The Surface-Segregated Nanostructure of Fluorinated Copolymer

pubs.acs.org/Langmuir © 2009 American Chemical Society

The Surface-Segregated Nanostructure of Fluorinated Copolymer-Poly(dimethylsiloxane) Blend Films Jerzy A. Mielczarski,*,† Ela Mielczarski,† Giancarlo Galli,*,‡ Andrea Morelli,‡ Elisa Martinelli,‡ and Emo Chiellini‡ † ‡

LEM, Nancy-Universit
e, CNRS, 15 avenue du Charmois, B.P. 40, F-54501 Vandoeuvre l es Nancy, France and Dipartimento di Chimica e Chimica Industriale and UdR Pisa INSTM, Universit a di Pisa, via Risorgimento 35, 56126 Pisa, Italy Received August 6, 2009. Revised Manuscript Received October 5, 2009

Two fluorinated/siloxane copolymers, O5/19 and D5/3, carrying 6 and 8 CF2 groups in the fluoroalkyl tail, respectively, were used as the surface-active components of cured poly(dimethylsiloxane) (PDMS) blends at different loadings (0.3-5.0 wt % with respect to PDMS). The surface chemical composition was determined by angle-resolved X-ray photoelectron spectroscopy at the takeoff angles θ of 0°, 60°, and 75°. It was found that the fluorinated copolymer was surface-segregated, and in-depth segregation (∼5 nm) depended upon the chemical structure of the copolymer. The surface fluorine atomic percentage of the blends with D5/3 was up to 3 orders of magnitude higher than the theoretical value expected for ideal homogeneous samples. Moreover, small amounts of the copolymer in the blends were sufficient to saturate the outermost surface in fluorine content. The chemical composition of the surface-segregated nanostructure of the films was also proven to be affected by external environment, namely, exposure to water.

Introduction Surface self-organization of polymer films can be driven by several “bottom-up” mechanisms,1 including phase separation of block copolymers, liquid crystallinity, demixing of polymer blends, and incorporation of nanosized fillers in a polymer matrix. Surface segregation of low surface tension polymers, notably fluorinated polymers, may be an additional powerful tool to nanostructure a thin film with minimized surface energy.2-5 The inherent nonwetting, nonstick properties and the nanoscopically resolved morphology and topography of films of various architectures of phasesegregated fluorinated polymers6-10 have lately been regarded as special features to combat biofouling. Biofouling is a natural phenomenon, which affects all the manmade aquatic structures, causing serious practical and economical consequences.11 Its control has imposed environmental burdens through the use of toxic biocidal antifouling paints, some of which are now banned.12 In recent years, research has focused on the *To whom correspondence should be addressed. Email: jerzy.mielczarski@ ensg.inpl-nancy.fr. Email: [email protected]. (1) Teo, B. K.; Sun, X. H. J. Cluster Sci. 2006, 17, 529. (2) Li, X.; Andruzzi, L.; Chiellini, E.; Galli, G.; Ober, C. K.; Hexemer, A.; Kramer, E. J.; Fischer, D. A. Macromolecules 2002, 35, 8078. (3) Granville, A. M.; Boyes, S. G.; Akgun, B.; Foster, M. D.; Brittain, W. J. Macromolecules 2004, 37, 2790. (4) Fujimori, A.; Shibasaki, Y.; Araki, T.; Nakahara, H. Macromol. Chem. Phys. 2004, 205, 843. (5) Makal, U.; Uslu, N.; Wynne, K. J. Langmuir 2007, 23, 209. (6) Gudipati, C. S.; Greenlief, C. M.; Johnson, J. A.; Prayongpan, P.; Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 6193. (7) Gudipati, C. S.; Finlay, J. A.; Callow, J. A.; Callow, M. E.; Wooley, K. L. Langmuir 2005, 21, 3044. (8) Krishnan, S.; Ayothi, R.; Hexemer, A.; Finlay, J.; Sohn, K. E.; Perry, R.; Ober, C. K.; Kramer, E. J.; Callow, M. E.; Callow, J. A.; Fischer, D. A. Langmuir 2006, 22, 5075. (9) Krishnan, S.; Wang, N.; Ober, C. K.; Finlay, J. A.; Callow, M. E.; Callow, J. A.; Hexemer, A.; Sohn, K. E.; Kramer, E. J.; Fischer, D. A. Biomacromolecules 2006, 7, 1449. (10) Martinelli, E.; Agostini, S.; Galli, G.; Chiellini, E.; Glisenti, A.; Pettitt, M. E.; Callow, M. E.; Callow, J. A.; Graf, K.; Bartels, F. W. Langmuir 2008, 24, 13138. (11) Callow, M. E.; Callow, J. A. Biologist 2000, 1, 49. (12) Champ, A. M. Sci. Total Environ. 2000, 258, 21.

