Improvement of mechanical properties and thermal stability of biodegradable rice starch–based films blended with carboxymethyl chitosan

Industrial Crops & Products 122 (2018) 37–48

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Improvement of mechanical properties and thermal stability of biodegradable rice starch–based films blended with carboxymethyl chitosan Rungsiri Suriyatema, Rafael A. Aurasb, Pornchai Rachtanapunc,d,

T

⁎

a

Division of Food Science and Technology, Faculty of Agro-Industry, Chiang Mai University, Chiang Mai 50100, Thailand School of Packaging, Michigan State University, East Lansing, MI 48824, USA c Division of Packaging Technology, Faculty of Agro-Industry, Chiang Mai University, Chiang Mai 50100, Thailand d Center of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand b

A R T I C LE I N FO

A B S T R A C T

Keywords: Compostability Swelling Morphology Barrier Bio-based films

Biodegradable blend films from rice starch (RS) and carboxymethyl chitosan (CMCh) were produced and characterized. Color, opacity, mechanical properties, thermal properties, swellability, oxygen and water permeability, and biodegradability of the RS–CMCh blend films are reported. Interaction and compatibility of films components were evaluated by using Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. Increased yellowness, total color difference and transparency, and decreased redness, lightness and whiteness index were observed in the blend films as incorporation of CMCh increased. Addition of 50% w/w of CMCh into the RS matrix increased the tensile strength of the RS–CMCh blend film by 35% and the elongation at break by 28%. Addition of CMCh improved the thermal stability of the RS–CMCh films. Incorporation of 12, 33 and 50% w/w CMCh in the blend films increased the swelling ratio by around 850%, 3985% and 3404% at 24 h, respectively, when compared with the RS film. The oxygen permeability of all the films increased as relative humidity increased. The FTIR spectra suggested that interactions may be present between the eOH groups of RS and the COOe groups of CMCh. Scanning electron microscopy images revealed that the cross-sectional fracture surfaces of all the films were smooth and homogenous. The RS film exhibited a priming effect in the biodegradation study. The addition of 50% w/w CMCh led to a decrease in mineralization of the blend films.

1. Introduction Among the several types of biomaterials, the polysaccharide starch is an attractive material due to its good film-forming capability, biocompatibility, relatively low cost, renewability and abundance. Rice starch (RS) is a by-product of rice processing. RS is considered to be a relatively cheap source of starch. RS contains around 30% amylose, which is the linear and the more readily crystallizable component of starch compared with amylopectin. RS can be used to produce biodegradable films (Janjarasskul and Krochta, 2010; Wittaya, 2012). Polysaccharide films are good oxygen barriers with suitable optical properties and moderate mechanical properties at low relative humidity (RH) (Bourtoom and Chinnan, 2008; Laohakunjit and Noomhorm, 2004; Whistler et al., 1984). However, starch-based films are brittle and hydrophilic, which affect and limit their processing and application (Mendes et al., 2016). Plasticizers such as glycerol, sorbitol and polyethylene glycol have been used to overcome the brittleness of starch films (Suppakul et al., 2013; Wittaya, 2012), but their use decreases the tensile strength of the films (Tantala et al., 2012a). ⁎

Blending, grafting, and compounding with other materials have been used as the main techniques to overcome this shortcoming (Al-Hassan and Norziah, 2012; Bourtoom and Chinnan, 2008; Dias et al., 2010; Ghanbarzadeh et al., 2010; Mathew et al., 2006; Saberi et al., 2016). Chitosan is a cationic polysaccharide containing β (1–4)-2-amino-2deoxy d-glucopyranose repeating units (Wu et al., 2013). Chitosan is a deacetylated product of chitin, and is abundantly available in nature, nontoxic and biodegradable. Chitosan has high quality film-forming capability (Wu et al., 2013); however, it dissolves in acidic water, which affects the finished product by causing a bad odor (Ferreira et al., 2009). Carboxymethyl chitosan (CMCh) is a water-soluble etherified chitosan. Applications for CMCh have been reported for food (Carolan et al., 1991), drugs (Chen et al., 2004), cosmetics (Wannaruemon et al., 2013), and agriculture (Dau et al., 2016). CMCh has the capability to form films and gels, is biodegradable and soluble over a wide range of pH, has high viscosity, biocompatibility and antimicrobial activity, and low toxicity (Bukzem et al., 2016; Tantala et al., 2012b). The introduction of CMCh in blend films can increase water solubility, improve tensile strength, and provide some antimicrobial capability (Fan

Corresponding author at: Division of Packaging Technology, Faculty of Agro-Industry, Chiang Mai University, Chiang Mai 50100, Thailand. E-mail address: [email protected] (P. Rachtanapun).

https://doi.org/10.1016/j.indcrop.2018.05.047 Received 29 September 2017; Received in revised form 6 April 2018; Accepted 20 May 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.

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80 °C using a magnetic stirrer for 15 min. The various combinations of RS and CMCh solutions were mixed for 30 min using a magnetic stirrer, and glycerol (25% wt of solid content) was added as a plasticizer. Each solution was degasified, cooled to 25 °C and poured onto an acrylic casting plate (0.15 m × 0.15 m). To form and dry the film, the plate was placed in a control room at 25 ± 1 °C and 55% ± 5% RH for 36 h. The films were peeled from the plates, placed in sealed aluminum bags, and stored in an environmental chamber at 25 ± 1 °C and 55% ± 2% RH until use. Selected samples were stored at −20 ± 1 °C and removed for conditioning before testing for mechanical, thermal and barrier properties, swelling and biodegradation. All the films were pre-conditioned at 23 ± 2 °C and 50% ± 10% RH for 48 h before testing.

et al., 2006; Tantala et al., 2012b). Previous studies have shown that the incorporation of chitosan or CMCh can improve the mechanical properties of rice starch–chitosan blend films (Bourtoom and Chinnan, 2008), corn starch–chitosan blends (Mendes et al., 2016), and pullulan–CMCh blend films (Wu et al., 2013). Tantala et al. (2012b) showed that CMCh has some antimicrobial capabilities against several strains of lactic acid bacteria but not against foodborne pathogens. Thus, incorporating CMCh into rice starch–based films could provide appealing film properties, and it is the focus of this work. To the best of our knowledge, there is no published research on the development and characterization of RS-based films that incorporate CMCh, or on understanding their end-of-life scenario and biodegradability. Thus, the purpose of this work was to evaluate the effect of incorporating CMCh on the physical and thermal properties, permeability and biodegradability of RS–CMCh blend films. The interactions and compatibility between these two polysaccharides were also studied by Fourier transform infrared spectroscopy and X-ray diffraction.

