Surface Self-Assembly of Fluorosurfactants during Film Formation of

Oct 20, 2004 - School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science. Research Center, Department of Polymer Science, ...
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Langmuir 2004, 20, 10455-10463

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Surface Self-Assembly of Fluorosurfactants during Film Formation of MMA/nBA Colloidal Dispersions W. R. Dreher and M. W. Urban* School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, Department of Polymer Science, The University of Southern Mississippi, Hattiesburg, Mississippi 39401 Received February 26, 2004. In Final Form: July 6, 2004 These studies focus on the behavior of fluorosurfactants (FS) containing hydrophobic and ionic entities in the presence of methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal dispersions stabilized by sodium dodecyl sulfate (SDS). The presence of FS significantly not only alters the mobility of SDS in MMA/nBA films, but their hydrophobic and ionic nature results in self-assembly near the film-air (F-A) interface leading to different surface morphologies. Spherical islands and rodlike morphologies are formed which diminish the kinetic coefficient of friction of films by at least 3 orders of magnitude, and the presence of dual hydrophobic tails and an anionic head appears to have the largest effect on the surface friction. Using internal reflection IR imaging, these studies show that structural and chemical features of FS are directly related to their ability to migrate to the F-A interface and self-assemble to form specific morphological features. While the anionic nature of FS allows for SDS migration to the F-A interface and the formation of stable domains across the surface, intermolecular cohesion of nonionic FS allows for the formation of rodlike structures due to inability to form mixed micelles with SDS. These studies also establish the relationship between surface morphologies, kinetic coefficient of friction, and structural features of surfactants in the complex environments.

Introduction The ability to form organized interfacial features is one of the prerequisites for obtaining surfaces that exhibit desirable properties.1 If such properties result from an interplay between chemical composition of starting materials and chemical-physical processes occurring during film formation, stimuli-responsive characteristics may be achieved.2,3 Previous studies have indicated that surfactants utilized in the synthesis of colloidal dispersions may exhibit various behaviors which depend on coalescence conditions, particle morphologies, pH, and temperature, just to name a few.4-7 More recently,8 the mobility of sodium dodecyl sulfate (SDS) surfactant in styrene/2ethylhexyl acrylate/methacrylic acid (Sty/EHA/MAA) dispersions was shown to be greatly affected by the presence or absence of ionic or covalent cross-linkers. When adipic dyhadrizide and diacetone acrylamide were incorporated into Sty/EHA/MAA particles as covalent cross-linkers, annealing temperatures of 90 °C were necessary to facilitate SDS stratification at the film-air (F-A) interface, but ambient conditions resulted in SDS migration to the F-A interface when no cross-linkers were present. Similarly, the presence of cosolvents such as propylene glycol (PG) in MMA/n-butyl acrylate (nBA)/acrylic acid (AA) colloidal dispersions not only facilitates coalescence but also effectively contributes to the displacement of interactions between SDS and AA resulting in the * To whom correspondence should be addressed. (1) Zheyuan, H.; Wang, R.; MacDiarmid, A.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480. (2) Mendez, S.; Ista, L.; Lopez, G. Langmuir 2003, 19, 8115. (3) Zhu, X.; Jun, Y.; Staarup, D.; Major, R.; Danielson, S.; Boiadjiev, V.; Gladfelter, W.; Bunker, B. G. A. Langmuir 2001, 17, 7798. (4) Zhao, Y.; Urban, M. W. Macromolecules 2000, 33, 8426. (5) Zhao, Y.; Urban, M. W. Macromolecules 2000, 33, 2184. (6) Zhao, Y.; Urban, M. W. Langmuir 2000, 16, 9439. (7) Zhao, Y.; Urban, M. W. Langmuir 2001, 17, 6961. (8) Dreher, W. R.; Zhang, P.; Urban, M. W. Macromolecules 2003, 36, 1228.

