Langmuir 2008, 24, 7939-7946
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Probing Molecular-Level Surface Structures of Polyethersulfone/ Pluronic F127 Blends Using Sum-Frequency Generation Vibrational Spectroscopy Qing Shi,†,‡ Shuji Ye,‡ Cornelius Kristalyn,‡ Yanlei Su,† Zhongyi Jiang,*,† and Zhan Chen*,‡ Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China, and Departments of Chemistry, and Macromolecular Science and Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed February 21, 2008. ReVised Manuscript ReceiVed April 26, 2008 We blended Pluronic F127 into polyethersulfone (PES) to improve surface properties of PES, which has been extensively used in biomaterial and other applications. The molecular surface structures of PES/Pluronic F127 blends have been investigated by sum-frequency generation (SFG) vibrational spectroscopy. The molecular orientation of surface functional groups of PES changed significantly when blended with a small amount of Pluornic F127. Pluronic F127 on the blend surface also exhibited different features upon contacting with water. The entanglement of PES chains with Pluronic F127 molecules rendered the blends with long-term surface stability in water in contrast to the situation where a layer of Pluronic F127 adsorbed on the PES surface. Atomic force microscopy (AFM) and quartz crystal microbalance (QCM) measurements were included to determine the relative amount of protein that adsorbed to the blend surfaces. The results showed a decreased protein adsorption amount with increasing Pluronic F127 bulk concentration. The correlations between polymer surface properties and detailed molecular structures obtained by SFG would provide insight into the designing and developing of biomedical polymers and functional membranes with improved fouling-resistant properties.
1. Introduction Interfacial properties of polymers are of considerable fundamental and practical interest in many different research areas, especially in the field of biomedical materials.1,2 Polymers have been extensively used as implants in the human body because of their versatility in the production of various devices which are required to have a wide range of excellent mechanical properties.3,4 These mechanical properties are determined by the optimized bulk structures of these polymers. At the same time, these polymers are required to present outstanding surface properties. Polymeric implants directly contact biological systems; therefore, the polymer-body interactions determine the biocompatibility. Clearly, such interactions are mediated by the surface structures of polymer implants. To achieve excellent biocompatibility, it is necessary to design polymer surfaces with desired structures. Blending different polymers together is an effective method to design polymer materials with both preferred surfaces and bulk structures. It is well-known that, driven by thermodynamic forces, the low surface energy component(s) likes to segregate to the surface to minimize the total free energy of the entire blend system. Therefore, it is feasible to develop an optimized surface by blending a surface-active polymer into a base polymer matrix.5–9 * To whom correspondence should be addressed. E-mail:
[email protected] (Z.C.) or
[email protected] (Z.J.). † Tianjin University. ‡ University of Michigan. (1) Ratner, B. D.; Castner, D. G. Surface Modification of Polymeric Biomaterials; Plenum Press: New York, 1996. (2) Silver, F. H. Biocompatibility: Interaction of Biological and Implantable Materials; VCH: New York, 1989. (3) Freij-Larsson, C.; Nylander, T.; Jannasch, P.; Wessle´n, B. Biomaterials 1996, 17, 2199–2207. (4) Moro, T.; Takatori, Y.; Ishihara, K.; Konno, T.; Takigawa, Y.; Matsushita, T.; Chung, U.; Nakamura, K.; Kawaguchi, H. Nat. Mater. 2004, 3, 829–836.
Polyethersulfone (PES) is a high-temperature engineering thermoplastic with high ductility, good hydrolysis, and chemical resistance. It has been widely used in many biomedical related processes, such as bioseparation, biopurification, and hemodialysis. However, research shows that it exhibits poor biocompatibility due to inherent high hydrophobicity. Therefore, modification of PES to achieve better biocompatibility has been the focus of many studies.10–14 Commercially available poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronics) are wellknown nonionic and water-soluble macromolecular surfactants. They have been successfully used in biomedical applications to reduce nonspecific protein adsorption and cell adhesion.3,15–18 When a small amount of Pluronics is blended into a polymer matrix, it segregates to the surface to improve biocompatibility of such a polymer matrix. Surface structures/properties of polymer matrices of Pluronics blends have been extensively characterized using X-ray photoelectron spectroscopy (XPS), scanning electron (5) Restos, H.; Margiolaki, A.; Messaritaki, A.; Anastasiadis, S. H. Macromolecules 2001, 34, 5295–5305. (6) Walton, D. G.; Mayes, A. M. Phys. ReV. E 1996, 54, 2811–2815. (7) Johnson, W. C.; Wang, J.; Chen, Z. J. Phys. Chem. B 2005, 109, 6280– 6286. (8) Wang, Y.; Wang, T.; Su, Y.; Peng, F.; Wu, H.; Jiang, Z. Langmuir 2005, 21, 11856–11862. (9) Chen, Z. Polym. Int. 2007, 56, 577–587. (10) Taniguchi, M.; Pieracci, J.; Samsonoff, W. A.; Belfort, G. Chem. Mater. 2003, 15, 3805–3812. (11) Susanto, H.; Ulbricht, M. Langmuir 2007, 23, 7818–7830. (12) Pieracci, J.; Crivello, J. V.; Belfort, G. Chem. Mater. 2002, 14, 256–265. (13) Wang, Y.; Su, Y.; Ma, X.; Sun, Q.; Jiang, Z. J. Membr. Sci. 2006, 283, 440–447. (14) Shi, Q.; Su, Y.; Zhu, S.; Li, C.; Zhao, Y.; Jiang, Z. J. Membr. Sci. 2007, 303, 204–212. (15) Green, R. J.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B. J. Biomed. Mater. Res. 1998, 42, 165–171. (16) Yang, Z. H.; Sharma, R. Langmuir 2001, 17, 6254–6261. (17) Amiji, M.; Park, K. Biomaterials 1992, 13, 682–692. (18) Norman, M. E.; Williams, P.; Illum, L. Biomaterials 1992, 13, 841–849.
