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Langmuir 1998, 14, 1656-1663
r-Amino Acid-β-Hydroxysiloxanes on E-Glass Fibers Arthur Provatas,† Janis G. Matisons,* and Roger St. C. Smart Polymer Science Group, Ian Wark Research Institute, University of South Australia, The Levels, Pooraka, South Australia, Australia 5095 Received June 3, 1997. In Final Form: November 17, 1997 R-Amino acid-β-hydroxysiloxanes prepared previously by our group were coupled to E-glass fibers to gain a better understanding of the surface processes at the polymer-glass interface of composite materials. Use of DRIFT (diffuse reflectance infrared Fourier transform spectroscopy), XPS (X-ray photoelectron spectroscopy), and SEM techniques allowed for the qualitative and quantitative determination of such processes. Our studies indicate that binding to the E-glass is occurring through different mechanisms, primarily hydrogen bonding, as evidenced by DRIFT and SEM. XPS studies confirm that the amino group is binding to the surface, as evidenced by the formation of two amino states, a covalent nonprotonated and an ionic protonated form. A theoretical model depicting the binding of polymer to E-glass fibers has also been proposed.
Introduction The adsorption of proteins on polymer or biopolymer surfaces is of great significance in a wide range of biological and nonbiological processes, where amino acids, proteins, and peptides interact with biological systems having different surfaces. The surface properties of such systems play an important role, as protein molecules are more influenced by nonionic or hydrophobic surfaces than polar or hydrophobic surfaces. Commercially, protein-polymer systems are of importance in the personal care industry, where many shampoos, conditioners, cosmetic, and topical pharmaceuticals contain adsorbed proteins. Pharmaceutical glass containers are coated with siloxane polymers to prevent drug adsorption and inactivation during storage. Siloxanes are known to repel water, thereby decreasing adsorption.1 Polymer-immobilized enzymes and serological tests which contain immunoproteins adsorbed onto polystyrene latex2 are now established in biotechnology processes. Metal and polymer surfaces aside, adsorbents such as alumina,3 titanium dioxide,4 mica,5 montmorillonite,6 cation-exchange resins,7 and charcoal8 have all been studied as possible amino acid adsorbent sites. Silica, as an adsorbent surface, has received considerable attention in the literature, with the adsorption of amino acids on dehydrated silica surfaces characterized by XPS9 and IR * To whom correspondence should be addressed. † Present address: DSTO, Weapons Systems Division, PO Box 1500, Salisbury, SA, Australia 5108. (1) (a) Mizutani, T. J. Pharm. Sci. 1981, 70, 493. (b) Packham, M. A.; Evans, G.; Glynn, M. F. J. Lab. Clin. Med. 1969, 73, 686. (c) Shortman, K.; Williams, N.; Jackson, H.; Russell, P.; Byrt, P.; Diener, J. Cell. Biol. 1971, 48, 566. (2) Soderquist, M. E.; Walton, A. G. J. Colloid Interface Sci. 1980, 75, 386. (3) Kaur, P.; Mundhera, G. L.; Kar, H. S.; Tiwari, J. S. Fundamentals of Adsorption, 3rd International Conference; Bavaria, Germany; The Foundation: New York, 1984; pp 379-394. (4) Ozazaki, S.; Aoki, T.; Toni, K. Bull. Chem. Soc. Jpn. 1981, 54, 1955. (5) Morrisey, B. W.; Han, C. C. J. Colloid Interface Sci. 1978, 65, 423. (6) Tarasevich, Yu, I.; Rak, V. S.; Telichkun, V. P. Kolloidn. Zh. 1979, 39, 1190. (7) Haynes, J. J. Colloid Interface Sci. 1970, 32, 282. (8) Lundstro¨m, I.; Ivarsson, B.; Jo¨nsson, U.; Elwing, H. In Polymer Surfaces and Interfaces; Feast, W. I.; Munro, H. S., Eds.; Wiley: New York, 1987; Chapter 11. (9) Wu, C. R.; Nilsson, J. O.; Salaneck, W. R. Phys. Scr. 1987, 35, 586.
