Self-Organized Films from Cellulose I Nanofibrils ... - ACS Publications

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Biomacromolecules 2010, 11, 872–882

Self-Organized Films from Cellulose I Nanofibrils Using the Layer-by-Layer Technique Christian Aulin,*,†,‡ Erik Johansson,‡ Lars Wågberg,‡ and Tom Lindstro¨m§ BIM Kemi AB, Box 3102, SE-443 03 Stenkullen, Sweden, Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, The Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Innventia AB, Box 5604, SE-114 86 Stockholm, Sweden Received October 22, 2009; Revised Manuscript Received February 19, 2010

The possibility of forming self-organized films using only charge-stabilized dispersions of cellulose I nanofibrils with opposite charges is presented, that is, the multilayers were composed solely of anionically and cationically modified microfibrillated cellulose (MFC) with a low degree of substitution. The build-up behavior and the properties of the layer-by-layer (LbL)-constructed films were studied using a quartz crystal microbalance with dissipation (QCM-D) and stagnation point adsorption reflectometry (SPAR). The adsorption behavior of cationic/anionic MFC was compared with that of polyethyleneimine (PEI)/anionic MFC. The water contents of five bilayers of cationic/anionic MFC and PEI/anionic MFC were approximately 70 and 50%, respectively. The MFC surface coverage was studied by atomic force microscopy (AFM) measurements, which clearly showed a more dense fibrillar structure in the five bilayer PEI/anionic MFC than in the five bilayer cationic/anionic MFC. The forces between the cellulose-based multilayers were examined using the AFM colloidal probe technique. The forces on approach were characterized by a combination of electrostatic and steric repulsion. The wet adhesive forces were very long-range and were characterized by multiple adhesive events. Surfaces covered by PEI/anionic MFC multilayers required more energy to be separated than surfaces covered by cationic/anionic MFC multilayers.

Introduction Cellulose is probably the most frequently occurring biopolymer on earth. It is a long-chain polysaccharide composed of β-1,4-linked D-glucose rings. Cellulose fibrils of partially crystalline cellulose form the primary structural component in plant cell walls. The fibrils have excellent mechanical properties1 due to the crystalline arrangement of the cellulose molecules in the fibrils, where the crystals are held together by both van der Waals forces and hydrogen bonding. In most plants and cellulose-containing organisms, the fibrils are then merged into larger fibril aggregates held together by hydrogen bonding and van der Waals forces.2 The potential utilization in advance applications of the unique properties of microfibrillated cellulose (MFC) as a renewable, abundant, and biodegradable natural product has attracted an increasing interest. The preparation of MFC derived from wood was introduced by Turbak et al.3 and Herrick et al.4 more than two decades ago. Through a homogenization process, wood fibers are disintegrated, to give a material where the fibers are moderately degraded and where their substructural fibrils and microfibrils are liberated from the mesostructure of the fiber wall. The microfibrils are 5-10 nm thick and have a length of up to 1 µm and can, as such, be regarded as nanofibers. Despite their nanodimensions, the term “microfibrillated cellulose” is kept due to the earlier definitions by Turbak et al.3 and Herrick et al.4 Their strength, flexibility, and aspect ratio are, needless to say, interesting in large-scale applications. For example, MFC has a large potential to improve the tensile strength of paper5 and for applications in highstrength biocomposites.6-10 From the chemical structure of * To whom correspondence should be addressed. E-mail: caulin@ polymer.kth.se. † BIM Kemi AB. ‡ The Royal Institute of Technology. § Innventia AB.

cellulose, it is obvious that the surfaces of cellulose microfibrils carry a large number of hydroxyl groups, which makes the microfibrils suitable for the introduction of new functionalities via different reactions.11 Past work on model cellulose surfaces has mostly been conducted with regenerated cellulose materials.12-15 In these earlier publications, both modified and unmodified celluloses have been used. Dong et el.16 used sulphated microcrystalline cellulose to produce cellulose I model surfaces and most of the sulfate groups could be removed by a simple heat treatment. Fa¨lt et al.12 used spin-coating of cellulose dissolved in N-methyl morpholine oxide (NMMO) to produce surfaces composed of cellulose II, and Buchholz et al.17 prepared regenerated cellulose II films from silylated celluloses, TMSC, via Langmuir-Blodget deposition. Exposure of the formed TMSC films to wet HCl vapor cleaves the TMS side groups and this reaction makes possible the in situ conversion of TMSC films to thin films of cellulose. In the present work, an aqueous charge-stabilized colloidal dispersion of MFC in the native “cellulose I” crystal form14 has been used, continuing the studies on model cellulose I surfaces developed in our laboratories.14,18 The MFC dispersions are stabilized by either positive or negative surface charges, by respectively a covalent modification of the wood fibers using a quaternary amine, as described in this paper, or from the carboxymethylation of the wood fibers.18 This gives the MFC a charged surface useful in the build-up of polyelectrolyte multilayer (PEM) coatings using the layer-by-layer (LbL) deposition technique described by Decher et al. in the early 1990s.19 To produce LbL surfaces a negatively charged substrate surface (SiO2) is sequentially immersed in solutions containing cationic and anionic polyelectrolytes. Each adsorption step with charged polymer leads to a charge reversal, which promotes the adsorption of an oppositely charged polymer in the next

