ZnO-Ag NPs and their effect on the properties of carboxymethyl cellulose-based nanocomposite film

Accepted Manuscript Title: Preparation of multifunctional chitin nanowhiskers/ZnO-Ag NPs and their effect on the properties of carboxymethyl cellulose-based nanocomposite film Authors: Ahmed A. Oun, Jong-Whan Rhim PII: DOI: Reference:

S0144-8617(17)30431-9 http://dx.doi.org/doi:10.1016/j.carbpol.2017.04.042 CARP 12230

To appear in: Received date: Revised date: Accepted date:

16-1-2017 14-4-2017 18-4-2017

Please cite this article as: Oun, Ahmed A., & Rhim, Jong-Whan., Preparation of multifunctional chitin nanowhiskers/ZnO-Ag NPs and their effect on the properties of carboxymethyl cellulose-based nanocomposite film.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.04.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation of multifunctional chitin nanowhiskers/ZnO-Ag NPs and their effect on the properties of carboxymethyl cellulose-based nanocomposite film

Running title: CMC-based multifunctional bio-nanocomposite films

Ahmed A. Oun1,2, Jong-Whan Rhim3*

1

Department of Food Engineering and Bionanocomposite Research Institute, Mokpo National

University, 61 Dorimri, Chungkyemyon, Muangun, 534-729 Jeonnam, Republic of Korea 2

Food Engineering and Packaging Department, Food Technology Research Institute,

Agricultural Research Center, Giza, Egypt 3

Department of Food Science and Human Nutrition, KyungHee University, 26 Kyungheedae-ro,

Dongdaemun-gu, Seoul 120-701, Republic of Korea

* Corresponding author Phone: +82-61-450-2423 Fax: +82-61-454-1521 E-mail: [email protected]

Highlights 

Hybrid ZnO-Ag nanoparticles were prepared.



The ZnO-Ag NPs were formed on the surface of the chitin nanowhiskers (ChNW).



The hybrid NPs were used for the preparation of CMC-based multifunctional nanocomposite films.



Tensile strength, elastic modulus, water vapor barrier, and thermal stability properties of the nanocomposite films increased significantly.



The nanocomposite films exhibited strong antibacterial activity against both Gram+ and Gram- bacteria.

Abstract Chitin nanowhiskers (ChNW) were isolated and used for the synthesis of hybrid ChNW/ZnO-Ag NPs. The hybrid nanoparticles were used for the preparation of multifunctional carboxymethyl cellulose (CMC) films. A ChNW was needle shape with the width of 8-40 nm, the length of 150260 nm, and crystallinity index of 93.6%. The ZnO-Ag NPs were spherical with the diameter of 10.5-16.2 nm. STEM, EDX, XRD, and UV-vis analyses confirmed the formation of ZnO-Ag NPs on the surface of ChNW. The thermal stability of ChNW was increased by incorporation of ZnO-Ag NPs. A CMC-based nanocomposite film incorporated with 5 wt% of ChNW/ZnO-Ag NPs was homogeneous and showed the high UV-barrier property. The tensile strength (TS) and elastic modulus (E) of the composite film increased by 18-32% and 55-100%, respectively, while the elongation at break (EB) decreased by 23-33%. CMC composite films showed strong antibacterial activity against E. coli and L. monocytogenes.

Keywords: Chitin nanowhiskers; ZnO-Ag nanoparticles; Hybrid nanoparticles; Carboxymethyl cellulose; Multifunctional nanocomposite; Antimicrobial activity

1. Introduction Growing concerns about environmental pollution and exhaustion of natural resources caused by the petroleum-based plastic packaging materials accelerated the development of ecofriendly packaging materials using biodegradable materials (Reddy, Vivekanandhan, Misra, Bhatia, & Mohanty, 2013). Several biopolymers such as polysaccharides (cellulose, chitin, starch and others), proteins (casein, whey protein, collagen, soybean protein, corn protein, wheat gluten), and lipids have been exploited to develop eco-friendly food packaging materials (Rhim, Park, & Ha, 2013). As one of such biopolymers, carboxymethyl cellulose (CMC) has been widely used to produce eco-friendly food packaging films (Oun & Rhim, 2015b). CMC has been used in the food processing, cosmetics, and biomedical industries as a thickener, edible coating, and film-former, a carrier for functional materials, an emulsion stabilizer, a hydrogel for wound dressing, a drug delivery system (Biswal & Singh, 2004; Oun & Rhim, 2015b). However, the use of CMC-based packaging films has been limited because of their poor mechanical, water vapor barrier, and heat stability properties. Several nano-sized reinforcing fillers such as nanoclay (Almasi, Ghanbarzadeh, & Entezami, 2010), metallic nanoparticles (Kanmani & Rhim, 2014), cellulose nanocrystals (Oun & Rhim, 2015b, 2016), and chitin nanofibers (Li et al., 2016) have been used to enhance the film properties through the formation of nanocomposites. Nanocomposites exhibit increased gas barrier properties and mechanical strength and improved modulus, dimensional stability, and heat resistance compared to their neat polymer films (Oun & Rhim, 2016). The properties of nanocomposites are strongly dependent upon the type of nano-fillers, their size, and shape, as well as their interfacial characteristics (Xu et al., 2013a). Also, homogeneous dispersion of nano-fillers in polymer matrices is a key factor affecting the

functional properties of the polymer nanocomposites. However, it has frequently been observed that the incorporation of the low volume fraction of a single type of nano-filler does not improve the properties enough for industrial applications of films (Azizi et al., 2014a). To solve this problem and to take full advantage of nanocomposite technology, mixtures of various nanofillers have been used for the preparation of multifunctional nanocomposites with highperformance properties (Azizi et al., 2014b). Multifunctional nanocomposites prepared with two or more nano-fillers to combine the merits of the individual components have received attention due to their synergistic effects with enhanced properties (Sahay, Reddy, & Ramakrishna, 2014). Among the hybrid nano-fillers, ZnO-Ag nanoparticles have received considerable attention since the functional properties of ZnO NPs such as photocatalytic and antimicrobial activities can be improved by doping with silver (Aladpoosh & Montazer, 2016). ZnO-Ag nanoparticles have been synthesized by various techniques such as microwave irradiation (Roberson, Rangari, Jeelani, Samuel, & Yates, 2014) and electrospinning (Lin, Wu, Zhang, & Pan, 2009). Also, Azizi et al. (2016) synthesized ZnO-Ag NPs by an eco-friendly method using essential oil of ginger. Cellulose nanocrystals have been used to prevent aggregation and improve the dispersion of ZnO-Ag nanoparticles (Azizi, Ahmad, Hussein, & Ibrahim, 2013; Azizi et al., 2014b). Cellulose nanocrystals/ZnO and cellulose nanocrystals/ZnO-Ag nanoparticles have been used as nanofillers to improve the mechanical properties and provide the antimicrobial function of poly(vinyl alcohol)/chitosan blend films (Azizi et al., 2014a, 2014b). Also, silver-copper alloy and ZnO nanoparticles have been incorporated into plasticized poly(lactic acid) (PLA) matrix (Ahmed, Arfat, Castro-Aguirre, & Auras, 2016). Chitin nanowhiskers are used for the same purposes as cellulose nanocrystals to prepare for the multifunctional nanocomposites, but chitin nanowhiskers have more functional groups

