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Photoelectron Spectroscopy Studies of the Functionalization of a Silicon Surface with a Phosphorylcholine-Terminated Polymer Grafted onto (3-Aminopropyl)trimethoxysilane Emma M. E. Kristensen,*,† Fredrik Nederberg,‡ Håkan Rensmo,† Tim Bowden,‡ Jo¨ns Hilborn,‡ and Hans Siegbahn† Department of Physics, Box 530, and Department of Materials Chemistry, Polymer Chemistry, Box 538, Uppsala UniVersity, SE-751 21 Uppsala, Sweden ReceiVed March 3, 2006. In Final Form: September 1, 2006 The structure of a biomimetic phosphorylcholine (PC)-functionalized poly(trimethylene carbonate) (PC-PTMCPC), linked to a silicon substrate through an aminolysis reaction at 120 °C with (3-aminopropyl)trimethoxysilane (APTMS), was studied using photoelectron spectroscopy. Two chemical states were found for the unreacted APTMS amine, a neutral state and a protonated state, where the protonated amine on average was situated closer to the silicon substrate than the neutral amine. The experiments also indicated the presence of a third chemical state, where amines interact with unreacted silanol groups. The PTMC chains of the grafted films were found to consist of only 2-3 repeat units, with the grafted chains enriched in the zwitterionic end group, suggesting that these groups are attracted to the surface. This was further supported by the experiments showing that the PC groups were situated deeper within the film.
Introduction Biomimetic phosphorylcholine (PC)-functionalized poly(trimethylene carbonate), PC-PTMC-PC, polymers (Figure 1a) combine the biodegradability of poly(trimethylene carbonate) with the PC group,1,2 which is the hydrophilic moiety in the cell membrane. PC-functionalized surfaces have proven to improve blood biocompatibility: Studies of PC-PTMC-PC surfaces indicated enhanced hemocompatibility compared to that of nonfunctionalized PTMC and the low-activating poly(vinyl chloride) (PVC) when formation of thrombin-antithrombin complexes and platelet reduction were compared.1 PC-PTMCPC-coated surfaces also showed reduced cell adhesion and protein adsorption compared to nonfunctionalized PTMC-coated surfaces.3 The nonbiodegradable PC-functionalized polymer APC ((2-(acryloyloxy)ethyl)phosphorylcholine)4 reduces cell adhesion,5 as do other PC-ended polymers.6 Ishihara et al.7 suggest that this is due to hydration of the PC group with a large fraction of free water, i.e., water with weak interaction with the polymer. This leads to a large excluded volume and a decreased risk of protein adsorption and denaturation.7 Various PC-functionalized polymers have been studied by XPS to determine the surface composition or obtain conformational information.1,3,5,8-14 In the present study PC-PTMC-PC was covalently linked to a * To whom correspondence should
[email protected]. † Department of Physics. ‡ Department of Materials Chemistry.
be
addressed.
E-mail:
(1) Nederberg, F.; Bowden, T.; Nilsson, B.; Hong, J.; Hilborn, J. J. Am. Chem. Soc. 2004, 126, 15350. (2) Nederberg, F.; Watanabe, J.; Ishihara, K.; Hilborn, J.; Bowden, T. Biomacromolecules 2005, 6, 3088. (3) Nederberg, F.; Watanabe, J.; Ishihara K.; Hilborn, J.; Bowden, T. J. Biomater. Sci., Polym. Ed. 2006, 17, 605. (4) Ishihara, K.; Ueda, T.; Nakabayashi, N. Polym. J. 1990, 22, 355. (5) Ruiz, L.; Hilborn, J. G.; Le´onard, D.; Mathieu, H. J. Biomaterials 1998, 19, 987. (6) Matsuda, T.; Nagase, J.; Ghoda, A.; Hirano, Y.; Kidoaki, S.; Nakayama, Y. Biomaterials 2004, 24, 4517. (7) Ishihara, K.; Nomura, H.; Mihara, T.; Kurita, K.; Iwasaki, Y. J. Biomed. Mater. Res. 1998, 39, 323. (8) Hayward, J. A.; Durrani, A. A.; Lu, Y.; Clayton, C. R.; Chapman, D. Biomaterials 1986, 7, 252.
