Cellulose is considered to be the most abundant organic compound derived from biomass. In addition to wood, which cellulose is its main building material, there are other major sources for cellulose such as plant fibers, marine animals, or algae, fungi, invertebrates, and bacteria. Cellulose is an unbranched homopolysaccharide composed of -D-glucopyranose units linked by (14) glycosidic bonds (O’Sullivan 1997). The rings are believed to be in the chair conformation 4C1, where the hydroxyl groups are positioned in an equatorial places. The degree of polymerization (DP) for Cellulose chains are approximately 10, 000 glucopyranose units in wood and 15, 000 for native cellulose cotton. Basic chemical structure of cellulose is a dimer called cellobiose (Figure ). Figure . Repeating unit of cellulose chain (O’Sullivan 1997)Cellobiose consists of three hydroxyl groups. Presence of these hydroxyl groups facilitates the monomer to create strong hydrogen bonds which give cellulose unique properties: Multi-scale microfibrillated structure, which means cutting down the microfibrillated cellulose structure retains the properties of the original cellulose, in addition, offers new properties that are specially present in smaller size scales; duality in characteristics (crystalline vs. amorphous regions), which donates strength and flexibility at the same time; and highly cohesive nature (with a glass transition temperature higher than its degradation temperature (Lavoine et al. 2012).
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Microfibrillated cellulose and nanocellulose
Microfibrillated celluloses (MFC), which is assembled into cellulose fibers, are the largest unit of elementary fibrils or microfibrils who are generated through combination of about 36 individual cellulose molecules. MFC, which is also called cellulose microfibril, microfibrillar cellulose, or more currently, nanofibrillated cellulose (NFC), is cellulosic product with greatly expanded surface area that is obtained from mechanical disintegration of cellulosic materials. Different methods are already developed for producing MFC. Turbak was the first who introduced homogenizer method. The method consists of successive disintegration of very dilute suspension of wood pulps in high-pressure in order to obtain a viscous and shear thinning aqueous gel. In an alternative method, so called Microfluidizer, the wood pulp is forced through thin z-shaped chambers under pressures as high as 30, 000 psi. Consequently, it is possible to produce more uniform nanocellulose that has thinner dimensions. In this method, the mechanical treatment is repeated with different chamber sizes to increase the degree of fibrillation. This latter process produces more uniformly sized fibers. The diameter of elementary fibrils is about 5 nm whereas the MFC has diameters ranging from 20 to 50 nm. The microfibrils which are several micrometers in length are forming after biosynthesis of cellulose. Microfibrils can be considered as strings of ordered cellulose crystals linked along the microfibril axis by disordered amorphous domains. The cellulose chains are tightly packed together and stabilized by strong and complex network of hydrogen bonds (Lavoine, Desloges et al. 2012). Cellulose nanocrystals (CNC) might be called with different names: rod-like colloidal particles, nanocrystalline cellulose, cellulose whiskers, cellulose microcrystallites, microcrystals, microfibrils. CNCs are high purity single crystals of cellulose structure. They are produced mainly by acid hydrolysis under controlled conditions of temperature, agitation, and time. Acid hydrolysis dissolves the amorphous regions, which are acquired weak bonds, leaving the crystalline regions, the more resistant domain, intact. Afterward, the resulting suspension is washed several times with distilled water to remove any remained acids (Lavoine, Desloges et al. 2012).
