Arabinoxylan/nanofibrillated cellulose composite films
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- Stevanic, J.S., Bergström, E.M., Gatenholm, P. et al. J Mater Sci (2012) 47: 6724. doi:10.1007/s10853-012-6615-8
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There is an increasing interest in substituting petroleum based polymer films, for food packaging applications, with films based on renewable resources. In many of these applications, low oxygen permeability and low moisture uptake of films are required, as well as high enough strength and flexibility. For this purpose, rye arabinoxylan films reinforced with nanofibrillated cellulose was prepared and evaluated. A thorough mixing of the components resulted in uniform films. Mechanical, thermal, structural, moisture sorption and oxygen barrier characteristics of such films are reported here. Reinforcement of arabinoxylan with nanofibrillated cellulose affected the properties of the films positively. A decrease in moisture sorption of the films, as well as an increase in stiffness, strength and flexibility of the films were shown. From these results and dynamic FTIR spectra, a strong coupling between reinforcing cellulose and arabinoxylan matrix was concluded. Oxygen barrier properties were equal or better as compared to the neat rye arabinoxylan film. In general, the high nanofibrillated cellulose containing composite film, i.e. 75 % NFC, showed the best properties.
Production of environmentally friendly packaging materials, based on polymers from renewable resources, is increasingly demanded. Films made of plant-derived non-food polysaccharides, such as cellulose and hemicelluloses (i.e. xylans and glucomannans), excluding storage polysaccharides, such as starch, may provide a potential alternative for today’s petroleum based packaging materials. These polysaccharide polymers are hydrophilic, and have good barrier properties against oils and fats, but are by its nature less efficient as moisture and water vapour barriers . Also, oxygen barrier properties are good at low or moderate relative humidities, particularly for xylans [2–4]. The mechanical strength of pure hemicellulose-based films does however vary considerably and are generally considered to be too low without additives. One possibility to enhance the performance of such hemicellulose films is by reinforcing them with cellulose, thus initiating this study.
Nanofibrillated cellulose (NFC) is a specifically prepared cellulosic material composed of liberated semicrystalline nanosized cellulose microfibrils with a high aspect ratio (i.e. length to with ratio). The lateral dimension of the NFC nanofibrils and nanofibril aggregates is in the order of 5–20 nm, respectively  and, if further aggregated, up to 40 nm  (cf. for kraft pulp fibres, the lateral dimension of microfibrils and microfibril aggregates is about 4 and 20 nm, respectively ), a value highly dependent upon the preparation technique used. Most frequently, various chemical and/or enzymatic pre-treatments before an intensive mechanical fibrillation are used for preparation of the NFC. The pre-treatments are introduced to purify the origin material from other cell wall components, such as hemicelluloses and lignin, but also to significantly reduce the energy consumption during the microfibril liberation process . The NFC, at the time termed microfibrillated cellulose (MFC), was first introduced by Turbak et al.  and Herrick et al. . An improved MFC was later prepared [9, 15] using a combination of mechanical and enzymatic pre-treatments, followed by a high pressure homogenization. The NFC used in the present study was prepared according to such a procedure outlined by Henriksson et al. . This NFC forms a stable hydrocolloidal dispersion and forms a gel already at fairly low concentration. The gel consists of a strongly entangled and disordered network of cellulose nanofibrils and nanofibril aggregates. Filtration and drying can be used to form a nanopaper structure  in the form of a NFC fibril network of high tensile strength (>200 MPa), high stiffness (~15 GPa) and relatively high strain-to-failure (6–10 %). In addition, NFC from wood pulp is characterised by biodegradability, it is from renewable resources, and the existing infrastructure for commercial wood pulping also makes NFC attractive as an easily available resource to be used as reinforcement component for biocomposites.
The goal of the present study was to investigate the morphology, physical and mechanical properties of biocomposite films, based on rye arabinoxylan (rAX) and NFC, and to examine effects of addition of NFC.
