. ReVised Manuscript ReceiVed August 26, 2008 In the present paper, thermosensitive coatings are prepared by deposition of P(NIPAM-co-AAc) microgel particles on precoated silicon wafers. The effect of pH, substrate precoating, and preparation technique is studied. The pH value is found to significantly influence the adsorption density, while the substrate surface charge is less important. Hence, the electrostatic contribution of the particle-particle interaction seems to play a more pronounced role for the adsorption density than at least the electrostatic part of the particle-surface interaction. For the latter, also nonelectrostatic contributions like hydrogen bonding and surface roughness play an important role. Immersion of the prepared polyelectrolyte/microgel layers in buffers leads to a reorganization of the adsorbed particles at the surface.
1. Introduction Microgels are gels having colloidal dimensions and are swollen in a suitable solvent. Due to their specific viscoelastic and rheologic properties, microgels have a huge potential with respect to the use in technical applications, e.g. in the coating industry as additive in paints and films. In addition, microgels may have sensitivity to various environmental parameters such as temperature, pH, or ionic strength and react upon changes of these in terms of changes in their state of swelling. A prominent example of such “intelligent” materials are colloidal poly-N-isopropylacrylamide (PNIPAM) particles, which show a completely reversible phase transition above 32 °C at which the polymer network collapses and the gel volume decreases drastically.1-4 Due to this specific thermoresponsive behavior, macroscopic PNIPAM hydrogels have attracted great interest related to their potential as functional materials for thermo-optical, biomedical, and thermomechanical devices.5-7 However, it can take several hours or even days for macroscopic gels to swell or shrink to their maximum or minimum volume, whereas microgels show rapid, but still rather sharp, response to external stimuli.8 Hence, PNIPAM microgels are more promising candidates for applications as responsive material, especially when a fast response upon changes of external parameters is required. Further future applications of PNIPAM microgels include microencapsulation,9,10 removal of heavy metal ions from wastewater,11 * Corresponding author. E-mail:
[email protected]. Tel: ++49-30-31423476. † Max-Planck-Institut fu¨r Kolloid- and Grenzfla¨chenforschung. ‡ Present address: Universita¨t Bayreuth, Physikalische Chemie II. § Universita¨t Bayreuth. ⊥ Technische Universita¨t Berlin.
(1) Murray, M.; Snowden, M. AdV. Collloid Interface Sci. 1995, 54, 73–91. (2) Pelton, R. AdV. Colloid Interface Sci. 2000, 85, 1–33. (3) Nayak, S.; Lyon, L. A. Angew. Chem., Int. Ed. 2005, 44, 7686–7708. (4) Ballauff, M.; Lu, Y. Polymer 2007, 48, 1815–1823. (5) Tanaka, T. Phys. ReV. Lett. 1978, 40, 820–823, no copy. (6) Shibayama, M.; Tanaka, T.; Han, C. C. J. Chem. Phys. 1992, 97, 6829– 6841. (7) Dusek, K. ResponsiVe Gels: Volume Transitions I, 1st ed.; Advances in Polymer Science; Springer Verlag: Berlin, 1993; Vol. 109. (8) Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214–1218. (9) Nayak, S.; Gan, D.; Serpe, M. J.; Lyon, L. A. Small 2005, 1, 416–421. (10) Greinert N, R. W. Colloid Polym. Sci. 2004, 282, 1146–1149.
and microgel supported catalysis.12,13 All of these applications benefit from the fact that the microgels can be functionalized by inclusion of comonomers in the network. Another advantageous feature of PNIPAM microgels is the ease of synthesis and working up as particles with low polydispersity are obtained via emulsion polymerization without using detergents. In order to functionalize surfaces, several groups started to deposit microgel particles at surfaces during the past few years.14-16 With respect to confinement effects on the thermosensitive behavior, the particle density is an important parameter. Lyon and co-workers fixed microgel particles on a poly(ethylene terephthalate) (PET) surface by spin coating and covalent tethering afterward.17 The distribution was irregular in this work. For optical applications, e.g. surfaces with specific reflectivity properties,18-20 it is of interest to produce colloidal 2D lattices of particles with a controlled distance between the particles. In some studies, the particles were densely packed on a hexagonal lattice.15,21,22 A 2D monolayer of PNIAPM microgel beads was prepared by Yoshida and co-workers via a very complex preparation method: double-template polymerization.21 Kawaguchi and co-workers18,23 prepared films of PNIPAM particles upon air-drying and got highly ordered 2D colloidal cyrstals with a controllable distance. They claim a balance between capillary attraction and steric repulsion as the origin for the regular distance (11) Morris, G. E.; Vincent, B.; Snowden, M. J. J. Colloid Interface Sci. 1997, 190, 198–205. (12) Kawaguchi, H.; Fujimoto, K.; Mizuhara, Y. Colloid Polym. Sci. 1992, 270, 53–57. (13) Ballauff, M. Prog. Polym. Sci. 2007, 32, 1135–1151. (14) Serpe, M. J.; Jones, C. D.; Lyon, L. A. Langmuir 2003, 19, 8759–8764. (15) Nerapusri, V.; Keddie, J. L.; Vincent, B.; Bushnak, I. A. Langmuir 2006, 22, 5036–5041. (16) Sorrell, C. D.; Lyon, L. A. J. Phys. Chem. B 2007, 111, 4060. (17) Singh, N.; Bridges, A. W.; Garcia, A. J.; Lyon, L. A. Biomacromolecules 2007, 8, 3271. (18) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21, 8439. (19) Suzuki, D.; McGrath, J. G.; Kawaguchi, H.; Lyon, L. A. J. Phys. Chem. C 2007, 111, 5667. (20) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 5292–5293. (21) Sakai, T.; Takeoka, Y.; Seki, T.; Yoshida, R. Langmuir 2007, 23, 8651. (22) Meng, Z.; Cho, J. K.; Debord, S.; Breedveld, V.; Lyon, L. A. J. Phys. Chem. B 2007, 111, 6992. (23) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21, 2434.