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development of innovative, non-toxic environmentally friendly coatings, able to repel marine foulers or minimize their adhesion strength.13 Many physical and chemical parameters are relevant to create such a “fouling-release” coating, i.e., elastic modulus,14 wettability,15 and a complex surface profile in the nanoscale regime.16-18 Our interest is in developing novel marine fouling-release coatings by nanostructuring the surface of elastomeric films, which would be able to resist attachment of the foulers and specifically enhance dislodging of those that do adhere. This may be achieved by incorporation of a fluorinated copolymer into a poly(dimethylsiloxane) (PDMS) matrix in order to create low elastic modulus and low surface energy materials, in which selfsegregation and self-organization of the fluorinated moieties can add in a synergistic way to form an ordered surface structure over different length scales.19 In particular, we incorporate -(CF2)nF (n = 6-10) fluorinated chain segments as side group substituents of a polymer backbone. It has been shown that relatively long perfluorinated segments self-assembled at the outer surface of their polymer films in ordered structures, which were ultimately determined by the achievable bulk order.20-22 By contrast, an (13) Genzer, J.; Efimenko, K. Biofouling 2006, 22, 339. (14) Chaudhury, M. K.; Finlay, J. A.; Chung, J. Y.; Callow, M. E.; Callow, J. A. Biofouling 2005, 21, 41. (15) Ista, L. K.; Callow, M. E.; Finlay, J. A.; Coleman, S. E.; Nolasco, A. C.; Simons, R. H.; Callow, J. A.; Lopez, G. P. Appl. Environ. Microb. 2004, 70, 4151. (16) Schumacher, J. F.; Aldred, N.; Callow, M. E.; Finlay, J. A.; Callow, J. A.; Clare, A. S.; Brennan, A. B. Biofouling 2007, 23, 307. (17) Schumacher, J. F.; Carman, M. L.; Estes, T. G.; Feinberg, A. W.; Wilson, L. H.; Callow, M. E.; Callow, J. A.; Finlay, J. A.; Brennan, A. B. Biofouling 2007, 23, 55. (18) Kerr, A.; Cowling, M. J. Phil. Mag. 2003, 83, 2779. (19) Marabotti, I.; Morelli, A.; Orsini, L. M.; Martinelli, E.; Galli, G.; Chiellini, E.; Lien, E. M.; Pettitt, M. E.; Callow, M. E.; Callow, J. A.; Conlan, S. L.; Mutton, R. J.; Clare, A. S.; Kocijan, A.; Donik, C.; Jenko, M. Biofouling 2009, 25, 481. (20) L€uning, J.; St€ohr, J.; Song, K. Y.; Hawker, C. J.; Iodice, P.; Nguyen, C. V.; Yoon, D. Y. Macromolecules 2001, 34, 1128. (21) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takahara, A. Langmuir 2004, 20, 5304. (22) Nishino, T.; Urushihara, Y.; Meguro, M.; Nakamae, K. J. Colloid Interface Sci. 2005, 283, 533.

