Sum Frequency Generation Studies on the Surface Structures of

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Anal. Chem. 2003, 75, 3275-3280

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Sum Frequency Generation Studies on the Surface Structures of Plasticized and Unplasticized Polyurethane in Air and in Water Matthew L. Clarke, Jie Wang, and Zhan Chen*

Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109

This study characterizes the molecular surface structures of polyurethane (PU) and plasticized PU films in air, in water, and in albumin solution in situ using a nonlinear optical technique, sum frequency generation (SFG) vibrational spectroscopy. Two different plasticizers are investigated: dioctyl sebacate (DOS) and o-nitrophenyl octyl ether (NPOE). Plasticization of PU is common for biosensors to achieve better adhesion, malleability, elasticity, and permeability; however, this can adversely affect biocompatibility. Our research indicates that plasticizers can segregate to the PU surface not only in air but also in water. In addition, plasticizer content can affect protein adsorption behaviors of PU surfaces. This is the first time surface-sensitive SFG has been applied to deduce plasticizer surface behavior in situ. More specifically, we found that DOS dominates the surface of the film with DOS surface concentrations higher than DOS bulk concentrations both in air and in contact with an aqueous environment. NPOE has a reduced effect at the surface compared to DOS in both air and aqueous environments. Addition of either DOS or NPOE to PU was also found to cause a change in albumin adsorption.

in biomedical applications including coatings for cardiac pacemaker leads, infusion pumps, cardiovascular catheters,2 and artificial hearts due to their enhanced biocompatibility.3 PU has also been widely used in ISE membranes, due not only to its biocompatibility but also to other favorable bulk properties such as stability, durability, and adhesiveness.4 Since the surface properties of a biomaterial determine its biocompatibility, efforts have been made to further improve the biocompatibility of PU by modifying the polymer surface structure. These efforts include immobilization of heparin on the PU surface,5 attachment of poly(dimethylsiloxane) end groups to the polymer chain,6,7 and optimization of annealing techniques.8 The perfection of biocompatibility or surface structure is not the only concern for constructing an ISE; positive mechanical and other bulk properties are also required for ISE polymer membranes. Therefore, plasticizers are often added to polymers to improve the bulk properties of the finished polymer material. For example, incorporating different amounts of plasticizer into the polymer film can modify the adhesiveness and permeability of a polymer film.8 Research shows that adding plasticizers to the membrane matrix of an ISE can improve the sensitivity and selectivity of the sensor.4 These added plasticizers act by disrupting interchain bonding of polymer molecules, resulting in a more

Ion selective electrode (ISE)-based biosensors have the potential to monitor blood electrolytes in vivo.1 As with other biomedical devices, ISEs need to be biocompatible. For this reason, polymers exhibiting enhanced biocompatibility are favorably employed in the construction of ISE membranes for bloodcontacting devices. Polyurethanes (PUs) have been widely used

(2) Szycher’s Handbook of Polyurethanes; Szycher, M., Ed.; CRC Press: New York, 1999; Chapter 22. (3) Yang, M. J.; Zhang, Z.; Hahn, C.; King, M. W.; Guidoin, R. J. Biomed. Mater. Res. 1999, 48, 648-659. (4) Yun, S. Y.; Hong, Y. K.; Oh, B. K.; Cha, G. S.; Nam, H.; Lee, S. B.; Jin, J. I. Anal. Chem. 1997, 69, 868-873. (5) Bae, J. S.; Seo, E. J.; Kang, I. K. Biomaterials 1999, 20, 529-537. (6) Chen, Z.; Ward, R.; Tian, Y.; Malizia, F.; Gracias, D. H.; Shen, Y. R.; Somorjai, G. A. J. Biomed. Mater. Res. 2002, 62, 254-264. (7) Berrocal, M. J.; Badr, I. H. A.; Gao, D. O.; Bachas, L. G. Anal. Chem. 2001, 73, 5328-5333. (8) Takahashi, A.; Kita, R.; Kaibara, M. J. Mater. Sci.: Mater. Med. 2002, 13, 259-264.