2.4. Film color and opacity Lightness (L*) and chromaticity parameters, a* (redness) and b* (yellowness), of the films were measured using a colorimeter (CR-10, Minolta, Japan). A white standard (tile: L* = 98.35, a* = 0.00, and b* = 1.08) was used to calibrate the color reader before measurements were taken. Samples were tested in triplicate. Total color difference (ΔE) and whiteness index (WI) were calculated using Eqs. (1) and (2), respectively:

2. Materials and methods 2.1. Materials Native RS powder (Rose 100R) was purchased from Thai Flour Industry Co., Ltd. (Bangkok, Thailand). Shrimp chitosan flake (molecular weight range 900,000–1,300,000 Da; degree of deacetylation 98%) was purchased from Taming Enterprises (Samut Sakhon, Thailand). Sodium hydroxide, glacial acetic acid, isopropanol, ethanol and methanol were purchased from RCI Lab-scan Co., Ltd. (Bangkok, Thailand). Glycerol was purchased from Union Science Co., Ltd. (Chiang Mai, Thailand). Monochloroacetic acid was purchased from Sigma Aldrich (Steinhiem, Germany). All reagents were of analytical grade and used as received. Cellulose powder (20-mm grade) was purchased from Sigma Aldrich (St. Louis, MO, USA). Compost was obtained from the Michigan State University commercial composting facility (East Lansing, MI, USA).

ΔE =

(ΔL*)2 + (Δa*)2 + (Δb*)2

WI = 100 −

(100 − L*)2 + a* 2 + b* 2

(1) (2)

where ΔL*, Δa* and Δb* are the differences between the color parameter of the samples and of the RS film. Film opacity (Op) was evaluated by measuring the absorbance at 550 nm (A550) (Al-Hassan and Norziah, 2012) using a Spectro SC spectrophotometer (Labomed, Los Angeles, USA). The Op of the films was calculated using Eq. (3): Op = A550/x

(3)

where x is the film thickness (mm). According to this equation, a higher value of Op indicates a lower degree of transparency. All tests were run in triplicate.

2.2. Synthesis of carboxymethyl chitosan CMCh was synthesized as previously described by Suriyatem et al. (2015). Chitosan powder (25 g), with a particle size less than 60 mesh, was suspended in a medium consisting of NaOH–isopropanol–distilled water at a ratio of 50 g:400 mL:100 mL. The suspension was stirred continuously for 1 h at 50 °C using a hotplate stirrer (IKA C-MAG HS7, Wilmington, NC, USA). Monochloroacetic acid (50 g) was dissolved in isopropanol (50 mL) and gradually introduced to the suspension as an etherifying agent. The mixture was then stirred continuously for 4 h at 50 °C and then held until the two phases separated. The liquid phase was removed, and the solid phase was suspended in methanol and neutralized with glacial acetic acid to stop the reaction and then filtered. The residue was washed with 70% v/v ethanol (500 mL) five times to remove the byproduct NaCl and then washed once with 95% v/v ethanol (500 mL). The final product, CMCh, was dried in an oven at 55 °C for 18 h and then stored in a high-density polyethylene container containing silica gel packs. The degree of substitution of CMCh was measured by the potentiometric titration procedure as detailed elsewhere (Jaidee et al., 2012).

2.5. Tensile tests Tensile properties of the films (i.e., tensile strength, break elongation and modulus of elasticity) were measured using a universal testing machine (United Calibration Corp. and United Testing Systems, Huntington Beach, CA, USA). The measurements were conducted according to ASTM-D882-12 (2012). Five rectangular specimens (0.025 m × 0.100 m) were cut from each film. A load cell of 5 kN and a cross-head speed of 0.050 m/min were used. The initial grip separation was set at 0.050 m due to the sample size restriction and because we did not find statistically significant differences when testing films with an initial grip separation of 0.050 and 0.100 m. The specimens were conditioned at 23 ± 2 °C and 50% ± 10% RH for 48 h prior to testing. 2.6. Fourier transform infrared (FTIR) spectroscopy Transmission infrared spectra of films were obtained with a FTIR spectrometer (Equinox 55, Bruker, USA) in the range of 4000–400 cm−1, with a resolution of 4 cm−1 using a DTGS detector and Omnic software. The film was mounted directly in the sample holder.

2.3. Preparation of films Film-forming solutions (3% w/v) with different RS and CMCh contents were prepared to yield five different blend films. The RS/ CMCh ratios (wt/wt) of the blend films were 100:0 (RS film), 88:12 (RS/12CMCh film), 67:33 (RS/33CMCh film), 50:50 (RS/50CMCh film) and 0:100 (CMCh film), indicating g CMCh/100 g solid. To prepare the solutions, RS was dispersed in distilled water; the mixture was heated at 85 − 90 °C with constant stirring using a magnetic stirrer (IKA C-MAG HS7) for 15 min. CMCh was dissolved in distilled water and heated to

2.7. X-ray diffraction (XRD) X-ray diffraction patterns of the powders (RS, CMCh and chitosan) and films (RS, CMCh and RS–CMCH blends) were recorded in the reflection mode on a X-ray diffractometer (MiniFlex II, Rigaku, Japan). The scattering angle (2θ) was from 5 to 60° at a scan rate of 5°/min. 38

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2.8. Scanning electron microscopy (SEM)

2.12. Water vapor permeability

Morphological investigation of the surface of the film samples was performed using a scanning electron microscope (JSM-5910LV, JEOL, USA). Fracture surfaces were prepared in liquid nitrogen, cut using a sharp razor blade, and mounted on specimen stubs with carbon tape. The sample was sputter-coated with a thin layer of gold using a sputter coater (SPI-Module, USA). The samples were observed using accelerating voltages of 15 kV with 500× and 1500× magnification.

