Biomacromolecules 2005, 6, 1628-1634
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Covalent Immobilization of Cellulose Layers onto Maleic Anhydride Copolymer Thin Films Uwe Freudenberg,† Stefan Zschoche,† Frank Simon,† Andreas Janke,† Kati Schmidt,‡ Sven Holger Behrens,‡ Helmut Auweter,‡ and Carsten Werner*,†,§ Leibniz Institute of Polymer Research and The Max Bergmann Center of Biomaterials Dresden, Hohe Str. 6, 01069 Dresden, Germany, BASF Aktiengesellschaft, Polymer Physics, GKP/D-J 542 S, 67056 Ludwigshafen, Germany, and Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada Received November 26, 2004; Revised Manuscript Received February 7, 2005
Thin films of cellulose are advantageous for analytical studies in aqueous environments to investigate various factors determining the performance of cellulose-based products. However, the weak fixation of cellulose layers on common carrier materials often limits this approach. To address this problem, we suggest a novel maleic anhydride copolymer precoating technique which allows for the covalent attachment of cellulose thin films through esterification. Maleic anhydride copolymers were deposited and covalently bound onto planar, aminosilane-modified glass or silicon oxide surfaces. Cellulose was subsequently immobilized on top of the copolymer precoatings by spin coating from N-methylmorpholine-N-oxide/dimethyl sulfoxide solutions. The resulting cellulose films were thoroughly characterized with respect to layer thickness, morphology, chemical constitution, and electrical charging. The stability of the layers against shear stress was demonstrated in aqueous solutions and the covalent attachment of the cellulose to the copolymer films was proven by means of dissolution experiments followed by ellipsometry and high-resolution X-ray photoelectron spectroscopy. Introduction Properties and interactions of cellulose surfaces are of a great technical interest for a wide variety of applications including paper, textiles, and pharmaceutical products. Thin cellulose model films provide valuable options for the investigation of chemical and physical characteristics of cellulose-based products in different environments. In that context, cellulose-water interactions, swelling, and adsorption phenomena as occurring in paper making/recycling, and textile production or washing, are of interest. Also, such interactions are important for cellulose products in biomedical applications such as temporary artificial skin made from bacterial cellulose and cellulose materials for acoustic membranes or for the immobilization of microorganisms.1 Therefore, the preparation of cellulose model surfaces for experimental studies received more and more attention throughout the past few years.2-10 Beyond that, thin cellulose layers are very promising for the surface modification of dispersed or macroscopic materials. However, a fundamental difficulty of creating cellulose layers is caused by the limited solubility of cellulose in most of the common organic and inorganic solvents. This is the reason for the preferential use of easily dissolving cellulose derivates such as trimethylsilyl * To whom correspondence should be addressed. Phone: +49 351 4658531. Fax: +49 351 4658533. E-mail: Carsten.Werner@ mbc-dresden.de. † Leibniz Institute of Polymer Research and The Max Bergmann Center of Biomaterials Dresden. ‡ BASF Aktiengesellschaft. § University of Toronto.