* To whom all correspondence should be addressed. E-mail: [email protected]. Fax: 734-647-4865. (1) Bakker, E.; Diamond, D.; Lewenstam, A.; Pretsch, E. Anal. Chim. Acta 1999, 393, 11-18. 10.1021/ac034417+ CCC: $25.00 Published on Web 06/06/2003

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flexible material.9 Unfortunately, while plasticized PU films may exhibit bulk properties favorable for an ISE membrane, biocompatibility studies on such films have shown that increasing the plasticizer content of a film can lead to a greater biological response.10 This loss of biocompatibility may be caused by the leaching of plasticizer into the biological environment, or by a change in the surface properties of the film, or both. Traditionally, polymer ISEs were made using a 2:1 plasticizer-to-polymer mass ratio. A recent study has shown that this high degree of plasticization is not necessary for PU films with high soft segmentto-hard segment mass ratios.4 However, the study also concluded that complete elimination of plasticizer from the matrix resulted in a loss in sensor performance. There are many plasticizers commercially available, each with its own unique behavior. Our research here examines the behavior of PU films plasticized with either dioctyl sebacate (DOS) or o-nitrophenyl octyl ether (NPOE), each a widely used plasticizer. The PU chosen for this study is Tecoflex SG-80A, which has been shown to exhibit reduced activation of adhered platelets when compared to structurally similar aromatic PU.11 Tecoflex SG-80A is an aliphatic polyether urethane synthesized from methylene bis(cyclohexyl) diisocyanate, poly(tetramethylene ether glycol) (PTMEG), and chain extender 1,4-butanediol.12 The polymer contains 27% (w/w) hard segment and previously has been processed with reduced amounts of plasticizer without loss in sensor performance.4 Since this investigation will focus on plasticizer behavior, higher plasticization levels (25 and 50% plasticizer (w/w)) have been used. Recent studies have applied techniques such as static secondary ion mass spectrometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy,13 neutron reflectivity,14 and second harmonic generation15 to study the surface of plasticized poly(vinyl chloride) membranes. However, surface studies of plasticized PU are limited,16 and there still remain questions about the chemical behavior of plasticizers on the surface of the film in the wet biological environment. Conventional surface-sensitive techniques cannot provide in situ information for such surfaces, because these techniques normally operate under high vacuum. In recent years, the nonlinear optical technique sum frequency generation (SFG) vibrational spectroscopy has become a very powerful and highly versatile spectroscopic tool for surface and interface studies, which not only permits identification of surface molecular species but also provides information about surface structure, such as coverage and orientation of surface functional groups.17-29 The SFG technique can be used in ambient conditions (9) Sears, J. K.; Darby, J. R. The Technology of Plasticizers; John Wiley & Sons: New York, 1982; Chapter 1. (10) Lindner, E.; Cosofret, V. V.; Ufer, S.; Buck, R. P.; Kao, W. J.; Neuman, M. R.; Anderson, J. M. J. Biomed. Mater. Res. 1994, 28, 591-601. (11) Espadas-Torre, C.; Meyerhoff, M. E. Anal. Chem. 1995, 67, 3108-3114. (12) Mowery, K. A.; Meyerhoff, M. E. Polymer 1999, 40, 6203-6207. (13) Ye, Q. S.; Keresztes, Z.; Horvai, G. Electroanalysis 1999, 11, 729-734. (14) Ye, Q. S.; Borbely, S.; Horvai, G. Anal. Chem. 1999, 71, 4313-4320. (15) Tohda, K.; Umezawa, Y.; Yoshiyagawa, S.; Hashimoto, S.; Kawasaki, M. Anal. Chem. 1995, 67, 570-577. (16) Reichmuth, P.; Sigrist, H.; Badertscher, M.; Morf, W. E.; De Rooij, N. F.; Pretsch, E. Bioconjugate Chem. 2002, 13, 90-96. (17) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984. (18) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327-350. (19) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1999, 103, 3292-3307. (20) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281-1296.

