Langmuir 1995,11, 2009-2017
2009
Aminohydroxysiloxanes on E-Glass Fibers Pawittar S. Arora, Janis G. Matisons,* Arthur Provatas, and Roger St. C. Smart Polymer Science Group, School of Chemical Technology, University of South Australia, The Levels, S A 5095, Australia Received November 14, 1994. I n Final Form: February 21, 1995@ Nonionic and cationic aminohydroxysiloxanes 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) and XPS (X-ray photoelectron spectroscopy) techniques allowed for the qualititative and quantitative determination of such surface processes. DRIFT analysis indicated that coupling between the glass and polymer occurred. XPS experiments confirmed that the amino group was binding to the surface, evidenced by the the formation of two amino states, a covalent nonprotonated and an ionic protonated form. Our studies clearly indicate that the nonionic aminohydroxysiloxanes bind more effectively to the glass surface than their cationic counterparts.
Introduction The need for coupling agents to combat the deleterious effects of water on the mechanical properties of glass fiber composites has been extensively documented.1!2 Coupling agents have been used for several decades and are generally applied in glass fiber reinforced polymers to provide a water resistant bond between the polymer and the glass f i b e ~ - ,thereby ~ - ~ enhancing the overall composite performance. The most common coupling agents used today, the silane^,^ readily adhere to both the inorganic glass fiber or filler surfaces and the organic polymer matrix, providing a water resistant interfacial bond. The nature of the silane chemical structure to the fiber or filler surface a t the interface and its bonding mechanism is still a subject of considerable debate.8 Of all the silanes investigated, aminosilanes have not only provided a focus for such debates, but have also been the subject of extensive spectroscopicand surface-property investigations probing their structure a t solid-liquid interfaces. Aminofunctional silanes such as y-aminopropyltriethoxysilane, are commonly used as coupling agents to enhance the durability of glass fiber reinforced composi t e ~ The . ~ exact mechanism is unclear. It is thought that the hydrolyzable ethoxy groups of y-aminopropyltriethoxysilane condense with the surface silanols, releasing ethanol and leaving the amino end group free. The free amino group is then capable of binding to the polymeric resins ofthe composite. However, amines are also known to bind to surface s i l a n o l ~ , via ~ J ~the formation of very strong hydrogen bonds. Fowkes et al.ll has found that y-aminopropyltriethoxysilaneadsorbs onto glass surfaces through the amino groups, as evidenced by the formation Abstract published in Advance A C S Abstracts, May 1, 1995. (l)Atkins, A. G. J. Mater. Sci. 1975,10,819. (2) Outwater, J. 0. J. Adhes. 1970,2,242. (3) Holister, G. S.; Thomas, C. Fiber Reinforcement of Materials; Elsevier: New York, 1966. (4) Manson, J. A.; Sperling, L. H. Polymer Blends and Composites; Plenum: New York, 1976. ( 5 ) Chen, R.; Boerio, F. J . J. Adhes. Sci. Technol. 1990, 4, 453. (6) Walker, P. J . Adhes. Sci. Technol. 1991,5,279. (7) Plueddemann, E. P. Silane CouplingAgents;Plenum: New York, 1982; Chapter 1. (8) Plueddemann, E. P. InInterfaces in Polymer, Ceramic and Metal Matrix Composites; Ishida, H., Ed.; Elsevier: New York, 1988;pp 1734. (9) Child, M. J.; Heywood, M. J.; Pulton, S. K.; Vicary, L. A.; Yong, G. H.; Rochester, C. H. J. Colloid Interface Sci. 1982,89,203. (10)Zhdanov, S. P.; Koshleva, L. S.;Titova, T. I. Langmuir 1987,3, 960. (11)Fowkes, F. M.; Dwight, D. W.; Cole, D. A.; Huang, T. C. J. NonCryst. Solids 1990,120,47. @
0743-746319512411-2009$09.00/0
