A Novel Modified Starch/Carboxymethyl Cellulose ...

doi 10.1515/ijfe-2012-0197

International Journal of Food Engineering 2014; 10(1): 121–130

Babak Ghanbarzadeh*, Hadi Almasi, and Seyed Amir Oleyaei

A Novel Modified Starch/Carboxymethyl Cellulose/Montmorillonite Bionanocomposite Film: Structural and Physical Properties Abstract: A novel glycerol-plasticized and citric acid (CA)modified starch/carboxymethyl cellulose (CMC)/montmorillonite (MMT) bionanocomposite films were prepared from corn starch by casting, to study the effect of the 10% CA, 10% CMC and four different loadings of MMT on the properties of starch films. Atomic force microscopy surface analysis showed that starch/CMC/MMT films had the highest roughness. X-ray diffraction test showed that the clay nanolayers formed an intercalated structure in the bionanocomposites. However, completely exfoliated structure formed only in the pure starch/MMT nanocomposites (without CA and CMC). CA, CMC and MMT improved mechanical properties of starch films. MMT had the greatest effect on the mechanical properties. The MMT addition at content of 7% caused to increase in ultimate tensile strength by more than threefold in comparison to modified starch/CMC films. The water vapor permeability (WVP) decreased significantly (p < 0.05) by the addition of CA and CMC. When the MMT content of the starch films reached to 7%, the WVP decreased about 75% in comparison to the neat starch film. However, the hydrophilic character of bionanocomposites increased as the increasing of MMT content. Keywords: corn starch, montmorillonite, bionanocomposite, atomic force microscopy, physical properties *Corresponding author: Babak Ghanbarzadeh, Department of Food Science and Technology, Faculty of Agriculture, University of Tabriz, Tabriz, Iran, E-mail: [email protected]; [email protected] Hadi Almasi, Department of Food Science and Technology, Faculty of Agriculture, University of Urmia, Urmia, Iran, E-mail: [email protected] Seyed Amir Oleyaei, Department of Food Science and Technology, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran, E-mail: [email protected]

1 Introduction Over the past few years, there has been a renewed interest in films made from renewable, cheap, biodegradable

and abundant natural polymers [1]. Edible and biodegradable films are not meant to totally replace the synthetic packaging films. However, the latter do have the potential to replace the conventional packaging in some applications. Usually, the film-forming substances are based on proteins, polysaccharides, lipids and resins or on a combination of these [2, 3]. Among the natural biodegradable polymers and renewable resources, starch has been considered as one of the most promising candidates. It is a versatile biopolymer with low price and abundant availability, not depending on fossil sources, for use in the nonfood industries. Starch has received considerable attention during the past three decades as a raw material for biodegradable edible films [4–6]. Nevertheless, starch exhibits several disadvantages such as a strong hydrophilic character (water sensitivity) and poor mechanical properties compared to conventional synthetic polymers [7], which make it unsatisfactory for some applications such as packaging purposes. Generally, many approaches are suggested to mitigate these shortcomings. Polymer–clay nanocomposites have generated enormous interest since Toyota researchers in the late 1980s showed that as little as 5 wt% addition of nano-sized clays to nylons doubled their moduli, increased their heat distortion temperature and decreased water vapor permeability [8]. Montmorillonite (MMT) is the most commonly used layered silicates because it is environmentally friendly and readily available in large quantities with relatively low cost. Therefore, it is possible to improve the properties of starch polymer by the addition of small amounts of MMT [9, 10]. Other approach to improve the functional properties of the starch films is to blend starch with other biopolymers. When another biopolymer is mixed with starch, the mechanical properties of the resulted polymer blend are obviously improved, because the chemical similarities of starch and other biopolymers provide a good interaction [11, 12]. Carboxymethyl cellulose (CMC) is a cellulose ether that exhibits thermal gelation and forms excellent films. CMC was able to improve the mechanical and

