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Frone, A. N., Panaitescu, D. M., Donescu, D., Spataru, C. I., Radovici, C., Trusca, R., and Somoghi, R. (2011). "Preparation and characterisization of PVA composites with cellulose nanofibers obtained by ultrasonication," BioRes. 6(1), 487-512.

Abstract

Cellulose nanofibers were obtained from microcrystalline cellulose (MCC) by the action of hydrodynamic forces associated with ultrasound. Nanofibers isolated from MCC by applying different ultrasonication conditions were characterized to elucidate their morpho-structural features by field emission scanning electron microscopy, atomic force microscopy, X-ray diffraction, and dynamic light scattering. Several differences were observed regarding the size of the nanofibers obtained in different ultrasonic conditions, but no significant changes in the crystalline structure of cellulose nanofibers were detected. The obtained cellulose fibers were used at low levels (1 to 5 wt.%) as reinforcements in a poly(vinyl alcohol) (PVA) matrix. The mechanical and thermal properties of the PVA/cellulose fibers nanocomposites films were determined. The tensile strength and modulus of the PVA film were significantly improved by the addition of cellulose nanofibers. Slightly higher onset degradation temperatures were obtained for PVA composites in comparison to neat PVA, showing an increase of the thermal stability caused by the addition of cellulose fibers.


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PREPARATION AND CHARACTERIZATION OF PVA COMPOSITES WITH CELLULOSE NANOFIBERS OBTAINED BY ULTRASONICATION

Adriana N. Frone,a Denis M. Panaitescu,*a Dan Donescu,a Catalin I. Spataru,a Constantin Radovici,a Roxana Trusca,b and Raluca Somoghi a

Cellulose nanofibers were obtained from microcrystalline cellulose (MCC) by the action of hydrodynamic forces associated with ultrasound. Nanofibers isolated from MCC by applying different ultrasonication conditions were characterized to elucidate their morpho-structural features by field emission scanning electron microscopy, atomic force microscopy, X-ray diffraction, and dynamic light scattering. Several differences were observed regarding the size of the nanofibers obtained in different ultrasonic conditions, but no significant changes in the crystalline structure of cellulose nanofibers were detected. The obtained cellulose fibers were used at low levels (1 to 5 wt.%) as reinforcements in a poly(vinyl alcohol) (PVA) matrix. The mechanical and thermal properties of the PVA/cellulose fibers nanocomposites films were determined. The tensile strength and modulus of the PVA film were significantly improved by the addition of cellulose nanofibers. Slightly higher onset degradation temperatures were obtained for PVA composites in comparison to neat PVA, showing an increase of the thermal stability caused by the addition of cellulose fibers.

Keywords: Cellulose nanofibers; Ultrasonication; Poly(vinyl alcohol); Nanocomposites; Mechanical properties

Contact informations: a: National Institute for Research and Development in Chemistry and Petrochemistry, Polymer Department, 202 Spl. Independentei, 060021, Bucharest, Romania; b: METAV Research and Development SA, 31 C.A Rosetti Street, Bucharest, Romania; *Corresponding author: panaitescu@icf.ro

INTRODUCTION

In the last decade, increasing environmental awareness has led to growing interest in the development of materials with eco-friendly attributes. Owing to their good mechanical properties, polymer composites with cellulose fibers (with micro or nano size) are able to substitute for glass-fiber-containing composites in some important applications such as automotive or construction and have found potential applications in biomedical and cosmetic industries, the electrical and electronic field, and the paper, and packaging industry (Hoenich 2006; Lee 2006; Petersson and Oksman 2006; Dong and Roman 2007; Panaitescu et al. 2007a,b,c; Wang and Sain 2007; Hubbe et al. 2008; Ioelovich 2008; Kamel 2009).

The production of nano-scale cellulose fibers and their application in composite materials has gained increasing attention in recent times. Two different types of nanoreinforcements can be isolated from a cellulosic source: nanofibers and nanocrystals/ nanowhiskers (Petersson et al. 2009; Elazzouzi-Hafraoui et al. 2008). Nanofibers are fibrilar units resulting from the linear combination of cellulose macromolecules that contain both amorphous and crystalline regions of cellulose and have the ability to create entangled networks. Cellulose nanocrystals can have a perfect crystalline structure and high modulus, close to the theoretical modulus of cellulose (Eichhorn et al. 2010).