10.1021/la800570a CCC: $40.75 2008 American Chemical Society Published on Web 07/11/2008
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and quartz crystal microbalance (QCM) have been used in this study to evaluate the fouling-resistance of the blends.
2. Experimental Section
Figure 1. Molecular structures of PES and Pluronic F127.
microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and contact angle goniometry.8,19–21 It is widely accepted that the hydrophobic PPO chains provide the necessary anchor for the polymer molecule to segregate to the surface, while the highly hydrophilic, brushlike PEO chains extend freely into the aqueous medium, resulting in the protein-resistant ability for surface segregated Pluronics. However, molecular details of the surface structures of these polymer blends containing Pluronics have not been fully elucidated. Many traditional surface analysis techniques have limitations such as the lack of surface sensitivity or require high vacuum to operate. Here, we want to examine the surface structures of such polymer blends in aqueous environments or after immediate removal from these aqueous environments. In the past decade or so, a second-order nonlinear vibrational spectroscopic technique, sum-frequency generation (SFG) vibrational spectroscopy, has been developed into a powerful tool to study surface structures of polymer systems at the molecular level in situ.22–25 SFG not only identifies the surface molecular species but also provides information about surface structures, such as orientation and orientation distribution of a functional group. According to the selection rule of this technique, SFG signals can only be generated from media with no inversion symmetry (under the electric dipole approximation). In many cases, SFG spectra can be selectively collected from surfaces and interfaces, as most bulk materials have inversion symmetry that is only broken at the surface or interface. Both experimental results and theoretical calculations indicate that SFG is submonolayer surface sensitive and can be used to detect molecular structures at various surfaces and interfaces. In addition, SFG has many other advantages: nondestructive, high sensitivity, with good spatial, temporal, and spectral resolution.26 In this research, we have studied molecular surface structures of PES/Pluronic F127 blends using SFG. Pluronic F127 was chosen from a series of Pluronics because of its excellent foulingresistant property according to our previous studies.8,13 The molecular structures of PES and Pluronic F127 are presented in Figure 1. SFG results indicated that PES/Pluronic F127 surface structures in air can be varied by altering the bulk concentrations of the two components in the blend. Also, surface restructuring has been observed using SFG on the blend surface after water exposure. In addition to SFG, atomic force microscopy (AFM) (19) Kiss, E´.; Taka´cs, M. G.; Berto´ti, I.; Vargha-Butler, E. I. Polym. AdV. Technol. 2003, 14, 839–846. (20) Kiss, E´.; Berto´ti, I.; Vargha-Butler, E. I. J. Colloid Interface Sci. 2002, 245, 91–98. (21) Lee, J. H.; Ju, Y. M.; Kim, D. M. Biomaterials 2000, 21, 683–691. (22) Zhang, D.; Gracias, D. H.; Ward, R.; Gauckler, M.; Tian, Y.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 1998, 102, 6225–6230. (23) Chen, Z.; Ward, R.; Tian, Y.; Eppler, A. S.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 1999, 103, 2935–2942. (24) Gracias, D. H.; Chen, Z.; Shen, Y. R.; Somorjai, G. A. Acc. Chem. Res. 1999, 32, 930–940. (25) Shen, Y. R. Nature 1989, 337, 519–525. (26) Chen, Z.; Ward, R.; Tian, Y.; Baldelli, S.; Opdahl, A.; Shen, Y. R.; Somorjaii, G. A. J. Am. Chem. Soc. 2000, 122, 10615–10620.
