Orientational Ordering of Structured Polymeric Nanoparticles at

Mar 9, 2002 - BASF Aktiengesellschaft, Polymer Research and Product Development Adhesives and. Construction Chemicals, 67056 Ludwigshafen, ...
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Orientational Ordering of Structured Polymeric Nanoparticles at Interfaces A. Pfau,*,† R. Sander,† and S. Kirsch‡ BASF Aktiengesellschaft, Polymer Research and Product Development Adhesives and Construction Chemicals, 67056 Ludwigshafen, Germany Received June 21, 2001. In Final Form: December 11, 2001 Straight acrylic composite nanoparticles with a mean diameter of 132 nm and a mushroomlike particle morphology consisting of a soft poly(n-butyl acrylate) phase with a caplike poly(methyl methacrylate) hard phase covering one side of the particle were prepared by emulsion polymerization. The particles show oriented adsorption when deposited from a diluted dispersion as single particles onto flat substrates. On hydrophilic surfaces, the hard phase is preferentially oriented toward the substrate, whereas on hydrophobic substrates the soft phase is preferentially oriented toward the surface. Orientation at the film substrate interface and at the outer surface is also found when depositing dispersion films with thicknesses in the micron range. However, here the orientation does not change with the character of the surface and always the more hydrophobic soft phase is exposed. These findings are discussed in a framework of hydrophilic and hydrophobic interactions.

1. Introduction Although they are long-established areas of research, ordering or self-assembly phenomena have in recent years attracted increasing attention given the emergence of smart materials. Ordering is of prime interest for quite a number of systems, for example, three-dimensional ordering in block copolymers [e.g., ref 1] or in polyelectrolyte surfactant complexes [e.g., refs 2 and 3], twodimensional ordering in all sorts of self-assembly layers [e.g., refs 4 and 5] or ordered layers6 or possibly multilayer systems,7 and one-dimensional ordering as in the micelles of surfactants8 or block copolymers. Perhaps the most common case is the ordering of small molecules (e.g., surfactants), where ordering in solution and at boundary interfaces according to the geometry and polarity or hydophilicity/hydrophobicity of their parts is well understood.9,10 When proceeding toward bigger entities, such as copolymers, they are known to form ordered structures in solution, which may persist upon adsorption to a surface and may show lateral ordering.6 However, the interaction with the surface may also lead to a rearrangement as for example in the case of the adsorption of micelles formed from poly(tert-butylstyrene)-sodium polystyrenesulfonate (Mw ) 87-160 kDa) on silica. Here, the micelles rearrange such that poly(tert-butylstyrene) blocks, which are hydrophobic, face the substrate thus †

BASF Aktiengesellschaft, Polymer Research. BASF Aktiengesellschaft, Product Development Adhesives and Construction Chemicals. ‡

(1) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. (2) Ober, C. K.; Wegner, G. Adv. Mater. 1997, 9, 17. (3) Antonietti, M.; Maskos, M. Macromolecules 1996, 29, 4199. (4) Chechik, V.; Scho¨nherr, H.; Vancso, G.; Stirling, C. J. M. Langmuir 1998, 14, 3003. (5) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719. (6) Spatz, J. P.; Mo¨ller, M.; Noeske, M.; Behm, R. J.; Pietralla, M. Macromolecules 1997, 30, 3874. (7) Decher, G. Layered Nanoarchitectures via Directed Assembly of Anionic and Cationic Molecules. In Comprehensive Supramolecular Chemistry; Sauvage, J. P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9. (8) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1995; Part 3, p 341 ff. (9) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (10) Manne, S.; Gaub, H. E. Science 1995, 270, 1480.