of two amino species, a free non protonated and a protonated amino species. Use of angle-resolved X-ray photoelectron spectroscopy allowed Fowkes et al.ll to show that protonated amino groups were located close to the surface with the free amino groups interacting with the free surface. Similar results have now been observed on inorganic oxides using X P S 2 and solid state NMR,13 as well as on metal surfaces.14 We wish to present in this paper our findings that amino functional siloxanes, which do not contain any alkoxy groups, can bind effectively to surfaces. Our research has focussed on siloxanes, which, like silanes, are strongly water resistant polymers capable of binding tenaciously to a variety of surfaces, including glass.15-18 Siloxanes exhibit remarkable backbone flexibility19g20 enabling them to be used as surfactants as well as coupling agents linking polymers to inorganic surfaces. Such backbone flexibility, results from both the electronic and structural properties of the Si-0 and Si-C bonds, which permit unhindered rotation about the siloxane backbone.lg The freedom of rotation gives ideal screening for the polar Si-0-Si backbone, by the non polar methyl groups, thereby giving the polymer excellent film forming properties. As a result, siloxanes have very low surface tensions usually between 20 and 25 mN m-l, which promote their use as surfactants in personal care p r o d u ~ t s and ~ ~ -in~ the ~ textile i n d ~ s t r y . ~ ~ - ~ ~ (12) Kallury, K. M. R.; Macdonald, P. M.; Thompson, M. Langmuir 1994,10,492. (13) Caravajal, G. S.;Leyden, D. E.; Maciel, G. E. In Silanes, Surfaces, and Interfaces: Leyden, D. E., Ed.: Gordon and Breach: New York, 1986; p 283. (14) Homer. M. R.: Boerio. F. J.: Clearfield. H. M. J. Adhes. Sci. Technol. 1992,’6 , 1. (15)Bennett, D. R.; Matisons, J. G.; Netting, A. K. 0.;Smart, R. St. C.; Swincer, A. G. Polym. Int. 1992,27,147. (16) Britcher, L. G.; Kehoe, D. C.; Matisons, J. G.; Smart, R. St. C.; Swincer, A. G. Langmuir 1993,9,1609. (17) Cosgrove,T.; Prestidge, C. A.;Vincent, B. J. Chem. SOC.Faraday Trans. 1990,86,1377. (18)Auroy, P.; Auvray, L.; LBger, L. J. Colloid Interface Sci. 1992, 150,187. (19) Owen, M. J. Ind. Eng. Chem. Prod. Res. Deu. 1980,19,97. (20)Kendrick, T. C.; Parbhoo, B.; White, J. W. In The Silicon Heteroatom Bond; Patai, S., Rappoport, Z., Eds.;John Wiley: New York, 1991; Chapter 3. (21) Chandra, G.; Kohl, G. S.; Tassoff, J. A. US Patent 4 559 227, 198.5.
(22) Cornwall, S. M.; Homan, G. R. US Patent 4 586 518, 1986. (23) Fridd, R.; Taylor, R. M. GB Patent 2 186 889, 1987. (24) Cook, J. R. J. Text. Inst. 1984,75,191. (25) Joyner, M. M. Text. Chem. Color. 1986,18,34.
0 1995 American Chemical Society
2010 Langmuir, Vol. 11, No. 6, 1995
Arora et al.
Table 1. Aminohydroxysiloxanes Used to Treat E-Glass Fibers"
Amino-hydroxy siloxane CH3
I
I
0-Si-
CH3
(?H2)3
0
0
CH2
CH2
I
I
I
OH- CH
I
CH2
CH2
NH
NH
R
R
I
I I
0
Methyl
1
Diethyl
2
Hexyl
3
Benzyl
4
Anilino
5
Dic yclohexyl
6
Hexyl
7
CH-OH
I
CH3
I
(FHA3
I
CH3-Si- I
Number *
CH3
I
CH3-Si-
R
I
1:-
O l S iI - CH3
(CHA3
CH3
0
I
CH2
I
CH -OH
I
CH2
I
NH
I
R
0
I
CH -OH
I
CH2
I
NH
I
R a
Polymers 9-16 are cationic derivatives of compounds 1-8, respectively, and are not listed above.
Organofunctional siloxanes also have the latent ability to function as effective coupling agents, if the correct selection of two pendant functional groups is made. In this paper our research examines the binding properties of aminohydroxysiloxanesto glass fibers. Aminohydroxy functional siloxanes (Table 1)are prepared by a threestep process, initially involving attachment of an epoxy (26)Makino, S.;Moriga, H. Jap. Patent 3 294 522,1991;Chem.Abstr. 116.20. 196181r. (27)Nomura, K.;Tsurumi, H. Jap. Patent 2 045 789,1987;Chem. Abstr. 106,26,2154732.
group to a siloxane backbone by hydrosilylation, followed by the attachment of an amine group via ring opening of the epoxy group. The final step involves the formation of a cationic amino species via the addition of a haloacid, in this case, hydrogen bromide.2s This new class of aminosiloxanes offer dual functionality containing both an amino and a hydroxyl group pendant to the siloxane backbone. Such aminohydroxysiloxanes are now being investigated as coupling agents in composite materials. (28)Matisons, J . G.;Provatas, A. Macromolecules 1994,27, 3397.
