Cellulose (2013) 20:1459–1468 DOI 10.1007/s10570-013-9923-5
ORIGINAL PAPER
Nanofibrillated cellulose/carboxymethyl cellulose composite with improved wet strength Nikolaos Pahimanolis • Arto Salminen • Paavo A. Penttila¨ • Juuso T. Korhonen Leena-Sisko Johansson • Janne Ruokolainen • Ritva Serimaa • Jukka Seppa¨la¨
•
Received: 23 November 2012 / Accepted: 3 April 2013 / Published online: 11 April 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract In this paper, nanofibrillated cellulose/ carboxymethyl cellulose (CMC) composite films were prepared using tape casting. The obtained transparent films showed shear induced partial alignment of fibrils along the casting direction, resulting in birefringence in cross polarized light. The carboxyl groups of CMC could be further utilized to create ionic crosslinking by treatment with glycidyl trimethyl ammonium chloride (GTMA). The GTMA treated composite films had improved mechanical properties both in wet and dry state. The chemical composition and morphologies of composites were analyzed with X-ray photoelectron spectroscopy, elemental analysis, scanning electron microscopy and wide-angle X-ray scattering. N. Pahimanolis A. Salminen J. Seppa¨la¨ (&) Polymer Technology, Department of Biotechnology and Chemical Technology, Aalto University School of Chemical Technology, P.O. Box 16100, 00076 Aalto, Finland e-mail:
[email protected] P. A. Penttila¨ R. Serimaa Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland J. T. Korhonen J. Ruokolainen Molecular Materials, Department of Applied Physics, Aalto University School of Science, P.O. Box 15100, 00076 Aalto, Finland L.-S. Johansson Department of Forest Products Technology, P.O. Box 16100, 00076 Aalto, Finland
Keywords Nanofibrillated cellulose Nanocellulose Carboxymethyl cellulose Ionic crosslinking Orientation Tape casting
Introduction There is an increasing interest to utilize the potential of natural fibers due to their abundant availability and biodegradability. Nanofibrillated or microfibrillated cellulose can be produced by mechanical disintegration of pulp fibers though a process first described by Turbak et al. (1983) and Herrick et al. (1983) and the process was further developed with oxidation or enzymatic pretreatments (Saito et al. 2006; Pa¨a¨kko¨ et al. 2007). Characteristics of the obtained fibrils are a small diameter and high aspect ratio and because of its high strength, it is an interesting material as a reinforcing element in polymer composites for various applications. However, nanofibrillated cellulose (NFC) is hydrophilic and obtained as a dilute water suspension usually below 2 wt% of solid content, having a flocculated, entangled network structure (Saarikoski et al. 2012; Karppinen et al. 2012; Plackett and Siro´ 2010) This makes it challenging as a composite material. Polymer composites of NFC have been produced via solvent-exchange procedures or by chemical modification of the surface of NFC fibrils to obtain better compatibility with polymers (Plackett
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and Siro´ 2010). However, using solvents other than water to disperse NFC might lead to partial loss of the original fibril nanostructure by promoting bundle and aggregate formation (Johansson et al. 2011). Shaping the NFC composite directly from its aqueous suspension is environmentally benign and maintains the nanofibril structure. In addition, water soluble polysaccharides, such as carboxymethyl cellulose can be used to modify the suspension rheology and also introduce functional groups to cellulosic surfaces (Beghello 1998; Yan et al. 2006; Eronen et al. 2011; Filpponen et al. 2012; Liu et al. 2011; Orelma et al. 2012a). A key aspect of strong composites in addition to good dispersion and adhesion is the orientation of the reinforcing element. Thin films of cellulose micro and nanocrystals have been obtained by shearing, and oriented cellulose films with improved mechanical properties have been produced by colddrawing (Gindl and Keckes 2007; Yoshiharu et al. 1997; Hoeger et al. 2011; Sehaqui et al. 2012). In a recent paper, a similar approach was successfully applied for cellulose whiskers in polyvinyl alcohol matrix by wet spinning and hot drawing (Uddin et al. 2011). Cellulose microfibrils and nanowhiskers have also been reported to align in strong magnetic and electric fields (Habibi et al. 2008; Csoka et al. 2011; Kimura et al. 2005; Cranston and Gray 2006). This phenomenon has been utilized for inducing crystal alignment in all-cellulose and polyvinyl alcohol composites (Kvien and Oksman 2007; Pullawan et al. 2012). However, composites with shear induced alignment of nanofibrillated cellulose have not been reported so far. Our approach was to produce all cellulose nanocomposites from NFC and carboxymethyl cellulose via tape-casting. With this technique, the aqueous suspension can be easily processed to films and the shear forces during tape-casting introduce alignment of fibrils along casting direction, providing interesting optical properties. The carboxymethyl groups of CMC can be further utilized for wet strength improvement by treatment with glycidyltrimethylammonium chloride (GTMA). The obtained wet strength increase was attributed to the resulting ionic crosslinking and could be an interesting way to improve the wet strength of cellulose materials and provide bio-based materials for e.g. wound dressing applications.
