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Langmuir 2009, 25, 2248-2257
Surface Segregation of Fluorinated Moieties on Random Copolymer Films Controlled by Random-Coil Conformation of Polymer Chains in Solution Dongwu Xue,† Xinping Wang,*,† Huagang Ni,† Wei Zhang,† and Gi Xue‡ Department of Chemistry, Key Laboratory of AdVanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech UniVersity, Hangzhou 310018, People’s Republic of China, Department of Polymer Science, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed October 14, 2008. ReVised Manuscript ReceiVed NoVember 20, 2008 The relationship between solution properties, film-forming methods, and the solid surface structures of random copolymers composed of butyl methacrylate and dodecafluorheptyl methylacrylate (DFHMA) was investigated by contact angle measurements, X-ray photoelectron spectroscopy, sum frequency generation vibrational spectroscopy, and surface tension measurements. The results, based on thermodynamic considerations, demonstrated that the random copolymer chain conformation at the solution/air interface greatly affected the surface structure of the resulting film, thereby determining the surface segregation of fluorinated moieties on films obtained by various film-forming techniques. When the fluorinated monomer content of the copolymer solution was low, entropic forces dominated the interfacial structure, with the perfluoroalkyl groups unable to migrate to the solution/air interface and thus becoming buried in a random-coil chain conformation. When employing this copolymer solution for film preparation by spin-coating, the copolymer chains in solution were likely extended due to centrifugal forces, thereby weakening the entropy effect of the polymer chains. Consequently, this resulted in the segregation of the fluorinated moieties on the film surface. For the films prepared by casting, the perfluoroalkyl groups were, similar to those in solution, incapable of segregating at the film surface and were thus buried in the random-coil chains. When the copolymers contained a high content of DFHMA, the migration of perfluoroalkyl groups at the solution/air interface was controlled by enthalpic forces, and the perfluoroalkyl groups segregated at the surface of the film regardless of the film-forming technique. The aim of the present work was to obtain an enhanced understanding of the formation mechanism of the chemical structure on the surface of the polymer film, while demonstrating that film-forming methods may be used in practice to promote the segregation of fluorinated moieties on film surfaces.
1. Introduction Since nonlithographic techniques that lead to controllable and potentially functional structures occupy a fundamental niche in modern materials science, the study of the formation mechanism of polymer surface patterns has attracted considerable interest.1-5 However, the formation of chemical structures on polymer surfaces during processing remains largely unstudied and poorly understood despite the immense importance of the surface chemical structure in practical applications, especially when compared to the vast theoretical and empirical literature available on bulk structures. There is thus a fundamental interest regarding the nature of the formation mechanism of the chemical structure on the surface of a polymer film. The surfaces of fluorinated polymers have attracted much attention due to their exceptional properties, which originate from the C-F bond. The low wettability, low adhesion, and low coefficient of friction associated with fluorinated surfaces play essential roles in microelectronic, antifouling, and medical applications. These properties depend not only on the coverage of the surface by fluorocarbons, but also on the degree of order.6-11 Mach et al.12 observed a dramatic change in the surface energy * To whom correspondence should be addressed. Tel/Fax: +86-5718684-3600. E-mail address:
[email protected]. † Zhejiang Sci-Tech University. ‡ Nanjing University.
(1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (2) Cheyne, R. B.; Moffitt, M. G. Langmuir 2006, 22, 8387. (3) Zhu, J.; Eisenberge, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (4) Cheyne, R. B.; Moffitt, M. G. Langmuir 2005, 21, 10297. (5) Chung, B.; Choi, M.; Ree, M.; Jung, J. C.; Zin, W. C.; Chang, T. Macromolecules 2006, 39, 684.