Aminohydroxysiloxanes on E-Glass Fibers In an attempt to gain an understanding of the bonding mechanism of our nonionic and cationic aminohydroxysiloxanes on glass fibers, both types of surfactant were coupled onto E-glass and studied by two surface analytical techniques (diffusereflectance infrared Fourier transform spectroscopy, DRIFT, and X-ray photoelectron spectroscopy, XPS). Early attempts at detecting the presence of polymers adsorbed on glass fibers were restricted largely to infrared methodsz9that 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 spec i e ~ Conventional .~~ infrared techniques like transmission IR and ATR (attenuated total reflectance) exhibit a high proportion of scattering, when used for the analysis of polymers on glass fibers. X P S 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 surf a c e ~ .Combination ~~ of both techniques has allowed polymer chemists to gain a better insight of polymersurface interfaces. The pairing of DRIFT and XPS techniques has now allowed for ready characterization of siloxane polymers on glass ~ u r f a c e s . ~DRIFT ~ J ~ was used as an initial qualititative check to determine if adsorption between the aminohydroxysiloxanes and the glass fibers had occurred. The glass fibers were then analyzed by X P S to determine the elemental composition of the aminohydroxy siloxanes on the glass and their surface coverage. In addition, scanning electron microscopy, SEM studies have commenced to establish the surface morphology of the siloxanes on the glass fibers.
Experimental Section The aminohydroxysiloxanes used in the coupling studies were prepared as outlined previouslyz8 and are shown in Table 1. E-glass fibers (water washed only) were supplied by ACI Fiberglass (Australia) using standard production methods a s outlined in the 1iteratu1-e.~~ 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%SiOz; 21.5% CaO; 14.5%&03; 6.0%B&; 0.8% NazO; 0.6%MgO; 0.4% Fez03; 0.3% TiOz; 0.6% Fz; 0.3% FOz; 0.1% KzO. All reagents were of at least laboratory grade. Toluene and tetrahydrofuran (ACE Chemicals) were dried by distilling (bp 110 and 67 "C, respectively)over sodium wire with benzophenone. Surface coating was performed by immersing the glass fibers (1g) in dry, distilled toluene containing (80 mL) 0.8-1.5% (wt/ wt) of the appropriate aminohydroxysiloxane (the actual percentage was varied in accordance with the number of functional groups the siloxane possesses). The solutions were allowed to stand a t room temperature for 3 h, decanted from the glass fiber, and the wet fibers were washed with toluene (2 x 40 mL). The fibers were then left t o dry in the oven a t 110 "C for 3 h. The immiscibility of the cationic aminosiloxanes with toluene was overcome by the addition of 80 mL of dry distilled tetrahydrofuran to the toluene. The XPS data were analysed using a Perkin-Elmer PHI 5100 XPS system with a concentric hemispherical analyser and a Mg Ka X-ray source functioning a t 300 W,15 kV, and 20 mA. High vacuum pressure achieved during analysis varied between lo-* (29) Hair, M. L.Infrared Spectroscopy in Surface Chemistry;Marcel Dekker: New York, 1967; p 315. (30) Culler, S. R.;McKenzie, M. T.; Fina, L. J.; Ishida, H.; Koenig, J. L.Appl. Spectrosc. 1984,38, 791. (31) Pekala, R. W.; Merrill, E. W. J. Colloid Interface Sci. 1984,101, 120. (32) Doyle, P.J. GlassMaking Today;Pbrtcullis: Clarendon, England, 1979.
Langmuir, Vol. 11, No. 6, 1995 2011 to Torr. The angle between the X-ray source and analyzer was fixed a t 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 single-beam BIORAD Model FTS 65 spectrometer in the wavenumber region 4000400 cm-l, at a resolution of 4 cm-l, with the use of a MCT liquidcooled detector. Samples were mounted on a Spectra Tech diffuse reflectance apparatus and scanned 256 times. The fibers were placed at an angle of 90" with respect t o the IR beam (for maximum signal to noise ratio), in a specially constructed sample holder designed for the analysis of glass fibers.
