Preparation and Characterization of Cellulose Nanofiber Reinforced ...

Fibers and Polymers 2014, Vol.15, No.2, 347-354

DOI: 10.1007/s12221-014-0347-0

Preparation and Characterization of Cellulose Nanofiber Reinforced Thermoplastic Starch Composites B. Nasri-Nasrabadi, T. Behzad*, and R. Bagheri Department of Chemical Engineering, Isfahan University of Technology, Isfahan 8415681167, Iran (Received April 18, 2013; Revised June 30, 2013; Accepted July 23, 2013) Abstract: In the present study, cellulose nanofibers composite films were manufactured based on thermoplastic starch. Nanofibers were extracted from rice straw employing a developed chemo-mechanical method. In the chemical step, almost all of non-cellulosic components were removed and a white pulp of cellulose microfibers was obtained. Then, a diluted suspension of fibers was ultrasonicated to destruct intermolecular hydrogen bonds achieving nanofibers networks. Afterward, bio-nanocomposites were prepared by film casting. In order to study the effect of nanofibers content on the composite properties, the mechanical and dynamic mechanical properties, morphology, humidity absorption, and transparency of films were investigated. The yield strength and Young modulus of nanocomposites were satisfactorily enhanced compared to the pure thermoplastic starch film. The glass transition temperature of films was shifted to higher temperatures by increasing nanofibers contents. The uniform dispersion of the nanofibers was investigated using SEM images. The humidity absorption resistance of films was significantly enhanced by using 10 wt% cellulose nanofibers. The transparency of the nanocomposites was reduced compared to the pure starch films. Keywords: Nanocomposites, Cellulose nanofibers, Ultrasonication, Thermoplastic starch, Film casting

important challenge for fabrication of nanofibers composites is dispersion of nanofibers in matrix. The film casting process is a common method for manufacturing composites based on hydro soluble polymers [14]. In this study, the cellulose nanofibers were extracted from rice straw. Starch thermoplastic composites reinforced by nanofiber were manufactured by casting method. To investigate the properties of the composite mechanical and dynamic-mechanical properties, humidity absorption, morphology, and transparency of specimens were measured and elucidated.

Introduction In the last decade, due to environmental challenges and non-renewability of fossil sources, synthetic polymers have been replaced by biopolymers in many applications where the long term-durability is not necessary. Thermoplastic starch materials are wildly used in various applications such as food containers and packing industries [1]. Within the wide family of biopolymers, the starch has the high usage because of its availability, biodegradability, biocompatibility, and ease of chemical modification [2-5]. Starch is extensively found in seeds, stems, leaves, tubers, and fruits of plants [4]. Starch is a hydrophilic polymer that consist of anhydroglucose unites linked by α-D-1,4-glycosidic bonds. Starch contains of two different main phases of amylose (linear) and amylopectin (non-linear). Biological interactions, post modifications, and processing conditions are the most effective parameters in arrangement of these distinct phases [6]. Starch can be converted to a thermoplastic starch via incorporation of plasticizers such as water and or polyalcohols [2]. Compare to synthetic polymers, starch has some disadvantages such as high water sensitivity, brittleness, low mechanical properties, and poor barrier properties [3,7]. To overcome these problems, some solutions such as blending with other biopolymers [8,9], chemical modifications [5, 10], and also reinforcing with nanofillers such as nanoclay [11,12] and cellulose nanofibers (CNF) [13,14] have been suggested. Cellulose nanofibers, in addition to biodegradability, biocompatibility, have high crystallinity, high aspect ratio, and low density (compared to glass fibers) that cause noticeable increase in stiffness of matrix [15]. The most

Experimental Materials The cellulose nanofibers were extracted from rice straw obtained from local source (Isfahan farms, Iran). Potato starch, glycerol, and other chemicals used for extraction and characterization of CNF were supplied from Merck Company, Germany. Preparation of Cellulose Nanofibers (CNF) Suspension From Rice Straw The suspension of cellulose nanofibers were prepared by a two-step method (Figure 1). In the first step, a chemical purification was performed to remove lignin, hemicelluloses, and extractives from rice straw. Rice straws were cut to 4-5 cm length and soaked in 17.5 wt% NaOH solution for 2 h followed by washing in abundant distilled water until the pH of fibers becomes natural. The swollen fibers were blended in a blender and dried at room temperature. Subsequently, the dried pulp was hydrolyzed by dilute HCl (2 M) solution at 80±5 oC for 2 h. To remove the soluble lignin and remained extractives, the hydrolyzed pulp was alkaline-