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to probe any surface or interface accessible by light. It has all the common advantages of optical techniques: it is nondestructive, is highly sensitive, and provides good spatial, temporal, and spectral resolution. Here, we have applied SFG to examine the molecular surface structures of plasticizer-modified PU films in air and water. The fact that different polymers have varying degrees of biocompatibility has been widely observed. As with other biomaterials, the biocompatibility of a polymer is determined by the interactions between the surface and adsorbed proteins. The different interactions at polymer surfaces are due to the differences in their various surface properties. The adsorbed configurations of such protein films mediate the potential long-term effects of biomaterials on host response, thrombus formation, blood clotting, immunological mechanisms, or future carcinogenicity.30-32 For this reason, the effect that surface plasticizer has on the adsorption of bovine serum albumin (BSA) has been studied. Albumin adsorption leads to a passivating layer on the polymer surface, preventing platelet adhesion, and is known to favorably bind to the surface of PU. Therefore, differences in the adsorption of albumin to the plasticized PU surface compared to that of unplasticized PU could indicate a difference in biocompatibility. EXPERIMENTAL SECTION Reagents. A biomedical grade PU, Tecoflex SG-80A (Thermedics, Woburn, MA) was used. DOS, tetrahydrofuran (THF), phosphate buffer (pH 7.4), and BSA were purchased from Aldrich (Milwaukee, WI). NPOE was purchased from Fluka (Milwaukee, WI). Sample Preparation. PU, DOS, and NPOE were dissolved in THF (1-2%, w/w). Blends of PU with plasticizer were made by mixing these solutions. Fused-silica substrates (for SFG) and glass microscope slides (for contact angle) were cleaned by sulfuric acid bath saturated with potassium dichromate and thoroughly rinsed with water. Polymer/plasticizer films were prepared by spin coating polymer/plasticizer solutions at 3000 rpm onto the substrate. Samples were found to be adequately free of solvent without curing of the polymer film. Films cured overnight at 80 °C to ensure complete solvent evaporation gave the same spectral responses as uncured films. To create DOS- or NPOEcoated PU films, DOS and NPOE were dissolved in methanol (2% (21) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343-1360. (22) Walker, R. A.; Gruetzmacher, J. A.; Richmond, G. L. J. Am. Chem. Soc. 1998, 120, 6991-7003. (23) Gracias, D. H.; Chen, Z.; Shen, Y. R.; Somorjai, G. A. Acc. Chem. Res. 1999, 32, 930-940. (24) Shultz, M. J.; Schnitzer, C.; Simonelli, D.; Baldelli, S. Int. Rev. Phys. Chem. 2000, 19, 123-153. (25) Pizzolatto, R. L.; Yang, Y. J.; Wolf, L. K.; Messmer, M. C. Anal. Chim. Acta 1999, 397, 81-92. (26) Kim, J.; Cremer, P. S. J. Am. Chem. Soc. 2000, 122, 12371-12372. (27) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L. J. J. Phys. Chem. B 2001, 105, 2785-2791. (28) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Phys. Rev. Lett. 2000, 85, 3854-3857. (29) Lo ¨bau, J.; Wolfrum, K. J. Opt. Soc. Am., B 1997, 14, 2505-2512. (30) Baier, R. E. Applied Chemistry at Protein Interfaces; Advances in Chemistry Series 145; American Chemical Society: Washington, DC, 1975. (31) Brash, J. L., Horbett, T. A., Eds. Proteins at Interfaces, Physicochemical and Biochemical Studies; ACS Symposium Series 343; American Chemical Society: Washington, DC, 1987. (32) Horbett, T. A., Brash, J. L., Eds. Proteins at Interfaces II. Fundamentals and Applications; ACS Symposium Series 602; American Chemical Society: Washington, DC, 1995.

w/w). Then DOS/NPOE in methanol solution was spin coated onto a PU film at 3000 rpm. A 1 mg/mL BSA solution was made by dissolving BSA in phosphate buffer (PBS). Instrumentation. Detailed explanations of the SFG technique have been published before and will not be repeated here.17-29 SFG spectra were collected using a custom-designed EKSPLA system. Details about the system have been described in our previous publications.33,34 SFG spectra presented in this work were collected using the ssp polarization combination of the input and output laser beams (s-polarized SFG output, s-polarized visible input, and p-polarized IR input). SFG spectra were also collected using other polarization combinations of laser beams including sps and ppp. Such spectra supported the ssp results and are not shown in this paper. Spectra were normalized for variations in the visible and IR beam powers. For each type of sample, several spectra were collected at each interface and averages of at least three samples were taken. Concentrations of surface plasticizer in different chemical environments were estimated using spectral fitting. In this experiment, the intensity of the ssp SFG signal at a given IR wavelength (ω) can be estimated by

Issp(ω) ∝ |χyyz,nr +

Ayyz,q

∑ω - ω q

q

+ iΓq

|2

(1)