silicon surface functionalized with (3-aminopropyl)trimethoxysilane (APTMS), Figure 1. Aminosilanes, such as APTMS, are widely used as coupling agents between inorganic and organic materials (such as polymers and proteins).15-21 In this study the monolayer polymer grafts were prepared by casting followed by heating to effect an aminolysis reaction between the aminosilane and the carbonate repeat unit in PTMC. Several groups have studied aminosilanes at surfaces with various techniques.19,22-36 In the present study (9) Gribbin Marra, K.; Winger, T. M.; Hanson, S. R.; Chaikof, E. L. Macromolecules 1997, 30, 6483. (10) Orban, J. M.; Faucher, K. M.; Dluhy, R. A.; Chaikof, E. L. Macromolecules 2000, 33, 4205. (11) Tegoulia, V. A.; Rao, W.; Kalambur, A. T.; Rabolt, J. F.; Cooper, S. L. Langmuir 2001, 17, 4396. (12) Feng, W.; Brash, J.; Zhu, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2931. (13) Chung, Y. C.; Chiu, Y. H.; Wu, Y. W.; Tao, Y. T. Biomaterials 2005, 26, 2313. (14) Nederberg, F.; Bowden, T.; Hilborn, J. Polym. AdV. Technol. 2005, 16, 108. (15) Plueddemann, E. P. Silane coupling agents; Plenum Press: New York, 1982. (16) Chang, Y.-C.; Frank, C. W. Langmuir 1998, 14, 326. (17) Jonas, U.; del Campo, A.; Kru¨ger, C.; Glasser, G.; Boos, D. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5034. (18) Sapsford, K. E.; Ligler, F. S. Biosens. Bioelectron. 2004, 19, 1045. (19) Zhang, L.; Chen, Y.; Dong, T. Surf. Interface Anal. 2004, 36, 311. (20) Nakagawa, T.; Tanaka, T.; Niwa, D.; Osaka, T.; Takeyama, H.; Matsunaga, T. J. Biotechnol. 2005, 116, 105. (21) Wilson, D. J.; Chenery, D. H.; Bowring, H. K.; Wilson, K.; Turner, R.; Maughan, J.; West, P. J.; Ansell, C. W. G. J. Biomater. Sci., Polym. Ed. 2005, 16, 449. (22) Chiang, C.-H.; Ishida, H.; Koenig, J. L. J. Colloid Interface Sci. 1980, 74, 396. (23) Ishida, H.; Chiang, C.-H.; Koenig, J. L. Polymer 1982, 23, 251. (24) Kang, H.-J.; Blum, F. D. J. Phys. Chem. 1991, 95, 9391. (25) Horner, M. R.; Boerio, F. J.; Clearfield, H. M. J. Adhes. Sci. Technol. 1992, 6, 1. (26) Eldridge, B. N.; Chess, C. A.; Buchwalter, L. P.; Goldberg, M. J.; Goldblatt, R. D.; Novak, F. P. In Silanes and other coupling agents; Mittal, K. L., Ed.; VSP BV: Zeist, The Netherlands, 1992; pp 305-321. (27) Wang, D.; Jones, F. R.; Denison, P. In Silanes and other coupling agents; Mittal, K. L., Ed.; VSP BV: Zeist, The Netherlands, 1992; pp 345-364. (28) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Ha¨hner G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (29) Kowalczyk, D.; Slomkowski, S.; Chehimi, M. M.; Delamar, M. Int. J. Adhes. Adhes. 1996, 16, 227.
10.1021/la060586o CCC: $33.50 © 2006 American Chemical Society Published on Web 10/12/2006
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Figure 1. (a) PC-PTMC-PC, n ) 19,1 and aminolysis, forming the grafted surface where (b) PC-PTMC-PC is linked to the Si(100) surface through APTMS.