Materials and methods
Preparation of rAX solution
A high molecular weight rAX (lot 20601a) was used (Megazyme International Ireland Ltd); molar mass (Mw) 289,300 g/mol . The arabinose/xylose (Ara/Xyl) ratio of the rAX was 0.50. An aqueous arabinoxylan solution was prepared by dissolving the rAX flour, 2 g, (pre-wetted with 95 % C2H5OH) in deionised water, heating it at 100 °C under magnetic stirring for 10 min. The solution was allowed to cool down under magnetic stirring to room temperature and its concentration was adjusted to 2 % (w/v).
Preparation of nanofibrillated cellulose
NFC was prepared from a never-dried bleached sulphite pulp based on Norwegian spruce (Picea abies) (kindly provided by Nordic Paper Seffle AB, Sweden), containing 13.8 % hemicelluloses and 0.7 % lignin. A combined pre-treatment, i.e. an enzymatic degradation and a mechanical beating, was used, followed by a disintegration into NFC by a homogenisation, i.e. passing several times through a microfluidizer, as described by Henriksson et al. [15, 16] The solid content of the NFC (deionised water dispersion) was adjusted to 1.4 % (w/v). The charge density of the NFC was 50 μequiv/g, and it contained 0.8 % arabinoglucuronoxylan and 2.5 % galactoglucomannan (most likely a low Gal substituted fraction of galactoclucomannan). This lower amount of hemicelluloses as compared to the original pulp can be explained by the fact that much of the galactoglucomannan has been released upon the liberation of the cellulose microfibrils during the NFC preparation process. The crystallinity of the NFC may be estimated to around 8–12 % using NMR, based on previous studies , where the NMR crystallinity value corresponds to an X-ray crystallinity of ca. 50 %, due to the contribution of the paracrystalline material.
Preparation of films
Codes and descriptions of rAX, NFC and composite films thereof; % is percent by weight
100 % Arabinoxylan
75 % Arabinoxylan + 25 % nanofibrillated cellulose
50 % Arabinoxylan + 50 % nanofibrillated cellulose
25 % Arabinoxylan + 75 % nanofibrillated cellulose
100 % Nanofibrillated cellulose
In order to produce homogenous films, a thorough mixing is necessary obtained by a homogenization and further dispersion of the rAX/NFC mixtures in three steps: (a) using an Ultra-Turrax® 9500 rpm for 5 min, (b) exposing the mixture to magnetic stirring for 3 days, and (c) once again using the Ultra-Turrax® 9500 rpm for 10 min; adopting method developed by Svagan et al. . The resulting dispersions were exposed to vacuum under magnetic stirring for 24 h to remove air bubbles. The homogenized dispersions were then cast into Teflon (PTFE) coated polystyrene petri dishes (Ø 8.4 cm). These were left to evaporate at 23 °C and 52 % RH (relative humidity) for 48 h. The thickness of the resulting films was approximately 20 μm (measured using a micrometre screw gauge).
Scanning electron microscopy (SEM)
SEM images of the films, brittle fractured in liquid N2, were collected with a LEO ULTRA 55 FEG SEM (LEO electron microscopy LTD, Cambridge, UK) equipped with a field emission gun and an Inlens detector. The acceleration voltage was set to 5 kV. Prior to the SEM analysis, the films were glued to aluminium specimens, mounted with a colloidal silver liquid and sputter coated with a thin layer of gold.
Atomic force microscopy (AFM)
Dynamic vapour sorption (DVS)
Mechanical testing (stress–strain scanning) was carried out on a Single Column Table Top Instron 5944 (Instron Ltd., Coronation Road, UK), with a load cell of 2 kN, operating in tension mode. Film stripes cut to the size of 5 × 50 mm (width × length), of 20 μm thickness, were used for testing at 50 % RH and 23 °C. The applied loading rate was 10 % of the span length/min, where the initial span length of films was 30 mm. Ten replicates from each film type were tested. The Young’s modulus (E) was taken as the initial slope of the linear part of the stress–strain curve. The stress at break (σb) and strain at break (εb) were evaluated based on the initial sample dimensions. Data were presented as average values from ten measurements.