spectra.10,11 However, very little published data are available monitoring amino acid adsorption onto E-glass fibers.12 Our interest in E-glass fibers stems from their application in composites, where the addition of coupling agents to glass fiber reinforced composites provides a water impermeable bond that enhances the overall composite performance. Yosoyima et al.12 examined the adsorption of amino acids onto glass fibers pretreated with diisocyanate groups in the presence of triethylamine, in an effort to understand the reactivity of surface silanol groups to various organic functionalities, in their case, amino acids and diisocyanates. Glass fibers were heated at 30 °C under nitrogen, in the presence of amino acids and catalyst, washed sufficiently with dioxane, and vacuum-dried at 40 °C. Characterization of glass fiber surfaces was by weight change and infrared spectrometry. The isocyanates react with protic compounds; the reactivity decreasing according to the following series:
amine > alcohol > water > thiol > carboxylic acid Carboxyl groups were protected by esterification. Both the glycine methyl ester and the methionine methyl ester were adsorbed over 12 h onto glass fibers using an equimolar solution of triethylamine in either DMSO or DMF. Temperatures of around 75 °C give good yields of isocyanate groups (75% yield) with glycine methyl ester. Organo-functional siloxanes have the latent ability to function as effective coupling agents, if the correct selection of two pendant functional groups is made. In this research we examine the binding properties of R-amino acid-βhydroxysiloxanes to E-glass fibers. R-Amino-β-hydroxy functional siloxanes bound to E-glass fibers have already been studied by XPS, DRIFT, and SEM and found to possess binding strengths equal to those of their silane counterparts.13 The R-amino acid-β-hydroxysiloxanes are prepared by a three-step process, initially involving attachment of 3-butenoic acid to the tert-butyl ester (10) Baysuk, V. A.; Gromovoi, T. Yu.; Chuiko, A. A. Zh. Prikl. Speklrosk. 1991, 55, 877. (11) Baysuk, V. A.; Chuiko, A. A. Zh. Prikl. Speklrosk. 1990, 52, 134. (12) (a) Yosomiya, R.; Morimoto, K. Polym. Bull. 1984, 12, 41.; (b) Yosomiya, R.; Morimoto, K.; Suzuki, T. J. Appl. Polym. Sci. 1984, 29, 671. (13) Arora, P. S.; Matisons, J. G.; Provatas, A.; Smart, R. St. C. Langmuir 1995, 11, 2009.
S0743-7463(97)00580-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/03/1998
R-Amino Acid-β-Hydroxysiloxanes
protected amino acids to generate an allyl functionalized amino acid.14 Dicyclohexylcarbodiimide (DCC) is used to couple both reagents in high yield and readily precipitates out of solution as dicyclohexylurea (DCU), a white solid. The second step involves the hydrosilylation of the allyl functionalized amino acids to Si-H functional siloxane backbones, using a platinum catalyst. Deprotection of the tert-butyl ester group with trifluoroacetic acid leaves an R,ω-telechelic amino acid functionalized siloxane as the final product. Such R-amino acid-β-hydroxysiloxanes are now being investigated as coupling agents in composite materials. To gain an understanding of the bonding mechanism of R-amino acid-β-hydroxysiloxanes on glass fibers, the polymers were coupled onto E-glass and studied by surface analytical techniques, (diffuse reflectance infrared Fourier transform spectroscopy, DRIFT, X-ray photoelectron spectroscopy, XPS, and scanning electron microscopy SEM). Early attempts at detecting the presence of polymers adsorbed on glass fibers were restricted largely to infrared methods15 that excluded DRIFT. DRIFT uses the diffuse reflected light from a sample surface, which is collected by mirrors and transmitted to a detector, to spectroscopically detect and measure the adsorbed species.16 Conventional infrared techniques such as transmission IR and ATR (attenuated total reflectance) exhibit a high proportion of scattering when used for the analysis of polymers on glass fibers. FT-IR microspectroscopy, and two-dimensional mapping experiments have also been used to characterize epoxy resin-glass fiber reinforced composites, giving good spectral data (with low scattering).17 XPS on the other hand, can be applied to the analysis of surfaces at the nanometer level and enables the quantification of elements at the surface. As a high vacuum technique, any contamination present is kept relatively low. X-ray photoelectron spectroscopy has steadily become a valuable technique for the examination and characterization of polymers on glass surfaces.18 Combination of both techniques has allowed polymer chemists to gain a better insight of polymer-surface interfaces. The use of DRIFT, XPS, and SEM techniques has now allowed for ready characterization of siloxane polymers on glass surfaces.19,20 DRIFT was used as an initial qualitative check to determine if adsorption between the R-amino acid-β-hydroxysiloxanes and the glass fibers has occurred. The glass fibers were then analyzed by XPS to determine the elemental composition of the R-amino-βhydroxysiloxanes on the glass, and their surface coverage. In addition, scanning electron microscopy (SEM) analyses have been carried out to establish the surface morphology of the siloxanes on the glass fibers. Experimental Section The R-amino acid-β-hydroxy functional siloxanes used in the coupling studies were prepared as outlined previously and are shown in Figure 1.14 E-glass fibers (water washed only) were (14) Matisons, J. G.; Provatas, A. Macromolecules, in press. (15) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967; p 315. (16) Culler, S. R.; McKenzie, M. T.; Fina, L. J.; Ishida, H.; Koenig, J. L. Appl. Spectrosc. 1984, 38, 791. (17) Pekala, R. W.; Merrill, E. W. J. Colloid Interface Sci. 1984, 101, 120. (18) Doyle, P. J. Glass Making Today; Portcullis: Redhill, England, 1979. (19) Bennett, D. R.; Matisons, J. G.; Netting, A. K. O.; Smart, R. St. C.; Swincer, A. G. Polym. Int. 1992, 27, 147. (20) Britcher, L. G.; Kehoe, D. C.; Matisons, J. G.; Smart, R. St. C.; Swincer, A. G. Langmuir 1993, 9, 1609.
