diamond nanocomposites for packaging applications

Diamond & Related Materials 99 (2019) 107523

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Mechanical, rheological and oxygen barrier properties of ethylene vinyl acetate/diamond nanocomposites for packaging applications ⁎

T

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Majed Aminia, Ahmad Ramazani S.A.a, , Seyyed Arash Haddadia,b, , Amanj Kheradmanda,c a

Chemical and Petroleum Engineering Department, Sharif University of Technology, Tehran, Iran School of Engineering, University of British Columbia, Kelowna V1V 1V7, Canada c School of Engineering, Macquarie University, Sydney, NSW 2109, Australia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Surface modification Diamond nanoparticles EVA Packaging

In this work, the effects of the surface-modified nanodiamond particles (NDs) on the barrier, rheological, mechanical and thermal properties of ethylene vinyl acetate (EVA) composites for the packaging applications were investigated. Fourier transform infrared spectroscopy, as well as thermal gravimetric analysis were employed to study the grafting of vinyltriethoxy silane (VTS) on the surface of NDs. Afterwards, EVA samples containing 0, 0.1, 0.5, 1, 1.5 and 2 wt% of surface-modified NDs were prepared by a two-stage process including the solution and injection processes. In order to evaluate the physicochemical, rheological, mechanical and thermal properties of the EVA/NDs samples, field emission scanning electron microscopy, contact angel measurement, rheometric mechanical spectroscopy, tensile test, differential scanning calorimetry, and thermogravimetric analysis were employed. The result showed a more uniform dispersion of modified NDs in the EVA matrix. Also, the oxygen permeability rate of EVA films in the presence of NDs was reduced remarkably. Furthermore, the mechanical characteristic and thermal stability of the EVA matrix were enhanced significantly in the presence of the modified NDs and the EVA/NDs nanocomposite films have this capability to use for the packaging applications.

1. Introduction Polymer composites are a class of material in which one or more types of fillers disperse into the polymer matrix to improve the mechanical and physical properties of the polymer [1]. Nanocomposites refer to a class of composites in which particles are at least with nanoscale dimensions in one direction. The reinforcing effect of nanoparticles is due to the high ratio of length to diameter, therefore in very small size, they can have very high surface area compared with conventional reinforcement materials such as glass fiber. In low quantities, nanocomposites are likely to achieve the desired properties [1–3]. Over the past decade, the incorporation of nanoparticles into polymeric materials has attracted attention of scientists and has become a fascinating topic, which is due to the role of nanoparticles to increase the properties of polymeric composition and increase the compatibility between polymeric components due to the presence of nanoparticles with the high surface area [4–6]. The structure of nanoparticles in a polymeric composition can be determined with thermodynamic and kinetic parameters. The most common methods to detect nanoparticles

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structure in a polymer matrix are transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM). NDs due to the lower toxicity compared to other carbon isotopes have numerous applications in the medical and pharmaceutical industries [7,8]. As nanoscale materials, nanodiamonds and its composites are also used in electrical energy storage, wastewater treatment, and bioapplications. Nanodiamonds synthesized by detonation have already been commercialized and are most frequently employed in polymer composites, energy, environmental, and electrical applications [9]. One major difference between NDs and other carbon materials is that NDs possess various functional groups such as hydroxyl, carboxyl and amine on their surface expanding their scope of applications. In addition, these functional groups existing on the surface of NDs provides appropriate active sites for unification and changing the surface chemistry without altering in their bulk physical properties such as the surface morphology, surface roughness, and particle size distribution [9]. Hydrophobicity or hydrophilicity of the nanoparticles surface has a significant impact on its dispersion and compatibility with the host polymer matrix. NDs have an unusual tendency to agglomerate and

Corresponding author. Corresponding author at: Chemical and Petroleum Engineering Department, Sharif University of Technology, Tehran, Iran. E-mail addresses: [email protected] (A. Ramazani S.A.), [email protected], [email protected] (S.A. Haddadi).

