Atomic Force and Ultrasonic Force Microscopy Investigation of

Nov 10, 2007 - Acoustics and atomic force microscopy for the mechanical characterization of thin films. Daniele Passeri , Andrea Bettucci , Marco Ross...
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Langmuir 2007, 23, 12973-12983

12973

Atomic Force and Ultrasonic Force Microscopy Investigation of Adsorbed Layers Formed by Two Incompatible Polymers: Polystyrene and Poly(butyl methacrylate) V. N. Bliznyuk,*,† Y. S. Lipatov,‡ N. Ozdemir,† T. T. Todosijchuk,‡ V. N. Chornaya,‡ and S. Singamaneni† College of Engineering and Applied Sciences, Western Michigan UniVersity, Kalamazoo, Michigan 49008, and Institute of Macromolecular Chemistry, NASU, KieV, 02160 Ukraine ReceiVed June 4, 2007. In Final Form: August 31, 2007 Atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) have been used to study the properties of adsorption layers formed by two incompatible polymers, polystyrene and poly(butyl methacrylate), in the course of simultaneous adsorption on the surface of silica (naturally oxidized surface of a silicon wafer). The adsorption was performed from solutions containing both of the components in a common solvent (carbon tetrachloride) in dilute and semidilute concentration regimes. It was discovered that in both cases the structure of adsorption layers has a complex mosaic structure, the details of which depend on solution composition, on the solution concentration regime, and on the ratio of the components in the adsorption layer. The observed structural inhomogeneity on the length scale of ∼200-500 nm (distribution of segment density revealed by UFM) appears as result of thermodynamic incompatibility in the system and is conditioned by changes in the conformation states of the adsorbed macromolecules in the route of competitive adsorption of the components. The adsorbed polymer films with thicknesses of ∼20-500 nm appeared to have fractal properties and could be characterized with fractal dimensions dependent on the ratio of the components at the interface and the adsorption conditions.

Introduction Formation of thin polymer layers at the surface of a solid is a valuable technique of producing nanoscale films with the possibility of fine control of their thickness and structure.1 Characteristic features of the adsorption process are dictated by the dependence of the thickness and other properties of the adsorbed layer on the conformation state of the polymer chains, and they have received considerable attention from both a theoretical and experimental point of view.2,3 Depending on the solution regime (dilute, semidilute, or concentrated), the structure of the adsorption layer can be varied in a wide range. In the concentration region near and above the critical concentration of the coil overlapping, C*, macromolecular clusters of a fluctuation nature are formed as a result of intermolecular interactions. These clusters can pass almost unchanged to the adsorbent surface and therefore determine the structure of the adsorption layer. The dimensions of the molecular adsorption coils are in the range of several tens of nanometers,4 whereas the cluster dimensions are in the range of 40-300 nm.5 These molecular and supramolecular dimensions predetermine the formation of nanoscale adsorption layers. Consequently, the adsorption from polymer solution is a universal process, giving a great variety of structures, formed by one and the same polymer. Scanning force microscopy studies on polymer adsorption can be divided into three categories. The first one is devoted to * Corresponding author. † Western Michigan University. ‡ NASU. (1) Lipatov, Y. S. Colloid Chemistry of Polymers; Elsevier: Amsterdam, 1988. (2) Skan, K. J.; Blokhus, E. M.; van Male, J. Macromolecules 2004, 37, 1969. (3) Lipatov, Y. S.; Todosijchuk, T. T.; Chornaya, V. N. Polymer Interfaces and Emulsions; Esumi, K., Ed.; Marcel Dekker: New York, 1999. (4) Kawaguchi, M. A.; Takahashi, A. AdV. Colloid Interface Sci. 1992, 37, 219. (5) Lipatov, Y. S. Adsorption of Polymer in Mixed Polymer Systems. In Encyclopedia of Surface & Colloid Science; Hubbard, A., Ed.; Marcel Dekker: New York, 2002.

studies of polyionomer adsorption. Both in situ and ex situ studies have been performed, and details of adsorption kinetics and conformational changes in the route of adsorption have been recognized.6-9 The second set of publications is connected to the adsorption of proteins and some other biologically important molecules (DNA, etc.) typically on specially functionalized polymer surfaces.10-12 In both cases, aqueous-based systems are used, which simplifies significantly the experimental procedure. Nevertheless, in situ measurements of the adsorption process remain a challenging task,13-15 as the atomic force microscopy (AFM) tip also provides an appropriate active surface site for possible adsorption and its contamination with the adsorbate species is difficult to avoid. The third direction of AFM research in the field is dealing with ultrahigh resolution (molecular resolution) imaging of individual polymer chains.14,15 The latter represents a special case of sample preparation from ultradiluted solutions and therefore does not stand for practically important cases of polymer adsorption but rather fundamental questions of conformational behavior of the macromolecules at a solid surface. Several special scanning force microscopy modes have been applied in previous studies to decrease possible damage produced (6) Samoshina, Y.; Nylander, T.; Claesson, P.; Schillen, K.; Iliopoulos, I.; Lindman, B. Langmuir 2005, 21, 2855. (7) Liu, G. M.; Yan, L. F.; Chen, X.; Zhang, G. Z. Polymer 2006, 47, 3157. (8) Minko, S.; Roiter, Y. Curr. Opin. Colloid Interface Sci. 2005, 10, 9. (9) Tsukruk, V. V.; Bliznyuk, V. N.; Visser, D.; Campbell, A. L.; Bunning, T. J.; Adams, W. W. Macromolecules 1997, 30, 6615. (10) Tulpar, A.; Henderson, D. B.; Mao, M.; Caba, B.; Davis, R. M.; Van Cott, K. E.; Ducker, W. A. Langmuir 2005, 21, 1497. (11) FreijLarsson, C.; Nylander, T.; Jannasch, P.; Wesslen, B. Biomaterials 1996, 17, 2199. (12) Zhang, G.; Yan, X.; Hou, X. L.; Lu, G.; Yang, B.; Wu, L. X.; Shen, J. C. Langmuir 2003, 19, 9850. (13) Hamley, I. W.; Connell, S. D.; Collins, S. Macromolecules 2004, 37, 5337. (14) Yamada, T.; Shiratori, S. Electr. Eng. Jpn. 2002, 141, 1. (15) Minko, S.; Kiriy, A.; Gorodyska, G.; Sheparovych, R.; Lupitskyy, R.; Tsitsilianis, C.; Stamm, M. ACS Symp. Ser. 2005, 897, 207.