cellulose (TMSC) to produce cellulose model surfaces by the Langmuir-Blodgett (LB) technique2,3,5,8 or by spin coating.8,9 However, cellulose layers were also prepared from NaOH/urea aqueous solution6 or from nanocrystalline suspensions by hydrolyzing dissolving pulp with sulfuric acid.10 Dissolving cellulose in the recently introduced N-methylmorpholine-N-oxide (NMMO)/water system and using the relatively simple method of spin coating to get thin cellulose films was described by Gunnars and co-workers.7 To obtain tightly bound cellulose layers on inorganic substrates (e.g., oxidized silicon wafers), thin polymer films were suggested to enhance the physisorption of the cellulose to the carrier surface.7 Although providing valuable new options, this approach is limited to noncovalent binding of the cellulose to the substrate and the resulting layers suffer from a lack of stability against delamination in aqueous solutions. In this context, we report a new method for the preparation of covalently immobilized cellulose layers on top of inorganic substrates (silicon wafers or glass carriers). Maleic anhydride copolymer films covalently attached to glass carriers or oxidized silicon wafers were used as anchor films which bind the cellulose layer by esterification between the hydroxyl groups of the cellulose and the anhydride groups of the maleic anhydride copolymers. The preparation of maleic anhydride copolymer films covalently bound to aminefunctionalized surfaces and the characterization of these layers have been recently reported elsewhere.11 Esterification of the anhydride groups of maleic anhydride or maleated polypropylenes with surface hydroxide groups
10.1021/bm0492529 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/23/2005
Covalent Immobilization of Cellulose Layers
of solid cellulose materials (e.g., cellulose fibers) was described in several earlier publications. Yang and coworkers12 showed that cross-linking of cotton cellulose for stabilization by polycarboxylic acids (e.g., poly(maleic acid)) occurs as a two step reaction: (1) Formation of a cyclic anhydride intermediate (activation) and (2) reaction of the anhydride intermediate with cellulose by esterification. Also, the use of maleic anhydride or maleated polypropylenes in cellulose reinforced thermoplastic composites was reported in several variants13-17 assuming an esterification between the “solid” cellulose and the anhydride moiety. The abovementioned two-step mechanism was confirmed by Felix and Gatenholm.13 They studied the modification of cellulose fibers with graft copolymers of polypropylene and maleic anhydride (GPPMA) at 100 °C (5 min) in toluene and figured out that activated GPPMA (i.e., the anhydride form, obtained upon heating to 170 °C for 5 min prior to the reaction with the cellulose) provided a higher degree of esterification than nonactivated (hydrolyzed) GPPMA. The formation of ester bonds between “mechanically activated” hydroxyl groups of cellulose and maleic anhydride groups of GPPMA (freshly tempered at 120 °C) during ball milling at room temperature (for 10 h) was discussed by Qiu and co-workers.18 They demonstrated the importance of destroying the hydrogen bond network during ball milling of the highly crystalline cellulose to yield free (activated) hydroxyl groups. Alternatively, the chemical modification of cellulose by maleic anhydride to cellulose maleate was also carried out successfully in pyridine at 100 °C.19 The above-mentioned reaction between cellulose and anhydride functions is also applied in our approach to produce stable cellulose thin films. The use of alternating maleic anhydride copolymers as anchoring layers was considered advantageous since (i) they can provide a variety of physicochemical characteristics depending on the comonomer and spontaneously form covalent bonds to any kind of amine-functionalized surfaces (e.g., inorganic oxide substrates after aminosilane pretreatment or ammonia plasma treated polymer materials) and (ii) the high reactivity of the remaining anhydride groups of the surface-bound copolymer allows for the creation of ester bonds to cellulose layers subsequently deposited on top of the copolymer films. The composition of the resulting cellulose coating is schematically shown for the example of a silicon oxide carrier material in Figure 1. Materials and Methods Maleic Anhydride Copolymer Film Preparation. Freshly cleaned planar glass (polished glass carriers 10 × 20 mm, Herbert Kubatz GmbH Co., Berlin, Germany) or silicon oxide carrier materials (silicon wafers 10 × 20 mm, Sico Wafer GmbH, Jena, Germany) oxidized in a mixture of aqueous ammonia solution (Acros Organics, Geel, Belgium) and hydrogen peroxide (Merck, Darmstadt, Germany) were functionalized by reaction with 3-aminopropyl-dimethylethoxysilane (ABCR, Karlsruhe, Germany). Poly(propene-alt-maleic anhydride) (PPMA) MW ) 39 000, poly(styrene-alt-maleic anhydride) (PSMA) MW ) 100 000 (both are special products
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Figure 1. Schematic structure of cellulose layered substrates as prepared and characterized in this study.