The interests of the barrier properties of bio-based materials are increasing in order to develop environmentally friendly and efficient materials for different purposes. MFC, as one of the promising bio-based materials, is explored in the forms of films, nanocomposites, and paper coating. With regard to MFC films, WVTR and WVP were the first barrier properties that were studied. One objective was to use these films in barrier packaging to replace the current packaging materials, in a so called modified atmosphere packaging (MAP). The influences of the types and chemical compositions of wood sources on the WVTR were studied in details. Comparisons between the original pulps and MFC showed the reduction in WVTR. Among the MFC, derived from different sources of wood, MFC from bleached hardwood has the highest water vapor barrier property (200 gm-2day-1). As compared with the WVTR of low density polyethylene (20 gm-2day-1), MFC shows 10 times lower barrier property (Lavoine, Desloges et al. 2012). It was also shown that the water absorption increases with lignin content in the original pulps, however, after pre-treatment and after homogenization, water absorption is not a function of lignin content anymore. This was explained based on the structural difference between original pulp and MFC films, where the MFC films are believed to be more compact, and less water can penetrate into the film even in high lignin content samples (Spence et al. 2010). Barrier property is greatly affected by the physical structure of MFC. The degree of crystallinity strongly influences the barrier properties of a cellulosic material. Generally, it is believed that higher crystallinity is associated with a lower permeability (Aulin et al. 2010). The degree of crystallinity increases from fibers to MFC and CNC (Lavoine, Desloges et al. 2012). Comparing the barrier properties of cellulose whiskers films (a typical of CNC structure) with MFC films showed that cellulose whiskers films absorbed as much water as MFC films. It is also accompanying with higher diffusion coefficient of cellulose whiskers films comparing with MFC films. However, it was expected from cellulose whiskers films to provide more resistance against water absorption. This suggests that, there might be other important parameters such as nanoporosity and tortuosity of diffusion pathway that exert a more significant influence on water barrier than crystallinity (Belbekhouche et al. 2011). Several studies focused on the influence of the modifications of MFC by pre-treatments and post-treatments, on the water vapor permeability. Despite of some improvements, the WVTRs of MFC films still remain high as compared to those of other polymer films (Lavoine, Desloges et al. 2012). In food packaging industry, oxygen barrier property has a key role. The recommended threshold for Oxygen Transmission Rate (OTR) value is as low as 10-20 mL m-2 day-1. In one study for barrier properties of MFC films made from fully bleached spruce sulfite pulp (Syverud and Stenius 2009), it was reported that the OTR values for MFC films is in the range 17-18 mL m-2 day-1, for a thickness of between 20 and 30 µm. Compared to films made from synthetic polymers of approximately the same thickness, the MFC films are comparable with the best synthetic polymers with respect to OTR (9-15 mL m-2 day-1 and 3-5 mL m-2 day-1, for PVdC coated, oriented polyester and EVOH, respectively). It was reported that carboxymethylation pre-treatment of MFC has a positive impact on oxygen permeability: carboxymethylated MFC films exhibited approximately 5 times better oxygen barrier as compared to non-pretreated MFC films at 50% RH (Lavoine, Desloges et al. 2012). The influence of number of passes through the homogenizer on oxygen permeability was studied by Aulin. It was shown that by increasing the number of passes the components of the film microstructure became finer and smoother, however the OTR found to be very similar (Aulin, Gällstedt et al. 2010). One important aspect is the effect of RH on oxygen barrier property in MFC, either in a film form or for paper coating. Studying the MFC films in different RHs revealed that the OTR increases significantly with RH and in higher than 70% RH values, the OTR increases sharply. Comparisons with hydrophilic polymers and edible films exhibit similarities in this behavior, which can be explained by recalling the abrupt changes in polymeric chains mobility in high RH values. Amorphous domains in the MFC structure are plasticized by absorbing water molecules, and consequently the hydrogen bonds and fibril-fibril joints become weak. Hence the fibril network mobility increases. As a result new sites for permeation of oxygen are created in the polymer network. Moreover, in addition to structural modifications of the fibril network, oxygen solubility is also increased in presence of higher amount of moisture (Aulin, Gällstedt et al. 2010). The study of MFC films in presence of additives were also the focus of a number of researchers. In a recent study, clay mixed with MFC produced a favorable gas barrier property. At 0% RH and 50% RH, low oxygen permeability values were confirmed for a nanocomposite composed of half MFC and half clay (0. 001 and 0. 045 mL mm m-2 day-1 atm-1, respectively). At 95% RH, the oxygen permeability value increased up to 3. 5 mL mm m-2 day-1 atm-1. Comparing with the results from pure MFC films, one finds the inhibiting effects of clay particles for oxygen permeation in medium and high RH values (6×10-5, 0. 085 and 17. 5 mL mm m-2 day-1 atm-1, for 0%, 50% and 95% RH, respectively) (Lavoine, Desloges et al. 2012). To summarize, MFC shows a good barrier property for oxygen; however it is not yet an attractive alternative among other water vapor barriers. On the other hand, oxygen barrier property is drastically affected in an environment of higher relative humidity.