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Spectroscopic Characterization of Zirconia Coated by Polymers with Amine Groups Bernard Chaufer,*,† Murielle Rabiller-Baudry,† Anne Bouguen,† Jean Pierre Labbe´,‡ and Auguste Que´merais§ Lab. des Proce´ de´ s de Se´ paration, UA 991 University Rennes I-Inra, Campus de Beaulieu, CS 74 205, 35042 Rennes Cedex, France, Lab. de Corrosion, ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France, and PALMS, UMR 6627 CNRS-University Rennes 1, Campus de Beaulieu, CS 74 205, 35042 Rennes Cedex, France Received April 20, 1999. In Final Form: October 13, 1999 The interaction between zirconia and polyethyleneimine (PEI) has been characterized using Fourier transform infrared analysis (FTIR) and X-ray photoelectron spectroscopy (XPS). For FTIR, polyvinylimidazole with an aromatic amine group and tetraethylenepentamine were added for an in-depth study: two new broad bands due to nitrogen coordinated with zirconium are seen in the 1580-1560 cm-1 region and 1450-1350 cm-1 region, respectively. XPS shows that both ZrOH and N+ were involved in a minor part only, in good accordance with FTIR. After a cross-linking step of the adsorbed PEI on zirconia, a ratio of about one molecule of cross-linker to five PEI units was obtained by both methods. From XPS Zr 3d signal attenuation, the PEI thickness on plates is about 1.2 and 3.5 nm before and after the cross-linking step, respectively; the cross-linked PEI thickness on zirconia membrane is larger because of the procedure of coating or porous and tubular character of membranes.
Introduction Mineral oxides are extensively used in separation techniques, particularly because of their ability to withstand applied pressure. Ceramic properties (mechanical, thermal, and chemical stability) of silica and alumina are extended by a zirconia layer for extreme pH in liquid chromatography and membrane filtration, respectively. Membrane applications have originated several works on the electrokinetic properties of zirconia.1-4 By chemical modification of the surface groups, new separation properties (ion-exchange, hydrophobic groups, activated groups) are obtained. The most widely used mineral oxide is silica with two alternative routes: covalent bonding to organic groups5 and coating with polymer.6 (Co)polymer coatings were mostly described through their applications in chromatography rather than their attachments on the mineral oxide surface. Polymer adsorption on silica with a further cross-linking reaction has been described as the way for obtaining a suitable material for HPLC.7 Application of this chemical modification to polyethyleneimine (PEI), the target polymer for membrane application in the food industry, was extended successfully to various mineral oxides.8 Polyvinyl imidazole (PVI) coating on silica and zirconia, according to this route, has been previously described and characterized by diffuse reflectance infrared * To whom correspondence should be addressed. † Lab. des Proce ´ de´s de Se´paration. ‡ Lab. de Corrosion. § PALMS. (1) Blesa, M. A.; Maroto, A. J. G.; Passagio, S. I.; Figliola, N. E.; Rigotti, G. J. Mater. Sci. 1985, 26, 4601. (2) Randon, J.; Larbot, A.; Guizard, C.; Julbe, A.; Cot, L. Key Eng. Mater. 1991, 61-62, 155. (3) Dumon, S.; Barnier, H. J. Membr. Sci. 1992, 74, 289. (4) Khayat, C.; Vatelot, A.; Decloux, M.; Bellon-Fontaine, M. N. J. Membr. Sci. 1997, 137, 219. (5) Weetal, H. H.; Filbert, A. M. Methods Enzymol. 1974, 34, 59. (6) Alpert, A. J.; Regnier, F. E. J. Chromatogr. 1979, 185, 375. (7) Kopaciewicz, W.; Regnier, F. E. J. Chromatogr. 1986, 358, 107. (8) Chicz, R. M.; Shi, Z.; Regnier, F. E. J. Chromatogr. 1986, 359, 121.