of Leuna-Werke AG, Germany), and poly(ethylene-alt-maleic anhydride) (PEMA) MW ) 125 000 (Aldrich, Munich, Germany) were dissolved for subsequent spin coating of the amine-modified carriers. For preparation of thin polymer films concentrations of 0.1 wt % of PPMA in 2-butanone p.a. (Fluka, Deisenhofen, Germany), 0.15 wt % of PEMA in a 1:2 acetone p.a. (Merck)/tetrahydrofuran p.a. (THF, Fluka) mixture, and 0.12 wt % of PSMA in THF were used, respectively. After spin coating (RC 5 Suess Microtec, Garching, Germany, 4000 rpm/30s), the polymer-coated samples were annealed at 120 °C to convert the spontaneously formed amide in the more stable five-membered cyclic imide with the amino-silane on the silica substrate. Rinsing of the polymer films in the polymer solvent removed traces of noncovalently bonded polymer. For further information on the preparation and characteristics of maleic anhydride copolymer films, the reader is referred to refs 11 and 20. A large excess of the anhydride functions of the copolymer films is not converted for the anchorage to the carrier (as demonstrated by dedicated XPS experiments in ref 11) and remains available for the covalent attachment of subsequently deposited reactive molecules containing amine or hydroxyl groups (e.g., cellulose). Cellulose Film Preparation. Materials. Microcrystalline cellulose “Avicel” (Fluka) DP ) 215-240 was dissolved in different concentrations in NMMO-monohydrate (Merck) (1 wt %; 2 and 3.8 wt % of the complete NMMO/DMSO mixture). Dimethyl sulfoxide p.a. (DMSO) (Fluka) was used for diluting the spin coating solution. Propyl gallate (PG) p.a. (Merck; 1 wt % of the NMMO) was used as antioxidant to stabilize the cellulose/NMMO mixture.21 Dissolution Process. For spin coating solutions (i.e., 3.8 wt %) 0.60 g Avicel, 9.0 g of NMMO and 0.09 g of PG (alternative: without PG) were heated to 100 °C during stirring for 30 min. (Temperatures above 130 °C have to be avoided to prevent the separation of the mixture and thermally induced side reactions.) Solutions without PG appeared clear and yellowish, and if PG was added, the solutions were clear and brownish. At this stage, 6.0 g of
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DMSO was added to the solution. After cooling to 70 °C, the solution was ready to use. Spin Coating/Washing. The polymer-coated (Si wafer or glass) carriers were tempered at 120 °C for 2 h just prior to spin coating to regenerate the anhydride form of the copolymers. Subsequently, the carriers (preheated at 45-50 °C) were spin coated with the hot cellulose solution with 3000 rpm for 15 or 60 s, respectively. (Temperature control was found to be very important with respect to the reproducibility of the layer preparation.) After spin coating, the samples were immersed into a beaker with MilliQ water (deionized, decarbonized water, R ) 18.2 MΩ) for precipitation of the cellulose layers. Air drying overnight followed by vacuum-drying for 2 h at 90 °C completed the preparation procedure. Excessive washing in MilliQ water (3 times for 1 h) was further applied to remove solvent residues. All samples were dried again at 30 °C under vacuum prior to any other measurement. Methods. Ellipsometry. Determination of interfacial layer thicknesses of polymer-coated Si wafers in the dry state was performed by variable angle spectroscopic ellipsometry in air at room temperature (VASE 44 M, Woollam, Lincoln, NE). Experimentally determined refractive indices at 630.1 nm were 1.50 ( 0.01 for the maleic anhydride copolymer layer and 1.54 ( 0.01 for the cellulose film, which agrees well with previously published data covering a range between 1.53 and 1.60 (cellulose spin coating layers, no information about the wavelength22) and between 1.51 and 1.58 (1.52 at 550 nm) (cellulose LB films23). Average values of 3 or more different samples were determined. Atomic Force Microscopy (AFM). Morphological features of the cellulose layers were characterized by atomic force microscopy in tapping mode (Nanoscope IIIa Dimension 3100, Veeco, Santa Barbara, CA) with Pointprobe silicon cantilevers (Nanosensors, Wetzlar, Germany). The rootmean-square-roughness (Sq) of a 4 × 4 µm2 scanned area was calculated using the Nanoscope III software according to the following equation: Sq )
[ ]
∑(zi)2 1/2 N
(1)