forming a bilayer.11 However, to our knowledge only little is known about an oriented adsorption (this means here rotational orientation) of bigger structures such as polymer latex particles, despite their being of great technical interest and having many possible applications.12 In this paper, we will show how dispersion particles with asymmetric structures, suitable for ordering at surfaces and interfaces, are synthesized, give direct microscopic evidence for the proposed ordering toward a substrate surface in a highly diluted state, and show that ordering is also found at the surfaces and film/substrate interface of concentrated systems. The findings and the concept behind them are discussed within a simple framework of hydrophobic/hydrophilic interactions. 2. Experimental Section 2.1. Particle Synthesis. Synthesis was performed in a 2000 mL four-necked flask equipped with a reflux condenser, N2 gas inlet tube, blade stirrer running at 150 rpm, and inlet funnels to feed the pre-emulsions and initiator solution. The poly(n-butyl acrylate)/poly(methyl methacrylate) (PnBA/ PMMA) composite latex particles were prepared by a conventional, in situ seeded, semibatch emulsion polymerization process. In the first stage, PnBA (glass transition temperature Tg approximately -8 °C) was prepared by reacting 30 g of nBA, 417 g of water, 7.5 g of a 15 wt % aqueous solution of SDS, and 12 g of the initiator solution (0.545 mM aqueous solution of NaPS) at 85 °C for 15 min. The first stage pre-emulsion was prepared from 230 g of water, 95 g of SDS solution, 78 mM acrylic acid, and 4.39 mol nBA. The first stage was buffered by adding 50 g of a 0.2 wt % solution of NaPP. The pre-emulsion and 45 g of initiator solution were fed for 2 h to the reaction mixture. The second stage pre-emulsion consisted of 90 g of water, 10 g of SDS solution, 78 mM acrylic acid, and 1.87 mol MMA. The second stage material (Tg of PMMA ) +105 °C) was fed after the end of the first stage with 18 g of initiator solution for another 45 min into the reaction vessel. After the polymerization, the dispersions were neutralized to pH 7 and then allowed to cool to room temperature (RT). The final particles are of a phase ratio of 75% soft (low Tg) phase and 25% hard (high Tg) phase. (11) Amiel, C.; Sikka, M.; Schneider, J. W., Jr.; Tsao, Y.-H.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125. (12) Baumstark, R.; Kirsch, S.; Schuler, B.; Pfau, A. Farbe & Lack 2000, 106, 125.

10.1021/la010942x CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002

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Figure 1. TEM picture (left) and schematic representation (right) of the particle morphology of the sample with 75 parts soft phase and 25 parts hard phase. The particle size and particle size distribution were measured by means of dynamic light scattering (DLS) and capillary hydrodynamic fractionation (CHDF). The mean diameter of the particles was 132 nm with a standard deviation of 5.9. 2.2. Particle Deposition and Sample Preparation for Atomic Force Microscopy (AFM). Particle coverages in the monolayer or submonolayer range were prepared by highly diluting the latex dispersions with purified water and placing a drop of this solution on the respective substrate. The surplus water was blown off the substrates with dry nitrogen, and the substrates were then left to dry at ambient conditions. Thicker films in the micron range were prepared by placing a drop of concentrated dispersion (solid content approximately 50% by mass) onto the substrate and drawing it down with a glass slide. Also, the thicker films were left to dry under ambient conditions. The substrates used were muscovite mica (Balzers Union, Lichtenstein), silicon wafers with native SiO2 (Wacker, Germany), and single crystalline (10-14) CaCO3 surfaces (Kristallhandel Kelpin, Leimen, Germany). The mica was freshly cleaved before use. The silicon wafers and the CaCO3 were cleaned in MilliQ water and ethanol. The SiO2 surfaces were silanized using n-octadecyltrichlorosilane (Aldrich) and a standard silanization procedure.13 The CaCO3 was in some cases modified with a dispersant, that is, a shortchain acrylic (Polysalz S, BASF-AG, Germany), by saturation adsorption and consecutive washing with MilliQ water. The surface preparation and modification were checked by static contact angle measurements using water. 2.3. AFM. The AFM measurements were performed in air on a Nanoscope Dimension 3000 SPM (Digital Instruments) using Si cantilevers. For working in Tapping Mode (TM), hard Si levers were used (35 N/m, ν0 approximately 300 kHz, tip radius 10 nm, Nanoprobe; operation at the low-frequency side of the resonance), whereas for pulsed force mode (PFM)14 softer levers of the same type were employed (2 N/m, ν0 approximately 75 kHz, tip radius 10 nm, Nanoprobe). The system was equipped with a phase extender box that allows the simultaneous acquisition of height and phase data in TM operation. For PFM, a commercial add-on by Witec (Ulm, Germany) was used. All images were acquired in ambient conditions at room temperature.