Water vapor permeability (WVP) was investigated according to the gravimetric modified cup method based on ASTM-E96/E9M-16 (2016). The film specimens were cut into circle shapes with a diameter of 0.07 m. Each test cup was filled with dried silica gel (10 g) as desiccant. The cups were covered with a film specimen, sealed with paraffin wax, weighed, and stored in a desiccator at 25 °C; the desiccator contained saturated sodium nitrite (NaNO2) to provide a constant 65% RH. Changes in weight of the cups were recorded as a function of time every 24 h. Slope of the linear relationship between weight gains vs. time was calculated. All measurements were performed in triplicate, and WVP was calculated by Eq. (6):

2.9. Thermal properties 2.9.1. Differential scanning calorimetry (DSC) The thermal profile of the films was determined with a DSC Q100 (TA Instruments, New Castle, DE, USA). The films were pre-conditioned at 23 ± 2 °C and 50% ± 10% RH for 48 h before testing. Film samples of 5–10 mg were cut into small pieces (∼0.001 m × 0.001 m) and placed into a sample aluminum pan. An empty pan was used as reference. The transition profiles of the first heating cycle are reported. The first heating cycle was from −50 to 250 °C, at a scanning rate of 10 °C/min under N2 atmosphere. A minimum of two samples was tested per film.

WVP =

2.13. Biodegradability Biodegradability of the films was monitored for 87 days under simulated composting conditions (using manure compost in bioreactors) and measuring the release of carbon dioxide (CO2) from the bioreactors. Testing was carried out using an in-house built direct measurement respirometric (DMR) system according to the procedure described by Kijchavengkul et al. (2006). The initial carbon content of the films was measured using a Perkin Elmer CHN analyzer (Waltham, MA, USA) prior to the test. Film samples (8 g) were cut into pieces of around 0.01 m × 0.01 m. Each bioreactor was made from a glass jar and loaded with 400 g of compost (wet basis). Saturated vermiculite, premium grade (Sun Gro Horticulture Distribution Inc., Bellevue, WA, USA) was added to the compost (1:4, vermiculite wt/dry weight compost) to provide better aeration. The compost was filled with test material (film samples pieces) and mixed thoroughly. Cellulose powder (8 g) was used as positive control. Blank bioreactors contained only compost. Details of the apparatus can be found elsewhere (Kijchavengkul et al., 2006). The test was operated according to ASTM-D5338-15 (2015). The temperature of the environmental chamber was set at 58 ± 2 °C. The airflow rate was set at 40 cm3/min and the air humidity was adjusted to 50% ± 10% RH. The bioreactors were incubated in the dark for 87 days. Each test was run in triplicate. The amount of evolved CO2 for each reactor was recorded by a computer program written using Labview (Labview software version 7.1, National Instruments) and exported to the Microsoft Excel program. Percent mineralization was calculated using Eq. (7):

2.10. Swelling ratio The swelling behavior of the films was determined according to the method described by He et al. (2010), with slight modifications. Duplicate films (0.02 m × 0.02 m) were initially weighed (Wb) and immersed in 30 mL of distilled water at 23 °C for specific time intervals. At regular time intervals, each film sample was removed from the water, wiped with a piece of Kimwipes® paper to absorb excess water on the film surfaces, and then reweighed (Wa). The swelling ratio of the samples was calculated with Eq. (4).

Wa − Wb × 100 Wb

(4)

%Mineralization = 2.11. Oxygen permeability

O2 TR × L Δp

sCO2 − bCO2 W×

%C 100

×

44 12

× 100 (7)

where sCO2 is the amount of CO2 from the sample reactor; bCO2 is the amount of CO2 from the blank reactor; W is the weight of the sample (g); and %C is the % carbon in the sample obtained from the CHN analyzer.

Oxygen permeability (O2P) was measured using an oxygen permeation analyzer (Model 8001, Illinois Instruments Inc., Johnsburg, IL) in accordance with ASTM-D3985-05 (2005). The specimen was prepared by masking with aluminum tape to obtain an exposure area of 3.14 cm2. The test was run continuously at 23 °C, 50% RH and 100% O2 until steady state was achieved. O2P was calculated with Eq. (5).

O2 P=

(6)

where WVTR is the rate of vapor transmission; S is saturation vapor pressure at the test temperature 25 °C (23.756 mmHg); R1 is the RH in the desiccator; R2 is the RH in the test cup, which is assumed to be 0% since the cup is filled with desiccant.

2.9.2. Thermogravimetric analysis (TGA) A TGA Q50 (TA Instruments) was used to characterize the degradation and thermal stability of the films. Each film sample was cut into small pieces (∼0.001 m × 0.001 m) and 5–10 mg was used for testing. The samples were heated from 30 to 600 °C at 10 °C/min in the presence of N2 at 70 mL/min. Thermal degradation temperatures at 5% (Td5) and 50% weight loss (Td50) were determined from the TGA curves, and maximum decomposition (Tmax) from the derivative thermogravimetric (DTG) curves. A minimum of two samples were tested per film.

Swelling ratio =

WVTR S × (R1 − R2)

2.14. Statistical analysis SPSS software (Version 11, SPSS Inc., Chicago, IL) was used to analyze the data. One-way analysis of variance (ANOVA) was carried out using Duncan’s multiple range test (p ≤ 0.05).

(5) 2

where O2TR is the oxygen transmission rate (cc/m .day) at steady state, as determined by the analyzer; L is the film thickness; Δp is partial pressure (101,325 Pa). The RS, CMCh and RS/50CMCh films were the representative films used to study the effect of various RH (0, 50, 90% RH) on the O2P of the films. Duplicate samples were tested.