10.1021/bm100075e  2010 American Chemical Society Published on Web 03/02/2010

Self-Organized Films from Cellulose I Nanofibrils

treatment step. The LbL technique presents a great advantage over many other coating techniques in that it is possible to control and finely tune the thickness and structure of the coating on a nanometer scale, simply by adjusting factors such as the ionic strength and deposition pH and the number of dipping cycles.20 PEM treatment is already used in a wide range of applications, such as tissue engineering,21 immunosensing,22 and the incorporation and immobilization of proteins/polypeptides.23,24 Charge-stabilized colloid dispersions such as clay platelets,25,26 inorganic sheets,27 and dye particles28 have also been used to form multilayered structures. Previous work has incorporated the linear polymer cellulose sulfate into polyelectrolyte multilayer films,29,30 and Cranston and Gray have described the incorporation of colloidal cellulose nanocrystals.31 The use of MFC as an anionic colloid for the build-up of multilayers on a silica substrate has been reported by Wågberg et al.18 and by Aulin et al.32 Within the pulp and paper field, PEMs have also been formed on cellulose fibers.33-36 The primary objective of such research has been to increase paper strength by the sequential treatment of a fiber suspension with polyelectrolytes before sheet formation. PEM treatment of fibers has also been used to prepare electrically conductive fibers and papers, as reported by Wistrand et al.37 and Agarwal et al.38 Recent work has indicated that the deposition of multilayers on the surface of cellulose fibers will significantly increase the adhesion between the fibers.39 This also shows that the application of multilayers has a large potential to control the adhesion between solids. Extensive work has been undertaken on the measurement of the forces between surfaces with adsorbed, weak polyelectrolytes as a function of pH and ionic strength.40-42 To the knowledge of the authors there are only a few publications on the mechanical properties of wet multilayer films.43-46 In two recent publications, Richert et al.47 and Picart et al.,48 applied a contact mechanics evaluation of AFM measurements with a colloidal probe technique for films of polylysine and hyaluronan and microinterferometry for films of polystyrene sulfonate (PSS) and poly(allylamine) (PAH), respectively. Earlier investigations have indicated that it is possible to form multilayers from cationic polyelectrolytes and MFC.18,32 In the present study, the possibility of creating a multilayer using only charge-stabilized MFC dispersions is presented. The build-up and the properties of the LbL-constructed films were investigated using a quartz crystal microbalance with dissipation (QCM-D) and stagnation point adsorption refletrometry (SPAR). The adhesive properties of the MFC LbL-assembly, the surface morphology and film roughness were studied in detail by atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) was also used to evaluate the surface composition of the films. A detailed description of the viscoelastic properties of the layers and a careful evaluation of the interactions between the surfaces carrying multilayers will be important for optimizing the influence of the multilayers on adhesion. The surface composition and adsorption behavior of cationic/anionic MFC and polyethyleneimine (PEI)/anionic MFC have also been compared. The build-up of a multilayer using only charged colloids is very rare,49,50 and to the best of our knowledge, multilayer build-up using only charged cellulose I dispersions has not previously been reported.

Experimental Section Chemicals. Polyethyleneimine (PEI; Mw ) 60000, 50% aqueous solution according to the manufacturer) was purchased from Acros Organics, U.S. The molecular weight of the PEI sample was determined

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to be about 40500 g/mol and the polydispersity was 7.7, using gel permeation chromatography according to a previously described method.51 Because PEI contains primary, secondary, and tertiary amino groups, its charge will vary with pH. The charge density of PEI at pH 6.5 and 10, respectively, was determined to be about 9.5 and 1.8 meq/g using polyelectrolyte titration.52 The cationic MFC dispersion used was prepared by first reacting a dissolving pulp (Domsjo¨ dissolving plus, Domsjo¨ Mills, Sweden) with N-(2,3 epoxypropyl)trimethylammonium chloride. To 10 g (dry) of a 20% aqueous pulp dispersion 5 g N-(2,3 epoxypropyl)trimethylammonium chloride (g90%, Sigma Aldrich, Germany) was added together with 0.8 g NaOH dissolved in 0.8 g water. The dispersion was diluted with 100 mL isopropanol and was allowed to react at 50 °C for 2 h, after which the cationic pulp was washed with an excess of deionized water. The anionic MFC was prepared in a manner similar to a previously described procedure53 but using a carboxymethylation18 pretreatment of the fibers. In brief, the dissolving pulp (Domsjo¨ dissolving plus, Domsjo¨ Mills, Sweden) was first dispersed in deionized water at 10000 revolutions in an ordinary laboratory reslusher. The fibers were than solvent-changed to ethanol by washing in ethanol four times with an intermediate filtration step. The fibers were thereafter impregnated with a solution of monochloroacetic acid in isopropanol. This carboxymethylation reaction was allowed to continue for 1 h. Following this carboxymethylation step, the fibers were filtered and washed in three steps: first with deionized water, then with acetic acid (0.1 M), and finally with deionized water. The fibers were then impregnated with a NaHCO3 solution (4 wt % solution) to convert the carboxyl groups to their sodium form. Finally, the fibers were washed with deionized water and drained on a Bu¨chner funnel. Neither the cationic nor the anionic fibres were dried before use in the homogenization. After these treatments, the cationic/anionic fibers were homogenized using a high-pressure fluidizer (Microfluidizer M-110EH, Microfluidics Corp). Cellulose slurries containing a 2% pulp fiber suspension in deionized water were processed through the homogenizer. A total of six passes through the homogenizer were carried out, each with a subsequent dilution step, to ensure a proper dispersion of the microfibrils. The initial MFC dispersion concentration was 2%, via 1.5, 1, 0.5, 0.2, and 0.1%, to a final concentration of 0.01%. The surface charge densities of the cationic and anionic MFC at pH 7.2 and 6.5, respectively, was determined to be about 322 and 426 µeq/g using polyelectrolyte titration.52 The concentration of PEI used in the different experiments was 100 mg/L at a pH of 10. Silica Substrates. Silica was used as the substrate in all the reflectometry, QCM-D, and AFM experiments. AT-cut crystals (5 MHz resonant frequency) with an active surface of sputtered silica were supplied by Q-Sense AB for the QCM-D measurements. Immediately before use, the active surface was cleaned by rinsing with a water/ ethanol/water sequence. The surface was then made hydrophilic by a 3 min treatment in a plasma oven (PDC-002, Harrick Scientific Incorp, US) operating at 30 W under reduced air pressure. Polished silicon wafers (p-doped with boron) were obtained from MEMC Electronic Materials SpA (Novara, Italy) and were used as the base substrate for the reflectometry and AFM experiments. Three hours of oxidation at 1000 °C resulted in an oxide layer approximately 75 nm thick. Each substrate was measured with ellipsometry to determine the actual thickness of the SiO2 layer according to a procedure described earlier.54 The substrates were cut into strips or squares and cleaned in the same manner as above. Stagnation Point Adsorption Reflectometry (SPAR). A stagnation point adsorption reflectometer (SPAR) from the Laboratory of Physical Chemistry and Colloidal Science, Wageningen University, The Netherlands, was used to study the formation of the MFC-based multilayers on the silica substrates. The theory of the method is thoroughly described elsewhere.55,56 A brief description of the theory is available in the Supporting Information. Quartz Crystal Microgravimetry with Dissipation (QCM-D). Adsorption was also studied using a quartz crystal microbalance with