10.1021/la701644n CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2007

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to gentle polymer-solvent or polymer-polymer layers during scanning, increase the contrast, and distinguish the compositional distribution based on the different chemical natures of the components. Those include variations of force modulation mode scanning probe microscopy (SPM): pulsed force mode AFM16 and ultrasonic force microscopy (UFM)17,18 and also variants of chemical contrasting by employing the difference in the attraction and repulsion interactions between the tip and the components.19,20 Importantly, scanning probe microscopy provides more physical information about the surface than just imaging. For example, details of polymer-substrate adhesive interactions have been revealed in specially designed experiments including those which addressed individual macromolecules in situ while they adsorb to a solid substrate.21,22 AFM investigation of polymer blend adsorption was performed earlier in combination with selective desorption of polymer components performed with selective solvents.23,24 These earlier experiments revealed characteristic morphological features of the adsorbed layers and posed additional questions concerning appropriate nondestructive procedures of the samples’ postadsorption treatments (washing, drying) to preserve the original structure for ex situ experiments.25,26 A separate set of studies deals with structural peculiarities of self-organization in ultrathin blended or block-copolymer films on various inorganic substrates.27-33 A tiny balance of polymerpolymer interactions, polymer-substrate interactions, and a thermodynamically unfavorable polymer-air interface in a confined geometry creates in this case a specific morphology, which is described in terms of spinodal dewetting of the polymer film from the surface.33-36 The features of such surface imposed phase separation (lateral and vertical) were recognized and studied both experimentally and theoretically depending on polymer type, film thickness, and film deposition technique.27,29,30,33,37,38 The latter was most typically a dip or spin-coating procedure.26,33 (16) Ebner, A.; Kienberger, F.; Stroh, C. M.; Gruber, H. J.; Hinterdorfer, P. Microsc. Res. Tech. 2004, 65, 246. (17) Porfyrakis, K.; Kolosov, O. V.; Assender, H. E. J. Appl. Polym. Sci. 2001, 82, 2790. (18) Dinelli, F.; Castell, M. R.; Ritchie, D. A.; Mason, N. J.; Briggs, G. A. D.; Kolosov, O. V. Philos. Mag. A 2000, 80, 2299. (19) Kiriy, A.; Gorodyska, G.; Minko, S.; Tsitsilianis, C.; Jaeger, W.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 11202. (20) Fleming, B. D.; Wanless, E. J. Microsc. Microanal. 2000, 6, 104. (21) Bremmell, K. E.; Scales, P. J. Colloids Surf., A 2004, 24, 19. (22) Friedsam, C.; Becares, A. D.; Jonas, U.; Seitz, M.; Gaub, H. E. New J. Phys. 2004, 6. (23) Zimin, D.; Craig, V. S. J.; Kunz, W. Langmuir 2004, 20, 8114. (24) Harton, S. E.; Luning, J.; Betz, H. Macromolecules 2006, 39, 7729. (25) Jacquemart, I.; Pamula, E.; De Cupere, V. M.; Rouxhet, P.; DupontGillain, C. C. J. Colloid Interface Sci. 2004, 278, 63. (26) Gesang, T.; Hoper, R.; Possart, W.; Petermann, J.; Hennemann, O. D. Appl. Surf. Sci. 1997, 115, 10. (27) Gesang, T.; Possart, W.; Hennemann, O. D.; Petermann, J. Langmuir 1996, 12, 3341. (28) Tanaka, K.; Yoon, J. S.; Takahara, A.; Kajiyama, T. Macromolecules 1995, 28, 934. (29) Li, Y. X.; Yang, Y. M.; Yu, F. S.; Dong, L. S. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 9. (30) Gutmann, J. S.; Muller-Buschbaum, P.; Stamm, M. Appl. Phys. A: Mater. Sci. Process. 2002, 74, S463. (31) Krausch, G.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J. Appl. Phys. Lett. 1994, 64, 2655. (32) Walheim, S.; Boltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995. (33) Heriot, S. Y.; Jones, R. A. L. Nat. Mater. 2005, 4, 782. (34) Liao, Y. G.; Su, Z. H.; Sun, Z. Y.; Shi, T. F.; An, L. J. Macromol. Rapid Commun. 2006, 27, 351. (35) Leonard, D. N.; Spontak, R. J.; Smith, S. D.; Russell, P. E. Polymer 2002, 43, 6719. (36) Rui, X.; Song, Z. W.; Jing, S.; Tian, D. C. Polymer J. 2005, 37, 560. (37) Neto, C.; Jacobs, K.; Seemann, R.; Blossey, R.; Becker, J.; Grun, G. J. Phys.: Condens. Matter 2003, 15 S421. (38) Ton-That, C.; Shard, A. G.; Teare, D. O. H.; Bradley, R. H. Polymer 2001, 42, 1121.

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Nanoscale adsorption layers can also be produced by adsorption from mixed polymer systems. A detailed description of the adsorption in mixed systems is given in ref 5. Of special interest are the adsorption layers formed by two different polymers in the course of simultaneous competitive adsorption.39 In this case, the adsorption is coupled to the interaction parameter between two dissolved polymers and the solution regime (dilute or semidilute one), and the thermodynamic compatibility of the two species in the solution exerts an essential influence on the final adsorption layer. Our paper presents the first attempt of scanning probe microscopy characterization of polymer adsorption and of the structural features of the mixed layers formed by simultaneous competitive adsorption of two immiscible polymers, polystyrene and poly(butyl methacrylate), from a common solvent in comparison to a solution casting procedure in a broad range of compositions and two practically important concentration regimes, a diluted solution adsorption (no overlap between polymer chains) and semidiluted regime with considerable overlap. A combination of the AFM and UFM techniques provided a unique nondestructive characterization tool and an opportunity to reveal both the topography and distribution of the polymer components within the adsorbed or cast films. We have also addressed the fractal properties of blended films formed through the adsorption process. Experimental Section We have studied the adsorption of two amorphous immiscible polymers, polystyrene (PS) and poly(butyl methacrylate) (PBMA), from their binary solutions and ternary solutions in a common solvent (carbon tetrachloride, CCl4) with component ratios of 1:3, 1:2, 1:1, 2:1, and 3:1. Chemicals were purchased from JSC STIROL, Ukraine (PS); ORGSTEKLO, Russia (PBMA); and NEFTEHIM, Russia (CCl4) and used without further purification. The polymer components chosen for this research were distinct in their polarity and energy of adsorption. The characteristics of PBMA and the PS are given as follows: PBMA: Mw ) 2.7 × 105 g/mol, Mw/Mn ) 1.9 and PS: Mw ) 2.2 × 105 g/mol, Mw/Mn ) 1.95. We used three different protocols of sample preparation as shown schematically in Figure 1: (1) adsorption from a polymer solution (“unwashed samples”); (2) adsorption from a polymer solution followed by rinsing the film with a pure solvent to remove polymer material which has not been adsorbed but rather trapped by the polymer solution liquid meniscus during preparation (“adsorbed films”); and (3) casting from a very dilute solution (“cast films”). Figure 1 also demonstrates the difference between in situ characterization (which has not been applied in this study) and ex situ characterization of the adsorbed films in comparison to the case of cast films (where the forced deposition technique was applied). In the latter case of deposition, interaction of the polymer chains with the substrate plays little or no role. The overall polymer solution concentration was kept in the range of 0.25-6.0 g/100 mL for protocols 1 and 2 but was much lower (0.025-0.05 g/100 mL) for case 3. Additionally, two different solution regimes were applied in case 1 and case 2 with the overall polymer concentration being lower (C < C*) or higher (C > C*) than the critical concentration of polymer chain overlap C*. C*PS ) 0.9 g/100 mL and C*PBMA ) 1.01 g/100 mL were used as the critical concentrations of the pure polymer components.3 The ranges of the overall solution concentration were 0.4-0.6 g/100 mL for C < C* and 4.5-6.0 g/100 mL for C > C* regimes of adsorption. The same concentrations have been used for the preparation of samples for AFM studies and in independent experiments for the measurement of the overall adsorbate amount. The surface of a silicon substrate was pretreated by immersion into a saturated solution of potassium bichromate in concentrated sulfuric acid for at least 2 h and then (39) Lipatov, Y. S.; Todosijchuk, T. T.; Chornaya, V. N. Compos. Interfaces 1994, 2, 53.

