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Influence of Surface Charge Density and Morphology on the Formation of Polyelectrolyte Multilayers on Smooth Charged Cellulose Surfaces Tobias Benselfelt, Torbjörn Pettersson, and Lars Wagberg Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04217 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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Influence of Surface Charge Density and Morphology on the Formation of Polyelectrolyte Multilayers on Smooth Charged Cellulose Surfaces Tobias Benselfelt,* Torbjörn Pettersson, and Lars Wågberg* Department of Fibre and Polymer Technology and Wallenberg Wood Science Center, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden KEYWORDS Carboxymethylation, Cellulose, Layer-by-Layer, NMMO, oxidation, pDADMAC, PSS, Surface charge, Surface potential, TEMPO. ABSTRACT In order to clarify the importance of the surface charge for the formation of polyelectrolyte multilayers, Layer-by-Layer (LbL) assemblies of polydiallyldimethylammonium chloride (pDADMAC) and polystyrene sulfonate (PSS) have been investigated on cellulose films with different carboxylic acid contents (20, 350, 870, and 1200 µmol/g) regenerated from oxidized cellulose. The wet cellulose films were thoroughly characterized prior to the multilayer deposition using quantitative nanomechanical mapping (QNM), which showed that the mechanical properties were greatly affected by the degree of oxidation of the cellulose. Atomic force microscopy (AFM) force measurements were used to determine the surface potential of the cellulose films by fitting the force data to the DLVO theory. With the exception of the 1200

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µmol/g film, the force measurements showed a second order polynomial increase in surface potential with increasing degree of oxidation. The low surface potential for the 1200 µmol/g film was attributed to the low degree of regeneration of the cellulose film in aqueous media due to increasing solubility with increasing charge. The multilayer formation was characterized using a quartz crystal microbalance with dissipation (QCM-D) and stagnation point adsorption reflectometry (SPAR). Extensive de-swelling was observed for the charged films when pDADMAC was adsorbed due to the reduced osmotic pressure when ions inside the film were released, and the 1:1 charge compensation showed that all the charges in the film were reached by the pDADMAC. The multilayer formation was not significantly affected by the charge density above 350 µmol/g due to interlayer repulsions, but it was strongly affected by the salt concentration during the layer build-up. INTRODUCTION Over the past decade, cellulosic nanomaterials i.e. cellulose nanofibrils (CNF), have received considerable attention as an interesting renewable material from trees and other cellulose-rich plants. The energy consumption required to disintegrate pulp fibres into single fibrils can be greatly reduced by oxidation of the hydroxyl groups into carboxylic acids to introduce charges in the fibre wall. The carboxylate entities create a swollen ionic network that is easier to separate into individual fibrils,1, 2 and the energy demand for the disintegration can be reduced from 30 000 kWh/ton down to less than 1000 kWh/ton.3 The oxidative pre-treatment results in highly charged nano-sized fibrils, that can be turned into materials with as many as 2300 µmol carboxylate entities per gram (µmol/g).4 CNF can be used to assemble thin films5,

6

or bulk

materials such as nano-papers,7, 8 aerogels,9, 10, hydrogels,11, 12 foams,13, 14 filaments,15, 16, 17 and they can also function as a reinforcement in composite materials.18 The diversity of CNF


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provides the basis for a new material platform of starting materials that can be tailored for different applications, which gives nanocellulose a key role in a broad range of applications in a bio-based economy. In order to adapt the CNF materials to different end-use applications, the surface chemistry, in all its different details, has to be tailored to ensure a maximum utilization of the excellent properties of CNF. As a result of an introduced charge, cellulose surfaces can efficiently be modified by assembling thin films of polymers or particles in a sequential procedure known as the Layer-byLayer (LbL) technique.19,

20, 21, 22

Despite the vast literature on the LbL technology,23

astonishingly little attention has been given to the influence of the surface charge on the formation of LbLs at the solid-liquid interphase. The use of highly charged CNFs to create new materials from an aqueous environment has made the surface charge an even more important property, and a series of well characterized cellulose model surfaces would be ideal for clarifying the importance of the surface charge for the LbL formation. Spin-coated cellulose II model surfaces, first described by Gunnars et al.24 and later characterized by Aulin et al.25 provide an opportunity to create well-defined and smooth cellulose surfaces with different charge densities for use in high resolution measurement techniques. This can be achieved by the water-assisted regeneration of oxidized pulp dissolved in a mixture of N-methylmorpholine N-oxide (NMMO) and dimethyl sulfoxide (DMSO). Cellulose model surfaces with charge densities in the range of 100 - 1000 µmol/g have previously been prepared from pulp,26,

27, 28

but there is no literature showing how the charge affects the

morphology and surface potential of the highly charged films (>500 µmol/g) or the LbL formation on these surfaces.


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The adsorption of polyelectrolytes to cellulose-rich fibre surfaces as retention aids29 and/or strength additives30 has been an important feature of papermaking for a long time. The fibre/fibre joint properties in the paper have been shown to be further improved by the LbL assembly of polyelectrolyte films on the fibre surface prior to the papermaking.31 The use of LbL films to introduce a specific functionality to the treated cellulose surface is an additional feature. Lightweight aerogels with a large specific surface area can be created from CNF to provide a new material platform that can be functionalized by LbL assemblies, with potential applications such as charge storage devices and fire-retarding insulation materials.4 The properties required of these materials will depend on the nanoscale assembly of the functional material, which is probably affected by both the surface charge and the micro- to nano-scale morphology. In this respect a fundamental knowledge of how the surface charge of the cellulose-based material influences the material properties and multilayer assembly is essential for understanding how to best tailor materials from the nano-scale components. The quartz crystal microbalance with dissipation technique (QCM-D) has been used to study the adsorption of polyelectrolytes with high charge density,27, 32, 33, 34 and low charge density,28, 33, 34 onto cellulose surfaces with a carboxylic acid content in the range of 0-1000 µmol/g. It was shown that a higher carboxylic acid content in the cellulose film lead to an increased adsorption of polyelectrolyte.32 An apparent de-swelling of the cellulose film was observed during the adsorption of high-charge-density polyelectrolytes, and was manifested as a positive frequency shift during the adsorption and an increase in the rigidity of the film. The literature shows that systems with many charged groups are difficult to investigate using QCM-D, since it is difficult to distinguish and draw any conclusion about the adsorption behavior when the mass change observed in QCM-D is affected or even dominated by the release of water from the cellulose