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Figure 1. SPAR data showing the relative change in the detected signal as a function of the adsorption and multilayer build-up of (a) cationic and anionic MFC and (b) PEI and anionic MFC. The multilayers were constructed using PEI solution and cationic/anionic MFC dispersions with concentrations of 100 mg/L. Rinsing with Milli-Q water was performed after each adsorption step.

dissipation (QCM-D) supplied by Q- sense AB (Va¨stra Fro¨lunda, Sweden). A brief description of the theory is available in the Supporting Information. X-ray Photoelectron Spectroscopy (XPS). XPS was used to characterize the chemical composition of the MFC-based multilayer films. A brief description of the method is available in the Supporting Information. Atomic Force Microscopy (AFM): Imaging. To characterize the film morphology and surface roughness of the multilayer films, AFM imaging was performed using a Nanoscope IIIa AFM (Vecco, Santa Barbara, CA). These images were scanned in tapping mode under ambient air conditions (23 °C and 50% relative humidity). RTESP silica cantilevers (Veeco, Santa Barbara, CA), each with a tip radius of 8 nm and spring constant of 40 N/m (values provided by manufacturer) were oscillated at their fundamental resonance frequencies, which ranged between 200 and 400 kHz. The roughness value of the prepared LbL films was determined from the height image over a 1 µm2 image and is presented as a root-mean-square (rms) value. No image processing except flattening was utilized. AFM: Force Measurements. Interactions between cellulose-covered surfaces were measured using a Nanoscope IIIa AFM with a Picoforce scanner (Veeco, Santa Barbara, CA). The general principle of force measurements using AFM is described in detail elsewhere57 and will not be further discussed here. The colloidal probe technique58 was used to measure the normal forces between two silica substrates covered with PEM. Borosilicate glass spheres (Duke Scientific, Inc., Fremont, CA) with a diameter of 10 µm were glued to standard V-shaped Si3N4 cantilevers (Veeco, Santa Barbara, CA) with a nominal spring constant of 0.12 N/m. For each individual measurement, the borosilicate probe diameter was measured using a light microscope (Nikon) and the normal spring constant was determined by the thermal noise method.59 The multilayers were formed in situ in the AFM liquid cell, so that both the flat silica substrate and the silica sphere were covered by multilayers and the substrates were never allowed to dry. The PEI solutions and MFC dispersions were injected into the liquid cell through a syringe filter with a 1.2 µm polyethersulfone membrane and were allowed to adsorb for 5 min, followed by rinsing with Milli-Q water for 5 min. The force-distance curves were then obtained for each layer in the multilayer at the end of the rinsing cycle. Representative force curves are shown.

Results SPAR. SPAR was used to study the multilayer formation of PEI/anionic MFC and cationic MFC/anionic MFC. The results are presented as the relative change in the reflected signal (∆S/ S0) to monitor the formation of multilayers. The exact determination of the adsorbed amount is difficult because the polyelectrolytes and microfibrils most probably diffuse into each other to a certain extent during the multilayer formation,60,61 producing a multilayer with a nondefined dn/dc value. The

results are presented in two separate figures (Figure 1a,b) due to the large differences in the reflected signals obtained and in the time scales of the experiments. Two experimental series were performed where the silicon oxide was consecutively treated with (1) cationic MFC (100 mg/L)/anionic MFC (100 mg/L) and (2) PEI (100 mg/L)/anionic MFC (100 mg/L), all in Milli-Q water. Following each polyelectrolyte/MFC treatment, a rinsing step was performed with Milli-Q water. It is important to rinse after each treatment to remove any nonadsorbing polyelectrolytes of fibrils and to ensure film stability.62 The relative change in the reflected signal, when the silicon oxide surface was consecutively treated with cationic MFC and anionic MFC, is shown in Figure 1a. A stable baseline was collected before treating the surface with the MFC. After 200 s, cationic MFC was introduced into the cell and the adsorption was followed to saturation. The equilibrium was reached after less than 100 s. The adsorption step was followed by a Milli-Q rinsing step after about 350 s, and a slight desorption could be detected. Anionic MFC was then introduced until adsorption saturation. The equilibrium was reached quite rapidly and, during the following rinsing, no desorption of the anionic MFC could be detected. Furthermore, there was an expected increase in the reflectometer signal (Figure 1) during each MFC adsorption step, and the signal detected for the cationic MFC during multilayer build-up was slightly higher than for the anionic MFC. The formation of a multilayer using PEI and MFC was followed by SPAR measurements. Figure 1b illustrates the change in the detected signal after the stepwise addition of PEI (100 mg/L, pH 10) and anionic MFC (100 mg/L, pH 6.5) with an intermediate rinsing step. As a first step, after stabilization of the baseline with Milli-Q water, PEI was added and the adsorption was followed to saturation. The equilibrium was reached very rapidly, the detected signal increased to approximately 0.02 and no desorption could be detected during the rinsing phase. A large increase in the detected signal was observed after injection of the MFC dispersion at 300 s. The adsorption process was slower than the adsorption of PEI, with a larger increase in the detected signal. As in the case of the PEI, no desorption of MFC was visible during rinsing. After the second addition of PEI, a small but rapid increase in the detected signal was observed. A larger increase in the detected signal was observed after the second addition of MFC than after the first MFC addition, but the adsorption was slower than for the first MFC adsorption and a longer time was necessary to reach saturation. This behavior became more pronounced as the number of adsorbed layers was increased. QCM-D. Figure 2a shows the LbL adsorption of cationic and anionic MFC from the normalized resonance frequency shifts corresponding to the overtones (at 15, 25, and 35 MHz)