Here χyyz,nr is the nonresonant background contribution; Ayyz,q, ωq, and Γq are the strength, resonant frequency, and damping coefficient of the vibrational mode q. Static water contact angle measurements were taken using a CAM 100 Optical Contact Meter (KSV Instruments). Three samples of each polymer blend were studied, and four contact angle measurements were taken for each of these samples. RESULTS AND DISCUSSION We have measured water contact angles on the surfaces of PU, PU plasticized with different amounts of DOS or NPOE, and PU coated with DOS or NPOE. We believe that the surfaces of plasticizer-coated PU are dominated by the plasticizer, and this was confirmed by our SFG studies, which will be shown later. The contact angle measurement results are shown in Figure 1. From Figure 1 we find that the water contact angle of PU is smaller than that for either plasticizer-coated surface, indicating that the plasticizer-dominated surfaces are more hydrophobic. Addition of plasticizer to the polymer film increases the water contact angle and, therefore, the surface hydrophobicity. This indicates that there is significant surface coverage of plasticizer on the surfaces of PU/plasticizer blends, resulting in increased hydrophobicity for these surfaces relative to the unmodified PU surface. Taking the plasticizer-coated PU to represent the contact angle of a plasticizer-saturated surface, we find that NPOE-coated surfaces are more hydrophobic than DOS-coated surfaces. However, by comparing PU samples containing similar degrees of plasticization, either DOS or NPOE, we find that the DOSplasticized films approach saturation at smaller concentrations of (33) Wang, J.; Chen, C. Y.; Buck, S. M.; Chen, Z. J. Phys. Chem. B 2001, 105, 12118-12125. (34) Wang, J.; Woodcock, S. E.; Buck, S. M.; Chen, C. Y.; Chen, Z. J. Am. Chem. Soc. 2001, 123, 9470-9471.

Figure 1. Contact angle measurements for PU films plasticized with DOS (circles) or NPOE (squares). Plasticizer-coated PU films are assumed to represent 100% plasticizer film.

plasticizer than the NPOE-plasticized films, indicating that DOS is more surface active than NPOE. It is also observed that there is little difference between the contact angle for a 50% plasticizer/ PU film and a plasticizer-coated PU film. This indicates that adding 50% plasticizer into the PU film results in a PU surface nearly covered by plasticizer. While contact angle measurements of the plasticized PU surfaces indicate surface segregation of both DOS and NPOE to the surface, there is no molecular level information about the surface structures of these materials and it is difficult to monitor the surface behavior of plasticized PU in water. As mentioned, ISEs are used in biologically relevant environments; thus, it is quite important to detect how plasticizers modify PU surfaces in water. We have previously demonstrated that SFG can detect polymer surface structures in air and in water with submonolayer surface specificity,35 and apply this technique here to monitor plasticized polymer surface structures. SFG spectra were collected from surfaces of PU, PU plasticized with different amounts of DOS, and DOS-coated PU (Figure 2). The DOS spectra were collected from the DOS liquid/air interface. The PU spectrum is dominated by soft segment groups (PTMEG). This result is consistent with published SFG36 and XPS studies16,37 on similar PUs. Peak assignments for PU are as follows: 2790 cm-1, R-CH2 symmetric stretch; 2850 cm-1, normal CH2 symmetric stretch; 2920 cm-1, normal CH2 asymmetric stretch; 2945 cm-1, Fermi resonance. 36 Compared to PU, DOS has a characteristic peak at 2880 cm-1 corresponding to the symmetric C-H stretch of CH3. Two other dominant peaks of DOS are seen: 2850 cm-1, symmetric stretch of CH2; 2950 cm-1, primarily Fermi resonance. The spectra of the plasticized films shown in Figure 2 indicate that DOS is present on the surfaces in air for both samples with 25% DOS and 50% DOS. Due to its more hydrophobic nature, DOS preferentially segregates to the PU surface. Our SFG studies show that PU samples with as little as 5% DOS (w/w) in the bulk give rise to a (35) Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z. J. Am. Chem. Soc. 2002, 124, 7016-7023. (36) Amitay-Sadovsky, E.; Komvopoulos, K.; Tian, Y.; Somorjai, G. A. Appl. Phys. Lett. 2002, 80, 1829-1831. (37) Lelah, M. D.; Grasel, T. G.; Pierce, J. A.; Cooper, S. L. J. Biomed. Mater. Res. 1986, 20, 433-468.