10.1021/la801770n CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
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between the particles. The latter one is controlled by the graft chain length of the hairy particles. The electrostatic repulsion between the particles is assumed to be minor, since their charge results only from the negatively or positely charged initiator. In the present paper, the charge of the microgel particles is increased by the addition of acrylic acid (AAc) monomers. P(NIPAM-co-acrylic acid) microgel particles containing 5% AAc monomers were synthesized. The introduction of acrylic acid comonomers allows controlling the particle charge.24-27 The aim is to control first the density of the adsorbed microgel particles in a reproducible fashion. In a next step, a regular particle distribution is desired, e.g. for design of 2D colloidal crystals. The challenge is to tune the particle-particle interaction and the substrate-particle interaction, in order to obtain the requested film properties. The present study comprises testing various particle deposition methods on bare and polyelectrolyte-precoated substrates using different particles and pH conditions. The structure of the films is characterized with scanning force microscopy and ellipsometry.
2. Experimental Section 2.1. Synthesis of Microgel Particles. The particles were synthesized via emulsion polymerization in water without using surfactants.28,29 N-isopropyl acrylamide (NIPAM), N,N′-methylenbisacrylamid (BIS), acrylic acid (AAc), and potassium persufalte were obtained from Sigma-Aldrich. All chemicals were reagent grade and used without further purification. The NIPAM obtained has a purity of at least 95%. The synthesis was performed in a single step preparation employing a conventional stirring technique (300 rpm). After dissolving 55.0 mmol NIPAM, the desired amount of BIS, and AAc in 500 mL of (Milli-Q) water, the mixture was heated to 70 °C under a nitrogen atmosphere to exclude oxygen. Then, 40 mg of potassium persulfate dissolved in 1 mL of triple-distilled water was added to start the polymerization. The reaction proceeded for 24 h at constant temperature. Thereafter, the microgel suspension was slowly cooled down to room temperature (4 h) under continued stirring. The final step of the preparation comprised of extensive washing of 250 mL of the reaction solution in an ultrafiltration cell (Millipore, 350 mL) using a celluloseacetate filter (Satorius, pore size 0.45 µm). In each washing cycle, 300 mL water was added and almost completely removed. Some of the water inevitably remained in the hydrogel particles. For the last two washing cycles, the conductivity of the outflowing water was constant at 3 µS/m. After carrying out five washing cycles in total, the particle suspension is freeze-dried. The yield was ≈85% with respect to the starting concentration of the monomers. 2.2. Preparation of Microgel Dispersions. Throughout this work, microgel dispersions with a concentration of 0.05 wt % were used. The microgel suspensions were vigorously shaken for 24 h to redisperse the freeze-dried microgel powder in solution. Then, the suspensions were filtered over a syringe filter with a 5 µm pore size to remove large aggregates. The pH adjustment of the particle suspensions was accomplished by using five different solutions as a dispersing medium. The respective pH of the aqueous P(NIPAMco-AAc) particle solutions is given in brackets: • 0.01 HCl (f pH 2.3) • Pure water (Milli-Q) (f pH 4.9) • Two phosphate buffer solutions (f pH 6.4 and pH 9.8) (24) Snowden, M. J.; Chowdhry, B. Z.; Vincent, B.; Morris, G. E. J. Chem. Soc., Faraday Trans. 1996, 92, 5013–5016. (25) Kim, J.-H.; Ballauff, M. Colloid Polym. Sci. 1999, 277, 1210–1214. (26) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf. A 2000, 170, 137–149. (27) Debord, J. D.; Lyon, L. A. Langmuir 2003, 19, 7662–7664. (28) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247–256. (29) Kratz, K. Intelligente Poly-N-Isopropylacrylamid-Mikrogele unterschiedlicher chemischer Zusammensetzung. Einfluss Von KonnektiVita¨t, Ladungsdichte und Ionensta¨rke auf das QuellVerhalten Von PNIPA-Kolloiden. Thesis, University of Bielefeld, 1999.