Results and Discussion The mechanism by which aminosiloxanes couple to glass remains to be determined, as no hydrolyzable groups are present to promote coupling to surfaces. Prior work carried out by our group15on amino-terminated siloxanes, has shown that these siloxanes bind tenaciously to glass surfaces. Since there is only one possible site of binding, the amino group, it was inferred that the amino group does indeed react with the surface silanols. Jalbert et al.33 utilized XPS to characterize surface segregation effects in amino-terminated siloxanes. In all cases the amino groups were depleted from the vacuum- polymer interface because of their relatively high surface energy. Such results suggest that aminosiloxanes attach to surfaces by a similar mechanism to that for aminosilanes." It is important to note that the aminohydroxysiloxanes discussed above have no hydrolyzable alkoxy groups, with only two possible sites of binding to the surface hydroxyl groups of glass fibers: the pendant amino and hydroxyl groups. The ability of the hydroxyl group, in a position 5, to the amino functionality, to bind to the surface is a matter of conjecture. Studies currently in progress in our laboratories have shown that hydroxy-terminated siloxanes bind to glass fibers as well as, but no stronger than, their silane counterpart^.^^ In order to evaluate aminosiloxane adsorption on glass surfaces, both nonionic and cationic aminosiloxanes were applied to E-glass fibers from their respective solvents, to examine the effects of varying the charge associated with the amino functionality in binding to the glass surface. Various amines, including methylamine, diethylamine, hexylamine, benzylamine, aniline, and dicyclohexylamine were attached to the siloxane backbones shown in Table 1.28These amines were chosen to display selectivity in their functionality (i.e.,primary, secondary),size and type of organic group attached to the amine (i.e., aliphatic or aromatic). In this paper, only 1,3-bis(3-glycidoxypropyl) tetramethyldisiloxane analogues of nonionic siloxanes 1, 2,3,4,5, and 6 and cationic siloxanes 9,10,11,12,13, and 14 are considered. In addition, both nonionic and cationic compounds of hexylamine coupled to poly(g1ycidoxypropylmethylsiloxane), dp = 33,and poly[(glycidoxypropylmethy1)-co-(dimethyl)lsiloxane,dp = 23, (7,8,15, and 16 respectively) were included to evaluate the effects of both the siloxane chain length and the frequency of pendant amino groups on siloxane adsorption to glass fibers. Nonionic aminosiloxanes were applied in toluene solution, while the cationic siloxanes were applied to glass fibers in tetrahydrofuran solution. The glass fiber solutions were allowed to stand for 3 h, decanted and the fibers washed with their respective solvents (2 x 40 mL) to (33)Jalbert, C. J.; Koberstein, J. T.; Balaji, R.; Bhatia, Q.; Salvati, L., Jr.;Yilgor, I. Macromolecules 1994,27,2409. (34) Britcher, L. G.;Kehoe, D. C.; Matisons, J. G.; Swincer, A. G. Unpublished results.
Arora et al.
2012 Langmuir, Vol. 11, No. 6, 1995
I
I
I
I
I
I
35@0
3080
2500
2000
1500
1600
Wavenumbe r
s
Figure 1. DRIFT spectrum of siloxane 2 on E-glass fibers. Also shown is a clean E-glass fiber spectrum (bottom). remove any physisorbed species. Finally, the fibers were Subtraction DRIFT spectra of the nonionic aminohythen left to dry in an oven for 3 h at 110 “C. droxysiloxanes reveal many interesting details. The simplest of these aminohydroxysiloxanes, the methyDRIFT Analysis. The diffuse reflectance technique lamino derivative l, displays a large asymmetric methyl was developed many years ago;35however, it has only been in the past 15-20 years that it has been developed band at 2963 cm-l and a weak methylene band at 2935 for the analysis of polymers on surfaces.36DRIFT is a cm-l. The diethylaminohydroxysiloxane 2 likewise exhighly sensitive, nondestructive technique that is used to hibits asymmetric stretching vibrations of the methyl and gauge whether siloxane adsorption has occurred on the methylene groups at 2963 and 2935 cm-l, respectively. glass fiber surface. DRIFT allows for the qualitative Experiments show that the intensity of these groups analysis of surface treatments several microns thick, a increases with larger alkyl chains. The hexylamino and common range for polymers adsorbed onto surfaces. While dicyclohexylamine derivatives 3 and 6, not only have DRIFT is a surface sensitive technique, the ability of the similar asymmetric C-H stretching vibrations of methyl DRIFT technique to analyze the surface of glass fibers is and methylene groups at 2963 and 2935 cm-l respectively, difficult below 1600 cm-l, due to the strong Si-0 and but also have methyl and methylene symmetric vibrations B-0 absorbances of the glass.37 Further, the DRIFT a t 2871 cm-l. Furthermore, the dicyclohexylamine despectra of glass generally exhibit broad peaks; a result of rivative 6, has the most intense methylene vibration at the irregular dispersions of the refractive index associated 2935 cm-l, arising from its four cyclohexyl groups. The