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film. The present work shows that the use of an optical measurement technique can circumvent the problem of the swelling in QCM-D, making it possible to quantify polyelectrolyte adsorption onto cellulose film with high charge densities. The salt concentration of the solution in which the LbL assembly takes place is an important factor that controls the swollen cellulose structure, the assembly kinetics, and the morphology of the adsorbed polyelectrolyte layers.22 Polydiallyldimethylammonium chloride (pDADMAC) and polystyrene sulfonate (PSS) have been used as a model system to understand fundamental aspects of LbLs. Since they are both strong polyelectrolytes, in that they carry charges over the greater part of the pH range, it is possible to deduct the pH dependence of the polyelectrolyte from the adsorption behavior. Several reports have shown that pDADMAC has an adsorption maximum on cellulose model surfaces32,

34

, cellulose-rich fibres,35 and a self-assembled

monolayer with carboxylic acid end groups36 in the monovalent salt concentration of 10-100 mM. Pure electrostatic simulations made by J. Forsman37 showed that the adsorption maximum is reached at salt concentrations in the range of 5 to 20 mM, depending on the molecular weight of the adsorbed polyelectrolyte. For cellulose fibres and model surfaces, the adsorption declines and reaches a value close to 0 in the range 0.3-1 M NaCl.32, 34, 35, 38 Despite all the literature on the adsorption profiles of pDADMAC and PSS, no systematic studies have been carried out to show how different densities of charged groups will affect the adsorption and LbL formation. In the present work, multilayers of pDADMAC and PSS were assembled onto differently charged cellulose films at different salt concentrations. The adsorption was quantified using QCM-D and stagnation point adsorption reflectometry (SPAR), which in combination can also be used to estimate the degree of de-swelling upon adsorption. The cellulose films were characterized using atomic force microscopy (AFM), AFM force measurements and Fourier


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transform infrared spectroscopy (FTIR) to investigate the properties of the highly charged cellulose model surfaces. The aim was to evaluate how the LbL assembly is affected by the charge of materials created from highly charged cellulose fibres and nanofibrils. EXPERIMENTAL SECTION Preparation of charged cellulose Never-dried dissolving pulp was provided by Domsjö Fabriker AB, Örnsköldsvik, Sweden, with a carboxylic acid content of roughly 20 µmol/g. The dissolving pulp was further oxidized to three different carboxylic acid contents: 350 µmol/g, 870 µmol/g and 1200 µmol/g. The 350 µmol/g pulp was prepared using carboxymethylation by a previously described procedure6. The 870 and 1200 µmol/g pulps were prepared at pH 6.8 and pH 10 respectively using a 2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO)-mediated oxidation developed by Saito et al.1, 2 The 1200 µmol/g pulp (4 g) was reduced for 1 hour in room temperature in 1 L Milli-Q water containing dibasic sodium phosphate dihydrate (0.01 M, 1.42 g, Sigma Aldrich) and sodium borohydride (0.053 M, 2 g, Sigma Aldrich) in order to remove aldehydes or ketones created due to incomplete oxidation. Detailed descriptions of the three oxidation procedures can be found in the Supporting Information. The charge was determined by conductometric titration39 of 0.5 g pulp in duplicate for each pulp sample. The values given are the average values from repeated measurements with a deviation of less than 3% for all samples. This was also done after reduction of the aldehydes and ketones in the 1200 µmol/g pulp to show that the concentration of carboxylic acids was the same before and after the reduction. The dry content of the pulps was measured using a HB43-S Halogen Moisture Analyzer from Mettler Toledo. The pulps were further characterized using FTIR with


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an ATR module (Perkin-Elmer Spectrum 2000) to identify any differences between the prepared pulps. Cellulose model surfaces Cellulose model surfaces were prepared on Silicon wafers (boron-doped, p-type, thickness 610640 µm) used for reflectometry measurements supplied by MEMC electronic materials Novara Italy (double-side polished) or Addison Engineering Inc. San José, US, (single-side polished). Cellulose model surfaces were also prepared on QCM-D crystals coated with silicon oxide (QSX 303 Sensor Crystal), purchased from Q-Sense AB, Göteborg, Sweden. The silicon wafers were washed with a sequence of water, ethanol, and water, dried with N2 gas and oxidized at 1000 °C for 1 h, which resulted in an oxide layer of 40 ± 2.5 nm for the doubleside polished wafers and 31 ± 2.5 nm for the single-side polished wafers, determined with nullellipsometry (model 43702-200E from Rudolph Research, Flanders NJ, US). The wafers were cut into pieces roughly 10 x 10 mm in size for AFM measurements, or strips of approximately 10 x 50 mm for use as reflectometry substrates. The oxidized wafers were hydrophilized by submersion in a 10 wt % sodium hydroxide solution for 30 s before rinsing and drying with N2. Both oxidized silicon wafers and QCM-D crystals were placed in the plasma cleaner (PCD 002, Harrick Scientific Corp., Ossining, NY, US) for 2 min prior to dipping in a polyvinyl amine (PVAm) solution (0.1 g/L, pH 7.5, Lupamin 9095, BASF) for 15 min to produce an anchoring layer for the cellulose film, rinsed with Milli-Q water and dried with N2. Cellulose II surfaces were regenerated from a solution in NMMO (50 wt % in water, Sigma Aldrich) and DMSO (99 %, Sigma Aldrich) onto PVAm-treated oxidized silicon wafers, by a procedure previously described by Gunnars et al.24 Two different methods to dissolve the


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cellulose were developed due to the change in solubility and sensitivity of the cellulose with increasing charge density, i.e. faster dissolution and more intense yellowing of the solution were observed with the oxidized pulps. The detailed descriptions of the dissolving procedure can be found in the Supporting Information. Thin cellulose film was regenerated from the NMMO/DMSO solution onto PVAm treated silica by spinning at 1500 rpm for 15 seconds followed by 3500 rpm for 30 seconds, using a spincoater (KW-4A-2, Chemat Technology, Northridge, CA, US). The films were precipitated in Milli-Q water for 1 hour and rinsed in fresh milliQ for at least 1 more hour, dried with N2 gas and the cellulose was cured at 105 °C for 6 hours to improve the wet stability. AFM and Colloidal Probe measurements The cellulose film thickness was measured using a scratching procedure with atomic force microscopy (AFM) MultiMode 8 (Bruker, Santa Barbara, CA), both in the dry state in air under ambient conditions and in the swollen state induced by a 10 mM NaCl solution. Cellulose films were prepared on silicon wafers with a thin oxide layer in order to minimize the effect of scratching the brittle silica layer. The wafers were scratched using a sharp blade and the thickness was measured at three points on each surface on three different surfaces using the SCANASYST mode with SCANASYST-AIR, TAP150 and SCANASYST-FLUID+ cantilevers (Bruker, Camarillo, CA). The surface roughness was determined over an area of 40 µm2 to include large-scale variations in the roughness. PeakForce quantitative nanomechanical mapping (QNM) measurements were used to compare the moduli of the different films in the swollen state. SCANASYST-FLUID+ tips were used with the spring constant and tip radius given by the