Schmidt et al. • 0.01 NaOH (f pH 11.5) The ionic strength of the buffer solutions was 0.025 M. The water used in all experiments was deionized with a Purelab Plus system (USF Seral) and had a conductance lower than 55 nS/cm. 2.3. Surface Modification. As substrates, either bare silicon wafers or polymer precoated silicon wafers were used. The silicon wafers were a gift from Wacker Siltronic AG (Burghausen, Germany) and they were cleaned with a 1:1 mixture of H2O2 and H2SO4 for 30 min. In water (pH ) 5.5), the silicon wafer is negatively charged. The 1-1.5 nm thick native silicon oxide layer is neglected in the following notation. For coating the wafers, the layer-by-layer (lbl) method30-33 was used. Thereby, polyanions and polycations are deposited alternately from aqueous solutions. The first layer is a (cationic) polyethylene imine (PEI) layer (SI/PEI). In order to get negatively charged surfaces again, in some cases a polystyrene sulfonate layer was added (Si/PEI/PSS). Beside (Si/PEI) also other polycation terminated coatings were studied: (Si/PEI/PSS/PAH) or (Si/PEI/PSS/PDADMAC) (PAH poly(allylamine hydrochloride); PDADMAC poly(diallyldimethylammonium chloride)). PEI, PSS, and PAH were purchased from Sigma-Aldrich. PEI and PAH were used without further purification, and PSS was dialyzed against Millipore water and freeze-dried before use. PDADMAC was a gift from W. Jaeger from the Fraunhofer Institute for Applied Polymer Research in Golm (Germany). The molecular weights are the following: PEI 75.000 g/mol, PAH 65.000 g/mol, PSS 70.000 g/mol, and PDADMAC 100.000 g/mol. The aqueous polyelectrolyte solutions contained 10-2 monomol/L (referring to the monomer concentration) of the respective polyelectrolyte and 0.5 NaCl with the exception of PEI which was dissolved in pure water. The wafers were dipped in the respective polyelectrolyte solution for 20 min. 2.4. Preparation of P(NIPAM-co-AAc) Films. In the present work, the microgel particles were dip-coated from aqueous solutions on silicon wafers. A dipping robot was used to achieve a better reproducibility of the film preparation procedure. It allows moving the wafer in and out of the solutions with a defined speed. The washing solutions were the same solutions, which were used for preparation of the particle suspensions (i.e., same pH). Three standard dipping procedures were used: (I) dipping and drying, (II) dipping, drying, washing in water, and drying or (III) dipping, washing in water, and drying. The wafer was dipped into the solution or into water with a speed of 20 mm/s and taken out with 0.01 mm/s. The wafer remained in the particle solution for 30 min and in water for 12 h. 2.5. Methods. (a) ζ Potential Measurements. ζ potentials of the microgels were measured using Malvern zeta Sizer 3000 HS. (b) Scanning Force Microscopy (SFM). Tapping mode scanning force microscopy (TA-SFM) was employed to characterize the morphology of dried mircogel particles adsorbed on the wafer. The SFM imaging was performed with a Multimode (nanoscope III) from Veeco in air using a tapered Si-cantilever (Nanosensors, PPPNCHR-50) with a tip radius smaller than 10 nm. All SFM images shown in this work were processed using a topography scale from 0 to 100 nm. The scanning size of all images shown in work was 20 × 20 µm2. Cantilevers with a nominal resonance frequency 40 kHz (air) were operated in tapping mode. (c) Ellipsometry. Ellipsometry measurements were carried out with a Multiscope from Optrel.34,35 Since the polymer films were very thin both, before and after the deposition of microgel particles, only one polarizer angle changed with respect to the bare silicon substrate, which allowed the determination of only one parameter (thickness or refractive index). Therefore a refractive index of n ) (30) Decher, G. Science 1997, 277, 1232–1237. (31) von Klitzing, R.; Tieke, B. In Polyelectrolytes with Defined Molecules Architecture I; Schmidt, M., Ed.; Advances in Polymer Science; Springer: Heidelberg, 2004; Vol. 165, p 177. (32) Neff, P. A.; Naji, A.; Ecker, C.; von Klitzing, R.; Bausch, A. R. Macromolecules 2006, 39, 463–466. (33) von Klitzing, R. Phys. Chem. Chem. Phys. 2006, 8, 5012–5033. (34) Harke, M.; Teppner, R.; Schulz, O.; Motschmann, H.; Orendi, H. ReV. Sci. Instrum. 1997, 68, 3130–3134. (35) Teppner, R.; Motschmann, H. Ellipsometry in interface science; Elsevier B.V.: New York, 2001.