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Figure 1. R-Amino acid-β-hydroxysiloxanes used to treat E-glass fibers. supplied by ACI Fiberglass (Australia) using standard production methods as outlined in the literature.18 The constituent elemental profile of the glass fibers as determined by X-ray fluorescence measurements carried out by the supplier is as follows: 55.0% SiO2; 21.5% CaO; 14.5% Al2O3; 6.0% B2O3; 0.8% Na2O; 0.6% MgO; 0.4% Fe2O3; 0.3% TiO2; 0.6% F2; 0.3% FO2; 0.1% K2O. All reagents were of at least laboratory grade. Silica and titanium dioxide were used as supplied (BDH). Toluene and tetrahydrofuran (ACE Chemicals) were dried by distilling (bp 110 and 67 °C, respectively) over sodium wire with benzophenone until a violet color had developed, indicating the formation of the benzophenone ketyl radical. Surface coating was performed by immersing the glass fibers (1 g) in dry, distilled toluene (80 mL) containing 0.8-1.5% (wt/ wt) of the appropriate amino acid functional siloxane. The solutions were allowed to stand at room temperature for 3 h and decanted from the glass fiber, and the wet fibers were washed with toluene (2 × 40 mL) and tetrahydrofuran (2 × 40 mL). The fibers were then left to dry in the oven at 110 °C for 3 h. The XPS data were analyzed using a Perkin-Elmer PHI 5100 XPS system with a concentric hemispherical analyzer and a Mg KR X-ray source functioning at 300 W, 15 kV, and 20 mA. High vacuum pressure achieved during analysis varied from 10-8 to 10-9 Torr. The angle between the X-ray source and analyzer was fixed at 54.6°. Surface charging was corrected to the adventitious carbon 1s peak (284.6 eV). The glass fibers were carefully cut and placed on a metallic sample holder with a molybdenum cover plate securing the fibers. Care was taken such that the X-ray beam was only on the fibers and not on the molybdenum cover plate. DRIFT samples were analyzed with a Nicolet Model 750 spectrometer in the wavenumber region 4000-650 cm-1, at a resolution of 4 cm-1, with the use of a MCT liquid-cooled detector. Samples were mounted on a Spectra Tech diffuse reflectance apparatus and scanned 256 times. The interferogram was apodized using the boxcar method installed in the FT-IR software (OMNIC FT-IR software version 2.0). Signal-to-noise ratio is generally better than 15 000 to 1 with 256 scans. The fibers were placed at an angle of 90° with respect to the IR beam (for maximum signal-to-noise ratio), in a specially constructed sample holder designed for the analysis of glass fibers. The background spectrum was analyzed from IR high purity grade potassium bromide (Merck), placed in a sample cup, and leveled to the top of the cup using a spatula. No pressure was applied to the KBr powder in the cup, during packing or leveling. Sample spectra were obtained by subtracting a pure KBr background spectrum. Absorption band positions, areas and heights were determined using OMNIC software, available from OMNIC. A Cambridge stereoscan 100 SEM was used for scanning electron micrographs. The treated fibers were coated with a thin (∼200 Å) evaporated carbon layer to reduce the effects of charging.
Results and Discussion A number of factors affect the adsorption of polymers onto surfaces.21,22 These include: the solvent used, molecular weight and polydispersity of the polymer, the
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polymer concentration, time allowed for adsorption, number of reactive or surface interactive groups per polymer molecule, the reaction temperature, the pH in aqueous systems, the type of posttreatment, and the substrate. This study primarily focuses on the binding characteristics of the available functional groups (carboxylic acid, hydroxyl and amino groups) on the siloxane backbone. The siloxanes used are not water soluble, so pH effects are less prevalent, and difficult to evaluate. Polymer adsorption upon E-glass fibers is somewhat different to that on silica, primarily because of the small surface area of glass (∼0.12 m2/g) relative to silica (∼141182 m2/g)23 and the mixtures of various oxides inherent to bulk E-glass. The R-amino acid-β-hydroxysiloxanes were all adsorbed onto E-glass fibers, and subsequently characterized by DRIFT, XPS, and SEM to assess adsorption. Solvents were evaluated on the basis of their Hildebrand and Hansen solubility parameters, where toluene [18.2 and 18.2 (cal cm-3)1/2, respectively] and tetrahydrofuran [18.6 and 19.4 (cal cm-3)1/2, respectively] are chosen.24 Hansen solubility parameters, which take into account hydrogen bonding effects, are considered more reliable indicators of solubility, as the amino acid siloxanes used contain strong hydrogen bonding groups. The mechanism by which amino acid functional siloxanes bind or adsorb to glass surfaces is unknown. Prior work carried out by our group on amino-terminated siloxanes19 and R-amino-β-hydroxysiloxanes13 has shown that these siloxanes bind tenaciously to glass surfaces. The R-amino-β-hydroxysiloxanes have no hydrolyzable alkoxy groups, with only two possible sites of binding to the surface hydroxyl groups of glass fibers: the amino and hydroxyl groups. The ability of the hydroxyl group, in a position β to the amino functionality, to bind to the surface is a matter of conjecture. Studies currently in progress in our laboratories have shown that hydroxyterminated siloxanes bind to glass fibers as well as, but no stronger than, their silane counterparts.25 Aminosiloxanes bind strongly to glass fibers, so it appears that the amino groups drive the adsorption and binding of such functionalized siloxanes to glass fibers. The synergistic effects toward strong hydrogen bonding of a hydroxy group β to the amino group cannot be discounted in rationalizing the binding of R-amino acid-β-hydroxysiloxanes to E-glass surfaces. The R-amino acid-β-hydroxysiloxanes used herein, contain amino, hydroxyl, and carboxylic acid groups and should therefore bind readily to inorganic surfaces such as E-glass. To evaluate amino acid siloxane adsorption on glass surfaces, siloxanes were applied to E-glass fibers from toluene solution. Various pendant amino acids, including alanine, glycine, leucine, phenylalanine, and valine, were attached to the siloxane backbones shown in Table 1. Such amino acids were selected to display the differences in size and type of organic R group attached to the siloxane (i.e. aliphatic or aromatic) has on the adsorption process. Utilizing the R,ω-telechelic tetramethyldisiloxane, derivatives of amino acid functional siloxanes (1-4) have been prepared. The glass fiber solutions were allowed to stand for 3 h and decanted and the fibers washed with toluene (2 × 40 mL) to remove any (21) Cohen Stuart, M.; Cosgrove, T.; Vincent, B. Adv. Colloid Interface Sci. 1986, 24, 143. (22) Kawaguchi, M.; Takahashi, A. Adv. Colloid Interface Sci. 1992, 37, 219. (23) Little, L. Infrared Spectra of Adsorbed Species; Academic: London, 1966. (24) Grulke, E. A. In Polymer Handbook; 3rd ed., Brandrup, J.; Immergut, E. H., Eds.; Wiley-Interscience: New York, 1989.