than cellulose nanocrystals (Ifuku, 2014). Since chitin is renewable, environmentally-friendly, biocompatible, biodegradable, and biofunctional, it has been used as a chelating agent, water treatment additive, drug carrier, wound-healing agent, to prepare biodegradable pressuresensitive adhesive tape and membranes, and other applications (Zargar, Asghari, & Dashti, 2015). The advantages of isolated chitin nanowhiskers such as nontoxicity, low density, biodegradability, and easy surface modification, are expected to be used in wide application areas such as nanocomposites, food packaging, cosmetics, drug delivery and tissue engineering (Zeng, He, Li, & Wang, 2012). Several methods like acid hydrolysis, TEMPO-mediated oxidation, and mechanical methods have been employed to obtain chitin nanocrystals (ChNC) and chitin nanofibers (ChNF) (Ifuku, 2014; Salaberria, Fernandes, Diaz, & Labidi, 2015). The ChNC and ChNF have been used as nano-reinforcing fillers in various polymers such as thermoplastic starch (Salaberria, Labidi, & Fernandes, 2014), chitosan (Ifuku et al., 2013; Li et al., 2011), poly(vinyl alcohol) (Junkasem, Rujiravanit, & Supaphol, 2006), CMC (Li et al., 2016), and PLA (Herrera et al., 2016). However, only a few works have been published on the isolation of nanocrystals from chitin and their application for the preparation of multifunctional hybrid nanomaterials. In the present study, chitin nano-whisker (ChNW) was used as a stabilizing agent to prevent aggregation of ZnO-Ag nanoparticles and improve their dispersion in the CMC polymer matrix to prepare multifunctional bio-nanocomposite films. The effect of the hybrid nanofiller of ChNW/ZnO-AgNPs on the film performance and antimicrobial properties of CMC-based bionanocomposite films were evaluated.

2. Materials and methods 2.1. Materials Sodium carboxymethyl cellulose (CMC) (average Mw: 250,000, the degree of substitution: 0.9), and zinc acetate dihydrate (Zn(CH3COO)2·2H2O) were purchased from Junsei Chemical Co., Ltd. (Tokyo, Japan). Silver nitrate (AgNO3), glycerol, and hydrochloric acid (HCl) were obtained from Daejung Chemicals & Metals Co., Ltd. (Siheung, Gyeonggi-do, Korea). Chitin (viscosity, 0.5% ascorbic acid at 20°C: 800-940 cPs; ash < 5%; particle size: 3 mm) obtained from crab shell was procured from YB Bio Co., Ltd. (Youngduk, Kyungbuk, Korea). Escherichia coli O157: H7 (ATCC 43895) and Listeria monocytogenes (ATCC 15313) were obtained from the Korean Collection for Type Culture (KCTC, Seoul, Korea). Both bacterial strains were grown on BHI and TSB agar plates and stored at 4 oC for further test.

2.2. Isolation of chitin nanowhiskers Chitin nanowhiskers (ChNW) were isolated from chitin using acid hydrolysis. For this, 5 g of finely ground chitin powder was hydrolyzed using 150 mL of 3 M HCl at 100 °C for 90 min with strong agitation. The reaction was quenched by adding an excess of distilled water and centrifuged at 4000 rpm for 20 min using a bench-top centrifuge (Hanil Scientific Centrifuge, Incheon, Gyeonggi-do, Korea). The addition of distilled water and centrifugation was repeated until the pH of the suspension was above 5. The pellet was re-suspended in distilled water and dialyzed against water (cellulose membrane tube with a molecular cut-off of 14,000, SigmaAldrich, St. Louis, MO, USA) for 5 days. After that, the suspension was filtered through Whatman 541 filter paper to remove large particles, and stored in a refrigerator at 4 °C until use.

2.3. Preparation of ChNW/ZnO-Ag NPs. ChNW/ZnO-Ag NPs were prepared following the method reported by Azizi et al. (2013) with a minor modification. Zinc acetate dihydrate (0.25 g) was dissolved in 10 mL of ethanol. This solution was added (while stirring) to 100 mL of a suspension containing different amount of ChNW (0.75, 1.0, and 1.5 g). The resulting mixture was heated to 80 °C. Then, 1 M sodium hydroxide solution was added dropwise to the above mixture until the pH reached 10. After the mixture had become white (indicating the formation of ZnO NPs), different amount of silver nitrate (0.125, 0.25, and 0.5 g) dissolved in 2 mL water were added dropwise into suspensions containing 0.75, 1.0, and 1.5 g of ChNW, respectively. The suspensions were heated continuously at 80 °C for 2 h and washed with distilled water for several times by centrifugation (4000 rpm/20 min) until the pH became 7-8. The insoluble part was collected and dried at 100 °C for 1 h to obtain hybrid nanoparticles. The weight ratio of zinc acetate dihydrate to silver nitrate was 1:0.5, 1:1, and 1:2 w/w, respectively, so the hybrid nanoparticles were designated as ChNW/ZnO1-Ag0.5, ChNW/ZnO1-Ag1, ChNW/ZnO1-Ag2, respectively. The probable chemical processes to form ChNW/ZnO-Ag NPs are as follows (Azizi et al., 2013; Conde et al., 2011): Zn2+---ChNW + 2OH−  Zn(OH)2 ---ChNW

(1)

∆ Zn(OH)2 ---ChNW → ZnO---ChNW + H2O

(2)

2ChNW---ZnO + 2Ag+ + 2(OH)− → 2 ChNW---ZnO---Ag° + H2O + 1/2O2

(3)

where (2OH−) represent of sodium hydroxide.