3. Results 3.1. Particle Structure. The most common way to synthesize composite latex particles is a semibatch emulsion polymerization process.15 This technique allows the creation of particles with well-defined particle size distribution and chemical composition. By using different monomers at different stages in a seeded emulsion polymerization, complex particle morphologies (phase distributions within a particle) can be achieved. In this

process, the structure of the multiphase particles is influenced by an interplay of thermodynamic and kinetic factors.16-19 Hence, in most cases the resulting particle morphology cannot be described simply as a core/shell type. Transmission electron microscopy (TEM) measurements were performed to obtain independent information on the particle morphology. To enhance phase contrast in this system of pure acrylics, negative staining with UAc was used in combination with preferential staining by RuO4 as described elsewhere.20 Owing to the polymer itself and polymer surface tension of PnBA and PMMA, phase separation should occur from the thermodynamic point of view. When kinetic control is predominant, we would expect a thin PMMA shell due to the more hydrophilic character.21 In Figure 1 (left), the result from the TEM measurement is shown. The light areas on top of the particles are the phase-separated PMMA which is not stained by RuO4. Two particles were merged together due to the low Tg of the soft, first-stage PnBA. As the TEM result (Figure 1, left) clearly shows, the impact of the thermodynamic factor is predominant. A phase-separated, hemisphere-like structure is observed. Figure 1 (right) gives a schematic representation of the particle structure. 3.2. Detection of Orientational Order. Figure 2 shows mica surfaces covered with dispersion particles in the submonolayer range. The phase-contrast images are dominated by light annular features, which are surrounded by structureless dark areas. Here, the light and dark areas correspond to positive and negative phase shift, respectively. On the basis of the overall composition of the particles, it is straightforward to assign the light phase to the PMMA (hard) and the dark phase to the PnBA (13) Gelest Manual; Gelest Inc.: Tullytown, PA, 1995. (14) Rosa-Zeiser, A.; Weiland, E.; Hild, S.; Marti, O. Meas. Sci. Technol. 1997, 8, 1333. (15) Lee, D. I. Makromol. Chem., Macromol. Symp. 1990, 33, 117. (16) Dimonie, V. L.; Daniels, E. S.; Shaffer, O. L.; El-Aasser, M. S. In Emulsion Polymerisation and Emulsion Polymers; Lovell, P. A., ElAasser, M. S., Eds.; Wiley: Chicheston, 1997; Chapter 9, pp 293-326. (17) Kirsch, S.; Pfau, A.; Landfester, K.; Shaffer, O.; El-Aasser, M. S. Macromol. Symp. 2000, 151, 413. (18) Durant, Y.; Sundberg, D. C. J. Appl. Polym. Sci. 1995, 68, 1607. (19) Schuler, B.; Baumstark, R.; Kirsch, S.; Pfau, A.; Sˇ andor, M.; Zosel, A. Prog. Org. Coat. 2000, 40, 139. (20) Kirsch, S.; Landfester, K.; Shaffer, O.; El-Aasser, M. S. Acta Polym. 1999, 50, 347. (21) Kirsch, S.; Pfau, A.; Stubbs, J.; Sundberg, D. Colloids Surf. 2001, 183-185, 713.

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Figure 2. AFM-TM images of a mica surface covered with dispersion particles in the submonolayer range ((a) 2 µm × 2 µm and (b) 5 µm × 5 µm). The schematic drawings indicate the orientation of the particles at the surface.

Figure 3. AFM-TM height, amplitude, and phase images of a mica surface with a dispersion particle in the mug geometry. In addition, cross sections of height and phase are given along the marked directions.

(soft). (This is backed by experiments also involving the variation of the phase ratio between PMMA and PnBA and analyzing film surfaces and ultracryomicrotomed cross sections. Furthermore, the phase contrast was crosschecked with PFM adhesion and stiffness images.) Bearing in mind the particle structure, as determined by TEM (Figure 1), the ringlike structures can be accounted for by a model as shown in Figure 2 in the lower right (mug). The hard-phase partial shell is oriented with its outside toward the substrate. The soft phase fills the “mug” or “bowl” to its edge, and the surplus material forms a “blob” around the partial shell. In this geometry, the edges of

the mug are not covered by the soft phase and are detected as light rings in the phase images. Inverse orientation can also be found as indicated in Figure 2. In this model, the particle resides with its soft phase oriented toward the substrate and exposes its “hard cap”, which is detected as a light area in the TM-AFM phase images. To back the interpretation of the rings as mugs in Figure 3, height, amplitude, and phase data of this type of particle orientation are shown, together with cross sections of height and phase. The amplitude data show the rim of the hard-phase mug. It is also detected as a feature in the

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Figure 4. AFM-PFM adhesion (light ) higher adhesion) and stiffness (light ) higher stiffness) images of a mica surface covered with dispersion particles in the submonolayer range (1 µm × 1 µm).