AFM and UFM InVestigation of PS/PBMA Layers

Figure 1. Schematic of polymer sample preparation by adsorption from solution (steps 1 and 2) or solution casting (step 3). The washing and drying procedure removes polymer chains or clusters, which are not tightly attached to the substrate. Simultaneously, a minor rearrangement and considerable shrinking in thickness should be observed, which makes ex situ structural studies different but relevant to a virgin structure of the adsorbate. On the contrary, the solution casting procedure is a forced deposition where polymer chains and clusters are first confined in a thin layer of concentration solution (with nonequilibrium increasing concentration) and then into a solid film with a morphology dictated by phase separation of immiscible components and phase segregation due to different affinities of the components to the substrate (see more explanations in the text). washing with deionized water. This treatment allowed simultaneous cleaning (degreasing) and hydrophilization of the surface. In a typical adsorption procedure (case 1 or 2), a piece of cleaned and hydrophilic silicon substrate was put (with the working surface up) on the bottom of a beaker filled with a polymer solution of a desirable concentration and polymer-polymer ratio for 4-6 h to ensure adsorption equilibrium.40 The substrates were then quickly removed from the solution and allowed to dry naturally in a horizontal position to remove the solvent (case 1), or rinsed with pure solvent to remove polymer chains not tightly attached to the solid surface during adsorption (case 2).41 It should be stressed that in case 2 the substrate was transferred from the polymer solution to a beaker with pure solvent instantly (i.e., in such a way that the polymer solution layer trapped in the meniscus has not been dried within this procedure, but only after washing of the adsorbed layer with the pure solvent). The main difference between the diluted and concentrated solutions in the sense of the final structure of the adsorbate is expected to be the presence of polymer clusters in the latter case. The clusters are formed in the solution state, and then they can be transferred as a whole to the substrate during the deposition process. Reduction of the adsorbate thickness under drying due to evaporation of the solvent is also expected as shown in Figure 1. In case of solution casting (case 3 in Figure 1), the silicon substrate was put on a horizontal surface and then a small amount (∼0.1 mL) of highly diluted polymer solution was deposited on top and naturally dried due to solvent evaporation. Our structural studies were focused on the samples prepared by protocol 2 (adsorbed layers) as the samples prepared (40) Lipatov, Y.; Todosijchuk, T.; Chornaya, V. J. Colloid Interface Sci. 1995, 174, 361. (41) Lipatov, Y. S.; Bliznyuk, V. N.; Todosijchuk, T. T.; Chornaya, V. N.; Kattumenu, R. Colloid Polym. Sci. 2006, 284, 893.

Langmuir, Vol. 23, No. 26, 2007 12975 under the most equilibrium conditions, while the two other types (unwashed and cast films) were used mainly for comparison purposes. The thickness of the adsorption layers, surface morphology, and structural features (the domain size and type) of the polymer blends adsorbed on the silicon substrate were addressed with the atomic force microscopy (AFM) technique. An Autoprobe CP (Thermomicroscopes) instrument was employed in non-contact operation mode or in contact mode in the case of ultrasonic force microscopy (UFM) studies. Non-contact mode was used for the thickness and surface root-mean-square (rms) roughness measurements and surface structure and morphology estimations, while UFM (which is analogous to force modulation mode AFM) was applied for investigation of the actual ratio and distribution of the two polymer components within the ultrathin polymer films formed as a result of adsorption. Non-contact mode scanning was performed using silicon cantilevers with a typical resonance frequency of 250 kHz, a spring constant of 7-10 N/m, and a tip radius of 3-10 nm (MikroMasch). On the other hand, UFM is a special modification of the standard contact mode of AFM where the sample is oscillated at a high frequency (compared to the resonance frequency of the cantilever) by an additional piezo resonator.18 It is well-known that the cantilever exhibits nearly 102-104 times higher dynamic stiffness at higher frequencies compared to low frequencies. The sample oscillating at these higher frequencies exerts a constant additional force on the “apparently stiff” cantilever, elastically indenting itself into the tip. The vertical deflection of the cantilever was fed to the lock-in amplifier (Stanford SR830 system) along with the reference frequency supplied to the sample. The modulation of sound waves passing through the sample thickness due to the varying local stiffness is detected as the modulation of the cantilever deflection (voltage of the photodetector) and then amplified by a lock-in amplifier to give the surface stiffness distribution in arbitrary units.18 For UFM imaging, soft silicon nitride cantilevers with a spring constant of 0.3 N/m, a resonance frequency of 20 kHz, and a tip radius of 3-10 nm (MikroMasch) have been employed. A 50 kHz sinusoidal signal has been applied to the piezo resonator on which the sample was mounted. The typical sample surface penetration depth during UFM imaging can be estimated as 1-2 nm. The surface topography of the sample was simultaneously acquired as an average cantilever deflection. AFM and UFM images were analyzed with Thermomicroscopes IP2.1.15 Image Processing and Data Analysis software to extract information on average domain size and composition. The latter was based on the consideration of histograms and surface coverage and was possible because of the high UFM signal contrast due to differences in the polymer component stiffness. The rms roughness has been measured at 20 × 20 µm2, 10 × 10 µm2, 5 × 5 µm2, 2 × 2 µm2, and 1 × 1 µm2 scan sizes to explore surface fractal properties. The thickness measurements of the polymer films were done by using AFM as follows: First, intentional scratches were produced with a sharp steel needle on the polymer surface down to the substrate, and then the thickness was extracted from cross sections of the AFM scans performed across these scratches. The film thickness was estimated as an average distance in the z-direction (height) between the bottom of the scratch (substrate level) and the film surface from the cross sections of the AFM images.

Results and Discussion The sample preparation procedure is schematically shown in Figure 1. Postadsorption washing of the samples is necessary to remove the polymer chains, which were occasionally trapped in the liquid meniscus during substrate withdrawl from the polymer solution. Without such procedure, the adsorbate layer would not be different from the solution cast films. The same solvent should be used for this procedure to ensure that the adsorbed molecules would not be desorbed. Generally, the desorption processes are thermodynamically disadvantageous and very unlikely and therefore proceed very slowly if they proceed at all.5 Drying of the adsorption layer proceeds very fast (a few seconds). Taking into account that this time is a priori drastically shorter than the

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Figure 2. Typical morphology of mixed adsorption films (PS/ PBMA ) 1:3) revealed by AFM for low concentration (a-c) and high concentration (d-f) adsorption regimes. Scan size is 2 × 2 µm2 for images (a) and (d), 5 × 5 µm2 for images (b) and (e), and 10 × 10 µm2 for images (c) and (f). Vertical scale is 60 nm for images (a)-(c) and and 200 nm for images (d)-(f).