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Figure 2. Formation of a multilayer from cationic and anionic MFC evaluated by the QCM-D technique. (a) Change in normalized frequency, ∆f/υ (proportional to the adsorbed mass), as a function of the adsorption and build-up of multilayers of cationic and anionic MFC (from the third, fifth, and seventh overtone). The MFC concentrations were 100 mg/L at pH 7.2 and 6.5 for the cationic and anionic MFC, respectively. (b) Change in energy dissipation through the multilayer build-up. (c) Change in energy dissipation as a function of change in frequency for the multilayer build-up for the third overtone.

measured by the QCM-D. A rinsing step, with Milli-Q water, was included between the cationic MFC and anionic MFC adsorption stages. A stable baseline was established prior to adsorption of the cationic MFC. At time t ) 5 min, a finite amount of cationic MFC (>2 mL, pH ) 7.2, 100 mg/L) was injected into the measurement chamber of the QCM-D, and a rapid decrease in frequency was observed due to the adsorption of the cationic MFC onto the anionic silica substrate. Cationic MFC was allowed to adsorb until a steady-state signal was reached. Upon rinsing with more than 2 mL (approximately 10 times the volume of the QCM-D measuring chamber) of the rinsing solution, no significant change in frequency was observed. At t ) 12 min, the rinsing solution was replaced by a dispersion of anionic MFC (100 mg/L). A distinct decrease in the resonance frequency was observed, due to the adsorption of the anionic MFC fibrils onto the cationic MFC layer. Again, free anionic MFC fibrils from the cell were rinsed away before the next adsorption step. The adsorption rate was fairly constant for the adsorption of each new MFC layer. This is not totally in agreement with the adsorption kinetic profile for the LbL adsorption of cationic and anionic MFC, as measured by SPAR, where the adsorption until saturation of the cationic MFC was slightly slower than that of the anionic MFC (Figure 1a). The decrease in frequency was almost the same for each new MFC layer deposited. The decrease in frequency upon adsorption of the first layer of cationic MFC, shown in Figure 2a, is associated with an increase in energy dissipation in the layer, as shown in Figure 2b. The fairly large change in dissipation for the initial cationic MFC layer implies that the layer is essentially soft and clearly viscoelastic. The increase in energy dissipation due to the adsorption of anionic MFC was slightly larger than that of the cationic MFC throughout the multilayer build-up, which suggests that the anionic MFC forms a more extended and/or more open structure than the cationic MFC. There is a general nonlinear increase in the dissipation as a function of the bilayer number, due presumably to the increasing load of MFC on the sensor crystal, and the magnitude of the dissipation increases for the third, fourth and fifth bilayers. A plot showing normalized

frequency data, ∆f/υ, for different overtones in the QCM-D, Figure 2a, shows that the data do not fall on a single line, indicating that there is a slight inhomogeneity in the LbL structure in the z-direction (Figure 2a). When the dissipation value, as in this case, typically reaches a level of about 1 × 10-6 per 10 Hz, the film is too soft to function as a fully coupled oscillator, that is, the upper parts of the adsorbed layer, far from the surface, do not couple elastically to the oscillation of the sensor. This also means that the most common way of calculating the mass directly from the change in frequency, the Sauerbrey equation, then slightly underestimates the mass.63 Hence, the Sauerbrey relationship can be used only as an approximation to convert the observed frequency shift to the adsorbed mass. The data were therefore analyzed using the Johannsmann model, which is described elsewhere.64 This will be further discussed later in the paper. Additional information on the build-up of the layers can be obtained by plotting the change in the dissipation factor as a function of the change in frequency (see Figure 2c). The increase in energy dissipation is proportional to the decrease in frequency during the multilayer build-up. This data indicates that all the adsorbing microfibrils adopted a similar surface conformation, regardless of the layer in which they were adsorbed. Figure 3a shows the LbL adsorption of PEI and anionic MFC from the normalized frequency shifts corresponding to the measured overtones of the resonance frequency. PEI and anionic MFC were adsorbed from 100 mg/L solutions at pH ) 10 and 6.5 respectively. The LbL adsorption was carried out with the same technique as for the cationic/anionic MFC deposition; i.e. by consecutive treatment with PEI and MFC. All solutions or dispersions were retained in the measuring chamber long enough to reach saturation adsorption. Large and distinct decreases in the resonance frequency were observed, especially following the adsorption of the anionic MFC fibrils onto the cationic PEI layer. The rate of adsorption decreased with each successive adsorption of a new MFC layer. The decrease in frequency was associated with an increase in energy dissipation in the adsorbed

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Figure 3. Formation of a multilayer from PEI and anionic MFC evaluated by the QCM-D technique. (a) Change in normalized frequency, ∆f/υ (proportional to the adsorbed mass), as a function of the adsorption and build-up of multilayers of PEI and anionic MFC (from the third, fifth, and seventh overtone). The PEI and anionic MFC concentrations were 100 mg/L at pH 10 and 6.5, respectively. (b) Change in energy dissipation through the multilayer build-up. (c) Change in energy dissipation as a function of change in frequency for the multilayer build-up for the third overtone. Figures adopted from Aulin et al.32