Fourier transform (DRIFT) and zeta potential;9 DRIFT allows the cross-linking reaction to be evaluated in good accordance with elemental analysis; it was not the scope of this paper to study the attachment of amine groups of the polymer on the mineral oxide. Acidity and basicity of metal oxide surfaces were studied by different physicochemical techniques in a limited domain of solid and experimental procedures.10 Characterization of acid sites was performed with probe molecules such as amine, pyridine, and ammonia. The main adsorption mechanisms of ammonia (pyridine) on the different types of adsorption sites for metal oxides (acidic proton, electron acceptor sites, and hydrogen bond from hydrolyzed groups) were schematically shown.10 A review of species arising from ammonia adsorption on 11 metal oxides has been published.11 They were detected by infrared spectroscopy; coordinated nitrogen (Lewis base site) was shown to be involved in most oxides, and among them zirconia, for which details are given in Table 7 (see Discussion). Microcalorimetry data10 showed a heterogeneity of the strong adsorption sites of zirconia, in agreement with infrared spectroscopic data. Accordingly, FTIR spectroscopy was selected for in-depth characterization of interactions between zirconia and amine groups. X-ray photoelectron spectroscopy (XPS) (or ESCA) is a well-known spectroscopic technique to characterize surface coatings, for example, a polymer film on a glass surface. XPS was successfully used in a biomedical application for the characterization of soft and hard segments of polyurethanes from the chemical shifts of the main atoms (C 1s, N 1s, O 1s) and signal deconvolution.12 The adsorption (9) Millesime, L.; Amiel, C.; Michel, F.; Chaufer, B. Langmuir 1996, 12, 3377. (10) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371. (11) Tsyganenko, A. A.; Pozdnyakov, D. V.; Filimonov, V. N. J. Mol. Struct. 1975, 29, 299. (12) Ratner, B. D.; McElroy, B. J. Electron Spectroscopy for Chemical Analysis: Application in the Biomedical Sciences. In Spectroscopy in Biomedical Sciences; Gendrau, R. M., Ed.; CRC Press: Cleveland, 1985; Chapter 5.
10.1021/la990482w CCC: $19.00 © 2000 American Chemical Society Published on Web 12/22/1999
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of a modified PEI and a hydrophobic cross-linked PEI on mica was studied by XPS.13 The comparison of C 1s and N 1s signals evidenced an adsorption of PEI derivatives on mica (highly negatively charged surface) due to electrostatic interactions between N+ groups and mica with the charge neutralization condition imposing restriction of the polymer chain mobility. The first aim of this study was to use Fourier transform infrared analysis (FTIR) for in-depth characterization of the bond or the adsorption sites between zirconia and a polymer bearing either aliphatic amine (PEI) or aromatic amine groups (PVI). XPS was used both for the characterization of the PEI coated on zirconia and of the (crosslinked) polymer network on ZrO2, which is similar to a zirconia-based membrane able to withstand extreme pH conditions. Experimental Section Chemicals. Powder of cubic zirconia (yttrium doped, P 316) and inorganic membrane (Carbosep M1 type, cutoff 150 000 g/mol) made of an active layer of zirconia on carbon support kindly provided by the membrane manufacturer (P 316, Tech-Sep, Miribel, France) were used as substrates for FTIR study. Zirconium and monoclinic zirconia (ZrOx, substoichiometric in oxygen)14 plates were used for XPS studies. These materials were modified according to the route for PVI coating and cross-linking steps described elsewhere.9 For PEI (Polymin P, BASF, 40 000 g/mol or PEI 600 Polysciences, 600 g/mol), 2 g of zirconia, previously treated with 0.1 M