*Corresponding author: [email protected] 347

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Figure 1. Schematic procedure for extraction of cellulose nanofibers.

treated with 2 wt% NaOH at 80±5 oC for 2 h followed by washing in distilled water. Afterward, the dried pulp was bleached by sodium chlorite solution at 50 oC for 1 h to eliminate insoluble lignin (Klason lignin). In the second step, a mechanical technique was employed to separate and isolate nanofibers by applying a high shear force. A dilute suspension of bleached pulp (cellulose microfibers) was ultrasonicated by a Hielscher UP400s, at 400 W and 20 kHz for 30 min. This treatment was conducted in an ice bath to control the temperature. Composite Preparation The composite mixture was prepared based on the procedure described by Chen et al. and Lu et al. [2,16]. The fabrication of thermoplastic starch/cellulose nanofibers composite films was based on film casting and evaporation process. Deionized water, modified starch, cellulose nanofibers suspension, and glycerol were mixed together to obtain a homogenous dispersion. The mixture contained 90 wt% deionized water, 7 wt% starch and nanofibers, and 3 wt% glycerol. Starch and nanofibers were fixed at weight ratios of 100:0, 95:5, 90:10, and 85:15. To gelatinize the stock, the starch was heated at 100 oC for 30 min with constant stirring. Afterward, the mixture was cast in a petridish and degassed at 70 oC under vacuum for 2 h. The compound was dried at 40 oC and 50 % relative humidity for 2 days to obtain films with a thickness of approximately 0.3 mm. Chemical Composition of Fibers Since the main part of non cellulosic materials is removed during alkali treatment in chemical stage, the chemical composition of untreated and alkali treated fibers were determined by NREL/TP-510-42618 standard [17]. In this test, first, the fibers were hydrolyzed by H2SO4 72 % w/w to decompose hemicellulose and cellulose to their monomers. Then, hemicellulose and cellulose contents were determined by an HPLC analyzer (HPLC-RL UV-VIS Detector, Jasco). In order to measure lignin content, the acid soluble lignin was measured by IR spectroscopy of filtrated liquid at 240 nm peak following equation (1): UVabs × volumefiltrate Acid Soluble Lignin % = --------------------------------------------ε × ODV where, UVabs:Average UV absorbed at 320 nm

Volumefiltrate:volume of filtrate (ml) ε:Absorptivity of biomass at specific wavelength (30 l/g cm) ODW: Oven Dried Weight of sample (g) Also, to determine the insoluble lignin, solid residue of filtrate was placed in a chamber at 105 oC for 4 h. The weight of solid in this step is the summation of insoluble lignin and ash. To measure the ash content, solid was heated at 575 oC for 24 h. The residue solid after this process is the ash content. The silica content of rice straws and swelled fibers were measured by performing a developed method [18]. In this technique, fibers are digested using a solvent combined by NaOH and H2O2 in an autoclave. Then, the silica content of samples is measured by a standard colorimetric method. Morphology of Fibers and Nanofibers In this study the rice straw and bleached fibers were analyzed by Scanning Electron Microscopy (SEM, ZEISS 1450EP) to investigate the effect of chemical treatment on morphology and dimension of fibers. Also to estimate the dimension of ultrasonicated fibers, at first the water of the suspension of sonicated fibers was replaced by t-butyl alcohol and then subject to freeze-drying. Afterward Field Emission Electron Microscopy was applied on dried fibers sheet using HITACHI-S4160. Powder X-ray Diffraction (PXRD) Powder X-ray diffraction was performed to investigate the crystallinity of untreated, bleached, and ultrasonicated fibers. PXRD pattern of each sample was determined by a Philips instrument equipped with a Cu Kα radiation (λ=1.5418 Ao). The diffraction data were recorded at 40 kV voltage and 30 mA intensity with diffraction angle range (2θ) from 10 o to 30 o. Crystallinity index (CI) of cellulose fibers were calculated from equation (2): I200 −Iam - × 100 CI ( % ) = ----------------I200

(2)

where I200 is the high of the peak at 2θ =22.6 o and Iam is the diffraction intensity at 2θ =18 o.

(1) Mechanical Properties of Nanocomposites The Young’s modulus, tensile strength, and elongation at

Cellulose Nanofibers/Starch Composite


break of pure starch and starch/CNF reinforced films were determined based on ASTM D 638 (type V), using Zwick Universal Testing Machine – 1446-60, with a load cell of 10 kN and crushed speed at 5 mm/min. These tests were carried out at least for five repetitions under the same conditions (relative humidity of 43 % and 25 oC) to give an average value of the data and standard deviation.