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Table 1. ζ Potential Measurements at Room Temperature of (a) PNIPAM-co-AAc Particles in Solution of Varying pH and (b) PNIPAM Particles Containing No Comonomer particle PNIPAM-co-AAc PNIPAM-co-AAc PNIPAM-co-AAc PNIPAM-co-AAc PNIPAM-co-AAc PNIPAM PNIPAM
pH
ionic strength [M]
PNIPAM-co-aac particles (5%) 2.3 0.01 (5%) water 10-5 (5%) water 0.025 (5%) 6.4 0.025 (5%) 9.8 0.025 PNIPAM particles 2.0 0.01 water 10-5
ζ [mV] -0.5 ( 1 -27 ( 3 -10 ( 2 -16 ( 2 -16 ( 2 -0.5 ( 2 -18 ( 2
1.5 was assumed, and all thicknesses were calculated for this fixed refractive index.
3. Results 3.1. Characterization of Particles in Particles in Bulk. Table 1 summarizes the ζ potential measurement on various PNIPAMco-AAc particles in solutions of varying pH (a). Measurements of PNIPAM particles containing no AAc comonomers are presented in part b. (a) PNIPAM-co-AAc at Varying pH. One can see that already at pH 6.4 complete dissociation of carboxylic acid groups is achieved. There is no further decrease in ζ potential at beyond this pH for the same ionic strength. The ζ potential of microgels is higher in buffer solutions than in water of the same ionic strength indicating that not all AAc groups are charged in water [The Milli-Q water has a pH 4.9, i.e. very close to the pKa of AAc (4.5) and poly(acrylic acid) (5.1)]. Increasing the ionic strength leads to a decrease in ζ potential as can be seen from measurements in water in the absence and presence of added salt. Complete protonation of carboxylic acid groups at low pH leads to nearly uncharged particles and a ζ-potential of approximately 0 (some residual surface charges are caused by unhydrolysed sulfate groups on the particle surface stemming from the radical starter used in the synthesis). (b) Pure PNIPAM particles. For pure PNIPAM microgels in deionized water, a negative ζ-potential of -18 mV is found due to the fact that the PNIPAM particles contain negatively charged sulfate groups originating from potassiumperoxodisulfate as initiator. However, in solutions of low pH, the sulfate groups are easily hydrolyzed to form uncharged hydroxyl groups
Hydrolysis of sulfate groups: H+
R-OSO3- + H2O f R-OH + HSO4which then results in a decrease to ζ ≈ 0. This is in agreement with the observations made for case a. 3.2. Characterization of Thin Films of Microgel Particles. (a) Effect of the pH Value of the Solution. Figure 1 shows SFM micrographs of Si/PEI wafers coated with P(NIPAM-co-AAc) particles from 0.05 wt % dispersions adjusted to pH 2.3, 6.4, 9.8, and 11.5. Dip coating procedure I was used where the wafers were not washed but directly investigated after pulling out of the particle solution. All SFM micrographs were recorded after drying the film in air. If the particles are deposited from pH 2.3, a nearly close packed film with a small number of defects will be formed (Figure 1a). The packing density in this case is similar to one produced by spin-coating.15,36 With increasing pH, the packing density decreases. Moreover, the particles obviously assembles (36) Schmidt, S.; Motschmann, H.; Hellweg, T.; von Klitzing, R. POLYMER 2008, 49, 749–756.