2.4. Preparation of CMC/ChNW/ZnO-Ag nanocomposites films.

Neat CMC and CMC nanocomposite films were prepared using a solution casting method as described earlier (Oun & Rhim, 2015b). The CMC film dispersion was prepared by dissolving 4 g (2%, w/v) of CMC and 1.2 g of glycerol as a plasticizer (30 wt% of CMC) into 200 mL distilled water with stirring and heating to 90 °C for about 20 min to get a clear dispersion. The CMC film dispersion was cast onto a leveled glass plate (24 cm × 30 cm) coated with Teflon film (Cole-Parmer Instrument Co., Chicago, IL, USA) and dried at room temperature for 2 days. For the preparation of nanocomposite film suspension, 0.2 g of ChNW or ChNW/ZnO-Ag (5 wt% of CMC) was dispersed into 200 mL of distilled water with stirring for 2 h at room temperature. And the suspension was homogenized using a high shear mixer (T25 basic, Ika Labotechnik, Janke & Kunkel Gmbh & Co., KG Staufen, Germany) at 10,000 rpm for 10 min, followed by sonication for 20 min using a bath-type ultrasonicator (FS 140H, Ultrasonic Cleaner, Fisher Scientific, Pittsburg, PA, USA). Then, 1.2 g of glycerol and 4 g of CMC were dissolved into the suspension with stirring and heating at 90 °C for about 20 min and followed the same procedure as the preparation of the neat CMC film. All dried films (neat and nanocomposite) were peeled off and kept in a humidity chamber (model FX 1077, Jeio Tech Co., Ltd., Ansan, Gyeonggi-do, Korea) controlled at 25 °C/50% RH for 4 days before further characterization.

2.5. Characterization of ChNW and ChNW/ZnO-Ag NPs The microstructure of the ChNW and ChNW/ZnO-Ag NPs was observed using a fieldemission scanning electron microscope (FE-SEM, S-4800, Hitachi Co., Ltd., Matsuda, Japan) in the transmission mode of the scanning transmission electron microscope (STEM). The STEM samples were prepared by dropping about 8 μL of the sample suspension on a 300 mesh nickel grids and allowed to dry at room temperature. The length and diameter of samples were

determined using the internal scale of the instrument, and the average values of 50 measurements in three different points were presented. The presence of ZnO and Ag NPs were evaluated by energy dispersive X-ray spectroscopy (EDX) in the SEM instrument. Morphology of the surface and cross-sections of CMC-based films were examined using the same FE-SEM at an acceleration voltage of 10 kV and a current of 10 μA after coating the samples with osmium (Os) using a vacuum sputter coater. Fourier transform infrared spectroscopy spectra (FTIR) of the samples were obtained using an FTIR spectrophotometer (TENSOR 37 spectrophotometer with OPUS 6.0 software, Billerica, MA, USA) in the range of 4000-600 cm-1. X-ray diffraction patterns of chitin, ChNW, and ChNW/ZnO-Ag NPs were analyzed using an XRD diffractometer (PANalytical Xpert Pro MRD diffractometer, Amsterdam, Netherlands). Sample powder was placed on a glass slide, and the diffraction pattern was obtained at diffraction angles between 2θ = 5° - 50° for the chitin and ChNW samples, and 2θ = 30° - 80° for the ChNW/ZnO-Ag samples with a scanning speed of 0.4°/min at room temperature. Crystallinity index (CI) of the samples was calculated using the following equation (Park, Baker, Himmel, Parilla, & Johnson, 2010): 𝐂𝐈 =

𝐈𝟏𝟏𝟎 – 𝐈𝒂𝒎 𝐈𝟏𝟏𝟎

× 𝟏𝟎𝟎

(𝟒)

where I110 was the maximum intensity of the diffraction peak (110) at 2θ = 19.3° for the chitin and ChNW, and Iam was the minimum intensity corresponding to the amorphous structure. Crystallite size (D) of the samples was calculated using the Scherrer equation (Nazir, Wahjoedi, Yussof, & Abdullah, 2013). 𝐃=𝛃

𝐊𝛌

𝟏/𝟐

𝐂𝐨𝐬𝜽

(𝟓)

where K was a constant (0.94), λ was the X-ray wavelength (λ = 0.154056 nm), and β1/2 was the full width at the half maximum of the diffraction peak (FWHM). The FWHM values were determined at diffraction peaks at 19.3° for ChNW (110), at 36.3° for ZnO (101), and at 38.15° for Ag (111) plains. The FWHM was adjusted by subtracting the amorphous peak (Iam) from the maximum intensity peak. Thermal stability of the samples was analyzed using a thermogravimetric analyzer (TGA; Hi-Res TGA 2950, TA Instrument, New Castle, DE, USA). About 10 mg of each film sample or nanoparticle powder was placed in a standard aluminum pan and heated from room temperature to 600 °C with a heating rate of 10 °C/min under a nitrogen flow of 50 cm3/min; an empty pan was used as a reference. The DTG curve was obtained using differentials of TGA values, which were calculated using a central finite different method as follows (Oun & Rhim, 2015a): DTG = (wt+Δt - wt-Δt) /2Δt

(6)

where wt+Δt and wt-Δt were the residual weights of samples at time t+Δt and t-Δt, respectively, and Δt was the time interval for reading residual sample weight. The onset, end, and mid-point (T0.5) of the decomposition temperature of samples were determined from the DTG curves, and the weight loss (%) and char content (terminal residue at 600 °C) were determined from the TGA curves.

2.6. Optical properties Light absorption spectra of ChNW/ZnO-Ag NPs suspensions (0.05%) was determined using a UV-vis spectrophotometer (Model 8451A, Hewlett-Packard Co., Santa Alara, CA, USA) at the wavelength of 200-800 nm using distilled water as a blank. Light absorption and

transmittance of CMC/ChNW/ZnO-Ag NPs composite films were examined using the same spectrophotometer at the wavelength of 200-800 nm. Film sample (7 cm × 7 cm) was directly located between spectrophotometer magnetic cells and measured using air as a blank.

2.7. Mechanical properties of films Mechanical properties of the films such as tensile strength (TS), elongation at break (E), and elastic modulus (EM) were evaluated using an Instron Universal Testing Machine (Model 5565, Instron Engineering Corporation, Canton, MA, USA) according to ASTM Method D 88288. Film samples were cut into rectangular strips (2.54 cm × 15 cm) and the thickness was measured using a digital micrometer (Digimatic micrometer, Quantumike IP65, Mitutoyo, Japan) with an accuracy of 0.001 mm. The mechanical properties were measured by operating the machine in the tensile mode with an initial grip separation of 50 mm and a crosshead speed of 50 mm/min (Oun & Rhim, 2016). The TS was determined by dividing the maximum load (N) by the initial cross-sectional area (m2) of the film, the E was determined by dividing the extension at the rupture of the film by the initial length of the film (50 mm) multiplied by 100, and the EM was determined from the slope of the initial portion of the stress-strain curve, which corresponds to the stress divided by the strain of the film. Fifteen specimens were tested for each film sample, and the measured values were averaged.

