Structure Evolution in Layers of Polymer Blend Nanoparticles

Nanoscale Control over Phase Separation in Conjugated Polymer Blends Using Mesoporous Silica Spheres. Timothy L. Kelly , Kazuhisa Yano and Michael O...
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Langmuir 2007, 23, 7235-7240

7235

Structure Evolution in Layers of Polymer Blend Nanoparticles Joanna Raczkowska,*,† Rivelino Montenegro,‡ Andrzej Budkowski,† Katharina Landfester,‡,§ Andrzej Bernasik,| Jakub Rysz,† and Paweł Czuba† Smoluchowski Institute of Physics, Jagellonian UniVersity, Reymonta 4, 30-059 Krako´ w, Poland, Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany, Department of Organic Chemistry III - Macromolecular Chemistry and Organic Materials, UniVersity of Ulm, 89081 Ulm, Germany, and Faculty of Physics and Applied Computer Science, AGH - UniVersity of Science and Technology, Mickiewicza 39, 30-059 Krako´ w, Poland ReceiVed September 27, 2006. In Final Form: April 12, 2007 The early stages of phase evolution, not available for nanometer polymer blend films spin-cast from solutions of incompatible mixtures, have been examined for films prepared from nanoparticles of deuterated polystyrene/ poly(methyl methacrylate) blends (1:1 mass fraction of dPS/PMMA) with PS-PMMA diblock copolymer additives. The initial phase arrangement, confined to the size of nanoparticles, has provided the homogeneity of the initial film composition. The early stages of structure formation, promoted by annealing and traced with atomic and lateral force microscopy (AFM, LFM) as well as secondary ion mass spectroscopy (SIMS), resulted in bilayers, observed commonly for as-prepared solvent-cast blends. The initiated capillary instability of the upper dPS-rich layer depended on copolymer additives, which enhanced the lateral structures pinning the dewetting process.

Introduction Polymer nanoparticles, typically prepared using miniemulsion polymerization,1-4 because of their ability to combine properties of different polymers or materials in one particle, are widely used, particularly in biomedical applications such as drug delivery systems, molecular recognition, and powerful tools for medical diagnosis.1,5-8 In turn, nanometer layers of polymer blends based on nanoparticles, prepared using miniemusification process,9-12 are expected to improve the efficiency of various optoelectronic devices.10-12 Furthermore, similar systems of very thick (tens of micrometers) layers of nanosized latex particles have become important for the coatings industry as a result of environmental concerns (waterborne coatings are preferred over those containing volatile organic compounds).13,14 In this article, we examine the morphological coarsening of nanometer layers based on nanoparticles, which are composed * Corresponding author. E-mail: [email protected]. † Jagellonian University. ‡ Max Planck Institute of Colloids and Interfaces. § University of Ulm. | AGH - University of Science and Technology. (1) Solans, C.; Izquierdo, P.; Nolla, J.; Azemar, N.; Garcia-Celma, M. J. Curr. Opin. Colloid Interface Sci. 2005, 10, 102-110. (2) Asua, J. M. Prog. Polym. Sci. 2002, 27, 1283-1346. (3) Schork, F. J.; Poehlein, G. W.; Wang, S.; Reimers, J.; Rodrigues, J.; Samer, C. Colloids Surf., A 1999, 153, 39-45. (4) Landfester, K. AdV. Mater. 2001, 13, 765-768. (5) Eguchi, M.; Du, Y.-Z.; Ogawa, Y.; Okada, T.; Yumoto, N.; Kodaka, M. Int. J. Pharm. 2006, 311, 215-222. (6) Ito, F.; Makino, K. Colloids Interfaces B 2004, 39, 17-21. (7) Zheng, W.; Gao, F.; Gu, H. J. Magn. Magn. Mater. 2005, 288, 403-410. (8) Lorenz, M. R.; Holzapfel, V.; Musyanovych, A.; Nothelfer, K.; Walther, P.; Frank, H.; Landfester, K.; Schrezenmeier, H.; Maila¨nder, V. Biomaterials 2006, 27, 2820-2828. (9) Landfester, K.; Montenegro, R.; Scherf, U.; Gu¨ntner, R.; Asawapirom, U.; Patil, S.; Neher, D.; Kietzke, T. AdV. Mater. 2002, 14, 651-655. (10) Kietzke, T.; Neher, D.; Landfester, K.; Montenegro, R.; Gu¨ntner, R.; Scherf, U. Nat. Mater. 2003, 2, 408-412. (11) Kietzke, T.; Neher, D.; Kumke, M.; Montenegro, R.; Landfester, K.; Scherf, U. Macromolecules 2004, 37, 4882-4890. (12) Kietzke, T.; Stiller, B.; Landfester, K.; Montenegro, R.; Neher, D. Synth. Met. 2005, 152, 101-104. (13) Oh, J. K.; Yang, J.; Rademacher, J.; Farwaha, R.; Winnik, M. A. Macromolecules 2004, 37, 5752-5761. (14) Wu, J.; Tomba, J. P.; Winnik, M. A.; Farwaha, R.; Rademacher, J. Macromolecules 2004, 37, 2299-2306.