where zi are the height deviations with reference to the mean of N data points. X-ray Photoelectron Spectroscopy (XPS). The chemical composition of the cellulose layers was determined using an Axis Ultra spectrometer (Kratos Analytical, Manchester, United Kingdom) equipped with a monochromatized Al KR X-ray source of 300 W at 20 mA. The kinetic energy of photoelectrons was assessed using a hemispherical analyzer with constant pass energy of 160 eV for survey spectra and 20 eV for high-resolution spectra. For a take off angle (angle between sample surface normal and the electron optical axis of the spectrometer) of 0°, the maximum information depth of the measurements was approximately 8 nm.24 All spectra were charge corrected by setting the energy of the Cy-Hx bond to 285.0 eV. Quantitative elemental compositions were determined from the peak areas of the survey spectra using experimentally determined sensitivity factors and the spec-
Table 1. Survey of Properties of the Copolymer Films
POMA PSMA PPMA PEMA
Mwa
Tdryb
θa(H2O)c
Sqd
50 000 100 000 39 000 125 000
3.5 ( 0.5 5.0 ( 0.5 3.0 ( 0.5 4.8 ( 0.5
100° 75° 52° 57°
0.32 0.31 0.34 0.80
a M : (mass based) molecular weight average of the copolymers, g/mol. w Tdry: thickness of the copolymer films determined by ellipsometry in dry state (average of the measurements of 5 different samples of each copolymer surface), nm. c θa(H2O): water contact angle of the copolymer layers, deg.11 d Sq: root-mean-square-roughness obtained by AFM, nm.11
b
trometric transmission function. The high-resolution spectra were deconvoluted using software routines supplied by Kratos Analytical. Fitting parameters were the binding energy, peak height, full width at half-maximum, and Gaussian-Lorentzian ratio of the component peaks. Zeta Potential Measurements. The zeta potential ζ of cellulose-coated carriers was obtained applying the in house developed Microslit Electrokinetic Setup.25 Streaming current measurements were performed across a rectangular streaming channel (length, L ) 20 mm; width, b ) 10 mm; height, h ) 50 µm) formed by two parallel sample carriers. The zeta potential was calculated from streaming current data by use of the following equation: ζ(IS) )
η L dIS 0r bh dp
(2)
where IS is the streaming current, p is the pressure drop across the streaming channel, η is the dynamic viscosity of the fluid, 0 is the permittivity of vacuum, and r is the dielectric constant of the fluid. The solutions for the electrokinetic measurements were prepared from vacuum-degassed MilliQ water by addition of 0.1 mol/L potassium chloride, potassium hydroxide, and hydrochloric acid solutions. Results and Discussions The properties of the used maleic anhydride copolymer precoatings are summarized in (Table 1). Comparing the preparation of cellulose films at different maleic anhydride copolymer films it turned out that NMMO-based cellulose solutions do not spread on the hydrophobic poly(octadecenealt-maleic anhydride) (POMA) films while the more hydrophilic PSMA, PPMA, and PEMA layers were sufficiently wetted by the cellulose solutions. Thus, PSMA, PPMA, and PEMA could be used as anchoring polymer coatings to produce smooth cellulose layers. Layer Thickness. Variation of spin coating parameters conveniently allowed for the adjustment of the thickness of the cellulose films within a range of 10-280 nm. Spin coating conditions such as rotating speed, concentration and viscosity of the solution are well-known to have a strong influence on the layer thickness. As shown in Table 2, increasing layer thicknesses were obtained with increasing cellulose concentration of the solution and with decreasing the spin coating time. Film Morphology. The AFM measurements at 4 × 4 µm2 area showed Sq values in the range from 2.7 to 3.9 nm. This range is in good agreement with the average value of 5 nm
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Covalent Immobilization of Cellulose Layers Table 2. Dependence of the Layer Thickness on the Cellulose Concentration of the Spin Coating Solution and on the Spin Coating Time
Ccellulose [wt %]a 1 2 3.8
spin coating time [s]
layer thickness [nm]b
15 60 15 60 15 60
22 ( 2 16 ( 2 58 ( 4 39 ( 2 274 ( 8 172 ( 4
aC cellulose [wt %]: concentration of cellulose in the spin coating solution, weight %, anchoring copolymer: PEMA b Layer thickness [nm] (average of the measurements of 5 different samples).