formation of SDS crystallites at the F-A interface.9 These changes are stimulated by minute chemical alterations of chemical composition or particle morphology of colloidal dispersions and effectively alter surface and/or interfacial properties such as adhesion and surface friction. In contrast to molecules and macromolecules containing hydrocarbon chains, the presence of fluorinated species offers another level of challenges and opportunities in colloidal dispersions. Incorporation of fluorinated species into polymeric systems has been of considerable interest because fluorocarbons not only alter thermal stability and chemical resistance but also, due to low surface tension, may effectively change friction.10-12 In contrast to the extensive studies on hydrocarbon colloidal dispersions, limited knowledge exists pertaining to F-containing colloidal dispersions primarily due to considerable synthetic challenges. Low surface tension of fluorocarbons makes these species noncompatible in aqueous environments because fluorocarbon moieties are capable of forming significant intramolecular bonds but relatively weak intermolecular interactions. Thus, synthetic efforts are not straightforward; in particular, fluorocarbons often form bilayers of fluorinated amphiphiles which tend to segregate to internal hydrophobic and lipophobic films with enhanced stability and reduced permeability.13 Consequently, limited knowledge exists concerning processes governing film formation, where low surface tension and solubility are the primary causes14,15 for limited dispersibility and stability.11 (9) Dreher, W. R.; Zhang, P.; Urban, M. W. Langmuir 2003, 19, 10294. (10) Munekata, S. Prog. Org. Coat. 1988, 16, 113. (11) Banks, R. E.; Smart, B. E.; Tatlow, J. C. Organofluorine Chemistry Principles and Commercial Applications; Plenum Press: New York, 1994. (12) Parker, H.; Lau, W.; Rdenlind, E. S.; Rohm and Haas Co. U.S. Patent No. 6,218,464, 2001. (13) Kraft, M. P.; Riess, J. G. Biochemie 1998, 80 (5-6), 489-514.

10.1021/la049494u CCC: $27.50 © 2004 American Chemical Society Published on Web 10/20/2004

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Chart 1. Structural Features of Fluorosurfactants: FSA, FSO, and FSP

Figure 1. ATR FT-IR spectra of MMA/nBA films recorded from the F-A interface: (A) 0.18 µm, (B) 0.32 µm, (C) 0.4 µm, (D) 0.54 µm, and (E) 0.69 µm from the F-A interface and (F) SDS reference spectrum.

Although several useful attempts have been made to prepare fluorinated dispersions,12,16-19 limited experimental approaches did not allow elucidation of processes leading to film formation as well as determination of surface properties. As indicated by numerous studies, morphological information obtained from surfaces is not sufficient for understanding the role of fluorocarbons in colloidal dispersions.20,21 However, coexistence of hydrocarbon and fluorosurfactants may provide the environment for mixed micelles to form prior to polymerization of colloidal particles. As a result, reduced electrostatic forces may possibly overcome strong hydrophobic interactions that cause poor dispersibility in an aqueous phase thus disturbing monomer-micelle equilibrium, which should alter particle morphology. With this in mind and in an attempt to develop polymeric surfaces with desirable morphologies and film friction properties, these studies examine the effect of fluorosurfactants on interfacial properties of MMA/nBA colloidal dispersions and ultimately mechanisms responsible for their film formation. Experimental Section MMA, nBA, potassium persulfate (KPS), and SDS were purchased from Aldrich Chemical Co. Fluorosurfactants utilized for the purpose of these studies are tridecafluoro-octyl sulfanyl propionic acid lithium salt (FSA), tridecafluoro-octyloxy ethylene (FSO), and phosphoric acid bis(tridecafluoro-octyl) ester am(14) Barthelemy, P.; Tomao, V.; Selb, J.; Chaudier, Y.; Pucci, B. Langmuir 2002, 18, 2557. (15) Hiyama, T. Organofluorine Compounds; Springer-Verlag: Berlin, 2000. (16) Kim, C. U.; Lee, J. M.; Ihm, S. K. J. Appl. Polym. Sci. 1999, 73, 777. (17) Marion, P.; Beinert, G.; Juhue, D.; Lang, J. Macromolecules 1997, 30, 123. (18) Marion, P.; Beinert, G.; Juhue, D.; Lang, J. J. Appl. Polym. Sci. 1997, 64, 2409. (19) Tsuda, N.; Iwakiri, R.; Yonew, Y.; Imoto, K.; Shimizu, Y.; Takayuki, A.; Kondo, M.; Daikin Industries, Ltd. U.S. Patent No. 5,804,650, 1998. (20) Linemann, R. F.; Malner, T. E.; Brandsch, R.; Bar, G.; Ritter, W.; Mulhaupt, R. Macromolecules 1999, 32, 1715. (21) Thomas, R. R.; Lloyd, K. G.; Stika, L. M.; Stephans, L. E.; Magallanes, G. S.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. Macromolecules 2000, 33, 8828.