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barrier properties of starch-based films [13]. This fact has been attributed to the chemical similarity of starch and CMC, providing good compatibility between them. Citric acid (CA) has been proved as a cross-linking agent to modify the starch macromolecules [14]. Because of the multi-carboxyl structure, CA may serve as a crosslinking agent and the carboxyl groups on CA can form stronger hydrogen bonds between the hydroxyl groups on starch molecules and hence it may improve the mechanical properties and water resistibility [15]. The incorporation of layered silicate in polymers and biopolymers has already demonstrated significant enhancements in a large number of physical properties, including barrier properties, mechanical properties and thermal and environmental stability [16–19]. The clay dispersion state has a great effect on all of these properties. Improved properties have been usually obtained for exfoliated structures [20]. Among all the studies concerned with starch nanocomposites [21–25], very few studies have been related to starch and secondary polymer blends nanocomposites. There are few papers about the use of CMC as a secondary polymer in starch matrix. On the other hand, to the best of our knowledge, there is no specific study on the effects of nanoclay on the physical properties of starch/ CMC polymer blend. The aim of this study was to prepare environmentally friendly composites from biodegradable CA-modified starch, CMC and nanoclay and to investigate the role played by the presence of CA and CMC in the dispersion of nanoclay. Furthermore, effect of nanoclay content on the physical properties of modified starch/CMC/MMT bionanocomposite films was studied.

2 Materials and methods 2.1 Materials Corn starch (12% moisture) was provided from Glucosan Industry (Ghazvin, Iran). Glycerol, calcium sulfate, potassium sulfate and calcium nitrite (analytical grade) were purchased from Merck (Darmstadt, Germany). CMC, with an average molecular weight of 41,000 (practical grade), was purchased from Caragum Parsian Corporation (Tehran, Iran). CA (food grade) was prepared from Tianjin Chemical Reagent Factory (Tianjin, China). Sodium montmorillonite (Cloisite® Na þ ) with structural formula Na0.33(Al1.67 Mg0.33)Si4O10(OH)2, a cation exchange capacity (CEC) of 92.6 meq/100 g clay and

particle size (90% < ) of 13 μm was supplied by Nanocor Inc. (Arlington Heights, IL).

2.2 Preparation of films Five grams of starch were mixed with distilled water (100 ml) and 2 ml glycerol (40 ml/100 g starch) and 0.5 g CA (10% wt of starch) at room temperature (25°C) for 5 min. This suspension was transferred to a water bath at 90°C for 30 min and agitated by magnetic stirrer (500 rpm). 10% W/W CMC was solubilized in 75 ml of water at 75°C for 10 min. On the other hand, the MMT was dispersed in distilled water by sonication during 10 min at room temperature. The clay dispersion was added to the aqueous dispersion of starch and the mixing was continued for 10 min. The starch/clay ratios were 100/0, 99/1, 97/3, 95/5 and 93/7 wt/wt, relative to dry starch. CMC and starch/MMT solutions were mixed together and stirred at 75°C for 10 min. Dispersion was then cooled at 40°C and mixed gently for 20 min to release all air bubbles. Then, about 70 ml of the sample was poured into a Teflon casting tray resulting in films with 0.08 6 0.01 mm thickness, measured with a Alton M820-25 hand-held micrometer (China) having a sensitivity of 0.01 mm and then dried at 60°C in an oven to cast the films.

2.3 Atomic force microscopy The atomic force microscopy (AFM) provided topographic images and roughness of scanned samples. The surface morphology of the films was analyzed using dynamic scanning probe microscope (SPM) acquisition mode and contact SPM acquisition mode (AFM, Nanotec Electronica S.L., Madrid, Spain) with different scan sizes: 1 1, 2 2, 5 5 and 10 10 μm and 500 500 nm. Two kinds of cantilevers were used. An NSC12 cantilever (MicroMasch) with a spring constant of 14 N/m, and an AC160TS-2 cantilever (Olympus) with a spring constant of 42 N/m. Three different images were taken of each sample at five scan sizes. The most frequently quantitative parameters of roughness, Rq, were calculated using the data from the images with appropriate software. Average roughness (Ra) is the arithmetic mean of the height deviations from the profile mean value (Z). Ra is written as [26]:


Ra ¼

XN Zi Z =N i¼1

ð1Þ


where: Z¼

XN i¼0

Zi =N

ð2Þ

and Zi ¼ the height values in profile (histogram; nm), Z ¼ arithmetic means of heights (nm) and N ¼ number of data points in the profile. The root mean square (RMS) of roughness (Rq) is the root of mean square of height deviations from the mean of heights: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX 2 Rq ¼ t ðZi ZÞ =N ð3Þ i¼1