Table 3. Dependence of the Layer Roughness on the Concentration of the Spin Coating Solution and on the Spin Coating Time
Ccellulose [wt %]a 2 3.8
spin coating time [s]
Sqb [nm]
15 60 15 60
3.8 3.5 2.7 2.9
aC cellulose [wt %]: concentration of cellulose in the spin coating solution, weight %, anchoring copolymer: PEMA. b Sq: root-mean-square-roughness obtained by AFM.
reported earlier for spin coated cellulose layers obtained from NMMO solutions.7 There was no obvious trend in the dependence of the roughness on the spin coating parameters time or solution concentration (see Table 3). The type of copolymer precoating also did not significantly influence the roughness of the cellulose layer as shown in Figure 2. Chemical Composition. The widescan spectra of the cellulose layers showed carbon and oxygen as chemical species, and only traces of nitrogen (solvent residuals) were found (XPS is insensitive to hydrogen). The absence of silicon in the spectrum demonstrates the complete coverage of the carrier with cellulose. A patch-like structure of the
Figure 3. Wide scan X-ray photoelectron spectrum (XPS) of a 40 nm cellulose layer on PPMA. Only oxygen, carbon, and traces of nitrogen occur in the spectrum.
layer would manifest itself in the XPS wide scan by showing a silicon signal (see Figure 3). All of the films showed a higher carbon/oxygen ratio than expected according to the elemental composition of cellulose. However, carbon enriched surfaces were previously concluded to be caused by ubiquitous hydrocarbon impurities.9,10 The high-resolution C1s spectra could be resolved into four distinguishable species (A, C, D, and E) according to the different local chemical environment of the carbon atoms. The A peak, carbon bound to carbon and hydrogen only (CyHx), has a binding energy of 285 eV. This peak represents hydrocarbon impurities as discussed above. Increasing the number of carbon-oxygen bonds results in an increased binding energy. Therefore, the peaks could be characterized by the number of the involved carbon-oxygen bonds: A, no carbon oxygen bond; D, one carbon oxygen bond (carbon hydroxyl groups, C-OH); E, two carbon oxygen bonds (acetal groups, O-C-O or carbonyl groups, CdO); and C, three carbon oxygen bonds (carboxylic acid groups, OdCs OH). The resolved carbon spectra (see Figure 4) reveal the chemical structure of the cellulose layer. The ratio of E to D is in a range from 0.228 to 0.23 (average values obtained by 3 or more independent measurements of cellulose layers on each copolymer), which is very close to the theoretical
Figure 2. AFM images of cellulose films on different maleic anhydride copolymer films. The concentration of the cellulose in the spin coating solution was 2 wt %.
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Figure 4. High-resolution C1s X-ray photoelectron spectrum of a 40 nm cellulose layer on PPMA showing the different local environment of the cellulose carbon atoms.
Figure 5. pH dependence of the zeta potential (ζ) for a cellulose layer (170 nm on PEMA) at 10-3 mol/L KCl background concentration.
value of 0.2 in pure cellulose. Possible reasons for the marginal excess of peak E could be the subsequent reaction between formaldehyde (derived of the autocatalytic degradation of NMMO; Polonowski-type reaction) and hydroxyl groups of the cellulose to form acetal groups. Also, cellulose may form carbonyl groups through oxidation processes.26 The small C peak (atomic concentration: 2.97%) is attributed to oxidation products of the cellulose (carboxylic acid groups). Due to the fact that NMMO is well-known as an oxidizing agent for cellulose, oxidation could occur during the dissolution process in the NMMO mixture.21,26 However, the XPS spectrum of the cellulose raw material also showed a small C peak (atomic concentration: 1.58%) pointing at the fact that the cellulose was already partly oxidized prior to the dissolution. Therefore, adding PG (1%) as antioxidant to the spin coating solution did not considerably influence the content of carboxylic acid groups in the cellulose layer. Zeta Potential Measurements. Zeta potential (ζ) measurements were performed at varied solution pH to determine the isoelectric point (IEP) for the cellulose layer. IEP values of about 2.0 (in 10-3 mol/L aqueous KCl solutions, pH-titration from basic to acidic values) were obtained (Figure 5). Since polymers without dissociable surface groups (e.g., unoxidized cellulose) exhibit IEP of about 4.0 in comparable solutions the results indicate the acidic characteristics of the prepared cellulose layers.27 This finding is in line with the XPS data hinting at the presence of carboxylic acid surface sites. Layer Stability. The electrokinetic measurements using the Microslit Electrokinetic Setup apply shear stress (up to 3 × 104 s-1) to the layered samples in aqueous electrolyte
Freudenberg et al.