Published on Web 11/09/2009

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isotropic bulk phase could effect a low surface order only, when relatively short perfluorinated chain segments were used.20 In this work, we investigated polymer thin films comprising a cured PDMS matrix and a dispersed surface-active fluorinated/ siloxane copolymer by probing their surface nanoscale structure by angle-resolved X-ray photoelectron spectroscopy. We found that the surface-active copolymer exhibited a remarkable tendency to segregate at the outer surface layer (∼5 nm) of the blend films. This feature can open new prospects for application of the blends as (bio)fouling-release coatings and allows one to conceive other applications that require self-repairing properties.

Experimental Section Materials. 1H,1H,2H,2H-Perfluorooctyl acrylate (AF6) and 1H,1H,2H,2H-perfluorodecyl acrylate (AF8) (from Fluorochem) were used without further purification. Monomethacryloxypropyl-terminated poly(dimethylsiloxane) (SiMA, Mn = 5000 g mol-1), poly(diethoxysiloxane) (ES40, 134 g mol-1), and bis(silanol)-terminated poly(dimethylsiloxane) (0.1% OH, Mn = 26 000 g mol-1) (PDMS) were used as received (from ABCR). 2,20 -Azobis(isobutyronitrile) (AIBN) (from Fluka) was recrystallized from methanol and stored at 4 °C before use. Dibutyltin diacetate (DBTDA) (from Fluka) was used as received. Synthesis of Fluorinated/Siloxane Copolymers. All the polymers were synthesized by using a free radical process. In a typical synthesis of copolymer O5/19, a solution of AF6 (6.25 g, 14.95 mmol), SiMA (3.75 g, 0.75 mmol), and AIBN (100 mg) in trifluorotoluene (TFT) (5 mL) was introduced in a Pyrex vial and degassed by several freeze-thaw pump cycles. The vial was sealed under vacuum, and the polymerization reaction was allowed to proceed for 48 h at 65 °C. The polymer was purified by repeated precipitations from TFT solutions into methanol. The obtained copolymer O5/19 was characterized by an AF8/SiMA molar ratio of 19: Mn = 95 kg mol-1, Mw/Mn = 2.3 (by size exclusion chromatography, with polystyrene calibration). The fluorinated monomer AF8 was used to prepare the copolymer D5/3, with an AF8/SiMA molar ratio of 3: Mn = 80 kg mol-1, Mw/Mn = 2.1 (by size exclusion chromatography, with polystyrene calibration). Preparation of Blend Films. In a typical preparation of the blend film B6/0.4, copolymer O5/19 (0.4 wt parts) was dissolved with the bis(silanol)-terminated PDMS (100 wt parts), the ES40 cross-linker (2.5 wt parts), and the DBTDA catalyst (0.5 wt parts) in xylene (50 wt parts). The xylene solution was cast on glass slides (76  26 mm2), that were previously rinsed with acetone and dried at room temperature, to produce uniform and well-adhered films. The blend was cured at room temperature overnight, and the cross-linked film was then annealed at 70 °C for 48 h to fully evaporate the solvent and finish the cure reaction. The thickness of the films was 100-150 μm. The blends based on the copolymer D5/3 were prepared according to the same procedure using either xylene or chloroform as solvents. Various loadings Y of D5/3 were used to prepare the B1/Y films (Y = 0.3, 0.4, and 5.0 wt % with respect to PDMS). X-ray Photoelectron Spectroscopy (XPS). AXIS NOVA photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with monochromatic Al KR (hν = 1486.6 eV) anode was used in the studies. The kinetic energy of the photoelectrons was determined with the hemispheric analyzer set to the pass energy of 160 eV for wide scan spectra and 20 eV for highresolution spectra. Electrostatic charge effect of sample was overcompensated by means of the low-energy electron source working in combination with magnetic immersion lens. The hydrocarbon carbon C 1s line with position at 284.6 eV was used as a reference to correct the charging effect. Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity factors and spectrometer transmission function. Spectrum background was subtracted 2872 DOI: 10.1021/la902912h