2.8. Water vapor permeability (WVP) of films Water vapor permeability of films was determined following the standard method of ASTM E96-95 with modification (Gennadios, Weller, & Gooding, 1994). The WVP cups made of poly(methyl methacrylate, 2.5 cm in depth, and 6.8 cm in diameter) containing 18 mL of

distilled water were covered with film samples (7 cm × 7 cm) and sealed to prevent leakage of water vapor. The assembled cups were weighed and placed in a humidity chamber (model FX 1077, Jeio Tech Co., Ltd. Ansan, Korea) controlled at 25◦C and 50% RH with air movement of 198 m/min for 8 h. At every 1 h, weight loss from each cup was measured. The water vapor transmission rate (WVTR; g/m2s) of the film was determined from the slope of the plot of weight change of the cup vs. time. Then the WVP (g.m/m2.s.Pa) was calculated as follows: 𝑾𝑽𝑷 = (𝐖𝐕𝐓𝐑 × 𝐋)/∆𝐩

(7)

where L is the thickness of the film (m), and ∆p is partial water vapor pressure difference (Pa) across the film.

2.9. Water contact angle (CA) of films Water contact angle of film samples was determined using a CA analyzer (model Phoenix 150, Surface Electro-Optics Co., Ltd., Kunpo, Gyeonggi-do, Korea). Film samples were cut into rectangular pieces (3 cm × 10 cm), which were fixed on the horizontal movable stage of the CA analyzer. A drop of water (about 10 μL) was placed on the surface of the film using a micro-syringe. The CA of samples was determined by averaging 15 measurements for each film sample.

2.10. Antimicrobial activity of films Antibacterial activity of the CMC-based films against food-borne pathogenic bacteria, L. monocytogenes and E. coli, was evaluated using the viable cell colony count method (Shankar, Wang, & Rhim, 2016). A single colony of test organism (L. monocytogenes or E. coli) that had been grown overnight on an agar plate was aseptically inoculated with 20 mL of BHI (Brain

Heart Infusion) and TSB (Tryptic Soy Broth) broth, respectively, and the plates were incubated at 37 °C for 16 h with mild shaking at 150 rpm. The bacterial cells were pelleted by centrifugation at 2000 rpm and subsequently suspended in 100 mL of sterile TSB and BHI broth, respectively. The broths were diluted 10 times with sterile distilled water, and 50 mL of diluted broth (106-107 colony-forming units/mL bacteria) was incubated with 500 mg of the film at 37 °C for 12 h under mild shaking. The same diluted broth without film sample was used as the control. The samples (100 μL) were taken out at 3 h interval, serially diluted using a saline solution and plated on BHI and TSB agar plates. The plates were incubated at 37 °C for 16-18 h, and the cell viability was calculated by counting bacterial colonies on the plates. All experiments were carried out in triplicate with individually prepared films.

2.11. Statistical analysis The measurements of each property of the films were performed in triplicate with individually prepared film samples. The mean values and standard deviations were reported. One-way analysis of variance (ANOVA) was performed, and the significance was determined (p < 0.05) with Duncan’s multiple range test using the SPSS statistical analysis computer program for Windows (ver. 12.0, SPSS Inc., Chicago, IL, USA).

3. Results and discussion

3.1. Morphology properties of ChNW and ChNW/ZnO-Ag NPs ChNW isolated from the chitin (pale red flakes of crab shell) by acid hydrolysis formed a homogeneous milky dispersion. Fig. 1 shows the morphology and size of the chitin, ChNW, and

ChNW/ZnO-Ag NPs observed by STEM. The ChNW exhibited needle-like crystals without entanglement as shown in the STEM images. Width (w) and length (L) of the ChNW ranged from 8 to 40 nm and 150 to 260 nm, respectively, with an average aspect ratio (L/W) of about 8.8. Reportedly, the aspect ratio of ChNW isolated by acid (HCl) hydrolysis varied from about 5 (Salaberria, Labidi, & Fernandes, 2014) to 16 (Nair & Dufresne, 2003), while that of chitin nanocrystals isolated from crab shell by the TEMPO-mediated oxidation was about 42.5 (Fan, Saito, & Isogai, 2008). Li et al. (2016) isolated chitin nanofiber (ChNF) from a speckled swimming crab shell using a mechanical disintegration method (wet grinding and high-pressure homogenization) with an average aspect ratio of 83.6. Whereas the average aspect ratio of ChNF isolated from yellow lobster wastes using a dynamic high-pressure homogenization method was about 60 (Salaberria, Labidi, & Fernandes, 2014). It can conclude that ChNW isolated by acid hydrolysis method showed a rod-like shape with low aspect ratio and formed homogeneously dispersed suspension while ChNF isolated by mechanical methods exhibited a web-like structure with high aspect ratio and highly entangled nano-size fibrils. For this, size and distribution of nanochitin are dependent on several factors such as the source of chitin, isolation method, and isolation conditions (e.g. acid concentration, chitin/acid ratio, hydrolysis temperature and time). The isolated ChNW was used as a stabilizer for the preparation of hybrid nanoparticles, ZnO-Ag NPs. The presence of ChNW during synthesis of ZnO-Ag NPs might act as the nucleation controller as well as the particle stabilizer (Lokanathan, Uddin, Rojas, & Laine, 2014). As shown in STEM images, ZnO-Ag NPs were closely attached on the surface of ChNW. The average particle size of ChNW/ZnO1-Ag0.5, ChNW/ZnO1-Ag1, and ChNW/ZnO1-Ag2 nanoparticles was about 10.5±3.6 nm, 14.7±5.1 nm, and 16.2±5.0 nm, respectively. The particle size was slightly increased with the increase of Ag NPs, presumably due to the formation of Ag