3. Results and discussion CMCh was synthesized from chitosan via carboxymethylation using sodium monochloroacetate as the etherifying agent. The degree of 39

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Fig. 1. Optical properties of RS–CMCh blend films: (a) L* value, (b) a* value, (c) b* value, (d) ΔE, (e) WI, and (f) Op, opacity. Data are fitted with linear models. The best fitted line and the confidence and prediction intervals are provided.

difference (ΔE) and whiteness index (WI) of the RS film, CMCh film and RS–CMCh blend films. All films were visually transparent, with small differences between L* values. The RS film had higher a* and lower b* values compared with the CMCh film. The a* of the blend films decreased with an increase in CMCh content, while b* conversely increased, indicating a more pale yellow-like appearance due to the nature and amount of CMCh in the blend films. A more yellow color was also observed in pullulan–chitosan and pullulan–carboxymethyl chitosan blend films reported by Wu et al. (2013). The ΔE value displays the total difference in color of the RS film with respect to the other films. The ΔE of the blend films increased with incorporation of CMCh (Fig. 1d). A similar tendency was observed for RS–chitosan blend films by Bourtoom and Chinnan (2008).

substitution (DS) of the CMCh was found to be 0.49. The critical DS value at which CMCh becomes soluble in water is in the range of 0.40–0.45. Above this range, CMCh solubility increases with an increase in the DS value (Chen et al., 2003). This means that the synthesized CMCh was water-soluble. 3.1. Optical properties Color measurement was carried out on the prepared films to determine changes in color after blending RS and CMCh. Experimental data was fitted to linear models. Fig. 1(a–e) shows the color parameters, including L* (0 = black, 100 = white), a* (−60 = greenness, +60 = redness), b* (−60 = blueness, +60 = yellowness), total color 40

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The WI value refers to the degree to which a surface is white, so it corresponds to the formation of a white matrix in the films. The RS–CMCh blend films containing lower levels of starch showed decreasing WI values (Fig. 1e). A similar observation was reported for pea starch–guar gum films by Saberi et al. (2016). Transparency is an important attribute of films, especially if they are used as a coating. A higher value of Op means a lower degree of transparency. The transparency value of the CMCh film was around 5 times higher than that of the RS film (Fig. 1f). The lower transparency of the RS film may be due to the presence of residual RS crystals. Evaluation of film transparency can assist in determining the miscibility of biopolymer blends (Wang and Su, 2014). When the CMCh content in the blends increased, the transparency increased. The increase in transparency of the RS–CMCh blend films is probably due to the dilution effect of the CMCh phase, indicating good compatibility of the RS–CMCh blend films, as previously observed by Wu et al. (2013) for pullulan–CMCh blend films. Fig. 3. Stress (MPa) vs strain (%) of RS and CMCh films and RS–CMCh blend films.

3.2. Mechanical properties Fig. 2 shows the tensile strength (TS), elongation at break (EB) and Young’s modulus (E) of the RS and CMCh films and RS–CMCh blend films, and Fig. 3 shows characteristic stress-strain curves of these films. Addition of CMCh to the RS matrix induced a different mode of failure for the blend films, from a brittle to more ductile behavior. A strainhardening behavior was observed in the blend films due to the increased amount of CMCh. The RS/50CMCh film had optimal TS and EB values due to the original attributes of the RS and CMCh. The TS of the RS film was lower than that of the CMCh film. The TS of the blend films tended to increase in films with ≥50 g CMCh/100 g solid; however, no significant difference was found for films with 0–33 g CMCh/100 g solid (p > 0.05). The enhanced TS for the RS/50CMCh film may be attributed to the reaction between the COO− groups of the CMCh backbone and the OHe groups of the RS, as previously reported by Wu et al. (2013) for pullulan–CMCh blend films. The researchers found that when the CMCh-to-pullulan ratio (1:3, 2:2 and 3:1) of the blend films increased, the TS increased. Bourtoom and Chinnan (2008), studying on the effect of chitosan on rice starch films, also showed an increase in tensile strength after incorporation of chitosan. The RS film had lower EB than the CMCh film, imparting flexibility to the RS film. The EB of each blend film was higher than that of the RS film (p < 0.05) and comparable to that of the CMCh film (p > 0.05). Wu et al. (2013), working with pullulan–CMCh blend films, showed a similar effect of CMCh on the final film blends. The E value of the RS film was around 7 times higher than that of CMCh film. This higher E value may be due to the presence of hydrogen bonding created between the glycerol and starch molecules in the RS film, and may also be attributed to the Vh-type crystals of complex between amylose and glycerol (Mendes et al., 2016), as demonstrated in the XRD patterns in Section 3.4. The E of the RS–CMCh blend films

decreased with the addition of CMCh, indicating softening of the blends. The addition of CMCh might lead to greater movement of the polymer chains, enhancing film flexibility. Mendes et al. (2016) reported that the addition of chitosan decreased the elastic modulus of corn starch–chitosan blends. Effect of chitosan concentration on mechanical properties of corn starch–chitosan films was studied by Ren et al. (2017). They reported that incorporation of chitosan resulted in an increase TS and EB of the films and a decrease in E.

3.3. FTIR FTIR spectroscopy was carried out to determine the interactions between RS and CMCh. Fig. 4 shows the FTIR spectra corresponding to the RS and CMCh films and RS–CMCh blend films. In the RS film spectrum, a broad band at 3303.46 cm−1 was attributed to OeH stretching; the peak at 2933.20 cm−1 corresponded to CeH stretching of the CH2 of starch (Bourtoom and Chinnan, 2008); and the band at 1650.77 cm−1 was assigned to the OeH bending of water (Mathew et al., 2006). In the CMCh film spectrum, the absorption band at 3324.68 cm−1 corresponded to the eNH2 and eOH groups of CMCh (Duan et al., 2011); the peaks at 2923.57 cm−1 and 2881.13 cm−1 are due to the asymmetric and symmetric CeH stretch vibrations, respectively; and the peaks at 1595.80 cm−1 and 1412.60 cm−1 were assigned to antisymmetric and symmetric vibrations for the COOe group, respectively (Wu et al., 2013). The interaction of RS with CMCh in the blends was evidenced by the shift in wavenumbers for these two peaks of COOe vibrations. In the spectra of the RS–CMCh blend films, the antisymmetric and symmetric vibrations for the COOe peaks of the CMCh, which were

Fig. 2. Mechanical properties: (a) tensile strength; (b) elongation at break; (c) Young’s modulus of RS and CMCh films and RS–CMCh blend films. 41

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Fig. 4. FTIR spectra of RS and CMCh films and RS–CMCh blend films.