Figure 2. Schematic illustration of different adsorption mechanisms proposed in the literature by Kowalczyk et al.,29 Horr et al.,31 and Chiang et al.22 The silane could (a) be covalently linked by a siloxane bond and terminated by a free amine, (b) have the amino groups protonated by unreacted silanol groups of other silanes or the substrate surface and have the inverse orientation, (c) interact with silanol groups by strong hydrogen bonding while bending down toward the surface, (d) have the amino groups intramolecularly hydrogen bonded to silanol groups, or (e) be in direct interaction of the amine with the surface silicon atoms.
the molecular surface structure and chemical constituents of APTMS and PC-PTMC-PC covalently bound to APTMS were studied using synchrotron radiation or monochromatized Al KR based photoelectron spectroscopy. Two chemical states of nitrogen in aminosilanes are generally observed using photoelectron spectroscopy.26,28,29,32,34-36 However, both one and three chemical states of the amine have also been reported.19,30 Several chemical states and interactions are proposed for (3-aminopropyl)(30) Harder, P.; Bierbaum, K.; Woell, Ch.; Grunze, M.; Heid, S.; Effenberger, F. Langmuir 1997, 13, 445. (31) Horr, T. J.; Arora, P. S. Colloids Surf., A 1997, 126, 113. (32) Moon, J. H.; Kim, J. H.; Kim, K.-J.; Kang, T.-H.; Kim, B.; Kim, C.-H.; Hahn, J. H.; Park, J. W. Langmuir 1997, 13, 4305. (33) Siqueira Petri, D. F.; Wenz, G.; Schunk, P.; Schimmel, T. Langmuir 1999, 15, 4520. (34) Wei, Z. Q.; Wang, C.; Zhu, C. F.; Zhou, C. Q.; Xu, B.; Bai, C. L. Surf. Sci. 2000, 459, 401. (35) Magalha˜es, J. L.; Moreira, L. M.; Rodrigues-Filho U. P.; Giz M. J.; Pereirada-Silva M. A.; Landers R.; Vinhas R. C. G.; Nascente P. A. P. Surf. Interface Anal. 2002, 33, 293. (36) Allen, G. C.; Sorbello, F.; Altavilla, C.; Castorina, A.; Ciliberto, E. Thin Solid Films 2005, 483, 309.
triethoxysilane (APS) and APTMS in the literature. Some of these are illustrated in Figure 2 as an aid to the reader. Kowalczyk et al.29 suggested the following possible adsorption states for APS on quartz: (i) linked at the silicon and terminated by a free amine, (ii) the amines protonated by unreacted silanol groups of other silanes or the substrate surface, (iii) the amino groups intramoleculary hydrogen-bonded to silanol groups. For APTMS Horr et al.31 suggested a combination of mechanisms (i) and (ii) and hydrogen bonding between amine groups and surface silanol groups as well as direct interaction between the amine group and the surface silicon atoms. The latter configuration was proposed by Plueddemann as a cyclic structure in aqueous solution.37 This was contradicted by Chiang et al.,22 and to our knowledge it has not been shown to exist for thin films. Chiang et al.22 proposed a surface conformation where the silanol group is reacted with the substrate and the amino group interacts with the substrate (37) Plueddemann, E. P. In Composite materials; Plueddemann, E. P., Ed.; Volume 6: Interfaces in polymer matrix composites; Broutman, L. J., Krock, R. H., Eds.; Academic Press: New York, 1974.
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surface through hydrogen bonding. Since aminosilanes are widely used as coupling units for organic molecules, they have large importance for the design of surface properties. Their composition and structure are crucial to optimize and understand the performance of the films. In the present study the aminosilane film was combined with biodegradable and blood-biocompatible PC-functionalized molecules with potential blood-contacting applications. The aim of this study is therefore to determine the molecular surface structure, using photoelectron spectroscopy, of the novel PC-functionalized film grafted onto a silicon substrate through an aminosilane. Experimental Section Preparation of APTMS-Treated Si(100) Wafers. For grafting of APTMS onto Si(100), wafers were cut using a diamond edge tool, repeatedly cleaned in a piranha solution (1 part 30% H2O2(aq)/2 parts concentrated H2SO4), and rinsed with distilled water. APTMS was dissolved in ethanol/water (95/5 by volume) to give a final concentration of 2 vol % (e.g., B. Arkles, Chemtech Vol 7,766-778, 1977). Cleaned wafers were immersed for 5 min to allow the silanization to occur. Silanized wafers were rinsed and sonicated for 10 min in ethanol, further washed in ethanol, and dried under a flow of nitrogen gas. To modify the films, a number of samples were immersed into 0.1 M HCl(aq) for 2 min and into 0.1 M NaOH(aq) for 2 min. Grafting of PC-PTMC-PC onto Si(100). The PC-PTMCPC polymer was prepared according to ref 1 and was dissolved in chloroform to give a final polymer concentration of 5 wt % (chosen to obtain a sufficient amount of polymer on the surface without making the solution too viscous). The solution was cast onto the aminosilanized wafers and