of deuterated polystyrene/ poly(methyl methacrylate) blends (1:1 mass fraction of dPS/PMMA) with admixed PS-PMMA block copolymer. Typically, nanometer films of immiscible polymer blends, which are widely employed for technological applications,15-20 are prepared by spin-casting from a solution of polymers dissolved in a common solvent. This technique is considered to be extremely attractive for the industry because it is easy, cheap, and allows for the fabrication of films with ordered domains of numerous polymers, which play potentially different roles in high-tech devices. Despite the simplicity of spin-casting, the phase separation that occurs in its course, promoted by solvent extraction, is a complex process that has not yet been completely resolved, mainly because of the great number of relevant parameters that change continuously during film deposition. Furthermore, rapid structure formation results in film morphologies corresponding to the late periods of phase evolution; therefore, its mechanisms may be proposed only on the basis of the impact of the modified parameters on the examined system.21-25 To study the early stages of phase separation, nanometer films of polymer mixtures were cast from aqueous dispersions of nanoparticles composed of a model PS/PMMA blend (also with PS-PMMA diblocks), where the phase domains (15) Haupt, M.; Miller, S.; Ladenburger, A.; Sauer, R.; Thonke, K.; Spatz, J. P.; Riethmu¨ller, S.; Mo¨ller, M.; Banhart, F. J. Appl. Phys. 2002, 91, 6057-6059. (16) Moons, E. J. Phys.: Condens. Matter 2002, 14, 12235-12260. (17) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123-2126. (18) Snaith, H. J.; Arrias, A. C.; Morteani, A. C.; Silva, C.; Friend, R. H. Nano Lett. 2002, 2, 1353-1357. (19) Urbas, A.; Sharp, R.; Fink, Y.; Thomas, E. L.; Xenidou, M.; Fetters, L. J. AdV. Mater. 2000, 12, 812-814. (20) Walheim, S.; Scha¨ffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520-522. (21) Affrossman, S.; O’Neill, S. A.; Stamm, M. Macromolecules 1998, 31, 6280-6288. (22) Gutmann, J. S.; Mu¨ller-Buschbaum, P.; Stamm, M. Faraday Discuss. 1999, 112, 285-297. (23) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 32323239. (24) Walheim, S.; Bo¨ltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995-5003. (25) Raczkowska, J.; Bernasik, A.; Budkowski, A.; Sajewicz, K.; Penc, B.; Lekki, J.; Lekka, M.; Rysz, J.; Kowalski, K.; Czuba, P. Macromolecules 2004, 37, 7308-7315.