according to Shirley.23 The high-resolution spectra were analyzed by means of spectra deconvolution software (Vision 2, Kratos Analytical, UK). In order to study the molecular structure of the top surface layers (a few nanometers), angle-resolved XPS was applied and spectra were recorded for the three takeoff angles θ of 0°, 60°, and 75°. The experimental results at 60° are not reported here, since they did not generate additional or different conclusions from those for 75°. The takeoff angle is defined as the angle between the normal to the surface of the sample and the electron optical axis of the spectrometer. The effective information depth varies according to d = d0 cos θ, where d0 is the maximum information depth (d0 ∼ 10 nm for the C 1s line by employing an Al KR source). Maximum depth of analysis was obtained at 0°, while at 75°, the depth of analysis was limited to 1/4 of the value at 0°, giving information from the very thin outermost layer.

Results and Discussion Design of Copolymer for Surface Nanostructuring of PDMS Matrix. The fluorinated/siloxane random copolymers O5/19 and D5/3 (Figure 1) were synthesized by free radical polymerization, using 1H,1H,2H,2H-perfluorooctyl acrylate (AF6) or 1H,1H,2H,2H-perfluorodecyl acrylate (AF8) and monomethacryloxypropyl-terminated poly(dimethylsiloxane) (SiMA) as starting monomers. Therefore, the two copolymers substantially differed for the number (n) of CF2 moieties in the fluoroalkyl side chains and the molar ratio (y/(1 - y)) between the siloxane and fluorinated repeat units, which were 6 and 19 for O5/19 and 8 and 3 for D5/3, respectively. The two copolymers were used as the surface-active components of PDMS-based cured blends at different loadings. The blends of copolymer D5/3 (B1/Y) and copolymer O5/19 (B6/Y) are identified here by their different wt % content (Y) with respect to the PDMS matrix (Y = 0.3, 0.4, and 5.0). The copolymers were dissolved in either chloroform (B1/0.3 and B1/5.0) or xylene (B1/0.3, B1/0.4, and B6/0.4) together with the bis(silanol)terminated PDMS and the ES40 cross-linker, in the presence of dibutyltin diacetate as a curing catalyst. The solution was cast on glass slides and cured at room temperature according to a typical condensation mechanism of inter- and intrachain reactions. Moreover, condensation of PDMS with the silanol groups of the glass surface ensured firm anchorage by covalent bonding of the film to the substrate. According to this procedure, the copolymer was incorporated into a semi-interpenetrated network of the PDMS matrix (100-150 μm thickness). In the early stages of the cross-linking process, the polymer chains have a high mobility, and the fluorinated segments could migrate to the film-air interface driven by their lowest surface energy.24 PDMS chains are also known to favor low surface tension behavior, albeit this character is not so distinct as for fluorinated chains,25 and have been introduced in several types and architectures of polymers to modify the surface properties.26 The physicochemical properties of the copolymers and their respective films have been discussed previously.19 It is worth noting here that the films were both hydrophobic and lipophobic with a resultant low surface energy behavior. For one example, the contact angles of blend B1/0.3 with water, n-hexadecane, and (23) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (24) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321. (25) Bertolucci, M.; Galli, G.; Chiellini, E.; Wynne, K. J. Macromolecules 2004, 37, 3666. (26) Dou, Q.; Wang, C.; Cheng, C.; Han, W.; Th€une, P. C.; Ming, W. Macromol. Chem. Phys. 2006, 207, 2170.