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https://doi.org/10.1016/j.diamond.2019.107523 Received 18 May 2019; Received in revised form 9 August 2019; Accepted 26 August 2019 Available online 28 August 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.

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prepared nanocomposite films was purposed. In another study [31], the rheological and mechanical properties of the same nanocomposites were also investigated. A series of single-step melt processed polylactic acid (PLA)/EVA immiscible blends and their organo-clay (Cloisite 30B) filled nanocomposites in the presence and absence of potassium titanate whisker (PTW) as a compatibilizer were analyzed in terms of morphological, rheological and thermal properties. By studying the rheological properties, it was found that by comparing trends of the storage modulus of the neat blend and its associated compatibilized and uncompatibilized nanocomposites at the high and low frequencies, crossover points between G′ and G″ curves, localization of organo-clay and effect of compatibilizer on its microstructure could be well explained and correlated with the morphological findings. In this study, the effects of the surface-modified NDs with VTS on the characteristics of the EVA matrix were investigated for packaging applications. For this purpose, the nanocomposite films of EVA with different NDs contents were prepared using a two-step process (solution and injection methods). A self-designed and self-made setup was used to evaluate the barrier properties and gas permeability coefficients of the prepared nanocomposite sheets. Comprehensive physicochemical, rheological, mechanical and thermal studies were carried out on the prepared EVA/NDs sheets.

aggregate in the polymer matrix due to the specific surface characteristics and the existence of oxygen-containing surface functional groups [10–12]. Since the aim of the present work is to fabricate EVA/NDs nanocomposites for packaging applications, dispersion of NDs in the EVA matrix has significant effects on the overall properties of the fabricated Nanocomposites [13]. Yinhang Zhang et al. [10,14] showed that epoxy nanocomposites exhibited the high thermal stability and conductivity. In addition, inserting nanodiamond particles between BN and graphene oxide layers prevented the BN and graphene oxide nanosheet from forming agglomerates. Hajiali et al. [15] showed that surface modification of NDs with VTS enhanced both mechanical and thermal properties of the prepared polydimethylsiloxane (PDMS)/NDs nanocomposites due to the enhanced dispersion of modified NDs compared to unmodified NDs due to the presence of hydrocarbon species on the surface of NDs, which are more compatible with the hydrophobic EVA chains. Surface functional groups including fluoro-, alkyl- and aminohave been examined and discussed for NDs [6,16–19]. In other researches, Osswald et al. [20], Neitzel et al. [21], Zhang et al. [9,10,14], and Szunerits et al. [22] investigated the properties and applications of NDs. Szunerits et al. [22] introduced NDs as suitable materials for medical and biomedical application due to non-toxicity and unique properties. There are several methods to produce NDs. Among these methods, the explosive method is more interesting due to the production of highquality nanoparticles with particle sizes between 4 and 6 nm [23]. As mentioned above, diamond has various surface functional groups; therefore these modifiable particles have the capability to be distributed uniformly in the polar and non-polar environment [24]. Khalilzadeh et al. [25] studied the effects of surface-modified NDs on polyacrylic acid/ND composites. They observed that surface modification can improve the dispersion of nanoparticles in polymer and increase its stability in the matrix leading to improvement in its scratch resistance. EVA is a widely used material as a matrix to prepare polymer films for packaging applications used in food industries [26]. In fact, excellent prevention for the permeability of gases, good toughness in low temperature, acceptable resistance to fragility, high transparency, and excellent mechanical properties for film casting [26,27]. Furthermore, for packaging application, layered nanostructures such as graphene [2,28], graphene oxide [13], and organo-clays (Cloisite 20A) [29] sheets present considerable gas barrier properties compared to other nanoparticles. The effect of nanoclay (Cloisite 15A) on the permeability of EVA/clay film nanocomposite has been examined by Shafiee et al. [13]. In another study, Molina et al. [3] studied the antibacterial properties of low-density polyethylene (LDPE)/EVA films filled with Ag/TiO2 nanoparticles. To fabricate such antimicrobial nanocomposites, LDPE/EVA samples were processed with Ag nanoparticles grafted on TiO2 particles as an inorganic carrier substance. The results showed that the incorporation of Ag/TiO2 nanoparticles in the polymer matrix improved the antimicrobial properties enhancement. In another research, Xu et al. [30] investigated the properties and applications of packaging films of modified polyethylene in the presence of silica/EVA nanoparticles. In this work, to improve the polarity and comprehensive performance of LDPE films, gas barrier properties of LDPE films were modified by a co-precipitation method in the presence of the silica/EVA nanoparticles. Dadfar et al. [7] studied the permeability properties of nanoclay (Cloisite 15A) filled high-density polyethylene (HDPE)/EVA nanocomposites and a model for the migration of oxygen through the