Dynamic mechanical analysis (DMA)
Dynamic mechanical testing (humidity scanning) was carried out using a Perkin-Elmer DMA 7e (PerkinElmer Corp., Norwalk, CT, USA), operating in tension mode. Film stripes cut to the dimensions of 3 × 15 mm (width × length), of 20 μm thickness, were first conditioned at 5 % RH for 3 h; (the hysteresis was negligible due to the low moisture content of ca. 3 at 5 % RH). The samples were then scanned in the range of 5–95 % RH at a speed of 1 % RH/10 min at a temperature of 30 °C, a speed low enough to maintain equilibrium conditions during scanning . The desired relative humidities were achieved by mixing dry air and water-saturated air using a Wetsys humidity generator (Setaram Instrumentation, Caluire, France). The static load was adjusted to be equal to 140 % of the dynamic load, the amplitude was set to be constant at 5 μm corresponding to a deformation of 0.033 %, at a frequency of 1 Hz. Three replicates from each film type were tested. The storage modulus (E’) and the loss tangent (tan δ) were recorded using a Pyris DMA 28 software (PerkinElmer Corp., Norwalk, CT, USA).
Dynamic mechanical testing (temperature scanning) was carried out using a Perkin-Elmer DMA 7e (PerkinElmer Corp., Norwalk, CT, USA), operating in tension mode. Film stripes cut to the dimensions of 5 × 20 mm (width × length), of 20 μm thickness, were first conditioned at a relative humidity of 0 % RH in a He atmosphere, a temperature of 30 °C for 30 min and then scanned in a range of 30–300 °C at a speed of 2 °C/min. The static load was adjusted to be equal to 120 % of the dynamic load, the amplitude was set to be constant at 3 μm, at a frequency of 1 Hz. Three replicates from each film type were tested. The storage modulus (E′) and the loss tangent (tan δ) were recorded using a Pyris DMA 28 software (PerkinElmer Corp., Norwalk, CT, USA).
Dynamic FTIR spectroscopy
Dynamic FTIR (Fourier Transform InfraRed) spectra were recorded on a Varian 680-IR spectrometer (Varian Inc., Santa Clara, CA, USA) in transmission mode. A liquid nitrogen cooled MCT (Mercury Cadmium Telluride) detector was used and the IR radiation was polarized by a KRS5 wire grid polarizer at 0° in relation to the stretching direction. An optical filter was added after the polarizer to reduce the spectral range (3950–700 cm−1). The interferometer was run in step-scan mode at a phase modulation frequency of 400 Hz (the moving mirror oscillates around each step point with a frequency of 400 Hz). Thin sheets with a dimension of 10 × 20 mm (width × length) and of 20 μm thickness were mounted between two parallel jaws in a specially constructed polymer modulator (PM-100). The PM-100 was placed in a temperature control system (TC-100) (MAT-Manning Applied Technology Inc., Troy, ID, USA). A modular humidity generator (MHG-32) (Projekt Messtechnik, Ulm, Germany) was connected to the TC-100. Spectra were recorded with a resolution of 8 cm−1; one scan per measurement. To ensure the linear deformation during the collection of the dynamic FTIR spectra, the amplitude of the applied sinusoidal strain was less than 0.3 % of the sample length at a frequency of 16 Hz. The dynamic changes in the IR spectrum were divided into two orthogonal spectra: the in-phase spectrum indicating immediate changes or elastic responses and the out-of-phase spectrum representing the time-delayed changes or viscous responses using Varian Resolutiom Pro 5.1 software (Varian Inc., Santa Clara, CA, USA). The resulting interferograms were Fourier transformed and a Norton–Beer medium apodization function was used. The in-phase and out-of-phase spectra obtained were base-line corrected at 1800 cm−1. They were then converted into a phase spectrum and a magnitude spectrum, where the converted magnitude spectrum was normalised to 1.0 at 1160 cm−1. New in-phase and out-of-phase spectra were calculated from these. Two parallel measurements were made at 50 % RH and 30 °C. The average in-phase and out-of-phase spectra were then calculated.