Results and Discussion

Dynamic Mechanical Analysis of Nanocomposites The dynamic mechanical properties of the TPS and nanocomposite films were determined using a dynamic mechanical analyzer (DMA-Triton, UK). The composite films were cut into small slabs (20×5 mm) and conditioned at relative humidity of 43 % for three weeks. The measurements were performed at strain amplitude of 0.01 % and a constant frequency of 1 Hz. The temperature range was 0 to 160 oC with a heating rate of 5 oC/min. Moisture Absorption of Films To estimate moisture absorption of nanocomposites, the method of Kaushik et al. [19] was employed. The samples were cut at approximate dimension of 30×10 mm and vacuum-dried until their weight became kept constant. They were then conditioned at 75 % relative humidity and 25 oC and weighted at desired intervals. The water absorption (WA%) of samples at 0, 5, 10, and 15 wt% cellulose nanofibers was calculated by the following equation: Wt −W0 × 100 WA % = ---------------W0

Chemical Composition and Morphology of Fibers The morphology of the untreated and chemically treated fibers was investigated by SEM micrographs. The cellular structure in raw material is covered by an outer epidermis that has a concentrated layer of silica on the surface (Figure 2) which was reported by other researchers as well [21]. After the chemical treatment, the silica content of straw was reduced from 1.8±0.3 % to almost 0 %. Moreover, after the chemical purification, almost all of lignin, hemicelluloses, and other impurities were removed resulted in microfibers with a dimension of around 5-20 µm (Figure 3). Individual nanofibers bundles on the surface of microfibers that associated together by hydrogen bonds can also are observed. These results can be proved by chemical characterizations of the fibers. The chemical composition of untreated straw and extracted nanofibers was presented in Figure 4. The cellulose content was enhanced from 46.5±0.7 % to 79.3±0.8 %. The amount of hemicellulose and lignin was decreased to 78 % and 45 %, respectively. The FE-SEM micrograph of sonicated fibers was shown in Figure 5. The sonication power destructed the intermolecular hydrogen bonds between the cellulose

(3)

Where Wt and W0 are the weight of humid specimens after time t and dry sample, respectively. These measurements were continued till attained to constant weight. Also, all measurements were performed at three replicates. Morphology of Nanocomposites Scanning electron microscopy images were employed to characterize the morphology of TPS and TPS+15 wt% CNF films. The samples were frozen in liquid nitrogen and then broken immediately. The cross section of films was coated by gold and images were taken at 20 kV acceleration voltage.

Figure 2. SEM image of rice straw cross section.

Light Transmittance Testing of the Films The light transparency of TPS and nanocomposite films with a thickness of 0.3 mm was analyzed using a UV-VIS spectrometer (UV-240, 1990 Shimidzu, Kyoto Japan, 1990) at a visible light wavelength of 600 nm as follow to equation (4): T600 A600 - or --------Tr = −log -------X X

(4)

where Tr is transparency index, T600 and A600 are the transmittance and absorbance at wavelength of 600 nm, respectively and X is the thickness of the film (mm) [20].

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Figure 3. SEM image of fibers after chemical purification.

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Figure 4. Chemical compositions of untreated and alkali treated fibers.


Figure 6. XRD patterns of the untreated, bleached, and ultrasonicated (cellulose nano) fibers.

Figure 7. Stress-strain curves of TPS and (TPS-CNF) nanocomposites.

nanofibers bundles and reduced the dimensions of fibers to nano- scales [22].

crystallinity index of cellulose fibers after sonication was determined to be 68 %. The achieved results indicates that the chemical treatment increases the crystallinity and consequently rigidity of cellulose fibers which can be more effective to reinforce composites [19,23].

PXRD Analysis Powder X-ray diffraction was used to investigate the effect of chemical purification and ultrasonication on crystallinity of rice straw fibers. It is clear from Figure 6 that the XRD pattern of the raw material shows diffraction peaks around 2θ =16.3 o and 22.6 o which typically represent cellulose type I. It can be noticed that after chemical treatment this pattern was changed to cellulose type II with a split peak around 2θ =20 o and 21.7 o. This may be justified by transformation and regeneration of cellulose chains after chemical treatments. Also from the results, it was found that the crystallinity was increased from 54 % for raw materials to 69 % for bleached fibers which is due to the removal of amorphous regions of fibers (lignin, hemicelluloses, and …). On the other hand, as it can be seen (Figure 6) the PXRD pattern of sonicated fibers does not show a noticeable difference with respect to chemically treated fibers and the