2.4 X-ray diffraction X-ray diffraction (XRD) studies of the samples were carried out using a Bruker D8 Advance X-ray diffractometer (Karlsruhe, Germany) operating at CuKα wavelength of 0.1539 nm. The samples were exposed to the X-ray beam with the X-ray generator running at 40 kV and 40 mA. Scattered radiation was detected at ambient temperature in the angular region (2θ) of 1–20° at a rate of 1°/min and a step size of 0.05°.

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was maintained using anhydrous CaSO4 in the cup. Each cup was placed in a desiccators containing saturated K2SO4 solution in a small beaker at the bottom. A small amount of solid K2SO4 was left at the bottom of the saturated solution to ensure that the solution remained saturated at all times. Saturated K2SO4 solution in the desiccator provides a constant RH of 97% at 25°C. The desiccator was kept in an incubator at 25.0 6 0.1°C. Cups were weighed every 24 h and water vapor transport was determined by the weight gain of the cup. Changes in the weight of the cup were recorded as a function of time. Slopes were calculated by linear regression (weight change vs. time). The water vapor transmission rate (WVTR) was defined as the slope (g/h) divided by the transfer area (m2). WVP (g m−1 h−1 Pa−1) was calculated as: WVP ¼ ðWVTR=PðR1 R2 ÞÞ:X

ð4Þ

where P is the saturation vapor pressure of water (Pa) at the test temperature (25°C), R1 is the RH in the desiccator, R2, the RH in the cup and X is the film thickness (m). Under these conditions, the driving force [P(R1_R2)] is 3073.93 Pa. All measurements were performed in three replicates.

2.7 Contact angle 2.5 Tensile properties Ultimate tensile strength (UTS) and strain to break (SB) of the films were determined at 21 6 1°C using a tensile tester (Zwick/Roell model FR010, Germany) according to ASTM standard method D882-91 [27]. After conditioning in RH ¼ 55% for 24 h, three film specimens, 8 0.5 cm dumbbelly forms, were cut from each of film samples and were mounted between the grips of the machine. The initial grip separation and cross-head speed were set to 50 mm and 5 mm/min, respectively.

2.6 Water vapor permeability Water vapor permeability (WVP) tests were carried out by ASTM method E96 [28] with some modifications [29]. Special cups, with an average diameter of 2 cm and a depth of 4.5 cm, utilized to determine WVP of films. Films were conditioned in a desiccator containing calcium nitrite saturated solution at 20–25°C to ensure a relative humidity of 55%. Films were cut into discs with a diameter slightly larger than the diameter of the cup. After placing 3 g of anhydrous CaSO4 in each cup, they were covered with edible films of varying composition. RH 0

The sessile drop method is basically an optical contact angle method, which is the most frequently used method to estimate wetting properties of a solid surface. A droplet of distilled water (~1 μL) was deposited on the film surface with a precision syringe. The contact angles made by the drops of water were measured with a camera MV-50, zoom 6 and acquired with the software Adobe Acrobat 8 Professional. The method is based on image processing and curve fitting for contact angle measurement from a theoretical meridian drop profile, measuring contact angle between the baseline of the drop and the tangent at the drop boundary. Contact angle measurements were obtained by analyzing the shape of a sessile drop after it had been placed over the samples for 0 and 60 s. For each film type, at least three measurements were made and the average was taken.

2.8 Statistical analysis Statistics on a completely randomized design were performed with the analysis of variance (ANOVA) procedure in SPSS (Version 11.5, SPSS Inc., Chicago, IL) software. Duncan’s multiple range test (p < 0.05) was used to detect differences among mean values of films properties.