10.1021/la062844n CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007

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Figure 1. AFM (a-d) and LFM (e-h) images of free surfaces of thin polymer films prepared from nanoparticles containing a symmetric dPS/PMMA mixture with φc ) 0.05 of PS-PMMA additives. The images were recorded on as-prepared film (a, e) after annealing at T ) 146 °C for 8 (b, f), 44 (c, g), and 130 h (d, h).

were smaller than the size of multicomponent nanoparticles. They were also distributed randomly, at least on scales larger than the nanoparticle size. In this way, the early stages of phase evolution could be initiated by the annealing of blend nanoparticle layers. Although there have been many previous studies of PS/ PMMA thin film blends,24,26-35 they have never been specifically directed to examination of the early stages of structure formation, which were not available for films prepared conventionally by spin-casting from solution. The topographical and compositional data that are presented in this article were recorded with atomic and lateral force microscopy (AFM and LFM, respectively) as well as secondary ion mass spectroscopy (SIMS). They indicate the merging of the initial nanoparticles followed by the growth of phase domains and the formation of the bilayer structure of the PS/PMMA// substrate, which is prone to dewetting instability. The main effect of the copolymer additives on the phase evolution in the investigated system is an observed significant retardation of said process.

To trace the structure evolution, all samples were annealed in vacuum at 146 °C. The free surface topography and the phase arrangement of the examined polymer films were determined using a CP Park Scientific Instruments microscope working in AFM and LFM modes, respectively. To discern properly between topographical and compositional contributions to the LFM image, LFM pictures recorded twice (i.e., for forward and backward scan directions) were analyzed. The average film thicknesses, determined after the partial removal of the polymer film using a scalpel scratch,36 were equal to 130 ( 15, 120 ( 15, and 160 ( 20 nm for blends with φc ) 0, 0.05, and 0.10, respectively. The characteristic length scale R of the surface undulations was determined from examining 2D fast Fourier transforms (FFT) of AFM images.36,37 The vertical phase domain arrangement of the polymer blend films was determined through the use of the depth profiling mode of SIMS. The data were obtained using a VSW apparatus equipped with a high-resolution ion gun (liquid metal, Fei company) and Balzers quadrupole mass spectrometer.38

Experimental Section The measurements reported here correspond to thin polymer blend films, spin-cast (spin-casting speed ω ) 4200 rpm for 60 s) from aqueous dispersions of nanoparticles (total concentration cp ) 50 mg/mL) and composed of a symmetric mixture (1:1 mass fraction) of dPS (molecular weight Mw ) 174 000, polydispersity index Mw/ Mn ) 1.03) and PMMA (Mw ) 149 000, Mw/Mn ) 1.1) with a varied amount of diblock copolymer PS-PMMA (Mw ) 680 000, Mw/Mn ) 1.26, Mw(PS block) ) 442 000, Mw(PMMA block) ) 238 000) added (weight fraction φc ) 0-0.10). All said polymers were purchased from Polymer Standard Service (Mainz, Germany) and used as received. Silicon wafers covered in layers of evaporated Au were used as the substrates. The polymer blend nanoparticles were prepared using a miniemulsification process.9-11 Polymers, in this method, were dissolved in chloroform, added to an aqueous solution containing surfactant (sodium dodecyl sulfate SDS), and stirred for 1 h for pre-emulsification. Ultrasonicating the mixture (with a W450 Branson sonifier) resulted in the appearance of stable miniemulsions of small polymer droplets. The remaining solvent was removed by stirring the miniemulsion at elevated temperature (62 °C), and as a result, a stable dispersion of solid polymer blend nanoparticles was obtained. The sizes of the nanoparticles, determined with the use of a Nicomp particle sizer,10 were 60 ( 20, 120 ( 20, and 95 ( 25 nm for blends with φc ) 0, 0.05, and 0.10, respectively.