Figure 2. ESEM micrographs of the F-A interface of MMA/ nBA film: (A) without fluorocarbon surfactant, (B) FSA, (C) FSO, and (D) FSP. monium salt (FSP), DuPont Co., and their corresponding structures are shown in Chart 1. Colloidal dispersions were synthesized under monomer-starved conditions using a semicontinuous polymerization process,22 as described elsewhere.4 The following concentration levels (% w/w) of each ingredient were utilized in synthesis: water (58), MMA (20), nBA (20), KPS (0.24), and SDS (1.66). Colloidal sphere-shaped particles with a diameter of 110 nm dispersed in an aqueous phase (no organic solvent) were obtained, and the main reason for postadding fluorosurfactants was to avoid the particle size effect. Addition of F-containing surfactants from 0 to 2% (w/w) was examined for each surfactant, but concentration levels exceeding 1% for FSA and 1.25% for FSO resulted in colloidal particle coagulation; thus these dispersions were not utilized in further analysis. Colloidal (22) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers; John Wiley & Sons: New York, 1997.

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Figure 3. ATR FT-IR spectra recorded from various depths from the F-A interface of MMA/nBA containing FSA: (A) 0.18 µm, (B) 0.32 µm, (C) 0.4 µm, (D) 0.54 µm, and (E) 0.69 µm from the F-A interface; (F) MMA/nBA reference spectrum; (G) FSA reference spectrum; (H) SDS reference spectrum.

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Figure 4. ATR FT-IR spectra recorded from various depths from the F-A interface of MMA/nBA containing FSO: (A) 0.18 µm, (B) 0.32 µm, (C) 0.4 µm, (D) 0.54 µm, and (E) 0.69 µm from the F-A interface; (F) MMA/nBA reference spectrum; (G) FSO reference spectrum; (H) SDS reference spectrum.

dispersions were cast on a poly(vinyl chloride) (PVC) substrate and allowed to coalescence at 80% relative humidity (RH) for 3 days at 24 °C to obtain approximately 20 µm thick films. Polarized attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra were collected using a Bio-Rad FTS-6000 FT-IR single-beam spectrometer set at a 4 cm-1 resolution. A 45° face angle Ge and KRS-5 crystal with 50 × 20 × 3 mm dimensions was used. This configuration allows for the analysis at approximately 0.2-0.7 µm from the film-air. Each spectrum represents 100 coadded scans ratioed against the same number of reference scans collected using an empty ATR cell. All spectra were corrected for spectral distortions and optical effects using Q-ATR software.23 The average depth thickness in ATR was determined using the following relationship:

dp )

λ0 2Πη1(sin2 θ - η21)1/2

where λ0 is the wavelength of light, η1 is the refractive index of the crystal, θ is the angle of incidence, and η12 is the refractive index ratio of sample and crystal. All spectra were corrected using the Urban-Huang algorithm.23 In a typical experiment, by varying the angle of incidence as well as the ATR crystal (typically Ge or KRS-5) one can obtain the depth penetration data from surfaces ranging from a monolayer to about 4 µm thickness into the surface with a spatial resolution of about 100 nm. The latter depends on the precision of the cell, and the depth profiling is accomplished by a stepwise approach.22 Figure 6 shows the relative amounts of SDS as a function of depth and was obtained by normalizing ATR spectra to the acrylic CdO vibrations at 1728 cm-1, and the symmetric S-O stretching band at 1084 cm-1 was used to estimate obtain relative concentrations of SDS. IR images were obtained using internal reflection IR imaging (IRIRI) with a Ge internal reflection element. This system consists of a Bio-Rad FTS 6000 spectrometer, a UMA 500 microscope, an ImagIR focal plane array image detector, and a semispherical Ge crystal which facilitates a spatial resolution of about 1 µm.23 IRIR images were collected using the following spectral acquisition parameters: undersampling ratio ) 4, step-scan speed ) 2.5 Hz, number of spectrometer steps ) 1777, number of images per step ) 64, and spectral resolution ) 8 cm-1. In a typical experiment, a spectral data set acquisition time was approximately 15 min. Image processing was performed using ENVI (23) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers: Theory and Practice; American Chemical Society: Washington, DC, 1996.