NaOH, rinsed with water and dried overnight at 100 °C, were stirred with about 25 mL of PEI in water (11 g/L) for 4 h. After three cycles of washing with water and centrifugation, a final rinsing with absolute ethanol was performed before PEI/zirconia drying overnight at 100 °C. A part (0.5 g) was then allowed to react with 15 g/L bisphenol A diglycidyl ether (DGEBA, Epikote 828 LV, Shell) in ethanol for a night at room temperature followed by heating at 70 °C for 4 h; the crosslinked polymer is abbreviated KPEI in this paper. Tetraethylenepentamine (TEPA, Janssen) was distilled before use in the same conditions as for PEI. Membrane modification with PEI is described elsewhere.15 Chemical formulas of PVI, PEI, DGEBA, and TEPA are given in Figure 1 for a better understanding. Infrared Measurements and Procedure. The infrared measurements were performed on an FTIR spectrometer (Bruker IFS 48) with typically 240 scans and 8 cm-1 resolution in dry air atmosphere. Quantitative measurements for small amounts of solids dispersed in transparent media (alkali halides) were not obtained by using Beer’s law, as it is usually expressed for solutions (homogeneous isotropic media). The pellet diameter (limiting the beam diameter if all the substance is to be measured), through its area S, is the important variable to point out: smaller diameter values enhance sensitivity and transparency when the signal-to-noise ratio can be controlled. If the pellet volume is Sb (b: optical path length) and m the amount of substance, LambertBouguer’s law in fact means that b is not taken into account:16
A ) km/S
Figure 1. Chemical formulas of polyvinylimidazole (PVI), polyethyleneimine (PEI), tetraethylenepentamine (TEPA), and polymer cross-linker: diglycidyl ether of bisphenol A (DGEBA). 10-12 mg of dry CsBr (carefully weighed) and introduced into the pressing mold. Grinding an additional milligram of CsBr in the same mortar ensured a quantitative transfer into the mold, which is then pressed (4000 N force) for 5 min into a 3-mm diameter pellet. Each spectrum was registered three times, rotating the disk in its plane by 120° increments for each sample to obtain a reproducible mean value (isotropy requirement). To register higher values of absorbance for amines adsorbed on ZrO2, the maximum amount of zirconia was determined: the various bands of PVI stayed linear at least up to 600 µg of ZrO2. Further experiments were performed with an amount m1 (carefully weighed) of ZrO2 close to 500 µg and m (about 10 µg, exactly known) of various substances (s) to be examined. Two kinds of difference spectra were registered: (a) the effect of ZrO2 was easily canceled from an absorbance difference D (spectra are given in brackets) with a zirconia blank of mass m2 close to m 1:
D ) [ZrO2(m1) + s(m)] - (m1/m2) [ZrO2(m2)]
(2)
(b) a double difference (∆) was susceptible to compensate for the (s) contribution:
∆ ) D - R(m/m′)[s(m′)]
(3)
(1)
As the homogeneity hypothesis does not hold, k is not constant for solid grains of various sizes. As predicted by theory16 and shown experimentally,17 k varies toward a constant value when the size of the grains approaches zero (better yield). In this work, the repeatability of k was ensured by standardizing the grinding time at 10 min in a corundum mortar.18 Known amounts of the substances to be registered were ground with (13) Go¨lander, C. G.; Eriksson, J. C. J. Colloid Interface Sci. 1987, 117, 38. (14) Debuigne, J. Met. Corros. Ind. 1967, 42, 89. (15) Lucas, D.; Rabiller-Baudry, M.; Millesime, L.; Chaufer, B.; Daufin, G. J. Membr. Sci. 1998, 148, 1. (16) Duyckaerts, G. Spectrochim. Acta 1955, 7, 25. (17) Bonhomme, J. Spectrochim. Acta 1955, 7, 32. (18) Labbe´, J. P.; Que´merais, A.; Michel, F.; Daufin, G. J. Membr. Sci. 1990, 51, 293.