Mechanical Properties of the Samples The mechanical properties of the starch based nanocomposites were investigated by tensile testing. Figure 7 shows the typical stress-strain curves of the samples films measured at the same conditions. Results in Table 1 confirmed that the nanofibers content had a satisfactorily effect on the mechanical properties and demonstrated a high compatibility between the starch and cellulose nanofibers. By increasing the nanofibers content from 0 to 15 wt%, the Young’s modulus was significantly increased (from 36±3.4 to 160±16.4) and the yield strength was enhanced from 3.1±0.22 to 5.01±0.67. Whereas, the elongation at the break was decreased from 126 % to 61 %. This is probably attributed to the restriction of the starch molecular mobility due to the strong interaction between the nanofibers and matrix and a good stress transfer from the matrix to fibers [2,13]. At higher percentage of the cellulose nanofibers, due to low dispersion and agglomeration

Figure 5. FE-SEM image of ultrasonicated (cellulose) nanofibers.



of nanofibers in the matrix, no improvement in the properties can be observed [16]. Dynamic Mechanical Characterization In this step, the temperature dependence of dynamic mechanical properties of the nanocomposites were studied and compared with the pure thermoplastic starch films. The storage modulus (E') value is attributed to the load bearing ability of the films. The trend of the storage modulus variations with temperature can indicate the reinforcement effect of nanofibers at various contents. The E' value of the pure starch film at low temperature is almost constant but at around 30 oC a rapid drop is noticed which can explain the transition from a brittle to a rubbery state (see Figure 8(a)). In the composite films, especially at higher filler contents (10 and 15 wt% CNF), the rubber plateau region was

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increased and the storage modulus was almost constant at an extensive range of temperature compared to the pure starch film. This can be due to the reinforcement effect of the cellulose nanofibers network that have strong hydrogen bonds between their components [24] and also a strong interaction between the starch molecules as well as a large specific surface of the nanofibers [20]. Figure 8(b) shows the variation of Tan δ versus temperature. For starch films there is a transition peak at 73 oC that could be attributed to Tg of the reach-starch regions. Results in Table 1 shows that by increasing the fiber content this peak shifted to higher temperatures (121 oC for TPS+15 wt% CNF) which demonstrates lower mobility of amylopectin chains due to the presence of nanofibers [25,26]. Kinetic Studies of Moisture Absorption of the Films Starch is very sensitive to moisture; therefore humid conditions adversely influence the mechanical and physical properties of the starch based materials. Thus, improvement of the water resistance of starch products is highly important. The effect of CNF addition on the moisture resistance of starch films is investigated employing the Fick’s second law. This law is used for plane sheet geometry at constant temperature and it is assumed that migration of water molecules was controlled by the diffusion mechanism based on the following equation: 2 M 8- 8 -------------------1 ( 2n + 1 ) -t⎞ 2 ⎛D -------t = 1− ----------------------------2 exp 1− π Σ 2 n 2 2 ⎝ ⎠ M∞ n ( 2n + 1 ) h

(5)

By defining of the parameters for short times, equation (5) can be converted to equation (6): M -------t = kt1/2 M0

(6)

where Mt and M are the moisture content of the samples at time t and equilibrium conditions, respectively, and k is: 4 D 1/2 K = ⎛⎝ ---⎞⎠ ⎛⎝ ----⎞⎠ h π

(7)

where h is the film thickness and D is diffusion coefficient. Also, permeability coefficient (P) of the samples can be calculated by: Figure 8. Temperature dependent of (a) Log storage modulus and (b) tan δ of (TPS- cellulose nanofibers) composite films.

P = S×D

(8)

Table 1. Mechanical properties and dynamical-mechanical results of nanocomposites Material TPS TPS - 5 wt% CNF TPS - 10 wt% CNF TPS - 15 wt% CNF