Figure 5. ATR FT-IR spectra recorded from various depths from the F-A interface of MMA/nBA containing FSP: (A) 0.18 µm, (B) 0.32 µm, (C) 0.4 µm, (D) 0.54 µm, and (E) 0.69 µm from the F-A interface; (F) MMA/nBA reference spectrum; (G) FSP reference spectrum; (H) SDS reference spectrum. (The Environment for Visualizing Images, Research Systems, Inc.) version 3.5, and when necessary, baseline correction algorithms were used to compensate for a baseline drift.24 Environmental scanning electron microscopy (ESEM) experiments were performed using a FEI Quanta 200 microscope, and ESEM micrographs were obtained from the F-A interface of F-containing films using a 400× magnification. Thermal transitions were recorded using a TA Q100 differential scanning calorimeter (DSC) by heating the samples from -90 to 200 °C at a rate of 10 °C/min. A Qualitest 1055 friction tester was utilized to determine the kinetic coefficient of friction for each film.25 In the friction experiment, each film was placed on a movable sled and friction was determined by measuring the force required to move the specimen against the stationary 200 g weight.

Results and Discussion As stated in the Introduction, primary interest in F-containing polymers rises from their unique surface and interfacial properties, and the objective of these studies (24) Otts, D. B.; Zhang, P.; Urban, M. W. Langmuir 2002, 18, 6473. (25) ASTM Standard Test D 1894-01.

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Figure 6. Relative concentration levels of SDS plotted as a function of depth from the F-A interface for (A) MMA/nBA containing FSA, (B) MMA/nBA containing FSO, and (C) MMA/ nBA containing FSP.

is to incorporate these species into aqueous colloidal dispersions. Furthermore, film formation processes as well as surfaces with desirable friction properties are also of interest. Ultimately, one would like to establish their role and interactions with other components in colloidal dispersions. To set the stage, we prepared films from MMA/ nBA dispersions synthesized in the presence of SDS and identified spectroscopic features characteristic of MMA/ nBA copolymers at various depths from the F-A interface. For that purpose and similarly to the previous studies, we utilized ATR FT-IR. Figure 1, traces A-E, illustrates a series of spectra recorded at 0.18, 0.32, 0.40, 0.54, and 0.69 µm from the F-A interface of MMA/nBA films. Trace F represents the spectrum of SDS. As shown in traces A-E, increasing intensities of the 1084 and 1220 cm-1 bands due to SDS26,27 indicate that the concentration level of SDS decreases as the distance from the F-A interface increases. As the distance from the F-A interface increases, the 1165 and 1146 cm-1 bands attributed to nBA and MMA of the copolymer matrix also exhibit changes. These changes may be attributed to compositional drift which may occur during the emulsion polymerization process, therefore producing a slightly phaseseparated system.22,28 This conclusion is further supported by DSC data (not shown) which indicated the presence of the Tg at -46 °C29 due to the softer nBA-rich copolymer component. As shown in the previous studies,4,5,8,9 stratification near interfaces may be driven by a number of factors, and the surface energy driven by the chemical makeup of the surface species is one of the prevailing attributes. Thus, chemical structures of surfactants in a given environment will dictate their ability to alter particle interfacial properties. In an effort to perturb particle interfacial regions, we postadded 1% w/w of fluorocarbon surfactants to MMA/nBA SDS-stabilized colloidal dispersions and allowed films to coalesce. ESEM images recorded from the F-A interface are illustrated in Figure 2A-D, for MMA/nBA and MMA/nBA containing FSA, FSO, and FSP surfactants, respectively. As seen in Figure 2A, MMA/ (26) Lin-Vien, D.; Colthup, N. B.; Fateley, W.; Grasselli, J. The Handbook of Infrared and Raman Characteristic Frequenxies of Organic Molecules; Academic Press: San Diego, 1991. (27) Roeges, N. P. G. A Guide to the Complete Interpretation of Infrared Spectra of Organic Structures; Wiley: New York, 1994. (28) Odian, G. Principles of Polymerization, 3rd ed.; John Wiley & Sons: New York, 1991. (29) Brandrup, J.; Immergut, E. H. Polymer Handbook, 2nd ed.; John Wiley & Sons: New York, 1975.