2.1. Sample Preparation. All the samples and chemicals used in this research except PES were purchased from Sigma and used as received. PES 6020P was purchased from BASF Co. (Germany) and was dried at 110 °C for 12 h before use. The fused silica (1 in. diameter, 1/8 in. thickness, ESCO Products Inc.) substrates were cleaned thoroughly by heating them in a potassium dichromate/ sulfuric acid solution for about 30 min and stored in the same solution for 24 h to ensure that the surface was free of contamination. The substrates were then rinsed repeatedly using deionized water and finally dried in a dry N2 stream. The fused silica substrates were used immediately after cleaning to minimize airborne contamination. Polymer films of different blend compositions were prepared by spin-coating from N,N-dimethylformamide (DMF) solutions (2%, w/w) onto fused silica substrates. Syringe filters of 0.45 µm pore size were used to remove possible impurities of the solutions before spin-coating. The samples were spun at 2000 rpm for 30 s using a spin-coater purchased from Specialty Coating System. The polymer blend film thicknesses were about 100 nm as measured by an ellipsometer. Polymer blend samples, PES/PEG (Mn ) 400) and PES/PPG (Mn ) 425), were also prepared in the same manner for use as reference materials. In the literature, poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) are often used interchangeably with PEO and PPO, respectively, even though usually PEO and PPO refer to those with higher molecular weights. All spin-cast samples were dried in air for 24 h before analysis. Bovine fibrinogen solution was prepared by dissolving fibrinogen in phosphate buffer saline (PBS) with a pH value of 7.4. For AFM experiments, the fused silica substrates coated with polymer blends were immersed in a 1.0 µg/mL fibrinogen solution at 4 °C for 8 min. The substrates were then removed from the solution, rinsed gently with PBS and deionized water (to remove excess salt), and dried in a nitrogen stream. For QCM measurements, 10 MHz quartz crystals (Universal Sensors) were rinsed repeatedly using DMF. Polymer films were then made by spin-coating 2% (w/w) polymer blend solutions in DMF on these quartz crystals. The concentration of fibrinogen solution used for QCM measurements was 1.0 mg/mL. 2.2. SFG Vibrational Spectroscopy. In our SFG system, visible and tunable IR laser beams were spatially and temporally overlapped on the polymer blend surfaces with incident angles of 60° and 54° to the surface normal, respectively. The diameters of both beams on the samples were about 500 µm. The pulse energies of the visible and the IR beams were ∼200 and ∼100 µJ, respectively. Details of the SFG technique and experimental SFG setup used in this study can be found elsewhere27–30 and will not be repeated. In this study, the SFG spectra were collected in air with the input beam traveling through the fused silica to the polymer blend-air interface, as reported before.31 SFG signals from the sample surfaces were normalized by the power of the input laser beams. All SFG spectra shown were collected with the ssp (s-polarized SFG signal, s-polarized visible, and p-polarized IR beam) polarization combination. SFG spectra were recorded over the C-H stretching frequency range of 2800-3100 cm-1. We used the following equation to fit the SFG spectra:
|
I(ωsum) ) χNR(2) +
∑ ωIR - ωqq + iΓq | A
2
(1)
q
where Aq, Γq, and ωIR are the strength, damping constant, and frequency of a single resonant vibration, respectively. χNR(2) is the nonresonant part of the signal. (27) Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z. J. Am. Chem. Soc. 2002, 124, 7016–7023. (28) Wang, J.; Chen, C. Y.; Buck, S. M.; Chen, Z. J. Phys. Chem. B 2001, 105, 12118–12125. (29) Clarke, M. L.; Wang, J.; Chen, Z. J. Phys. Chem. B 2005, 109, 22027– 22035.
Surface Structures of PES/Pluronic F127 Blends 2.3. Atomic Force Microscopy (AFM). The AFM images were collected in air under ambient conditions, using a molecular imaging picoscan. The images were taken with magnetic AC (MAC) mode with magnetically coated silicon nitride cantilevers. The tips had an average spring constant of 2.8 N/m. 2.4. Quartz Crystal Microbalance (QCM). QCM was used as a supplemental technique to measure the relative amount of fibrinogen adsorbed onto PES/Pluronic F127 blend surfaces. The QCM equipment used in this study was purchased from Maxtek Inc. (Cypress, CA). The relative fibrinogen adsorption amounts were compared by assuming a linear relationship between the crystal frequency change and the adsorbed mass change.32 The quartz crystals coated with PES/Pluronic F127 blends were first contacted with PBS for 30 min. The buffer solution was then replaced by fibrinogen solution while ensuring the blends remained wet in the process. The fibrinogen solution was kept in contact with the blends for 30 min, and then protein-free PBS was injected to remove the loosely adsorbed protein.
3. Results and Discussion 3.1. Surface Structures of PES/Pluronic F127 Blends in Air. As a surface-specific vibrational spectroscopy, SFG is a powerful tool to identify surface functional groups, since different chemical groups have characteristic vibrational modes. In addition, SFG can measure orientation and even orientation distributions of these surface functional groups. SFG has been extensively used to study polyether materials,33–37 including PEG, PPG, and Pluronics. Peak assignments for vibrational spectra of such polyethers in the C-H stretching region have been studied and were used to assign peaks in the SFG spectra. From previous SFG studies,34,37 we found that PEG has a peak at ∼2860 cm-1 upon contacting hydrophobic media. This resonance has been treated as the characteristic peak of the symmetric stretch of C-H bonds in the hydrophilic ethylene glycol group (-OCH2CH2-).38,39 For PPG, SFG spectra indicated methyl groups oriented more or less along the surface normal and give a strong SFG ssp signal. For Pluronics, we found that methyl groups in PPO blocks liked to segregate on both Pluronics solution-air and solid-Pluronics solution interfaces. By changing the solution concentration or solid contacting media, methyl side chains showed different orientations.34,37 In this research, PES/PEG and PES/PPG blend samples are used as references for later PES/Pluronic F127 studies. SFG spectra collected from PES, PES/PPG blend, and PES/PEG blend surfaces in air are displayed in Figure 2. No SFG C-H stretching signal was detected from the PES surface using the ssp polarization combination (Figure 2a), indicating that PES adopts an orientation which does not generate a ssp SFG signal. This may be because PES molecules lie parallel to the surface, have a random orientation, or both. Otherwise, the SFG ssp signal of aromatic C-H signal which is above 3000 cm-1 should be observed. (30) Wang, J.; Woodcock, S. E.; Buck, S. M.; Chen, C. Y.; Chen, Z. J. Am. Chem. Soc. 2001, 123, 9470–9471. (31) Chen, C. Y.; Wang, J.; Even, M. A.; Chen, Z. Macromolecules 2002, 35, 8093–8097. (32) Sauerbrey, G. Z. Physica 1959, 155, 206–222. (33) Zolk, M.; Eisert, F.; Pipper, J.; Herrwerth, S.; Eck, W.; Buck, M.; Grunze, M. Langmuir 2000, 16, 5849–5852. (34) Chen, C. Y.; Even, M. A.; Wang, J.; Chen, Z. Macromolecules 2002, 35, 9130–9135. (35) Kim, J.; Koffas, T. S.; Lawrence, C. C.; Somorjai, G. A. Langmuir 2004, 20, 4640–4646. (36) Even, M. A.; Chen, C. Y.; Wang, J.; Chen, Z. Macromolecules 2006, 39, 9396–9401. (37) Chen, C. Y.; Even, M. A.; Chen, Z. Macromolecules 2003, 36, 4478– 4484. (38) Chen, Q.; Zhang, D.; Somorjai, G.; Bertozzi, C. R. J. Am. Chem. Soc. 1999, 121, 446–447. (39) Dreesen, L.; Humbert, C.; Hollander, P.; Mani, A. A.; Ataka, K.; Thiry, P. A.; Peremans, A. Chem. Phys. Lett. 2001, 333, 327–331.