height cross section (arrows). As the phase image suggests, the mug is still filled with soft material, while the topographic data show that it is actually a drop of soft phase residing on the hard phase. PFM14 adhesion and stiffness images were recorded (see Figure 4) to obtain additional data, independent of tapping mode and its possible artifacts22,23 or difficulties in interpretation of phase contrasts (e.g., refs 24 and 25). PFM basically yields the same features as the tapping mode images with the PMMA phase showing the higher stiffness and adhesion values in comparison with the PnBA. The proposed muglike structures are clearly shown in accordance with the data presented so far. (The rims of the mugs appear to be somewhat broader which we think is due to the stronger repulsive forces we used for mapping in PFM as compared to tapping. Thin layers of PnBA are penetrated by the tip and the underlying hard PMMA dominates the signal, which results in the observed broadening of the hard-phase features.) A close look at Figure 2 and Figure 4 reveals that the mugs are the dominant hard-phase features, but not the only ones however. In some particles, there is an additional small feature of light phase contrast. We interpret it as hard phase, not entirely localized in the cap due to kinetic reasons. From the phase and height images, however, an upper limit for its volume can be determined which is in the order of only 5% as compared with the nominal full volume of the hard phase (25% of the particle volume), and hence it will be neglected in the following discussion of particle orientation. In summary, Figure 2 shows that in the monolayer range orientational order can be generated by depositing composite latex particles onto mica surfaces and that this ordering is quite complete (rough estimate, >90%). Apart from the reasons for this ordering, which will be discussed in section 4, these findings brought up some quite basic questions: Does the ordering depend on the substrate? Is it present at film substrate interfaces and film surfaces or only in the submonolayer range? Data (22) Ku¨hle, A.; Sorensen, A. H.; Bohr, J. Appl. Phys. 1997, 81, 6562. (23) Ku¨hle, A.; Sorensen, A. H.; Zandbergen, J. B.; Bohr, J. Appl. Phys. A 1998, 66, 329. (24) Bar, G.; Thomann, Y.; Brandsch, Y.; Cantow, H.-J.; Whangbo, M.-H. Langmuir 1997, 13, 3807. (25) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, L385.

that address these questions will be presented in the following two paragraphs. 3.3. The Influence of the Substrate. Figure 5 shows the orientation of the particles on SiO2 (θ ) 50° with θ being the contact angle against water) and hydrophobized SiO2 (θ ) 100°). In comparison to the dominant mug orientation on mica (Figure 2), here the dominant orientation is cap, with the soft phase facing the substrate. In general, for native SiO2, which represents the intermediate case, the orientation does not seem to be as complete as in the extreme cases of mica and hydrophobized SiO2. To back the interpretation of the light circular areas in the phase images as caps, height, amplitude, and phase data of this type of particle orientation are shown (the images show just like the TEM image in Figure 1 two particles adjacent to one another with merged soft phases) in Figure 6 together with cross sections of height and phase. The amplitude data reveal clearly the edge of the hard-phase cap. It is also detected as a feature in the height cross section (arrows). In amplitude, there is also no double feature as the rim of the mug shows in Figure 3. To complete the image and to expand the data set to surfaces of more practical relevance than the model surfaces discussed so far, CaCO3 (often used as a pigment in various applications) was included in the study. The surface of the CaCO3 was first cleaned with purified water, ethanol, and acetone. In addition, some of the CaCO3 specimens were treated by saturating their surface with acrylic acid, a typical dispersing agent for pigments. Again, in all cases the surfaces were smooth and regular enough to enable the determination of the orientation of the particles. Preferential orientation was observed. In the case of clean CaCO3 (θ ) 70°), the cap orientation is dominant. In the case of the acrylic acid covered surface (θ ) 20°), the mug orientation is found. All these findings, that is, dominant orientation as a function of contact angle of the substrates against water, are plotted in Figure 7. As this graph indicates, there seems to be a close correlation of dominant orientation of the particles and contact angle of the surfaces and a transition between mug and cap for surfaces with contact angles around 30°. 3.4. Ordering at Interfaces of Thick Films. Films of the composite latex with thicknesses in the range of

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Figure 5. TM-AFM phase images (2 µm × 2 µm) of composite latex particles on native SiO2 and hydrophobized SiO2. These surfaces show an increasing contact angle toward water as compared to mica, which is wetted. The models below the images show the orientation of the particles at the surfaces.