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nBA surfaces are featureless, as shown by the image without contrast. Although spectroscopic analysis (Figure 1) indicated the presence of SDS at the F-A interface, it should be realized that ATR FT-IR penetrates deeper into the F-A interface and thus is capable of detecting surfactants imbedded in the film, whereas ESEM measurements allow for detection of surface morphological features. Examination of ESEM images in Figure 2B-D, however, indicates that the addition of fluorosurfactants results in distinctly different surface morphologies. Specifically, depending upon the chemical structure of the F-containing surfactants, the morphological features appear as spheres (Figure 2B), rodlike morphologies (Figure 2C), and spheres again (Figure 2D). Although these results clearly demonstrate the effect of F-containing surfactants on film morphologies, the question of immediate importance is the origin of these features and their formation. Although one would assume that the addition of F-containing species will generate their stratification near the F-A interface, another issue is how the presence of fluorosurfactants alters SDS distribution and ultimately surface morphologies. For that reason, we conducted a series of depth profiling ATR FT-IR experiments allowing us to obtain chemical information from 0.18 to 0.70 µm near the F-A interface. Figures 3-5 illustrate a series of ATR FT-IR spectra recorded from the F-A interfaces of MMA/nBA films coalesced under the same conditions containing FSA, FSO, and FSP surfactants, respectively. For each figure, traces A-F represent ATR FT-IR spectra of respective colloidal films collected at 0.18, 0.32, 0.40, 0.54, and 0.69 µm for the F-A interface, and traces G and H are the reference spectra of the corresponding Fcontaining surfactant and SDS. Analysis of the S-O stretching vibrations of SDS at 1084 cm-1 used to monitor the mobility of SDS in the presence of alternate fluorosurfactants clearly illustrates significant differences not only in the mobility of SDS but also in the surface properties. For example, Figure 3 shows that the 1145 cm-1 band due to C-F stretching vibrations of FSA is detected at the F-A interface, but its stratification near the F-A interface suppresses the mobility of SDS. This is shown by the decrease of the 1084 cm-1 band. On the other hand, as shown in Figure 4, FSO exhibits a significant increase in the degree of stratification at the F-A interface for both SDS and FSO. This is demonstrated by an increase of the 1220 and 1080 cm-1 bands and a decrease of the 1728 cm-1 carbonyl bands due to the copolymer matrix. Finally, in the presence of FSP (Figure 5) FSP and SDS are simultaneously present at the F-A interface, as manifested by the shoulder at 1205 cm-1 and the 1066 cm-1 band due to FSP as well as the 1120 and 1080 cm-1 SDS bands. At this point, let us focus on the distribution of SDS as a function of depth from the F-A interface in the presence of F-containing species. This is illustrated in Figure 6, which shows that SDS concentration levels are significantly higher near the F-A interface and their magnitude depends on the type of fluorosurfactant. These curves were obtained by measuring the area under the S-O stretching bands at 1084 cm-1 in SDS, which allows us to determine relative surface concentration of SDS. For example, curve A represents the distribution of SDS for unmodified MMA/ nBA films, and SDS is detected in excess at the F-A interface. However, at greater distances from the F-A interface, its concentration decreases. In contrast, the presence of FSO (curve B) further increases the content of SDS near the F-A interface, but the presence of FSP

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Figure 7. IRIR images recorded from the F-A interface of MMA/nBA containing FSA: (A-C) Images obtained by tuning into 1728, 1145, and 1080 cm-1 IR bands, respectively; (D) IR spectra recorded from areas labeled 1 and 2 in image A. For reference purposes, the IR spectra of MMA/nBA, FSA, and SDS are included.