2. Experimental 2.1. Raw materials EVA pellets with 28% vinyl acetate were purchased from Hyundai Petrochemical of Korea. Chloroform was obtained from Merck and used as the EVA solvent. Detonated NDs were purchased from Notrino Company (Iran) with the properties tabulated in Table 1. Surface modification of NDs was performed using VTS with a molecular weight of 190.31 g cm−3. VTS was purchased from Merck (Germany). The properties of the purchased organosilane are shown in Table 2. Ethanol (99.6%), acetic acid (13%) and ammonia (28%) were purchased from Mojallali Co. (Iran). All raw materials were used without further purification.

2.2. Surface modification of NDs For purification of NDs prior to the surface modification process, thermal oxidation was employed. For this purpose, NDs were placed in a furnace at 420 °C for 2 h to eliminate the organic impurities and extend the carboxyl groups onto the surface of NDs. Surface modification of NDs was performed based on the sol-gel method used by Haddadi et al. [32,33]. In the first step, ethanol (42.5 mL) was mixed with deionized water (5 mL). Then, VTS (2.5 mL) was added into the previous mixture under stirring (200 rpm) at the ambient conditions for 20 h to complete the hydrolysis reactions of the organosilane. The pH of the solution in this step was adjusted approximately to 4 using the acetic acid solution (1 M). Next, to carry out condensation reactions of the hydrolyzed organosilane on the surface of NDs, 1 g of oxidized NDs was added into the previous solution under stirring (250 rpm) for 4 h at 65 °C. 28% ammonia solution was used to adjust the pH of the solution to 8. Finally, the sediments were centrifuged, washed with ethanol and placed in an oven at 110 h for 5 h. The thermal oxidation and surface modification steps of NDs are illustrated schematically in Fig. 1.

Table 1 Properties of NDs as received. Sample

Bulk density (g cm−3)

True density (g cm−3)

Particle size (nm)

Specific surface area (m2/ g)

Purity (%)

ND

0.11

3.05

2–6

485

97

2

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Table 2 The physical properties of the organosilane. Chemical name

Vinyltriethoxy silane

State

Clear liquid

Density (g cm−3) 0.97

Purity (%)

97–98

Storage temperature (°C) 2–8

2.3. Preparation of nanocomposite samples In the first step of the preparation of EVA films, NDs were dispersed into chloroform at concentration of 0.1, 0.5, 1, 1.5 and 2% (w/w), based on the EVA weight, using a probe sonicator (FAN AZMA, Iran) for 10 min at 100 W and 25 kHz frequency. Sonication of chloroform solutions containing NDs could certainly lead to the elimination of NDs agglomeration, resulting in almost a uniform solution. Then, EVA pellets were added slowly to the previous solutions under stirring using a high shear mixer (4000 rpm) at 40 °C temperature for 6 h. After complete dissolving of EVA pellets, the solutions were remained under stirring for 5 h resulted in the uniform EVA/NDs solutions. EVA nanocomposite films were prepared using the casting of the prepared solution at ambient conditions (24 ± 1 °C and 30 ± 2 RH). However, after casting of the EVA/NDs solutions, air bubbles were formed in the casted films affecting the overall mechanical and thermal characterization of the EVA films. Consequently, to remove the air bubbles and prepare