Figure 6. High-resolution C1s X-ray photoelectron spectrum (XPS) of a thin covalently linked cellulose layer on PEMA obtained after rinsing of the films in a hot NMMO/DMSO mixture, take off angle 75°, ellipsometrically determined layer thickness: 2.5 ( 0.2 nm.
solutions of varied pH. Therefore, the stability of the maleic anhydride copolymer based cellulose layers against delamination and desorption could be evaluated by the comparison of the ellipsometrically determined layer thickness prior to and after the zeta potential measurements. The layer thickness was found to be invariant, also the layer roughness did not change significantly during this experiment. This was considered convincing evidence for the stability of the prepared cellulose films. Proof of Covalent Attachment. Dissolution experiments were realized to prove the covalent attachment of the cellulose to the maleic anhydride copolymer films. These experiments are based on the fact that out of a cellulose layer of some 200-300 nm thickness only a thin monolayer of a few nanometers thickness can be expected to become covalently linked while the rest of the cellulose layer is immobilized by cohesion within the cellulose layer and should rapidly dissolve in a hot NMMO/DMSO mixture. To analyze this directly attached layer, several PEMA covered wafers either bearing the reactive anhydride or the nonreactive diacid surface sites were coated with a 172 ( 4 nm cellulose layer (3.8 wt. % cellulose solution, spin coating time 60 s). The cellulose-coated wafers were immersed three times for 5 min in a 80 °C NMMO/DMSO mixture (analogous to the spin coating solution), followed by three cycles of extensive washing in MilliQ water for 1 h. After drying for 2 h at 30 °C under vacuum the wafers were analyzed by ellipsometry and XPS measurements. A remaining thin cellulose layer of 2.5 ( 0.2 nm was determined ellipsometrically for the cellulose films deposited onto anhydride-containing copolymer films. In contrast, for the case of the hydrolyzed maleic acid copolymers (diacid form, nonreactive toward cellulose) only a very thin cellulose layer (0.8 ( 0.2 nm) remained after this procedure. However, since the conditions of the dissolution process (80 °C) can induce the formation of the anhydride form of the copolymer, even in this case, the low fraction of retained cellulose might be caused by esterification occurring during the dissolution. Angle dependent XPS measurements were performed at 0°, 60°, and 75° to identify the remaining thin films on the anhydride-containing copolymer as a cellulose layer (2.5 ( 0.2 nm thickness). The related maximum information depth of the measurements was 8, 4, and 2 nm, respectively.24 The shape of the high-resolution C1s spectrum (Figure 6) reveals the superposition of the spectra of cellulose and poly(ethylene-alt-maleic acid) (Figure 7, structure 1) or poly(ethylene-alt-maleic anhydride) (Figure 7, structure 2):
Covalent Immobilization of Cellulose Layers
Biomacromolecules, Vol. 6, No. 3, 2005 1633 Table 4. Angle Dependent High-Resolution XPS Data of the Covalently Attached Cellulose Layer on PEMA after Washing
take off angle information depth [nm] Acellulosea Xcelluloseb
Figure 7. Structures of the poly(ethylene-alt-maleic acid) (1), poly(ethylene-alt-maleic anhydride) (2), and the ester of cellulose and poly(ethylene-alt-maleic anhydride) (3) contributing to the XPS C1s spectrum. For explanation see text.