Provatas et al. Table 1. Atomic Concentrations of Elements in E-Glass Fiber element (line)
binding energy (eV)a
fiber concn (%)
concn after etch (%)
C 1s O 1s Si 2p Na 1s Ca 2p Al 2p Cl 2p F 1s
284.6 531.5 102.2 1071.9 347.7 74.0 197.6 684.9
29.1 49.7 12.2 0.9 2.8 3.5 0.6 0.4
10.6 61.2 15.6 1.5 4.1 4.1 0.6 0.4
a Corrected for carbon binding energy of 284.6 eV (i.e. a -3.9 eV correction was applied).
physisorbed species. Finally, the fibers were dried in an oven for 3 h at 110 °C. DRIFT Analysis DRIFT is a highly sensitive, nondestructive infrared technique that can be used to gauge whether organo functional siloxanes adhere to E-glass. DRIFT allows for the qualitative analysis of surface treatments several micrometers thick, a common range for polymers adsorbed onto surfaces, but can only analyze the surface of glass fibers above 1600 cm-1, because of the strong Si-O and B-O absorbances of the glass. Further, DRIFT spectra show broad peaks, a result of the irregular dispersions of the refractive index associated with the adsorption of the various glass components.26 Unlike silica, E-glass fibers exhibit only very broad Si-OH vibrations27 rendering it difficult to monitor the interactions between the polymer and specific surface silanol groups. In a previous paper,13 DRIFT was used to qualitatively show that both nonionic and cationic R-amino-β-hydroxysiloxanes bind to the glass surface. In a similar manner, amino acid functionalized siloxanes have been analyzed by DRIFT, in the region 2500-3100 cm-1 to qualitatively determine their coupling. Since amino functional siloxanes find uses in personal care products, particularly hair conditioners,28 quantitative studies by DRIFT to characterize their coupling to keratin fibers have been previously carried out.29 Adsorption occurs, and amino groups are detected in the infrared spectrum. Similar work, using amino acid functional siloxanes on keratin fibers, remains to be carried out. Keratin fibers are largely proteinaceous,30 and consequently surface forces between amino acid siloxanes and keratin will involve complex protein interactions beyond those occurring on E-glass surfaces. Despite the considerable progress in the development of surface sensitive techniques to accurately characterize surface binding forces, protein surfaces containing secondary and tertiary structures, as found on keratin fibers, are still beyond the scope of such surface techniques. All R-amino acid-β-hydroxysiloxanes show coupling to E-glass by DRIFT analysis. Typical vibrations between 2963 and 2865 cm-1 due to C-H stretching are observed. All functional siloxanes show that the intensity of the (25) Britcher, L. G.; Kehoe, D. C.; Matisons, J. G. Unpublished results. (26) Condrate, R. A., Sr. In Introduction to Glass Science; Pye, L., Stevens, H., La Course, W., Eds.; Plenum: New York, 1972; p 123. (27) Lipp, E.; Smith, A. L. In The Analytical Chemistry of Silicones; Smith, A. L., Ed.; Wiley: New York, 1991; pp 49, 325-333. (28) Berthiaume, M. D.; Merrifield, J. H.; Riccio, D. A. J. Soc. Cosmet. Chem. 1995, 46, 231. (29) Klimisch, H. M.; Kohl, G. S.; Sabourin, J. S. J. Soc. Cosmet. Chem. 1987, 38, 247. (30) Morrison, R. T.; Boyd, R. N. Organic Chemistry; Allyn and Bacon: Boston, MA, 1963.
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Figure 2. DRIFT spectrum of siloxane 3, 1,3-bis(3-leucine-β-hydroxyglycidylpropyl)tetramethyldisiloxane.