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Experimental Materials Nanofibrillated cellulose (NFC) was provided by UPM Corporation (Helsinki, Finland) and prepared by mechanical disintegration of once dried bleached birch kraft pulp (BHKP), which was pre-treated with a Voith refiner to a target 90°–94°SR using a nominal energy consumption of 0.3 kWh/kg and then fluidized by six passes through a M7115 fluidizer (Microfluidics Corp. Newton, MA, USA) with an operating pressure of 1,850 bar. The dilute hydrogel (solids content 1.1 wt%, xylan content 25 wt%, conductivity 162 lS/cm) was concentrated to a 2.2 % solid content by centrifugation at 4,000 rpm for 30 min prior to use. Sodium carboxymethyl cellulose (CMC, Sigmaaldrich, Mw = 250 kg/mol with degree of substitution 0.75 determined by titration) was dried in vacuum before use and glycidyltrimethylammonium chloride (GTMA, Sigma-Aldrich C90 %) was used as received.
Nanocomposite film preparation All nanofibrillated cellulose water dispersions were prepared as 25 g batches. Carboxymethyl cellulose solution (2 wt%) was added to NFC suspensions to obtain desired CMC content and distilled water was added until the concentration of NFC was 1.0 wt%. The suspensions were mixed for 24 h with a magnetic stirrer at 1,200 rpm and ultrasonicated at 20 kHz with a power of 20 W with 0.5 s active/passive pulses for 10 min under agitation at 1,200 rpm. The temperature of the suspension rose to around 55 °C during this time. The mixing was continued for an additional hour and the suspensions were immediately loaded into the tape-casting apparatus (Fig. 1) and cast onto polyimide supports. Casting speeds of either 0.30 or 1.9 m/s were used and approximately 50 cm long samples were produced. The cast suspensions were let to dry overnight at 22 °C and 40–60 % relative humidity and subsequently in oven at 75 °C for 24 h. The GTMA treatment of films were done for dried films by immersing them into ethanol:water 1:1 mixture containing 0.1 M GTMA for 10 min and subsequently washed with ethanol:water 1:1 mixture three times over the course of 3 h. The films were then
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Fig. 1 Geometry of the tape-casting unit used in this study. Dimensions H = 28.8 mm, h = 1.95 mm, d = 30.6 mm, l = 42.0 mm (glass surface), width = 24.4 mm
dried at 22 °C and 40–60 % relative humidity overnight and subsequently in oven at 75 °C for 24 h. Based on gravimetric measurements, no significant loss of CMC was found to take place during the washings. Characterization The morphology of the samples was studied using a field emission scanning electron microscope (FESEM, JEOL JSM-7500FA) operating at 5 kV. The samples were made conductive by Au/Pt sputtering. The tensile tests were performed using an Instron 4204 universal testing machine with a crosshead speed of 2 mm/min. At least six composite samples (approximately 15 9 0.010 9 5.6 mm3) were tested for each composite and the samples were conditioned at 23 °C and 50 % relative humidity for at least 24 h before testing. The wet strength measurements were done for samples immersed in distilled water for 24 h and the dimensions were measured from dry samples. Elemental analysis of the composites was performed using a Perkin Elmer 2400 Series II CHNS equipment. 1 H-NMR analysis was done using a Bruker Avance III spectrometer. Typically, the composite samples (25 mg) were extracted at room temperature with 1 ml of deuterium oxide containing 55 mg NaCl for 2 h. X-ray photoelectron (XPS) spectroscopy was chosen for chemical characterization of the cast film surfaces. The experiment was performed using a Kratos AXIS 165 electron spectrometer with monochromatic irradiation at 100 W and charge neutralization, on analysis spots less than 1 mm in diameter. Elemental compositions were determined from low resolution survey scans combined with extended regional scans on trace elements (N, Na, and Cl). O