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Figure 2. SFG spectra of PU, DOS/PU blends, DOS-coated PU, and DOS in air.

discernible CH3 peak (data not shown). DOS-coated PU films give rise to spectra that are very similar to those collected from the DOS surface, indicating that DOS covers the surface of the PU films. Small differences in the SFG signals from DOS/air and DOScoated PU/air interfaces may be attributed to weak interference with the signal from the PU/DOS interface. This interface was measured by contacting PU with DOS liquid and collecting the SFG spectrum. The signal is at least 1 order of magnitude weaker, but discernible. However, in this paper, we still use the SFG signal from the plasticizer-coated PU to approximate a plasticizer-covered PU surface. We believe that this approximation is acceptable to deduce the semiquantitative surface coverage of different components on the plasticized PU surface. These SFG studies indicate that DOS preferentially segregates to the polymer surface in air, supporting the contact angle measurements quite well. As mentioned, ISEs are applied in wet biologically relevant environments. Thus, it is necessary to investigate surface structures of PU and plasticized PU in water. We have collected SFG spectra from PU, DOS-plasticized PU, and DOS-coated PU sample/water interfaces in situ using a previously published experimental geometry.34,38,39 The results are shown in Figure 3. They are markedly different from those spectra collected in air in both intensities and spectral features. Such changes cannot be solely due to the optical constant (e.g., Fresnel coefficient) difference between the polymer/air and polymer/water interfaces. These changes indicate that surface restructuring has occurred for all polymer materials. For example, on the PU surface in air and in water, the orientation of surface-dominating methylene groups changed. In water, the DOS-coated PU film gives rise to a spectrum that is different from that of uncoated PU. We assume that the surface is still dominated by DOS and consider the spectral features observed for the coated film to be indicative of a plasticizer-covered film. It is evident that the plasticized PU films share spectral features of both the PU/water and DOS-coated PU/water interfaces. This suggests that DOS has remained at the surface of the plasticized PU film. Upon removal from the aqueous environment, (38) Wang, J.; Buck, S. M.; Chen, Z. J. Phys. Chem. B 2002, 106, 11666-11672. (39) Wang, J.; Buck, S. M.; Even, M. A.; Chen, Z. J. Am. Chem. Soc. 2002, 124, 13302-13305.

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Figure 3. SFG spectra of PU, DOS/PU blends, and DOS-coated PU in water.

the surface spectra of all the samples return to the original surface/air spectra, indicating such surface restructuring is reversible. No spectral differences were observed when water was replaced by PBS as the contacting medium. In the above discussions, we show qualitatively that the addition of DOS to the polymer matrix affects the surface structures of PU both in air and in water. Now we want to fit the SFG spectra and provide more semiquantitative information. The spectra of PU in air and in water and of DOS-coated PU in air and in water have been fitted according to eq 1. Using the resonant frequencies, damping coefficients, and strengths for each mode in these spectra obtained by fitting, we can estimate the SFG spectra of plasticized PU films. To generate estimated SFG spectra for a DOS-plasticized PU film with a particular surface coverage of DOS in air, the PU and DOS-coated PU spectra in air were combined, assuming the functional group orientation as well as the resonant frequencies and damping coefficients of each mode does not change in the blend, and attenuating the strengths of the modes for each species by the percentage surface coverage for that species (Figure 4). Similar estimations can be made for the samples in water (Figure 5). Comparing the experimental data shown in Figures 2 and 3 to the estimated spectra displayed in Figures 4 and 5, we see that the DOS plasticizer concentration at the surface is greater than the bulk concentration. It can be estimated that the surface concentration of DOS in air and in water on the 25% DOS/PU film is near 75% and is near 85% for the 50% DOS/PU film. This spectral estimation based on the combination of the fitting results for PU and DOS-coated PU assumes that the concentration of the surface groups of PU (or DOS) vary identically (i.e., for PU, the surface concentrations of the OCH2 and CH2 groups from PU will both decrease from 75 to 25% as the surface concentration of PU is reduced from 75 to 25%) and that the orientations of different groups are the same at different surface coverages. This assumption is not entirely valid, as can be seen by the inability of the fitting to more precisely match the data. However, these fitting estimations are still valuable in approximating the surface behavior of each component. Our results here show that the surface concentration of DOS for DOSplasticized PU in both air and water is higher than its bulk concentration. Therefore, adding the plasticizer DOS into the PU

Figure 4. Estimated SFG spectra for DOS/PU blends in air at different percentages of DOS surface coverage. Estimations are based on mixing fitting results from experimentally collected PU and DOS-coated PU spectra in air.

Figure 6. SFG spectra of PU, DOS/PU blends, and DOS-coated PU in albumin solution.