Results and Discussion The free surface evolution of the film composed of nanoparticles with φc ) 0.05, presented in Figure 1, is representative of all mixtures based on nanoparticles. The contrast in each LFM picture (Figure 1e-h) corresponds essentially to the change in (26) Raczkowska, J.; Budkowski, A.; Rysz, J.; Czuba, P.; Lekka, M.; Bernasik, A. J. Nanostruct. Polym. Nanocompos. 2005, 1, 25-35. (27) Zhu, S.; Liu, Y.; Rafilovich, M. H.; Sokolov, J.; Gersappe, D.; Winesett, D. A.; Ade, H. Nature 1999, 400, 49-51. (28) Harris, M.; Appel, G.; Ade, H. Macromolecules 2003, 36, 3307-3314. (29) Li, X.; Han, Y.; An, L. Polymer 2003, 44, 8155-8165. (30) Li, X.; Han, Y.; An, L. Appl. Surf. Sci. 2004, 230, 115-124. (31) Mitov, Z.; Kumacheva, E. Phys. ReV. Lett. 1998, 81, 3427-3430. (32) Sprenger, M.; Walheim, S.; Budkowski, A.; Steiner, U. Interface Sci. 2003, 11, 225-235. (33) Ton-That, C.; Shard, A. G.; Teare, D. O. H.; Bradley, R. H. Polymer 2001, 42, 1121-1129. (34) Ton-That, C.; Shard, A. G.; Bradley, R. H. Polymer 2001, 43, 49734977. (35) Heriot, S. Y.; Jones, R. A. L. Nat. Mater. 2005, 4, 782. (36) Cyganik, P.; Budkowski, A.; Raczkowska, J.; Postawa, Z. Surf. Sci. 2002, 507-510, 700-706. (37) Raczkowska, J.; Rysz, J.; Budkowski, A.; Lekki, J.; Lekka, M.; Bernasik, A.; Kowalski, K.; Czuba, P. Macromolecules 2003, 36, 2419-2427. (38) Bernasik, A.; Rysz, J.; Budkowski, A.; Kowalski, K.; Camra, J.; Jedlin´ski, J. Macromol. Rapid Commun. 2001, 22, 829-834.

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Figure 2. Surface (rms) roughness of thin polymer films that were prepared from nanoparticles containing the dPS/PMMA/PS-PMMA blend with copolymer weight fractions of φc ) 0 (b), 0.05 (2) and 0.10 (9) and annealed at T ) 146 °C. Dashed lines serve only as a visual guide.

Figure 3. Evolution of the characteristic length scale of free surface undulations for films composed of blend nanoparticles (with copolymer fractions φc ) 0, b; 0.05, 2; and 0.10, 9) that have been annealed at T ) 146 °C. The dashed lines correspond to the function R(t) ∝ tn, with n ) 0.44 ( 0.05, 0.37 ( 0.04, and 0.48 ( 0.04 for φc ) 0, 0.05, and 0.10, respectively.

the film surface composition because the dominating (scandirection-dependent) frictional component of the topographical contribution39 has been removed through the subtraction of the image recorded for the tip moving left to right from the one taken in the opposite direction. The high LFM sensitivity to the phase contrast of identical film blends, prepared conventionally (by spin-casting from solution), was demonstrated in our previous work.26 Initially, the polymer film consists of small spherical features resembling the original nanoparticles (Figure 1a). The surface phase arrangement, traced through LFM (Figure 1e), cannot be resolved at this stage because the phase separation process, which is confined to the dimensions of nanoparticles, results in phase domains that are too small to be distinguished. Therefore, no frictional contrast can be observed in LFM pictures, where only weak features, due to the lateral force (modified by the local slope of the topography39) are visible. However, this situation changes with the annealed films: after 8 h of annealing, spherical features merge to form an undulated surface (Figure 1b), and the distinct surface phase domains, showing no correlation with the surface undulations, become visible as a result of the frictional LFM contrast (Figure 1f). This picture remains very similar for films annealed for 44 h (Figure 1c,g) and changes noticeably for samples annealed for 130 h, when the significant growth of undulations (Figure 1d) is accompanied by the disappearance of surface phase domains that are distinguishable by LFM (Figure 1h). The corresponding density of surface domains, determined using an integral geometry approach37 (Figure S1 and Note S1 in Supporting Information) from LFM data, decreases monotonically from 89 ( 10 to 3 ( 1 µm-2 for annealing times that increase from 8 to 130 h (Figure S2). The observed merging of initial nanoparticles, followed by the formation and growth of surface undulations, can also be traced by surface (rms) roughness analysis (Figure 2). For films with φc ) 0.05 (triangles in Figure 2), the rms value decreases rapidly from 9 nm (for as-prepared films) to 2 nm (for films annealed for 8 h) and increases to 5 nm (for t ) 44 h). Longer annealing times show no noticeable effect, which indicates the stable amplitude of surface undulations.