Fig. 2. Steps of the nanocomposite samples preparation.

uniform EVA films, injection molding was carried out as the postmixing process. Then, via injection molding of the casted EVA films at 80 °C°C, uniform melted EVA samples were injected to a silicon mold until the complete cooling. Moreover, the injection molding process was conducted to ensure the uniformly complete dispersion of NDs in the EVA matrix. The same sample preparation procedure was performed to prepare an EVA nanocomposite sheet containing 2 wt%

Fig. 1. Schematic illustration of the thermal oxidation(a), hydrolysis of VTS (b) and grafting of VTS on ND surface via condensation reactions (c). 3

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unmodified NDs for TEM test. Fig. 2 illustrates the nanocomposite samples preparation. After the preparation of the samples, EVA samples containing 0, 0.1, 0.5, 1, 1.5 and 2 wt% of ND were denoted as EVAND0, EVAND-0.1, EVAND-0.5, EVAND-1, EVAND-1.5, and EVAND-2, respectively. 2.4. Methods 2.4.1. Field emission scanning electron microscopy (FE-SEM) FE-SEM analysis was employed to investigate the morphology of the EVA/NDs nanocomposites by a field-emission scanning electronic microscope (Mira 3-XMU, TESCAN, Czech Republic). Prior to FE-SEM observations, the nanocomposite sheets (2 × 1 cm2) were immersed into the liquid nitrogen for 30 s and then were broken by two pliers. The cross-section area of the prepared samples was double-coated with gold using an EM ACE600 sputter coater (Leica Co., Canada). Fig. 3. Schematic illustration of the setup used to determine the O2 transmission rate through the prepared films.

2.4.2. TEM analysis The morphology of crystalline structures of the unmodified and modified NDs sample was investigated using TEM analysis (TEM, Zeiss EM900 under 200 kV). To prepare unmodified and modified NDs filled EVA samples for TEM test, the EVA nanocomposite sheets containing 2 wt% unmodified and modified NDs were prepared. After placing the samples in liquid nitrogen, they were broken. Then, a thin-section microtomy process was performed on the samples.

performed at the temperature range of 10–100 °C in two heatingcooling cycles. The first cycle was performed to remove the thermal history of samples, and the second cycle was conducted in order to evaluate both endothermic and exothermic behavior of the samples.

Crystallinity (%) = 2.4.3. Fourier-transform infrared spectroscopy (FTIR) spectroscopy Grafting of VTS onto the surface of NDs after the surface modification was evaluated using FTIR spectroscopy (Tensor 27, Bruker, Germany) within the wavenumber range of 400–4000 cm−1 using KBr pellets.

∆Hmsample × 100% ∆Hm∗

(1)

in which ΔHm∗ is the melting enthalpy for totally crystalline EVA. Assuming the same melting enthalpy for all crystalline forms of EVA, ΔHm∗ will be 283 J/ g. ΔHmsample is the melting enthalpy of each sample through the crystallization in DCS curves.

2.4.4. Contact angle measurements In order to study the effects of the silanized NDs on the wetting and surface characterization of the EVA matrix, water contact angle measurements were conducted (SHARIF AZMA, CA-1, Iran). The films dimensions were 4 × 5 cm2. After dropping a water droplet (10 μL) on the surface of the EVA films and allowing some time to pass to achieve a thermodynamic equilibrium, the images of the water droplets on the surface of the EVA films were recorded and the obtained images were analyzed using Dino-Lite software. The water contact angle values of the right and left sides of the water droplets were precisely measured.