in domains of higher particle density and order, which becomes very pronounced at pH 11.5 (Figure 1d). There, the particles have a defined distance of about 400 nm to each other within the domains. This is different in the pH interval from pH 2.3 to 9.8, where the distance is smaller. The change in packing density was followed more quantitatively by ellipsometry measurements, shown in Table 2. Ellipsometry measurements on the respective wafers against air give values for a thickness that correspond to the packing density (apparent thickness) and averages over approximately 1 mm2 sample area. Under the assumption that the shape of the particles is more or less the same in air irrespective of the pH during preparation, the ellipsometry results indicate the change in packing density generated at different pH values. The relative changes in average film thickness are consistent with the results from SFM imaging shown in Figure 1, i.e. dense packing and highest film thickness at pH 2.3, reduced packing density at pH 11.5, and intermediate packing density for pH 6.4 and 9.8. (b) Effect of the Preparation Method. Figure 2 shows SFM micrographs of Si/PEI wafers coated with P(NIPAM-co-AAc) particles deposited from 0.05 wt % dispersions adjusted to pH 2.3 and 6.4. The wafers were either dried after particle deposition and then washed (procedure II, micrographs in the upper row) or directly washed after particle deposition (procedure III, micrographs in the bottom row). The SFM micrographs were again recorded with dry films. The averaged film thicknesses measured with ellipsometry are listed in Table 2. As already observed for films prepared with dip coating procedure I, the films prepared at pH 2.3 show a higher film thickness and a higher packing density as compared to the preparation from pH 6.4. Furthermore, the average film thicknesses achieved with procedure II are very similar to the ones obtained with procedure I, but the distributions look different for both procedures. At pH 2, the particles are regularly arranged for procedure I, while for procedure II, the particle layers are more inhomogeneous. At pH 6.4, the distribution after washing (procedure II and III) is much more regular than with dipping procedure I. For procedure III at pH 2.3, the particle density is obviously reduced with respect to I and II. To summarize, at pH 2.3, washing of a still-wet microgel film decreases the particle density, while dry films appear to be more stable upon reimmersing in liquid. At 6.4, the packing density on the surfaces remains constant (II) or is only slightly reduced (III), but a reorganization of the surface structure and particle distribution is induced for these samples when immersed into aqueous buffers. An increase of the 2D order is generated by this treatment. (c) Effect of the Substrate Precoating. Five different substrates have been prepared and used for microgel deposition from solutions all having a pH value of 6.4. Table 3 is listing the respective substrates and the film thickness before and after dip coating into the PNIPAM-co-AAc microgel dispersion. Measurements performed prior to dip coating give the thickness of the polyelectrolyte layer (PEL) only. It can be seen that by consecutive deposition of polyelectrolytes a gradual increase in thickness of the polyelectrolyte (multi)layer is achieved. Ellipsometry measurements done after deposition of microgels on negatively charged bare Si surfaces and Si/PEI/PSS show the smallest film thickness. On the positively charged substrates Si/PEI and Si/PEI/PSS/PDADMAC, the averaged film thicknesses of the microgels are similar, whereas the film thickness of Si/ PEI/PSS/PAH system is clearly larger. SFM micrographs of microgel particles adsorbed on different substrates are presented in Figure 3. The relative changes of the adsorbed amounts of particles observed in the SFM images are
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Figure 1. SFM images of P(NIPAM-co-AAc) miicrogels deposited on Si/PEI according to dip procedure I. The 0.05 wt % particle suspension was adjusted to pH 2.3 (a), 6.4 (b), 9.8 (c) and 11.5 (d). Dark lines are due to a line flattening artifact during image processing. Also for the sake of comparability, we always applied the same flattening procedure to all AFM images. The fitting procedure comprised of subtraction of a first order polynomial fit for each line on the fast scanning axis using the Nanoscope III software v5.21 (digital instruments). This fitting procedure proved to be the best compromise for the whole series of AFM images. Clearly, other fitting procedures would give better results but only for individual scans. Table 2. Average PNIPAM-co-AAc Layer Thicknesses for Wafers Prepared According to Dip Coating Procedures I,II, and III Using 0.05 wt % Particle Suspensions Adjusted to Different pH Values pH of particle suspension procedure I procedure II procedure III 2.3 6.4 9.8 11.5
27.1 ( 2 nm 25.1 ( 2 nm 16.6 ( 1 nm 10.6 ( 1 nm 10.6 ( 1 nm 7.5 ( 1 nm 10.6 ( 1 nm 7.3 ( 1 nm
in good agreement with averaged film thickness measurements done by ellipsometry. It is remarkable that the negatively charged particles adsorb also on negatively charged substrates. On each substrate, we observe quite regular packing of microgels with the exception of the microgel film prepared on the Si/PEI/PSS/PAH substrate. Here, the particles form aggregates on the surface which leads to a higher packing density and an unexpectedly high film thickness as well. Next, in order to study the differences in packing density of the PAH and PDADMAC terminated microgel substrates the surfaces imaged via liquid SFM before the microgel deposition at pH 6.4 solution. Figure 4 shows that the PAH terminated surface appears much rougher compared to the PDADMAC terminated surface. Calculations of the rms [root-mean-square deviation from the mean height of the surface] values confirm this finding: PDADMAC and PAH terminated surfaces possess
rms values of 0.6 and 1.2 nm, respectively. That is, the rougher surface leads to a higher amount of adsorbed particles. The formation of microgel films prepared with nearly uncharged microgels at pH 2.3 is weakly influenced by the type of terminating polyelectrolyte on the surface as shown in Figure 5. Regardless of the type of substrate and surface charge, similar film structures were found. In line with SFM micrographs, ellipsometry measurements give similar film thicknesses of about 25 nm. The adsorbed amount of particles is generally higher when films are prepared at pH 2.3 compared to preparation in pH 6.4.