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1 s and C 1 s regions were recorded in high resolution mode for a more detailed analysis of surface chemical compositions. No sample degradation due to high vacuum or X-rays was detected. Furthermore, according to the data from samples and in situ reference [100 % cellulose filter paper, see Johansson and Campbell (2004)], vacuum conditions remained clean and stable throughout the experiment. Two dimensional X-ray diffraction patterns were measured in perpendicular transmission geometry from 6 film pieces stacked on top of each other with uniform casting direction (/ = 0°). The X-ray setup consisted of a Seifert ID 3003 X-ray generator (voltage 36 kV, current 25 mA) equipped with a Cu ˚ ), a Montel multilayer tube (wavelength 1.542 A monochromator and a MAR345 (Marresearch) image plate detector. The measured diffraction patterns were corrected for the electronic background noise of the detector and normalized with the transmission of the direct beam, and the corresponding air scattering background was subtracted. Normalized azimuthal intensity distributions were computed on a ring between radial angles 2h = 22.1 and 24.1°, corresponding to the equatorial 200 reflection of cellulose Ib. The degree of orientation was quantified by calculating the Herman’s orientation function f (Gedde 1995), which gives the value f = 0 for randomly oriented fibers and f = 1 for fibers perfectly aligned in the direction of / = 0. For a sample with perfect fiber orientation in the direction of / = 90°, the value is f = -1/2 (Gedde 1995). Here the orientation function was computed from the normalized azimuthal distributions n(/) on sectors / = 90°–180° and / = 270°– 360° and the average of these two results was taken. Rheological measurements were performed using a TA Instruments AR-G2 rheometer equipped with a vane-cup geometry operating at 25 °C (vane diameter 28 mm, vane length 42 mm, cup diameter 30 mm, gap 1 mm). The NFC suspensions were prepared as in the composite films preparation procedure and 20.0 ml sample volumes were used. The dynamic viscoelastic measurements were performed at the linear viscoelastic region. This was determined by strain sweep from 0.02 to 10,000 % at 1 Hz, and a strain amplitude of 0.5 % was chosen. In order to introduce equal shear history, a peak hold step at shear rate of 500 1/s for 30 min was done followed by a time sweep with 1 Hz with 0.5 % strain for 2 h in order to recover the structure. The frequency sweeps were performed at
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The effect of film preparation method was studied using the tape-casting technique commonly used for producing ceramic substrates and composites with ordered structures (Hotza and Greil 1995; Libanori et al. 2012; Lanticse et al. 2006; Korkut et al. 2011; Van Opdenbosch et al. 2012). The produced films (Fig. 3a) are smooth with controlled thickness and it is also possible to introduce shear-induced alignment of reinforcing elements during the casting. The orientation by hot or cold-drawing has been commonly used in paper making processes and recently also reported for NFC, which has a positive effect in mechanical properties along the orientation direction (Sehaqui et al. 2012). In rheological experiments, nanocellulose structures have been shown to orient during high shearing, so in principle, the production of films having preferential alignment of fibrils should be possible (Yoshiharu et al. 1997; Hoeger et al. 2011; Lasseuguette et al. 2008; Ebeling et al. 1999; Orts et al. 1998). However, NFC suspensions have strong interfibrillar interactions and high shearing forces are
needed to break the formed flocks of fibrils. This problem can be overcome by introducing charges on fibril surfaces, which yields electrostatic stabilization of the gels. CMC has been shown to reduce friction of cellulosic surfaces and facilitate dispersion of nanofibrils (Beghello 1998; Yan et al. 2006; Zauscher and Klingenberg 2001; Ahola et al. 2008). The charges can be introduced either chemically, by oxidation or by covalent attachment of charged groups on cellulosic surface. In addition, cellulosic surfaces can be modified by adsorption of polysaccharides, which is a very simple and convenient way to introduce chemical functionalities (Eronen et al. 2011; Filpponen et al. 2012; Liu et al. 2011; Orelma et al. 2012a; Laine et al. 