with the adsorption ofthe various components of the glass. benzylamine and aniline derivatives 4 and 5, also show A typical DRIFT spectrum is shown in Figure 1where characteristic weak, aromatic C-H stretching vibrations adsoprtion of the siloxane to the glass fiber is indicated a t 3060 and 3033 cm-’. by the appearance of an enhanced broad band between Figure 2 displays the subtracted spectra of nonionic 3200 and 3400 cm-l due to the amine and alcohol groups aminohydroxysiloxanes 3,7,and 8 having the same amino of the siloxane. The clean E-glass fiber spectrum is shown substituent (hexylamine),but with three different siloxane in Figure 1 (bottom) and clearly highlights whether backbones (see Table 1). The very intense asymmetric adsorption has occured after solvent washing. This is methyl band at 2967 cm-’ for siloxane 8 results from the indicated by the appearance of asymmetric and symmetric higher proportion of methyl groups present in the backbone C-H stretching vibrations of the hydrocarbon groups of siloxane 8. In Figure 2 the small band at 3019 cm-’ attached to the siloxane backbone at 2963 and 2906 cm-l for siloxanes 3 and 7 is due t o residual toluene solvent respectively. The absorption band at 2671 cm-l arise from used in the washing process. the first overtone of the B-0 stretchingvibration present The cationic derivatives do not show marked spectral on the glass surface.3s Asubtraction technique to remove deviations in the infrared from their aminohydroxysiloxthe background glass fiber is needed to examine the ane counterparts. Generally, for all cationic siloxanes spectral region between 3100 and 2700 cm-’ in detail. analyzed several “salt”deformation stretches 2750-2421 The technique involves subtraction of the clean glass fiber cm-l were detected. Such “salt” deformation stretches DRIFT spectrum from the DRIFT spectrum of the coated are typical of cationic amines.39 Further, the appearance glass fibers by the area and intensity of the B-0 overtone of very broad OH and NH2+ bands around 3400 cm-l is of each sample. consistent with the spectral characteristics associated with (35) Kubelka, P.; M u d , F. Z.Tech. Phys. 1931, 13, 593. cationic organic ammonium groups.39 (36) Ishida, H.; Chiang, C.; Koenig, J. L.Polymer 1982,23, 251. ~~
(37) Condrate, R. A., Sr. InIntroduction to Glass Science; Pye, L. D., Stevens, H. J.,La Course, W. C.,Eds.; Plenum: New York, 1972;p 101. (38) Haaland, D. M. Appl. Spectrosc. 1986,40, 1152.
(39)Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic Chemistry; 2nd ed.; McGraw-Hill: New York, 1973; Chapter 3.
Aminohydroxysiloxanes on E-Glass Fibers
Langmuir, Vol. 11, No. 6, 1995 2013
0.20-1
I
r
-0.18
J
/
I
I
I
I
I
I
I
I
3000
2950
2900
2850
2800
2750
2700
2650
Wavenumbe r s
Figure 2. SubtractedDRIFT spectra of siloxanes 3,7,and 8: (A) 1,3-bis(3-~hexylamino~hydroxypropyl)tetramethyldisiloxane; (B and C) hexylamine analogs of siloxane polymers-poly(glycidoxypropylmethylsi1oxane) and poly[(glycidoxypropylmethyl)-co(dimethy1)lsiloxane.
Experiments conducted with DRIFT qualitatively show that both nonionic and cationic aminosiloxanes bind to the glass surface and cannot be removed even after several washing cycles. Coupling is noted by the appearance of distinct vibrations after a subtraction technique is applied to the DRIFT spectra. X P S Analysis. X P S 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 versus bulk concentration^.^^ The XPS technique has since been applied to the surfaces of glass fibers as a means to differentiate between nonbridging and bridging oxygen^.^^ Recently, we have examined low molecular weight functionalized siloxanes, including nonionic aminosiloxanes and found that the amino group seems to have a strong binding preference for the glass surface. l5 A clean unsized glass fiber sample was analyzed by XPS before and after a 5-min etch was applied (Figure 3). The 5-min etch is calculated to have a depth of 50 nm for calibrated sputter rates on standard thin film Taz05 films.41 Changes of the sputtering rate with varying sample composition must be considered, as this may result in changes to the etching depth. Atomic concentrations for both the clean glass fiber and the etched glass fiber are shown in Table 2. Etching is accomplished by depth profiling, a useful technique in separating any contamination effects from surface effects. Boron concentrations were not included, due in part to its low sensitivity (factor = 0.129) and its proximity to chlorine (binding energies for boron and chlorine are 193 and 198 eV, respectively). The carbon content of a clean glass fiber is adjusted to a binding energy of 284.6 eV, to remove surface charging effects. The silicon binding energy of the clean glass fiber is equal to 102.2 eV, which is less than that for pure quartz (SiOd at 103.7 eV and results from decreasing binding energy effects with the sodium, calcium and aluminum (40) Nichols, G. D.; Hercules, D. M. Appl. Spectrosc. 1974,28, 219. (41)Bruckner, R.;Chun, H. U.; Goretzki, H.; Sammet, M. J. NonCryst. Solids 1980,42, 49.