2.4.8. Thermal gravimetric analysis (TGA) TGA was conducted to study the thermal degradation and char yields of the EVA/NDs samples using a LINSEIS TG analyzer (STA PT1600, USA). TGA analysis was performed with a heating rate of 10 °C min−1 and the temperature range of 25–600 °C under a pure nitrogen atmosphere.

2.4.9. Oxygen barrier measurement Fig. 3 illustrates the scheme of the used setup (SHARIF AZMA, Iran) for determination of the O2 transmission rate through the prepared EVA/NDs films. The gas permeation device consisted of two compartments separated by the EVA films with an effective area of 3 cm2. The gas was introduced in the upstream compartment under a pressure of 5 bars. The exiting gas was directed to a bubble flow meter at the atmospheric pressure to define the gas volumetric flow rate in a steady state. To measure the permeability coefficient, the following equation was used [13]:

2.4.5. Rheometeric mechanical spectrometer (RMS) RMS was used to evaluate the rheological properties of the EVA/ NDs samples both in steady and dynamic modes using a rheometer (Anton Paar, Physical MCR 301, Austria). The steady and dynamic rheological studies of the EVA samples were carried out at 75 °C and within a shear rate range of 0.001–100 s−1 and the angular frequency range of 1000–0.1 s−1, respectively. All the rheological measurements were performed in the parallel plate geometry and the gap distance of 0.8 mm.

P=

2.4.6. Tensile test A universal tensile machine (H10KS, HOUNSFIELD, Germany) by a force load cell of 500 N capacity was used to investigate the mechanical characteristics of the EVA/NDs nanocomposite films. Three stripshaped specimens (ASTM D638) were tested for each sample at a loading rate of 1 mm min−1 at 27 °C. Triplicates were prepared for each sample and the mean values were reported.

N×L ∆P

(2)

In Eq. (2), the permeability coefficient is P, L is the film thickness, N is the rate of gas flow per unit area and ΔP is the pressure difference on either side of the film. Three circular samples for each EVA film were prepared with partially uniform thicknesses and the mean values were reported. The thickness of each sample was measured at ten different places using a digital thickness gauge at a horizontally flat position before running the tests and the mean thickness value was used for measuring the oxygen permeability coefficient.

2.4.7. Differential Scanning Calorimetry (DSC) measurements The thermal characterization of EVA/NDs nanocomposite samples was evaluated using a DSC TA analyzer (Q100, USA) under a pure nitrogen atmosphere at a heating rate of 10 °C min−1 The crystallinity of the samples was calculated using Eq. (1) [34]. The DSC tests were 4

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Fig. 5. TGA thermograms of the unmodified and modified NDs.

Fig. 4. FTIR spectra of the as-received, oxidized and modified NDs.

3.1.3. TEM TEM images of the unmodified and modified NDs are presented in Fig. 6. These images show that both unmodified and modified nanoparticles have aggregated structures with primary particles size approximately between 4 and 10 nm. Considering that according to numerous published documents in literature, unmodified NDs have a great tendency to form the agglomerates and aggregates. In fact, NDs with a great specific surface area of ≥200 m2/g have a higher surface reactivity due to the existence of oxygen-containing surface functional groups [20]. Thus, altering the surface chemistry of NDs can decrease their tendency to form the agglomerates and aggregates. Agglomeration and aggregation of unmodified NDs in the polymer matrix negatively affect their reinforcing abilities. Hajiali et al. [37] proved that the addition of unmodified NDs in the PDMS matrix has no considerable effects on the mechanical properties of PDMS/NDs nanocomposites; while the silanization of NDs enhanced both mechanical and thermal characterization of the samples compared to unmodified NDs in the same concentration. Silanization of NDs not only enhances the interfacial interactions and bonding with the polymer chains but also leads to the substantial deagglomeration of NDs [15,38]. Also, the dispersion of surface-modified NDs in a polymer matrix may lead to the fewer and also smaller agglomerated and aggregated structures. Surface modification of NDs with VTS leads to the surface energy and chemistry altering and the formation of many hydrocarbon species, which are more compatible with the hydrophobic EVA chains. Consequently, their tendency to disperse in the polymer matrix increases [9,14]. In TEM analysis, the agglomerates can be observed by the darker zones which are greater in size for the unmodified NDs. In fact, after surface modification of NDs, the number of NDs in the clusters is declined and the clusters become smaller. As shown in Fig. 6b, the image is more uniform and has less dark spots than Fig. 6a. This shows that NDs have this tendency to disperse and distribute in the EVA matrix.