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3 Results and discussion 3.1 Atomic force microscopy AFM can be used in the structural investigation of biopolymer specimens and surfaces. Figure 1 shows that the starch films and starch/CMC polymer blends have a relatively smooth and continuous matrix with low valleys and hills and with good structural integrity. However,

(a)

when the nanoclay added to the film mixture, sharp projections in the surface of the films were formed, thus increasing the surface roughness. Therefore, the starch/ CMC/MMT bionanocomposites had the highest projections (Figure 1(c)). Table 1 summarizes the roughness parameters (Ra and Rq) of different films in three scan sizes. As shown, in all scan sizes, the starch/CMC films and starch/CMC/MMT bionanocomposite films had the lowest and highest Ra and Rq, respectively, and the pure modified starch films had medium values.

[435 nm] 527 nm [ nm ]

10.0

[435 nm] 527 nm

400 300

[µm ]

200 10.0

10.0 [µm ]

[420 nm] 456 nm

µm

.0

10

[ nm ]400 350 300 250 200 150 100

[420 nm] 456 nm

0

[µm ]

(b)

µm .0 10

100

0 0

m

.0µ

10

m

.0µ

10

50

[µm ]

(C)

10.0

0

0 0

[838 nm] 881 nm

[838 nm] 881 nm [ nm ]

10.0

µm

.0

10

700 600 500 400 300 200 100

[µm ]

m

.0µ

10

0

0 0

[µm ]

10.0

Figure 1 AFM topography images of CA-modified starch film (a), CA-modified starch/CMC polymer blend film (b) and CA-modified starch/ CMC/MMT bionanocomposite film at 10 10 μm scan size. These three images were chosen as representatives from each sample scanned at different scan sizes



Table 1 Comparison of Ra and Rq values obtained from AFM images of different films Sample Starch/10% CA

Starch/10% CA/ 10% CMC Starch/10% CA/10% CMC/5% MMT

Scan size 10 10 22 500 500 10 10 22 500 500 10 10 22 500 500

μm μm nm μm μm nm μm μm nm

Ra

Rq

80.0 6 2.5 11.2 6 1.4 5.6 6 0.7 75.6 6 4.4 16.4 6 2.1 4.3 6 1.0 132.1 6 6.3 24.0 6 2.3 3.3 6 0.3

99.6 6 5.8 13.7 6 1.3 7.1 6 2.0 92.4 6 3.5 22.6 6 1.3 4.5 6 1.0 162.4 6 4.2 29.3 6 2.6 4.0 6 0.9

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(a)

(b)

The phase images presented in Figure 2 describe the uniformity of the starch/CMC/MMT mixtures. The white regions in all images could be attributed to the excess CA. Linear and brown zones in the image of starch/CMC film (Figure 2(b)) may be attributed to CMC-rich areas in starch matrix. However, light-phase variations in the phase image of starch/CMC polymer blend indicate the good dispersion of CMC in the starch matrix. The high dispersion of the nanoclay in starch/CMC/MMT films was also verified by AFM observations. Narrow and dark zones observed in the phase image of starch/CMC/MMT films (Figure 2(c)) indicate the MMT platelets. Figure 3 shows phase image of starch/CMC/MMT bionanocomposite films in three different scan sizes. Homogenous and parallel distribution of shown zones in the starch matrix indicates the presence of intercalated clay layers dispersed into the matrix. However, for the precise studding of nanoclay dispersion in the polymer matrix, meticulous analyses such as X-ray diffraction and TEM microscopy are required.

(c)

3.2 X-ray diffraction In order to investigate the dispersion of the MMT layers in polymer matrix, X-ray diffraction analyses were performed on the nanocomposites. Figure 4 shows the XRD patterns for the pristine and ultrasonically treated MMT, the starch/CMC/MMT bionanocomposites and the pure starch/MMT nanocomposite. The XRD patterns revealed that when MMT was sonified in an ultrasonic bath, the diffraction peak of MMT (001) crystal plane moved from 7.43 to 6.04°. According to the Bragg diffraction equation: 2d sinθ ¼ λ, the d-spacing between the layers was 1.18 and 1.46 nm for the pristine and ultrasonically treated MMT, respectively, which indicated that the use

Figure 2 AFM phase images of CA-modified starch film (a), CAmodified starch/CMC polymer blend film (b) and CA-modified starch/CMC/MMT bionanocomposite film at 10 10 μm scan size. These three images were chosen as representatives from each sample scanned at different scan size. Arrows in figs a, b and c show the CA, CM and MMT, respectively

of ultrasonic was advantageous for d-spacing increasing of MMT layers. In the starch/CMC/MMT bionanocomposites, the d001 diffraction peak of the MMT was shifted to lower angle, regardless of the clay content. All