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Figure 2. SFG spectra (ssp) at the polymer-air interface for (a) PES, (b) PES/PEG, and (c) PES/PPG. The concentrations of PEG and PPG in the polymer blends were 10 wt %.
The ssp SFG spectrum collected from the PES/PEG blend surface also shows no C-H stretching signal (Figure 2b). This is different from those collected from PEG alone, as reported previously by our group as well as other research laboratories,34,35,40,41 but similar to that from the PES surface shown above. We believe that, on the PES/PEG blend surface in air, PES covers the surface. When PPG was blended with PES, PES could not cover the surface, because the SFG ssp spectrum collected from the PES/PPG blend surface shows several distinct peaks: The strongest peak is located at 2945 cm-1, which is assigned to the CH3 Fermi resonance peak [CH3(FR)]. A weaker and broader peak at around 2870 cm-1 is also detected, which is primarily due to the methyl symmetric stretching [CH3(s)] and may include a small contribution from the symmetric stretch of methylene adjacent to oxygen atoms [-OCH2-(s)]. A shoulder observed at 2970 cm-1 in the spectrum is ascribed to the methyl asymmetric stretching [CH3(a)]. This spectrum is not very different from those collected from the PPG surface,34 showing that the PES/PPG surface is dominated by the PPG component. Particularly, the surface is mainly covered by methyl groups, which orient preferentially along the interface normal. The different surface behaviors of PEG and PPG when blended with PES are due to their different surface activities. Previous SFG studies reported that functional groups or components with lower surface energies generally prefer to segregate to the surface to minimize the free energy of the entire system.24 At room temperature, the surface tension of PES, PEG, and PPG in air were measured to be 0.041, 0.044, and 0.031 N/m, respectively.42,43 PPG has the lowest surface energy among these three polymers, whereas PEG has the highest surface energy. Therefore, for the PES/PEG blend in air, PES likes to segregate to the surface while PEG is buried in the bulk. By contrast, for the PES/PPG blend in air, the surface is dominated by PPG, which has a lower surface energy than PES. As the SFG data show, the methyl groups on the PPG backbone dominate the PES/PPG blend surface with a relatively ordered structure. As a general phenomenon observed on many polymer surfaces by SFG, the side chains (i.e., methyl side chains and pendant phenyl groups) tend to extend outward from the surface into the air to form a “hydrophobic contact” with air to lower the free energy.31,44,45 (40) Kim, J.; Opdahl, A.; Chou, K. C.; Somorjai, G. A. Langmuir 2003, 19, 9551–9553. (41) Casford, M. T. L.; Davies, P. B. Langmuir 2003, 19, 7386–7391. (42) James, S. G.; Donald, A. M.; Miles, I. S.; Mallagh, L.; Macdonald, W. A. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, 221–227.
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Shi et al. Table 1. Fitting Results of SFG Spectra in Figure 5 using eq 1 |Assp/Γ| wavenumber 2860 2875 2920 2945 2970
Figure 3. SFG spectra (ssp) of the PES/Pluronic F127 blends as a function of bulk Pluronic F127 concentration.
Figure 4. (a) Model molecule used in the calculation of PES vibrational frequencies by semiempirical quantum chemistry (MP3) methods and (b,c) schematics of aromatic C-H stretching modes at frequencies of 3069.11 and 3069.62 cm-1, respectively.
Figure 3 shows SFG spectra collected from surfaces of PES/ Pluronic F127 blends with different Pluronic F127 bulk concentrations. The SFG spectrum collected from the PES/ Pluronic F127 blend with 0.5 wt % Pluronic F127 bulk concentration is dominated by a peak at 3075 cm-1. The peak at approximately this wavenumber has been extensively observed at the polystyrene-air interface and was assigned to the V2 phenyl mode.35,46,47 We believe the peak centered at 3075 cm-1 in this study is also contributed by an aromatic C-H stretching mode from the PES backbone. To confirm this point, we calculated normal mode vibrational frequencies using a model molecule of PES, as shown in Figure 4a. The calculation was performed using the software HyperChem 7.0,48 with the semiempirical quantum chemistry (MP3) methods and Amber force fields. Some calculated vibrational modes are shown in Table 2. As indicated in Table 2, the peak at 3075 cm-1 is most probably associated (43) Sprung, M.; Seydel, T.; Gutt, C.; Weber, R.; DiMasi, E.; Madsen, A.; Tolan, M. Phys. ReV. E 2004, 70, 051809. (44) Opdahl, A.; Somorjai, G. A. Langmuir 2002, 18, 9409–9412. (45) Liu, Y.; Messmer, M. C. J. Phys. Chem. B 2003, 107, 9774–9779. (46) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L. J. J. Phys. Chem. B 2001, 105, 2785–2791. (47) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Phys. ReV. Lett. 2000, 85, 3854–3857. (48) HyperChem 7.0; Hypecube Inc.: Gainesville, FL, 2001.