3. Results and discussion 3.1. Characterization of the surface-modified NDs 3.1.1. FTIR FTIR analysis was performed to evaluate the quality of organosilane grafting onto the surface of NDs. Fig. 4 shows the FTIR spectra of both unmodified and modified NDs. FTIR spectra of unmodified and modified NDs have been normalized base on absorbance band of –OH bands around 3430 cm−1 [35]. For the oxidized NDs, the main characteristic peaks observed at 1273 and 1758 cm−1 were associated to the asymmetric stretching of COOH and CeO bands illustrating the formation of carboxyl groups after the oxidization process of unmodified NDs. In fact, after the oxidization process, the functional groups of the surface of unmodified NDs were unified remarkably to the carboxylic group. As it can be seen from Fig. 4, for the modified NDs, the main characteristic peaks observed at 1128 cm−1 are linked to the Si-O-Si asymmetric band. Furthermore, the observed peak in the wavenumber range of 790–820 cm−1 can be attributed to the Si-O-Si network, which proves the grafting of organosilane onto the surface of NDs. Also, the observed peak at 3451 cm−1 is related to hydroxyl groups and absorbed moisture on the surface of nanoparticles. The absorption bands at 2800, 2900 and 1620 cm−1 were defined as asymmetric stretching of CeH in CH3 section of organosilane [32,33,36]. Observation of characteristic peaks of organosilane onto the surface of the modified NDs proved the silanization of NDs. 3.1.2. TGA TGA was performed in the temperature range of 25–500 °C with the heating rate of 10 °C min−1 for the unmodified and modified NDs to quantify the content of the grafted silane onto the surface of the modified NDs. Fig. 5 presents the TGA thermograms of the unmodified and modified NDs. As shown in Fig. 5, both modified and unmodified samples have shown slight weight loss due to evaporation of absorbed water on the NDs surface which is approximately 1 wt% weight loss of silane-modified nanoparticles is less than unmodified nanoparticles, due to the formation of silane bonds network. This network prevents the easy removal of these compounds from NDs caused by the increased temperature. The weight difference between the samples was intensified from the temperature of 210 °C and modified nanoparticle showed more weight loss due to degradation of organic as a result of surface modification on the surface of the diamond. Reducing the weight of the modified sample up to about 450 °C continued and then reached a plateau. The difference of the remained weight for the unmodified and modified NDs at 470 °C can be attributed to the thermal decomposition of the grafted VTS onto the surface of NDs which is about 4%.

3.2. Characterization of the prepared EVA/NDs samples 3.2.1. Contact angle measurements Hydrophobicity or hydrophilicity of a surface has a significant impact on its applications. Since the aim of present work is to make nanocomposite for packaging applications, these parameters are vital to determine the surface characterization of the prepared nanocomposite samples. Triplicates were prepared for each sample and Contact angle measurements were performed on the various parts of each nanocomposite film (at least 5 parts) and the mean values of left and right sides of water contact angle were reported. Fig. 7 shows the contact angle images and contact angle values for the different samples. The obtained results from water contact angle measurements were confirmed by calculation of the work of adhesion (Wa) and the surface 5

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Fig. 6. TEM images of the unmodified (a) and modified (b) NDs.