located at 1595.80 cm−1 and 1412.60 cm−1, respectively, shifted to 1594.84 cm−1 and 1411.64 cm−1 for both RS/12CMCh and RS/ 50CMCh, and to 1597.70 cm−1 and 1411.64 cm−1 for RS/33CMCh. In addition, the intensity of absorption in these COOe bands increased with increasing the CMCh content (Fig. 4). This result was indicative of interactions between the eOH groups of RS and the COOe groups of CMCh. However, the hydrogen bonds acting on the COOe groups in the blends (except RS/33CMCh) may be weaker compared with those in the CMCh film, according to the shift in peaks to the lower wavenumbers (Tong et al., 2008). Interaction between eOH and COOe groups in other blends has been observed using FTIR measurement: for example, in maize starch–CMCh (Duan et al., 2011) and pullulan–carboxymethyl cellulose (Tong et al., 2008). 3.4. XRD XRD analysis was used to investigate the crystal structure and compatibility of the RS and the CMCh phases. Fig. 5 compares the XRD patterns of RS powder and film, CMCh powder and film, RS–CMCh blend films as well as chitosan. The diffractogram of the RS film presented reflection peaks and a broad amorphous background, indicating that the RS film was a semi-crystalline material. The RS film showed peaks at 2θ of 13.8°, 17.3°, 20.0° and 21.9°; the peaks at 13.8° and 21.9° were attributed to Vh-type crystals of complexation between amylose and glycerol induced by heat treatment (Mendes et al., 2016). The absence of A-type crystals, which belong to the cereal starch granule (Luchese et al., 2017), indicated that the native RS structure was completely destroyed during the film preparation, as was supported by SEM results (see Section 3.5). The diffraction pattern of the CMCh film had two small broad peaks around 2θ = 11.9° and 20.0°, indicating a mostly amorphous structure (Fan et al., 2006). In the conversion of chitosan to CMCh, the matrix of CMCh underwent destruction creating the amorphous structure (Wu et al., 2013). The patterns of RS–CMCh blend films are similar to that of the CMCh film. The Vh-type crystallinity peaks of starch disappeared in the blends, which could be due to the decrease in formation of amylose-glycerol complexes. In addition, it is possible that intermolecular interactions between RS and CMCh diminished the crystalline regularity of the RS film. The results indicated good compatibility of the components in the blend films and supported

Fig. 5. XRD patterns of RS powder and film, CMCh powder and film and RS–CMCh blend films.

the FTIR evidence that indicated good miscibility between RS and CMCh owing to their intermolecular interaction. Similar observations were made by Ren et al. (2017) and Xu et al. (2005) who reported that addition of chitosan to corn starch decreased the crystallinity of the blend film. Wu et al. (2013) reported that pullulan films and CMCh films were amorphous materials; in their blends, a peak around 2θ = 21.4° for CMCh disappeared. Our results indicated a probability of hydrogen bond interactions between the two main components. 3.5. SEM SEM was used to investigate the film morphology. Fig. 6 shows SEM micrographs of native rice starch and CMCh powder (at 500× magnification) and the cross-sectional fracture surfaces of RS film, CMCh film and RS–CMCh blend films (at 500× and 1500× magnification). Higher magnification levels could not be used since the films can be degraded due to the high voltage of the SEM (Dias et al., 2010). Compared with 42

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Fig. 6. SEM micrographs (at 500× or 1500× magnification) for (a) native rice starch (500×), (b) CMCh powder (500×), and cross-sectional surfaces of (c) RS film (500×), (d) RS film (1500×), (e) CMCh film (500×), (f) CMCh film (1500×), (g) RS/12CMCh film (500×), (h) RS/12CMCh film (1500×), (i) RS/33CMCh film (500×), (j) RS/33CMCh film (1500×), (k) RS/50CMCh film (500×), and (l) RS/50CMCh film (1500×).

43

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44

0.4 a 0.4 b 0.4 c 0.4 d 0.3 e ± ± ± ± ± 40.8 39.7 38.5 37.5 33.5 324.9 ± 0.3 a 298.0 ± 0.2 b 286.5 ± 0.9 c – – 0.4 2.6 0.7 1.0 ± ± ± ±

Values within a column followed by the same letter are not significantly different (p > 0.05). Td5 and Td50 are thermal degradation temperatures at 5% and 50% weight loss, respectively.

320.2 299.4 297.9 297.2 333.4 160.4 145.6 150.4 219.2 358.1 RS RS/12CMCh RS/33CMCh RS/50CMCh CMCh

0.128 0.129 0.136 0.143 0.159

± ± ± ± ±

0.005 0.001 0.019 0.013 0.004

a a a ab b

(J/g) (mm)

± ± ± ± ±

5.5 1.3 4.0 3.6 3.7

a b b c d

87.2 ± 2.8 a 61.3 ± 32.0 a 74.5 ± 7.7 a 91.1 ± 10.4 a 86.2 ± 16.9 a

(°C) (°C)

Td5 ΔH

The DSC curves of RS film, CMCh film and RS–CMCh blend films (Fig. 7) were analyzed to determine the differences in thermal behavior. The single endothermic peak for all films in the DSC scans, which ranged from 103.7 to 116.2 °C, indicates the evaporation of water. The enthalpy of vaporization (ΔH) values for the films are presented in Table 1. The water content of the films was approximated from the moisture sorption isotherms, as measured previously (Suriyatem et al., 2015). The CMCh film had higher ΔH than the RS film since it had 30% higher moisture content due to its higher hydrophilicity. The RS/ 12CMCh and RS/33CMCh films had lower ΔH than the RS film, likely because of a decrease in free hydrophilic groups due to the intermolecular bonding between the main components. No Tg was detected in the thermograms of all films by using conventional DSC. Ghanbarzadeh et al. (2010) suggested that the Tg of plasticized starchbased films is sometimes difficult to determine by DSC analysis due to quite low heat capacity change. The presence of glycerol may further depress the Tg. DMA is a precise technique for measuring Tg because it involves a clearer associated signal change for starch-based materials (López et al., 2011). Peaks of the tan δ and the Eʺ curve are used to

Thickness

3.6. DSC

Film sample

Table 1 Thermal properties of RS and CMCh films and RS–CMCh blend films.