C-H stretching vibrations increases as the size of the pendant R group of the amino acid increases in its alkyl content. Further, the phenylalanine derivative shows aromatic C-H stretching vibrations at 3060 cm-1. The secondary amine and hydroxyl functionalities as well as the carboxylic acid hydroxyl group vibrations are all found at approximately 3400 cm-1. A representative DRIFT spectrum of siloxane 3, 1,3bis(3-leucine-β-hydroxyglycidylpropyl)tetramethyldisiloxane is shown in Figure 2. The absorption band at 2650 cm-1 is due to the first overtone of the boron oxide stretching vibration present on the glass surface.31 A subtraction technique is required to remove the background glass fiber absorptions between 2700 and 3500 cm-1. This is done by matching the area of the B-O overtone of each sample, and subtracting the clean E-glass fiber DRIFT spectrum from the DRIFT spectrum of the coated glass fibers. Large vibrations between 2963 and 2865 cm-1 are due to C-H stretching. Experiments on all amino acid functional siloxanes show that the intensity of the these vibrations increase as the R group associated with the amino acid increases in alkyl content. The phenylalanine analogue, also reveals aromatic C-H stretching vibrations at 3060 cm-1. The secondary amine vibration is coincident with the carboxylic acid vibration at 3320 cm-1. Experiments conducted with DRIFT qualitatively show that all R-amino acid-β-hydroxysiloxanes bind to the glass surface and remain adhered to the glass surface after several washing cycles. Coupling is affirmed by the appearance of distinct vibrations after adsorption of the functionalized siloxanes. XPS Analysis The glass surface is a dynamic system capable of altering with changing conditions. Not only is there an accumulation of water on the glass surface and diffusion of water into, or out of the glass (depending on relative humidity) but also alkali ions (from the bulk) and adventitious carbon (from surrounding atmosphere and sample handling) aggregate extensively at the surface. (31) Haaland, D. M. Appl. Spectrosc. 1986, 40, 1152.
Over time, the adventitious carbon can increase and the distribution of surface elements undergo change. Surface concentrations of the glass can also vary from the bulk composition depending on several factors; thermal history of the glass, relative humidity, surface diffusion, and surface treatment to which the glass has been subjected to during melting and cooling processes.32 The fibers used were passed through pure water as they came from the furnace. No special cleaning treatment after manufacture or prior to analysis was applied. The thermal history of the glass fibers was not disclosed. XPS was first applied to the study of glass fibers to examine not only the effect of chemical reagents on surfaces but also the role organic functional groups can play on the surface, as well as providing a measure of surface vs bulk concentrations.33 All XPS samples were subjected to a 5 min argon ion etch after initial XPS analysis. The sputter rate of this 5 min argon ion etch has been previously calibrated as sufficient to remove a 5 nm Ta2O5 film34 (not 50 nm as reported previously35) upon a tantalum metal surface (see Table 1). Sputtering greatly reduces the amount of adventitious carbon and water multilayers on the surface, together with a thin film of Si(OH)x which forms on the surface. The amount of calcium, sodium, and aluminum all increase as more of the glass-silicate is exposed after etching. The carbon content of a clean glass fiber, is adjusted to a binding energy of 284.6 eV, to correct for surface charging effects. The silicon binding energy of the clean glass fiber is 102.2 eV, which is less than that for pure quartz (SiO2) at 103.7 eV, and the decrease results from the sodium, calcium, and aluminum oxide components in glass.36 Furthermore, the presence of moisture also lowers the silicon binding energy through the formation (32) Rosington, D. In Introduction to Glass Science; Pye, L., Stevens, H., La Course, W., Eds.; Plenum: New York, 1972; p 101. (33) Nichols, G. D.; Hercules, D. M. Appl. Spectrosc. 1974, 28, 219. (34) Smart, R. St. C. University of South Australia, personal communication, 1996. (35) Wagner, C. D.; Passoja, D. E.; Hillery, H. F.; Kinsky, T. G.; Six, H. A.; Jansen, W. T.; Taylor, J. A. J. Vac. Sci. Technol. 1982, 21, 933. (36) Lam, D. J.; Paulikas, A. P.; Veal, B. W. J. Noncryst. Solids 1980, 42, 41.
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Table 2. Atomic Concentrations of Amino Acid Functional Siloxanes Coated (1-5) on E-Glass Fibers atomic concn (%) siloxane no.