Figure 6. AFM-TM height, amplitude, and phase images of a mica surface with two merged dispersion particles in the cap geometry. In addition, cross sections of height and phase are given along the marked directions.

several microns were prepared by depositing concentrated dispersion with a draw-down bar onto native SiO2 substrates. Once dry, these films could be partly peeled off the substrate. AFM images were taken of the substrate side of the films (Figure 8a) and of the SiO2 substrate. The images of the substrate in the peeled-off area showed no significant residual material thus indicating an adhesion failure. On the other hand, Figure 8a shows dominant round features of soft phase (dark areas, negative phase shift) with hard phase (light areas, positive phase shift) forming the borders between the soft-phase areas. The round shape and the size of the soft areas correspond perfectly to the soft-phase particle cores. The hard-phase

segments fit well into the image of caps touching the substrate with their edges. Figure 8b shows the dry films after drying at room temperature. The explanation of the features, which are very similar to the ones in Figure 8a, is basically the same. (Some much smaller features than the large hard-phase segments are due to small bits of additional hard phase as already discussed in connection with Figure 2 and Figure 4.) In summary, also at the film-substrate interface and at the surface of thick films, with thicknesses in the micrometer range rather than in the submonolayer range, ordering takes place to a considerable degree.

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Figure 7. Compilation of particle orientations at the particle-substrate interface in the range of submonolayer coverages versus contact angle θ of the substrates against water. Mica was freshly cleaved. SiO2 and CaCO3 were cleaned consecutively in deionized water, ethanol, and acetone. The hydrophobization of SiO2 was done with n-octadecyltrichlorosilane following a standard procedure given in ref 13. CaCO3 + PS denotes a surface of CaCO3 which was dipped into a concentrated solution of dispersing agent Polysalz S (BASF-AG, Germany), a short-chain acrylic (3 kDa), and then rinsed with pure water and dried.

Figure 8. (a) A 2 µm × 2 µm TM-AFM phase image of the interface of a several micrometer thick dispersion film upon drying at room temperature and delamination from the substrate surface (native SiO2). (b) A 2 µm × 2 µm TM-AFM phase image of the surface of a several micrometer thick dispersion film on native SiO2 upon drying at room temperature. The images were both taken upon a short rinse with water, which brings out the structures more clearly as partially adhering surfactant is removed. However, this treatment does not significantly change the phase distribution of the polymers.

The particles of PnBA/PMMA latex on native SiO2 orient themselves at both interfaces during the drying procedure in such a way that their soft-phase part faces the interface or the surface, respectively. Mica substrates show a different orientation of single particles than SiO2 substrates. However, for thick films the findings are the same as for SiO2. At the outer surface and substrate interface, the PnBA phase is oriented toward the interface. This was unexpected for the film-substrate interface as such an occurrence could not have been predicted from the results of the submonolayer experiments. In all cases, the observed features were stable at room temperature over a period of weeks. From this and from the fact that at ambient temperature the Tg of the soft phase is well below 20 °C (i.e., fairly mobile), we conclude

that the presented structures are equilibrium structures with regard to the soft-phase distribution and the hardphase orientation. 4. Discussion The effects outlined are obvious, but the reason for the orientation still has to be discussed. One way to approach this problem is to discuss it in terms of hydrophobicity or hydrophilicity of the particular substrate surfaces and the two phases on the latex particles. As the surface tension of PMMA with respect to water is lower than for the more hydrophobic PnBA (surface tension: PMMA/H2O, 19 mN/m; PnBA/H2O, approximately 33.5 mN/m28), the particle surface may not be uniform in terms of hydrophobicity. This somewhat

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Figure 9. Model for the preferential orientation of the dispersion particles at the surface and interface of thick films.

Figure 10. Model for the orientation of single separated dispersion particles: (a) structure formation on hydrophilic substrates where the surface is wetted and the water traps the hydrophilic hard phase; (b) structure formation on hydrophobic substrates where the surface dewets and droplet formation takes place.

contradicts the general notion that the particle surface contains surfactants and charged groups of acrylic acid, which largely determine the surface properties. However, it is plausible to assume that the character of the base polymer still “leaks” through to a certain extent and also influences the surface coverage by surfactant and charged groups. (The space blocked by e.g. SDS on PMMA is 93 Å2, but on PnBA it is only 47 Å2, reflecting these differences.28) This in the end means that it is plausible to propose that the “mushroom” particles have a dipolar character in terms of hydrophobicity and hydrophilicity. The question now is how this leads to the observed orientation and the influence of the substrates and their contact angles on it (Figure 7). In the colloidal state, quite a number of different forces possibly act between the particles and the surfaces, even if surfactant and salt ions are neglected for simplicity. However, standard Derjaguin-Landau-Verwey-Overbeek (DLVO) forces (van der Waals forces and ionic forces) can hardly account for rotational order in the discussed systems. However, in view of the asymmetry of the particles regarding hydrophilicity and hydrophobicity, solvation forces and possibly OH bridges may play an important role.8,26,27 On hydrophobic substrates, attractive solvation forces (hydrophobic forces) should favor the orientation of the (26) Papastavrou, G.; Akari, S. Nanotechnology 1999, 10, 1. (27) Papastavrou, G.; Akari, S. Colloids Surf., A 2000, 164, 175. (28) Durant, Y.; Sundberg, D. C. J. Appl. Polym. Sci. 1985, 68, 1607.