typical relaxation time of confined polymer chains, one should not expect a significant structural reconstruction of the adsorption layer during this operation. Previous comparative studies of in situ and ex situ structures of ultrathin polymer films have confirmed such theoretical considerations.13-15,24,27 Contrary to the adsorption process, solution cast polymer films are formed under conditions of fast solvent evaporation and nonequilibrium drying. Polymer chains are trapped in a thin liquid solution layer and forced to rearrange under intermolecular interactions and the surface tension of the liquid meniscus. Thermodynamic immiscibility of the polymer components together with their interaction with the solid surface will cause a special situation of microphase separation and self-ordering with the formation of a heterogeneous structure different from the adsorption layer but with a composition imposed by the ratio of the components in the initial solution.5 Figure 2 shows the representative morphology of a PS/PBMA layer obtained as a result of adsorption from a joint solution with the initial ratio PS/PBMA ) 1:3 at different scales of observation (AFM scan size). One can recognize the characteristic feature of the morphology: appearance of round, ellipsoidal, or elongated domains. Also, an increase of the solution concentration above C* produces a more homogeneous morphology. The distribution of the sizes and the shape of the domains depend on the original ratio of the components as is evident from Figure 3. Three different kinds of domain structures have been observed: island-type structures mostly seen in C > C* films (with the exception of the sample prepared from the PS/PBMA ) 2:1 binary solution at high concentration), interpenetrated domain structures for the C < C* regime, which are represented in Figure 4, and lamella type structures with an alternation of the PS and PBMA domains, which can be found mainly for the 1:1 ratio of the components and have been described in our previous publication.41 Maximum domain sizes were found in samples prepared from binary solutions with the ratios PS/PBMA ) 1:2 and PS/PBMA ) 3:1. The morphology and domain size in their dependence on the compositions for both the low and high concentration regimes of adsorption are summarized in Table 1. Domain sizes I and II stand for the maximum and minimum values, respectively, observed for the same sample. In the case of the elongated domain shape (such as that in Figure 3), the averages of the longest and the shortest axes were calculated, and they are shown in Table 1. The effect of the diminishing adsorption rate of the preferably

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Figure 3. AFM images of adsorption films formed from mixed polymer solutions in carbon tetrachloride with component ratios of PS/PBMA ) 1:3 (images a and b) and PS/PBMA ) 1:2 (images c and d) under different concentration regimes of adsorption: C < C* (a and c) and C > C* (b and d). Scan size is 10 × 10 µm2 and vertical scale is 300 nm for all images.

Figure 4. AFM images of adsorption films formed from mixed polymer solutions in carbon tetrachloride with component ratios of PS/PBMA ) 2:1 (images a and b) and PS/PBMA ) 3:1 (images c and d) under different concentration regimes of adsorption: C < C* (a and c) and C > C* (b and d). Scan size is 10 × 10 µm2 and vertical scale is 120 nm for all images. Table 1. AFM Measured Domain Sizes in Mixed Adsorbed Layers Depending on the Composition of the Binary Polymer Solution and the Concentration Regime of Adsorption

PS/PBMA ratio in the original solution 1:0 3:1 2:1 1:1 1:2 1:3 0:1

domain size I (nm) (C < C*)

domain size II (nm) (C < C*)

no domains 500-600 200 200 200 500 300 1000 no domains

domain size I (nm) (C > C*)

domain size II (nm) (C > C*)

no domains 300 2000 1000 200 500 4000 400-500 no domains

adsorbing polymer component by introduction of the second polymer into the binary solution is observed for both the miscible and immiscible polymer pairs.5 As reported in previous publica-

AFM and UFM InVestigation of PS/PBMA Layers

Figure 5. AFM topography (a-c) and UFM (d-f) images of PS/ PBMA ) 1:3 polymer blends adsorbed from solution C < C* (a and d) and adsorbed from solution C > C* (b and e), and a film cast from solution C , C* (c and f). Scan size is 10 × 10 µm2 for all images. Vertical scale is 80 nm for all AFM topography images and is 0.2 V in arbitrary units of voltage measured by using a photodetector for UFM images.

Figure 6. AFM topography (a-c) and UFM (d-f) images of PS/ PBMA ) 2:1 polymer blends adsorbed from solution C < C* (a and d) and adsorbed from solution C > C* (b and e), and a film cast from solution C , C* (c and f). Scan size is 10 × 10 µm2 for all images. Vertical scale is 120 nm for all AFM topography images and is 0.2 V in arbitrary units of voltage measured by using a photodetector for UFM images.

tions, a poor quality of the mixed solvent resulted in a reduction of the dimensions of the coils and of aggregate size.42 Also, the adsorption values drop when passing through C*. The difference in adsorption in the regions below and above C* should be connected to a variation of the solvent quality. A higher probability for the formation of entanglements between macromolecular coils above C* may bring either diminishing of the adsorption rate or, on the contrary, formation of bigger domains in the adsorption layer. The latter effect is due to the formation of entangled network clusters in the solution and depends on several factors including the overall concentration of the solution and the ratio of the polymer components as discussed above. The UFM mode was used to obtain data on density distribution and phase separation of two polymers of the PS/PBMA polymer blend system by exploiting their different stiffness properties. The results obtained were analyzed under the consideration of given solution ratios. Figures 5 and 6 show examples of recorded topography (top) and UFM (bottom) images of polymer films under study adsorbed from PS/PBMA ) 1:3 and PS/PBMA ) (42) Kuleznev, V. N.; Wolf, B.; Pozharnova, N. A. Polymer Science 2002, 44, 67.

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Figure 7. AFM topography (a-c) and UFM (d-f) images of symmetrical PS/PBMA ) 1:1 polymer blends adsorbed from solution C < C* (a and d) and adsorbed from solution C > C* (b and e), and a film cast from solution C , C* (c and f). Scan size is 10 × 10 µm2 for images (a), (c), (d), and (f) and 6 × 6 µm2 for images (b) and (e). Vertical scale is 80 nm for all AFM topography images and is 0.2 V in arbitrary units of voltage measured by using a photodetector for UFM images.

2:1 polymer solutions. PS appears bright in the UFM images due to the fact that it is stiffer and has higher mechanical strength than PBMA. The percentage of the PS and PBMA distribution on the surface can be different compared to the known polymer solution ratios. The interaction between components, the free surface energy of the polymers at the film-air interface, and the interfacial interaction at the polymer-substrate boundary affect the observed distribution of polymers. Also, the amount of a polymer component may be difficult to estimate correctly from UFM data in some cases. PS has a high modulus of elasticity and therefore should look brighter in UFM images in comparison to the PBMA component. Unfortunately, the situation may be more complex in the case of a stratified (alternated) placement of the polymer components along the normal to the film substrate. In this case, only the upper layer will be “visible” in the UFM mode and the real content of the polymer component may be under- or overestimated. For example, in Figure 5a, the percentage of PS estimated visually and with image analysis software is ∼40% rather than 25% (the value expected from the initial PS/ PBMA ratio), and in Figure 6b the estimated adsorption amount of PS is below 66% while the initial ratio of the components in the joint polymer-polymer solution is PS/PBMA ) 2:1. Other examined samples showed distributions of the components close to the expected ratios. Figure 7 shows AFM/UFM data for a symmetrical (PS/PBMA ) 1:1) composition. A mosaic morphology reported by us before41 is typical for this case. The size of the individual phase domains, the rms roughness, and the films thickness are reduced for this composition in comparison to asymmetrical compositions (Figures 5 and 6). The distribution of the polymer components revealed by UFM images clearly shows the alternation of PS and PBMA domains in the plane of the film. Interestingly, solution cast films of the same initial solution composition look dissimilar to the adsorption films. They show very low contrast in UFM and appear different (more uniform morphology) in the AFM tapping mode. One can suggest that the in-plane lamellar structure typical for the adsorbed films (Figure 7a and b) is rotated by 90° (i.e., with a layered distribution of the components along the normal to the film surface). Additional arguments for such arrangement of the components will be given below after a short review of the thermodynamics of polymer adsorption.