by replacing a single atom within a submolecular length scale at the surface. At the same time, a close-packed, uniformly organized array of perfluorolauric groups created a surface with the lowest critical surface energy of 6 mN/m.7 Understanding the correlation between the physical/chemical surface properties and the surface chemical composition is of paramount importance, in particular when control of the surface properties is desired. In this respect, numerous papers dealing with the correlation of surface energy with surface structure, composition, and functionality of polymer systems have been published.13-16 The study of surface structure formation during the solidification of polymer solutions is extremely challenging. In general, the surface structure formation is governed by thermodynamics, but the kinetics for the segregation and array processes, determined by fixed process methods and conditions, also play an important (6) Wang, J.; Mao, G.; Ober, C. K.; Kramer, E. J. Macromolecules 1997, 30, 1906. (7) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (8) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y Langmuir 1999, 15, 4321. (9) Ulman, A. An Introduction to Ultrathin Organic Films from LangmuirBlodget to Self Assembly; Academic Press: New York, 1991. (10) Wang, X. F.; Ni, H. G.; Xue, D. W.; Wang, X. P.; Feng, R. R.; Wang, H. F. J. Colloid Interface Sci. 2008, 321, 373. (11) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221. (12) Mach, P.; Huang, C. C.; Nguyen, H. T. Phys. ReV. Lett. 1998, 80, 732. (13) Van de Grampel, R. D.; Ming, W.; Gildenpfennig, A.; van Gennip, W. J. H.; Laven, J.; Niemantsverdriet, J. W.; Brongersma, H. H.; de With, G.; van der Linde, R. Langmuir 2004, 20, 6344. (14) Chaudhury, M. K.; Whiteside, G. M. Science 1992, 255, 1230. (15) Wang, J.; Ober, C. K. Macromolecules 1997, 30, 7560. (16) Luning, J.; Stohr, J.; Song, K. Y.; Hawker, C. J.; Iodice, P.; Nguyen, C. V.; Yoon, D. Y. Macromolecules 2001, 34, 1128.
10.1021/la803409c CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
Segregation of Fluorinated Moieties
role in the ultimate surface structure formation. The influence of the preparation method on the surface properties of block polymers has been reported in several publications,10,17-20 and it was found that various structures on the film surface could be obtained by controlling film-forming conditions, such as the nature of the solvent and the temperature. For example, superhydrophobic surfaces with a micronanostructure can be obtained by controlling the solvent type and temperature.20 The nature of the solvent also affects the component or group segregation on the surface as well as the ordering of the group packing on the topmost surface layer,10,21 which has a great influence on the surface performance.13 Within the scope of a previous study performed by our group,10 it was discovered that a relatively perfect closepacked and well-ordered structure of the perfluoroalkyl side chains at the surface of end-capped poly (methyl methacrylate) film was formed when the film was cast from a benzotrifluoride solution, as opposed to being cast from cyclohexanone and toluene solutions. To the best of our knowledge, a majority of the investigations have determined polymer surface properties of model systems prepared from spin-coated films. In part, spin-coated films very often lead to dewetting processes, whereas, for industrial coating applications, it is more appropriate to study thick coating layers. However, only a few studies have dealt with the effect of these two film-forming methods on the resulting film surface structure and properties. Studies of fluorinated block copolymers and endfluorinated oligoesters have shown that the contact angle and the segregation degree of the fluorinated component in films prepared by spin-coating were far lower than those of films prepared by casting or melting. This was due to higher concentrations of methacrylate components being localized at the surface due to micelle exposure.17-19 The present work reports on the influence of film-forming methods on the surface structure and properties of fluorinated random copolymer films. In contrast to fluorinated block copolymers, the contact angle and segregation degree of the fluorinated component in fluorinated random copolymers are far greater in spin-coated films as opposed to those of their cast counterparts. The surface properties and structures of this polymer, both in the solid state and in solution, were explored with contact angle measurements, X-ray photoelectron spectroscopy, sum frequency spectroscopy (SFG), and surface tension measurements. The aim of this work was a systematic investigation of the influence of the preparation conditions and polymer solution properties on the processes of fluorine surface segregation as well as on the surface properties of the fluorinated random copolymers. This study may help to enhance the understanding of the formation of surface structures during the solidification of polymer solutions.