The characteristic length scale R of surface undulations for films where φc ) 0.05 increases monotonically with time (triangles in Figure 3), following the equation R(t) ∝ tn, where n ) 0.37 ( 0.04. Similar structure evolution can be observed for films composed of nanoparticles where φc ) 0 (Figure 4a-c) and 0.10 (Figure 4e-g). The characteristic length scale of surface undulations R presented in Figure 3 (circles for φc ) 0 and squares for φc ) 0.10) grows monotonically with time, obeying the law R(t) ∝ tn, where n ) 0.44 ( 0.05 and 0.48 ( 0.04 for φc ) 0 and 0.10, respectively. Although the overall structure evolution of all films is similar, the surface undulations of samples with no copolymer additives grow faster with time (cf. Figure 4b,f), achieving R ≈ 2.8 µm after 254 h of annealing (Figure 4c), whereas for blends with diblocks added the characteristic length scale remains smaller by half (R ≈ 1.4 µm, Figure 4g). The change in surface roughness as a function of annealing time for films where φc ) 0 and 0.10 (presented by circles and squares in Figure 2, respectively) indicates behavior analogous to that of blends where φc ) 0.05. LFM observations reveal, for samples with no block copolymer, the distinct surface phase domains for an annealing time of t ) 8 h (Figure 5b). Frictional contrast, disappearing for t ) 44 h (Figure 5d), indicates the formation of a film structure with only one phase-facing sample surface. (This situation holds for longer annealing times as shown by LFM images in Figure S3, Supporting Information). For the blends where φc ) 0.10, this process is strongly retarded. Different surface phases, which are clearly visible for annealing periods of 130 h (Figure 5f), start to vanish in films annealed for 173 h (Figure 5h). For comparison, films of nanoparticles where φc ) 0.05 show distinct, dense surface domains for times ranging from 8 to 44 h (Figure 1f, g and Figures S1 and S2). The free surface evolution presented above implies the coalescence of both the nanoparticles and the domains inside the nanoparticles (formed from the solvent quench in the course of the miniemusification process). Different polymer phases at surface are visible at intermediate annealing times. Later on, only one surface phase can be identified on the basis of the LFM data. The explanation for this is bilayer formation, driven by PMMA wetting of the Au substrate, because the interactions of the more polar PMMA with Au are stronger than between Au and dPS.24 Partial support for this formation mechanism can be

(39) Meyer, E.; Hug, H. J.; Bennewitz, R. Scanning Probe Microscopy; Springer: Berlin, 2004.

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Figure 4. AFM images of the topography of thin polymer films prepared from nanoparticles with copolymer fractions of φc ) 0 (a-d) and 0.10 (e-h) after spin-casting (a, e) and annealing at T ) 146 °C for 87 (b, f) and 254 h (c, g). Images d and h correspond to c and g, respectively, after the removal of the dPS-rich phase through immersion in cyclohexane for 5 min. The maximal amplitude of surface undulations is equal to 20 (c), 25 (d), 20 (g), and 35 nm (h).