Considerable research has been done regarding the extraction of cellulose nanofibers from different sources and on preparing polymer composites with these fibers (Nakagaito et al. 2004; Yano et al. 2005; Nakagaito and Yano 2008; Nogi et al. 2006; Chakraborty et al. 2005). The mechanical and chemical treatments have been the most applied methods to obtain cellulose nanofibers. Nakagaito and Yano (2004) obtained cellulose nanofibers from kraft pulp after repeated passes (16 to 30) through a refiner and prepared a composite with improved mechanical properties based on a phenolic resin reinforced with these fibers. Chakraborty et al. (2005) applied combined mechanical techniques to obtain cellulose microfibers from bleached kraft pulp, employing severe shearing in a refiner, followed by high-impact crushing under liquid nitrogen. Cellulose whiskers with a length between 200 and 400 nm were isolated from microcrystalline cellulose (MCC) by acid hydrolysis using sulphuric acid with a concentration of 63.5% (Bondenson et al. 2006). By a combination of chemical and mechanical treatments, Jonoobi et al. (2009) isolated nanofibers from unbleached and bleached kenaf pulp. The obtained nanofibers showed higher crystallinity and thermal stability as compared to the raw kenaf. Lee et al. (2009) obtained nanocellulose by acid hydrolysis of MCC using different hydrobromic acid concentrations. PVA composites prepared with these nanocelluloses showed significantly improved tensile and thermal properties. Panaitescu et al. (2007b) prepared polypropylene composites with cellulose fibers obtained through mechano-chemical treatment of bleached pulp and α-cellulose. These composites exhibited higher tensile strength and elastic modulus than neat polypropylene.

All of the above mentioned methods used for cellulose nanofibers isolation have some drawbacks: they involve high consumption of energy for such processes as mechanical treatments by refining, homogenisation, or grinding (Nakagaito and Yano 2004), or the fabrication processes can cause deterioration of the environment, as in the case of chemical treatments (De Souza Lima et al. 2004). Current research has been focused on finding new environmentally friendly methods to isolate cellulose fibers, characterized by high efficiency and low costs. To address this goal, ultrasonication has been employed alone or in combination with acid hydrolysis to obtain cellulose fibers in a few laboratory tests (Filson and Dawson-Andoh 2009; Zhang et al. 2007; Cheng and Wang 2008; Oksman et al. 2006; Li and Renneckar 2009). High intensity ultrasonication can be a powerful and clean method of defibrillation of cellulose sources. Ultrasonic waves produce strong mechanical stresses due to cavitation, causing disaggregation of cellulose fibers into smaller entities (Zhang et al. 2007; Wang and Cheng 2009).

Several attempts to prepare polymer composites with cellulose nanofibers using polymeric matrices with hydrophilic character such as starch, polyethylene oxide, poly (vinyl alcohol), or polyethylene glycol have been reported (Kvien et al. 2007; Alemdar and Sain 2008; Yano and Nakahara 2004; Famá et al. 2009; Azizi et al. 2005; Kvien and Oksman 2007; Roohani et al. 2008; Yan and Gao 2008; Yuan and Ding 2006). Nanocomposite films with improved mechanical properties were obtained by dispersing cellulose nanofibers (5 %) into a starch matrix, emphasizing the reinforcing effect of cellulose nanofibers (Kvien et al. 2007).

Poly(vinyl alcohol) (PVA) is a water-soluble and biodegradable polymer with excellent chemical resistance; as such it is an interesting material for high-tech applications (Zhang et al. 2009). PVA has no toxic action on the human body and is used to manufacture medicines cachets, yarn for surgery, and controlled drug delivery systems (Tang et al. 2009). Development of eco-friendly packaging materials is a challenging area, and many studies have been focused on the improvement of PVA’s mechanical and barrier properties by combination with other polymers or fillers in order to use it in the packaging industry (Sedlarik et al. 2006). For many other applications, the mechanical properties of PVA should be substantially improved without damaging its other valuable properties such as transparency and flexibility.

The main goal of this work was to employ different ultrasonic conditions in order to isolate nanofibers from microcrystalline cellulose and to evaluate the reinforcing ability of small amounts of these fibers in a PVA matrix. A low concentration of cellulose fibers was preferred in order to preserve the transparency and flexibility of PVA films. PVA was chosen as a matrix because of the above specified advantages and the expected interaction of its hydroxyl groups with the hydrophilic surfaces of the cellulose nanofibers, leading to strong hydrogen bonding. Although several attempts to separate the cellulose nanofibers from various sources of cellulose using only high power ultrasonication have appeared in the literature (Wang et al. 2009; Wu et al. 2009), especially in recent years, it has not been tried yet, as our knowledge, to use fibers obtained in this way to reinforce PVA, and the influence of ultrasonic conditions on the properties of obtained fibers and composites have not yet been investigated.

Nanofibers were characterized by field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and dynamic light scattering (DLS). The mechanical and thermal properties of PVA nanocomposites prepared with these nanofibers were also determined.