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Article Table 1. XPS Atomic Surface Concentrations of B1/0.3, B1/0.4, B1/5.0, and B6/0.4 Blends, Copolymer D5/3, and PDMS Matrix sample

θ

B1/0.3

F 1s (%) Si 2p (%) O 1s (%) C 1s (%) theoretical

75° 0° B1/0.4

Figure 1. Chemical structures of fluorinated/siloxane copolymers D5/3 (y/(1 - y) = 3) and O5/19 (y/(1 - y) = 19).

isopropanol were 122°, 62°, and 62°, respectively, and the surface tension was 12 mN/m. This was attributed to the surface segregation induced enrichment of the fluorinated chain segments of the copolymer in the blend films.27 In fact, while PDMS surfaces are hydrophobic, fluorinated surfaces are both hydrophobic and lipophobic.25,28 Surface Segregation of Fluorinated Copolymer. Surface segregation of the blend films was investigated by angle-resolved X-ray photoelectron spectroscopy (XPS) at takeoff angles θ of 0°, 60°, and 75°. The XPS atomic surface composition of the blend films B1/0.4 and B6/0.4 at 0° and 75° is reported in Table 1. For comparison, the expected theoretical values are also reported for ideal homogeneous samples and a pure PDMS sample as references. Apparently, the fluorinated chains in both B1/0.4 and B6/0.4 are preferentially accommodated within a few nanometers at the top surface layers. Nonetheless, the differences in atomic surface composition and structure between these two samples are enormous. Observed surface enhancement in fluorine content (F 1s) is very large, 20 times, for B6/0.4, and exceptionally high, 560 times, for B1/0.4. For the latter sample, a very strong decrease in siloxane signals (Si 2p and O 1s) was also observed, despite the vast majority (99.6%) of the PDMS content in the blend. This indicates that almost all the added D5/3 copolymer is selectively segregated within a few nanometers of the top surface. Pure PDMS shows XPS results typical for a homogeneous-type sample (Table 1). Copolymer D5/3 exhibited a much stronger surface segregation than copolymer O5/19 because of its longer perfluorinated chain -(CF2)8F. Such an intrinsic tendency was predominant in the former copolymer despite its lower F content per siloxane repeat unit. This is also reflected in higher surface order parameters and lower surface tension for films of polymers containing longer perfluoroalkyl chains.20 The C 1s high-resolution lines for B6/0.4 and B1/0.4 are shown in Figure 2 for a comparison. B6/0.4 presents no signal from fluorinated fragments in the C 1s line because of a very low concentration of copolymer O5/19 at the polymer-air interface. The presence of fluorinated chains was detected only from the F 1s line at a binding energy around 689.0 eV, which is more sensitive in XPS spectrum. The sensitivity factor of the F 1s line is 5 times larger than that of the C 1s line. The recorded C 1s line at 283.6 eV for B6/0.4 (Figure 2a) is due to carbon atoms bonded to the siloxane chain of PDMS matrix. The small components at 284.6 and 285.8 eV are due to residual C-C and C-O groups of the cross-linking reagent. In the case of B1/0.4 (Figure 2b), the C 1s line shows different components characteristic of all specific molecular groups of the blend: (i) CF3 at 293.6 eV, (ii) CF2 at 291.1 eV, (iii) COO at 288.5 eV, (iv) C-CF2 at 286.4 eV, (v) C-O at 285.6 eV, (vi) C-C at 284.6 eV, and (vii) C-Si at 283.6 eV. (27) Johnston, E.; Bullock, S.; Uilk, J.; Gatenholm, P.; Wynne, K. J. Macromolecules 1999, 32, 8173. (28) She, H.; Chaudhury, M. K.; Owen, M. J. Polym. Prepr. 1998, 39, 548.