Figure 7. SFG spectra of PU, NPOE/PU blends, NPOE-coated PU, and NPOE in air. Figure 5. Estimated SFG spectra for DOS/PU blends in water at different percentages of DOS surface coverage. Estimations are based on mixing fitting results from experimentally collected PU and DOS-coated PU spectra in water.

bulk will greatly modify the surface properties of the resulting polymer. Since DOS segregates to the PU surface, the biocompatibility of DOS-plasticized PU may differ from that of pure PU. To evaluate the effect surface plasticizer may have on biocompatibility, SFG spectra from the interfaces between PU, DOS-plasticized PU, or DOS-coated PU and albumin solution were collected (Figure 6). Comparing the spectra collected from the PU/water (top spectrum in Figure 3) and PU/albumin solution (top spectrum in Figure 6), we see stark differences. Changes in the relative peak intensities indicate interference from the protein C-H stretches with the PU or plasticizer vibrational stretching modes, or both. Decreased intensities can be attributed to opposite orientation of C-H groups of the protein and polymer/plasticizer. An increase in the relative peak intensities at 2850 and 2950 cm-1 due to albumin adsorption is observed for PU. A negative peak at 3060 cm-1 is also observed and corresponds to the protein aromatic C-H stretch. As the amount of plasticizer is increased, the relative signal intensities become more similar to those seen in water, perhaps because there is lessened BSA adsorption or denaturing

on the plasticized films. Due to the spectral overlaps between the C-H stretches from BSA, PU, and DOS, more detailed structural information of adsorbed BSA is difficult to deduce. In the future, we will use deuterated protein, DOS, or PU in similar experiments to understand more details about protein adsorptions on the PU and DOS-plasticized PU surfaces. In addition to the research on the plasticizer DOS, we have also examined how the plasticizer NPOE affects the surface structures of PU materials. SFG spectra collected from PU, PU plasticized with different amounts of NPOE, and NPOE-coated PU are shown in Figure 7. NPOE can be characterized by two strong peaks: 2880 cm-1, CH3 symmetric stretch; 2950 cm-1, primarily Fermi resonance. An aromatic C-H stretch is observed at 3090 cm-1. In contrast to the behavior of DOS, NPOE does not dominate the surface of the polymer films at lower bulk concentrations of plasticizer. The 25% NPOE/PU film remains mostly covered by PU groups though the surface spectral features indicate the presence of some NPOE at the surface. The different surface activities between DOS and NPOE revealed by SFG correlate with the contact angle measurements. Figure 1 shows that the water contact angle on the DOS-plasticized PU surface increased faster toward the plasticizer-coated film than NPOEplasticized PU. Using spectral fitting and estimated spectra, we calculate the concentration of NPOE at the surface of the 25% Analytical Chemistry, Vol. 75, No. 14, July 15, 2003

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NPOE/DOS sample to be 35% in air and 55% in water, while the 50% NPOE/PU sample had a surface NPOE concentration of 85% in air and in water. The differences in the calculated surface concentration of the 25% NPOE/DOS sample in water compared to air may be due to additional NPOE molecules segregating to the surface or an orientation change in the polymer or plasticizer that cannot be accounted for with this model. CONCLUSIONS We have examined surface structures of PU and platicized PU in air, in water, and in contact with protein solution in situ. Our SFG studies demonstrate that surface restructuring behaviors occur for PU and plasticized PU upon contact with water, showing that it is important to characterize surface structures of biomedical materials in situ. Additionally, the enrichment of the plasticizers DOS and NPOE at the surface of the polymer film in air and in water has been observed. Such surface enrichment and subsequent change in surface characteristics may account for the loss in biocompatibility of ISE membranes upon addition of plasticizer. The differences in protein adsorption for the plasticized and unplasticized polymers, as indicated by our BSA studies using SFG, show that each film may have a unique interaction with

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biological media. Though the spectra of each film were recoverable after removal from the aqueous media, this does not rule out the possibility of small amounts of plasticizer leaching from the polymer matrix. Due to the spectral overlap of the C-H stretches of PU, plasticizer, and protein, future studies will employ the use of deuterated polymers, plasticizers, or proteins, to systematically and quantitatively understand the behavior of each component in each chemical environment. Future studies will also examine the possibility of plasticizer migration in plasticized PU with low bulk concentrations of plasticizer. ACKNOWLEDGMENT The authors thank the Meyerhoff group (University of Michigan) for the donation of the polyurethane. Funding for this research was provided by the start-up funds and the 2002 Rackham Grant of the University of Michigan.

Received for review April 22, 2003. Accepted May 16, 2003. AC034417+