NPs on the surface of ZnO NPs. In the present study, ZnO-Ag NPs prepared by treatment with NaOH produced spherical nanoparticles with a size of 10.5-16.2 nm. The shape and size of nanoparticles were influenced by the preparation method (chemical or physical). Roberson et al. (2014) reported that ZnO-Ag NPs prepared by a microwave irradiation method formed ZnO NPs in a rod shape with 10 nm in width and 50-200 nm in length, and Ag NPs in a spherical shape with the size of 5-25 nm. On the other hand, ZnO-Ag NPs prepared by a chemical method were spherical in shape with size varied from 8.8 nm to 34.7 nm (Azizi, Ahmad, Hussein, & Ibrahim, 2013). The EDX analysis of ChNW/ZnO-Ag NPs samples showed clear peaks for both ZnO NPs and Ag NPs, which confirmed the successful combination of the Ag NPs with ZnO NPs on the surface of ChNW (Data not shown). It is worth noting that the intensity of ZnO peaks decreased with increase the amount of silver. Change in the peaks intensity of ZnO-Ag NPs might depend on the coverage percentage of ZnO NPs by the Ag NPs. 3.2. FTIR and XRD analysis of ChNW and ChNW/ZnO-Ag NPs The change in the chemical structure of chitin nanowhiskers (ChNW) with ZnO-Ag NPs at different ratios are shown in Fig. 2a. The characteristic absorption peaks of ChNW such as OH stretching band at 3437 cm-1, NH stretching bands at 3257 and 3101cm-1, CH2 and CH3 stretching vibrations band at 2891 cm-1, amide I bands at 1654 and 1620 cm-1, amide II band at 1554 cm-1, C-H stretch of methyl groups at 1378 and 1310 cm-1, and amide III of the acetyl group band at 1260 cm-1, were clearly observed (Li et al., 2016; Shankar, Reddy, Rhim, & Kim, 2015). The peak at 1155 cm-1 corresponds to the glycosidic linkage (C-O-C bridge stretching). The strong peaks at 1010-1070 cm-1 are related to the saccharide structure of carbohydrate backbone. The peak at 897 cm-1 is attributed to the β-linkage of a glucopyranose of chitin

(Muzzarelli et al., 2007). The FTIR result of ChNW/ZnO-Ag samples showed decreased peak intensity with slight shifts in the position compared with the ChNW. The band intensity of carboxyl (−C=O), hydroxyl (−OH), and amine (N-H) groups significantly decreased, which was attributed to strong interactions between such groups with ZnO and Ag NPs (Yu, Chen, Wang, & Yao, 2015). The change in the position and intensity of ChNW peaks were influenced by ZnO-Ag NPs ratio. For example, the intensity of ChNW peak at 1620 cm-1 (amide I) was significantly decreased and shifted to 1624 cm-1 by incorporation of ZnO1-Ag0.5 and ZnO1-Ag1. When the ratio of AgNPs increased (ZnO1-Ag2), the peak height increased more and shifted to 1622 cm-1. Similar results of changes in peak height and peak position have been observed in the hybrid nanomaterials such as nanocellulose/ZnO and nanochitin/Ag NPs (Lefatshea, Muivaa, & Kebaabetswe, 2017; Solairaj & Rameshthangam, 2016). On the other hand, the presence of hydroxyl and acetamide groups within the structure of ChNW might play a significant role in attachment of ZnO and Ag NPs on the surface of ChNW. Awwad, Salem, & Abdeen, (2013) suggested that the presence of carboxyl, hydroxyl, and amine groups within the chemical structure of carob leaf extract might participate in the reduction of Ag+ ions to Ag° nanoparticles.

X-ray diffraction was carried out to investigate the change in the crystal structure of chitin, ChNW, and ChNW/ZnO-Ag samples (Fig. 2b,c). Chitin and ChNW showed six diffraction peaks at 2θ = 9.4° (020), 12.6° (021), 19.3° (110), 20.6° (120), 23.3° (130), and 26.3° (013) (Fig. 2b), which corresponded to those in the typical crystal pattern of α-chitin (Duan, Chang, & Zhang, 2014). The crystallinity index (CI) and crystallite size (D) of the samples were calculated using Eq. 4 and Eq. 5, respectively. The CI and D of chitin increased from 81.8% and 4.76 nm to 93.6% and 6.3 nm of ChNW after acid hydrolysis, respectively. The increase in the

CI and D of ChNW was due to the removal of amorphous regions from chitin during acid hydrolysis (Shankar et al., 2015). Fig. 2c shows the XRD diffraction patterns of ChNW/ZnO-Ag NPs samples with characteristic peaks of ZnO at 31.8 (#100), 34.5 (#002), 36.3 (#101), 47.5 (#102) 56.6 (#110), 62.8 (#103), and 67.9 (#112) planes (Azizi et al., 2013; Yu et al., 2015). Silver nanoparticle formation was evidenced by the peaks at 2θ values of 38.1 (*111), 44.2 (*200), 64.4 (*220), and 77.2 (*311) (Awwad, Salem, & Abdeen, 2013; Shankar, Wang, & Rhim, 2016). The diffraction peaks confirmed that both ZnO and Ag NPs were successfully synthesized and attached on the surface of ChNW. The crystallite sizes of ZnO NPs and Ag NPs were calculated using Eq. 5. The average particle size of ZnO NPs was about 22.40 nm, 16.80 nm, and 6.72 nm, while the crystallite size of Ag NPs was about 5.44 nm, 13.79 nm, and 14.65 nm for ChNW/ZnO1-Ag0.5, ChNW/ZnO1-Ag1, and ChNW/ZnO1-Ag2, respectively. The crystallite size of ZnO was reduced with an increase in the ratio of Ag. The intensity of peaks corresponding to ZnO became weaker and broader with an increase in the content of Ag. This was presumably due to the enclosure of ZnO NPs by Ag NPs, which inhibited the growth of crystal size of ZnO. Similar behavior was observed in ZnO-Ag NPs prepared with cellulose nanocrystals (Azizi et al., 2016).

3.3.Thermal stability analysis. Thermogravimetric analysis was performed to determine the effect of the ZnO-Ag NPs on the thermal stability of ChNW and the effect of ChNW/ZnO-Ag NPs on the thermal properties of CMC-based nanocomposite film; the results are shown in Fig. 3. The thermal decomposition of cellulose materials includes depolymerization, dehydration, and decomposition

of glycosyl units to leave a charred residue (Dahiya & Rana, 2004). As shown in Fig. 3a, the thermal degradation of ChNW and ChNW/ZnO-Ag NPs exhibited two main weight loss regions. The first weight loss was observed at a temperature range of 60-120 °C, which was due to evaporation of moisture and weakly attached bound water. The weight loss was about 7% for ChNW and 5% for the hybrid of ChNW/ZnO-Ag NPs. The second main weight loss was observed at 250-400 °C, which was attributed to degradation of the chitin structure (Shankar et al., 2015). The thermal parameters such as the onset temperature (Tonset), end temperature (Tend), mid-point of the degradation (T0.5), and the final weight left after thermal degradation at 600 °C (char at 600°C) are summarized in Table 1. ChNW and ChNW/ZnO-Ag showed the same starting degradation temperature (Tonset) at 252 °C, while the Tend and T0.5 were increased by incorporation of ZnO-Ag NPs. The T0.5 increased by 15 °C as the concentration of Ag increased (Azizi et al., 2013). The presence of ZnO-Ag NPs on the surface of ChNW might act as a thermal insulator and delay thermal decomposition (Azizi et al., 2014a; Yu et al., 2015). The final weight left after thermal degradation at 600 °C (char at 600 °C) increased with the incorporation of ZnO-Ag NPs and further increased with the increase in the concentration of Ag probably resulted from ZnO-Ag NPs ash content. A similar result of increased thermal stability and increased final residues has been observed in the hybrid nanoparticles composed of ZnO NPs, Ag NPs, and cellulose nanocrystals (CNC) (Azizi et al., 2013; Yang et al., 2016).