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theoretical 75° 0°

B1/5.0

theoretical 75° 0°

B6/0.4

theoretical 75° 0°

D5/3

theoretical 75° 0°

PDMS

theoretical 75° 0°

0.04 34.5 30.0 0.06 38.7 28.7 0.72 45.5 45.1 0.14 3.5 2.1 13.71 44.7 39.3

24.98 11.0 14.0 24.97 10.1 12.4 24.62 3.9 7.1 24.93 23.4 20.7 17.74 3.9 8.3 25.00 23.9 22.1

24.98 10.2 14.8 24.98 10.7 16.8 24.72 5.4 9.0 24.95 20.4 25.8 19.62 5.8 11.5 25.00 22.0 24.1

50.00 44.3 41.2 49.99 40.5 42.1 49.94 45.2 38.8 49.98 52.7 51.4 48.92 45.6 40.9 50.00 54.1 53.8

Figure 2. XPS C 1s line recorded at 0° for B6/0.4 (a) and B1/0.4 (b) blends.

Assignments of these components agree well with those reported for polymers with similar molecular groups.29 Detection of these seven carbon components supports the conclusion that D5/3 copolymer is especially able to migrate and self-assemble at the topmost layers of the PDMS surface.30 Effect of Copolymer Concentration on Surface Structure Formation. After proving the superior capability of the D5/3 copolymer to produce a film surface exceptionally enriched in the (29) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; The Scienta ESCA 300 Database; John Wiley & Sons, 1992. (30) Tsibouklis, J.; Graham, P.; Eaton, P. J.; Smith, J. R.; Nevell, T. G.; Smart, J. D.; Ewen, R. J. Macromolecules 2000, 33, 8460.

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fluorinated component, we studied how the amount of the copolymer in the PDMS blend could affect its tendency to surface segregate. Therefore, the surface composition of two different blends, B1/0.4 and B1/5.0, and the pristine copolymer D5/3 was investigated. A summary of the obtained results of the atomic surface concentration determined at two depths of analysis is presented in Table 1. For comparison, the theoretical values of atomic concentrations for hypothetical homogeneous sample are also presented in this table. The most striking finding is a tremendous increase in fluorine surface concentration for all samples. Moreover, the relative increase in the surface fluorine concentration is much higher when the added amount of copolymer is lower. For a hypothetical homogeneous sample of a PDMS blend with as low as 0.3 wt % copolymer content, the theoretical atomic concentration of fluorine (0.04%) is below the detection limit (Table 1). In reality, the experimental results show around a 750 times increase in surface fluorine concentration at 0° and 860 times at 75° compared with the theoretical value. This increase in fluorine concentration by almost 3 orders of magnitude made it possible to monitor the fluorinated chains in spite of the very low content of the copolymer in the blend. Obviously, this is further confirmation of the exceptional ability of D5/3 to diffuse to the outer interface of the PDMS blend. One can speculate about the possibility of self-repairing31 of the system to rebuild the nanoscale structure of the film surface. In the event that the outermost layers were accidentally depleted of fluorine content, the surface-active perfluorinated chains would rearrange from the underneath layers toward the outer layers in order to restore the initial low-energy properties of the material. While the intensity of the F 1s signal increases remarkably, the intensity of Si 2p and O 1s lines decreases significantly compared with theoretical values, indicating that the polysiloxane chains are located in the bulk. This conclusion is supported by the observation of a lower intensity of the silicon and oxygen lines when spectra are recorded at 75° and compared with those recorded at 0°. In the case of the carbon line, the apparent changes are more complex, with the carbon intensity being lower than that theoretically expected but only slightly lower when compared with those observed for oxygen and silicon. Moreover, there is more carbon detected in the spectra recorded at 75° than at 0°. All these findings suggest that the outermost layer is enriched in the hydrophobic ends of the polysiloxane chain. Hence, the outermost layer appears to be composed of two distinct species: (i) hydrophobic and lipophobic fluorinated side chains and (ii) hydrophobic hydrocarbon terminals of siloxane chains. The siloxane chains themselves can form an anchoring loop in the bulk owing to their inherent flexibility and high local mobility. A simplified scheme of a surface arrangement produced in the selfassembly of the siloxane/fluorinated copolymer in the PDMS blend is shown in Figure 3. The fluorinated chain segments are depicted in a liquid crystal-like structure consistent with their suitability to spontaneous organization in an ordered array in the bulk and at the surface.19-22 When the amount of copolymer added to PDMS was significantly higher (5.0 wt %), the surface concentration in fluorinated chain is 70 times higher, whereas concentrations of silicon and oxygen (siloxane chain) decrease 2.5-3 times compared with the theoretical values expected for a perfectly homogeneous sample (Table 1). Even at this high content of copolymer in the blend, the surface segregation is exceptional. The proposed molecular (31) Yuan, Y. C.; Yin, T.; Rong, M. Z.; Zhang, M. Q. eXPRESS Polym. Lett. 2008, 2, 238.

