State of the art

Cellulose was rediscovered at the end of the last century due to nanoscience and improved technologies available for the disintegration of cellulose in submicron and nanosize fibers. The native cellulose molecule consists of linear glucan chains with repeating (1-4)-|3- glucopyranose units. The supramolecular structure of cellulose is very complex and it has been the subject of many studies (Eichhorn et al., 2010).

In living plants cellulose occurs in fibers, the cell wall of wood fibers consisting of repeated crystalline structures resulted from the aggregation of cellulose chains, termed microfibrils. These microfibrils, with high aspect ratio, are surrounded by an amorphous matrix of hemicelluloses and lignin. Besides wood, bast fibers (Cao et al., 2008; Li and Pickering, 2008; Shaikh et al. 2009; Belhassen et al., 2009), agricultural residues (El-Sakhawy and Hassan, 2007; Fama et al., 2009), leaf fibers (de Rodriguez et al., 2006; Zuluagaet al., 2009), bacterial cellulose (Nakagaito et al., 2005; Yano et al., 2008), the shell of some fruits and vegetables (Chen et al., 2009; Rosa et al., 2010) or tunicates (Petersson and Oksman, 2006) were used as sources for cellulose nanofibers preparation. Among the methods of nanofiber isolation, the most used are: mechanical disintegration, acid hydrolysis and biological treatments. Mechanical treatments involve conventional refining procedures (Hubbe et al., 2008), crushing and cryocrushing (Bhatnagar and Sain, 2005; Panaitescu et al., 2007a; Alemdar and Sain, 2008) or high pressure homogenization of cellulose source suspended in water (Lee et al., 2009a). Chakraborty isolated cellulose fibers with submicron diameters by combining the severe shearing in a refiner with high-impact crushing under liquid nitrogen (Chakraborty et al., 2005).

Mechanical treatments have some disadvantages related to the high energy required in the process and to the high degree of inhomogenity, the resulted material containing larger fibrils in addition to microfibrils (Nakagaito and Yano, 2004; Nakagaito and Yano, 2005; Andresen et al., 2006; Andresen and Stenius, 2007; Stenstad et al. 2008).

Chemical treatments involve mainly acid and alkaline hydrolysis. Acid hydrolysis leads to the isolation of micro and nanofibers with a high degree of crystallinity by removing the amorphous regions of the raw cellulosic material. Using sulfuric acid, a negatively charged surface of the cellulose fibers can be obtained, through the esterification of hydroxyl groups by the sulfate ions, which prevents the agglomeration of fibers.

Many researchers have successfully used this method, alone or in combination with others methods, to obtain cellulose nanofibers (Zhang et al., 2007; El-Sakhawy et al., 2007; Moran et al., 2008; Chen et al., 2009): Bondenson et al. (2006) treated microcrystalline cellulose (MCC) with sulfuric acid in concentration of 63.5% (w/w) and isolated cellulose whiskers with an aspect ratio between 20 and 40, Lee et al. (2009b) obtained nanocellulose fibers by hydrobromic acid hydrolysis of MCC, an increasing in MCC crystallinity being reported, Rosa et al. (2010) obtained cellulose whiskers with diameters as low as 5 nm and aspect ratio of up to 60 by sulfuric acid hydrolysis of coconut husk fibers.

Degradation of the cellulosic substrate may also occur in the presence of microorganisms (fungi, bacteria) or, directly, with enzymes. Enzymatic treatment of cellulose was performed by Henriksson et al. (2007) who reported the obtaining of cellulose nanofibers from two different commercial bleached wood sulphite pulps. Li et al. (2008) reported that removal of non-cellulosic components from cellulose fibers by enzyme treatment can increase the crystallinity, thermal stability and the amount of – OH groups of the treated fibers.