Td50

± ± ± ± ±

1.7 a 1.6 b 7.6 b 5.4 b 30.9 ab

water peak

native rice starch granules (Fig. 6a), no residual granular structure was apparent in the continuous phase of the RS film (Fig. 6c, d), due to the high temperature and shear of film preparation melting and breaking up the starch granules into small fragments. A similar result was observed with the CMCh film (Fig. 6e, f), as no residual flake structure of CMCh (Fig. 6b) was apparent in the continuous phase of CMCh film. However, the CMCh film morphology revealed some cracks in the continuous phase, perpendicular to the drying direction. These surface cracks may be attributed to the brittleness of CMCh. Similar fracture surfaces were reported by Mendes et al. (2016) for chitosan film. The fracture surface of all the blend films was smooth and homogenous (Fig. 6g–l), which indicated the high miscibility between RS and CMCh. The homogenous matrix was attributed to good interfacial adhesion between the RS and CMCh phases. This is a good indicator of the structural integrity responsible for films with good mechanical properties (Dias et al., 2010). The fracture surface of the RS/50CMCh film (Fig. 6k, l) appeared to be slightly rougher than those of the other blend films, which may be due to the flexibility of the film. Ren et al. (2017) observed similar structures for corn starch–chitosan films.

158.4 ± 5.2 a 144.1 ± 1.5 b 143.3 ± 2.6 b – –

glycerol peak

Fig. 7. DSC thermograms of RS and CMCh films and RS–CMCh blend films.

72.9 ± 5.1 a 81.1 ± 9.4 ab 95.5 ± 1.8 bc 104.1 ± 3.7 c 84.2 ± 6.6 ab

Tmax (°C)

a b b b – 241.9 254.1 254.5 258.0

90.6 77.7 74.9 70.6 65.4

± ± ± ± ±

3.0 0.5 1.1 2.5 5.5

a b c d d

(%) (%) starch peak CMCh peak

Final weight loss

C content

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Fig. 9. Swelling behavior of RS and CMCh films and RS–CMCh blend films in water at 23 °C.

3.8. Swelling ratio The swelling behavior of the RS film, CMCh film and RS–CMCh blend films in water as a function of time is shown in Fig. 9. The swelling ratios of the films increased and equilibrated over 24 h. The CMCh film did not crack into smaller pieces until 172 h, while the RS/ 12CMCh, RS/33CMCh and RS/50CMCh films began to crack and disintegrate in water at 48, 72 and 72 h, respectively. Although the tests were stopped at those times, the films had already reached a plateau sorption stage. The RS film had not disintegrated at the end of the test (172 h). The equilibrium swelling ratio (ESR) of the RS film was 85%. The CMCh film had an ESR of 3872%, which was around 45 × higher than that of the RS film. When compared with the RS film, all RS–CMCh blend films had higher ESR values (ranging from 778 to 4488%); the highest ESR value belonged to the RS/33CMCh film (4488%). Swelling capacity refers to the water retaining capacity of the film; the blend films are dominated by free hydrophilic groups and free volume (He et al., 2010). Accordingly, the swelling capacity of the blend films is a function of their hydrogen bonding and crystallinity (He et al., 2010). The increased ESR of the RS–CMCh blend films may be attributed to an increase in the intermolecular interactions between RS and CMCh in the film structure and the free volume in the amorphous region. The amorphous region of the blend films may hold the water while some intermolecular hydrogen bonds may create the framework for the films to swell. This increase in swelling could be considered as indirect evidence of the reaction between the functional groups of RS and CMCh, supporting the FTIR observations. A similar result was obtained for blend films made of corn starch and poly(vinyl alcohol) (Yoon et al., 2007), who found that the swelling capacity of the blend films was related to the strong functional groups capable of forming hydrogen bonds. Likewise, high concentrations of starch decreased the swelling power of blend films because of the inhibitory effect of amylose on swelling capacity (Thakur et al., 2016). In the RS–CMCh blend films, the swelling ratio increased when CMCh content increased (0–33 g/ 100 g solid) and then decreased when CMCh content exceeded 33 g/ 100 g solid. Although the RS/50CMCh film may have more free hydrophilic groups (eCOO) than the RS/33CMCh film, the hydrogen bonds between RS and CMCh were probably weaker, according to the FTIR results. Thus, the capacity of the RS/50CMCh film to retain water occupied by the large amorphous area may be lower. He et al. (2010) reported that silk fibroin–CMCh blend films with 5–10% CMCh content had lower swelling capacity; swelling capacity increased in the blend films with more than 15% CMCh but decreased when CMCh content was over 55%.

Fig. 8. Thermal stabilities of RS and CMCh films and RS–CMCh blend films: (a) TGA thermograms and (b) DTG thermograms.

distinguish Tg. However, in this work, those peaks did not occur for the films tested. This result emphasized that the change in the heat capacity of these films might be too weak to be detected or that the Tg is below our detection range (−50 °C for DSC and 0 °C for DMA). 3.7. TGA The thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves of all films in the presence of N2 are shown in Fig. 8a and b, respectively. The degradation below 100 °C and a broad peak around 140–160 °C were related to water loss and volatilization of plastizicer glycerol, respectively. Peaks at 325 and 258 °C were related to the decomposition of rice starch and CMCh, respectively (Fig. 8b). The temperatures at maximum rate of weight loss (Tmax) for the films are presented in Table 1. The addition of CMCh to the RS-based film resulted in differences in the thermal stability properties between the starch and the blend films. A complete overlap of the starch peak and CMCh peak was only found in the RS/50CMCh film at 254 °C. The glycerol peak and starch peak were shifted to lower temperatures with an increase in CMCh content (12–33 g/100 g solid), which was attributed to the lower thermal stability of CMCh compared with starch. Although the RS film showed a higher degree of resistance to thermal degradation at the beginning, the CMCh film and the blend films had higher thermal stability than the RS film at higher temperatures (350–600 °C), resulting in the lower mass loss (Fig. 8a). The final weight loss of the blend films decreased with an increase in CMCh content (Table 1). 45

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Fig. 10. Barrier properties of RS–CMCh blend films: (a–b) O2 permeability (O2P) at 23 °C, 50% RH and various %RH (0, 50 and 90% RH), respectively; (c) water vapor permeability (WVP) at 25 °C, 65% RH. *Values with the same lowercase letter (above a column) within the same %RH, and the same uppercase letter (above a column) within the same film type are not significantly difference (p > 0.05).