oxide components in glass.42 Furthermore, the presence of moisture also lowers the silicon binding energy through the formation of a thin film of Si(0H)x on the surface. After etching, the binding energy increases to 102.9 eV, which is consistent with the removal of the &(OH), thin film. The etching process also increases the amounts of calcium, sodium and aluminum observed (see Table 2). Calcium provides the most information on the adsorption of siloxanes onto glass fibers. Its 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. The atomic concentration of calcium therefore, provides an estimate of the degree of siloxane coating on the glass fiber surface. Atomic concentrations for the nonionic and cationic aminohydroxysiloxanes are tabulated in Tables 2 and 3, respectively. The following general observations can be made for both nonionic and cationic siloxanes. For al,l siloxanes analyzed, the silicon binding energy increases on etching (from 102.2 to 102.9 eV) consistent with removal of siloxane from the glass surface, which exposes more of the glass-silicate network. The silicon binding energy of 102.2 eV is nondiagnostic, and does not by itself distinguish between a hydrated silicate surface and a siloxane bound to the surface. The calcium, sodium, aluminum, and oxygen atomic concentrations all increase after etching, indicating that the adsorbed siloxane is present as a thin layer and is quickly etched away to reveal the bare glass surface, resulting in an increase in glass elements Ca, Na, Si, Al,and 0. The calcium content again, shows itself to be an accurate indicator of surface coverage (see Tables 3 and 4); where calcium concentrations of all siloxane samples increase upon etching, which is consistent with removal of siloxane from the glass fiber. Significantly, the cationic siloxanes have larger Ca 2p values than their nonionic counterparts, suggesting that they do not bind as well as the nonionic siloxanes (which have a higher degree of coverage). Table 2 also shows that sodium and aluminum are reasonable indicators of siloxane surface coverage. Sodium though, can easily (42) Lam, D.J.; Paulikas, A. P.; Veal, B. W. J. Non-cryst. Solids 1980,42, 41.
Arora et al.
2014 Langmuir, Vol. 11, No. 6, 1995
0
c
II
i
DlllUlllU CIICRUIi EV
0 0
On
Figure 3. XPS spectraof (A) unsized E-glass fibers and (B) E-glass fibers after 50 nm etch (* indicates Mo signal from cover plate). Table 2. Atomic Concentrations of Elements in E-Glass Fiber element (line)
binding
fiber
energy (eVIa
concn (%)
concn after etch (%)
284.6 531.5 102.2 1071.9 347.7 74.0 197.6 684.9
29.13 49.75 12.20 0.94 2.85 3.53 0.60 0.40
10.68 61.29 15.68 1.59 4.18 4.19 0.67 0.42
a Corrected for carbon binding energy of 284.6 eV (ie. a -3.9 eV correction was applied).
leach from the surface during washing with a succession of solvents,43while aluminum concentrations are also severely reduced by silane d e p o ~ i t i o nas , ~silanes ~ prefer to adsorb onto AI-OH sites rather than the surface silanols. Both aluminum and sodium were therefore not considered as accurate surface indicators in our experiments. More significantly, the Ca 2p line has now emerged as a commonly used standard X P S indicator of glass fiber coverage.15,16,45 To determine whether amino groups adsorb to other glass surface sites, particularly aluminum and boron, an (43)Mohai, M.; Bert6ti, I.; Revesz, M. Surf:InterfaceAnal. 1990,15, 364. (44) Wesson, S. P.; Jen, J. S.;Nishioka, G. N. J.Adhes. Sci. Technol. 1992, 5, 843. (45) Pantano, C . G.; Wittberg, T. N. Surf:Interface Anal. 1990, 15, 498.