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(a)


(b)

(c)

Figure 3 AFM phase images of CA-acid modified starch/CMC/MMT bionanocomposite film at different scan sizes (5 5 μm (a), 10 10 μm (b) and 500 500 nm (c)). Arrows show the MMT layers in starch matrix

bionanocomposites showed a sharp d001 at around 2θ ¼ 5.25° corresponding to a d-spacing of 1.69 nm. These results indicate that either the starch or the CMC polymer chains or both entered into the silicate layers forming intercalated starch/CMC/MMT bionanocomposites, without reaching complete exfoliation. Also intercalation of glycerol could be another reason for shift of 2θ to low angle. Both molecular weight of polymer and polar interactions between the clay and the polymer could influence polymer intercalation [30]. Strong polar interactions between the hydroxyl groups present in the starch and CMC chains and in the silicate layers probably caused to intercalation of biopolymer chains into MMT layers galleries [24]. Comparison of d001 peaks of pristine clay and bionanocomposites shows that probably only single chain of starch intercalated into clay layers. Starch is a linear molecule consisting of ring-like monomer with size of about 0.55 nm [31] (that is equal to d-spacing difference of pristine clay and bionanocomposites). Although, Huang et al. [9], and Cyras et al. [21] observed intercalated form in starch–MMT nanocomposites, however, some researchers reported that starch chains can be exfoliated completely in the starch-unmodified MMT [23, 25, 32, 33]. It seems that large numbers of inter- and intrahydrogen bonds formed among and within starch molecules (caused by CA (Ma et al. [15]; Shi et al. [34]) and

CMC (Ma et al. [13])), reduce the molecular mobility which in turn prevent complete exfoliation in nanocomposite matrix. In order to approval of this assumption, the films without CA and CMC were prepared and the dispersion pattern of MMT layers was investigated. As shown in Figure 4, there was no any sharp d001 peak in XRD pattern of pure starch–MMT nanocomposite. This result confirms the extensive diffusion of polymer chains inside galleries of nanoclay and starch chains exfoliated completely in the pure starch–MMT nanocomposites. These results indicate that, although CA and CMC are able to improve mechanical and barrier properties of starch films [13, 34], however, the possibility of extensive intercalation of starch biopolymer chains between silicate sheets could decrease due to cross-linking effect of CA and strong interactions between CMC and starch chains.

3.3 Mechanical properties Mechanical properties resulted from the tensile test show an improvement in the mechanical strength by the addition of CA, CMC and MMT. The UTS and SB of the different samples are shown in Table 2. The mechanical properties of the starch films were improved by addition of 10% CA. This could be attributed to the cross-linking caused by the CA. As well as, CA can probably hydrolyze



Pure starch, 5% MMT

5.29° 7% MMT 5.25°

Intensity (a.u.)

5% MMT

5.2° 3% MMT 5.28°

1% MMT

6.04° 7.43° Ultrasonically treated MMT

Pristine MMT

0

2

4

6

8

10

12

14

16

18

2°(degree) Figure 4 XRD patterns for the pristine MMT, ultrasonically treated MMT, CA-modified starch/CMC/MMT bionanocomposites and pure starch/MMT nanocomposite

Table 2 Ultimate tensile strength (UTS) and strain to break (SB) of different films Sample Starch Starch/10% Starch/10% Starch/10% Starch/10% Starch/10% Starch/10%

CA CA/10% CA/10% CA/10% CA/10% CA/10%

CMC CMC/1% MMT CMC/3% MMT CMC/5% MMT CMC/7% MMT

UTS (MPa)

SB (%)

4.42 6 0.6a 6.57 6 0.7b 9.83 6 0.5c 13.13 6 0.8d 16.48 6 1.3e 21.94 6 0.7f 27.55 6 1.1g

85.17 6 5.5a 66.18 6 3.4b 63.52 6 6.1b 43.87 6 4.9c 36.76 6 1.1d 24.19 6 5.3e 18.25 6 2.6f

Note: Means with different letters within a column indicate significant differences (p < 0.05).

branched chains of starch molecules that induce formation of highly linear structure which in turn allow forming more hydrogen bonds between the starch chains and increasing the tensile strength in the resulted films [15].