Figure 8. Structures of the pure cellulose (1) and the ester of cellulose and maleic anhydride copolymers (2) contributing to the XPS C1s spectrum. For explanation see text.
Peak A results from saturated hydrocarbons (Cy-Hx) of the copolymer plus adsorbed hydrocarbon contaminations. Peak C represents the carbon of the functional group of the copolymer, carboxylic acids (OdCsOH), anhydrides (OdCsOsCdO) or/and ester groups (OdCsOsCs) (see Figures 6 and 7), whereas the binding energy does not, unfortunately, allow for an unambiguous identification of the relative contribution of either one of these species. Component peak B reflects carbon atoms standing in the β-position of the functional groups of the copolymer, and its area agrees well with peak C according to the stoichiometry of the copolymer. The second sub-spectrum contains the two component peaks D and E, which are well-known for cellulose or its derivatives. Component peak D reflects the carbon of C-OH groups and component peak E has to be attributed to the acetal groups (O-C-O) of the polysaccharide. The ratio of the C-OH and the O-C-O groups was found to be 5:1, which agrees excellently with ratio expected from the cellulose structure. For a more quantitative evaluation of the angle dependent XPS C1s spectra, the ratio of the cellulose carbon signal areas (component peaks area D and E) to the total C1s peak area was denoted as Acellulose and the molecular ratio of the cellulose (component peaks area E or D/5) to the copolymer (PEMA, component peaks area B/2 or C/2) as Xcellulose. Acellulose and Xcellulose for angle dependent XPS data revealed that the cellulose contribution (see Table 4) grows with increasing angle from 0° to 60° but remains rather constant from 60° to 75°. It can be concluded that cellulose is localized in the outermost surface of the layered substrate. Although the maximum information depth at 75° is approximately 2 nm and the thickness of the cellulose layer was concluded to be about 2.5 nm (ellipsometric data), there is also a significant contribution of the copolymer to the C1s spectrum (Figure 6) taken at this angle. This finding points at a two-component layer at the outermost interface of the
0° ≈8 0.23 0.52
60° ≈4 0.28 0.60
75° ≈2 0.28 0.60
aA cellulose: ratio of the component peaks area D and E to the total carbon C1s peak area. b Xcellulose: ratio of the component peak E area to component peak C/2 area, ellipsometrically determined layer thickness: 2.5 ( 0.2 nm
sample containing both cellulose and the copolymer. During the dissolution experiments the covalently linked PEMA and cellulose molecules obtain a substantially elevated mobility in the NMMO mixture. This could be the reason for the enhanced interpenetration of PEMA and cellulose during the dissolution experiments. From the ratio of the ellipsometrically determined thicknesses of the PEMA layer (4.8 nm) and the cellulose layer (2.5 nm), and assuming similar densities of both components, the total layer should contain 34.2% cellulose. Calculating the mass ratio of cellulose vs PEMA from the molecular ratio Xcellulose for the case of XPS measurements at 0° (where the information depth comprises the total PEMA + cellulose layer), a very similar percentage of 36.8% is obtained for the cellulose component in the total layer. This confirms the results of the ellipsometric measurements and permits to identify the layer remaining on top of the PEMA film after the extraction procedure as covalently bound cellulose. Altogether, this demonstrates the efficiency of the esterification of the cellulose with the anhydride form of the copolymer precoatings. Conclusions and Perspective The introduced method for covalent attachment of cellulose thin films on solid supports using maleic anhydride copolymer precoatings was demonstrated to provide welldefined, smooth layers distinguished by an excellent stability against shear stress in aqueous solutions. These layers are advantageous for future model studies with cellulose surfaces to unravel important mechanistic aspects of processing and application characteristics of cellulose-based products. Beyond that, a wide variety of inorganic (e.g., oxide) or organic (e.g., polymer) materials can be efficiently modified with the suggested technology. The substantially enhanced stability of these coatings may even open up new applications for cellulose in various fields such as diagnostics or textile technology. Acknowledgment. This work was financed by the BASF AG, Ludwigshafen. Contributions of C. Lehmann and R. Schulze (both Leibniz Institute of Polymer Research Dresden) to the cellulose layer preparation and ellipsometric measurements are gratefully acknowledged. References and Notes (1) Vandamme, E. J.; De Baets, S.; Vanbaelen, K.; Joris, K.; De Wulf, P. Polym. Degrad. Stab. 1998, 59, 93-99. (2) Schaub, M.; Wenz, G.; Wegner, G.; Stein, A.; Klemm, D. AdV. Mater. 1993, 5, 919-922.