XPS-controlled experiment was run on pure silica which is devoid of aluminum and boron attachment sites. XPS analysis of the nitrogen atomic concentration percentage for nonionic anilinohydroxysiloxane adsorbed onto silica gave a value of 1.5%. This value, although slightly lower than that of the identical experiment on glass fibers, nevertheless indicates a consistently high amount of nitrogen on the silica. A further XPS experiment was carried out to determine whether any effects arise from a dual solvent system comprising toluene and tetrahydrofuran upon adsorption of nonionic siloxanes on E-glass fibers. Observation of the XPS nitrogen atomic concentration percentage for anilino-hydroxysiloxane adsorbed onto glass gave similar levels of nitrogen (2.0%)as for that of the single solvent systems. From the relative nitrogen, silicon,and calcium binding energies of siloxanes 7 and 8,both high molecular weight polymers (Table 2), it can be inferred that siloxane 7 binds more tenaciously to the glass fiber than does siloxane 8. The very low calcium atomic concentration of siloxane 7, indicates a good surface coverage of aminosiloxane on the glass. Further, the calcium concentration dramatically increases on etching the sample, clearly indicating that the aminosiloxane coating is extensive. Polymer 7 contains a relatively higher proportion of pendant organofunctional groups than polymer 8, and would therefore be expected to bind more effectivelyto the surface. Both cationic aminosiloxane counterparts, 15 and 16, show appreciable levels of calcium and aluminum, confirming
Aminohydroxysiloxanes on E-Glass Fibers
Langmuir, Vol. 11, No. 6, 1995 2015
Table 3. Atomic Concentrations of Nonionic Siloxanes 1-8 Coated on E-Glass Fibers concn (%) siloxane no.
1 after etching
2 after etching
3 after etching
4 after etching 5
after etching
6 after etching
7 after etching 8
after etching
c Is
0 Is
32.47 23.71 48.32 31.52 52.76 26.22 34.96 28.16 61.61 56.92 44.19 25.93 63.96 59.76 32.63 14.53
48.07 53.81 31.40 42.19 33.50 49.50 44.27 49.33 24.08 25.35 36.39 47.53 24.90 26.34 47.09 58.13
Si 2p 13.47 14.21 13.62 15.34 8.95 14.45 13.00 12.44 9.83 12.52 13.16 16.06 7.24 9.66 14.37 17.10
that considerable parts of the glass fiber silicate network remains exposed after siloxane adsorption. For all siloxanes analyzed, the carbon 1s multiplex spectra show the presence of both hydrocarbon peaks (284.6 eV) and carbon bound to single oxygen (286.5 eV). The latter peak arises from the pendant organofunctional group, which contains both electronegative ether and hydroxyl groups, that shift the binding energy ofthe carbon 1s multiplex upfield. Not suprisingly, nitrogen was detected in all samples analyzed, possibly indicating that the amino group binds to the glass surface. Even after a 5-min etch, the nitrogen content was still significant (Tables 3 and 41, suggesting substantial aminohydroxysiloxane coverage of the surface. Earlier work carried out by our group on aminofunctional siloxanes that are bound to E-glass fibers, has revealed similar levels of nitrogen in the X P S as for the aminohydro~ysiloxanes.~5 Jalbert and c o - ~ o r k e r shave ~ ~ examined terminated aminosiloxanes adsorbed onto sapphire surfaces by angleresolved X P S , and discovered that at the surface the terminal amino groups are present in lower quantities than in the bulk. Although we did not utilize angleresolved X P S , we have observed that after etching the elemental nitrogen percentage decreases consistent with more of the glass fiber surface being exposed. For the nonionic aminosiloxanes, the binding energy (399.9 eV) is in good agreement with the binding energy for a secondary amine.46 The affinity of amines for silica surfaces has been previously described,47and the X P S evidence presented here confirms the amine as a binding group in the attachment of aminosiloxanes to glass fibers. The glass fibers were washed with their respective solvents to remove any physisorbed species, thus any nitrogen detected in the XPS indicates binding of the aminohydroxysiloxanes to the glass. Polydimethylsiloxanes, physisorbed onto glass surfaces, are readily removed during the washing sequence. Such a result, is in general agreement with our earlier observations on the adsorption of various functionalized siloxanes on glass fiber surfaces. Closer examination of the nonionic XPS multiplex spectra, reveals that there are two nitrogen species present; a peak we have ascribed to NH species (binding energy = 399.9 eV) and a peak ascribed to NH2+ species (binding energy = 402.0 eV). TOdetermine the relative amounts of the two nitrogen peaks, a peak stripping routine, involving the use of asymmetrical Gaussian(46) Briggs, D.; Seah, M. P.Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy;2nd ed.;John Wiley: New York, 1983. (47) Knozinger, H.In The Hydrogen Bond; Schuster, P., Zundel, B., Sandorfy, C., Eds.; North Holland: Amsterdam, 1976; p 1263, Vol. 3.