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Also, addition of 10% CMC improved the mechanical properties of the films. This is in agreement with Ma et al. [13] who reported a significant improvement of water resistance and mechanical properties of starch films are achieved by adding relatively small amounts of CMC (till 10%). It was observed an important increase of two- and threefold in the UTS when 3 and 7%W/W of MMT was added to the starch/CMC polymer blend, respectively. With the increase of the MMT concentration from 0 to 7%, the UTS increased significantly (p 0.05) from 9.83 to 27.55 MPa; however, the SB decreased noticeably (p 0.05) from 63.52 to 18.25%. A similar behavior in the UTS increment was also observed by other authors [35]; Chen and Evans [9, 22, 23]; in plasticized starch/clay system. This behavior was expected and was attributed to the resistance exerted by the clay itself and to the orientation and aspect ratio of the intercalated silicate layers. In addition, the stretching resistance of the oriented backbone of the polymer chain in the gallery bonded by hydrogen interaction also contributed to enhancing the tensile strength. The layered silicate acts as a mechanical reinforcement of starch reducing the flexibility of the polymer. The main reason for this improvement in the mechanical properties is the stronger interfacial interaction between the matrix and layered silicate due to the vast surface exposed of the clay layers. During the processing and drying of the composites, the original hydrogen bonds formed between the starch molecules were replaced by the new hydrogen bonds formed between the hydroxyl groups in starch molecules, the hydroxyl and carboxyl groups in CMC and the hydroxyl groups in MMT. The existence of these new hydrogen bonds would improve the mechanical properties.

3.4 Water vapor permeability As shown in Table 3, the WVP of the plasticized starch film without CA (control sample) was 4.64 10−7 g/mhPa and adding 10% CA caused to significantly decrease in the WVP of the films. This might be attributed to the hydrophilic OH groups substitution with hydrophobic ester groups or increasing interchain interactions in the starch structure. By addition of 10% CMC, WVP of CA-modified starch film decreased significantly. The decrease of WVP by incorporation of secondary biopolymer is in agreement with results usually reported for polymer blends which


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Table 3


Water vapor permeability (WVP) of different films WVP (10−7 g/mhPa)

Sample Starch Starch/10% Starch/10% Starch/10% Starch/10% Starch/10% Starch/10%

CA CA/10% CA/10% CA/10% CA/10% CA/10%


4.64 6 0.02a 2.61 6 0.05b 2.44 6 0.07c 2.39 6 0.08c 1.69 6 0.10d 1.46 6 0.07e 1.13 6 0.03f

Note: Means with different letters indicate significant differences (p < 0.05).

are studied for packaging applications [13, 36, 37]. Water resistance of CMC biopolymer is better than starch biopolymer [13]. This could be attributed to the highly crystalline and hydrophobic character of the cellulose fibers in comparison to starch polymer. The addition of CMC could introduce a tortuous path for water molecule to pass through [38]. WVP of the bionanocomposite films diminished with the increase of MMT content as can be seen in Table 3. WVP was 2.44 10−7 g/mhPa for the films containing 0% MMT and decreased to 2.39 and 1.69 10−7 g/mhPa for the films containing 1 and 3% MMT, respectively. At the level of 7% MMT, the starch/CMC/ MMT films showed the lowest WVP values at 1.13 10 −7 g/mhPa. The decrease of WVP by incorporation of MMT is in agreement with results usually reported for nanocomposites which studied the effect of clays on barrier properties of starch films [24, 39]. Similar results have been reported for the effect of nanoclays on the WVP of poly(lactic acid) [40], polystyrene [41] and whey protein isolate [42] films. These results attributed first of all to the fact that in nanocomposites, gas molecules have to take a long and tortuous way around the impermeable clay layers which are distributed in the polymer matrix in comparison with pristine polymer where the penetration of the film is much easier [43]. This enhanced barrier characteristics of the starch/CMC/MMT bionanocomposites result from the better-ordered intercalated structure of the films that had high aspect ratio nanolayers of Na þ MMT in the films. However, Yano et al. [43] studied the correlation between the aspect ratio of the clay and the barrier properties and concluded that the fully exfoliated clay minerals give the best barrier properties in polymer nanocomposites. In addition, improving the barrier properties of bionanocomposites could attribute to the strong interactions of MMT with polymer chains. Generally, water vapor transmission through a