(cm-1
)
assignment (EO) CH2(s) CH3(s) CH2(as) CH3(FR) CH3(as)
(a)
(b)
(c)
1.09 0.81 1.87 0.80
1.00 1.65 1.41 2.12 0.16
0.51 1.57 1.76 2.04 0.18
with the aromatic C-H stretches at the ortho positions relative to the ether oxygen atom in the PES backbone. The normal mode eigenvectors for aromatic C-H stretching of ν72 and ν73 are shown in Figure 4b and c, respectively. The SFG spectrum of the PES/Pluronic F127 (0.5 wt %) blend is distinctly different from that observed from the pure PES surface, indicating that blending only 0.5 wt % Pluronic F127 greatly changed the surface structure. As discussed, the absence of C-H stretching signals on the pure PES surface is because the aromatic rings lie down or are randomly oriented on the surface. While after blending with a small amount of PES, the aromatic rings substantially change their surface orientation and generate strong SFG signals, a similar strong aromatic C-H stretching signal can also be observed on other PES/Pluronic F127 blend surfaces after PES blending with 1.0, 2.0, and 5.0 wt % Pluronic F127. When the Pluronic F127 bulk concentration reaches 10 wt %, the aromatic C-H stretching signal cannot be detected anymore. We believe that at this concentration Pluronic F127 covers the entire surface and PES stays in the blend bulk; thus, no PES signal can be observed in SFG. More details will be given below when discussing the results on surface Pluronic signals. The PES SFG signals indicate that when a small amount of Pluronic F127 (0.5-5.0 wt %) is blended into PES, the surface PES orientation is changed due to the PES-Pluronic F127 interactions on or near the surface. For a blend with 0.5 wt % Pluronic F127 bulk concentration, in addition to the strong 3075 cm-1 peak, which was discussed above, very weak signals around 2945 and 2875 cm-1 can also be observed. This shows that in addition to the surface coverage of aromatic rings of PES, some Pluronic F127 molecules can be present on the surface, especially those CH3 groups in the PPG component. As discussed earlier, PPG is more surface-active due to the low surface free energy of the side chain methyl groups. However, when a small amount (e.g., 0.5 wt %) of Pluronic F127 is blended, it cannot substantially cover the surface. This should be due to the fact that the other Pluronic component, PEG, tends to stay in the PES matrix in order to eliminate unfavorable interactions with air. The overall driving force for Pluronic F127 molecules to segregate to the surface in air thus must be much weaker than that of PPG molecules alone, resulting in weak aliphatic C-H stretching signals in this SFG spectrum. With the increase of the bulk Pluronic F127 concentration, the aliphatic C-H stretching signals, especially CH3 signals, become stronger, showing that more and more of the surface can be covered by the Pluronic F127. When the surface coverage of Pluronic F127 increases, the surface coverage of PES should decrease, resulting in decreased SFG signals from PES. However, we did not observe the decrease of aromatic C-H stretching signals. Figure 3 shows that, between the 0.5 and 5.0 wt % Pluronic F127 bulk concentration range, SFG signals of PES remain almost the same. Perhaps this indicates the orientation of the PES changes on the surface when different amounts of Pluronics are blended. The SFG signal is affected by both surface coverage and surface orientation. The varied surface coverage of PES contributing to similar SFG signal strength indicates
Surface Structures of PES/Pluronic F127 Blends
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Table 2. Harmonic Vibrational Frequencies of Aromatic C-H Stretching Modes Calculated by Semiempirical Quantum Chemistry (MP3) Methods Using a Model Molecule Shown in Figure 4a mode ν66 ν67 ν68 ν69 ν70 ν71 ν72 ν73 a
assignment and potential energy distribution (%)a 39.9 (C6-H22) + 36.3 (C13-H24) + 9.9 (C5-H21) + 8.4 (C12-H23) 25.6 (C12-H23) + 24.1 (C5-H21) + 23.9 (C13-H24) + 20.6 (C6-H22) 40.1 (C5-H21) + 34.7 (C12-H23) + 10.6 (C6-H22) + 7.6 (C13-H24) 26.5 (C13-H24) + 24.4 (C12-H23) + 23.1 (C6-H22) + 19.2 (C5-H21) 65.8 (C15-H26) + 23.9 (C4-H20) + 2.8 (C14-H25) + 2.5 (C12-H23) 65.7 (C4-H20) + 23.8 (C15-H26) + 2.8 (C3-H19) + 2.5 (C5-H21) 50.2 (C14-H25) + 40.4 (C3-H19) + 1.9 (C15-H26) + 1.6 (C4-H20) 50.2 (C3-H19) + 40.4 (C14-H25) + 2.0 (C4-H20) + 1.6 (C15-H26)
frequency (cm-1) 3021.08 3022.41 3023.86 3025.13 3059.44 3061.86 3069.11 3069.62
The symmetry coordinates contributing less than 1% are omitted. The atom designation and numbering used in this table are indicated in Figure 4a.