C 1s
O 1s
Si 2p
N 1s
Na 1s
Ca 2p
Al 2p
Cl 1s
B 1s
1 (%) after etching 2 (%) after etching 3 (%) after etching 4 (%) after etching 5 (%) after etching
25.7 5.1 30.1 8.0 32.6 5.4 46.4 16.2 31.7 6.5
45.1 58.2 41.7 57.3 41.1 58.2 30.5 50.3 41.8 57.6
17.0 20.8 14.9 19.4 14.5 19.9 19.4 18.4 14.0 19.3
3.5 1.9 5.8 2.8 3.0 1.6 5.4 2.1 4.8 2.4
0.7 0.8 0.5 0.8 0.8 1.6 2.9 1.9 0.7 1.8
1.4 3.4 1.9 3.9 1.6 4.6 1.7 3.6 0.8 1.4
2.6 4.3 3.2 5.2 3.2 2.3 2.4 4.9 3.5 5.7
0.4 0.3 0.3 0.1 0.5 0.4 0.8 0.4 0.6 0.3
1.4 2.4 1.3 2.3 1.4 2.3 0.9 1.6 1.5
of a thin film of Si(OH)x on the surface. After etching, the binding energy increases to 102.9 eV, which is consistent with the removal of the thin Si(OH)x film. The XPS atomic concentrations for the amino acid functionalized siloxanes (1 to 5) are tabulated in Table 2 and reveal several general trends. Not surprisingly, the siloxane coating on the E-glass fibers diminishes as etching takes place, and the silicon binding energy concomitantly increases from 102.2 to 102.9 eV (note that the silicon binding energy of 102.2 eV does not by itself distinguish between a hydrated silicate surface and a siloxane bound at the surface). An evaluation of all other atomic concentrations helps to establish whether siloxane coupling occurs. Boron concentrations are included, even though previous experiments on R-amino-β-hydroxysiloxanes to E-glass14 show no appreciable binding to boron. The glass elements; calcium, sodium, and aluminum, all increase after etching, consistent with the hypothesis that the R,ω-telechelic amino acid polymers form a thin layer on the E-glass. Both the aluminum and sodium concentrations increase with etching, though sodium is known to leach from the surface during washing37 and aluminum concentrations can be reduced by preferential silane deposition onto Al-OH surface sites.38 As a result, the calcium 2p line has now emerged as a commonly used XPS indicator for glass fiber coverage.39 Calcium’s low binding energy (351 and 347 eV) and high sensitivity factor (1.58) means that the calcium 2p photoelectrons are not as severely attenuated by the siloxane coating, as the other glass elements are. For example, the calcium concentration on adsorption of siloxane 1 is 1.4% (after etching 3.4%); whereas sodium concentrations are 0.7% before and 0.8% after etching and aluminum concentrations are consistently high (from 2.6 to 4.3%). Finally, all amino acid functional siloxanes analyzed reveal a carbon 1s multiplex spectra with hydrocarbon peaks (284.6 eV), carbon bound to a single oxygen (286.5 eV), and carbon bound to two oxygen atoms (289.2 eV). The latter peak arises from the free carboxylic acid group and has very weak intensity compared to the C-O and C-H peaks.40 The nitrogen becomes charged when a proton is transferred from the proximal carboxylic acid group to the nitrogen binding site. Proton transfer is facilitated by surface moisture which ensures a strong hydrogen bonding environment. Poly(dimethylsiloxane)s, physisorbed onto glass surfaces, are readily removed by washing with excess solvent, in agreement with our earlier observations on the adsorption of various functionalized siloxanes to glass fiber surfaces.19 (37) Mohai, M.; Berto´ti, I.; Re´ve´sz, M. Surf. Interface Anal. 1990, 15, 364. (38) Wesson, S. P.; Jen, J. S.; Nishioka, G. N. J. Adhes. Sci. Technol. 1992, 5, 843. (39) Pantano, C. G.; Wittberg, T. N. Surf. Interface Anal. 1990, 15, 498. (40) Ratner, B. D.; Leach-Scampavia, D.; Castner, D. G. Biomaterials 1993, 14, 148.
Nitrogen was also detected in all samples analyzed, indicating that the amino group alone or in conjunction with the carboxylic acid group, is binding to the E-glass surface. Even after the 5-min etch, the nitrogen content remains significant, suggesting that binding involves the secondary amine group of the amino acid siloxane. Nitrogen concentrations are typically greater than 3.0%, higher than that for the corresponding R-amino-βhydroxysiloxanes.13 An examination of the nitrogen XPS multiplex spectrum reveals two nitrogen species present, a peak ascribed to NH species (399.9 eV, after charge correction) and a protonated species, NH2+ (402.0 eV, after charge correction). To determine the relative amounts of the two nitrogen peaks, a peak stripping routine, involving the use of asymmetrical Gaussian-Lorentzian peak shapes41 was applied to the nitrogen signal of each amino acid siloxane polymer (see Figure 3). In general, the two peaks were found to have an averaged ratio of one NH to one NH2+ species. The formation of the NH2+ species is likely to be due to the zwitterionic nature of the amino acid, where the secondary amino group is in close proximity to the free carboxylic acid group. The R-amino-β-hydroxysiloxanes also show two nitrogen species when analyzed by XPS, similar to that described above for the amino acid siloxanes. Both amino acid and R-amino-β-hydroxysiloxanes have N 1s binding energies consistent with secondary amines,42 perhaps indicating that the amine retains its chemical integrity while adsorbing to the glass fibers. Bonding is likely taking place through the lone electron pair on the nitrogen atom. The affinity of amines for silica surfaces has been previously described by Knozinger,43 and the XPS evidence here confirms the amino binding group as one mode for the adsorption of amino acid siloxanes to glass surfaces. The presence of multiple water layers on the E-glass fiber surfaces promotes the leaching of glass elements in nonpolar solvents, such as toluene. Such water multilayers will, upon being placed into dry organic solvents, interact with the solvent, resulting in a loss of water from the glass fiber surface. The amount of surface water removed by the solvent, depends on the water-solvent interaction. The minor glass elements can also leach into the solvent, with the water. The degree and selectivity of such elemental leaching affects the glass surface potential and isoelectric point. Consequently, solely relying upon atomic concentrations to interpret the glass surface chemistry in the presence of siloxanes on glass surfaces is inadvisable, as elemental leaching can occur. Other techniques can be used to help understand the XPS trends, such as SEM and DRIFT analyses. (41) Desimoni, E.; Casella, G. I.; Cataldi, T. R.; Malitesta, C. Electron Spectrosc. Relat. Phenom. 1989, 49, 247. (42) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; John Wiley: New York, 1983. (43) Knozinger, H. In The Hydrogen Bond; Schuster, P., Zundel, B., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976.