layer, as shown in Figure 3b. The small change in dissipation associated with the initial PEI layer implies that the PEI layer is essentially rigidly attached to the surface. The energy dissipation due to the adsorption of MFC was much greater than that for the previous PEI layer, which suggests that the MFC forms a more extended and more open structure. The increase in dissipation throughout the multilayer build-up was almost entirely governed by the MFC adsorption steps, and the magnitude of the dissipation increased significantly for the fourth and fifth bilayers. Figure 3a also shows the normalized frequency shifts as a function of the multilayer build-up process, demonstrating that the film is highly viscoelastic and not completely homogeneous in the z-direction. The ∆D versus ∆f plot in Figure 3c shows a nonlinear increase, suggesting that the polymers/ fibrils adsorbed in the outer layers form a more flexible/open structure than in the initial layers. Surface Morphology of the Cellulose-Based Multilayer Films: AFM Imaging. AFM height and phase images were recorded to determine the morphology and surface roughness of the cellulose films (Figure 4). All images were recorded over an area of 1 µm2, and the images and height profiles presented in Figure 4 are representative of the films. AFM height and phase images of a five bilayer cationic/anionic MFC multilayer film show a fibrillar network structure with an average rms roughness of 2.3 nm, determined from 1 µm2 areas (Figure 4a). The microfibrils have a fairly constant width of about 20 nm and appear, as expected, as stiff rods. However, taking into consideration the broadening due to the geometry of the tip, direct measurement of the fibrils in Figure 4a shows an average width of 5 nm. This is in good agreement with earlier published TEM micrographs, indicating a microfibril width of about 5 nm for these filbrils that have been modified to contain higher amount of charged groups compared with native fibrils.18 Compared to the cationic/anionic five bilayer MFC film, the PEI/ancionic MFC film had a dense structure with an rms roughness value of 2.8 nm (Figure 4b). The microfibrils fully cover the silica surface. This is clearly seen in the phase images. Several 1 µm2 areas were measured, and no open spots were

detected. XPS was used to characterize the chemical composition of the MFC-based multilayer films. The XPS data supports the fibril coverage on the silica surface (Table S1). AFM Force Measurements. Figure 5 shows representative force-distance curves upon approach for silica substrates covered by MFC multilayers with different numbers of bilayers. The curves shown correspond to anionic MFC in the outermost layer. Because the adsorbed layers were thick and soft, a true constant compliance region was never reached and it was hence difficult to determine the point of zero separation. As proposed by Pettersson et al.,65 the pure silica-silica system was used to determine the deflection sensitivity of the cantilever, R (m/V), which was then applied for all subsequently adsorbed layers. To be able to compare the different force curves, it was also assumed that the point of zero separation corresponded to a normalized compression force of 0.9 mN/m and the maximum applied load was 3.6 mN/m. Thus, the parts of the force curves with negative values of apparent separation correspond to a compression of the adsorbed multilayers. Figure 5a shows that the forces on approach for the cationic/ anionic MFC multilayers were purely repulsive, and the range of repulsion increased somewhat with increasing bilayer number. The shape of the curves on a log-linear scale is fairly linear. On the negative apparent separation region of the curves, it can be seen that the compression increased with increasing number of adsorbed bilayers. For the first and second bilayers, the compression was about 10 nm at maximum load, and for the fourth bilayer, the maximum compression increased to about 20 nm. Figure 5b shows that, for the PEI/MFC multilayers, the approach forces for the fibril-containing LbL were also repulsive, but there was no clear relation between the layer number and range of the forces. The repulsive forces acted over a longer range for the PEI/MFC system than for the pure cationic/anionic MFC system but the PEI/MFC force curves lacked the linearity that was seen in the case of the cationic/anionic MFC multilayers. The compression of the PEI/MFC multilayers at maximum load increased with increasing layer number and was



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Figure 4. AFM tapping mode height and phase images of the multilayer films on silica. Height images: (a) cationic MFC/anionic MFC and (b) PEI/anionic MFC. Phase images: (c) cationic MFC/anionic MFC and (d) PEI/anionic MFC. The scanned surface areas were 1 µm2 and the z-ranges are 25 nm. Typical height profiles are also shown, all with a z-range of 25 nm.

around 40, 75, 125, and 175 nm for the first, second, third, and fourth bilayer, respectively. The compression was also much higher than for the cationic/anionic MFC, as expected, considering the higher adsorbed amounts for the PEI/MFC system, as shown by SPAR and QCM-D. Figure 6 shows a typical force-distance curve upon separation for anionic MFC in the outermost layer of the third bilayer of a PEI/anionic MFC multilayer. The appearance of this curve was typical for all force curves upon separation, both for the pure MFC multilayer system and for the PEI/MFC system. However, the range, magnitude, and force pattern were different for the different layers and different systems. The retraction force curves were characterized by multiple adhesive events, that is, the joints between the multilayer-covered silica substrates did not break in a single snap but instead broke in several steps. It should also be noted that the range of the adhesive forces on separation was very high, extending over several micrometers.

Discussion Adsorption Characteristics of Cellulose-Based Multilayer Films. In the present work, the interactions between PEI/ anionic MFC and cationic/anionic MFC as well as the multilayer build-up of these components have been investigated by means of SPAR, QCM-D, AFM, and XPS. The trends rather than the absolute adsorption values were of primary interest in this study. Because the change in the signal in the SPAR experiments can be transformed to an adsorbed amount, and because QCM-D detects both the absorbed polymer and the associated solvent,

it is to be expected that the sensed mass detected by the QCM-D would be larger than that detected by the SPAR. Furthermore, by combining the results obtained by the two techniques, it has been shown that it is possible to estimate the water content in the adsorbed layers.32,66,67 Silicon oxide surfaces were consecutively treated with cationic/anionic MFC and PEI/anionic MFC. The SPAR signal was converted to an adsorbed amount according to a four-layer optical model, described in the Supporting Information, and the results, adsorbed mass as a function of layer number, are presented in figure 7a. The adsorbed amount of cationic/anionic MFC seems to increase linearly with increasing number of layers and the total adsorbed amount of the five-bilayer film is estimated to be about 6 mg/m2. This is considerably lower than the adsorbed amount for the PEI/anionic MFC multilayer film, which was determined to be 30 mg/m2 for a similar number of layers. We suggest that the adsorption of the cationic MFC onto the anionic MFC network is not completely governed by the electrostatic driving force, but rather that there is a balance between a steric effect derived from the stiff and highly crystalline microfibrils inhibiting the adsorption and the electrostatic driving force promoting the adsorption. The higher mass adsorbed in the PEI/MFC than in the cationic/anionic MFC multilayer might also be a consequence of the difference in charge, flexibility, and geometrical restriction of the PEI molecules and MFC fibrils. PEI is a highly charged and branched molecule known to form almost spherical structures in water68,69 that might be able to penetrate into the

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Figure 5. Normalized force versus apparent separation upon approach for silica substrates covered by multilayers formed from (a) cationic MFC/anionic MFC and (b) PEI/anionic MFC. The PEI and cationic/anionic MFC concentrations were 100 mg/L at pH 10, 7.2, and 6.5 respectively. The force curves correspond to anionic MFC in the outermost layer of bilayer 1 (2), 2 (∆), 3 (b), and 4 (O).