4. Discussion 4.1. Effect of the pH Value of the Solution. Simplified View of the System. If the deposition of PNIPAM-co-AAc groups on a PEI coated substrate is controlled by electrostatic interaction, we can expect a strong influence on the adsorbed amount of particles varying the pH of the particle solution from values smaller than pKa of AAc comonomers to larger pH than the pKa of ammonium groups of PEI [The pKa is about 9-10]. Electrostatic interaction seems to be important here as electrophoretic measurements on microgels showed, that charged groups are likely to assemble in the outer shell of the particles, hence (37) Daly, E.; Saunders, B. R. Phys. Chem. Chem. Phys. 2000, 2, 3187–3193.
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Figure 2. SFM images of P(NIPAM-co-AAc) microgel particles deposited on Si/PEI according to dip procedure II (a and b) and III (c and d). The 0.05 wt % particle suspension was adjusted to pH 2.3 (a and c) and pH 6.4 (b and d). Table 3. Ellipsometry Measurement against Air: Film Thickness for Different Substrates, before and after Deposition of the PNIPAM-co-AAc Layer from pH 6.4 Solution substrate Si/PEL Si Si/PEI Si/PEI/PSS Si/PEI/PSS/PDADMAC Si/PEI/PSS/PAH
film thickness averaged film thickness PEL-layer [nm] PEL/microgel-layer [nm] none 0.6 ( 0.2 1.9 ( 0.3 3.1 ( 0.3 3.1 ( 0.3
8.9 ( 0.5 11.8 ( 1 8.0 ( 0.6 10.2 ( 1 20.2 ( 3
being able to interact with the surroundings and e.g. charged surfaces. This was also found by Daly and Saunders.37 The situation in terms of surface charge of the individual components (particles and substrate) is schematically shown in Figure 6. At pH 2.3, almost all acrylic acid groups (pKa ≈ 4.8) are protonated; the particles are more or less uncharged (see ζ potential measurements in Table 1). Moreover, in this pH range, the silanol groups are protonated and the surface of the silicon wafer is uncharged or even slightly positively charged.38,39 The PEI layer is positively charged at pH 2.3. From pH 6 to pH 10 both, the PEI layer and the microgel particles are charged. Above pH 11.5, the PEI layer is uncharged, while the particles remain charged. Influence of the Microgel and Substrate Charge on the Adsorption. From Figure 6, one can see that there are two counteracting electrostatic forces which control the distance between the particles: First, repulsion between the particles and (38) Bergna, H. E. The Colloid Chemistry of Silica; American Chemical Society: Washington, D. C., 1994. (39) Pfeuffer, A. Dissertation, CAU Kiel, 2006.
second, attraction of the surface and particles. Between pH 6 and 10, where both forces exist, there is a competition between the attractive surface-particle interaction and the interparticle repulsion. Which of the competing interactions previal and therefore control particle adsorption? From the results, we can conclude that rather the charge of the particles than the charge of the substrate controls the adsorbed amount of particles. This is confirmed by finding similar amount of particles on uncharged and charged PEI layer (pH 11.5 and pH 6-9) (see Figure 1). The electrostatic contribution of the substrate to the microgel adsorption seems to be less important in comparison to nonelectrostatic interactions. A clear argument for the pronounced influence of the repulsive particle T particle interactions on the adsorbed amount is given by the fact that only if the charge of the particles is “switched off” is a close-packed particle film formed. Furthermore, ellipsometry measurements indicate an increase in film thickness by a factor of 3 if the film is prepared with uncharged particles instead of charged particles. Close-packing of the particles at pH 2.3 is also supported by interparticle attractive hydrogen bonding between the protonated AAc groups of different particles. This observation is partially in agreement to findings by Lyon et al.14 for the particle deposition above the LCST. On the other end of the pH scale at pH 11.5, the PEI surface is uncharged, whereas the particles remain charged. These conditions favor low particle densities and also enhance particle mobility on the surface. This facilitates positioning of the particles to a more regular and loose structure. Such a rearrangement can be driven by the Coulombic repulsion between the microgel particles. Capillary attraction between the particles might
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Figure 3. P(NIPAM-co-AAc) microgel particles deposited on bare silicon Si (a), Si/PEI (b), Si/PEI/PSS (c), Si/PEI/PSS/PAH (d), and Si/PEI/ PSS/PDADMAC (e) by procedure II. Preparation at pH 6.4.