PnBA toward the substrate. At the same time, the repulsive solvation forces (hydration forces) between the hydrophobic substrate and the more hydrophilic parts of the particle are minimized. Direct bonding interactions between surface and particles due to hydrogen bonding are not possible for the hydrophobic substrate at all. On hydrophilic substrates, hydration forces are basically repulsive and are minimized for an orientation of the hydrophobic part (PnBA) of the particle toward the substrate surface. Unlike the first case, direct bonding interactions between surface and particles due to hydrogen bonding are possible. In summary, the colloidal forces between the particle and the substrate favor an orientation of the PnBA phase toward the substrate regardless of the nature of the substrate. This is found for thick films on mica and SiO2 (Figure 8a); however, it is not the case for single particles. An analogous reasoning can be applied to the hydrophobic water surface. The proposed dipolar character of the particles should orient the soft phase toward the water surface, which in fact is observed experimentally at the surface of dried thick films (Figure 8b), regardless of the substrates. So there is a coherent model for thick films (see Figure 9), basically stating that the composite latex particles behave like a surfactant with regard to their orientation at interfaces. (For the water surface and the hydrophobic

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surface, this comparison is straightforward. For the hydrophilic surface, this is not exactly the case; however, one should bear in mind that here the adsorption during drying is forced and any surfactant would be present in a micelle, avoiding the surface.) However, a coherent model is still lacking in the case of the single-particle orientation. The key to the understanding of the experimental findings in the case of the single-particle orientation is a close look at the drying procedure itself. During the drying process, in the case of single particles, the water surface approaches the substrate surface. For both substrate types, the hard-phase part should point toward the substrate and the soft-phase part upward; this leads to the observed orientation in the case of the hydrophilic substrate in a straightforward manner (see Figure 10a). In the ideal case, the film becomes thinner and thinner and finally there is just water in the gap between the particle apex and the substrate, which traps the hydrophilic cap. On the hydrophobic surface in the ideal case, droplet formation takes place instead of the continuous thinning of a water film. The droplets reside on the surface with a high contact angle and deposit the particles during shrinkage as depicted in Figure 10b, similar to the generation of a Langmuir-Blodgett (LB) film. (At the edges of the droplet, the situation is just as in a Langmuir trough where the substrate is pushed into the water phase. The pressure on the particles is generated by the shrinking surface due to water evaporation.) In this way, they are deposited in an orientation with the PnBA facing the substrate, with the surface water/air being the force imposing the ordering. The reasoning given above is greatly simplified and certainly gives room for further investigations. The main simplifications so far are that a chemically specific

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interaction between certain functional groups at the surfaces and the presence of surfactants and soluble polar oligomers are neglected. This is quite appropriate for the latter two points in the case of the deposition from highly diluted water phases (submonolayer coverages). However, when depositing thick films from concentrated dispersions (solid contents approximately 50%), this is not so obvious. 5. Conclusion Two-phase composite PnBA/PMMA latex particles with a mushroomlike particle structure show a preferential orientation when they are deposited onto surfaces, regardless if this is done from very diluted solution or at high concentration. The orientation is found to correlate with the surfaces’ or substrates’ hydrophobicity or hydrophilicity. In the case of isolated particles, the more hydrophobic PnBA is oriented toward surfaces with contact angles higher than 30° whereas the more hydrophilic PMMA is oriented toward more polar surfaces with contact angles smaller than 30°. At the interfaces of thick films to substrates and air, always a preferential orientation with the hydrophobic PnBA toward the interface is found, regardless of the type of interface, that is, hydrophobic or hydrophilic. In most cases, the latices behave like surface active components regarding their orientation at interfaces. To our knowledge, these orientational phenomena are the first described in the literature for polymer dispersion particles. Acknowledgment. The skillful experimental assistance of Mrs. Wagner (Polymer Physics Department) is gratefully acknowledged. LA010942X