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Table 2. Thickness of Adsorbed Films Measured from AFM Experiments PS/PBMA ratio in the original solution

thickness (nm) (C < C*)

thickness (nm) (C > C*)

1:0 3:1 2:1 1:1 1:2 1:3 0:1

25 ( 5 55 ( 6 80 ( 15 100 ( 50 92 ( 16 155 ( 8 22 ( 5

60 ( 5 640 ( 60 254 ( 30 200 ( 100 260 ( 17 375 ( 38 35 ( 5

Structural information obtained from AFM experiments on amorphous polymer layers formed during adsorption from a solution state can be represented quantitatively with several parameters including average domain size, average film thickness, surface roughness, and fractal dimensions. Let us consider these parameters separately. A very important parameter, which is considered in many theoretical and experimental works, is the thickness of the adsorption layer, which is determined by a conformational state of the adsorbed macromolecules. Recently, the determination of the thickness of adsorption layers of poly(2-vinyl pyrrolidone)polystyrene diblock copolymers formed in situ has been performed by ellipsometry.43 Two regimes of the adsorption process have been recognized: a fast initial adsorption, which is characterized with a submonolayer thickness of the adsorbed film, and a regime of relatively slow secondary adsorption, which starts as the thickness exceeds several radii of gyration and is characterized with a strong lateral overlap of the adsorbed chains. During this second regime of adsorption, the film becomes progressively more crowded and the 1/3 power law dependence of the film thickness on the amount of polymer material adsorbed is observed consistently with computer simulation and mean-field theoretical results.43 Therefore, both kinetic and thermodynamic factors appear to be important for the final polymer film thickness formation. In ref 13, AFM has been used to study the formation of micelles during the adsorption of a propylene oxide/ethylene oxide amphiphilic block-copolymer. Again, the adsorbed film thickness was found to be a complex function of the copolymer chemical composition, chemical nature of the substrate, and concentration of the polymer solution. Table 2 shows the results of the thickness measurements for the mixed polymer adsorption layer in the PS/PBMA-carbon tetrachloride-silica system. As is seen from Table 2, the thickness values of the adsorption are strongly dependent on the concentration of the solution and are generally in the range of 20-700 nm. The lower limit of this range is in agreement with the natural structural scale of polymer chains, the radius of gyration, which was calculated from the molecular weights to be 16 and 15 nm for PS and PBMA, respectively. The formation of aggregates with sizes of 40-250 nm was proved in previous studies (see, for example, ref 40) and can explain the upper limit of thickness variation. The thicknesses of the adsorption layers for the C > C* regime are 2-9 times higher as compared with those of the C < C* regime. These results conclude that C* is an important parameter affecting the thickness of the adsorbed layer. The reason for that fact can be explained as the size and number of macromolecular aggregates depend on the value of C*. The probability of polymer aggregate formation is much higher for the C > C* regime.5 Figure 8 shows the polymer film thickness dependence on the composition for low and high concentration regimes. Two maxima can be observed at the PS/PBMA ) 1:3 and PS/PBMA ) 3:1 ratios in the solution state (i.e., 25% and 75% concentration of (43) Toomey, R.; Mays, J.; Tirrell, M. Macromolecules 2004, 37, 905.

PS in the initial solution). Variation of the polymer component ratio in the solution affects the thickness of the adsorbed polymer material due to the fact that the kinetics of polymer adsorption for different polymer components is determined both by their affinity to the solid surface and on the parameter of their thermodynamical interaction.5 The minimum values of the thickness have been observed for pure polymer components. It should be mentioned that the adsorbed layers are not uniform in thickness, and therefore, the average values of the thickness are provided in Table 2. The difference between the thicknesses of the layers can be also related to the effect of incompatibility of the polymer components, which implies that the surface of the adsorbent has a mosaic structure with an alternation of regions formed by each polymer component.41 Experimentally measured surface rms roughness values collected for polymer blends formed by adsorption from concentrated solutions (C > C*) are given in Table 3. The polymer blend of PS and PBMA prepared by adsorption from a 1:1 component ratio solution and pure PS and PBMA surface rms roughness values at different solution concentration regimes were taken from a previous study.41 The rms roughness at the 10 × 10 µm2 scale depending on the ratio of the polymer components for two different solution adsorption regimes is presented in Figure 8. Table 3 demonstrates that the rms roughness values are increasing with increasing scan size for the same sample, which could be expected from theoretical considerations.44,45 Also, the rms roughness is higher for the high concentration regime in comparison to the low concentration regime of adsorption, and the shapes of the curves of the roughness versus composition dependence (a bimodal distribution with two broad maxima at around PS/PBMA ) 1:2 and PS/PBMA ) 3:1 polymer ratios) are similar for both concentration regimes. Qualitative theory of adsorption in mixed polymer systems (i.e., from solution of binary solute) has been suggested in ref 5. It stipulates that a solution of two polymers, A and B, in a common solvent may be considered as a solution of the polymer A in a “mixed solvent” (polymer B + common solvent) and vice versa. The thermodynamic quality of such mixed solvent differs from the quality of a pure solvent, and this fact dramatically affects the whole adsorption process. Strictly speaking, the process of adsorption of one of the polymer components depends on the presence of the second polymer through the thermodynamic quality of the mixed solvent, which in turn depends on the solution concentration and thus varies in the process of adsorption. As a consequence, the adsorption process will never proceed from the solvent of the same thermodynamic quality, as the concentration of the solution is constantly changing during the process of adsorption. The adsorption from the mixed solutions is also determined by the affinity of each polymer component to the surface. Chemically different polymers usually have different affinities to the surface, which leads to preferential adsorption of the polymer with the highest segmental adsorption energy. As a result of these factors, two simultaneous and competitive adsorption processes (of each polymer component) will take place with the possibility of a displacement adsorption of one of the components with another. Thus, the ratio of components in the adsorption layer does not necessarily correspond to that in the original solution. The situation where two or more polymeric species are competing for surface sites leads to greatly different adsorption characteristics for various polymers. A mixed adsorption layer is formed in which the properties of homopolymer (44) Meakin, P. Scaling and Growth Far from Equilibrium; Cambridge University Press: Cambridge, 1998. (45) Barabasi, A.-L.; Stanley, H. E. Fractal Concepts in Surface Growth; Cambridge University Press: Cambridge, 1995.