allowed to air-dry for 60 min. The chloroform then evaporated and left a thick PC-PTMC-PC film on the silanized wafer. The coated wafers were heat treated in an oven at 120 °C for 15 min (PC-PTMC-PC was stable up to 250 °C, tested using a TGA Q500 V6). After removal from the oven the wafers were allowed to cool to ambient temperature, and nongrafted polymer was removed by extensive washing in chloroform and acetone and in ethanol and hexane. The PC-PTMC-grafted Si(100) wafers were dried under a flow of nitrogen gas. Contact Angle Measurements. For contact angle measurements an FTÅ goniometry instrument with FTÅ 200 video software from First Ten Ångstrom Inc. was used. A 10 µL drop of water was applied to the substrate, and the contact angle was determined. Photoelectron Spectroscopy. The photoelectron spectroscopy (PES) measurements were performed with synchrotron radiation at BL I411 of the Swedish National Synchrotron Source MAX.38 The tunability of the synchrotron radiation photon energy allows for variation of the photoelectron kinetic energies and thus of the sampling depths. The samples were analyzed at a pressure of about 10-7 mbar, and the electron takeoff angle (TOA) with respect to the surface plane was 80° in all cases. In addition an ESCA-300 SCIENTA spectrometer was used, with monochromatic Al KR radiation (1486.7 eV).39 The TOA was 90° for maximum bulk sensitivity and 10° for surface sensitivity. The samples were analyzed at a pressure in the 10-10 mbar range. The spectra were energy referenced to the alkane C1s peak (set to 285.0 eV). Linear backgrounds were used for curve fits except for the P2p line, where the background of the APTMS samples was used. The peaks were fitted with symmetric Gaussians. The proportions between different elements were estimated using Al KR PES and a sensitivity factor table.40 In electron spectroscopic studies of organic surface layers, radiation damage is a possible complication. Before investigating these systems (38) Ba¨ssler, M.; Forsell, J.-O.; Bjo¨rnholm, O.; Feifel, R.; Jurvansuu, M.; Aksela, S.; Sundin, S.; Sorensen, S. L.; Nyholm, R.; Ausmees, A.; Svensson, S. J. Electron Spectrosc. Relat. Phenom. 1999, 953, 101. (39) Gelius, U.; Wannberg, B.; Baltzer, P.; Fellner-Feldegg, H.; Carlsson, G.; Johansson, C.-G.; Larsson, J.; Mu¨nger, P.; Vegerfors, G. J. Electron Spectrosc. Relat. Phenom. 1990, 52, 747. (40) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.
Figure 3. (a) Time development of the APTMS N1s spectrum at a photon energy of 504 eV. (b) Time development of the N1s spectrum of PC-PTMC-PC grafted onto APTMS at TOA90 (Al KR).
Figure 4. N1s spectrum for APTMS excited with a photon energy of 504 eV. The spectrum was fitted using two Gaussians of the same full width at half-maximum having binding energies of 401.5 and 399.7 eV. in detail, we therefore determined the effects due to radiation, Figure 3. The results presented below contain no such effects.
Results and Discussion APTMS. The main focus in studying the APTMS films was the N1s peak. Two different N1s signals were identified, as shown in Figure 4, having binding energies of 401.5 and 399.7 eV. This is in accordance the with literature (for APTMS and APS), where the two signals observed were attributed to protonated or hydrogen-bonded nitrogen (the high-energy signal) and to free amines or hydrogen-bonded amines (low-energy signal).25,28,29,35,36 The intensity ratio between the high- and the low-binding-energy peaks for these samples was about 1:1 (at a kinetic energy of 764 eV), with some minor variations. To look further into the peak identification and the modification possibilities of the surface, samples were immersed into dilute acid (0.1 M HCl) to protonate the amines or into dilute base (0.1 M NaOH) to provide for the nonprotonated amine. Protonation increased the intensity of the high-binding-energy signal (at 401.5 eV), Figure 5a, while the low-binding-energy signal (at 399.7 eV) decreased. On the contrary, exposure to base decreased the high-energy signal (at 401.5 eV) while increasing the low-binding-energy component (at 399.7 eV), as shown in Figure 5b. These results suggest that the high-energy peak mainly originates from protonated amines rather than hydrogen-bonded species or interactions with silicon atoms. We thus assign the
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Figure 7. As the obtained thickness of the APTMS film shows, it consists of 11/2-2 silane monolayers. We suggest an interaction between the first and the second layers; for example, hydroxyl groups in the second layer could be linked via hydrogen bonding to amine groups from the first layer. This is schematically illustrated here. Figure 5. (a) N1s spectrum for APTMS after immersion into 0.1 M HCl(aq) for 2 min. (b) N1s spectrum for APTMS after immersion into 0.1 M NaOH(aq) for 2 min. The photon energy was 504 eV.