2. Experimental Section 2.1. Materials. Butyl methacrylate (BMA) and dodecafluorheptyl methylacrylate (DFHMA) (shown in Figure 1, XEOGIA FluorineSilicon Chemical Company, China) were washed with a solution of 5 wt% NaOH and deionized water, dried with CaH2, and then vacuumdistilled prior to polymerization. Benzoyl peroxide (BPO) was recrystallized from methanol. Other reagent-grade chemicals were (17) Nishino, T.; Urushihara, Y.; Meguro, M.; Nakamae, K. J. Colloid Interface Sci. 2005, 283, 533. (18) Urushihara, Y.; Nishino, T. Langmuir 2005, 21, 2614. (19) Synytska, A.; Appelhans, D.; Wang, Z. G.; Simon, F.; Lehmann, F.; Stamm, M.; Grundke, K. Macromolecules 2007, 40, 297. (20) Xie, Q. D.; Fan, G. Q.; Zhao, N.; Guo, X. L.; Xu, J.; Dong, J. Y.; Zhang, L. Y.; Zhang, Y. J.; Han, C. C. AdV. Mater. 2004, 16, 1830. (21) Huang, H. Y.; Hu, Z. J.; Chen, Y. Z.; Zhang, F. J.; Gong, Y. M.; He, T. B. Macromolecules 2004, 37, 6523.
Langmuir, Vol. 25, No. 4, 2009 2249
Figure 1. A schematic representation of the DFHMA molecule. Table 1. Various Characteristics of the Polymers Used in This Study samples
DFHMA (mol %)a
PBMA no. 1 no. 2 no. 3 no. 4 no. 5 no. 6
0 2.0 9.0 18.8 28.4 38.3 46.6
a
Calculated from 1H NMR spectra.
Mn ( × 104) 6.34 5.78 5.97 5.62 4.16 6.92 4.29 b
b
Mw/Mnb 1.35 1.45 1.65 1.55 1.87 1.69 1.86
As determined by GPC.
purchased from Shanghai Reagent Co. and used without further purification. 2.2. Polymerization. The random copolymers were prepared by free radical copolymerization at 70 °C for 24 h in cyclohexanone using benzoyl peroxide (BPO) as the initiator. The products were purified by two reprecipitation cycles using THF and methanol. Their characteristics are listed in Table 1. 2.3. Film Formation. The glass plates were purchased from Fisher Scientific Co. (USA). The glass plates were washed three times with dichloromethane in an ultrasonic bath for 5 min and then in a sulfochromic solution for 24 h. The substrates were rinsed 5-6 times in deionized water and then dried in nitrogen flux. The polymer was dissolved in cyclohexanone in order to obtain a 4 wt% solution, which was then filtered through a porous PTFE filter (with pores of 0.25 µm in diameter). Two film-forming techniques were employed: the films were prepared either by casting of the solutions, or by spin-coating at 1000 rpm for 30 s on cleaned glass plates. Both types of films were first dried in air for 24 h and then in vacuum at room temperature for another 12 h. The films obtained by casting were denoted “cast”, and their spin-coated counterparts were denoted “spin”. 2.4. Characterization. The molecular weights and molecular weight distributions (MWDs) of the polymers were determined by gel permeation chromatography (GPC) using a Waters 1500 apparatus (with THF as the eluent at a flow rate of 0.5 mL/min). The GPC chromatograms were calibrated against polystyrene standards. 1H NMR spectra were recorded on a Bruker Advance AMX-400 NMR spectrometer in CDC13 with tetramethylsilane (TMS) as the internal standard. FTIR spectra of the copolymers were obtained on a Nicolet Avatar 370 Fourier Transform Infrared (FTIR) spectrometer. X-ray photoelectron spectroscopy (XPS) was performed using a PHI5000C ESCA System with an Mg KR X-ray source (1253.6 eV). The X-ray gun was operated at a power of 250 W and the high voltage was maintained at 140 KV with a detection angle of 54°. The chamber pressure during analysis was approximately 1 × 10-8 Torr. All spectra were calibrated by the C1s peak of the C-C bond at 285 eV. Contact angles (θ) of water and paraffin oil were measured by the Sessile drop method at room temperature and ambient humidity with a Kru¨ss (Hamburg, Germany) DSA-10 contact angle goniometer. The reported θ values were the averages of at least eight measurements taken within 10-20 s of applying each drop of liquid. Contact angles of water and diiodomethane were used to make estimates of the surface free energy for various samples according to the theory of Owens and Wendt.22 The experimental errors when measuring the θ values were evaluated to be less than ( 2°, thus indicating that the results were sufficiently accurate. (22) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.