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Figure 8. Dependence of the structural parameters of adsorbed films (the average film thickness and rms roughness) on the ratio of polymer components (PS/PBMA) in a mixed polymer solution. Table 3. AFM rms Roughness Measured at Various Scan Sizes for Adsorbed Polymer Layers on SiO2 rms roughness (nm) for C < C*

rms roughness (nm) for C > C*

PS/PBMA ratio in the original solution

1 × 1 µm2

5 × 5 µm2

10 × 10 µm2

1 × 1 µm2

5 × 5 µm2

10 × 10 µm2

1:0 3:1 2:1 1:1 1:2 1:3 0:1

3.1 ( 0.1 0.9 ( 0.1 1.1 ( 0.2 0.6 ( 0.1 6.5 ( 0.3 3.7 ( 0.5 0.4 ( 0.1

4.1 ( 0.2 1.9 ( 0.1 2.8 ( 0.7 0.7 ( 0.2 10.5 ( 0.5 7.5 ( 0.5 1.8 ( 0.2

4.4 ( 0.3 5.5 ( 1.5 4.1 ( 1.2 3.9 ( 0.3 11.5 ( 0.2 7.8 ( 0.5 2.6 ( 0.3

0.3 ( 0.1 1.3 ( 0.2 2.5 ( 1 0.2 ( 0.1 6.6 ( 3.7 1.6 ( 0.5 1.5 ( 0.1

0.5 ( 0.2 10.2 ( 0.5 12.8 ( 1.5 0.6 ( 0.2 48.5 ( 7 46 ( 4 2.6 ( 0.2

3.5 ( 0.3 25 ( 12.5 14 ( 0.7 1 ( 0.3 70.8 ( 4.7 60.5 ( 10 3.9 ( 0.3

chains are combined. All these effects can be additionally complicated by the dependence of the adsorption process of each component on its molecular-mass distribution. The theory of adsorption considers a dimensionless parameter of segmental adsorption energy (similar to the Flory and Huggins interaction parameter χAB introduced for polymer blends46), which characterizes the thermodynamic interaction between a polymer and a solid surface, χS, and can be introduced as follows:47

∆US ) (Ups - U1s) ) -χSkT

(1)

where Ups and Uls are the energies of interaction of the polymersurface and solvent-surface, respectively, ∆US is the difference in the interaction energy between situations when a polymer segment and a molecule of solvent are in contact with the solid surface, k is Boltzmann’s constant, and T is absolute temperature. Being expressed in such a way, a negative value of the interaction parameter χS favors adsorption. In an alternative consideration, the parameter of interaction with the surface can be introduced as an enthalpy exchange between the solvent molecules adsorbed on the adsorbent surface and polymer segment, which are to be adsorbed:48

χS ) [H1s - His + 1/2(H11 - Hii)]/kT

polymer and the surface, but rather as a parameter characterizing a given polymer-adsorbent-solvent system. This parameter depends on the nature of a solvent and the energy of interaction between the segment and solvent molecules. Application of the concept of the interaction parameter to adsorption from polymer mixtures needs the simultaneous knowledge of this parameter for each polymer component χSA and χSB. Due to the reasons considered above, these values will be dependent on the changing quality of the mixed polymer solution. Moreover, at conditions of competitive adsorption, it is necessary to take into account various interactions: interactions between each component and solvent, pair interactions between similar and dissimilar segments, and so forth. For an immiscible polymer pair (χAB > 0), independent adsorption of each polymer component should take place. However, as discussed above, such assumption can be confusing, as the thermodynamic quality of the solvent is constantly changing during adsorption. Nevertheless, χSA and χSB are important parameters in the theoretical description of the adsorption process. In a simplified form, these values may be represented in accordance to Silberg’s approach47 as follows:

∆USA ) (UAS - USL - UAB) ) -χSAkT

(2)

where H is the enthalpy of binary interactions with subscripts distinguishing the adsorbent surface (s), polymer chain (i), or solvent molecules (1). In such representation, a positive value of χS provides a condition for polymer adsorption. χS values may be found from displacement studies.49 However, they should not be considered as a universal measure of interaction between the (46) Tompa, H. Polymer Solutions; Butterworth: London, 1956; p 325. (47) Silberg, A. J. Chem. Phys. 1968, 48, 2835. (48) Lipatov, Y.; Chornaya, V.; Menzheres, G.; Todosijchuk, T. J. Colloid Interface Sci. 2004, 272, 281. (49) Kawaguchi, M. AdV. Colloid Interface Sci. 1990, 32, 1.

and

∆USB ) (UBS - USL - UAB) ) -χSBkT

(3)

where U represents different energies of interaction (indices A, B, L, and S belong to polymer components A and B, liquid surface L, and the adsorbent surface S, respectively) and ∆U gives the net variation of the free energy as a result of adsorption of one of the components. A complete description of the interaction parameters may be done in application of the FloryHuggins theory to a multicomponent system, which includes two polymers (solvent and adsorbent).5

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Table 4. Data for Adsorption in the PS/PBMA-Carbon Tetrachloride-Silica System (Summary of the Results Presented in Refs 3 and 48) initial solutionsa concentration, g/100 mL

concentration in the adsorption layer, g/100 mL

ratio of the PS/PBMA PBMA components

PS

adsorption layer

PS

PBMA

ratio of the PS/PBMA components

0.15 0.3 0.2 0.15 0.45

0.3 0.15 0.2 0.45 0.15

1:2 2:1 1:1 1:3 3:1

C< 0.08 0.16 0.08 0.04 0.12

0.2 0.12 0.16 0.25 0.08

1:2.5 1.3:1 1:2 1:6 1.5:1

1.5 3.0 1.5 4.5

3.0 1.5 4.5 1.5

1:2 2:1 1:3 3:1

C > C* 0.7 1.2 0.6 1.0

1.8 1.0 2.0 1.0

1:3.1 1.2:1 1:3.3 1:1

C*

a

The same solutions were used for AFM sample preparation.

Adsorption in the PBMA-PS-SiO2 (silica) system represents a traditional case for comparison of the abovementioned theoretical considerations with the experiment, and it has been addressed in several previous publications.48,50-52 The PBMA/ PS system Flory-Huggins interaction parameter χAB has been measured to be 0.0072.53 It is a small positive value denoting the immiscibility of the polymer components. The energies of adsorption interaction of the PS-carbon tetrachloride-silica substrate surface and PBMA-carbon tetrachloride-silica substrate surface have been measured in ref 48 as 16.8 and 25.6 kJ/mol, correspondingly, which presents a trend for a preferential adsorption of PBMA to silica from its joint solution with PS in carbon tetrachloride. Other reported experiments also demonstrated preferential adsorption of PBMA observed at any ratio of the components PBMA/PS from the joint solution in carbon tetrachloride.5 Our results of AFM/UFM studies of the morphology and structural parameters of mixed adsorbed layers of the PS/PBMA-carbon tetrachloride-silica system are also in favor of such conclusion. The observed two-maxima dependence of the structural parameters of the adsorbed layers (thickness, roughness, etc.) on composition can be generally explained from the above-discussed theoretical standpoint of a “mixed polymer solvent” and its quality variation in the system. Compositions with a nearly symmetrical ratio of the components represent the case of the worst quality of such mixed solvent, which means maximal contraction of the polymer chains. Such possibility has been previously discussed in literature (see, for example, ref 42). It is interesting to compare our direct AFM/UFM findings with those of other independent studies on the same system. Table 4, based on the results presented in ref 48, shows the compositions of adsorption layers at various adsorption regimes in comparison with the component ratio in solution. First of all, as can be seen in all the cases, PBMA is adsorbed preferentially compared to PS. Second, the adsorption of PBMA (rate) has a well pronounced non-monotonous dependence on the PS/PBMA ratio. The total amount of the adsorbed polymer material is (50) Lipatov, Y. S.; Todosijchuk, T.; Chornaya, V. J. Colloid Interface Sci. 1999, 215, 290. (51) Chornaya, V.; Lipatov, Y.; Todosijchuk, T.; Menzheres, G. J. Colloid Interface Sci. 2002, 255, 36. (52) Lipatov, Y.; Chornaya, V.; Todosijchuk, T.; Dudarenko, G. J. Colloid Interface Sci 2005, 285, 525. (53) Lipatov, Y. S.; Nesterov, A. E.; Ignatova, T. D.; Gudima, N. P.; Gritsenko, O. T. Eur. Polym. J. 1986, 22, 83.