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supplier. Deviations in the properties of the cantilevers were included in the error of the presented data. The colloidal probe measurements were conducted with atomic force microscopy MultiMode IIIa (Veeco Instruments Inc. Santa Barbara, CA) with the PicoForce extension. Tipless rectangular cantilevers (CLFC-NOCAL, Bruker) with a spring constant of approximately 0.18 N/m were calibrated in air under ambient conditions using the software AFM tune IT 2.5 (Force IT, Sweden).40 Silica particles (Duke Standards, Dry borosilicate glass Microspheres, Thermo Scientific) with a diameter of 10 µm were glued to the cantilevers with melting glue (Epikote 1001, Shell Chemical Co.) using a manual micromanipulator (HS 6 Manuell, Marzhauser Wetzlar GmbH & Co. KG) and a reflection microscope (Olympus). The data were fitted to a symmetric DLVO model41 using the AFM Force IT version 2.6 software with the plug-in dlvoIT (ForceIT, Sweden), and to an asymmetric model using the custom-made program Asymm_pc_v2_2 by Johan C. Fröberg, based on models created by Bell and Peterson42, and Devereux and De Bruyn43; the first for constant charge and the second for constant potential. The results were validated using the analytical approximation from The Colloidal Domain by Evans and Wennerström.44 Force curves were collected in 0.1 and 1 mM NaCl (traceSELECT, Sigma Aldrich) solutions at pH 5.8. Quartz Crystal Microbalance with Dissipation (QCM-D) The amount of adsorbed polyelectrolyte and associated water was measured using QCM-D (E4, Q-sense AB, Göteborg, Sweden). A detailed description of the method can be found elsewhere.45 Stagnation Point Adsorption Reflectometry (SPAR)


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The amount of adsorbed polyelectrolyte was determined using SPAR provided by the Laboratory of Physical Chemistry and Colloidal Science, Wageningen University, the Netherlands. A complete description of the method can be found in the original publication by Dijt et al.46 and is briefly summarized in the Supporting Information. A layered optical model was used to calculate the adsorbed amount of polyelectrolytes from the SPAR data and the input parameters are shown in the Supporting Information (Figure S1). Polyelectrolyte adsorption pDADMAC (400-500 kDa, Sigma Aldrich) and PSS (70 kDa, Sigma Aldrich) were prepared as 25 mg/L solutions with NaCl (99%, Sigma Aldrich) concentrations of 0, 10 mM, and 500 mM at pH 7 for the SPAR experiments, and solutions of 0.1 g/L with 10 mM NaCl and pH 7 for the QCM-D experiments. NaCl solutions with the same salt concentrations and pH were used for the baseline and as rinsing solutions. The baseline was allowed to equilibrate for approximately 1 h for SPAR measurements and 1-2 h for QCM-D measurements. The adsorption sequence: pDADMAC-Rinse-PSS-Rinse was repeated to form two bilayers, with a flow speed of 1 mL/min under ambient conditions for SPAR and 0.15 mL/min at 22°C for QCM-D. The ⁄ value is used to calculate the amount of polymer giving rise to the change in refractive index observed in the SPAR experiments. The values of ⁄ was 0.178 cm3 g-1 for PSS

47, 48

and 0.176 cm3 g-1

for pDADMAC 49. RESULTS Determination of the chemical composition of the oxidized pulps using FTIR The FTIR spectra of the oxidized pulps are shown in Figure 1a, where the main difference is in the absorbance from an increasing amount of carboxylic acids in the carboxylate-sodium form at


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1600 cm-1 and 1404 cm-1.50 A weak peak was observed at 1728 cm-1, assigned to either protonated carboxylic acids or residual aldehydes. The peak at 1643 cm-1 was assigned to water due to incomplete drying or to the sorption of moisture by the dried pulp samples during the sample preparation. The 1200 µmol/g pulp was reduced in order to remove aldehydes and ketones produced during the oxidation,51 in order to have as chemically uniform surfaces as possible. The non-reduced and reduced 1200 µmol/g pulps and films are referred to as N1200 and R1200. The small difference between the 870 µmol/g and the 1200 µmol/g carboxylate peaks at 1600 cm-1 may be an effect of the moisture content. Determination the chemical composition of the regenerated cellulose using FTIR Cellulose was regenerated in water from NMMO solutions, without adding DMSO, in order to investigate the effect of the dissolution and regeneration on the chemical composition of the cellulose. The full FTIR spectra of the regenerated celluloses are shown in Figure 1b, and the inset shows a comparison of the carboxylate peak. It can be seen that the N1200 and R1200 µmol/g cellulose had a lower absorbance on the regenerated cellulose at 1600 cm-1 (inset Figure 1b) than the pulp samples (inset Figure 1a). This indicates that some of the carboxylic carbonyls are lost during the regeneration, probably by dissolution of the highly charged cellulose chains in water during the regeneration. The dissolution in water is probably facilitated by both the high charge and the lower degree of polymerization (DP) as a result of the harsh oxidation at pH 10.2, 52, 53

The difference between N1200 and R1200 cellulose (Figure 1b) is not fully understood and

will be discussed later. The 350 µmol/g carboxylate peak on the regenerated cellulose seems larger than on the highly charged samples, and the 870 µmol/g surface may also have lost some charge.