with m′ the mass of (s) used as a second blank, R being a yield factor due to the screening effect of large amounts of ZrO2 toward small amounts of (s); R is smaller than 1. Then, the ∆ difference spectrum is expected to be zero except when additional bands due to the interactions between the organic substance (s) and zirconia are present. XPS Measurements. Membranes (M1 type) and flat plates were used as samples. Zirconia plates were zirconium samples oxidized at about 1000 °C in oxygen atmosphere,14 quenched in the air, and cut under water. Scanning electron microscopy (not shown) shows that both zirconia membranes and plates are rough surfaces with many small zirconia crystals. KPEI membrane (after polymer cross-linking) shows some zirconia grains appearing between polymer batches on the surface view and no polymer inside the pores in the cross-section view, thus validating plates as model material for spectroscopic membrane characterization. Zirconia plates were then modified according to the
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Figure 2. Difference spectra between PVI on zirconia and zirconia with a variable ratio (x) according to eq 2. x values are 0.9 for C1 D1 spectrum, and 1.0, 1.1, and 1.2 for the following spectra, respectively. (See text for details). same route as membranes (PEI 40 000 g/mol) before preparation for XPS measurements. Membranes or plates were cut with a jeweller saw (10 × 20 mm), then fixed on a transfer bar in the introduction/preparation bell (10-7 mbar). Samples were transferred, on a sample holder, to the measuring part and placed in the correct position to be studied (10-9 mbar). The surface under study was exposed to nonmonochromatized X-rays from a twin Mg-Al source (1253.6 eV Mg-KR). For insulating substances, shifts due to charged surfaces had to be taken into account. The energy calibration was then performed from the C 1s peak fixed at 285.0 eV. Measurements were taken through an HA 100 VSW hemispheric analyzer and pulses treated by a computer-controlled acquisition system. Results were usually obtained with 20 and 10 eV scanning energy, giving 1.4 and 1.1 eV resolution, respectively. The angle between the source and the analyzer lens axis was about 55° and the ejection angle of the electrons from the substrate was between 0° and 60-70° relative to the normal. The smaller this angle, the larger the extraction depth of the analyzed electrons. With a mean free path of about 2.5 nm, the analyzed depth is about 7-8 nm. When interpreting spectra, the layers were assumed to be homogeneous and the variations resulting from the different attenuation lengths were not corrected. Raw integrated signal values corresponding to O 1s, C 1s, N 1s, Zr 3d 5/2 structures were divided by sensitivity factors, taking into account ionization cross-sections and apparatus factors. The sensitivity factors are those experimentally determined according to Briggs and Seah.19
Results and Discussion 1. Interaction between Amine and Zirconia from FTIR Characterization. The aim of this IR study was to evidence new bands due to Lewis acid-base interactions between Zr atoms of zirconia and amine groups. Because of the large excess of zirconia with regard to organic (19) Briggs, D.; Seah, M. P. Practical Analysis, 2nd ed.; Wiley: Chichester, 1990; Vol. 1, p 635.
compounds, zirconia absorbance was too high and the difference of spectrum not significant; accordingly, the study was focused on amine groups. Owing to its sharp well-identified bands of its heterocyclic amine group, shown previously,9 PVI was chosen as test amine for the validation of the experimental procedure. 1.1. Validation of Zirconia Cancellation Procedure. As a very high amount of ZrO2 was chosen for a more favorable signal-to-noise ratio, the method of interactive difference, subtracting with a variable factor x (instead of m1/m2) until cancellation of zirconia absorbance, is not useful here because the magnitude of zirconia absorbance is too important (close to 13). The experimental validation of eq 2 was checked with sharp bands of PVI. Figure 2 shows the difference spectra according to x variation (zirconia contribution). Three peaks of PVI (pure PVI spectrum is shown in Figure 4) situated at 1499 (from the short tangent CiDi between 1550 and 1400 cm-1), 1285, and 1230 cm-1 [from a short tangent (not shown) between 1200 and 1400 cm-1] were selected. In fact, a difference spectrum D ) [A]-x[B] should be theoretically linear when [A] and [B] are the absorbances of PVI on zirconia and zirconia, respectively. But a break of symmetry is expected to be observed at x ) m1/m2 when only a part of the resulting D value is measured owing to the chosen tangent drawn between two points Ci and Di (isolating a characteristic part of PVI absorbance). The Ci and Di wave numbers are not constant, and a (variable) part of the PVI absorbance is then measured. The absorbance of each PVI band from difference spectra versus x is shown in Figure 3: the slope break determined the point where zirconia contribution is exactly canceled. As [B] values are important relative to [A] (500 µg of zirconia against 10 µg of PVI), a slope break of PVI bands was easy to find in the vicinity of the m1/m2 ratio. The
Spectroscopic Characterization of Zirconia
Figure 3. Absorbance of PVI (10 µg) for 1230 cm-1 (2), 1499 cm-1 (O), and 1285 cm-1 (9) bands obtained from difference spectra versus variable ratio (x) of subtracted zirconia.