Oxygen transmission rate (OTR) was measured according to an ASTM F—1927-07 standard method, using a Mocon Ox-Tran® 2/21, ML Master Base Control Module, (Mocon® 7500 Boone Avenue North, Minneapolis, Minnesota, USA) operating with a Mocon’s WinPerm™ permeability software. The measurements were done at controlled conditions of 50 % RH and 23 °C, using a coulometric detector. Duplicates from each film type of 20 μm thickness with an area of 5 cm2 were tested. Oxygen permeability (OP) was calculated from the film thicknesses and expressed as cm3 μm/m2 day kPa.
Results and discussion
Film formation and morphology
Mean roughness (Ra) of rAX, NFC and composite films thereof
Surface roughness, Ra (nm)
Moisture sorption isotherms
The NFC film displayed the lowest moisture adsorption, reflecting the crystalline nature of the microfibrillar cellulose. However, the present NFC contains a substantial amount of disordered cellulose as well as some hemicelluloses. The 100rAX film showed the highest moisture content, reflecting the highest amount of water sorption sites available. The composite films showed intermediate moisture sorption isotherms, where the moisture sorption isotherm successively decreased with increasing amount of NFC present.
Average values of Young’s modulus (E), stress at break (σb) and strain at break (εb)
Young’s modulus, E (GPa)
Stress at break, σb (MPa)
Strain at break, εb (%)
3.1 ± 0.2
62 ± 3
4.0 ± 0.9
4.8 ± 0.7
108 ± 17
6.0 ± 1.1
6.3 ± 0.4
135 ± 31
6.4 ± 2.4
7.3 ± 0.5
143 ± 15
7.2 ± 1.2
6.6 ± 0.5
107 ± 14
5.7 ± 1.9
The low mechanical properties for the 100NFC film were most likely due to high porosity, since NFC films are fibrillar network structures. One could also observe lamellar separation in the SEM micrographs, see Fig. 3. When smaller denser regions were sampled for DMA measurements, much higher Young’s modulus was measured at 50 % RH (i.e. 11.5 GPa). This was more in line with previously reported data on homogenous films with 19 % porosity, showing mechanical strength properties of approximately 15 GPa Young’s modulus, 200 MPa tension strength and 7 % strain-to-failure . With rAX, as an amorphous polymer [3, 21] present in the structure, a more uniform stress distribution results in the better strength properties.
Two transition points could be noted, a secondary transition around 20 % RH and a main transition  at around 80 % RH. The secondary transition was most apparent for the 100rAX film and may be attributed to the motion of the arabinose substituent . The main transition at 80 % RH was attributed to the moisture-induced glass transition of the xylan [24–26]. It is notable that already with an addition of 25 % NFC (25NFC film), the indication of the glass transition almost disappeared in the data. This may be due to possible interactions between NFC and arabinoxylan, limiting the moisture-induced mobility of the xylan chains .
Oxygen barrier properties
This study demonstrates that, with the addition of NFC to rAX, it is possible to produce composite films with greatly improved mechanical, thermal, moisture sorption and excellent oxygen barrier properties. Thorough mixing of NFC with rAX resulted in homogenous composite films.
In general, stiffer, stronger and more ductile films were produced when the NFC was added to the arabinoxylan; with a Young’s modulus of up to 7.2 GPa, a tensile strength of up to 143 MPa and a strain-to-failure of up to 7.3 %. The addition of the NFC to the arabinoxylan films caused a shift of the Tg to higher temperatures than the 200 °C of the pure xylan. This indicates a substantial xylan interphase region with reduced molecular mobility. A strong coupling between cellulose and arabinoxylan was also indicated. The moisture sorption and the associated moisture-induced softening were both reduced with increased NFC reinforcement content. With the reinforcement, the oxygen barrier properties of the arabinoxylan films were also improved, showing an oxygen permeability of 0.8 cm3 μm/m2 day kPa at 50 % RH in a film composed of 50–75 % NFC by weight.
The Knut and Alice Wallenberg Foundation are gratefully acknowledged for funding through the Wallenberg Wood Science Center. The authors thank Dr Aihua Pei for assistance in procuring the NFC and for fruitful discussions on the film casting techniques, Anders Mårtensson for providing the SEM micrographs and AFM measurements, Anne-Mari Olsson for performing the DVS and Therese Johansson for performing O2 permeability measurements.