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Figure 1. FTIR spectra of (a) oxidized fibres, and (b) NMMO-regenerated celluloses. The peak at 1600 cm-1 is assigned to the symmetric carbonyl stretch of sodium carboxylate and is affected by the peak at 1643 cm-1, which is assigned to bending vibrations of water. The insets are shown for comparison of the normalized carboxylate peaks at ca. 1600 cm-1. Determination of the cellulose film properties with AFM and PeakForce QNM Cellulose model surfaces were prepared by regenerating differently charged cellulose from the NMMO/DMSO solutions onto silica wafers. In order to calculate the amounts of pDADMAC and PSS adsorbed using reflectometry, the thickness of the films had to be determined and incorporated in the optical model shown in the Supporting Information (Figure S1). The data


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presented in Table 1 show a dry thickness of ca. 20 nm and a wet thickness of ca. 50 nm. The 1200 µmol/g films were slightly thinner in both the dry and the wet state, which again indicates that either the charge or the reduced DP affect properties important for the spin-coating and/or the regeneration. The R1200 µmol/g film was thinner in the wet state than the other model films, which may contribute to the difference in the FTIR spectra between the N1200 and the R1200 in Figure 1b. The roughness was similar for the different surfaces except for the 20 µmol/g film which was slightly rougher in both the dry and wet states. Table 1. Cellulose film dry and wet properties. AFM scratch-height thickness (d) and RMS roughness (40 µm2) in *air and ¤10 mM NaCl aqueous media. Charge (µmol/g)

d * (nm)

d ¤ (nm)

RMS * (nm)

RMS ¤ (nm)

20

23.8 ± 1.5

47.2 ± 6.6

5.4 ± 1.5

12 ± 2.0

350

20.4 ± 1.7

50.9 ± 3.2

3.7 ± 0.8

8.0 ± 2.0

870

24.0 ± 1.4

54.7 ± 9.4

4.1 ± 0.5

7.4 ± 1.0

N1200

16.8 ± 1.7

43.3 ± 4.9

3.5 ± 0.5

7.1 ± 0.5

R1200

16.8 ± 0.8

34.5 ± 4.6

4.1 ± 0.3

7.9 ± 0.5

The roughness differences in Table 1 are shown in the AFM images in Figure 2a-e, which shows the surface structure of the different films in the wet state. It can be seen that the 20 µmol/g film had larger cellulose bundles and that oxidation created a denser network of thinner bundles, which rendered the surfaces less rough in both the dry and wet states (Table 1). The structure changed as the degree of oxidation increased, probably as a result of the reduced drive to recrystallize the more highly charged cellulose. The effect of reducing the 1200 µmol/g pulp can be seen in Figure 2f and 3g, with the disappearance of the distinct yellow color seen in the dried N1200 µmol/g pulp. The yellowing was probably due to the presence of C6 aldehydes, in the range of 20-30%, or C2 and C3 ketones formed during the TEMPO-mediated oxidation under alkaline conditions.51,

52

It is suggested that these groups can form hemiacetals and that this


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contributes to the yellowing during drying.51 Reducing the aldehydes and ketones removed the yellowing and this pulp was further used to prepare model surfaces for the polyelectrolyte adsorption studies.

Figure 2. AFM images showing the wet state (10 mM NaCl) of the model surfaces prepared from oxidized pulp with charges of (a) 20 µmol/g, (b) 350 µmol/g, (c) 870 µmol/g, (d) R1200 µmol/g, and (e) N1200 µmol/g. The height of the images is 70 nm. The visual effect of the reduction of the 1200 µmol/g pulp can be seen in the photographs of (f) the non-reduced and (g) the reduced pulps. The Derjaguin-Muller-Toporov (DMT) modulus of the cellulose films (Figure 3a) was obtained using AFM in the PeakForce QNM mode in the wet state under a NaCl concentration of 10 mM. The modulus of the films decreased with increasing degree of oxidation, probably due to a


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softening of the films by water. It may also be due to difficulties in recrystallizing the cellulose during the regeneration, which is in agreement with the AFM images in Figure 2. The 1200 µmol/g films had a higher modulus and a lower deformation than the 870 µmol/g film, which further indicates that there was a significant difference in the films prepared from the pulp with the highest charge.

Figure 3. Mechanical properties of the cellulose films showing the relative DMT modulus (black) and the deformation (red), measured using PeakForce QNM in 10 mM NaCl. The error bars are 95% confidence intervals and the lines are only guides to the eye. Note that the left-hand ordinate has a logarithmic scale. Determination of the surface potential using colloidal probe measurements The AFM colloidal probe approach was used to determine the surface potential of the cellulose model surfaces. A typical force curve between a silica particle and a cellulose surface can be seen in Figure 4a, with three regions assigned to: (i) electrical double layer repulsion, (ii) compression of the cellulose film, and (iii) the hard wall contact measuring only the cantilever deflection. For the 20 and 350 µmol/g films these three phases were clearly distinguishable, but as the charge density increased the compressive resistance of the cellulose film became small in


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comparison to the double layer repulsion, making it more difficult to find phase (ii). However, for some curves, a slight change in the slope was an indication of the surface position and this was always observed at 10±2 nm for the different cellulose films. Therefore all the curves were fitted to DLVO theory with an offset of 10 nm (Figure 4a inset) to ensure consistency in the treatment of the data. The surface potentials of the different cellulose films were calculated by fitting the force curves to both a symmetric and an asymmetric DLVO model. The asymmetric model is the most valid representation, and the symmetric model was included as a reference to indicate the large error in using a symmetric model for an asymmetric system. The force curves were in both cases best represented by a constant charge boundary. In order to use the asymmetric model, the surface potential of the silica probe was determined by measuring the double layer force in a symmetric system consisting of two silica particles. The result was a surface potential of -40 mV in 0.1 mM NaCl and -20 mV in 1 mM NaCl (both pH 5.8), using a Hamaker constant of 4.6 x 10-21 J for the silica/water/silica configuration.54 These surface potentials were later used as the surface potential of the silica particle in order to fit the asymmetric silica/cellulose systems. Figure 4b shows the surface potentials for the differently charged cellulose surfaces at NaCl concentrations of 0.1 mM and 1 mM NaCl. The surface potential did not increase as expected between the 20 µmol/g and the 350 µmol/g films, but was surprisingly high for the 870 µmol/g film followed by a reduction for the R1200 µmol/g film. The surface potential followed a pattern similar to that of the DMT modulus and the deformation in Figure 3, which is consistent with the loss of carboxylate entities during the regeneration of the R1200 film.


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Figure 4. Colloidal probe data showing (a) the force curves between a 350 µmol/g surface and a silica particle in 0.1 mM NaCl with three distinct interaction regions (i-iii), and (b) Surface potential of the charged cellulose films at pH 5.8 in 0.1 mM (squares) and 1 mM (triangles) NaCl, calculated by fitting the force curves with a symmetric DLVO model (open/dashed symbols and lines) and an asymmetric DLVO model (solid symbols and lines), using a Hamaker constant (silica-water-cellulose) of 3.5 x 10-21 J.55 The inset in (a) shows the fitting to DLVO theory, where the solid line represents the constant charge boundary and the dashed line the constant potential boundary. The measured surface potentials are: symmetric 0.1 mM (-13, -23, 75, -57 mV), symmetric 1 mM (-7, -20, -45, -33 mV), asymmetric 0.1 mM (0, -13, -150, -83 mV), asymmetric 1 mM (-2, -19, -105, -55 mV).