Figure 6. N1s spectrum for different electron kinetic energies: (a) 100 eV, (b) 251 eV, and (c) 764 eV.
high-energy signal to protonated amines and the low-energy signal to free amines. The total nitrogen signal intensity decreased by about 15-30% after the NaOH(aq) treatment, while no substantial intensity change was observed after incubation in HCl(aq). The depth distribution of the different N1s species has been studied by other groups. Kowalczyk et al.29 and Horner et al.25 both found that the protonated amine was distributed further down in the film than the free amines (for APS), while Magalha˜es et al.35 found the opposite (for APTMS). Both groups used angleresolved PES. In the present study synchrotron radiation was used as the light source, where the photon energy and thus the electron kinetic energy could be varied and hence the sampling depth. The N1s signal assigned to protonated amines (at 401.5 eV) was found to increase with increasing photon energy, and thus the depth, relative to the free amine signal (at 399.7 eV) (Figure 6). From this it was concluded that the protonated amine was located closest to the silicon surface. This was the case for native films as well as for HCl- and NaOH-treated surfaces. The same result was obtained with angle-resolved PES using Al KR.
Figure 8. The curve fits of high-resolution N1s spectra suggest the presence of more than two N1s components. This is most obvious on the high-energy side where the flank of the original peak is steeper than the flank of the fitted curve. Here (a) a two-component curve fit is compared to (b) a three-component fit. Each spectrum was fitted with the same widths for all components. The shape of the high-binding-energy flank is impossible to reproduce with only two components (even using components with different Gaussians), and so is the high-energy side of the low-energy feature. The photon energy was 504 eV.
The thickness of the film was determined using two different methods. In the first method the silane film was compared to a monolayer of an ω-substituted alkanethiol, HS(CH2)16CN, on gold, the preparation of which is described in ref 41. The N1s (41) Liedberg, B.; Wirde, M.; Tao, Y.-T.; Tengvall, P.; Gelius, U. Langmuir 1997, 13, 5329.
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Figure 9. C1s, O1s, and P2p spectra (Al KR) obtained from silanized and functionalized samples at two different electron takeoff angles, TOA10 and TOA90. The upper figures in each frame are from the silanized samples (APTMS) and the lower figures from PC-functionalized samples (PC-PTMC-APTMS).
signal intensity (peak area) of our APTMS samples was compared to the intensity (peak area) of the alkanethiols HS(CH2)16CN on gold. The N1s signal intensity of APTMS was about 11/2-2 times that of the thiol. This suggests a coverage of approximately 11/2-2 monolayers of APTMS on our samples. This can then be compared to the thickness estimation obtained by using the substrate bulk signal (silicon). Two silicon signals were identified: silicon from the substrate bulk and silicon oxide (from the substrate surface and from APTMS). The silicon oxide/bulk silicon ratio decreased at greater sampling depths for both silanized and bare samples. Assuming that the substrate silicon oxide film was not affected by the silanization procedure, the thickness of the APTMS film may be obtained from
IS/IS° ) exp(-d/(λ sin θ))
(1)
where IS° is the substrate (bulk silicon) signal from a cleaned substrate sample and IS the substrate signal from a silanized sample. λ is the electron inelastic mean free path, d the thickness of the silane coating, and θ the electron takeoff angle relative to the surface plane. The data used were those obtained using the ESCA-300 instrument with Al KR radiation. λ was taken to be 39 Å according to Bierbaum et al.28 and references therein. The silane film in the present study was typically found to be 12 Å thick, as compared to the theoretical thickness for a monolayer (about 8.5 Å).28 This value of the thickness is comparable to the estimated 11/2-2 monolayers discussed above. This finding indicates that the substrate silicon oxide layer was not affected by the silanization process, since the attenuation of the substrate silicon signal would be affected by a change in the thickness of the silicon oxide. Bierbaum et al.28 obtained a