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(3) Buchholz, V.; Wegner, G.; Stemme, S.; O ¨ dberg, L. AdV. Mater. 1996, 8, 399-402. (4) Togawa, E.; Kondo, T. J. Polym. Sci. Pol. Phys. 1999, 37, 451-459. (5) Holmberg, M.; Berg, J.; Stemme, S.; O ¨ dberg, L.; Rasmusson, J.; Claesson, P. J. Colloid Interface Sci. 1997, 186, 369-381. (6) Zhang, L.; Ruan, D.; Zhou, J. Ind. Eng. Chem. Res. 2001, 40, 59235928. (7) Gunnars, S.; Wagberg, L.; Cohen Stuart, M. A. Cellulose 2002, 9, 239-249. (8) Rehfeldt, F.; Tanaka, M. Langmuir 2003, 19, 1467-1473. (9) Kontturi, E.; Thu¨ne, P. C.; Niemantsverdriet, J. W. Polymer 2003, 44, 3621-3625. (10) Edgar, C. D.; Gray, D. G. Cellulose 2003, 10, 299-306. (11) Pompe, T.; Zschoche, S.; Herold, N.; Salchert, K.; Gouzy, M.-F.; Sperling, C.; Werner, C. Biomacromolecules 2003, 4, 1072-1079. (12) Yang, C. Q.; Wang, X.; Lu, Y. J. Appl. Polym. Sci. 2000, 75, 327336. (13) Felix, J. M.; Gatenholm, P. J. Appl. Polym. Sci. 1991, 42, 609-620. (14) Hedenberg, P.; Gatenholm, P. J. Appl. Polym. Sci. 1995, 56, 641-651. (15) Kazayawoko, M.; Balatinecz, J. J.; Woodhams, R. T. J. Appl. Polym. Sci. 1997, 66, 1163-1173. (16) Matias, M.; De La Orden, M. U.; Gonzalez Sanchez, C.; Martinez Urreaga, J. J. Appl. Polym. Sci. 2000, 75, 256-266.
Freudenberg et al. (17) Rodriguez, C. A.; Medina, J. A.; Reinecke, H. J. Appl. Polym. Sci. 2003, 90, 3466-3472. (18) Qiu, W.; Zhang, F.; Endo, T.; Hirotsu, T. J. Appl. Polym. Sci. 2004, 91, 1703-1709. (19) Hadano, S.; Onimura, K.; Tsutsumi, H.; Yamasaki, H.; Oishi, T. J. Appl. Polym. Sci. 2003, 90, 2059-2065. (20) Osaki, T.; Werner, C. Langmuir 2003, 19, 5787-5793. (21) Rosenau, T.; Potthast, A.; Adorjan, I.; Hofinger, A.; Sixta, H.; Firgo, H.; Kosma, P. Cellulose, 9 2002, 283-291. (22) Gro¨be, A. Polymer Handbook, 3rd ed.; John Wiley: New York, 1989; pp V117-V170. (23) Bergstro¨m, L.; Stemme, S.; Dahlfors, T.; Arwin, H.; O ¨ dberg, L. Cellulose 1999, 6, 1-13. (24) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2-11. (25) Werner, C.; Ko¨rber, H.; Zimmermann, R.; Dukhin, S.; Jacobasch, H.-J. J. Colloid Interface Sci. 1998, 208, 329-346. (26) Rosenau, T.; Potthast, A.; Sixta, H.; Kosma, P. Prog. Polym. Sci. 2001, 26, 1763-1837. (27) Zimmermann, R.; Dukhin, S.; Werner, C. J. Phys. Chem. B 2001, 105, 8544-8549.
BM0492529