characterized with a bimodal distribution (i.e., this value is higher for the asymmetrical compositions PS/PBMA ) 2:1 and PS/ PBMA ) 1:3 in comparison to the symmetrical PS/PBMA ) 1:1 mixture) which is in good correspondence with the AFM data (Figure 8). Polymer films prepared through solution casting represent a different type of morphology. During the formation of such films, polymer components are trapped within a thin liquid film of polymer solution. In the process of solvent evaporation and formation of a solid film, polymer components compete for places at the solid-liquid and liquid-air interfaces while possessing enough mobility for rearrangement. Structural organization at the solid-liquid interface is governed by the cohesive energy of interaction with the substrate. On the contrary, formation of the polymer-air interface is determined by conditions of minimizing the surface free energy. As discussed before, the PBMA component is preferentially adsorbed on SiOx due to its higher cohesive energy in comparison to the PS component. PBMA is also characterized with a lower surface free energy (31.2 mN/m against 40.7 mN/m for PS54). This means that under equilibrium conditions the PBMA component has a tendency to occupy simultaneously the solid-liquid and liquid-air interfaces and PS is supposed to be “buried” inside between two PBMA layers. This situation has been previously observed for PS/PBMA block-copolymers where flat-top “table” domains or so-called relief 2D domains could be observed due to smectic type ordering (lamellar structure oriented parallel to the substrate) of twodimensionally confined polymer films. A similar self-organization process can be observed in our case. Figure 9 shows AFM topography and UFM images of a PS/PBMA ) 1:3 solution cast films. The characteristic flat domain morphology can be clearly seen on the topography image. A UFM image of the same place shows relatively uniform stiffness on top of the domains and between them but significantly reduced stiffness in places corresponding to the edge slopes of the domain (can be seen as ring structures in Figure 9b). Such characteristic morphology could be found only in a narrow composition range (with the majority being PBMA). Interestingly, while in block-copolymer systems the relief 2D domains are spontaneously formed at elevated temperature (T > Tg) and require monodisperse polymer chains, in our case they are formed at room temperature and are not so sensitive to polymer component polydispersity. A suggested distribution of the components within the film thickness is shown in Figure 9c. The vertical dimension of the domains is exaggerated in this scheme for clarity. In reality, the domains are very flat discs with a diameter of 1-2 µm and a thickness of ∼50 nm. Such characteristic “vertical phase separation” pattern is similar to the one proved for PS/PBMA block-copolymers.55 The observed distribution of the stiffness can also be explained based on this scheme. The elastic response to deformation should be similar for places 1 and 3 on the surface (in both cases, a thin PBMA layer is covering the stiffer PS or SiOx sublayer). Position 2 (on the slope of the domain) represents however a different situation when a much thicker PBMA layer can significantly damp the propagation of ultrasonic waves and therefore produce a negative contrast in the UFM images. Generally, one can see with this example that cast polymer films possess a morphology which is distinct from the films adsorbed from the same solution. In the former case, polymer chains are trapped in a thin liquid solution layer and forced to rearrange under intermolecular interactions and the surface tension (54) http://www.surface-tension.de/solid-surface-energy.htm. (55) Maaloum, M.; Ausserre, D.; Chatenay, D.; Coulon, G.; Gallot, Y. Phys ReV. Lett. 1992, 68, 1575.

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Figure 10. Characteristic morphology types of mixed adsorbed layers depending on the PS/PBMA composition for films with the majority being the PBMA component (a), for films of an approximately equal composition of PS and PBMA (b), and for films with the majority being the PS component (c).

Figure 9. AFM topography (a) and UFM (b) images of a PS/ PBMA ) 1:3, C > C* sample received by solution casting showing a characteristic structure of flat-top domains, which are formed due to the self-organization of immiscible polymer components confined within a thin polymer solution meniscus during solid film formation. Part (c) shows the suggested internal structural organization of the film with a lamellar arrangement of the components and a PS layer sandwiched between two PBMA layers due to the minimization of both the cohesive energy of the polymer film-substrate interface and surface free energy of the final polymer film during the solution casting process. Triangles show three distinct situations for polymer film stiffness mapped in the UFM mode (see additional explanations in the text).

of the liquid meniscus. Thermodynamic immiscibility of the polymer components together with their interaction with the solid surface will cause a special situation of microphase separation and self-ordering with the formation of a heterogeneous structure different from the adsorption layer but with a composition imposed by the ratio of the components in the initial solution.5 In the second case, the polymer film microstructure is governed only by the factors of polymer-polymer miscibility and polymer chain interactions with the substrate, which results in a different microstructure. Based on a comparative analysis of stiffness distribution available from the UFM and AFM topography images, one can suggest three distinct morphologies observed in the PS/ PBMA adsorbed layer depending on the PS/PBMA ratio (Figure 10): (a) elongated curved PS domains on top of a relatively uniform PBMA layer (this case is typical for polymer-polymer compositions with the majority being PBMA); (b) lamellar structures with alternating “vertical” domains typical for symmetrical compositions; and (c) wavy surface with roundly shaped domains typical for compositions with the majority being PS. Transition from the C < C* to C > C* adsorption regime leads to coarsening of these three nanostructures, leaving their characteristic features similar to those discussed above.

Additional insight into the self-organization of mixed adsorbed polymer layers can be given with consideration of the fractal properties of the latter. By now, it is well recognized that both individual macromolecular coils in a solution44,45,56 and amorphous polymer films formed from a solution57 are fractal objects. One can expect that the fractal characteristics of adsorbed layers as monomolecular-level thin films formed through the selfassembly process should be determined by the fractal dimensions of the macromolecular coils or clusters formed in the solution state.1 In accordance to the fractal theory approach, a polymer film can be characterized with a special parameter, a fractal dimension, Df, which shows the “degree of intrusion of the surface structure into the third dimension”.44,45 The fractal dimension of the adsorbed layer should depend strongly on several factors: the chain constitution, the conditions of the polymer film formation, the interactions of the polymer chains with the substrate, and the intermolecular interactions within the polymer film. As a mathematical description of the fractal properties of the surface, the roughness exponent R (or Hurst exponent in other notations) can be introduced through a scaling law describing the relationship between the average surface roughness W(L) and the lateral scale of its measurement, L:44,45

W(L) )

x

1

L

[h(i) - hh(i)]2 ∑ L i)1

(4)

where h(i) and hh(i) stand for the height of an individual point on the sample surface and the mean height of the surface, respectively (averaging is performed over a region of size L on the surface). The roughness exponent R can be found as a slope of log(W) versus log(L) dependence. On the other hand, R, the dimension of embedding space d (3 in our case), and the fractal (56) Kozlov, G. V.; Zaikov, G. E.; Novikov, V. U. Fractal analysis of polymers; Nova Science Publishers Inc.: New York, 2003. (57) Bliznyuk, V. N.; Burlakov, V. M.; Assender, H. E.; Briggs, G. A. D. Macromol. Symp. 2001, 167, 89.