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Figure 3. Peak stripping experiment applied to nitrogen multiplex of siloxane 1.
Further XPS investigations on pure silica, an acidic substrate devoid of elements such as aluminum and boron, helps to gauge the affinity of the R-amino acid-βhydroxysiloxanes for acidic surfaces by simply using nitrogen concentration. XPS analysis of siloxane 4 (phenylalanine derivative) reveals a 4.3% nitrogen concentration on pure silica, slightly lower than that for the same siloxane adsorbed onto E-glass fibers (N ) 5.4%). A consistently high amount of surface nitrogen still appears on the silica surface, indicating that amino groups effectively attach to acidic Si-OH sites. A comparison of the N/Si ratios (N/Si ratios ) 0.2%; after etching 0.1% for pure silica; N/Si ratios ) 0.3%, after etching 0.1% for E-glass), indicates a similar affinity of amino binding sites for silicon surface hydroxyl sites. The R,ω-telechelic amino acid functional siloxanes bind better to titanium dioxide than to silica, in keeping with the well-known reactivity of carboxylic acid groups to titania surfaces.44 XPS analysis of siloxane 4 (phenylalanine derivative) reveals a nitrogen concentration of 4.5% on titanium dioxide. Once again, this nitrogen value is slightly lower than that of the identical siloxane adsorbed onto E-glass fibers (N ) 5.4%) but higher than its corresponding equivalent on silica (N ) 4.3%). Such a result implies that these siloxanes with amino, hydroxyl, and carboxylic acid groups are capable of binding to most surfaces, whether such surfaces are basic, acidic, or neutral (i.e., TiO2, SiO2, and glass, irrespectively). The N/Si ratios are 0.3%, after etching 0.1% on titanium dioxide, and N/Si ratios are 0.3%, after etching 0.2% on E-glass. The binding of R-amino acid-β-hydroxysiloxanes to glass fibers is appreciable (as determined by both XPS and DRIFT). The majority of the R-amino acid-βhydroxysiloxanes display, by SEM, interesting surface morphology, as surface bubbles of varying dimensions appear after siloxane adsorption. This correlates well with the XPS and DRIFT results for such samples, which indicates substantial siloxane coverage onto the glass surfaces. After being washed with successive solvents (toluene and tetrahydrofuran), all five of the siloxane derivatives (1-5) display large polymeric patches on the glass fibers (patch size of no less than 0.4 µm; see Figure (44) Parfitt, G. D. Prog. Surf. Membr. Sci. 1976, 11, 181.
Figure 4. SEM micrograph of siloxane 1, 1,3-bis(3-alanineβ-hydroxyglycidylpropyl)tetramethyldisiloxane.
4). The average diameter of the treated fibers is 13.6 µm (ranging from 13.9 to 12.9 µm), while the untreated glass fibers have an average diameter of 12.4 µm. On the basis of XPS results, where high nitrogen concentrations are found, it would appear that the SEM patches form on top of a siloxane monolayer rather than forming patches on the bare glass surface. The chemical interactions between the siloxane and the glass surface will always be slower than the initial physisorption of the reagent upon the surface. The principal barriers to interaction between R-amino acidβ-hydroxysiloxanes and glass surface silanol groups are the water multilayers residing on the E-glass surface. The R-amino-β-hydroxysiloxanes can strongly hydrogen bond through the amino and hydroxyl groups either to glass surface hydroxyl groups or to surface moisture. The latter form of siloxane attachment can be readily reversed by washing treated surfaces with solvents having similar Hildebrand or Hansen solubility parameters to the adsorbed siloxanes (such as tetrahydrofuran). The relative adsorption of functionalized siloxanes (15) to glass fibers is revealed by plotting the N/Si and N/Ca XPS ratios as a function of amine type (for the various amino acid groups). Duplicate samples were analyzed,
1662 Langmuir, Vol. 14, No. 7, 1998
Provatas et al.
Figure 7. Proposed binding mechanism for carboxylate groups to inorganic oxide surfaces.
Figure 5. Graph of N/Si ratio vs amino acids having R,ωtetramethyldisiloxane as backbone: 4 ) amino acid siloxanes before etching; O ) amino acid siloxanes after etching. Figure 8. Proposed molecular orientation and coordination of R,ω-telechelic amino acid siloxanes to E-glass surface.