EXPERIMENTAL

Materials

Microcrystalline cellulose (MCC) with a mean particle size of 20 m and an aspect ratio of 2 to 4, purchased from Sigma-Aldrich, was used as raw material for the preparation of nanofibers. Poly(vinyl alcohol), PVA 120-99, 1200 polymerization degree and 99% hydrolysis degree, was purchased from Chemical Enterprise Râsnov (Romania) and was used for nanocomposites preparation.

Nanofibers Isolation

To obtain cellulose nanofibers from microcrystalline cellulose, the MCC was dispersed under continuous stirring in distilled water (1/500) and sonicated using an ultrasonicator type Vibra Cell VC505 (500 W, 20 KHz), equipped with a sonication probe of 19 mm. The concentration of MCC in distilled water was 0.2%, the size of a sample being ca. 500 mL. In order to prevent the uncontrolled increase of temperature, the beaker with the cellulose suspension was put in a water bath with controlled temperature. Different ultrasonication powers and times were used (Table 1). Sample temperature was measured at the end of the ultrasonic treatment and it did not exceed 50ºC. The appearance of non-settling turbidity in the supernatant was a definite indication of the presence of cellulose nanofibers (Chen et al. 2010; Petersson et al. 2007; Bondenson et al. 2006)

Table 1. Parameters of Ultrasonication Process and Cellulose Samples Symbols

Samples of cellulose nanofibers (supernantant) were obtained from the water suspensions by decanting the supernatant into other vessels, two hours after the sonication process ended. The cellulose samples U1, U2, U3, and U4, obtained under the conditions specified in Table 1, were characterized by means of dynamic light scattering, by microscopic techniques, and by X-ray diffraction.

Nanocomposite Films Preparation

In order to obtain PVA/cellulose fiber composite films with different filler concentrations, the required amount of PVA corresponding to a final concentration of 10% in the aqueous medium and the calculated amount of U1…U4 cellulose fibers suspensions to achieve 1, 3, and 5 wt% filler concentration in the final composite were mixed using a high speed stirrer (500 min-1). The stirring was performed at 80oC for 3 hours, and the resulting mixture was degassed for approximately 15 minutes in an Elmasonic S40 H ultrasonic bath. The films were cast on a PET plate and were kept at room temperature for two days until they were completely dried and then removed from the PET plates and placed in a desiccator for three days before mechanical characterization. Neat PVA films were obtained under similar conditions.

Experimental Methods

Cellulose fibers characterization

FE-SEM micrographs were obtained with a Quanta Inspect F Scanning Electron Microscope with a field emission gun having a resolution of 1.2 nm at an accelerating voltage of 30 kV. A droplet from the undiluted suspensions was put on a glass grid and dried under vacuum before FE-SEM analysis. All the samples were sputter-coated with gold before examination.

AFM images were captured in tapping mode by a MultiMode 8 atomic force microscope equipped with a Nanoscope V converter (Veeco, Santa Barbara, CA). Real time scanning was performed in air at room temperature with scan rates of 1 Hz and scan angle 0o. A silicon tip (nominal radius 8 nm from Veeco) with a cantilever length of 225 μm and a resonant frequency of about 75 kHz was used. The height and phase signal were recorded simultaneously and the images (256×256) were recorded and analyzed using the AFM software NanoScope version 1.20. Droplets of undiluted aqueous suspensions of U1 and U4 nanofibers were placed on a glass substrate and allowed to dry under vacuum before AFM analysis.

The crystallinity of cellulose fibers was determined by XRD with a DRON-UM diffractometer (horizontal goniometer Bragg-Brentano) using Co Kα radiation (wavelength λ = 1.79021 Å), scanning from the 2θ value of 4o to 36o at a scanning rate of 0.05o/5 sec. Samples were analyzed in reflection mode after drying 4 hours at 40oC.

The size distribution of cellulose aggregates was estimated by dynamic light scattering (DLS) using a Zetasizer Nano ZS instrument (Malvern, UK). The DLS measurements were performed with an angle of 170° by using a He–Ne laser (4 mW) operated at 633 nm, on the samples themselves (supernatant). DLS analysis has been developed for measuring the dimensions of spherical particles, so that the measurement of rod-shaped materials like our fibers will result only in approximate values. The size registered for the fibers will depend on their orientation in the fluid. Considering the tendency of agglomeration of cellulose fibers in spherical aggregates and the lack of adequate method for fibers size estimation, we used this analysis for internal comparisons.