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Investigation of the multilayer formation using QCM-D Multilayers of pDADMAC and PSS were formed to investigate how the charge density and morphological change due to the charge affected the LbL assembly. The adsorption of polyelectrolytes was initially investigated on cellulose films prepared on silica-coated QCM-D crystals. The frequency shift during the multilayer formation onto a 350 µmol/g cellulose film showed results that are not commonly observed for LbL assemblies; an initial increase in frequency during pDADMAC adsorption followed by a decrease in frequency during PSS adsorption, as shown in Figure 5. This behavior was not pronounced for a 20 µmol/g cellulose film and is associated with the charge in the cellulose network. The dissipation data (∆D) in Figure 5 show a significant decrease in the energy dissipation for the first pDADMAC layer, which is consistent with the increasing rigidity of the cellulose film. The increased rigidity is explained by the release of counter-ions from the interior of the cellulose film during pDADMAC adsorption, which leads to a reduced osmotic pressure and a concomitant deswelling. The dissipation reaches a constant value after two bilayers, which indicates that the multilayer formation is not affected by the surface charge after the initial four layers of pDADMAC and PSS. Note that the experiment was conducted in 10 mM NaCl where the cellulose film is already in a less swollen state than films at a lower ionic strength.56


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Figure 5. QCM-D data showing the normalized frequency shift (∆f/n and solid line) and the dissipation shift (∆D and dashed line) for the third overtone of the build-up of 6 bilayers with pDADMAC (grey regions) and PSS (white regions) on a 350 µmol/g cellulose film. The polyelectrolyte concentration was 0.1 g/L in a 10 mM NaCl aqueous solution at a flow rate of 0.15 mL/min. The adsorption sequence used was 10 min adsorption followed by 5 min rinsing for each layer. Investigation of multilayer formation using SPAR Reflectometry was used to measure the actual adsorbed amount and to avoid the influence of the de-swelling behavior observed with QCM-D. Optical methods such as SPAR measure the change in refractive index relative to that of water and the swelling behavior only slightly affects the thickness of the film. The change in thickness of the film has a small effect on the reflective properties at this length-scale and can therefore be neglected.32 The multilayer formation presented in Figure 6 shows quite different dynamics for the three salt concentrations, 0 mM, 10 mM and 500 mM NaCl, used as background ionic strength during the adsorption. There was a large adsorption in the primary layer in the 0 mM salt case (Figure 6a), after which the adsorption in the following layers was quite low. The large adsorption in the first layer was also seen in the 10 mM salt case (Figure 6b), and this can in both cases be related to a charge compensation where the adsorbed polyelectrolyte neutralizes the charges of the cellulose film.27, 32

The change in adsorption kinetics for the first layer at the different salt concentrations is also clear. Without added salt, the equilibrium time was 30 min, but saturation was reached within 10 min when 10 mM NaCl was added. This difference is probably due to a change in polymer


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conformation with increasing salt concentration and a reduced repulsion between the adsorbed polymer segments, which makes less rearrangement necessary to saturate the surface.57 In 10 mM NaCl, the adsorption in layers 2-4 increased for all samples except for the 20 µmol/g film. This behavior did not further increase for the higher charge density films, suggesting that the effect of the surface charge reaches a limit somewhere between 20 µmol/g and 350 µmol/g. The adsorption in 500 mM NaCl, Figure 6c, was quite low due to the reduction in the entropy gain of releasing counter-ions to a solution that already contained large amounts of ions. The amounts adsorbed in the presence of 500 mM salt is roughly the same for the differently charged surfaces and were surprisingly similar to the amount adsorbed onto the 20 µmol/g surface in 10 mM NaCl.


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Figure 6. SPAR adsorption data showing the reflectometry signal (∆!⁄!" ) for the multilayer formation of pDADMAC/PSS onto different cellulose films at the NaCl concentration (cs) of (a) 0 mM, (b) 10 mM, and (c) 500 mM. The polyelectrolyte concentration was 25 mg/L and the flow speed 1 mL/min. The adsorption sequence used was 10 min adsorption and 5 min rinse for each layer. DISCUSSION Chemical composition of the prepared films as determined with FTIR As mentioned earlier the infrared absorbance peak from water at 1643 cm-1 interferes with the carboxylate peak at 1600 cm-1 but, assuming that the water content is the same, the decrease in peak-height for the R1200 cellulose, in Figure 1, is significant. An even greater decrease in absorbance was seen for the N1200 sample, which is a strong indication that the charges are lost from the cellulose during the regeneration step. A reasonable explanation is the heterogeneous oxidation of the pulp due to the inaccessibility of the cellulose molecules in the crystalline domains, which results in a distribution of cellulose chains with different charge densities. In the fibres, the highly charged cellulose chains appear to be irreversibly locked inside the fibre wall, but when the pulp is dissolved in NMMO, these segments are liberated, which facilitates dissolution in water during the regeneration process. The solubility limit of carboxymethylated


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cellulose in cold water is a degree of substitution (DS) in the range of 0.5 to 1.2 with 3 being the maximum.58 This corresponds to a charge density in the range of 2500-5000 µmol/g for homogeneously oxidized cellulose, and it is therefore reasonable that a fraction of the heterogeneously oxidized 1200 µmol/g fibres will have a charge density within this range. The reduced DP as a consequence of the oxidation at pH 10 will further facilitate the dissolution and move the solubility limit closer to 2500 µmol/g. Usov et al. studied the morphology of TEMPOoxidized CNF using AFM.59 They observed cellulose chains splitting off from the fibrils and even free cellulose chains on the surface, which is in line with the suggested dissolution of charged cellulose. The data support the hypothesis of lost charge and the R1200 µmol/g surface had a decreased surface potential, an increased stiffness in the wet state, and a reduced adsorbed mass of pDADMAC. Mechanical properties of the cellulose films The consistency in the dimensions of the cellulose model surfaces made from differently charged pulp is remarkable. The only exception was the 1200 µmol/g surfaces which were slightly thinner in both the dry and wet states, probably due to the limited regeneration. The different water-expansion of the N1200 and R1200 films with lower swelling for the R1200 may be a result of the aldehydes or the ketones, as mentioned in the previous section. Removal of the aldehydes and the ketones will make the cellulose more similar to native cellulose and will favor a supramolecular organization. This will create a stronger network that requires a greater swelling force in order to expand. The AFM-images, the roughness, and the DMT modulus of the different films are consistent and reasonable. The limited regeneration of the oxidized pulp will create smaller and softer fibrillar