2000). We used carboxymethyl cellulose to reduce the fibril interactions, the hypothesis being that the resulting ionic repulsion between fibrils would make the fibril orientation possible with moderate shearing (Yoshiharu et al. 1997; Lasseuguette et al. 2008; Ebeling et al. 1999; Orts et al. 1998). The carboxyl groups of CMC also provide possibilities for further modifications of the composite. The rheological behaviors of the NFC suspensions with and without CMC can be seen in Fig. 2. All suspensions have a typical decreasing viscosity with increasing shear rate and, at rest, a gel-like behavior with storage modulus G0 being higher than loss modulus G00 (Pa¨a¨kko¨ et al. 2007; Agoda-Tandjawa et al. 2010). The addition of CMC to the NFC suspensions decreases both the
Fig. 2 Viscosity as a function of shear rate (left) and storage modulus (G0 , hollow symbols) and loss modulus (G00 , filled symbols) as a function of angular frequency (right) for 1.00 wt%
NFC suspensions. CMC dry-contents: (circle) 0 wt% (square) 5 wt% (inverted triangle) 10 wt% (diamond) 15 wt% (triangle) 30 wt%
0.02–100 rad/s. Shear rates of 0.01 to 1,000 1/s were used for shear viscosity studies. The samples were allowed to rest for 10 min between measurements.
Results and discussion NFC/CMC composite films
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storage and loss moduli, which is more pronounced at high CMC content. This is an indication of disruption of the fibril network, since the repulsive electrostatic double layer interactions create repulsion of negative charged surfaces of carboxyl groups of CMC, producing a lubricating effect of fibrils and facilitate breaking of fibril flocks (Eronen et al. 2011; Zauscher and Klingenberg 2001). Scanning electron microscopy images (Fig. 3c, d) and wide-angle X-ray scattering results (Fig. 4) from films containing CMC show anisotropy with higher casting speeds, compared to neat NFC. Based on the location of the maxima in the orientation distributions measured with WAXS, the cellulose crystals had preferred orientation in the direction parallel to the casting direction. Higher degrees of orientation (higher value of orientation function f) were observed in samples with higher casting speed and increased CMC content. Interestingly, the films
showed also birefringence in polarized microscope or when put between the polarized light of a cellphone display and a polarizing lens (Fig. 3b). Casting speeds higher than 1.9 m/s could not be achieved due to limitation of our tape-casting equipment, so the effect of higher casting speeds cannot be evaluated. However in previous rheological studies, high shearing rates up to 50 s-1 resulted in high anisotropy of charged fibrillated or microcrystalline cellulose suspensions (Lasseuguette et al. 2008; Ebeling et al. 1999). The limited orientation of our films may be due to relaxation phenomena during drying of the films, since it has been shown that relaxation after shearing depends on particle length (Orts et al. 1998). The anisotropy of the NFC/CMC composite appears to have a small increase on the mechanical properties along the casting direction (Table 2, entry 7) when 10 wt% CMC was used, but no further improvement in the
Fig. 3 a Photograph of NFC/CMC 10wt- % film obtained by tape casting. b Photograph of NFC/CMC 10 wt% tape cast films when put between a cellphone OLED display and a cross
polarizing lens, showing birefringence at 45° to the polarization axis. c, d SEM images of NFC/CMC 10 wt% films showing partial alignment of fibrils along casting direction
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Fig. 4 Azimuthal intensity distribution of cellulose 200 reflection from wide-angle X-ray scattering of tape cast films, shifted vertically for clarity. Herman’s orientation function f is given in parentheses. From top to bottom: CMC 0 wt% speed 0.3 m/s (f = -0.0006 ± 0.0003), CMC 0 wt% speed 1.9 m/s (f = 0.006 ± 0.003), CMC 10 wt% speed 0.3 m/s (f = 0.005 ± 0.003), CMC 10 wt% speed 1.9 m/s (f = 0.017 ± 0.003), CMC 15 wt% speed 1.9 m/s (f = 0.025 ± 0.003), CMC 30 wt% speed 1.9 m/s (f = 0.050 ± 0.003)