experimental x coefficients were 1.03, 1.04, and 1.02 for 1230, 1499, and 1285 cm-1, respectively. As the mass ratio was 1.05, then the selected x value was 1.04 (0.01). Thus,
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the m1/m2 ratio stays correct even with high unmeasurable absorbance values of material. Such a method is therefore interesting to avoid or overcome the possible interference of an important amount of substrate in the characterization of organics. 1.2. Difference Spectra of PVI-Modified Zirconia. A second pellet was prepared to get rid of the negative 1630 cm-1 absorbance (δHOH of water) visible on the difference spectra in Figure 2. Figure 4 shows the comparison between pure PVI and the difference spectrum after subtracting the contribution of ZrO2 (x ) 1.04); a deep alteration of PVI spectrum when coated on zirconia was evidenced. The broad 1640 cm-1 and sharp 1499 cm-1 bands of free PVI disappeared almost completely and new bands at 1560 cm-1 and 1465-1455 cm-1 due to interactions between PVI and ZrO2 were readily observed (with minor bands given in Table 1). Another way to show the modification of PVI spectrum when adsorbed on zirconia was to obtain a double difference spectrum. The m/m′ ratio of eq 3 was calculated by measuring the ratio of two PVI spectra: one for PVI coated on zirconia, the other being pure PVI. Because 1230, 1285, and 1420 cm-1 bands exist in both spectra (Figure 4), a mean value 0.84 was obtained. From the double difference spectrum {∆ ) D- 0.75 [PVI] (m′)} shown in Figure 5, the two main new bands of PVI when adsorbed on zirconia were pointed out at 1558 and 1456 cm-1: these new bands matched closely to the main new bands obtained in the difference spectrum of Figure 4. It was difficult to optimize further because the small 1116-1094 cm-1 doublet was still present, whereas the 1230 cm-1 band was negative (Table 1). In fact, the value is not the same for all peaks and is not really measurable because of the presence of new bands. Quantitative information was not readily performed because of the lack of an appropriate criterion for the baseline drift. Nevertheless, it was possible to compare band areas in each spectrum: the ratio of the new broad 1456 to 1558 cm-1 bands was about 1 for coordinated PVI,
Figure 4. Difference spectrum of PVI (7.54 µg) adsorbed on zirconia with the selected ratio x ) 1.04 of subtracted zirconia. Free PVI (7.50 µg) spectrum (dotted line) is shown for comparison.
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Table 1. Typical Wavenumbers of Free Amine and Adsorbed Amine Groups when Coated on Zirconia by Difference Spectrum According to eq 2 and by Double Difference Spectrum According to eq 3a amine TEPA
PEI
PVI
free amine spectrum
difference spectrum
double difference “new bands”
1680 s, b 1550 sh 1450 sh 1380 b 1663 s 1570 neglig.
1630 sh 1574/1580 b 1465 sh 1365/1375 1663 1570,7 b doublet 1450-1435.6 1400 sh 1116 w, b 1620 sh 1560 m, b 1499 sh 1465-1455 1420 1377 sh 1290 w 1240 w 1112 w,b
1680 negative 1579 m, b 1450 sh 1350 m,b negative 1560-1580 b doublet 1450-1435 canceled 1116 w, b canceled/negative 1558 m,b canceled/negative 1456 m,b canceled 1377 sh negligible negligible negligible
1450 w,b 1400 w,b 1110 w,b 1640 b 1499 s 1465 w 1420 m. 1377 w 1290 w 1240 s 1112 m
a
s, strong; m, medium; w, weak; sh, shoulder; b, broad.