slightly lower than values for rice starch films in previous reports (4.64 × 10−14–8.67 × 10−14 kg m/Pa m2 s) by Bourtoom and Chinnan (2008) and Dias et al. (2010). The WVP of the CMCh film was 6.65 × 10−14 kg m/Pa m2 s, which is within the range of values reported by Duan et al. (2011) and Wu et al. (2013) (3.61 × 10−19–2.36 × 10−13 kg m/Pa m2 s). Differences in WVP for the same type of film may be due to different contents of material and plasticizer. The RS film had a lower WVP value compared with the CMCh film, which may be because of the more hydrophilic nature (COOe groups) of CMCh. The WVP of the RS–CMCh blend films ranged from 4.39 × 10−14 to 4.73 × 10−14 kg m/Pa m2 s, which was not significantly different from the WVP of the RS film (p > 0.05). WVP of a film is believed to depend on the number of available polar groups that the polymer contains (Li et al., 2008). In this work, hydrogen bonds could form between COOe groups of CMCh and OHe groups of the starch, reducing the number of available polar groups. Thus, the WVP values of the blend films were lower than that of the CMCh film. Ren et al. (2017) also reported that the intermolecular hydrogen bond formation between chitosan and corn starch could decrease the number of available hydrophilic groups.

3.9. Oxygen permeability Fig. 10a shows the oxygen permeability (O2P) values of RS and CMCh films and RS–CMCh blend films. The O2P values of all films were not significant different (p > 0.05) at 50% RH, and ranged from 4.82 × 10−19 to 5.74 × 10−19 kg m/Pa m2 s. The values indicated that these films had moderate oxygen barrier properties. Krochta and De-Mulder (1997) defined the oxygen barrier level of biodegradable and edible films into three groups: good (O2P = 0.15 × 10−19–1.51 × 10−19 kg m/Pa m2 s), moderate (O2P = 1.51 × 10−19–15.1 × 10−19 kg m/Pa m2 s) and poor (O2P = 15.1 × 10−19–151 × 10−19 kg m/Pa m2 s) at around 25 °C, 0–50% RH. Compared with other biopolymer films, the RS–CMCh blend films had lower O2P than methyl cellulose films (30.2 × 10−19 kg m/Pa m2 s, at 25 °C, 52% RH), and comparable O2P to whey protein isolate films (2.81 × 10−19–11.57 × 10−19 kg m/Pa m2 s, at 23 °C, 50% RH) (Janjarasskul and Krochta, 2010). Compared with synthetic polymers, the RS–CMCh blend films had higher O2P than ethylene vinyl alcohol copolymer (0.02 × 10−19 kg m/Pa m2 s, at 25 °C, 50% RH) but much lower O2P than high-density and low-density polyethylene (64 × 10−19 kg m/Pa m2 s and 281 × 10−19 kg m/Pa m2 s, at 23 °C, 0% RH, respectively) (Almenar and Auras, 2010). In this study, the addition of CMCh to RS did not affect the O2P of the blend films, which may be attributed to the high miscibility between RS and CMCh in the blend film structures. Similarly, Samsudin et al. (2014) reported no significant difference between the O2P of poly (lactic acid) film and poly(lactic acid) film incoporated with marigold flower extract. Fig. 10b also shows the effect of relative humidity on the oxygen barrier properties of the RS, CMCh and RS/50CMCh films. Each film was a good oxygen barrier at 0% RH, indicating good oxygen barriers properties at low RH conditions. Higher RH was responsible for deterioration in the oxygen barrier for all films. Water causes a classic plasticizing effect, resulting in an increase of O2P (Gaudin et al., 2000). The RH dependence of O2P was previously observed in carbohydrate-based film by Gaudin et al. (2000) and in protein-based film by Mujica-Paz and Gontard (1997). At 0% RH, the RS/50CMCh film (0.56 × 10−19 kg m/Pa m2 s) had higher O2P than the RS (0.35 × 10−19 kg m/Pa m2 s) and CMCh films (0.35 × 10−19 kg m/Pa m2 s), which may be due to the formation of new hydrogen bonds between the eOH of RS and the eCOOH of CMCh. At 90% RH, the CMCh film exhibited lower O2P than the other films. This finding may be due to the hydrophilic and swell-ability nature of CMCh, resulting in a homogenous swollen structure that may act as a barrier to the oxygen at very high RH.

3.11. Biodegradability Biodegradability of starch-based composites is often studied in simulation tests in soil or compost environments (Vázquez et al., 2011). In this work, RS film, CMCh film and RS/50CMCh film were selected as representative films to study the biodegradation of the blend film in simulated composting conditions. Biodegradation of the RS and CMCh films and RS/50CMCh blend film, as measured by the DMR system with an incubation period of 87 days, is shown in Fig. 11. Cellulose was used as a positive control. Fig. 11a shows the mean amount of cumulative CO2 of each sample as a function of time. At the end of the test, the mean CO2 evolution of the RS film was the highest among all the films, even higher than with cellulose, while the CMCh film had the lowest evolution. However, the CMCh film emitted more CO2 than the compost alone (blank). The RS film exhibited a priming effect. Castro-Aguirre et al. (2017) explained that the priming effect is the over-degradation of the indigenous organic carbon present in the compost when testing samples like glucose and its polymers. Fig. 11b represents the percentage of mineralization of the film samples during the biodegradation test. After the test period of 87 days, the RS film can be defined as biodegradable since the% mineralization was greater than 90% and higher compared with the positive control, based on ASTM-D6400-12 (2012). The RS film had the most rapid and greatest degradation while the CMCh film had the slowest and least degradation over time. The greater biodegradability of the RS film compared with the CMCh film may be attributed to lower molecular weight and more OHe groups, so the RS film was easily

3.10. Water vapor permeability Fig. 10c shows the WVP values of RS and CMCh films and RS–CMCh blend films. The WVP of the RS film (4.16 × 10−14 kg m/Pa m2 s) was 46