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Table 5. Fractal Dimensions for Polymer Films Formed via Adsorption from Mixed PS/PBMA Polymer Solutions at Low and High Concentration Regimes PS/PBMA ratio in the original solution

fractal dimension C < C*

0:1 1:3 1:2 1:1 2:1 3:1 1:0

2.17 2.31 2.60 2.30 2.81 2.74 2.85 C > C*

0:1 1:3 1:2 1:1 2:1 3:1 1:0

2.6 2.50 2.25 2.30 2.51 2.07 2.04

dimension Df of the surface are connected through a simple equation:44,45

d ) Df + R

(5)

Fractal dimensions of the adsorbed polymer films have been calculated using the abovementioned equations from the rms roughness values estimated from the different scan sizes of AFM images. Df parameter data determined for the PS/PBMA-(carbon tetrachloride)-silica system are summarized in Table 5. Polymer films produced at the low concentration regime of adsorption have generally higher fractal dimension values (like 2.81) in comparison to those in the high concentration regime. As can be seen from Table 5, the fractal dimension values are monotonously decreasing with an increase of PS content in blended films for the high concentration regime of adsorption, while they are generally increasing (non-monotonously) in the case of low concentration regime adsorbed films. Some intermediate values of the fractal dimension are found for blended polymer films in comparison to pure polymer component films, which should be the result of a gradual variation of the conformation properties for the PS and PBMA components. In particular, PS chains can form special conformational features such as loops or mushrooms in a regime of dilute solution adsorption, which can be replaced with an ordinary random coil conformation of entangled polymer chains when the concentration is increased.41 On the contrary, PBMA chains have more bulky substitutes and cannot “close-pack” easily.41 The fractal behavior of films adsorbed from concentrated polymer solutions (at C > C* regime) is even more complicated. Despite the fact that polymer chains are prone to form aggregates, which are expected to additionally increase the fractal dimension of the adsorbed

films, this is obviously not the case for PS enriched films formed via adsorption from concentrated solutions. Df falls down to very low values of 2.04 for pure PS and 2.07 for PS/PBMA ) 3:1 compositions. These extremely small values correspond to an optically smooth surface of PS typically obtained under the solution casting regime. Our studies show that more experimental data for various polymer-polymer systems are required to make generalizations and draw conclusions of the actual dependence of the fractal properties of adsorbed polymer films on the composition and concentration regime of adsorption. Surface morphology, rms roughness, fractal dimension, and domain sizes are also studied for pure PS, pure PBMA, and PS/PBMA blends prepared by casting from a dilute polymer solution on silicon. Table 6 shows rms roughnesses measured at different scales for these samples. The correlation between the adsorbed and cast samples in the sense of surface morphology and domain sizes can be seen from a comparison of Figures 5 and 6. The morphology of the cast polymers and the polymers adsorbed from a high concentration solution generally consists of some islands with close domain size values (600and 650 nm). On the other hand, polymers adsorbed under the low concentration regime are generally characterized with an interpenetrated morphology and smaller domains when compared to the cast polymers and polymers adsorbed from a high concentration solution. The similarity between the cast samples and the polymers adsorbed from high concentration solution regimes can be explained as follows: During the casting process, the solvent starts to evaporate and the concentration of the polymer solution reaches first the low concentration regime and then a concentration close to the critical concentration of polymer chain overlapping. Finally, the high concentration regime is reached, which is close to or even characterized with a much higher concentration than that of the polymer films adsorbed from high concentration solution regimes.

Conclusions Our combined AFM/UFM study of the structure of polymer films formed as a result of adsorption from a polymer mixture revealed a complex mosaic structure of adsorbed layers. The features of the mosaic structure depend both on the ratio of the components on the adsorbent surface and on the concentration regime and composition of the original solution. The observed structural peculiarities appear as a result of the thermodynamic incompatibility of two different polymers and are conditioned by various possible conformation states of the adsorbing entities: isolated coils for dilute solutions or macromolecular clusters for semidilute solutions. Our experimental data also justify the fractal nature of the adsorbed polymer layers formed by two incompatible polymers on the surface of naturally oxidized silicon (SiO2 surface) in the course of adsorption from their joint solution. The fractal dimension parameter Df which shows the degree of

Table 6. rms Roughness, Domain Size, and Fractal Dimension Values for Solution Cast (C , C*) Amorphous Films of PS, PBMA, and Their Blends rms roughness (nm) composition of the solution (PS %) 100 75 66 50 33 25 0

fractal dimension 2.3 2.8 2.8 2.8

domain size I (nm)

domain size II (nm)

50 200 200 100 420 300

100 300 1000 300 2200 1600 250

1 × 1 µm2

2 × 2 µm2

5 × 5 µm2

10 × 10 µm2

20 × 20 µm2

0.8 ( 0.1 0.2 ( 0.1 6.7 ( 1.5 0.5 ( 0.1 0.2 ( 0.1 7.5 ( 1.5 4.5 ( 0.5

1 ( 0.1 2 ( 0.5 18 ( 0.5 1.2 ( 0.3 0.5 ( 0.1 17 ( 1 12.5 ( 1.2

1.7 ( 0.5 3 ( 0.5 23 ( 0.5 1.6 ( 0.2 1 ( 0.2 29.5 ( 0.3 23.8 ( 0.5

1.5 ( 0.2 8.3 ( 5.5 21.5 ( 1.3 1.5 ( 0.3 3.5 ( 1.5 33.5 ( 0.6 26.7 ( 0.7

2.2 ( 0.3 27.2 ( 4 22.5 ( 0.5 31 ( 0.2 31 ( 7 34.5 ( 3 28.5 ( 0.2

AFM and UFM InVestigation of PS/PBMA Layers

intrusion of the surface structure into the third dimension has been calculated from the data on rms roughness values estimated from the different scan sizes of AFM images. The fractal dimensions of adsorbed polymer mixtures are dependent on the ratio of the polymeric components at the interface and on the concentration regime of the solution (dilute or semidilute) applied in the adsorption experiments. Our data do not allow for distinguishing special conformations described in the literature for the early stages of adsorption of individual chains (trains, loops, or tails). Even in the limiting case of the very dilute solutions, the structures that are observed can hardly be interpreted in the framework of the classical picture of the conformations of the adsorbed chains mentioned above. On the contrary, it seems to be much more probable that polymer chains are adsorbed as individual polymer coils, which corresponds more to typical conformations of the chains in a dilute solution and does not require sharp changes of the conformation (i.e., their uncoiling at the interface) during the adsorption process. The description of the adsorption process as a macromolecular coil adsorption also explains better the observed variation in the structure of the adsorption layer caused by a transition to higher concentration regimes. The macromolecular coil adsorption is typical for adsorption from a concentrated solution according to

Langmuir, Vol. 23, No. 26, 2007 12983

ref 5. Moreover, the above considerations are relevant to adsorption in both the binary and the ternary systems. One more important conclusion can be drawn by analyzing the data on the roughness and fractal dimensions. Our results distinctly show the dependence of the roughness of the layer both on the adsorption conditions and on the ratio of two polymeric components in solution as well as an essential inhomogeneity of such dependence. Such an inhomogenity may be the result of various types of macromolecular packing at the interface, especially for adsorption of blends. Due to various dimensions of the coils of two different polymers, they are packed in a different way depending on their ratio in the solution. The dependencies of fractal dimensions on the same parameters (solution regime and component ratio) testifies both to the various modes of molecular packing and to the continuous changes of the characteristics of adsorbed chains. Thus, structural investigations of the adsorbed nanolayers obtained as a result of adsorption from the binary and the ternary systems demonstrate a hierarchy of the structures existing in adsorption layers and provide a route for the control of the structural organization of polymer nanolayers. LA701644N