thickness of 10 Å for their film, and on the basis of the NEXAFS results, the presence of more than one monolayer was proposed. Allen et al.36 reported homogeneous films with an estimated thickness of 12-16 Å, suggesting the film to be a single or double layer. To propose a model for the molecular configuration of the APTMS films consisting of 11/2-2 silane monolayers, we suggest an interaction between the first and the second layers; for example, hydroxyl groups in the second layer could be linked via hydrogen bonding to amine groups from the first layer. In the second layer of APTMS the amines are not expected to be protonated or interact further, since the low-energy peak is dominant for thicker coatings.31,35 This idea is illustrated in Figure 7 as an aid to the reader. The surface composition proposed by this model suggests the presence of a third N1s component. Interestingly, a closer look at the N1s curve fit reveals that the N1s spectrum consists of more than two components (Figure 8). The flanks of the N1s spectrum are steeper than the curves used to fit the peak when only two components are fitted. The binding energies obtained in curve fits with three components were approximately 401.5, 400.1, and 399.3 eV. Harder et al.30 observed three chemical states for (17-aminoheptadecyl)trimethoxysilane. These had binding energies of 402.2, 400.7, and 399.7 eV. The low-energy peak (at 399.7 eV) was assigned to free amines and the middle signal (at 400.7 eV) to amine groups hydrogen-bonded to the surface.30 The signal at 402.2 eV was assigned to protonated amines.30 Due to the resemblance between these results and those in the present study, we propose similar origins of the nitrogen signals. Our protonated amine might, however, interact more with the substrate, reducing its binding energy. The presence of
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Figure 10. C1s spectrum at TOA90 of Figure 9a for the PC-functionalized sample fitted with Gaussians of different widths. Three different carbon species are identified: carbonyl carbon (290.3 eV), carbon bound to one oxygen atom (286.5 eV), and alkane carbon (285.0 eV). The signals were identified by comparison with those of poly(alkyl carbonate) and poly(ethylene glycol) in the literature.42 Table 1. Average Relations between Different Elements of the Grafted APTMS-PTMC-PC Film at TOA90 P2p
O1s (PTMC-PC)
N1s (APTMS-PTMC-PC)
C1s (carbonyl)
1
10.7
1.5
3.3
a third component does not affect the conclusion in the present study that the protonated amine is found to be closer to the silicon substrate surface than the free amine. The presence of a third N1s peak supports the configuration shown in Figure 7. PC-PTMC-PC. The PC-PTMC-PC molecule grafted onto the silane surface was investigated by Al KR PES. The success of the grafting was confirmed by the presence of a P2p signal and the appearance of new features in the C1s and O1s signals (Figure 9). The contact angle measurements also corroborate this; values obtained were 30° (pure substrate), 98° (APTMS), and 68° (PC-PTMC-APTMS), indicating the exposure of more polar groups for the PC-functionalized surface. For this surface, counterions in the form of sodium and chloride were present, which was not the case for the samples which were only silanized. For carbon, three different chemical species were identified (Figure 10): carbonates and urethanes, ethers, and finally alkane carbon.42 Some variations in the results between the samples were found, which we interpret as due to a varying degree of homogeneity for the different sample films. The average ratios between different elements are shown in Table 1. The ratio between phosphorus and the carbonyl carbon was found to be between 1/2.7 and 1/4.0. This gives an average number of repeat units, n in Figure 1b, of 2-3. This is an interesting result, since if the PTMC chain had reacted at random, the relation between phosphorus and carbonyl carbon would be expected to be 1/20. One plausible explanation is an interaction between the PC group and the silane surface, both being polar in nature, favoring aminolysis close to the PC group. The PTMC not interacting with the surface would be the leaving group (Figure 11). The ratio between carbonyl carbon and phosphorus, as well as between carbonyl carbon and nitrogen, was about twice as large at TOA10 (surface-sensitive mode) as at TOA90 (bulk-sensitive mode). This is consistent with previous findings for monofunctional PTMC-PC and for bifunctional PC-PTMC-PC cast onto glass objective slides, where the PC group was found to be situated deeper in the film.2,14 Three nitrogen species could be expected on the basis of the chemical formula. In the N1s spectrum, however, only two signals (42) Beamson, G.; Briggs, D. High resolution XPS of organic polymers: The Scienta ESCA 300 database; John Wiley & Sons Ltd.: Chichester, England, 1992.