surface orientation variation of PES molecules. We tried to perform orientation analysis of PES molecules from SFG spectra collected using different polarization combinations of the input and output laser beams, but unfortunately, no discernible SFG signals were detected by using the ppp and sps polarization combinations. When the Pluronic F127 bulk concentration is increased to 10 wt %, the SFG spectrum is similar to that collected from the PES/PPG blend surface, showing that the surface is completely covered by the PPG blocks of Pluronic F127. The aromatic C-H stretching peak at 3075 cm-1 could not be detected. We believe that it indicates the absence of PES on the surface. SFG results displayed in Figure 3 demonstrate that Pluronic F127, especially its hydrophobic PPG segments, prefers to segregate to the PES/ Pluronic F127 blend surface even when a small amount of Pluronic F127 is blended. This is favorable to minimize the surface free energy of this binary macromolecular system. 3.2. Surface Restructuring of PES/Pluronic F127 Blends after Water Exposure. We demonstrated in our previous study that SFG is a powerful technique to investigate surface restructuring behaviors in water. We have successfully collected SFG spectra from interfaces between a variety of polymer materials and water in situ in real time.31,49 Here, we also tried to collect SFG signals from the PES/Pluronic F127 (10 wt %) blend-water interface. However, such SFG signals are extremely weak, perhaps because the blend surfaces are disordered. Instead of analyzing SFG spectra directly collected from the polymerwater interfaces, we investigated the surface restructuring behaviors of PES/Pluronic F127 blends after water exposure. To study such a water exposure effect, polymer blend films were immersed in water for 1 h. The sample surfaces were then rinsed copiously with deionized water. The SFG spectra of such polymer blend samples were collected immediately (Figure 5b). Comparing with the SFG spectrum collected in air before water exposure, the most striking feature of the observed spectral change after water exposure is the appearance of the pronounced 2860 cm-1 band of PEG. Therefore, the PES/Pluronic F127 blend surface undergoes molecular restructuring upon contact with water, due to the different surface-water and surface-air interactions. Since PEG is more hydrophilic, it prefers to migrate to the surface to contact water. However, surface PEG adopts a random or near random structure and thus cannot be observed at the polymer-water interface in situ. After water exposure, the surface does not revert back to that before contacting water; therefore, PEG surface coverage can be detected. If we leave the sample in air for 24 h at room temperature, as shown in Figure 5c, the methylene symmetric peak around 2860 cm-1 could hardly be detected. The signal around 2920 cm-1, which is characteristic (49) Chen, C. Y.; Clarke, M. L.; Wang, J.; Chen, Z. Phys. Chem. Chem. Phys. 2005, 7, 2357–2363.
for the CH2 asymmetric stretching [CH2(as)] mode, is increased. Such a change may indicate the surface orientation changes of PEG segments in air. In addition to such a qualitative discussion regarding the PEG segments, we will further quantitatively analyze the surface orientation of the methyl group in the PPG segment on the blend surface. To further understand structural information about Pluronic F127 on the surfaces of blends, the average orientation and orientation distribution of methyl groups were analyzed. The orientation distribution of methyl groups can be quantitatively determined by the measured results of |χyyz,as|/|χyyz,s| in the ssp SFG spectra. The details of this calculation have been described in previous publications.50,51 In brief, the relation between |χyyz,as|/ |χyyz,s| and the methyl group orientation angle is
| | |
χyyz,as 2r(〈cos θ〉 - 〈cos3 θ〉) ) χyyz,s 〈cos θ〉(1 + r) - 〈cos3 θ〉(1 - r)
〈cosn θ 〉 ) ∫π cosn θ f(θ) sin θ dθ 0
|
(2) (3)
where θ is the angle between the surface normal and the principal axis of the methyl group. x,y,z denote the laboratory frame coordinates, with the z axis along the surface normal. For methyl group vibrations, the values for r range from 1.6 to 4.2. A Gaussian function, f(θ) ) C exp[-(θ - θ0)2/2σ2], is introduced to describe the angle distribution, where C is the normalization constant, σ is the distribution width for the tilting angle θ0 with respect to the surface normal. The calculated ratio |χyyz,as|/|χyyz,s| as a function of θ0 for certain values of distribution width (σ ) 0°, 10°, 20°, 30°, 40°, and 50°) are shown in Figure 6 (r ) 2.9). Before analyzing the orientation of the methyl groups according to Figure 6, CH3(FR) should be carefully considered because it borrows intensity from CH3(s).52 To account for the lost CH3(s) intensity borrowed through CH3(FR), the strength of the CH3(FR) peak was added to that of CH3(s).53 According to Table 1, we know the measured results of |χyyz,as|/(|χyyz,s| + |χyyz,FR|) for methyl groups at the blend surface are about 0.27, 0.04, and 0.05 for the original, water-treated, and fully dehydrated samples, respectively, shown in Figure 6. We can see that before the PES/Pluronic F127 blend was contacted with water, the orientation angle θ0 is 30° with respect to the surface normal for the case of a δ function distribution, whereas, after water treatment, θ0 is much smaller. It is clear that the methyl groups tend to tilt more toward the surface normal after the water treatment, with a possible narrower distribution. Because the values of |χyyz,as|/(|χyyz,s| + (50) Chen, C. Y.; Wang, J.; Chen, Z. Langmuir 2004, 20, 10186–10193. (51) Gautam, K. S.; Dhinojwala, A. Macromolecules 2001, 34, 1137–1139. (52) Hirose, C.; Akamatsu, N.; Domen, K. J. Chem. Phys. 1992, 96, 997– 1004. (53) Harp, G. P.; Rangwalla, H.; Yeganeh, M. S.; Dhinojwala, A. J. Am. Chem. Soc. 2003, 125, 11283–11290.