Figure 6. Normalized force versus apparent separation upon retraction for silica substrates covered by MFC multilayers. The force curve corresponds to anionic MFC in the outermost layer of the third bilayer.

voids of the highly swollen MFC fibril network. Similar effects have been directly measured by neutron reflectivity on a close system by Jean et al.70 The higher mass adsorbed in the PEI/ MFC system thus indicate that PEI forms a very good link between the MFC fibrils, as was also reported by Wågberg et al.18 when studying the multilayer build-up of various cationic polyelectrolytes and anionic MFC using ellipsometry. The difference in charge density between PEI and MFC may explain the considerably smaller adsorbed amount of the cationic MFC/ anionic MFC compared with PEI/anionic MFC. The high charge

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of the PEI means that it can recharge the SiO2 surface more efficiently than the cationic MFC, allowing a higher saturation adsorption of anionic MFC in the second layer. This higher adsorbed amount will naturally propagate in the entire structure when a multilayer consisting of PEI and anionic MFC is compared with a multilayer of cationic and anionic MFC. As mentioned in the experimental section, the charge of the PEI was about 1.8 meq/g, which is around 6-9 times the charge of the cationic MFC. Apart from this, the spherical geometry of the PEI will contribute to a higher adsorbed amount.18 Aulin et al. studied the adsorption behavior of PEI/anionic MFC multilayers using the dual-polarization interferometry technique32 and reported an adsorbed mass of 32.6 mg/m2 for five bilayers. It is interesting to recognize that the adsorbed mass in the PEI/ anionic MFC multilayer build-up was about the same according to both SPAR and DPI measurements, thus supporting the accuracy in previous studies,32 despite the difference in, that is, optical model systems, flow rate, and flow cell structure used in the two techniques. The adsorbed amount of the first layer PEI (charge density ) 1.8 meq/g at pH 10) in the PEI/anionic MFC multilayer was 0.35 mg/m2 (Figure 7a) corresponding to 0.63 µeq/m2. After rinsing with Milli-Q water, the pH was lowered, followed by the adsorption of anionic MFC at pH ) 6.5. At a pH of about 6.5, the charge of PEI increases to about 9.5 meq/g, resulting in adsorbed PEI with a charge of 3.3 µeq/m2. The adsorbed amount of anionic MFC (charge density ) 426 µeq/g) was 3.17 mg/m2 corresponding to 1.35 µeq/m2. Thus, the high charge of the PEI-covered silica surface was not fully utilized by the anionic MFC, probably because of the geometric/steric restrictions imposed by the microfibrils and a neutralization of the PEI charges by the silicon oxide surface. Assuming that the microfibrils have a 5 × 5 nm square cross-section and a length of 1000 nm, the maximum adsorbed amount is 2 × 1014 microfibrils/m2 if the microfribrils are close-packed. If the density of the cellulose fibrils is 1600 kg/m3, the mass is 4 × 10-14 mg/microfibril. When this value is used, the adsorbed amount of 3.17 mg/m2 (Figure 7a) corresponds to 0.79 × 1014 microfibrils/m2. This is in fair agreement with the maximum possible adsorbed amount of 2 × 1014 microfibrils/m2 considering that the microfibrils are not adsorbed in a close-packed configuration, as indicated by the AFM images in Figure 5. This supports the hypothesis that the adsorption of anionic MFC in the PEI/anionic MFC multilayer is geometrically restricted rather than only limited by the accessible amount of PEI-charges. However, if PEI were adsorbed at pH 6-7, the higher charge density of the PEI (9.5 meq/g pH 6.5) and the lower surface charge of the silica surface would probably lead to a much lower adsorbed amount of PEI, and the adsorption of anionic MFC would then be limited by the amount of accessible charges rather than by the geometry. Aulin et al.32 recently showed that the adsorption of PEI at lower pH gives rise to a proportionate decrease in the adsorbed amount of the PEI/anionic MFC bilayer. A change from pH ) 10 for the adsorption of PEI to pH 6.5 for the adsorption of anionic MFC is favorable for high adsorbed amounts of the PEI/MFC multilayer. For the cationic/anionic MFC multilayer, the pH was almost neutral during the adsorption of both cationic and anionic MFC. Therefore, the changes in charge of the adsorbed layers after rinsing and following adsorption should be negligible. The adsorbed amount of the first layer of cationic MFC (charge density ) 322 µeq/g) was 0.99 mg/m2 corresponding to 0.32 µeq/m2. In the first layer of anionic MFC, with a charge density of 426 µeq/g, the adsorbed amount was 0.60 mg/m2 corre-



879

Figure 7. Estimated adsorbed amount of multilayers from (9) PEI/anionic MFC and (0) cationic/anionic MFC with (a) SPAR and (b) QCM-D. PEI (100 mg/L, pH ) 10) and cationic/anionic MFC (100 mg/L, pH ) 7.2 and 6.5, respectively) was used for multilayer build-up. Dry adsorbed mass as monitored by SPAR was estimated using a four-layer optical model. The adsorbed mass included trapped water was calculated by the Johannsmann model and monitored by QCM-D.