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Figure 3. Schematic representation of surface-segregated fluorinated/siloxane copolymer in PDMS matrix.

Figure 4. XPS lines of F 1s, O 1s, C 1s, and Si 2p of B1/0.3 recorded at 0° (black line, film as prepared; red line, film annealed; blue line, film exposed to water).

organization and mobility of the surface nanoscale layer seems to also apply for blends incorporating higher concentrations of the copolymer. The results obtained for the virgin copolymer D5/3 show that also in this case surface segregation is significant (Table 1). The fluorinated fragment is moving to the outermost layer (increase in fluorine concentration by 3 times) together with the aliphatic moiety of the siloxane chain terminal. At the same time, the siloxane chain remains hidden in the bulk, which is supported by an observation of lower surface concentrations of silicon and oxygen by 2-4 times. It was surprising to find out that the surface composition of the outermost very thin layer (experiments at 75°) of the pure copolymer is almost identical for all elements with that observed for B1/5.0. Hence, the addition of 5.0 wt % copolymer is already sufficient to produce a nanostructured surface (around 3 nm thick) identical to that observed for pure copolymer. This indicates that saturation is reached in the surface-segregation process. Surface Rearrangement after Annealing and Exposure to Water. Isothermal annealing of the films was carried out for 24 h Langmuir 2010, 26(4), 2871–2876

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Table 2. XPS Atomic Surface Concentrations of B1/0.3 and B1/5.0 Blends as Prepared, after Annealing, and after Water Exposure (Spectra Recorded at θ = 0°) B1/0.3

B1/5.0

sample

F 1s (%)

Si 2p (%)

O 1s (%)

C 1s (%)

F 1s (%)

Si 2p (%)

O 1s (%)

C 1s (%)

theoretical as-prepared annealed water-exposed

0.04 30.0 36.9 38.2

24.98 14.0 10.2 9.0

24.98 14.8 11.3 10.9

50.00 41.2 41.6 41.9

0.72 45.1 40.5 36.3

24.62 7.1 8.0 10.4

24.72 9.0 10.1 12.2

49.94 38.8 41.4 41.1

similar, suggesting migration of the siloxane chains to the outer interface, which is more pronounced after immersion in water than after annealing. Surprisingly, exposure to water of the two different samples (B1/ 0.3 and B1/5.0) results in very similar composition and structure of the outermost layers (Table 2). Hence, the developed fluorinated copolymer-PDMS blends show very specific characteristics. They are able to produce almost the same surface nanoscale structure over a wide range of copolymer loadings, from 0.3 to 5.0 wt %, after exposure to water. This finding is in agreement with the saturation effect after addition of 5.0 wt % D5/3 copolymer discussed above. It should be emphasized that all described surface rearrangements both after annealing and after exposure to deionized water are relatively small compared to that occurring during nanoscale structure formation by the self-assembling process. Exposure of polymer films to seawater is also able to induce significant changes to the nature of the film surface, including that of PDMS-based films.32 Therefore, long-term stability tests to seawater should be carried on the polymer films, when considering application in marine environment, e.g., for anti(bio)fouling application.10,33 In this context, there is increasing concern about bioaccumulation in the environment of biodegradation products of perfluorinated alcohols such as perfluorinated acids, e.g., perfluorooctanoic acid (PFOA).34 Therefore, after the water contact angles were measured the films were kept immersed in water at room temperature for up to 14 days. There was no evidence of the presence of fluorinated chemicals in the extraction waters, which suggests that hydrolysis or leaching did not occur to a sizable extent under the experimental conditions adopted. Figure 5. XPS lines of F 1s, O 1s, C 1s, and Si 2p of B1/5.0 recorded at 0° (black line, film as prepared; red line, film annealed; blue line, film exposed to water).