(curve D) and FSA (curve C) suppresses SDS concentration levels near the F-A interface. Although ESEM images may suggest that morphological features result from fluorosurfactants migrating to the surface, spectroscopic data indicate that this phenomenon is more complex. Furthermore, significant differences in morphological features as well as SDS distribution near the F-A interface raise the question as to what is the effect of ionic groups and hydrophobic tails of fluorosurfactants in the presence of ionic hydrocarbon surfactants on spatial distribution of these species near the F-A interface. As we recall, previous studies9 showed an inherent propensity for surfactants to form self-organized islands which influence a number of surface properties and ATR FT-IR measurements provide an average distribution at the F-A interface, whereas ESEM micrographs clearly indicated significant formation of heterogeneous domains. Thus, one of the desirable quantities is the chemical makeup of these surface heterogeneities. To identify their molecular origin, IRIRI was utilized. Figures 7-9 illustrate a series of IRIR images recorded from the F-A interface of MMA/nBA polymeric films containing 1% (w/ w) FSA, FSO, and FSP fluorosurfactants. Because IRIRI allows tuning into a given IR band associated with a specific chemical species, this approach facilitates chemical imaging of the F-A interface of each film in the xy directions and the following bands will be used as tracers of specific film components: CdO stretching at 1728 cm-1 due to MMA/nBA copolymer matrix, C-F stretching at 1145 cm-1 due to fluorosurfactants, S-O stretching at

1218 and 1084 cm-1 due to SDS, C-O stretching at 1090 cm-1 due to FSO, and P-O stretching at 1080 cm-1 due to FSP. As shown, Figure 7A-C illustrates IR images of the 1728, 1145, and 1080 cm-1 bands, and the red color represents the highest concentration levels of the band which was “tuned in” to, whereas black corresponds to the lowest concentration levels. Analysis of the image shown in Figure 7 indicates that the surface of the MMA/ nBA film containing FSA possesses a continuous copolymer matrix with the presence of phase-separated spherical domains. Further analysis of Figure 7B,C indicates that the origin of the heterogeneous regions is associated with both FSA and SDS, as illustrated by the presence of the 1145 and 1080 cm-1 IR bands. While Figure 7A-C provides spatial distribution of different entities near the F-A interface, Figure 7D represents IR spectra recorded from the regions labeled area 1 and area 2 in Figure 7A. For clarity, reference MMA/nBA, FSA, and SDS spectra are included. As previously determined, area 1 is due primarily to the presence of MMA/nBA polymer matrix, as shown by the intense 1728 cm-1 band, and area 2 represents an island that is detected upon tuning into the 1145 and 1080 cm-1 bands, thus attributed to the simultaneous presence of FSA and SDS. The same analysis was performed on MMA/nBA films containing FSO fluorosurfactant, and the results are shown in Figure 8. Similarly, Figure 8A-C illustrates the spatial distribution of 1728, 1218, and 1090 cm-1 bands due to the copolymer matrix, SDS, and FSO. As shown, the images display large heterogeneous domains which

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Figure 8. IRIR images recorded from the F-A interface of MMA/nBA containing FSO: (A-C) Images obtained by tuning into 1728, 1218, and 1090 cm-1 IR bands, respectively; (D) IR spectra recorded from areas labeled 1 and 2 in image A. For reference purposes, the IR spectra of MMA/nBA, FSA, and SDS are included.