The higher binding adsorption properties of the R-amino acid-β-hydroxysiloxanes would modify the series as follows:
R-amino acid-β-hydroxysiloxanes > R-amino-β-hydroxysiloxanes > methacrylsiloxanes > organofunctional silanes > epoxysiloxane
Figure 6. Graph of N/Ca ratio vs amino acids having R,ωtetramethyldisiloxane as backbone: 4 ) amino acid siloxanes before etching; O ) amino acid siloxanes after etching.
with reproducibility between samples quite high. The generally accepted precision of XPS atomic concentrations is (10% and applies to individual measurements, although in this case, a systematic series of adsorbate-adsorbent systems were studied under the same conditions of XPS measurement. We have therefore placed a reasonable estimate of (10% on each ratio as shown in Figures 5 and 6. The errors do not change the conclusion relating to the trend in N/Ca and N/Si ratios. In Figures 5 and 6, all the siloxanes have higher N/Si and N/Ca ratios prior to etching, irrespective of amino acid. Essentially then, the R-amino acid-β-hydroxysiloxanes can be considered as effective coupling agents for E-glass surfaces. The N/Si and N/Ca ratios show there is appreciable siloxane coverage on the surface. Where the N/Si ratio does not change appreciably on etching, substantial layers of functionalized siloxane must cover the surface. Furthermore, note that the N/Ca XPS ratio is a more reliable indicator when determining siloxane surface coverage on silicate mineral substrates. In our previous paper,13 the coupling effectiveness of R-amino-β-hydroxysiloxanes to E-glass fibers was assessed and follows the series (using both N/Si and N/Ca XPS ratios)
R-amino-β-hydroxysiloxanes > methacrylsiloxanes > organofunctional silanes > epoxysiloxane
Preliminary measurements with higher molecular weight siloxanes confirm the adsorption properties listed. Does the carboxylic acid group play a role in binding the functionalized siloxanes to glass surfaces? A variety of surface species may be formed through different coordination modes of carboxylate groups with the glass surface. On the basis of X-ray diffraction and infrared methods,45 a mechanism for carboxylate coordination to oxide surfaces has been proposed by Low et al.46 (see Figure 7). The unidentate structure III (Figure 7) is consistent with observed intensity ratios in the elemental XPS analysis (DRIFT spectra analyses were difficult to resolve experimentally). Low et al.46 established that asymmetric CO2- stretching vibrations have higher intensities when bidentate (I) or bridging (II) binding to surfaces occurs. Recent studies monitoring the adsorption of formic acid on oxygen covered platinum47 and copper48 surfaces substantiate that unidentate (III) binding occurs. A partially developed, surface ester bond forms when oleic acid is chemisorbed onto a Cu/Cu2O surface. Such results, together with our XPS and DRIFT findings, indicate the molecular orientation and coordination of amino acid siloxanes onto glass surfaces can occur by any of the possible conformations indicated in Figure 8 (where the R group corresponds to the amino acid side chain). The dynamic nature of the glass surface elements, as well as the large moisture content residing on the glass surface will both affect the binding. Such binding can occur either through hydrogen bonds (bond strengths of ∼10 to 40 kJ/ mol) or weaker van der Waals forces (∼1 kJ/mol).49 (45) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, Wiley: New York, 1963. (46) Low, M. J. D.; Brown, K. H.; Inoue, H. J. Colloid Interface Sci. 1967, 24, 252. (47) Avery, N. R. Appl. Surf. Sci. 1982, 14, 149. (48) Wadayama, T.; Hanta, Y.; Sue¨taka, W. Surf. Sci. 1985, 158, 579. (49) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Dekker: New York, 1974.
R-Amino Acid-β-Hydroxysiloxanes
Conclusions Combined together, XPS, DRIFT, and SEM are powerful analytical techniques for characterizing the adsorption processes of siloxane polymers bearing diverse functional groups upon E-glass fibers. Such characterization processes are essential to understanding the interfacial chemistry of commercial reactions, specifically in the composites industry, where E-glass fibers are used as reinforcing materials. The use of DRIFT to reveal siloxane adsorption after substrate subtraction is facile and straightforward. XPS studies quantify adsorbed siloxane layers, as well as revealing the identity of different chemical species on the glass surface. Telechelic siloxanes containing amino, hydroxy, and carboxylic acid functionalities show high levels of surface coating on E-glass fibers. The extent of siloxane binding can be monitored by comparing N/Si and N/Ca XPS ratios for the different amino acid derivatives. The N/Ca XPS ratio can be
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considered a good surface coating indicator of amino groups on glass. The binding and adsorbed layer thickness for these amino acid functional siloxanes is slightly better than that for the R-amino-β-hydroxysiloxanes on E-glass, suggesting that there are only minor adhesive differences between the amino acid and amino functionalities on siloxanes. Furthermore, strength of binding can be examined by washing the surface with solvents containing Hildebrand or Hansen solubility parameters similar to the adsorbed polymer, where such washing effectively removes physisorbed or weakly adsorbed siloxanes. Acknowledgment. The authors are grateful to the Australian Research Council for support (to A.P.) of a postgraduate award and to ACI Fiberglass for supply of glass fiber. The authors would also like to acknowledge Dr. P. Arora for XPS measurements and analysis. LA970580M