sponding to 0.25 µeq/m2. The amounts adsorbed in the subsequent layers were rather constant at about 0.2 µeq/m2. Assuming the same fibril dimensions as above, the adsorbed amounts correspond to 0.25 × 1014 fibrils/m2 and 0.15 × 1014 fibrils/m2 for the first layers of cationic and anionic MFC, respectively. The geometric maximum possible adsorbed amount of 2 × 1014 fibrils/m2 was far from being reached, suggesting that the adsorption in the cationic/anionic MFC system was restricted by electrostatic repulsion of the fibrils. Because there was no pH change to increase the charge of already adsorbed layers, it is expected that the adsorbed mass of the cationic/ anionic MFC multilayer would be much lower than that of the PEI/MFC multilayer. When the adsorbed amount obtained in the SPAR was compared with the Johannsmann mass calculated from the frequency shifts in the QCM-D, two distinct differences could be seen (Figure 7a,b). First, the QCM-D mass (Figure 7b) was substantially higher than the SPAR mass (Figure 7a). This is expected, as previously described, because the mass sensed by the quartz crystal is not only the mass of polymer but also the mass of the water molecules that are bound to or trapped within the polymer film.71,72 The trapped water may be due to hydration of the carboxymethylated MFC fibrils or water mechanically captured in the voids between the fibrils. The MFC multilayer films are highly swollen and viscoelastic, as indicated by the high dissipation values accompanying the adsorption of MFC, as shown in Figures 2 and 3. The high dissipation values may be because the MFC fibrils are oriented outward into the solution. The QCM-D and SPAR data in Figures 1-3 show the kinetics of the build-up of the multilayers as a function of time. The QCM-D and SPAR methods differ significantly in terms of how the adsorbate is transported to the surface. In SPAR, the solutions are constantly pumped through the cell, which in total is 100 µm thick. During adsorption, the laminar flow rate used in our experiments was 1 mL/min and, because new solution entered the measuring cell continuously, there was no depletion of the adsorbing species outside the surface. In contrast, no continuous material transport is applied in the QCM-D cell and, after the solutions are injected, the adsorbing species have to diffuse to the surface in order to be adsorbed. Despite these significant differences in the transport conditions of the species, there are very large similarities in terms of adsorption kinetics for the multilayer build-up indicating that other factors than the flow is controlling the multilayer formation. In the build-up of the PEI/MFC multilayer, the rate of adsorption of MFC decreased with each new deposited MFC layer. This may indicate conformational changes and/or a reorientation of MFC approaching the surface. Because one adsorbed layer of MFC does not fully cover the silica surface, as shown by the AFM

images presented by Aulin et al.,32 the next adsorbed layer of MFC may to some extent penetrate the voids between the previously absorbed microfibrils, and this could in principle lead to a roughening of the film, with some fibrils being oriented outward into the solution. Such a behavior would lead to large increases in dissipation, as were indeed observed in the QCM-D measurements shown in Figure 2b. The large number of fibrils needed to recharge the preadsorbed PEI layer also leads to a crowding at the surface where the adsorbing fibrils repel each other. This will lead to a slower adsorption, especially when new material is continuously being fed to the surface as in the SPAR experiments. In contrast to the slow adsorption of MFC onto PEI, the multilayer build-up of cationic/anionic MFC is considerably more rapid. This may be due to the smaller number of fibrils needed to recharge the surface in each adsorption step and, thus, a lower concentration of fibrils close to the surface and less hindrance to adsorption, resulting in a rapid and rather small amount adsorbed. The adsorbed masses of the five bilayers of cationic/anionic MFC and PEI/anionic MFC were estimated from SPAR to be about 6 and 30 mg/m2, and the QCM-D sensed mass were determined to be about 18 and 59 mg/m2, respectively. A common result for both the SPAR and the QCM-D experiments was that a higher mass was adsorbed in the system where PEI had been used. The total water content for the cationic/anionic MFC, PEI/anionic MFC multilayer films was approximately 70 and 50%. The AFM images (Figure 4) clearly demonstrate the differences in surface coverage of the MFC fibrils associated with the different strategies, that is, a higher surface coverage of MFC associated with the PEI/anionic MFC multilayer buildup. Assuming that the difference in the adsorbed mass calculated from the two methods, QCM-D and SPAR, corresponds to the mass of water within the film, a plot of the relative difference in adsorbed mass, as shown in Figure 8, would illustrate how the water content changed during multilayer build-up. The water content in the first three layers differed slightly when comparing the multilayer films. For the cationic/anionic MFC film, the water content seems to be rather stable at about 70%. The water content of the PEI/MFC multilayer initially dropped as more layers were added, indicating that the film density changes during multilayer build-up; particularly that the density increased in the first regime, that is, for the first three or four layers, as mentioned previously. However, as more layers were adsorbed, a water content of about 50% was found to be consistent. One explanation of the change in density during the build-up of the film is an increase in packing efficiency, as more and more fibrillar layers are formed. In the first layer, the entire surface is not covered, as mentioned earlier but, as more layers are formed, the gaps can be filled and the amount of water between

880


Figure 8. Water content in the multilayer films calculated as the relative difference between the SPAR mass and the Johannsmann mass calculated from QCM-D measurements: PEI/MFC (9) and cationic/anionic MFC (0).

the fibrils will hence decrease. For this to occur, it is obvious that a sufficiently high charge of the cationic component is needed. Interaction between MFC Multilayers. Apart from exploring the adsorption behavior of different MFC multilayers, one objective of the present work was also to study the interaction and the forces acting between MFC multilayers in Milli-Q water. This is especially interesting because it is, to the knowledge of the authors, the first time that pure cellulose surfaces have been prepared using the LbL technique. The high QCM energy dissipation values and the high estimated water contents of the films indicate that the films were thick and very soft and, as previously mentioned in the Results, it was therefore difficult to determine the point of zero separation. A force of 0.9 mN/m was used as a definition of zero separation. This zero separation force could of course have been differently chosen, and this would have led to slightly different absolute values of apparent separation but the trends would have been the same. The force curves on approach for the cationic/anionic MFC multilayer (Figure 5a) are fairly linear, indicating that electrostatic forces dominated the interaction. The magnitude of the forces was quite low, corresponding to a rather low surface potential, which corresponds well with the low adsorbed amounts and the low surface coverage found in the SPAR, QCM-D, and AFM imaging. The low adsorbed amounts suggest that the charge reversal upon adsorption of a new layer resulted in quite a low net charge of the multilayer. For the PEI/anionic MFC multilayers, the forces on approach were of longer range, see Figure 5b. This could be due partly to higher adsorbed amounts, as found in the SPAR and QCM-D measurements, thus resulting in higher net charge and higher surface potential, but the nonlinearity of the curves also suggests that steric forces added to the long-range repulsive forces on approach. The uneven slope of the PEI/anionic MFC approach curves and the fact that there was no relation between layer number and the range of the repulsive approach forces on approach for this system can possibly be explained by considering the shape of the retraction curve shown in Figure 6. The retraction curves were characterized by multiple adhesive events and the forces were very long-range, ranging over several micrometers. This has been seen for other systems such as chitosan-covered cellulose surfaces73 and surfaces covered with starch multilayers,45 and in these cases the shape of the curves has been interpreted in terms of molecular bridging. The MFC multilayers in the present study, however, require a somewhat different explanation. Expansion of the fibril network and flip up of single fibrils offer a possible explanation for the multiple adhesive

