High Oxygen Nanocomposite Barrier Films based on Xylan and Nanocrystalline Cellulose

The goal of this work is to produce nanocomposite film with low oxygen permeability by casting an aqueous solution containing xylan, sorbitol and nanocrystalline cellulose. The morphology of the resulting nanocomposite films was examined by scanning electron microscopy and atomic force microscopy which showed that control films containing xylan and sorbitol had a more open structure as compared to xylan-sorbitol films containing sulfonated nanocrystalline cellulose. The average pore diameter, bulk density, porosity and tortuosity factor measurements of control xylan films and nanocomposite xylan films were examined by mercury intrusion porosimetry techniques. Xylan films reinforced with nanocrystalline cellulose were denser and exhibited higher tortuosity factor than the control xylan films. Control xylan films had average pore diameter, bulk density, porosity and tortuosity factor of 0.1730 μm, 0.6165 g/ml , 53.0161% and 1.258, respectively as compared to xylan films reinforced with 50% nanocrystalline cellulose with average pore diameter of 0.0581 μm, bulk density of 1.1513 g/ml, porosity of 22.8906% and tortuosity factor of 2.005. Oxygen transmission rate tests demonstrated that films prepared with xylan, sorbitol and 5%, 10%, 25% and 50% sulfonated nanocrystalline cellulose exhibited a significantly reduced oxygen permeability of 1.1387, 1.0933, 0.8986 and 0.1799 cm⋅μm/m⋅d⋅kPa respectively with respect to films prepared solely from xylan and sorbitol with a oxygen permeability of 189.1665 cm⋅μm/m⋅d⋅kPa. These properties suggested these nanocomposite films have promising barrier properties.

DOI:10.3786/nml.v2i4.p235-241 http://www.nmletters.org glycerol blend were 4.6 cm 3 μm/m 2 dkPa [6]. The oxygen permeability of the GGM films was lower than that of glycerol-plasticized amylose and amylopectin films [11], but not as low as that of sorbitol-plasticized aspen glucuronoxylan films [7,12]. The oxygen permeability of oat spelt arabinoxylan films plasticized with 40% sorbitol was 4.7 cm 3 μm/m 2 dkPa, which is slightly lower than that of GGM films [13]. Biobased free-standing films and coatings with low oxygen permeability of 1 cm 3 μm/m 2 dkPa have been also prepared from a wood hydrolysate [14]. Films made from these polysaccharides are brittle and therefore to form cohesive films requires plasticizers such as sorbitol and xylitol. However, even with the plasticizers, the mechanical properties of these films have been considered to less than desirable. Nanocrystalline cellulose has been studied as reinforcements of various synthetic and some natural polymer matrices [15][16][17] to improve the strength properties owing to their high bending strength of 10 GPa, and elastic modulus of 143 GPa [18][19]. Lagaron et al. [20] discussed the role of crystalline structure of polymers and emphasized that high crystallinity improves barrier properties. Nanocrystalline cellulose is greater than 60% crystalline [21][22] and this property together with the resulting rigid hydrogen-bonded network of nanocrystallinecellulose can cause an increase in tortuousity and smaller pore size for nanocomposites which may be utilized to create high barrier materials. In a recent study, spruce galactoglucomannans (GGM) and konjac glucomannan (KGM) were mixed with nanocrystalline cellulose (NCC) to study the mechanical and barrier properties of the films [23,24].
The tensile strength of unplasticized KGM films increased by 30% but the mechanical properties of the plasticized films were not affected with increased in nanocrystalline cellulose. The presence of 5% of nanocrystalline cellulose did not significantly affect the oxygen permeability of the films.
Xylan is one of the most common hemicelluloses, is the most abundant polysaccharide in nature after cellulose, and is an attractive resource for film production [25][26][27]. Prior studies by Saxena et al. [28] have shown that the addition of 7% nanocrystalline cellulose to xylan-sorbital films increased the tensile energy absorption of the resulting films by 445% and the tensile strength of the film by 141%. Recent studies demonstrated that films with 10% nanocrystalline cellulose exhibited a 74% reduction in specific water transmission properties with respect to films prepared solely from xylan/sorbitol, and a 362% reduction in water transmission rate with respect to xylan films reinforced with 10% softwood kraft fibers [29]. The objective of the current study is to evaluate the oxygen barrier properties of xylan-nanocrystalline cellulose composite films.