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Figure 2. FTIR spectra of (a) PBMA, (b) poly (BMA-co-DFHMA) with 18.8 mol% DFHMA, and (c) poly (BMA-co-DFHMA) with 38.3 mol% DFHMA.
The surface tension was measured at 25 °C on a DCA-322 surface tensiometer (Cahn Instruments, USA) using a technique based on the Wilhelmy balance principle. Five parallels for each concentration were measured and the average and standard deviations were calculated from these parallels. 2.5. SumFrequencyGeneration(SFG)VibrationalSpectroscopy. Sum frequency generation (SFG) vibrational spectra were obtained using a custom-designed EKSPLA SFG spectrometer that has been described in a previous paper.10 Briefly, the visible input beam at 0.532 µm was generated by frequency doubling of part of the fundamental output from an EKSPLA Nd:YAG laser. The IR beam, tunable between 1000 and 4300 cm-1 (with a line width |4H|, and accordingly 4G >0. As a result, the perfluoroalkyl groups became buried in the close-packed random-coil chain as shown in Figure 8 (right). Since the concentration of fluorinated moieties in the copolymer chain was high, the migration of perfluoroalkyl groups at the solution/air interface resulted in |T4S| < |4H|. Therefore, it is reasonable to assume that the perfluoroalkyl groups would migrate to the solution/air interface with close-packed random-coil chains, as shown in Figure 8 (right). The concentration-induced chain conformation packing for the copolymer with low amounts of DFHMA was also confirmed by the surface structure on the cast film using various concentrations of copolymer with 9.0 mol% DFHMA, as shown in Table 3 and Figure 10. The F/C ratio and the oil contact angles on the films increased with decreasing copolymer concentrations. The question arises as to why the surface properties and composition of random copolymer films with high DFHMA contents were nearly independent of the film-forming method, while those with low DFHMA contents were greatly affected by the choice of film-forming technique. A plausible answer to this question comes from consideration of the random-coil chain conformation shown in Scheme 1. For the copolymers with low DFHMA contents, the entropy played a central role in the formation of the surface structure. During solidification, migration of fluorinated groups to the surface caused 4H < 0, and similarly, also gave rise to4S < 0 due to the increasing order of the polymer chains. However, 4H was small, since the DFHMA content was low in the copolymer, and accordingly |T4S| < |4H|. The entropy factor would therefore dominate this system, for which reason the perfluoroalkyl groups would be unable to segregate to the film surface (when prepared by casting) and would thus be buried in the random-coil chains, similar to the case in solution, as shown in Scheme 11a. This phenomenon was also observed by Carey and co-workers when they investigated the abnormal reconstruction of surface-oxidized 1,2-polybutadiene surfaces heated against water.56,57 The results showed that the process was dominated by entropy rather than enthalpic forces, since the migration of the polar groups to the polymer/water interface likely required the polymer chains to extend out of their entropically preferred, random-coil conformations. At elevated temperatures, however, entropy would cause these chains to recoil and rebury the polar groups, thus inducing a more hydrophobic surface when in contact with the hot water. When the films were prepared by spin-coating, the copolymer chains in solution were believed to extend due to the centrifugal force. This chain extension would weaken the entropy effect of the polymer chains. More specifically, the entropic cost of the fluorinated groups (56) Carey, D. H.; Ferguson, G. S. J. Am. Chem. Soc. 1996, 118, 9780. (57) Carey, D. H.; Grunzinger, S. J.; Ferguson, G. S. Macromolecules 2000, 33, 8802.
migrating to the polymer/air interface was reduced during the film-formation; giving rise to |T4S|