New and environmental friendly methods for cellulose nanofibers isolation were also tested in laboratory conditions. Ultrasonication had been used alone or in combination with other methods (e. g. acid hydrolysis) to obtain cellulose fibers. Filson and Dawson-Andoh (2009) obtained nanofibers with an average diameter between 21 and 23 nm applying this method. Application of cellulose nanofibers in polymer reinforcement is a relatively new research field. Two directions could be detected: research studies which explore the development of nano-bio-plastics as fully biodegradable nanocomposites and studies aimed at dispersing cellulose nanofibers in non-biodegradable, petroleum derived polymers. Polymer composites containing cellulose nanofibers were prepared with polyvinyl alcohol (PVA), polylactide (PLA), starch and polycaprolactone but also with polyethylene or polypropylene. Improvement in term of brittleness, thermal stability or barrier properties were signaled in case of biodegradable polymers reinforced with cellulose nanofibers (Iwatake et al., 2008; Chen et al., 2009; Fama et al., 2009; Pandey et al., 2009; Suryanegara et al., 2009; Bledzki et al., 2009; Chang et al., 2010). Cellulose fibers reinforcement could be a good solution for starch, which is a low cost source of biodegradable composites but a material with very poor mechanical properties. Dufresne and Vignon (1998, 2000) reported an improvement by a factor of 3.5 of the tensile modulus of starch at 50 wt% cellulose fibers addition. PLA is a commercially available biopolymer with similar properties to petroleum derived thermoplastics. Some drawbacks like brittleness and low thermal stability restrict its applications. Using cellulose fibers (10 wt%) and a solvent exchange method, Iwatake et al. (2008) succeeded in fabricating a composite sheet with uniform filler distribution showing an increase of Young’s modulus and tensile strength of 40 and 25% respectively. Chemical modification of cellulose has been explored as a route for improving filler dispersion in hydrophobic polymers such as polyethylene or polypropylene (Panaitescu et al., 2007b; Rahman et al., 2009; Yanjun et al., 2010).

PVA is a water-soluble and biodegradable polymer with excellent chemical resistance and an interesting material for biomedical applications. PVA has no toxic action on the human body being used to manufacture medicines cachets, yarn for surgery, controlled drug delivery systems (Tang et al., 2009). New fields of application regard cardiovascular devices (Millon and Wan, 2006), dialysis membrane, artificial cartilage, tissue engineering scaffold (Zhou et al., 2010). Development of ecofriendly packaging materials is still a challenging area and many studies are focused on the improvement of PVA 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, mechanical properties of PVA should be substantially improved without damaging its valuable properties. Low cellulose fibers addition could be an appropriate solution. Many studies emphasized the effectiveness of large amount of cellulose fibers in improving mechanical properties of PVA. Zimmermann et al. (2004) reported an improvement of the elastic modulus and tensile strength of up to five times and three times, respectively, in the case of dispersing 20 wt% cellulose fibers in PVA. An increase of about five times of the tensile strength relative to the reference polymer was reported by Bruce et al. (2005) at 50 wt% cellulose fibers in PVA and an increase by a factor of 3.5 at the same concentration of fibers in the work of Leitner et al.

(2007) . Nevertheless, no increase in tensile strength and modulus was observed by Lu et al.

(2008) above 10 wt% cellulose fibers in PVA.

Despite the important publication activity dealing with cellulose nanofibers and related nanocomposites the application of such materials is quite limited. There are several reasons for this situation. One of them is the difficulty of the separation of plant fibers into smaller constituents with uniform and reproducible characteristics. Another reason is the high energy demands for most isolation processes. Problems concerning interfacial adhesion and uniform dispersion of the cellulose fibers in the polymer matrix delayed also the wide application of these materials.

Moreover, results obtained in different laboratories on polymer composite containing cellulose fibers are often contradictory because of many factors controlling the process like filler size and content, interface adhesion, fiber aspect ratio and orientation, fiber dispersion in the matrix and stress transfer efficiency through the interface (Dufresne et al., 2003). However, the role of nanofiber characteristics and aggregation in influencing macroscale properties of polymer matrix is not yet well understood.

Important research work was focused on studying the changes induced in PVA characteristics by high concentration of cellulose fibers prepared by different processes. These results were obtained in different laboratories and different conditions were applied in order to prepare PVA composite films. PVA is a polymer sensitive to preparation conditions so that significant changes may occur depending on the working atmosphere and heat treatments applied. Knowing the high influence of preparing and characterization conditions on PVA properties, it is difficult to select the proper system PVA/cellulose fibers for a target application based on existing information. Moreover, achieving improved mechanical properties with the less concentration of cellulose fibers is desirable for preserving transparency and flexibility of PVA films. For these reasons, we further describe research results obtained in our laboratory for monitoring the influence of cellulose fibers obtained by different treatments on the microscopic and macroscopic properties of PVA, all the samples being prepared in the same conditions. PVA composites were prepared by solution casting using low concentration (<5 wt.%) of cellulose fibers obtained by mechanical treatment (L), acid hydrolysis (H) and ultrasound treatment (U). The isolated cellulose fibers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). PVA composites containing these fibers were characterized with the aim of determining thermal and mechanical behavior.