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bundles which lead to a smoother and softer surface in the wet state (Figure 3). Aulin et al.25 determined the degree of crystallinity to be 60% for NMMO-regenerated surfaces prepared from uncharged pulp. It is expected that the degree of crystallinity of the NMMO-regenerated films will decrease with increasing oxidation of the cellulose, and this can lead to the changes in DMT modulus and the structures observed in AFM, in combination with the water softening, due to lower network stability. Surface potentials from AFM colloidal probe measurements Earlier reports60,

61

have emphasized the difficulties with colloidal probe measurements of

cellulose model surfaces due to their comparably high degree of swelling in water, and according to Table 1, the water content is 50-60 % by volume. NMMO-regenerated cellulose films can be said to have a diffuse interface in aqueous media, which means that the force measurement is affected by steric forces when the swollen gel networks is compressed. The steric forces create difficulties in determining the surface position, and this makes it difficult to fit the force curves with the DLVO-theory with the desired accuracy. In this work, it is suggested that kinks in the force curves indicate the position of the external surface. The ability to find the different regions in Figure 4a depends on the modulus of the cellulose film and the spring-constant of the cantilever. Increasing the charge of the cellulose makes the surface softer and the borderline between the cellulose surface and the aqueous phase becomes more diffuse. An indication of this was observed in the slope of the force curves for the 870 and R1200 µmol/g films in 1 mM salt (data not shown), with a slope more similar to a 0.6-0.8 mM salt case. Long-range repulsion from a highly swollen charged surface layer would intuitively result in a shift in the slope of the force


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curves, and this phenomenon would more probably be observed at higher salt concentrations when the electrical double layer repulsion is not as great at a given distance. It can therefore be suggested that the surface potential for the 870 µmol/g and possibly for the R1200 µmol/g films was slightly overestimated. On the other hand, the calculated surface potential for the 870 µmol/g film fits quite well to the theoretical estimates made by Fall et al.62 with a surface potential of -100 mV at 0 mM NaCl and -70 mV at 10 mM NaCl, for a 600 µmol/g CNF surface. These estimates can be further used to suggest that the actual charge of the R1200 film is somewhere between 500 and 600 µmol/g. Notley27 measured the surface potential to be -55 mV at an ionic strength of 0.1 mM for a surface prepared from carboxymethylated cellulose with a charge density of 509 µmol/g. The measured value for the R1200 surface was -83 mV under similar conditions, which suggests that the R1200 film has a charge density of roughly 600 µmol/g. The highly charged films show a rather drastic decrease in surface potential by a factor of 0.7 between a salt concentration of 0.1 and 1 mM. Supporting information (Figure S2) shows colloidal probe measurements for silica/silicone surfaces commonly used to study LbL assemblies. The surface potential was in this case reduced by a factor of 0.9 between the two salt concentrations, and the difference between the ratios 0.7 and 0.9 indicates that the surface potential reduction for the cellulose surfaces includes a contribution from morphological changes at the higher salt concentration. Cellulose films are three-dimensional networks where the charges at a given distance inside the film contribute to the surface potential. As the salt concentration increases, charges are screened from the surface and outwards as well as from the inside to the surface of the film. The proportion of the film that affects the surface potential can reasonably be related to the Debye screening length and the swelling of the cellulose, which


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means that an increased in the salt concentration for a three-dimensional gel would have a greater impact on the observed surface potential than for an incompressible two-dimensional surface. The general relation between surface charge (#) and surface potential ($" ) is analytically represented by the equation:44 # = (8&'"∗ )" )* )+⁄, sinh 0

1234 ,56

7

(1)

where & is the Boltzmann constant, ' is the temperature (K), "∗ is the bulk ion concentration (m– 3

), )" is the vacuum permittivity, )* is the relative permittivity, 8 is the valency of the ions, and e

is the elementary charge. Supporting Information (Figure S3) shows a plot of the surface potential versus the surface charge for the three-dimensional cellulose film and the theoretical values from Equation 1 for a two-dimensional surface. The two-dimensional surface reaches an asymptotic value of surface potential with increasing surface charge density, in contrast to a second order polynomial growth relationship for the three-dimensional cellulose film. The increased charge of the cellulose is associated with an increased swelling, which means that the perceived surface charge density is reduced as the charges are distributed in a thicker, more swollen film. This effect is more significant at low charge densities and continues until the osmotic pressure and the swollen cellulose network pressure are in balance. The suggested mechanism would lead to a restricted surface potential increase at low cellulose charge densities and reach an asymptotic value, according to Equation 1, when pressure equilibrium is reached. The supramolecular structure is, however, also affected by the charge density, and this alters the


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network pressure until a point is reached where the film is no longer stable, which is indicated by the properties of the R1200 and the N1200 films. There is an interesting relation between the surface potentials and the DMT modulus of the cellulose films in Figure 7, showing a distinct reduction of the modulus as the charge density of the cellulose increases, which indicates that a cellulose model film prepared by regeneration from NMMO has an upper limit of charge when regenerated in water at neutral pH. The largest difference in the SPAR data observed between the 20 µmol/g and 350 µmol/g films is consistent with the largest differences in the relative DMT modulus. It thus, due to morphological changes, appears that both the charge and the specific surface area are important properties for polyelectrolyte adsorption onto charged cellulose surfaces. The shape of the curve in Figure 7 supports the suggested mechanism regarding the balance between osmotic pressure and network pressure, and this balance is reached at about -40 mV or a charge density of 400-500 µmol/g.

Figure 7. The relative DMT modulus of the different films in 10 mM NaCl as a function of the surface potential of the films in 1 mM NaCl. The line is an exponentially decreasing approximation. Using SPAR to quantify the solid adsorbed amounts and the charge compensation


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Using the optical model and the thicknesses from table 1, it is possible to calculate the amount of polyelectrolyte adsorbed in mg/m2. Figure 8 shows the adsorbed amount per layer, making it easier to compare different salt concentrations and individual layers. The most obvious result is the small difference in adsorbed amount in the first layer (pDADMAC 1) between the no salt case and the 10 mM salt case (Figure 8a and 8b), indicating that the adsorption capacity in the first layer is not noticeably affected by the salt although the time to reach equilibrium is affected. A reasonable explanation of this behavior is that pDADMAC has access to all the charges in the film and that only the outermost pDADMAC molecules are affected by the conformational changes due to the presence of salt, where the flat conformation of pDADMAC with no salt is replaced in the 10 mM NaCl case with a more extended pDADMAC layer with more loops and tails, as illustrated in the Supporting Information Figure S4.63 In order to establish to what extent the charges in the cellulose films are available to the pDADMAC in the first layer, the available charge was calculated based on the known dimensions of the films, the charge densities and the adsorbed amount pDADMAC from the SPAR measurement. Table 2 shows that the charge in the cellulose film was fully compensated or slightly overcompensated by pDADMAC charges, except for the theoretical calculation on R1200 where only about 50% of the charge was compensated. This fits well with the loss of charge hypothesis and the earlier prediction of a charge density close to 600 µmol/g for the R1200 film. The 1:1 compensation indicates that the polyelectrolyte can reach all the charges in the cellulose film, due to both a physical penetration into larger pores and the fact that the adsorbed polymer affects the cellulose film over a distance comparable to the Debye length of the solution, which is also illustrated in the Supporting Information Figure S4.