Figure 11. The short PTMC chains obtained could be explained by interaction between the silane surface and the PC groups. Any side of the chain can be the leaving group.
Figure 12. N1s spectrum at TOA90. The spectrum was fitted with two Gaussians of the same widths at binding energies of 401.5 and 399.6 eV.
could be resolved, at binding energies of 401.5 and 399.6 eV (Figure 12). The low-binding-energy signal was assigned to free amines. On the basis of the binding energies (other groups have found binding energies of 400-404 eV for the PC nitrogen)8,11-13 and on the ratio between phosphorus and the high-energy nitrogen signal (which was found to be 1/1.3 to 1/2.4 at TOA90), the high-binding-energy peak was assigned to both PC nitrogen and the linking unit. The fact that the ratio between phosphorus and the high-binding-energy nitrogen signal (at TOA90) was mostly found to be less than 1/2 could, according to a simple model based on eq 1, be explained such that nitrogen is situated deeper in the film than phosphorus. For the PC-PTMC-APTMS film the ratio between the high-binding-energy nitrogen signal (from the PC and linking groups) and phosphorus at TOA10 was about half the ratio at TOA90. Phosphorus is thus situated closer to the film-vacuum interface than nitrogen. For surfaces with a P/N ratio greater than 1/2 (at TOA90) a possible explanation is remaining unreacted, protonated aminosilane molecules. The free amine contributes about 10% of the total nitrogen signal (at TOA90) (Figure 12). To estimate to which degree unreacted, protonated APTMS molecules were still present at the surface, the total signal intensity (peak area) was compared at TOA90 for the silane film and the grafted film. The total nitrogen signal intensity at the grafted surface was found to be about 80% of the nitrogen signal intensity of the untreated silane films (at TOA90). According to a simple model based on eq 1, this result is consistent with approximately all of the original silane molecules remaining after grafting. The model is also consistent with the presence of two nitrogen atoms
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than they would have been had they been cut at random. According to the above, we also conclude that the PC groups bend down, which would give a somewhat thinner film. The calculated thickness depends, however, on λ, which differs with the film components and density, as well as on contaminations. An illustration of a possible model of the dominating molecular conformation of the film is presented in Figure 13.
Conclusions
Figure 13. Conceptual illustration of the proposed configuration of the PC-functionalized surface.
per phosphorus atom, which suggests a more or less complete reaction of the silane molecules with PC-PTMC-PC molecules. However, for spatial reasons it is not likely that all nitrogen moieties have reacted, and as mentioned above free amines were observed (Figure 12). The used model is not exact, but it can be concluded that a vast majority of the amines have reacted with PC-PTMC-PC. The thickness of the PC-PTMC-APTMS film was calculated using eq 1 and the same λ as for the silane film (39 Å). The thickness thus obtained was approximately 40 Å. This is about the same as the theoretical length of a molecule stretched out with four carbonyl carbons for each phosphorus. This thickness thus supports that the PTMC chains are substantially shorter
In this paper a PC-PTMC-PC polymer covalently attached to a silicon substrate surface via APTMS was studied as well as the unreacted APTMS film. The APTMS film was found to be 11/2-2 monolayers thick. Synchrotron-radiation-based photoelectron spectroscopy revealed that APTMS contained two or possibly three nitrogen species. These where identified as a protonated amine (closest to the substrate surface), a free amine directed toward the film-vacuum interface, and an amine interacting with silanol groups in the second layer of the film. The PC-PTMC-PC molecules were found to be successfully grafted onto the silane film, supporting an almost complete reaction of amines with PC-PTMC-PC. Also, the PTMC chains were found to be much shorter than expected from random scission: 2-3 polymer segments rather than the 20 segments expected from random attack. The APTMS surface molecular configuration is thus concluded to be of importance for the aminolysis reaction preferentially occurring close to the PC chain end. The thickness of the PC-functionalized film was estimated to be about 40 Å, and the PC groups were found to bend down and be situated deeper in the film. Acknowledgment. This work was funded by the Swedish Foundation for Strategic Research (SSF) and the Swedish Research Council (VR). LA060586O