Aulin et al.

Figure 9. Work of adhesion, WA, required to separate surfaces covered by multilayers constructed from PEI/anionic MFC (9) and cationic MFC/anionic MFC (0). The data points represent the average values and the bars represent the data range.

events seen in the shape of the retract curves, and considering that the length of the cellulose microfibrils can be up to several micrometers, this also explains the long-range of the adhesive forces. Fibrils protruding from the surface may also offer an explanation for the uneven approach curves of the PEI/anionic MFC multilayers. If protruding fibrils do not have time or are physically hindered from relaxing back into the multilayer, they might induce long-range steric forces that might not be very linear or predictable. Because the retraction curves looked similar for the pure MFC multilayers, it might have been expected that the approach curves for this system would not be so linear, but the lower adsorbed amounts possibly mean less mechanical blocking when the protruding fibrils try to relax back into the multilayers, giving less steric forces on approach. Figure 9 shows the work of adhesion required to separate the multilayer covered surfaces as a function of bilayer number. The work of adhesion was calculated by integrating the force curves on separation and was of the order of femtojoules. It can be seen in Figure 9 that a higher energy was required to separate the PEI/anionic MFC surfaces than to separate the cationic/anionic MFC surfaces. This suggests that the fibrils in the PEI/anionic MFC system required more energy to be pulled out from the multilayer and thus the relaxation of pulled-out fibrils should also be more energy-demanding in the PEI/anionic MFC system than in the cationic/anionic MFC system, which supports the hypothesis of a higher degree of mechanical blocking of protruding fibrils in the PEI/anionic MFC system. If MFC multilayers are to be considered for improving the adhesive properties of solid substrates, Figure 9 implies that PEI/anionic MFC multilayers are to be preferred rather than cationic/anionic MFC multilayers. When the multilayers were in contact and were compressed, the trends were as expected for both the cationic/anionic MFC and for the PEI/anionic MFC multilayers. Thicker layers appeared softer, enabling higher compression, and the slopes of the curves were less steep, indicating a lower elastic modulus. The compression was up to about 20 nm for the fourth bilayer of cationic/anionic MFC and 175 nm for the fourth bilayer of PEI/anionic MFC. In the QCM-D measurements, the thicknesses of the corresponding layers were about 10 and 50 nm, respectively, for the two interacting multilayers. These values are in reasonable agreement considering the difficulty of determining the point of zero separation and the possible fibrils protruding out from the surface. Indentation measurements using colloidal probe AFM have previously been used to calculate


the elastic modulus of adsorbed multilayers,47,74 but the films in those studies were several micrometers thick, whereas the thickness of the films in the present work was in the nanometer range. Therefore, in the present study, the influence of the hard underlying silica substrate had too large an influence on the film stiffness to enable the film modulus to be assessed. However, the higher compression of the PEI/anionic MFC layers also leads to a larger molecular contact area in the contact zone and this will give rise to a higher pull-off work of adhesion.

Conclusions SPAR and QCM-D measurements showed that biobased LbLfilms can be successfully constructed from cationic/anionic and PEI/anionic cellulose I nanofibrils. The adsorbed amount of the PEI/anionic MFC multilayer was approximately four times larger than that of the multilayer formed from cationic/anionic MFC. AFM tapping-mode imaging of MFC-based multilayer films showed randomly adsorbed microfibrils with a typical width of about 4 nm covering the silicon surfaces to various degrees. By combining SPAR and QCM-D measurements, the water content of the highly swollen PEI/anionic MFC and cationic/anionic MFC films were estimated to be about 50 and 70%, respectively. The high water content is probably associated with the water molecules entrapped in the pores formed among the MFC microfibrils in the multilayer structure and with the significant hydration of the carboxymethylated microfibrils. AFM colloidal probe measurements showed that the forces between two approaching MFC multilayers were repulsive, due to a combination of electrostatic and, especially for the PEI/ anionic MFC system, steric forces. When the multilayers were in contact, the compression of the multilayers at maximum load increased with increasing thickness of the multilayer films. The adhesive forces on separation of the surfaces were very longrange and were characterized by multiple adhesive events. The separation of PEI/anionic MFC-covered surfaces required more energy than the separation of surfaces covered by cationic/ anionic MFC multilayers. Acknowledgment. The authors thank BIM Kemi Sweden AB and the Knowledge Foundation through its graduate school YPK for financial support. E.J. acknowledges the Swedish Center for Biomimetic Fiber Engineering (Biomime) and Lyckeby Sta¨rkelsen AB for financial support. Dr. Torbjo¨rn Pettersson and ¨ dberg are greatly acknowledged for valuable Professor Lars O discussions. M.Sc. Mikael Ankerfors is greatly thanked for supplying the MFC samples. Dr. Andrei Shchukarev at Umeå University is acknowledged for performing the XPS experiments. Supporting Information Available. Theory of stagnation point adsorption reflectometry and quartz crystal microbalance with dissipation, and XPS experimental setup and results. This material is available free of charge via the Internet at http:// pubs.acs.org.

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