Figure 4. SFM images of Si/PEI/PSS/PDADMAC surface (left, rms roughness: 0.6 nm) and Si/PEI/PSS/PAH surface (right, rms roughness: 1.2 nm). Measurement at pH 6.4.
counteract this repulsion. During drying, a thin water wetting film (air/microgel dispersion/substrate) is formed in which capillary forces can act.40 This leads to areas of higher particle density separated by areas of lower density. A typical consequence is that increasing particle concentration does decrease the particle distance but increases the size of the areas of 2D colloidal crystals. Such a pattern is observed at pH 11.5. Tsuji and Kawaguchi concluded that electrostatic repulsion between the low charged particles has a minor effect and that the ordering is dominated by capillary attraction and steric repulsion.23 In their study the charge was only introduced by the starter. Clear evidence for the occurrence of capillary forces was the fact that the inter particle distance was independent of the particle concentration. Below full coverage, domains of 2D crystals were formed, and their size depends on the particle concentration. (40) Denkov, N. D.; Velev, O. D.; Kralchewsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183.
The maximum particle distance (surface-surface distance) found in the present work is about 650 nm. Electrostatic interaction could not act over this large range. The AFM images are recorded in the dry state. There the particles at the surface have a maximum diameter of about 650 nm and a maximum height of about 100 nm. It is assumed, that the particles contain 80-90% water36,41 in the swollen state, which is partially lost in ambient conditions. The particles should shrink at the surface. But, dynamic light scattering in aqueous solutions gives the same diameter of about 600-700 nm, depending on the pH. Two explanations are possible. Either the two effects of flattening during adsorption and shrinking after drying compensate each other or the particles stick at the surface and can only shrink perpendicular to the surface but not in lateral directions. The latter explanation contradicts the finding that obviously the particles can rearrange, i.e. move along the surface without changing their shape after washing processes. This paradox can only be clarified by SFMimaging under liquid. But so far, the lateral resolution was not sufficiently high for giving a clear statement on this point. Another repulsion could be induced by dipole-dipole interaction. In the thin film, which is formed during drying, a dipole at the particles occurs at the liquid/air interface due to different dielectric constants of the two adjacent media. The charge distribution around the particles differs in air and in water and leads to a dipole. In former studies, it was shown that water wetting films are stable on negatively charged surfaces, but unstable on cationic (41) Steitz, R.; Schmidt, S.; Hellweg, T.; Ecker, C.; von Klitzing, R. Experimental Report of the Hahn Meitner Institut; PHY-04-1228, 2006; in preparation.
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Figure 5. P(NIPAM-co-AAc) microgel particles deposited on bare silicon Si (a), Si/PEI (b), and Si/PEI/PSS (c) by procedure II. Preparation at pH 2.3.
Figure 6. Cartoon illustrating the surface charge of Si/PEI/PNIPAM layers at different pH.
ones.42 One explanation was the electrostatic interactions between the negatively charged air/water interface and the solid surface. In addition, contact angle measurements showed a lower contact angle for water droplets on polyanion terminated layers than on polycation terminated layers.43 Microscopic images of unstable water films on PEI showed that as the water film ruptures domains of wetted and dry areas are formed on the PEI precoated silicon wafer. The size of these areas is of the same order of magnitude as the formed particle domains observed when the deposition is done from solutions having pH 6.4, 9.8, or 11.5.42 When dip coating is done with a particle suspension at pH 11.5, the PEI layer is not charged. But also then no electrostatic repulsion between the two surfaces of the wetting film exist which could stabilize the wetting film. To summarize, the existence of a thin water film seems to be necessary in order to induce attractive capillary forces between the particles. 4.2. Effect of the Substrate. According to the sketch (Figure 6) using differently charged substrates should result in different adsorbed amounts of the also charged particles on the individual substrates. But, the fact that at pH 11.5 still particles adsorb on PEI indicates that the surface charge only marginally influences the adsorption.
We also find that the particles adsorb even on repulsive surfaces (bare silicon and PSS, Figure 3 and Table 3). The packing density is reduced only by 20% as compared to adsorption on a oppositely charged surface (PEI or PDADMAC). This implies that electrostatic interactions between particles and surface controls adsorption of the microgels only to a minor extent. Other interactions like hydrogen bonding seem to be important as well. On Si/PEI/PSS, one could argue that the negatively charged particles at pH 6.4 are electrostatically attracted by the underlying PEI layer. It is well-known that adjacent polyelectrolyte layers are strongly interdigitated,44,45 which could lead to locally opposite charges at the interface. But, the adsorption density on bare negatively charged silicon is quite similar, which contradicts the argument of interdigitation. Another important consequence of different precoatings might be a change in three-phase-contact line, when the wafer is pulled out of the dipping solution. But, the contact angle for PAHterminated precoatings shows an intermediate value (34°) with respect to the other used precoatings (24-40°),43 which cannot explain the difference in packing density. Another parameter that determines the packing density of the microgels is the substrate roughness (Figure 4). The PAH and
(42) Ciunel, K.; Armelin, M.; Findenegg, G. H.; von Klitzing, R. Langmuir 2005, 21, 4790–4793. (43) Ha¨nni-Ciunel, K.; Findenegg, G. H.; von Klitzing, R. Soft Mater. 2007, 5, 61.