Materials
Oat spelt xylan was obtained from Aldrich and was determined to contain 81.0% xylose, 9.8% arabinose, 7.6% glucose, 1.4% galactose and 0.2% of mannose. A commercial elemental chlorine-free (ECF) bleached softwood (SW) kraft pulp was used as received. Dialysis tubes were purchased from Spectrum Labs. All other reagents and solvents were purchased from Aldrich and used as received.

Preparation of Nanocrystalline Cellulose
Nanocrystalline cellulose was prepared following the procedure outlined by Pu [30]. In brief, softwood kraft pulp was

Oxygen permeability Analysis
The oxygen transmission of the films was measured using a Mocon Ox-Tran 2/21 l apparatus (Modern Controls Inc.,

Mercury Intrusion Porosimetry Analysis
Micromeritics' AutoPore IV 9500 Series was used to measure the porosity, bulk density, and average pore diameter and tortuosity factor of the control and nanocomposite films.

Results and Discussion
This study examines the oxygen barrier properties of xylan-nanocrystalline cellulose composite films. By AFM analysis, the sulfonated nanocrystalline cellulosic were observed to have rod like structure with an average length of 150-200 nm and a width of less than 20 nm (Fig. 1). AFM images, acquired using tapping mode, of the xylan/sorbitol films reinforced with nanocrystalline cellulose show well dispersed sulfonated nanocrystalline cellulose on xylan surface in comparison to more open structure of xylan/sorbitol control films (Fig. 2).
The specific oxygen transmission rate of the xylan nanocomposite films are shown in Table 1 [33] and the films made from microfibrillar cellulose [34], see Table 1.
Oxygen permeability values were calculated by dividing the oxygen transmission rates by the differential partial pressure of oxygen across the film (1 atm or 101.3 kPa) and multiplying by the film thickness in microns [5]. Table 2 summarized the oxygen permeability of some of the literature work and current work. The oxygen transmission rates as summarized in Table 1 at 25% and 50% dosage of nanocrystalline cellulose decreased drastically with respect to control xylan films and are the two lowest values that we obtained in this study. It will be an interesting subject to explore the porosity, bulk density and tortuosity factor at these two levels and the control xylan films.
As summarized in Table 3, the density and tortuosity factor of the composite film increased while the pore diameter and porosity decreased as the loading of sulfonated nanocrystalline cellulose increased in the xylan-based films.
SEM images of the control xylan film surface showed agglomerated structures on the surface in comparison to a more uniform surface for the nanocrystalline cellulose-xylan films ( Fig. 3 (a) and 3 (b)).
Oxygen transmission rate at 5% and 10% charge of nanocrystalline cellulose doesn't differ much but a significant drop of transmission rate as compared to control. We studied xylan-10% nanocrystalline cellulose film under SEM (Fig. 3) and AFM (Fig. 2b) and found that control xylan film surface in Fig. 3 (a) shows agglomeration in comparison to well dispersed sulfonated nanocrystalline cellulose on xylan surface in Fig. 3 (b). The uneven structure and agglomeration of the xylan can be the cause of higher oxygen transmission rate of control xylan film in comparison to xylan reinforced with 10% sulfonated nanocrystalline cellulose.
SEM cross-section images of freeze fracture of control xylan films showed a rough texture with small cracks in the film as summarized in Fig. 4(a) and 4(b). The same analysis for the xylan film reinforced with nanocrystalline cellulose exhibited smooth fractured surface and less porous structure (see Fig. 4(c) and Fig. 4(d)).  Oat spelt arabinoxylan films plasticized with 40% sorbitol 4.7 [13] Biobased free-standing films and coatings from a wood hydrolysate 1.0 [14] Xylan + 50% sulfonated cellulose whiskers 0.1799 (current study)