In the TGA thermograms of CMC/ChNW/ZnO-Ag nanocomposite films, two weight loss steps were also observed (Fig. 3b). The first weight loss was observed at 60-120 °C with a weight loss of 15%, 12.7%, 12.1%, 11.5, and 9.8% for CMC, CMC/ChNW, CMC/ChNW/ZnO1Ag0.5, CMC/ChNW/ZnO1-Ag1, and CMC/ChNW/ZnO1-Ag2, respectively. The weight loss at this

stage was mainly attributed to the evaporation of film moisture. The second weight loss was observed at 250-350 °C. The weight loss at this stage was due to the volatilization of glycerol, thermal degradation of carbohydrate polymers (Oun & Rhim, 2015b). The addition of ChNW and ChNW/ZnO-Ag NPs to the CMC film did not change Tonset and Tend temperatures. However, 1

the T0.5 increased by 12 °C compared with the neat CMC film when 5% of CMC/ChNW/ZnO 2

Ag was added. This result indicated that the Ag NPs gave higher thermal stability to the nanocomposite films than the ZnO NPs. Yu et al. (2014) reported that the thermal stability of PHBV films improved remarkably by the incorporation of cellulose nanocrystals (CNC)/Ag NPs and the increase in the thermal stability was dependent on the content of Ag NPs. Also, the incorporation of 1% of CNC/ZnO into poly(vinyl alcohol)/chitosan blend films increased the T0.5 of nanocomposite films, but it decreased with a further increase in the content of CNC/ZnO (Azizi et al., 2014a). The terminal residue left after thermal degradation increased with the incorporation of ChNW/ZnO-Ag.

3.4. Optical properties analysis The optical properties of samples were characterized using UV-vis absorption spectroscopy in the range of 200-800 nm (Fig. 4). As shown in Fig. 4a, the ChNW/ZnO-Ag NPs exhibited two absorption peak at 360 nm and 435 nm. The absorption peak at 360 nm was correlated with ZnO NPs, which shifted to 365 nm and the intensity was decreased with the increase in the ratio of Ag (Yu et al., 2015). The peak at 435 nm was associated with spherical Ag NPs (Shankar, Wang, & Rhim, 2016). It was clearly shown that the intensity of absorption peak decreased, became broader, and shifted to 425 nm with the increase in the ratio of Ag. Peak

shift of nanoparticles has been attributed to many factors such as capping ligands, particle size, surface oxidation, and the refractive index of the surrounding medium (Jiang & Hsieh, 2014). The broadening of peaks was possibly due to the growth of particle size resulted from the formation of Ag NPs on the surface of ZnO NPs or aggregation of nanoparticles as evidenced by the results of STEM and XRD. On the other hand, the intensity of absorption spectra at visible region (600 nm) was inversely proportional to the ratio of Ag NPs, which indicated an increase in particle size or aggregation of NPs capable of scattering of light (Lokanathan, Uddin, Rojas, & Laine, 2014). The light absorption properties of CMC/ChNW/ZnO-AgNPs nanocomposite film are shown in Fig. 4b. There were no distinct peaks in the region of 200-800 nm for the CMC and CMC/ChNW films. However, CMC/ChNW/ZnO-Ag had an absorption band at 430 nm, which was attributed to the presence of Ag NPs, and narrow peak at 360 nm due to the ZnO NPs. Compared to ChNW/ZnO-Ag samples, the weak of ZnO peak in the CMC nanocomposite film was possibly due to covering by Ag NPs. The transmittance of the CMC and CMC/ChNW/ZnO-AgNPs composite films are shown in Fig. 4c. The transmittance of the CMC film was significantly affected and particularly by incorporation of 5% of ChNW and ChNW/ZnO-Ag NPs. The transmittance at 660 nm (a measure of transparency) and 280 nm (a measure of UV barrier property) was decreased by incorporation of ChNW/ZnO-Ag NPs. Incorporation of ZnO-Ag NPs into CMC/ChNW film led to the absorption of about 94% of UV-light at 280 nm as compared to the neat CMC film and CMC/ChNW films, which indicates that the ZnO-Ag NPs have strong UV barrier properties. The effects of CHNW, ZnO, and Ag NPS on the UV-barrier, transparency, and surface color of different nanocomposite films have been reported (Kanmani & Rhim, 2014; Shankar et al.,

2016). Kanmani & Rhim (2014) reported that CMC/ZnO NPs nanocomposite film decreased about 90 % of UV light transmittance compared to the CMC film. Shankar et al. (2016) also found that alginate/silver composite film decreased UV-light transmittance by 52-90 % depending on the type of Ag NPs. 3.5. Morphology of nanocomposite films The microstructure of the neat CMC, CMC/ChNW, and CMC/ChNW/ZnO-Ag composite films are shown in Fig. 5. The neat CMC film had a smooth and homogeneous surface, and the surface of the CMC films was not affected by the incorporation of 5 wt% of ChNW and ChNW/ZnO-Ag NPs. This indicated that both ChNW and ChNW/ZnO-Ag NPs were well dispersed within the CMC matrix. The small dimensions of ChNW probably played a critical role in the good distribution without any agglomeration within the matrix. The cross-section of the neat CMC film was homogeneous without any cracks or agglomerations. No significant difference was observed after incorporation of ChNW and ChNW/ZnO-Ag NPs, indicating that the nanofillers were well dispersed through the polymer matrix with high compatibility between them. Salaberria et al. (2014) found that thermoplastic starch films containing chitin nanocrystals (ChNC) had a smooth, homogenous surface, while thermoplastic starch films containing chitin nanofibers (ChNF) had a web-like surface structure. However, Li et al., (2016) reported on the rough surface of CMC/ChNF composite films, which was mainly due to the agglomeration of ChNF.