(44) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058–7063. (45) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W.; Kjaer, K. Macromolecules 1998, 31, 8893–8906.
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PDADMAC terminated substrates were compared in terms of surface topography. It turns out that the rms roughness of the PAH terminated surface is twice as large as compared to the PDADMAC terminated surface. The microgel film thickness of the PAH terminated surface is increased by a factor of 2. Probably the rougher PAH surface offers more binding sites for microgel particles, which leads to a higher surface packing of the particles, while the smoother PDADMAC terminated surface shows reduced packing but a regular structure. Of course, the surface roughness is by 2-3 orders of magnitude lower than the diameter of water swollen hydrogel particles. That means that the surface should look flat on a length scale of the diameter of a particle. However, on a molecular level the particles are assumed to have a “hairy” surface with loops and dangling ends, which can locally interact stronger with a rough than with a smooth interface. It is assumed that the negative charge of the particles is compensated to a higher degree on rougher, positively charged surfaces. Therefore, repulsive electrostatic interactions between microgels are reduced and agglomeration (see SFM image) is rendered possible, as found for microgels on the PAH terminated substrate. At pH 2.3, the effect of the substrate is less pronounced due to a lack of particle charge. In this case, the nonelectrostatic contribution (e.g., hydrogen bonding) seems to dominate the interaction with the substrate, which eliminates the difference between negatively and positively charged substrate with respect to microgel adsorption. In addition, the reduced interparticle repulsion leads to formation of areas of high particle density. This is similar to the bulk behavior of these copolymer microgel particles.26,36 At low pH value, the particles form aggregates in solution, as the temperature is raised above the LCST of PNIPAM. Recently, thin films were prepared by the deposition of PNIPM block copolymer micelles.46,47 This leads to rather homogeneous films. The micelles are smaller and thermodynamically stable entities which might lead to a strong change in conformation or even destruction in front of the surface. In addition, the block(46) Wang, W.; Troll, K.; Kaune, G.; Metwalli, E.; Ruderer, M.; Skrabania, K.; Laschewsky, A.; Roth, S. V.; Papadakis, C. M.; Mu¨ller-Buschbaum, P. Macromolecules 2008, 41, 3209. (47) Troll, K.; Kulkarni, A.; Wang, W.; Darko, C.; Bivigou Koumba, A. M; Laschewsky, A.; Mu¨ller-Buschbaum, P.; Papadakis, C. M Colloid Polym. Sci. 2008, 286, 1079.
Schmidt et al.
copolymers are uncharged, which leads to a tight packing due to missing electrostatic repulsion.
5. Conclusions P(NIPAM-co-AAc) microgel particles are deposited on modified silicon wafers by three different dipping techniques at different pH values. The results show that the pH value of the dipping solutions has the most pronounced effect on the particle density at the surface. With increasing pH, the acrylic acid units become more charged and the microgel beads repell each other, which leads to a lower packing density. At the used concentration of 0.05 wt %, domains of particles are formed at higher pH, which is assumed to be caused by only partial wetting of the suspension on the substrate during preparation. The electrostatic particle-particle interaction seems to be much more important than the electrostatic interaction between the particles and the substrate. That is indicated by a pronounced adsorption of negatively charged particles on uncharged substrates and by the rather small effect of the sign of the substrate charge on the adsorbed amount (reduction by ca. 20%). Nonelectrostatic contributions like hydrogen bonding seem to play an important role. A higher surface roughness leads to aggregation of particles, which is due to the partial “wrapping” by the dangling ends and loops on such a rough surface and partial screening of the particle charge. This effect causes a higher packing density of adsorbed particles. On a molecular scale, this can be interpreted by a higher number of binding sites on rough surfaces in comparison to smooth ones. Washing after preparation affects the particle distribution. In general, the pattern becomes more ordered. The question is whether the distribution changes in water or when the wafer is pulled out. Studies on this topic are underway. Acknowledgment. The authors acknowledge the DFG-Priority program SPP 1259 “Intelligente Hydrogele”, the Fonds der Chemischen Industrie (FCI), and EU Cost action D43 for financial support. We are grateful to Wacker Siltronic for providing silicon wafers and to Werner Jaeger from the Fraunhofer Institute in Postdam for PDADMAC samples. LA801770N