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Figure 7. SFG spectra (ssp) for the PES-air interface (a) of original PES, (b) after contact with 0.1 wt % Pluronic F127 solution, and (c) after the washing procedure to remove the adsorbed Pluronic F127. Figure 5. SFG spectra (ssp) of the PES/Pluronic F127 blends with 10 wt % F127 bulk concentration collected (a) in air, (b) in air immediately after water exposure, and (c) in air after the water-treated film was fully dried. Circles represent experimental data points. Lines are the fits to the data obtained using eq 1. Schematic representations of the Pluronic F127 orientation at the polymer blend-air interface are also presented.
Figure 6. Calculated ratio |χyyz,as|/|χyyz,s| of the methyl group at the polymer blends surface as a function of tilting angle θ0 and angle distribution σ. The experimental values of |χyyz,as|/(|χyyz,s| + |χyyz,FR|) from Figure 5 are marked in the figure by the dashed lines.
|χyyz,FR|) are in the low orientation angle region, changing r values from 1.6 to 4.2 does not significantly affect the calculated tilt angles. Previously, we have examined orientations of side chain methyl groups on surfaces of a variety of polymers in air and in water. For some polymers, such as poly(n-butyl methacrylate) (PBMA) and poly(dimethyl siloxane) (PDMS), side chain methyl groups tend to stand up in air and lie down on the surface upon contacting water (because of the strong repulsion between the methyl groups and water molecules).27,30,31 We have quantitatively deduced the orientation distribution of methyl groups on PBMA surfaces in air and in water. We want to emphasize that here the situation is different. We collected SFG spectra after removing the sample from water. Perhaps the orientation of the methyl group has already changed immediately after the removal from water. Nevertheless, the surface orientation of methyl groups after water
exposure is changed. However, no further significant change of CH3 orientation is observed when the sample is completely dried (Figure 6b and c). For the methylene groups, the situation is more complicated because both PPO and PEO segments, possibly with different orientations on the surface, have methylene groups in their chains. The peak at 2920 cm-1 might contain the contribution of the asymmetric stretch of the CH2 groups in both PPO and PEO segments. Even though we will not conduct a quantitative analysis on the methylene orientation on surfaces, qualitatively we understand that methylene groups also change their coverage and orientation. Initially, no methylene signal is observed in air, indicating that they are absent from the surface, because the surface is covered by the methyl groups in the PPG segment. After water exposure, methylene signals can be detected, showing that they partially cover the surface. Upon drying, they change orientation again. Since methylene groups are in the backbone of the polymers, they may have a slower restructuring than that of the methyl groups. Therefore, after water exposure, the methyl groups do not exhibit an orientation change when the surface is completely dried, because it restructures faster and the restructuring is finished when the surface is removed from water. However, methylene continues to have surface restructuring, until the surface is fully dried. As mentioned above, SFG spectra from water-treated samples were collected after exhaustive water washing. Figure 5b and c shows that, after water treatment, no PES signal was observed, indicating that the blend surface is still fully covered by Pluronic F127. We believe that the films are stable to water exposure because Pluronic F127 molecules are entangled with PES chains rather than simply covering the surface with weak interactions with the PES matrix underneath. The high Tg of PES (∼230 °C) implies a rigid polymer structure at room temperature, which would restrict PPG blocks into the PES matrix. If Pluronic F127 molecules segregate to the surface and weakly adsorb on the surface, they should be able to be washed off from the surface using water. We designed a simple experiment to demonstrate this. We first collected a SFG spectrum from a PES film surface after contacting the surface with Pluronic F127 solution (0.1 wt %). This SFG spectrum is shown in Figure 7b, which is very similar to that collected from the PES/PPG blend surface shown above. This indicates that after PES contacts the Pluronic F127 solution, the PES surface can be covered by the PPG segment of Pluronic F127. This sample then undergoes an identical washing procedure as that for the PES/Pluronic F127 blends.
Surface Structures of PES/Pluronic F127 Blends
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Figure 8. Atomic force micrographs of fibrinogen molecules adsorbed on (a) PES, (b) PES/F127 blend with 5.0 wt % F127 bulk concentration, and (c) PES/F127 blend with 10 wt % F127 bulk concentration. Scan size ) 1 × 1 µm2.
After washing, the SFG spectrum collected becomes very weak (Figure 7c), demonstrating the removal of surface adsorbed Pluronic F127. This is different from the observation on the blend surface with water washing. We believe that after a simple “contact”, Pluronic F127 only weakly adsorbs on the PES surface and can be removed by water easily. While using the blending procedure previously described could result in surface segregated Pluronics that cannot be easily washed off, because they have stronger interactions with PES, such as chain entanglement. Previous studies showed the adsorption of Pluronics on hydrophobic substrates is a feasible method to form antiprotein-fouling surfaces.15 Our SFG results demonstrate that adsorbed Pluronics may not be stable enough to afford a long-term fouling-resistant ability, and perhaps this is due to the fact that the hydrophobic interaction between PES and Pluronic F127 is not strong enough. 3.3. Protein Adsorption on PES/Pluronic F127 Blend Surfaces. When PES/Pluronic F127 blends contact water, the surface can reconstruct to minimize the interfacial free energy. As shown above, using SFG, we demonstrated that PEO segments segregate more to the surface after water exposure because of the more favorable interaction between water and PEO segments. Here, we will show that such different surfaces exhibit differences in interacting with biological molecules such as proteins. We use fibrinogen as a model protein in this study. Fibrinogen (∼340 kDa) is a common blood protein and plays a vital role in blood clot formation on a surface.54 In this study, we applied AFM and QCM to compare adsorption amounts of fibrinogen on various surfaces. It has been widely reported that PEG has a unique protein resisting property; therefore, it is expected that the surface PEO of PES/Pluronic F127 blends would resist protein adsorption substantially. AFM images displayed in Figure 8 show a significant decrease in the number of fibrinogen molecules adsorbed on the blend surface with increasing Pluronic F127 bulk concentration. On the pure PES surface, fibrinogen adsorption was substantial, and almost a monolayer of adsorbed fibrinogen was observed. After PES blending with 5.0 wt % Pluronic F127, the surface was covered by PES and Pluronic F127 together, resulting in greatly decreased amounts of adsorbed fibrinogen, as shown in Figure 8b. When the bulk Pluronic F127 concentration reached 10 wt %, at which the blend surface could be fully covered by Pluronic F127, almost no fibrinogen adsorption was observed in the image (Figure 8c). The AFM studies clearly demonstrate that blending sufficient Pluronics into PES can significantly change the surface structure, improving protein adsorption resistance. AFM studies provide a clear comparison of protein adsorption on PES and PES/Pluronic F127 blend surfaces. To substantiate the AFM results, we also applied QCM to semiquantitatively measure the fibrinogen adsorption. QCM is a sensitive mass sensor which is capable of measuring the nanogram level of adsorbed material on a surface. Results from the QCM studies (54) Wang, J.; Lee, S. H.; Chen, Z. J. Phys. Chem. B 2008, 112, 2281–2290.