mechanical properties were observed with 5, 15 or 30 wt% CMC content. Wet-strength improvement of cellulose nanocomposite films The obtained NFC films have limited wet strength and the NFC-CMC composites were found to soften and disintegrate when wet and could not be subjected to mechanical testing. In previous studies, CMC treated cellulosic surfaces have been modified with cationic surfactants to improve mechanical properties, increase conductivity and surface hydrophobicity (Blomstedt and Vuorinen 2007; Tiitu et al. 2006; Xhanari et al. 2011). It has been shown that CMC combined with cationic polymers increase the dry and wet strength of paper due to ionic crosslinking (Hubbe 2006; Ga¨rdlund et al. 2003). Our approach was to utilize the
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carboxyl groups on the composite films to create ionic crosslinking (Fig. 5) using reactive epoxy-containing GTMA. After oven heating, the films had considerable dry and wet strength and remarkably good flexibility in wet state compared to untreated or neat NFC (Table 2, entries 5, 8, 9, 11). Films with 10 or 15 wt% CMC showed somewhat similar wet strength properties, whereas 5 wt% CMC content provided weaker films. Attempts to apply the GTMA treatment procedure to films with 30 wt% CMC were not successful, as the films swelled and softened during washing. Uneven, droplet-like swelling was also noted for the 15 wt% CMC content films during GTMA treatment. No considerable differences were found in water uptake of the wetted films, having a wet mass to dry mass ratio in the order of 2.7. FT-IR analysis of the GTMA treated films showed characteristic cellulosic peaks and the carboxyl group of CMC at around 1,600 cm-1, however peaks characteristic for the epoxide group did not appear, probably due to overlapping of cellulosic peaks. Despite this, the presence of the epoxide could be confirmed from 1H-NMR analysis of D2O/NaCl extract of room temperature dried composite films, yet after oven heating, no epoxy peaks could be detected at 2.80–2.90, 3.07–3.15 ppm. We speculate that this is an indication that the epoxide groups of GTMA reacted during oven drying of the films, yielding at least partial etherification of hydroxyl groups present in CMC and/or NFC surfaces and that the observed increase in strength of the films is due to formation of ionic cross-linking of the composite. The overall elemental composition (Table 1) of the GTMA treated samples showed an increase in nitrogen content and the amount of GTMA was calculated to be 29 mg/g for the 10 % CMC films and 42 mg/g for the 15 % CMC films. The etherification was further suggested by the XPS data. In the case of NFC-CMC composite, XPS survey spectra as well as the carbon and oxygen high
Fig. 5 Schematic presentation of GTMA treatment of NFC/CMC composites
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Cellulose (2013) 20:1459–1468 Table 1 Elemental composition of neat NFC, NFC/CMC and GTMA treated composites
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Sample
Overall elemental composition (wt%) C
H
N
NFC
43.19
5.66
0.03
NFC?CMC10 wt%
42.39
5.38
0.06
NFC?CMC10 wt%?GTMA NFC?CMC15 wt%?GTMA
43.04 41.40
5.90 6.05
0.35 0.51
Fig. 6 XPS wide-spectra of NFC/CMC 10 wt% composite (bottom), trace and HiRes spectra (top) after GTMA treatment (a) and without treatment (b)
resolution signals had components typical of cellulose (Johansson and Campbell 2004; Uschanov et al. 2010; Johansson et al. 2011). Due to the very dry conditions in ultra-high vacuum, non-anchored carboxylic groups are not seen in XPS (Johansson et al. 2011; Orelma et al. 2012b). Instead, CMC was indicated by the elevated CC component in C 1 s spectra, together with the appearance of the counter ion, sodium, at 0.4 at.%, see Fig. 6. After the addition of GTMA, sodium was
no longer observed. Furthermore, although the overall surface nitrogen levels were very low, only in the GTMA treated sample was there an additional nitrogen compound in the N 1 s line, indicative of the quaternary ammonium group (Beamson and Briggs 1992). No chlorine was detected in any of the samples, which could be an indication that the ammonium groups had a carboxyl groups as their counter-ion (Table 2).