whereas, even if the shape of the 1499 cm-1 band of free PVI is quite different from that of 1640 cm-1 band, the surface areas are of comparable magnitudes. The double difference is supposed to show a flat spectrum provided subtractions are well done. However, one can expect both positive contributions due to new bands and negative bands situated at the wave numbers of the free solute when interactions occur in the mixture. From this point of view, the double difference spectrum of Figure 5 could appear as a single new broad band weakened by the free PVI band near 1499 cm-1. In fact, the double difference spectrum of PVI could not give a
perfect idea of the actual new band(s) because of the artifact introduced by the subtraction of a free PVI band that was in their close vicinity. Then, aliphatic amine groups without any peak in the new bands region were selected to overcome this question closely related to the choice of a convenient baseline. 1.3. Difference Spectra of PEI and Aliphatic Amine-Modified Zirconia. Spectra of free and coordinated (difference spectrum) aliphatic PEI (600 g/mol) are shown in Figure 6 with an amount of PEI roughly 1.5 times higher when coordinated to zirconia for band intensity at 1600 cm-1, similar to that of free PEI. Bands of adsorbed PEI, observed in the 1560-1570 cm-1 and 1430-1450 cm-1 regions, were in fact doublets, rather broad and of comparable intensity. The double difference spectrum (easy to imagine from Figure 6) was featured by the same new doublet bands of similar intensity together with a negative peak near 1660 cm-1 (Table 1). TEPA, a linear low-molecular-weight amine, was used as a model molecule because it is an oligomer of PEI. The main bands of free and coordinated TEPA are shown in Figure 7: the difference spectrum shows clearly the cancellation of the main free band and new doublet bands situated at 1574-1580 cm-1 and 1365-1380 cm-1, respectively (Table 1). The double difference spectrum (not shown), in good accordance with the difference spectrum, confirms two new bands situated at lower wave numbers with a main new band at 1574 cm-1 and a second close to 1350 cm-1; a further important negative peak is seen at 1680 cm-1 from the free amine disappearance (Table 1). Contrary to imidazole amines, aliphatic species have no band close to 1500 cm-1. Accordingly, the new bands of the double difference spectra can be described as modification of N-H bonds due to zirconia, a possible artifact due to an overlapping of free and coordinated amine groups being thus avoided.
Figure 5. Double difference spectrum of PVI adsorbed on zirconia according to eq 3 with R ) 0.75.
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Figure 6. Difference spectrum of PEI (12.4 µg) adsorbed on zirconia with the selected ratio x ) 1.04 of subtracted zirconia. Free PEI (8.5 µg) spectrum (dashed line) is shown for comparison.
1.4. Ratio of Cross-Linker to PEI on Zirconia. For the determination of the ratio of cross-linker (DGEBA) to PEI, C-H stretching bands unaffected by the cross-linking reaction were selected. Noticing an important variation of the ratio of 2925 and 2855 cm-1 absorbances (A1 and A2, respectively) in both pure substances (DGEBA and PEI), the absorptivities were measured for both compounds at these wavenumbers. For 1 µg of total system, the masses of DGEBA (y) and PEI (1 - y) were determined with the following equations:
A1 ) 0.0031y + 0.0058(1 - y) A2 ) 0.00030y + 0.0067(1 - y) The system after cross-linking PEI with DGEBA on zirconia gives an experimental ratio A1/A2 ) 1.45 and a y value close to 0.6. The ratio of PEI (9.3 nM) to DGEBA (1.9 nM), close to 5, will be compared with XPS measurements. For PVI using a similar cross-linker it has been shown that this ratio was about 3 from elemental analysis;9 PVI has a vinyl polymer backbone that can be assumed less flexible, and a more accessible amine group than branched PEI. 2. XPS Characterization. XPS experiments were first performed on the nonporous and nonstoichiometric zirconia plates for in-depth characterization of curved porous membranes according to the chemical modification steps. 2.1. Overall Composition of Modified Zirconia Plates. Table 2 shows the elemental composition of zirconia, PEI/zirconia, and PEI/ZrO2 after DGEBA crosslinking. For bare zirconia, N 1s and C 1s contributions correspond to pollution. The N signal likely originates from a surface nitridation, due to the presence of a small amount of nitrogen during the oxidation step. As the O/Zr
ratio (2.6) is higher than expected (