Figure 9. Surface adsorbed fibrinogen amount on PES/Pluronic F127 blends determined by QCM as a function of bulk Pluronic F127 concentration.
are shown in Figure 9. As expected, with increased Pluronic F127 content in the blend bulk, a trend of decreasing adsorbed fibrinogen amount on the surface is observed. When the bulk Pluronic F127 concentration reached 10 wt %, the adsorption of fibrinogen on the surface was found to be about one tenth of the corresponding adsorption at the PES surface. Our fibrinogen adsorption studies yield results similar to those from previous research, in which the hydrophobic polystyrene surface was modified by the adsorption of Pluronics.55 It should be noted that the adsorbed fibrinogen layer is hydrated and has highly viscoelastic properties, which can cause energy dissipation significantly.56 In such a case, the linear relationship between the crystal frequency change and the adsorbed mass change may not be valid, and therefore, it is difficult to quantify the adsorption amount. However, combined with AFM results, we believe the data shown in Figure 9 can still give at least a semiquantitative measurement on the relative amount of fibrinogen present at each surface. Protein adsorption amount, and more importantly, adsorbed protein structure are important in determining biocompability. We have demonstrated in our previous studies that SFG is a powerful technique to elucidate protein structures at interfaces.57–59 We have shown that fibrinogen can adopt different structures on various polymer surfaces and exhibit varied timedependent structural changes after adsorption. SFG studies of adsorption of fibrinogen on PES and PES/Pluronics blend surfaces will be reported in the future.
4. Conclusions Surface structures of PES/Pluronic F127 polymer blends have been studied by SFG at the molecular level. It is found that the blending of Pluronic F127 can change PES surface molecular orientation. The aromatic C-H stretching mode, which cannot be detected on the pure PES surface, dominates the SFG spectrum when the Pluronic F127 bulk concentration is 0.5 wt %. As expected, the hydrophobic PPO segments, with lower surface free energy than that of PES, segregated to the surface, while (55) Marsh, L. H.; Coke, M.; Dettmar, P. W.; Ewen, R. J.; Havler, M.; Nevell, T. G.; Smart, J. D.; Smith, J. R.; Timmins, B.; Tsibouklis, J.; Alexander, C. J. Biomed. Mater. Res. 2002, 64, 641–652. (56) Kim, J. T.; Weber, N.; Shin, G. H.; Huang, Q.; Liu, S. X. J. Food Sci. 2007, 72, 214–221. (57) Chen, X.; Wang, J.; Paszti, Z.; Wang, F.; Schrauben, J. N.; Tarabara, V. V.; Schmaier, A. H.; Chen, Z. Anal. Bioanal. Chem. 2007, 388, 65–72. (58) Chen, X.; Tang, H.; Even, M. A.; Wang, J.; Tew, G. N.; Chen, Z. J. Am. Chem. Soc. 2006, 128, 2711–2714. (59) Wang, J.; Buck, S. M.; Chen, Z. Analyst 2003, 128, 773–778.
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the hydrophilic PEO segments, with higher surface free energy, are buried within the PES matrix. When the bulk Pluronic F127 concentration is higher than 10 wt %, the surface of the polymer blend is fully covered by hydrophobic PPO segments. Upon water exposure, the hydrophilic PEO segments tend to segregate to the blend surface because of the favorable interaction with water. From CH3(as) and CH3(s) signals in ssp spectra, we deduced that methyl groups tilted more along the surface normal with narrower orientation distribution after water exposure. The relative amount of protein that adsorbed to the blend surfaces was characterized by QCM and AFM. With increasing Pluronic F127 concentration in the bulk, the blends show a decreased fibrinogen adsorption amount, because more PEO
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segments would extend out of the surface and lead to foulingresistance. Correlation of surface structure with fouling-resistant ability aids the understanding of how molecular orientations at the surface control the polymer biocompatibility. Acknowledgment. This research is supported by the National Science Foundation (CHE-0449469), Office of Naval Research (N00014-02-1-0832), Research Fund for the Doctoral Program of Higher Education of China (20060056032), and the Program of Introducing Talents of Discipline to Universities (NO: B06006). Q.S. is supported by a CSC fellowship. LA800570A