are significantly different from the spherical domains shown in Figure 7A-C. Chemical analysis of areas 1 and 2 in Figure 8A-C is shown in Figure 8D and indicates that the surface of the MMA/nBA film consists of heterogeneous domains. The overall concentration of the copolymer matrix decreases in areas 1 and 2, as shown by the decrease of the 1728 cm-1 band, and the primary features are bands at 1218 cm-1 due to SDS and 1090 cm-1 due to FSO. The results of the IRIRI analysis of the MMA/nBA films containing FSP are illustrated in Figure 9. Again, Figure 9A illustrates spherical domains of alternate sizes embedded among a surface consistent with MMA/nBA, as shown by the presence of the intense red region when tuning into 1728 cm-1. When tuning into the 1218 cm-1 band (Figure 9B), it is clear that these regions contain SDS, and analysis of the P-O band at 1080 cm-1 indicates the presence of FSP (Figure 9C). Analysis of areas 1 and 2 in Figure 9A confirms these findings by the increases of the 1218 and 1080 cm-1 IR bands in area 2, as compared to area 1. Also, an increase of the 1145 cm-1 C-F band is observed, thus indicating an increase of fluorocarbon species at the F-A interface. In summary, these data clearly indicate the presence of self-assembled domains at the surface and, furthermore, the simultaneous presence of SDS and F-containing surfactants occupying the same space. At this point, let us examine how surface self-assembly may alter surface properties. The primary driving force for F-containing species to migrate to the F-A interface is their ability to compensate for an excess of surface

energy,29 and their effect on surface properties is significant. As illustrated in Figure 2, surfaces become morphologically heterogeneous yet chemically organized. For that reason, we prepared MMA/nBA colloidal dispersions containing FSA, FSO, and FSP ranging from 0.25 to 2% w/w concentrations with respect to the total emulsion solution and monitored the formation of heterogeneous surface domains at the F-A interface. Figure 10A-C illustrates IRIR images obtained from the F-A interfaces of MMA/nBA films containing FSA, FSO, and FSP, respectively. As seen, the presence of islands containing F-containing species is detected, and as the F-containing solution concentration increases, phase-separated domains become larger. [As stated in the Experimental Section, an excess of F-containing surfactant may result in unstable suspensions which result in nontransparent films. For that reason, the amount of F-containing species was limited to the values listed in Figure 10.] At this point, it would seem reasonable to determine the surface tension of these surfaces, but contact angle measurements were inconclusive. As one would expect, the overall surface tension has decreased upon addition of F-containing surfactants, but there were marginal differences between the surface tension of surfaces containing FSA, FSO, and FSP species. Another important surface property is the surface friction which is often expressed as the kinetic coefficient of friction. Therefore, the friction at the F-A interface was measured as a function of relative surface concentration of FSA, FSO, and FSP in MMA/nBA films. This is illustrated in Figure 11, curves A-C. Since the solution concentration of

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Figure 9. IRIR images recorded from the F-A interface of MMA/nBA containing FSP: (A-C) Images obtained by tuning into 1728, 1145, and 1080 cm-1 IR bands, respectively; (D) IR spectra recorded from areas labeled 1 and 2 in image A. For reference purposes, the IR spectra of MMA/nBA, FSA, and SDS are included.

fluorosurfactants listed in Figure 10A-C may not necessarily correspond to its surface concentration, we utilized IRIR imaging data to calculate relative surface concentrations which were used to generate the graph in Figure 11. The choice of using the 1728 cm-1 image was dictated by the fact that each surfactant exhibits different IR characteristics of the F-C vibrations. Therefore, to be able to normalize all three specimens, the 1728 cm-1 images were used. This was accomplished by the normalization of IR bands present in a given area occupied by F-containing species. As shown, addition of fluorosurfactants to MMA/nBA decreases the friction at the F-A interface, but each species results in significantly different friction coefficient profiles. For instance, although the surface concentration of FSP (Figure 10C) is initially the lowest, as compared to FSA and FSO (Figure 10A,B), its effect on the decrease of the kinetic coefficient of friction is the greatest, as illustrated by rapidly decaying curve C in Figure 11. In contrast, FSA and FSO are not as effective. The next question is the relationship between the levels of heterogeneity at the F-A interface and surfactant structures and differences in the kinetic friction coefficients. Since ionicity, hydrophilic headgroups, and hydrophobic tails are the driving forces for F-containing species to migrate to the F-A interface to compensate for an excess of surface energy,30 these structural features are responsible for surface characteristics. It has also been