Figure 5. AFM (a, c, e, g) and LFM (b, d, f, h) images of the free surfaces of thin polymer films prepared from nanoparticles with copolymer fractions of φc ) 0 (a-d) and 0.10 (e-h) after annealing at T ) 146 °C for 8 (a, b), 44 (c, d), 130 (e, f), and 173 h (g,h).

supplied by overall bilayer structures observed for various polymer blend films,25,34,40,41 including films of PS/PMMA that are spincast from toluene.26 However, different polymer solubilities in solvent can also result in bilayers because less-soluble polymer solidifies more quickly on the substrate, leaving the other one facing the free surface.26,33,40 The predicted formation of the bilayer phase arrangement has been verified (Figure 6) through the depth profiling mode of SIMS.38 The fractional composition of PMMA in the film is represented in linear plots by the original signal (C2H-, mass to charge ratio m/z ) 25, open squares in Figure 6) normalized by the m/z ) 24 yield. The carbon profile (C2-, m/z ) 24, solid squares in Figure 6) tracing all polymers in each film is shown on logarithmic plots. Both types of plots are presented to show the concentration profiles of PMMA and all polymers as a function of the depth in the samples. Composition profiles of the PMMA phase, as determined by an analysis of the films (as-prepared - Figure 6a,b; after additional annealing for 130 h - Figure 6c,d), confirm the bilayer structure (40) Bernasik, A.; Włodarczyk-Mis´kiewicz, J.; -Luz˘ ny, W.; Kowalski, K.; Raczkowska, J.; Rysz, J.; Budkowski, A. Synth. Met. 2004, 144, 253-257. (41) Geoghegan, M.; Jones, R. A. L.; Payne, R. S.; Sakellariou, P.; Clough, A. S.; Penfold, J. Polymer 1994, 35, 2019-2027.

that were significantly better developed for blends where φc ) 0 (Figure 6c) but also clearly visible for mixtures where φc ) 0.05 (Figure 6d). Phase stratification is driven by PMMA wetting the Au surface. Surface wetting by one of the polymer phases, occurring in phaseseparating film blends, is usually attributed42-46 to the hydrodynamic flow of the wetting phase through continuous tubes observed42,43,46 in an adjacent film region. Here such pathways of PMMA material flow can be shown in relation to domains of distinct phases that are visible, at intermediate annealing times, at the free surface (Figures 1f,g and 5b,f,h). Their lateral and vertical extensions are comparable to the film thickness. As the hydrodynamic flow, responsible for bilayer formation, is driven by the interfacial tension γ,44-46 the observed retardation in the vertical structure evolution in films where φc ) 0.05 (Figure 6) may be caused by the segregation of PS-PMMA additives at dPS/PMMA interfaces, taking place for annealed as well as spin(42) Rysz, J.; Ermer, H.; Budkowski, A.; Bernasik, A.; Lekki, J.; Juengst, G.; Brenn, R.; Kowalski, K.; Camra, J.; Lekka, M.; Jedlin´ski, J. Eur. Phys. J. E 2001, 5, 207-219. (43) Wang, H.; Composto, R. J. J. Chem. Phys. 2000, 113, 10386-10397. (44) Rysz, J. Ph.D. Thesis. Jagellonian University, Krako´w, Poland, 2001. (45) Tanaka, H. Phys. ReV. E 1996, 54, 1709-1714. (46) Wang, H.; Composto, J. Phys. ReV. E 2000, 61, 1659-1663.

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Figure 6. Depth profiles of PMMA (traced by the C2H-/C2- signal, 0) and all polymers (traced by the C2- signal, 9) determined for as-prepared (a, b) and annealed (for 130 h at T ) 146 °C) films (c, d) composed of blend nanoparticles with copolymer weight fractions of φc ) 0 (a, c) and 0.05 (b, d). Logarithmic and linear scales correspond to C2- and C2H-/C2- signals, respectively.