hydrophilic film depends on both diffusivity and solubility of water molecules in the film matrix. Reduction of OH groups of starch molecules caused by formation of hydrogen bonds with the hydroxyl groups of the MMT layers can reduce the solubility of water molecules. On the other hand, intercalation of MMT layers reduces the diffusivity by reduction of voids in polymer matrix.

3.5 Contact angle Protein and carbohydrate films are known to possess hydrophilic character. Water contact angles of films can be good indicator for determining the degree of hydrophilic nature of them [44]. The contact angle made by the drops of the water on the film surfaces is shown in Table 4. Presence of CA and CMC had no significant effect on surface wettability. The presence of MMT in the formulation tends to decrease surface hydrophobicity. The values of contact angles for starch/CMC/MMT bionanocomposites are found significantly lower than those of starch/CMC films. The initial contact angle and angle obtained after 60 s were 50.21° and 48.54° for the samples without MMT and decreased to 41.61° and 37.21° for the films containing 1% MMT, respectively. By increasing of MMT content, contact angle of water with film surfaces decreased significantly. A similar behavior in the contact angle diminution and wettability increment was also observed by Cyras et al. [21] in starch/clay nanocomposites. This behavior could be due to the hydroxylated silicate layers that make the MMT a hydrophilic compound. The confliction between results of WVP and contact angle tests could be an interesting result that revealed a hydrophilic material which may able to reduce water vapor permeability of the films. Table 4 Contact angle measured by the drops of water on the film surfaces in initial time (θt0) and after 60 s (θt60) Sample Starch Starch/10% Starch/10% Starch/10% Starch/10% Starch/10% Starch/10%

CA CA/10% CA/10% CA/10% CA/10% CA/10%


θt0

θt60

52.36 6 0.82a 52.13 6 1.75a 50.21 6 0.83b 41.61 6 2.36c 40.76 6 2.32c 29.05 6 3.24d 24.61 6 2.05d

51.78 6 1.00a 49.33 6 2.09b 48.54 6 1.63b 37.21 6 2.13c 37.29 6 1.44c 27.18 6 0.55d 21.61 6 2.03e

Note: Means with different letters within a column indicate significant differences (p < 0.05).



4 Conclusions New environmentally friendly bionanocomposites were prepared via a solution/casting method from a CAmodified corn starch matrix, CMC and MMT. The effect of these components on the structural and physical properties of the starch films was studied. AFM surface analysis showed that starch/CMC/MMT films and starch/CMC films had the highest and lowest roughness (Rq), respectively. Phase images showed that combination of the all ingredients produce a homogenous composite without any phase separation. An increment in the interlayer distance of the clay sheets in the bionanocomposites was observed by X-ray diffraction providing evidence that the clay nanolayer formed an intercalated structure without reaching a complete exfoliation. But there was no any sharp d001 peak in XRD pattern of pure starch/MMT nanocomposites. This result confirms the extensive diffusion of polymer chains inside galleries of clay.

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All the components (CA, CMC and MMT) demonstrated an important influence on the mechanical properties. Tensile strength of the films increased significantly by addition of these materials. Furthermore, an improvement in the barrier properties of the starch with the addition of CA, CMC and MMT was observed. However, the bionanocomposites showed a higher hydrophilic character than the starch and starch/CMC films due to the high polarity of the MMT added. It was revealed that enhanced barrier properties of bionanocomposites by addition of MMT depend on its layered structure and not on its chemical properties. The formation of a tortuous path-way by the presence of clay decreases the rate of water vapor transfer. As a result, simultaneous addition of CA, CMC and MMT to the starch matrix could improve the mechanical and barrier properties of the starch-based edible films. However, in the presence of CA and CMC, the possibility of complete exfoliation of starch biopolymer chains between silicate sheets could decrease due to cross-linking effect of CA and strong interactions between CMC and starch chains.

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