shown that fluorosurfactants possessing polar substituents such as carboxy, sulfur, or hydroxyl groups exhibit higher surface activity in aqueous solutions as compared to their nonfluorinated analogues.15 When the anionic, dual-tailed surfactant FSP is utilized, the presence of the polar headgroups increases its surface activity which gives rise to its stratification at the F-A interface, and in the presence of SDS spherical domains at the F-A interface of MMA/nBA are formed. Due to the fact that the fluorosurfactants were postadded and may either form mixed or separate micelles, it is reasonable to assume that alternate F-containing micelles are formed in an aqueous phase or simply exist as stabilizing agents. For example, it was shown30 that two oppositely charged surfactants such as FSO and SDS often form separate micelles due to incompatibility. On the other hand, anionic surfactants such as SDS, FSA, and FSP preferentially form mixed micelles which act as liquid crystalline domains above the critical micelle concentration (cmc).31 Therefore, the surface activity and concentrations in each case can differ, and this is reflected in the formation of rodlike and spherical morphological features shown in ESEM micrographs (Figure 2). These results combined with the previous literature reports indicate that the degree of phase separation at the F-A interface is related to the presence of fluorosurfactants, and for that reason film formation processes will also vary. As previous studies showed,8,9 film formation

(30) Eastoe, J.; Rankin, A.; Wat, R.; Bain, C.; Styrkas, D.; Penfold, J. Langmuir 2003, 19, 7734.

(31) Kissa, E. Fluorinated Surfactants; Marcel Dekker: New York, 1994.

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Figure 10. IRIR images obtained from (A) MMA/nBA containing 0.25-1.0% (w/w) FSA, (B) MMA/nBA containing 0.25-1.25% (w/w) FSO, and (C) MMA/nBA containing 0.25-2% (w/w) FSP.

illustrated in Figure 12C where both rodlike FSO species and free-standing SDS particles at the F-A interface are formed. Figure 12D depicts spherical surface morphological features in the presence of FSP where FSP and SDS coexist at the F-A interface, and in contrast to FSA, the stratification of SDS is not suppressed in the presence of FSP.

Figure 11. Kinetic coefficient of friction plotted as a function of relative surface concentration of FSA, FSO, and FSP.

of MMA/nBA colloidal dispersions in the presence of SDS may be governed by a number of factors including crosslinking agents, cosolvents, and acid functionalities. When considering MMA/nBA colloidal dispersions without fluorosurfactants, the absence of cross-linkers and acid functionalities gives rise to the stratification of SDS at the F-A interface. This is schematically illustrated in Figure 12A. On the other hand, the presence of the singletail fluorosurfactant (FSA) results in the formation of FSA/ SDS spherical domains at the F-A interface. This is illustrated in Figure 12B. In contrast, FSO exhibits strong intermolecular cohesion15 which generates rodlike entities (Figure 2C) and the film formation process is schematically

In summary, these studies illustrate that the presence of fluorosurfactants significantly alters surface properties and may result in the formation of liquid-crystalline-like domains which significantly alters surface friction properties. It appears that the most effective fluorosurfactant is FSP due to the fact that closely packed SDS/FSP domains are formed. Furthermore, the presence of fluorosurfactants also alters film formation processes. Conclusions Film formation processes of MMA/nBA colloidal dispersions are affected by the simultaneous presence of hydrocarbon and fluorosurfactants. Dispersions containing fluorosurfactants as postadditives are able to form films under ambient conditions, and structural and chemical compositions of the F-containing surfactants are directly related to their ability to migrate to the F-A interface. The anionic natures of FSA and FSP allow for migration of SDS and F-containing surfactants to the F-A interface and the formation of spherical domains across the surface of the films. In contrast, intermolecular

Surface Self-Assembly of Fluorosurfactants

Langmuir, Vol. 20, No. 24, 2004 10463

Figure 12. Pictorial representation of the film formation of MMA/nBA/SDS in the presence of (A) no fluorocarbon surfactant, (B) FSA, (C) FSO, and (D) FSP.

cohesion of nonionic FSO allows for the formation of rodlike structures at the F-A interface due to the inability of FSO to form mixed micelles with SDS. These studies also show the relationship between surface morphologies, kinetic coefficient of friction, and structural features of surfactants.

Acknowledgment. This work was supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR 0213883. The authors also thank DuPont Co. for their generosity in donating selected fluorosurfactants for this study. LA049494U