PVA composites characterization

Mechanical behavior of composite films at room temperature was tested in tensile mode according to ISO 527-1:1993 Part 1 and ISO 527-3:1995 Part 3 by an Instron 3382. The Universal Testing Machine was equipped with a video extensometer for strain measurement and with a load cell of 1 kN capacity. The following data: video axial strain at break (elongation at break), tensile stress at break (tensile strength), Young’s modulus (tensile modulus), and other optional features, were automatically displayed using the software of the Instron 3382, Bluehill 2 device. Young’s modulus on the strain channel with a start value of 0.05% and an end value of 0.25% was calculated by the system, applying the least squares fit algorithm to each region between the start and end value selecting the highest slope. The load cell of 1 kN capacity, used for PVA composites tensile properties measurements, has a sensitivity of 0.001 N.

Test specimens were cut to 110 mm (length) x 10 mm (width) x 0.035-0.045 mm (thickness) from the films maintained in desiccators and tested immediately after being removed from the dessicator. 7 specimens were tested for each sample. The conditions of mechanical testing were 50% relative humidity (RH) and a temperature of 23oC. Tensile modulus was determined at a crosshead speed of 2 mm/min, and tensile strength and elongation at 10 mm/min. Lower crosshead speed was necessary for correct determination of tensile modulus as stipulated by ISO 527 Part 1.

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on a SDT Q600 V20.9 from TA Instruments under helium flow (100 mL/min). The samples were dried for 24 h at 40 oC before testing. The samples weighing between 5 and 10 mg were packed in aluminium pans and placed in the DSC cell and were tested from the ambient temperature to 600 oC at a heating rate of 10oC/min. The DSC glass transition temperature (Tg) was taken at the onset of the glass transition endotherm, while the melting temperature (Tm) was taken as the peak temperature of the melting endotherm. The onset degradation temperature (Ton) was determined as the temperature corresponding to the crossover of tangents drawn on both sides of the decomposition trace, and the temperature of the maximum weight loss rate (Td) was taken as the peak temperature of the degradation endotherm. The degree of crystallinity (Xc) of the PVA component in the composites was obtained as follows,

 (1)

where ΔHf and ΔHo are the heats of fusion for PVA composites and 100% crystalline PVA, respectively, and w is the mass fraction of PVA in the composite. ΔHo was taken to be 150 J/g (Lu et al. 2008).

RESULTS AND DISCUSSION

Cellulose Fibers Characterization

The appearance of supernatant samples U1 through U4 one week after they were prepared is shown in Fig. 1. Different degrees of turbidity are visible in Fig. 1, indicating the highest light scattering for sample U4 and the weakest for sample U1, corresponding to different nanofibrillation degrees by applying different ultrasonication conditions.

Fig. 1. Cellulose nanofibers (U1…U4 samples) obtained by ultrasonication treatment

The presence of turbidity in the supernatant after one week is a strong indication of the presence of cellulose nanofibers, as observed by Chen et al. (2010). Moreover, Petersson et al. (2007) as well as Bondenson et al. (2006) obtained cellulose nanofibers by applying several cycles of centrifugation, noting that the supernatant remained turbid, considered the persistence of turbidity as a sure proof of the presence of nanocellulose fibers in the supernatant. The turbidity increased from U1 to U4 with the increase of power and time of sonication, showing the effectiveness of the sonication process.

The yield of the cellulose nanofibers was determined as a percentage of the cumulative dry weight of the supernatant (S1) and the sediment (S2). The yield of cellulose nanofibers (Y) was calculated as the ratio S1x100/(S1+S2), with the average of three determinations reported in Table 2.

The highest content of cellulose nanofibers was obtained for sample U4, an intermediate content for samples U2 and U3, and the lowest yield in the case of sample U1. The results are consistent with the observations of turbidity, as specified above.

Table 2. Yield of Cellulose Fibers

The temperature of cellulose fiber suspensions after the ultrasonication process changed depending on the power and time of ultrasonication, higher temperature being observed at higher values of the power and time of ultrasonication. An increase of the sample temperature was also obtained by Wang and Cheng (2009) when high intensity ultrasonication was applied to different cellulose sources.

FE-SEM analysis

FE-SEM micrographs of cellulose fibers U1 through U4 are shown in Fig. 2. The FE-SEM images demonstrate the effectiveness of the MCC defibrillation process, fibers with a diameter ranging from a couple of nanometers to 150 nm being observed.

Several differences were detected between the samples obtained with the application of different ultrasonic conditions. In the case of samples prepared with the lower ultrasonication power and the lower time of sonication (U1), pieces of undefibrilated MCC and many fibers with a diameter of 70 to 150 nm were observed. When a higher time of sonication was applied (U2), no pieces of undefibrilated MCC were detected, and the diameters of fibers were smaller, from 10 to 100 nm.

Between the samples prepared at higher ultrasonication power (U3 and U4) there were no significant differences. In the images one can see nanofibers of cellulose with very small dimension (<20 nm). Uniform dimensions can be noticed in these samples, especially in the case of sample U4.