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At first, it seems reasonable that penetration would explain the 1:1 charge stoichiometry. Horvath et al. studied the penetration of polyelectrolytes into the fibre wall of unbleached kraft pulp using fluorescent labelling,35,

38, 64

and showed that penetration was promoted by a low

molecular weight or a low charge density of the polyelectrolyte as well as moderate salt addition. These findings suggest that penetration of pDADMAC into the cellulose films is improbable since the pDADMAC used in this work had a relatively large Mw and a high charge density. Table 2. Compensation of the cellulose charge at 10 mM NaCl. Pulp charge (µmol/g)

20

350

870

R1200

600 estimate

Film thickness (nm)

23.8

20.4

24.0

16.8

16.8

0.036

0.031

0.036

0.025

0.025

31

30

15

2

Mass cellulose (g/m ) 2

Available charge (µmol/m ) pDADMAC charge (µmol/g) Adsorbed amount (mg/m2) Available charge w/ Cl (µmol /m2) Available charge w/o Cl (µmol/m2)

0.71

11

w/ Cl w/o Cl

6200 7900

0.31

2.2

4.8

2.5

2.5

1.9

14

30

16

16

2.4

17

38

20

20

Another interesting observation is the difference in adsorbed amount in the second layer (PSS 1) without or with salt shown in Figure 8a and 8b. The adsorbed mass differs quite a lot even though roughly the same amount was adsorbed in the first layer (pDADMAC 1). For LbL assemblies, the increased adsorption in the presence of salt is usually assigned to extended layers making it possible for the next layer to adsorb in a three-dimensional matrix rather than onto a two-dimensional surface.63 It has been shown65, 66 that, when the salt concentration is increased, there is a redistribution of the adsorbed segments from trains to tails making the adsorbed polymer available for adsorption of a second layer of polyelectrolyte. It has also been shown57 that the repulsion between the polyelectrolytes in the adsorbed layer decreased when the ionic strength is increased, and it can be suggested that the increase in adsorption of the PSS 1 layer at


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higher ionic strength is a combination of these two effects. According to this hypothesis, the low adsorption in the PSS 1 layer in the absence of salt is a result of less overcompensation of the charge due to the flat conformation on the surface, which makes it harder to reverse the surface potential, but it is also due to high repulsion between the adsorbing PSS segments when they have to compensate the charge located close together on a two-dimensional surface. This repulsion is of course also present for the pDADMAC 1 layer but it is overruled by the large attractive force from the highly charged cellulose film. It is also of value to point out the similarities between the pDADMAC 1 and the pDADMAC 2 layers in the 10 mM NaCl case (Figure 8b), which indicates that the surface had affected the multilayer assembly for at least the initial four layers. This has been suggested elswhere22, 28 and is also in accordance with the dissipation change shown in the QCM-D data (Figure 5) where the dissipation was different for the first two bilayers. The addition of 500 mM salt will reduce the entropic driving forces and screen interactions, and reduce the Debye length so that the charges inside the film become unavailable. This leads to a reduced adsorption of all layers onto the cellulose films, seen in Figure 6c and Figure 8c, which is in agreement with previously reported studies of adsorption onto cellulose surfaces.32, 35, 67 The adsorption in 500 mM NaCl was comparable for the differently charged films and similar to the adsorption onto the 20 µmol/g film in 10 mM NaCl, and this leads to the suggestion that a very weak non-electrostatic interaction may also be involved in the multilayer formation, in contrast to previous results.35 A high charge density with moderate amounts of salt resulted in a larger amount adsorbed in each bilayer, but the charge density appears to have no or little effect on the multilayer formation


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above a charge density of 350 µmol/g. The suggested explanation is that there is high repulsion and steric forces in the polyelectrolyte layer trying to compensate for this high surface charge, and this leads to limitations in the reversal of the surface potential. The practical implication is that a high charge density of CNF, CNC or other oxidized cellulose materials has little impact from an application point of view where several bilayers are usually applied.

Figure 8. Adsorbed amount of polyelectrolyte calculated using the optical model, showing the adsorption per layer for the different NaCl concentrations (a) 0 mM (b) 10 mM and (c) 500 mM.


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The error bars are the standard deviation of two repeated measurements to indicate the repeatability. Using the QCM-D and SPAR data to estimate the de-swelling during pDADMAC adsorption The observed de-swelling or collapse of the cellulose films following polyelectrolyte adsorption detected with QCM-D in Figure 5, has been reported by others.27, 28, 32, 68 It is unlikely that the release of water located at the surface of the films would cause such a drastic reduction in mass, suggesting that the de-swelling of a larger portion of the film, i.e. a collapse, is the underlying cause. Swerin et al.69 studied the de-swelling of unbleached hardwood kraft pulp due to the adsorption of polyelectrolytes, and were able to link the de-swelling to the size of the polyelectrolyte and the degree of beating of the pulp, which is consistent with penetration into pores and the release of counter-ions from the interior. On the other hand, Gawel et al.56 showed that a combination of molecular weight of the penetrating polyelectrolyte and the charge density in the polyampholyte network is important for the de-swelling rather than just the release of counter-ions. As mentioned before, it is unlikely that pDADMAC penetrates the cellulose film to any great extent. To explain the de-swelling, the initial distribution of charges in the lateral (x and y) directions should be taken into account, where the distance between charges is greater inside the cellulose film than in the adsorbed pDADMAC layer. This means that the unmatched positive charges on pDADMAC can start to compensate negative charges in the thickness direction of the cellulose film during the gradual de-swelling, and the distance over which this can take place is related to the Debye length of the liquid inside the cellulose film. Initially, the Debye length is