cast blends.26,47,48 Interfacial active diblocks lead to a considerable reduction in γ.26,27,44 Lowered interfacial energy makes the hydrodynamic flow less effective.45 This is confirmed here by the distinct surface domains, related to material flow pathways, that are visible for more extended annealing times in the samples with higher copolymer content φc (cf. Figures 5b,f and 1g). The images of the dPS/PMMA interface, recorded for samples annealed for 254 h after the selective dissolution of the dPS-rich phase, are presented in Figure 4d (φc ) 0) and h (φc ) 0.10). The topography of the examined interface manifests wavy undulations, which, for blends with copolymer additives, are accompanied by more distinct, smaller protrusions that could reflect the remnants of PMMA flow channels. Similar protrusions, in partially dissolved polymer blend films interpreted as flow channels,45 have been observed by Rysz et al.42 as well as by Wang et al.43,46 The origin of the undulations at the dPS/PMMA interface (Figure 4d,h) and free surface (Figure 4c,g) is expected to stem from long-range van der Waals interactions between air and the PMMA-rich phase across the destabilized dPS-rich layer. Longrange interactions, which destabilize multilayer structure,49 have recently been inferred for PS/PMMA//substrate bilayers annealed28,50 or cast from solution.24,26,35 The destabilization of PS/PMMA bilayers follows the general scenario51,52 for a thin liquid film not wetting another liquid (e.g., the PS layer does not wet PMMA).28 Also, in the PS/PMMA system this process is initiated by surface (interface) undulations.26,35 For much longer times, they are followed by the dewetting of an upper PS-rich layer.24,26,28,35,50 At a point very close to thermodynamic equilibrium, a PS-rich phase forms droplets on top of a PMMA layer, as shown by Harris et al.28 The characteristic length scales of interfacial and free surface undulations recorded at t ) 254 h are comparable only for film blends where φc ) 0 (cf. Figure 4c,d), whereas for mixtures containing block copolymers the undulation scales are larger for (47) Green, P. F.; Russell, T. P. Macromolecules 1991, 24, 2931-2935. (48) Green, P. F.; Russell, T. P. Macromolecules 1992, 25, 783-787. (49) David, M. O.; Reiter, G.; Sitthai, T.; Schultz, J. Langmuir 1998, 14, 5667-5672. (50) Lambooy, P.; Phelan, K. C.; Haugg, O.; Krausch, G. Phys. ReV. Lett. 1996, 76, 1110. (51) Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 36823690. (52) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084-1088.

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Figure 7. Schematic model of structure evolution in thin polymer films prepared from miniemulsions.

the dPS/PMMA interface (cf. Figure 4g,h). For developed bilayers, characteristic of blends where φc ) 0, the free surface features (of the dPS-rich phase) are expected to correspond to the underlying PMMA-rich phase surface topography (longitudinal peristaltic mode33,51). In turn, for blends with copolymer additives, different scales of surface and dPS/PMMA interface undulations have been observed. They may be related to the incomplete process of bilayer formation. In this case, the free surface topography may reflect surface undulations due to the dewetting instabilities of a structured film53,54 (rather than dPS) layer located between air and the PMMA lamella (adjacent to the Au substrate). The hydrodynamic flow channels, which were present longer in the complex layer of dPS/PMMA blends with diblocks added (φc ) 0.05, 0.10), can result in the pinning of capillary waves and in the reduction of characteristic length scale R as compared with that of the φc ) 0 samples (Figure 3). The analysis of topography presented above as well as the lateral and vertical phase arrangements allows us to introduce a schematic model of structure evolution in thin polymer films prepared from nanoparticles (Figure 7). Nanoparticles, initially arranged in layers (Figure 7a), merge in the course of annealing. Phase domains, confined to nanoparticles, start to grow to a size that is comparable to the film thickness. This stage is followed by the formation of a thin PMMA-rich layer wetting the Au substrate, reinforced by the hydrodynamic flow of PMMA through continuous tubes to the substrate (Figure 7b). Later on, long-range van der Waals interactions start to destabilize the bilayer phase arrangement, and spontaneous undulations of the dPS/PMMA interface, mirrored by the free surface, can occur (Figure 7c). The whole process is strongly retarded for blends containing copolymer additives, mainly as a result of the reduced interfacial tension, which is the driving force of the hydrodynamic flow responsible for bilayer formation. In addition, material flow tubes, which are present longer for film blends with diblocks, can hinder the temporal evolution of the scale of surface undulations R. Coarsening exponent n, observed in surface undulations of films prepared from nanoparticles (n ) 0.44 ( 0.05, 0.37 ( 0.04, and 0.48 ( 0.04 for films with φc ) 0, 0.05, and 0.10, respectively), is similar to the values observed previously for different stages of the spinodal dewetting of very thin PS (53) Limary, R.; Green, P. F. Macromolecules 2002, 35, 6486-6489. (54) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Lorenz-Haas, C.; Wunnicke, O.; Stamm, M.; Petry, W. Macromolecules 2002, 35, 2017-2023.