3.6. Mechanical properties of films

The effect of ChNW and ChNW/ZnO-Ag NPs on the tensile properties of the CMCbased film were tested, and the results were summarized in Table 2. In this study, 5 wt% of ChNW was used with different ratios of ZnO-Ag NPs because the tensile strength of the CMCbased films increased by incorporation of up to 5 wt% of ChNW then declined when more than 5 wt% of ChNW was incorporated. The decreased tensile properties at high concentrations were due to the formation of voids and cracks in the polymer matrix caused by the agglomeration of the NPs (Chang, Jian, Yu, & Ma, 2010; Oun & Rhim, 2015b). The thickness of the films increased with the addition of ChNW/ZnO-Ag NPs and increased further with the increased content of Ag, which was mainly due to the increased solid content. The tensile strength (TS) and elastic modulus (E) of the neat CMC film were 40.2 and 719 MPa, respectively. They were increased significantly (p<0.05) after formation of nanocomposite with ChNW and they increased further when hybrid nanoparticles were incorporated. The strength (determined by TS) and stiffness (determined by E) of the hybrid NPs incorporated nanocomposite films increased 1

2

with increased content of AgNPs. The CMC/ChNW/ZnO -Ag film exhibited the highest TS (53.1 MPa) and E (1444 MPa) values, which were1.32 and 2 times higher than those of the neat CMC film, respectively. Increased TS and E of CMC/ChNW composite film was presumably resulted from the formation of hydrogen bonds between amide groups of ChNW and hydroxyl groups of CMC, and the electrostatic attraction between the positively charged ammonium groups (NH3+) of ChNW and the negatively charged carboxyl groups (COO˗) of CMC (Li et al., 2016). Mechanical strength (TS) and stiffness (E) of the CMC/ChNW/ZnO-Ag NPs composite films increased with an increase in the content of Ag NPs, which was presumably due to the Ag NPs being more compatible with the ChNW and CMC polymer matrix than the ZnO NPs to form stronger interaction. Mechanical properties of nanocomposite films are influenced by various

factors such as chemical structure and size of nanofillers, compatibility between the nanofiller and polymer matrix, and concentration of nanofiller. For example, tensile strength of CMC film has been increased after reinforcing with 5 wt% of nanocelluloses isolated from cotton linter, rice straw, wheat straw, and barley straw, which increased by 23%, 45.7%, 25.2%, and 42.6%, respectively (Oun & Rhim, 2015b, 2016).

On the contrary, flexibility (determined by the EB) of the CMC film decreased significantly (p<0.05) after formation of a composite with ChNW and hybrid NPs. The EB of CMC film decreased significantly from 49.7% to 32.9% when NPs were incorporated. The EB of the nanocomposite films also affected by the content of Ag NPs. Li et al. (2016) also reported that elongation at break of CMC film decreased from 46.5 % to 25.1 % after incorporation of chitin nanofibers. Usually, the flexibility of polymeric film decreases with increase in the mechanical strength of the film as shown in the present results.

3.7. Water vapor permeability (WVP) and water contact angle (CA) of films The WVP of the CMC and CMC-based nanocomposite films are also shown in Table 2. −9

2

The WVP of the neat CMC film was 1.92×10 g.m/m .Pa.s, which was consistent with the previously reported value of CMC film (Kanmani & Rhim, 2014). The WVP of the CMC film −9

2

decreased significantly (p<0.05) to 1.47×10 g.m/m .Pa.s after incorporation of 5 wt% of ChNW, which was attributed to the formation of a tortuous path of water vapor by the crystalline form of chitin (Shankar et al., 2015). The WVP of CMC films blended with the hybrid NPs were −9

2

1

0.5

1

1

1

2

1.71, 1.60 and 1.52 ×10 g.m/m .Pa.s, respectively, when ZnO -Ag , ZnO -Ag , and ZnO -Ag

were used. The increase in the WVP of CMC/ChNW films after incorporation of ZNO-Ag NPs was possibly due to the decreased self-assembling of chitin at dry state, which might be caused by the adhered nanoparticles on the surface of ChNW to make pores between ChNW. With the increase of Ag NPs, the WVP of the nanocomposite films decreased. This was presumably due to the increased size of AgNPs which prevented penetration of water vapor through the pores. The present results also suggested that AgNPs were more effective than ZnO NPs to decrease the WVP of the nanocomposite films. Shankar, Wang, & Rhim (2016) have reported that incorporation of AgNPs decreased the WVP of an alginate-based film significantly, while the effect of incorporation of ZnO NPs into CMC film on the WVP was not significant (Kanmani & Rhim, 2014). The surface hydrophilicity/hydrophobicity of CMC and CMC-based composite films was tested by measuring the water contact angle (CA) (Table 2). The CA of the neat CMC film was 28.2°, which indicated that the surface of the CMC film hydrophilic. The hydrophilicity of CMC film was related to the presence of a huge number of hydroxyl and carboxyl groups in addition to the hydrophilic glycerol (Oun & Rhim, 2016). The CA of CMC film was not changed significantly (p>0.05) when blended with the ChNW only, but it increased significantly (p<0.05) by the incorporation of the hybrid NPs. Shankar et al. (2015) found that the CA of carrageenbased film decreased with a high concentration of chitin nanofibers (CNF), which was attributed to the highly polar and hydrophilic nature of the CNF. The increased CA of CMC/ChNW/ZnOAg NPs composite films was mainly due to the hydrophobic metallic NPs of ZnO and Ag (Kanmani & Rhim, 2014).

3.8. Antimicrobial activity of films

The antimicrobial activity of CMC nanocomposite films against food-borne pathogenic Gram-positive (L. monocytogenes) and Gram-negative (E. coli) bacteria was evaluated using the viable cell colony count method, and the results are shown in Fig. 6. The CMC/ChNW composite film showed slight antimicrobial activity against E. coli, while it did not show any antimicrobial activity against L. monocytogenes. The antimicrobial activity of the film was mainly due to the ChNW. The antimicrobial activity of the ChNW might be due to the interaction between the positive charge of chitin with the negative charge on the cell membrane of the Gram-negative bacterium, causing the leakage of intracellular constituents and cell death (Li et al., 2016; Shankar et al., 2015). The hybrid NPs incorporated nanocomposite films exhibited strong antimicrobial activity against both E. coli and L. monocytogenes. However, they showed higher antimicrobial activity against E. coli than L. monocytogenes. Similarly, Liu et al. (2012) and Xu et al. (2013b) also found that nanocomposite films incorporated with CNC/ silver nanoparticles showed stronger antibacterial activity against Gram-negative bacteria (E. coli) than Gram-positive bacteria (S. aureus). Though the antibacterial mechanism of the metallic nanoparticles has not been completely explained yet, it is commonly believed that metallic NPs interact with the outer membrane of bacteria, causing structural changes and destruction of the bacteria (Xu et al., 2013b). The antimicrobial activity of the tested films was affected by the content of Ag NPs. Antimicrobial activity of the composite films was gradually decreased with increase in the content of Ag NPs. Among the films, CMC/ChNW/ZnO1-Ag0.5 film exhibited the highest antimicrobial activity against both bacteria. The higher antimicrobial activity of the composite films with low content of Ag NPs might be due to the smaller particle size of ZnO-Ag NPs, which increased with increase in the content of Ag NPs as observed through the STEM micrographs (Fig. 1). The smaller sized ZnO1-Ag0.5 NPs might interact with and penetrate into