at 70 °C, while exposure to deionized water was for 48 h at ambient temperature. The samples with low (0.3 wt %) and high (5.0 wt %) loadings of copolymer were investigated, and the results obtained are presented in Figures 4 and 5 and Table 2. At low addition of copolymer (B1/0.3), annealing and exposure to water result in an additional increase in fluorine surface concentration (around 940 times of the theoretical value). At the same time, there is a decrease in silicon and oxygen surface concentrations. These observations suggest that the formation of a outermost layer, shown in Figure 3, was additionally enhanced after annealing or exposure to water. For both treatments, the recorded spectra (Figure 4) are similar, showing a limited variation in the surface concentration of individual molecular groups. At high copolymer concentration (B1/5.0), a reverse trend in changing the atomic surface concentrations was observed after the sample was annealed or immersed in deionized water: (i) the surface fluorine concentration is lower after annealing, and lowest after exposure to water, whereas (ii) silicon and oxygen surface concentrations increase (Table 2). Thus, the produced surfacesegregated film lost some degree of organization. The observed increases in silicon and oxygen surface concentrations are very Langmuir 2010, 26(4), 2871–2876

Conclusions Chemical design of the fluorinated/siloxane surface-active copolymer to produce nanoscale structured PDMS blend films was focused on the length and molar content of fluorinated side chain for one given length of the siloxane side chain. In fact, copolymer D5/3 with a longer pendant perfluorocarbon chain (8 CF2 groups) exhibited a more pronounced tendency toward surface segregation than O5/19 having a shorter perfluorocarbon chain (6 CF2 groups). The strong segregation process was responsible for the exceptional enrichment in fluorine concentration of the outermost surface layers with respect to the bulk composition. The F experimental/F theoretical ratio decreased with an increase in copolymer loading in the blend, being surprisingly high (∼800) for B1/0.3. Even at very low loading, the surface-active copolymer is able to saturate the surface of the PDMS blend films. Another specific characteristic of the blend films is their sensitivity to the environment. It was found that the produced surface-segregated nanostructures are practically chemically equal after contact with (32) Arce, F. T.; Avci, R.; Beech, I. B.; Cooksey, K. E.; Wigglesworth-Cooksey, B. Langmuir 2006, 22, 7217. (33) Guidipati, C. S.; Greenlief, C. M.; Johnson, J. A.; Pornpimol, P.; Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 6193. (34) Dinglasan, M. J. A.; Ye, Y.; Edwards, E. A.; Mabury, S. A. Environ. Sci. Technol. 2004, 38, 2857.

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water in the two films with surface-active copolymer contents differing over a wide range. Such surface-segregated films, which couple low elastic modulus and low surface tension properties, can be exploited as coatings for fouling-release application. The biological performance of the polymer films against marine organisms, like the macroalga Ulva linza and the barnacle Balanus amphitrite, has been reported in a previous paper.19

2876 DOI: 10.1021/la902912h

Mielczarski et al.

Acknowledgment. The work was funded by the EC Framework 6 Integrated Project ‘AMBIO’ (Advanced Nanostructured Surfaces for the Control of Biofouling) and the Italian MiUR (fondi PRIN). This article reflects only the authors’ views, and the European Commission is not liable for any use that may be made of information contained therein. The authors thank Dr. E. Ammannati (Argus Chemicals) for providing a sample of copolymer D5/3.

Langmuir 2010, 26(4), 2871–2876