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copolymer54 and homopolymer55 films. The early regime of the dewetting process has been characterized by the exponent 2/3, changing to 1/3 at a deviation time td,54 when the exponentially growing amplitude of surface undulations ∆h (or rms surface roughness) was considerably slowed. The deviation time td usually corresponds to the situation when the undulation amplitude ∆h approaches half the film thickness h.53-55 Moreover, rapid coarsening, characterized by the exponent 0.43, has been observed by Xie et al.55 when the PS films were ruptured into droplets for annealing times longer than break-up time tb (when undulation amplitude ∆h approaches film thickness h).53-55 In our case, maximal observed undulation amplitude ∆h ≈ 20-25 nm (Figure 4c,d for φc ) 0) is comparable to half the thickness of the destabilized PS layer (i.e., one-fourth of the total film thickness, 32 ( 4 nm). Therefore, our observations correspond to the transition beyond an early regime (around td) but well before the film ruptures into droplets. This is supported by the surface roughness data for films (φc ) 0) annealed longer than 44 h (Figure 2), which indicate a slowing down resembling that observed at td for the spinodal dewetting of polymer monolayers.53-55 Also, AFM images recorded for longer annealing times (Figure 1c,d) do not show isolated droplets, characteristic of the late dewetting stage.55 Only the occasional break-up of the top PS-rich layer is suggested by some LFM data (Figure S4, Supporting Information).

(admixed with various amount of diblock copolymers PS-PMMA) were prepared from suitably sized nanoparticles (dispersion casting) to initiate, through annealing, the early stages of phase evolution. The initial phase arrangement, confined to the size of nanoparticles and prepared using miniemulsification, was not affected by the preferential interactions between the substrate material and one of the blend components. The blend film composition is therefore homogeneous. For nanoparticle layers, the overall bilayer structure is obtained upon annealing, when initial nanoparticles merge, leading to the coalescence of phase domains followed by the hydrodynamic flow of PMMA to the wettable Au substrate. The resulting lamellar structure, with the polymer/polymer interface undulated as a result of preceding PMMA flows, is prone to the dewetting instability of the upper dPS-rich layer, which is visible as a surface with bicontinuous morphology. The copolymer additives strongly retard this phase evolution; therefore, the lateral structures (due to PMMA flow channels), which are present longer, pin the dewetting instability, reducing the scale of surface patterns R.

Conclusions

Supporting Information Available: Additional information on an integral geometry approach37 applied to the LFM data, corresponding to films with nanoparticles composed of dPS/PMMA/PSPMMA (φc ) 0.05) blends (annealed for 8, 24, 44, 87 and 130 h at T ) 146 °C), to visualize surface domains and to yield their surface density. Additional LFM images of the films prepared from nanoparticles with the dPS/PMMA/PS-PMMA blend (φc ) 0) after 44, 87, and 130 h of annealing. Additional AFM and LFM images of the same film annealed for 221 h. This material is available free of charge via the Internet at http://pubs.acs.org.

In this article, phase evolution in films composed of a symmetric dPS/PMMA blend was examined. Although the phase separation occurring in this system has been extensively investigated,24,26,28-35 the early stages of this process had not been studied because they are not available for blend films spin-cast from solutions. To resolve this problem, thin polymer films of a dPS/PMMA blend (55) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. ReV. Lett. 1998, 81, 1251-1254.

Acknowledgment. This work was partially supported by the State Committee for Scientific Research under project no. 1P03B01027 and the Reserve of the Rector of the Jagellonian University. J.R. is grateful to the Foundation for Polish Science for financial support.

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