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shorter inside the film due to the counter-ions and co-ions, but pDADMAC adsorption will gradually increase the Debye length inside the film and increase the distance over which charged groups can interact. It is reasonable to reflect on the fact that a dissipation change (∆D) of -2.5x10-6 in Figure 5 is a small change and that the word “collapse” is an exaggeration, but as the dissipation usually increases drastically due to water associated with the adsorbing polyelectrolyte, the total dissipation reduction was 2.5x10-6 plus the dissipation associated with the polyelectrolyte. It is possible to calculate the amount of water lost during the de-swelling by combining the QCM-D data and SPAR data. The mass loss measured with QCM-D can be estimated using the Sauerbrey equation70 which relates the frequency shift to the mass change, and this gives a mass loss of 3.8 mg/m2 for the 350 µmol/g cellulose film. The Sauerbrey equation applies to rigid films and systems with viscoelastic properties will lead to errors. However, several studies have shown that the difference between the Sauerbrey and more advanced viscoelastic models is small even at high dissipation,71, 72 and this makes the Sauerbrey equation sufficient and beneficial because of its simplicity. The amount of pDADMAC adsorbed is 2.2 mg/m2 according to the SPAR data, and this makes the total loss of water 6 mg/m2 during the polyelectrolyte-induced de-swelling. A thickness increase of 30 nm during the swelling, roughly estimated from table 1, corresponds to a water content of 30 mg/m2 in the cellulose film, which leads to the conclusion that about 20% of the water is lost during the de-swelling. This is a large quantity when the ionic swelling, driven by the ion concentration difference between the inside of the film and the surrounding solution, is already limited in the presence of 10 mM salt. SUMMARY AND CONCLUSIONS


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In this work, cellulose model surfaces with different charge densities were successfully regenerated from solvent mixtures of NMMO and DMSO. The model surfaces were extensively characterized using different AFM techniques and FTIR analysis, which showed that the mechanical properties of the wet cellulose films were related to the degree of oxidation of the cellulose. AFM colloidal probe measurements were used to estimate the surface potential of the regenerated films and showed a second order polynomial increase with increasing charge density. This behavior is in contrast to the theoretical relationship between the surface potential and the charge density for a flat surface, and it is suggested that the distribution of charges in film and the balance between the osmotic pressure and the network structure affect the perceived surface potential for a three-dimensional structure. The surface with the highest charge density, 1200 µmol/g, differed from the others with respect to its DMT modulus, charge density and surface potential. It was shown, by FTIR analysis, AFM colloidal probe, and charge compensation calculations based on the QCM-D and SPAR data, that some of the charge is lost during the regeneration process and that this is facilitated by a combination of a large amount of charged groups and reduced degree of polymerization following the oxidation at pH 10. Multilayers of pDADMAC and PSS were formed on the differently charged cellulose surfaces in order to evaluate the effect of the charge on the multilayer formation. It was shown that a large de-swelling was associated with a highly charged system, which makes said systems difficult to investigate using QCM-D. Reflectometry measurements were used to avoid the influence of the de-swelling and showed that the first pDADMAC layer could compensate for all the charges in the cellulose films, which is the reason for the extensive de-swelling. The multilayer formation was highly affected by the salt concentration but the effect of the surface charge was not


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significantly increased above a cellulose charge density of 350 µmol/g. The practical implication is that the surface charge in combination with salt can greatly increase the adsorption of polyelectrolytes in each layer, but that the surface charge density will not significantly affect this assembly above a certain charge density in the region of 350 µmol/g. ASSOCIATED CONTENT

Supporting information Descriptions of the different oxidation procedures, a description of the SPAR method, a figure showing the surface potentials of commonly used silica/silicone surfaces, an illustration of the nomenclature for the conformation of polymers at interfaces, and a plot of the relationship between the surface potential and the charge density according to Equation 1 are included in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHORS INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Author Contribution The manuscript was written through the contributions of all the authors. All authors have given their approval to the final version of the manuscript. ACKNOWLEDGMENTS


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The Wallenberg Wood Science Center (WWSC) and the Knut and Alice Wallenberg foundation are gratefully acknowledged for financial support. Jonatan Henschen is acknowledged for his help and input in the initial part of the project. Dr. Marcus Ruda is acknowledged for suggesting the rotoevaporatory technique for cellulose dissolution. REFERENCES 1. Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose. Biomacromolecules 2007, 8, 2485-2491. 2. Saito, T.; Hirota, M.; Tamura, N.; Kimura, S.; Fukuzumi, H.; Heux, L.; Isogai, A. Individualization of Nano-Sized Plant Cellulose Fibrils by Direct Surface Carboxylation Using TEMPO Catalyst under Neutral Conditions. Biomacromolecules 2009, 10, 1992-1996. 3. Siró, I.; Plackett, D. Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose 2010, 17, 459-494. 4. Hamedi, M.; Karabulut, E.; Marais, A.; Herland, A.; Nyström, G.; Wågberg, L. Nanocellulose Aerogels Functionalized by Rapid Layer-by-Layer Assembly for High Charge Storage and Beyond. Angew. Chem. Int. Ed. 2013, 52, 12038-12042. 5. Podsiadlo, P.; Choi, S.-Y.; Shim, B.; Lee, J.; Cuddihy, M.; Kotov, N. A. Molecularly Engineered Nanocomposites: Layer-by-Layer Assembly of Cellulose Nanocrystals. Biomacromolecules 2005, 6, 2914-2918. 6. Wågberg, L.; Decher, G.; Norgren, M.; Lindström, T.; Ankerfors, M.; Axnäs, K. The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationic Polyelectrolytes. Langmuir 2008, 24, 784-795. 7. Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindström, T.; Nishino, T. Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 2008, 9, 1579-1585. 8. Shimizu, M.; Saito, T.; Fukuzumi, H.; Isogai, A. Hydrophobic, Ductile, and Transparent Nanocellulose Films with Quaternary Alkylammonium Carboxylates on Nanofibril Surfaces. Biomacromolecules 2014, 15, 4320-4325. 9. Paakko, M.; Vapaavuori, J.; Silvennoinen, R.; Kosonen, H.; Ankerfors, M.; Lindstrom, T.; Berglund, L. A.; Ikkala, O. Long and entangled native cellulose I nanofibers allow flexible aerogels and hierarchically porous templates for functionalities. Soft Matter 2008, 4, 2492-2499. 10. Svagan, A. J.; Samir, M. A. S. A.; Berglund, L. A. Biomimetic Foams of High Mechanical Performance Based on Nanostructured Cell Walls Reinforced by Native Cellulose Nanofibrils. Adv. Mater. 2008, 20, 1263-1269.


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