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oxygen and water vapor permeability. The optimum CMCh content for the RS–CMCh blend film to provide strong, flexible and durable film was 50% w/w of CMCh; the RS/50CMCh film had the highest tensile strength and elongation at break, and lowest final weight loss by thermal degradation. At 50% w/w CMCh the biodegradation was delayed compared with the RS film, suggesting that it would be possible to tailor the environmental degradation of the blend films. The results indicated that all of the RS–CMCH blend films possess potential as biodegradable edible films and can replace the use of pure RS film. Conflict of interest The authors express no conflict of interest and no competing financial interest. Acknowledgments This work was supported by the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0063/2555) and the Graduate School and Faculty of Agro-Industry, Chiang Mai University. We wish to thank the Center of Excellence in Materials Science and Technology for financial support under the administration of the Materials Science Research Center, Faculty of Science, Chiang Mai University. References Al-Hassan, A.A., Norziah, M.H., 2012. Starch–gelatin edible films: water vapor permeability and mechanical properties as affected by plasticizers. Food Hydrocolloid 26, 108–117. Almenar, E., Auras, R., 2010. Permeation, Sorption, and Diffusion in Poly(lactic Acid), Poly(Lactic Acid). John Wiley & Sons, Inc, New York, USA, pp. 155–179. ASTM-D3985-05, 2005. Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor. ASTM International, West Conshohocken, PA. ASTM-D882-12, 2012a. Standard Test Method for Tensile Properties of Thin Plastic Sheeting. ASTM International, West Conshohocken, PA. ASTM-D6400-12, 2012b. Standard Specification for Labeling of Plastics Designed to Be Aerobically Composted in Municipal or Industrial Facilities. ASTM International, West Conshohocken, PA. ASTM-D5338-15, 2015. Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials Under Controlled Composting Conditions, Incorporating Thermophilic Temperatures. ASTM International, West Conshohocken, PA. ASTM-E96/E9M-16, 2016. Standard Test Methods for Water Vapor Transmission of Materials. ASTM International, West Conshohocken, PA. Bourtoom, T., Chinnan, M.S., 2008. Preparation and properties of rice starch-chitosan blend biodegradable film. LWT–Food Sci. Technol. 41, 1633–1641. Bukzem, A.L., Signini, R., dos Santos, D.M., Lião, L.M., Ascheri, D.P.R., 2016. Optimization of carboxymethyl chitosan synthesis using response surface methodology and desirability function. Int. J. Biol. Macromol. 85, 615–624. Carolan, C.A., Blair, H.S., Allen, S.J., McKay, G., 1991. N,o-carboxymethyl chitosan, a water soluble derivative and potential ‘green’ food preservative. Chem. Eng. Res. Des. 69, 195–196. Castro-Aguirre, E., Auras, R., Selke, S., Rubino, M., Marsh, T., 2017. Insights on the aerobic biodegradation of polymers by analysis of evolved carbon dioxide in simulated composting conditions. Polym. Degrad. Stab. 137, 251–271. Chen, L., Du, Y., Zeng, X., 2003. Relationships between the molecular structure and moisture-absorption and moisture-retention abilities of carboxymethyl chitosan. Carbohydr. Res. 338, 333–340. Chen, S.-C., Wu, Y.-C., Mi, F.-L., Lin, Y.-H., Yu, L.-C., Sung, H.-W., 2004. A novel pHsensitive hydrogel composed of N,O-carboxymethyl chitosan and alginate crosslinked by genipin for protein drug delivery. J. Control. Release 96, 285–300. Cuero, R.G., Osuji, G., Washington, A., 1991. N-carboxymethylchitosan inhibition of aflatoxin production: role of zinc. Biotechnol. Lett. 13, 441–444. Dau, H.A., Rachtanapun, P., Dumri, K., 2016. Fabrication of berberine modifying bentonite/carboxymethyl chitosan film as an absorbent to remove organophosphate insecticides from contaminated water. In: ICMMT 2016. Chiang Mai, Thailand. Dias, A.B., Muller, C.M.O., Larotonda, F.D.S., Laurindo, J.B., 2010. Biodegradable films based on rice starch and rice flour. J. Cereal Sci. 51, 213–219. Duan, B., Sun, P., Wang, X., Yang, C., 2011. Preparation and properties of starch nanocrystals/carboxymethyl chitosan nanocomposite films. Starch – Stärke 63, 528–535. Fan, L., Du, Y., Zhang, B., Yang, J., Zhou, J., Kennedy, J.F., 2006. Preparation and properties of alginate/carboxymethyl chitosan blend fibers. Carbohyd. Polym. 65, 447–452. Feng, Q.L., Wu, J., Chen, G.Q., Cui, F.Z., Kim, T.N., Kim, J.O., 2000. A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus. J. Biomed. Mater. Res. 52, 662–668.

Fig. 11. Biodegradation of films as quantified by: (a) Amount of evolved CO2 of RS and CMCh films, RS/50CMCh film, cellulose and compost alone (blank); (b) Percentage of mineralization of RS and CMCh films, RS/50CMCh film and cellulose at 58 ± 2 °C and ∼50% ± 10% RH in yard waste compost.

assimilated by the compost microcosmos. Chitosan itself has antimicrobial properties due to its eNH2 groups (Sabaa et al., 2015; Tantala et al., 2012b). The antimicrobial activity of CMCh has been reported, with positive results against gram-positive and gram-negative bacteria and also against fungi (Sabaa et al., 2015; Tantala et al., 2012b). The most acceptable mechanism of antimicrobial activity is the interaction between the positive charge of a material and negative charge of the microbial cell membrane (Feng et al., 2000). Another possible mechanism of antimicrobial activity is the chelation of metals, suppression of spore elements and binding of nutrients essential for microbial growth (Sabaa et al., 2015). The eCOOH groups of CMCh have excellent metal-binding capacity which explains their antibacterial abilities (Cuero et al., 1991). Therefore, the antimicrobial activity of CMCh film in this study may be attributed to (1) its eNH3+ groups converted from eNH2 reacting with its own eCOOH groups (Sabaa et al., 2015) and (2) its eCOOH groups. We observed a lower degradation of the CMCh film and RS/50CMCh blend film compared with the RS film. The biodegradation rate and extent of RS/50CMCh blend film was lower due to the presence of CMCh. Perotti et al. (2017) demonstrated that the addition of modified-clay and chitosan reduced the biodegradation of cassava starch films. Thus, the biodegradation of the blend films may be attributed to the biodegradability of their parent polymers. 4. Conclusions This work emphasized the advantages of the addition of CMCh into RS-based film. The RS-based blend films that incorporated various CMCh contents had better mechanical properties, transparency and thermal stability than the RS film, while the CMCh did not affect 47

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