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Table 2 Mechanical properties of films obtained by tape-casting Entry
Sample
Tested at 22 °C at 50 % relative humidity
Wet films soaked in distilled water for 24 h
Young’s modulus (GPa)
Young’s modulus (MPa)
Tensile stress at break (MPa)
Tensile strain (%)
Tensile stress at break (MPa)
Tensile strain (%)
1
NFC sp1a
8.1 ± 1.1
178 ± 14
7.0 ± 0.9
123 ± 18
3.1 ± 0.1
3.3 ± 0.9
2
NFC sp2a
4.2 ± 1.0 (5.5 ± 1.6) 6.6 ± 1.1
3.2 ± 0.1
4.2 ± 0.4
NFC sp2 GTMAb
150 ± 22 (144 ± 17) 154 ± 22
127 ± 15
3
9.5 ± 1.1 (8.4 ± 0.5) 8.9 ± 1.5
ND
ND
ND
4
NFC/CMC5 % sp2b
10.7 ± 0.8 (10.8 ± 1.0)
175 ± 20 (170 ± 45)
5.4 ± 1.0 (3.9 ± 1.4)
ND
ND
ND
5
NFC/CMC5 % sp2 GTMA
8.5 ± 2.1 (7.3 ± 0.7)
171 ± 43 (158 ± 24)
6.2 ± 0.8 (4.4 ± 1.2)
204 ± 115 (175 ± 58)
10 ± 4 (8 ± 4)
6.1 ± 1.1 (5.4 ± 2.2)
6
NFC/CMC10 % sp1b
9.4 ± 0.8
177 ± 33
5.0 ± 1.4
ND
ND
ND
7
NFC/CMC 10 %sp2b
12.2 ± 1.8 (11.4 ± 1.7)
214 ± 30 (201 ± 23)
4.3 ± 1.3 (3.7 ± 1.1)
ND
ND
ND
8
NFC/CMC10 % sp1 GTMA
11.1 ± 1.5
235 ± 10
6.4 ± 0.9
392 ± 36
29 ± 3
11.3 ± 0.5
9
NFC/CMC10 % sp2 GTMA
9.8 ± 1.6 (8.6 ± 1.0)
234 ± 29 (210 ± 10)
6.7 ± 1.0 (7.0 ± 1.2)
368 ± 22 (357 ± 13)
36 ± 9 (31 ± 10)
14.9 ± 2.6 (14.1 ± 3.3)
10
NFC/CMC15 % sp2b
8.7 ± 0.9 (9.8 ± 0.5)
193 ± 21 (167 ± 28)
7.6 ± 1.6 (4.2 ± 1.0)
ND
ND
ND
11
NFC/CMC15 % sp2 GTMA
9.3 ± 0.7 (10.0 ± 1.0)
198 ± 22 (172 ± 24)
6.4 ± 1.0 (3.9 ± 0.2)
285 ± 36 (255 ± 53)
42 ± 13 (28 ± 4)
18.7 ± 4.9 (14.9 ± 3.5)
12
NFC/CMC30 % sp2b
11.7 ± 0.7 (9.4 ± 0.7)
208 ± 39 (174 ± 51)
4.7 ± 1.1 (4.1 ± 1.7)
ND
ND
ND
Values of samples measured in cross direction to the casting are given in parentheses. Casting speed sp1 = 0.3 m/s, sp2 = 1.9 m/s a
For testing of wet films, four specimens were stacked together
b
The wetted films were too weak to be tested
Conclusions Partial alignment of nanofibrils was achieved by tape casting when carboxymethyl cellulose was mixed with NFC. SEM and wide-angle X-ray scattering showed partial alignment of the fibrils only when CMC was added to the suspensions. The aligned films have interesting optical and good mechanical properties. The carboxymethyl groups of CMC could be further utilized to create ionic crosslinking via glycidyltrimethylammonium chloride treatment, thus increasing the dry and wet strength of the composite films. Acknowledgments This work has been funded by the Graduate School for Biomass Refining (Academy of Finland) and the Finnish Funding Agency for Technology and Innovation (project ‘‘Tailoring of nanocellulose structures for industrial applications’’ NASEVA 2). We gratefully acknowledge Dr. Joseph Campbell for his contribution to the XPS measurements. UPM-Kymmene Corporation is acknowledged
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for supplying the UPM Fibril Cellulose raw material. This work made use of Aalto University Nanomicroscopy Center (AaltoNMC) facilities.
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