N 1s 1.49 0.88 2.53 2.19 3.27 1.49 3.44 2.67 2.14 1.99 1.68 1.26 3.16 2.95 1.18 1.07
Na 1s ‘0.05 0.87 0.36 1.00 X0.05 1.09 methacrylsiloxane > silane > epoxysiloxane
I
The results of this study, would see this series expanded to the following: aminosiloxane =- methacrylsiloxane > silane > cationic aminohydroxysiloxane > epoxysiloxane
Amine Type Figure 6. Graph of N/Ca ratio versus different amines having 1,3-bis(3-glycidoxypropyl)tetramethyldisiloxaneas backbone. A = nonionic aminohydroxysiloxanes; 0 = cationic aminohydroxysilanes. loxanes also have relatively low N/Si atomic concentration ratios (see Figure 5) with respect to their nonionic counterparts. The consistent, yet slightly elevated N/Si hexylamine result is currently being examined with other similar, aliphatic non cyclic aminosiloxanes. While the ratio of the two types of adsorbed nitrogen species may stay relatively constant, much more siloxane adsorption occurs with the nonionic counterparts. This conclusion remains valid, irrespective of the solvent the siloxanes are adsorbed from; the nonionic aminosiloxanes consistently adsorb in greater amounts onto glass surfaces than their cationic counterparts. Figure 4 suggests that the N/Si XPS ratio is a good indicator of aminohydroxysiloxane surface coverage. A plot of N/Ca XPS ratio versus amine, shown in Figure 6, further affirms that the nonionic aminosiloxanes adhere to the glass fibers in greater amounts than their cationic counterparts. The results collated in Figure 6 are in direct accordance with the results of Figure 5. The N/Ca XPS ratio makes use of a glass fiber element, calcium, as the surface coating indicator in place of silicon which is present as both siloxane and silicate in the glass network. The N/Ca XPS ratio therefore, is considered to be a more reliable indicator of siloxane surface coverage on silicate minerals. The extant literature reveals that the N/Si or N/Ca XPS ratios of most commercial silanes, in particular the amino, methacryl and mercapto s i l a n e ~ l fall ~ , ~between ~ the nonionic and cationic aminosiloxanes values that are shown in Figure 6. This would suggest that the nonionic aminosiloxanes preferentially bind, and in greater amounts, to the glass than the cationic aminosiloxanes, as well as most commercial silane coupling agents. Moreover, simply changing the charge of the aminofunctional group from a nonionic to a cationic charge, lowers the coupling effectiveness of the aminohydroxysiloxanes t o below that of most common commercial silane coupling agents today. Previously, we have assessed various siloxanes and silanes in terms of their coupling effectiveness to E-glass fibers,15and have found their relative effectiveness follows the series
Aminosiloxanes do not attach to glass surfaces by the hydrolysis mechanisms used by silane coupling agents, and so these siloxanes are different from their silane analogs. Consequently, reactivity and mechanical performances of the new siloxane-treated glass fiber composites are likely to be different from silane-impregnated glass fiber composites, and future work therefore focuses on such studies.
Conclusions The characterization of polymers chemically bonded to substrate surfaces is essential to understanding the interfacial chemistry of a variety of commercial processes and reactions, specifically in the composites industry where glass fibers are ofien used as reinforcing materials. In the past it has proved difficult to detect the presence of adsorbed polymers on glass fibers. With the emergence of direct surface analytical techniques, such as XPS and DRIFT the door has now opened for the facile characterization of polymers on glass fiber surfaces. DRIFT experiments indicate that coupling has taken place between the glass surface and various aminosiloxanes. Use of a subtraction technique allows for clearer examination of the characteristic C-H region, and allows relative amounts of adsorbed siloxanes to be estimated. XPS studies can not only quantify adsorbed siloxane amounts, but identify different amino species on the surface of the glass fibers. Nonionic aminosiloxanes bind more effectively to glass fibers than their cationic counterparts and conventional silane coupling agents. The consistent ratio of the two types of adsorbed species indicates that binding to the surface is the same from both types of aminosiloxane. Better adsorption occurs with the nonionic aminosiloxanes; yet their cationic aminosiloxane counterparts do adsorb to glass surfaces, possibly through an acid-base equilibration to their nonionic counterparts. The facility of the nonioniccationic aminosiloxane equilibration could then determine the amount of cationic aminosiloxane attachment to the glass fiber surface. The N/Ca XPS ratio can be considered as a good surface coating indicator of amino groups on glass. It is anticipated future studies will probe the adsoprtion of aminosiloxanes having other associated functional groups, which can also participate in coupling to an' inorganic surface.
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 untreated E-glass fiber. LA9409034