the cell membrane of the bacterium more efficiently than the larger NPs. The Ag NPs have a potent antibacterial activity due to their ability of penetration into the bacteria to damage DNA through interacting with sulfur- and phosphorus-containing moieties in the cell (Liu et al., 2012). Roberson et al. (2014) also reported that hybrid metallic NPs of ZnO and Ag with a ratio of 2:1 exhibited the strongest antibacterial and antifungal activity against E. coli, S. aureus, and C. albicans.

4. Conclusions Multifunctional hybrid nanofillers were prepared with chitin nanowhisker (ChNW) and ZnO-Ag NPs with different ratios of Ag NPs. The formation and attachment of ZnO-Ag NPs on the surface of ChNW were confirmed by STEM, EDX, and UV-vis spectra. The STEM results showed that the ChNW exhibited needle-like crystals without entanglement, and the ZnO-Ag NPs showed a spherical shape with different size depending on the ratio of ZnO and Ag NPs. The ratio of Ag NPs significantly affected the properties of both ChNW and CMC composite films. The thermal stability, mechanical strength, and CA of CMC/ChNW composite film increased by the addition of ZnO-Ag NPs, and they increased further with an increase of Ag NPs. On the contrary, UV-barrier, water vapor barrier, and antimicrobial activity of the nanocomposite films exhibited the highest effect with a low ratio of Ag NPs (ZnO1-Ag0.5). The CMC/ChNW/ZnO-Ag NPs composite films with improved mechanical, UV-screening, and potent antimicrobial activity have a high potential for application as an active food packaging as well as biomedical applications.

Acknowledgments This research was supported by the Agriculture Research Center (ARC 710003) program of the Ministry of Agriculture, Food and Rural Affairs, Republic of Korea.

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Fig. 1. STEM images of (1) ChNW, (2) ChNW/ZnO1-Ag0.5, (3) ChNW/ZnO1-Ag1, and (4) ChNW/ZnO1-Ag2, at 1 μm and 500 nm magnification for each sample.

Fig. 2. (a) FTIR of ChNW and ChNW/ZnO-Ag NPs, (b) XRD of chitin and ChNW, and (c) XRD of ChNW/ZnO-Ag NPs.

Fig. 3. TGA and DTG (inset) thermograms of (a) hybrid nanoparticles of ChNW/ZnO-Ag NPs, and (b) CMC/ChNW/ZnO-Ag NPs nanocomposite films.

Fig. 4. (a) Light absorption spectra of nanoparticle suspensions [(1) ChNW/ZnO1-Ag0.5, (2) ChNW/ZnO1-Ag1, (3) ChNW/ZnO1-Ag2], and (b) light absorption and (c) transmission patterns of CMC-based films [(1) neat CMC, (2) CMC/ChNW, (3) CMC/ChNW/ZnO1-Ag0.5, (4) CMC/ChNW/ZnO1-Ag1, (5) CMC/ChNW/ZnO1-Ag2].

Fig. 5. SEM images of surface and cross-section of (1) neat CMC, (2) CMC/ChNW, (3) CMC/ChNW/ZnO1-Ag0.5, (4) CMC/ChNW/ZnO1-Ag1, (5) CMC/ChNW/ZnO1-Ag2 nanocomposite films.

Fig. 6. Antimicrobial activity of CMC/ChNW and CMC/ChNW/ZnO-Ag nanocomposite films against E. coli and L. monocytogenes.

Table 1 Decomposition temperatures and terminal residues of hybrid nanoparticles and their CMC-based nanocomposite films. Decomposition Temperature (°C) Samples NPs

Tonset /Tend ChNW

T0.5

(%)

252/385

342

10.1

1

0.5

252/385

350

34.5

1

1

252/392

357

37.9

ChNW/ZnO -Ag

1

2

252/392

357

39.5

CMC

250/320

263

26.5

CMC/ChNW

250/320

269

28.7

ChNW/ZnO -Ag ChNW/ZnO -Ag

Films

Char at 600 °C

1

0.5

250/320

269

30.7

1

1

250/320

269

31.5

1

2

250/320

275

33.2

CMC/ChNW/ZnO -Ag CMC/ChNW/ZnO -Ag CMC/ChNW/ZnO -Ag

Tonset: starting decomposition temperature; Tend: End of decomposition temperature; T0.5: midpoint of the decomposition temperature; char at 600 °C: the final residue left after thermal degradation at 600 °C.

Table 2 Mechanical properties, water vapor permeability (WVP), and water contact angle (CA) of CMC and CMC/ChNW/ZnO-Ag NPs nanocomposite films.

Thickness (μm)

Films

CMC

66.4 ± 1.2

CMC/ChNW

67.3 ± 3.8

1

0.5

1

1

1

2

CMC/ChNW/ZnO -Ag

CMC/ChNW/ZnO -Ag

CMC/ChNW/ZnO -Ag

60.8 ± 2.3

63.7 ± 2.6

64.3 ± 2.6

a,b

a,b

a

a

a,b

TS (MPa)

40.2 ± 1.7

45.1 ± 1.2

47.5 ± 1.4

50.9 ± 1.2

53.1 ± 2.4

E (MPa)

−9

WVP (×10

EB (%)

2

g.m/m .Pa.s) a

b

c

c,d

d

719 ± 29

a

1093 ± 34

1121 ± 69

1201 ± 67

1444 ± 56

49.7 ± 1.4

b

b

b

c

40.3 ± 2.7

37.9 ± 3.6

36.8 ± 2.0

32.9 ± 2.4

c

b

a,b

a,b

a

1.92±0.15

1.47±0.22

1.71±0.16

1.65±0.08

1.52±0.11

d

a

b,c

b

a

CA (deg.)

28.2±0.8

29.3±1.4

34.9±0.9

39.3±1.3

36.3±1.3

a

a

b

c

b,c

*Values with the same superscript letter in the same column indicate that they are not statistically different (p < 0.05). TS: tensile strength; E: elastic modulus; EB: elongation at break; WVP: water vapor permeability; CA: water contact angle