Young’s modulus (MPa) 36±3.4 74±9.6 126±17.2 160±16.4

Yield strength (MPa) 3.1±0.22 4.02±0.36 4.47±0.32 5.01±0.67

Elongation at break (%) 126 98 75 61

Storage modulus at 25 oC (MPa) 60 80 152 234

Tan δ peak (oC) 71 89 102 121

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where S is the ratio of the mass of water at equilibrium (M) to dry sample (M0) [27,28]. The obtained results of the water absorption determination of pure starch and composite films were reported in Figure 9 and Table 2. The moisture uptake at equilibrium, M, was decreased from 10.98 to 6.76 % as nanofiber contents were increased from 0 to 10 wt%. Also, the diffusion coefficient was decreased about 46 %. These results indicated that the addition of cellulose nanofibers to starch improved the water resistance until a certain percentage that can be related to the good interaction of the nanofibers and starch and consequently formation of the rigid hydrogen bonding between hydroxyl and carboxyl groups of the cellulose and starch [29,30]. Since cellulose chains are less hygroscopic than starch molecules; addition of cellulose nanofibers had a pronounce effect on the moisture resistance of samples [19]. But, there is a dominant conflict about the behavior of nanocomposites at high nanofibers content. One the one hand increasing of cellulose content caused to the formation of denser nanofibers network and therefore the mechanical and dynamical-mechanical properties of samples were improved. This dens network also should decrease the permeability coefficient of nanocomposites. One the other hand high nanofibers content results in decreasing the homogeneity and cohesion of the composite structure due to agglomeration of cellulose nanofibers. Therefore, the nanofibers are not adequately wetted by starch which provides more penetration paths and cracks and consequently enhances the capillary action and permeability of moisture [19]. In this work, at cellulose content above 10 wt % this conflict becomes more evident. Although by more increasing the fiber content (above 10 wt%) the mechanical and dynamical-mechanical

performance of the composite is improved, the diffusivity and permeability of the composite is adversely increased. The plasticizer content, ratio of amylopectin to amylose, and environmental humidity are the other effective factors on the water absorption behavior of the cellulose-starch composites [31].

Figure 9. Moisture absorption of (TPS-cellulose nanofibers) composites films as a function of cellulose nanofibers content.

Figure 10. The SEM images of (a) pure starch and (b) composite of TPS+15 wt% CNF (cellulose nanofibers).

Morphology of the Nanocomposites To verify visually the fiber/matrix interaction, SEM images from the fracture surface of the TPS (Figure 10(a)) and TPS+15 wt% CNF (Figure 10(b)) films were investigated. The TPS composite film has rougher surface morphology compared to the pure starch specimen which is due to the presence of nanofibers and also interaction between TPS and CNF. It can be noticed that nanofibers, white domains, have uniform dispersion in the matrix and all the fibers were covered by matrix and no fiber pull out or debonding was observed.

Table 2. Barrier properties (75 % relative humidity) for nanocomposites at various nanofibers content CNF content 0% 5% 10% 15%

K×104 (s-1/2) 1.3 0.93 0.59 0.83

M (%) 10.98±0.11 8.16±0.07 6.76±0.15 7.37±0.23

D×108 (mm2 s-1) 2.48±0.13 2.31±0.06 1.33±0.17 2.26±0.12

S 0.53±0.02 0.49±0.01 0.46±0.20 0.47±0.20

P×108 (mm2 s-1) 1.31±0.08 1.14±0.04 0.62±0.16 1.06±0.06



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The transparency measurement of the composites revealed that due to the light scattering of nanofibers, the light transmittance of the films was decreased by increasing the cellulose nanofiber contents.

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Figure 11. Effect of nanofiller content on transparency of TPScellulose nanofibers films.

This strong adhesion between starch and nanofibers caused to improve the mechanical properties of the composite films compared to pure starch. Transparency Transparency of the starch based films, in some applications, in term of general appearance and consumer acceptance is one of the important properties. The transparency of the composite films was estimated by transparency index (Tr) [20]. The measurements indicated that the Tr of the composite films was increased from 1.2±0.07 to 3.35±0.09 by increasing the nanofiber content from 0 to 15 wt%. Compositing of TPS with CNF was significantly enhanced the Tr of the composite films because of light scattering and retarding of the light transmission, especially at low nanofiber content (5 wt%). However, further addition of nanofibers had no noticeable effect on the transparency of the composite films as can be observed from Figure 11. It may be related to the narrow diameter dispersity of the nanofibers, homogenous dispersion of the nanofibers within the TPS, and also a strong interaction between CNF and starch [16].

Conclusion In the present study, the bio-nanocomposite films were fabricated from rice straw cellulose nanofibers and plasticized thermoplastic starch. The chemical analysis of the fibers revealed that almost all of noncellulosic materials were removed after the chemical purification. The nonofibers were isolated by applying a high shear rate sonication. The young’s modulus and yield strength of the composite reinforced by 15 wt% CNF were increased by 233 % and 63 %, respectively compared to the pure starch specimen. Also, by increasing the nanofibers content, the dynamic mechanical properties of the composites showed a significant improvement. The humidity absorption tests demonstrated that the permeability coefficient (P×108) was decreased from 1.31 for pure starch to 0.62 for TPS+10 wt% CNF, but this value for TPS+15 wt% CNF was decreased to 1.06 again.

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