adhesion strength at the interface of the NDs and EVA matrix. 3.2.2. FE-SEM Excellent distribution of fillers into the polymer matrix is one of the vital and effective parameters in improving mechanical properties and adhesion of nanocomposites. For this reason, FE-SEM analysis was conducted in order to evaluate the distribution of NDs in the EVA matrix. Fig. 8 depicts FE-SEM images of pure EVA and EVA containing 0.5 and 2 wt% NDs captured from cross-section of the samples, fractured in liquid nitrogen. According to FE-SEM images, dispersion of the agglomerated and aggregated NDs in the EVA matrix is uniform and the few and small agglomerated structures of NDs could be observed. In prepared nanocomposites, NDs play a crucial role in determining the morphology of the sample. As can be seen in Fig. 8, the fracture surfaces are smooth and flat which indicates the high adhesion between the fillers and matrix [41]. Moreover, the created micro ruptures on the surface of EVAND-0 indicate that the blank EVA could not resist against the applied stresses. It is also clear from Fig. 8 that the addition of modified NDs led to decrease in the micro ruptures. Aggregation and agglomeration of nanoparticles in a polymer matrix lead to the formation of stress concentration points in the prepared nanocomposite when external stresses applied. Thus, these stress concentration points turn to the places for the mechanical failure of the nanocomposite. On the other hand, the uniform dispersion and appropriate distribution of compatible nanoparticles in the polymer matrix not only enhance the overall mechanical properties on the nanocomposite but also lead to the efficient transfer of applied stress from the matrix to the nanoparticles [32,42]. In fact, the nanocomposite samples containing modified NDs could damp the applied stresses due to the lower and fewer aggregation of the modified NDs [35]. Consequently, the lower micro rupture can be observed on the fracture surface of EVAND-2 compared to EVAND-0.5 and EVAND-0. The well-dispersed nanoparticles in a polymer matrix can create the stress concentration regions dissipating the applied stresses at the nanoparticle/matrix interface. This phenomenon can be enhanced with an increase in interactions between the nanoparticles and matrix due to the surface modification of NDs [12]. It is worth mentioning that FE-SEM can be used in order to determine the average particle size of clusters inside a polymer nanocomposite. But, the EVA matrix due to its lower glass transition temperature follows from the cold flow phenomenon. During FE-SEM analysis, EVA chains could cover the surface of NDs clusters due to their segmental mobility as a result of the cold flow phenomenon. Consequently, the obtained results for the particle size of NDs clusters are not reliable. That is why we did not determine the particle size of NDs clusters by FE-SEM.

Fig. 7. Shapes and values of water contact angles on the surface of the prepared nanocomposite samples.

free energy (γsv) data using Young (Eq. (3)) and Neumann (Eq. (4)) equations [33,39,40].

Wa = γLV (1 + cos θ)

(3)

Wa = 2(γLV γSV )0.5exp [−β (γLV − γSV )2]

(4)

where, γlv and θ are water surface tension and water contact angle, respectively. The values of γlv and β are 72.8 mJ/m2 and 0.001247 ± 0.000010 (mJ/m2)−2, respectively [39]. The values of water contact angles, Wa and γsv are tabulated in Table 3. According to Table 3, it is clear that by an increase of the NDs contents in the EVA matrix, contact angle increased and this improvement affects the hydrophobicity of surface which is important for the mentioned application. As shown in Table 3, the addition of the surfacemodified NDs with VTS can reduce the surface energy and increase the equilibrium angle of a water drop. Also, it reduces the surface energy difference of the EVA/ND samples and increases the interaction and Table 3 Water contact angles (left and right sides and average), work of adhesion and surface free energy values of the prepared nanocomposite samples. Sample type

EVAND-0 EVAND-0.1 EVAND-0.5 EVAND-1 EVAND-1.5 EVAND-2

θL/° (left side)

θR/° (right side)

96.7 98 101 102.8 108 110.5

94.1 98.6 102.6 106.2 106 112.7

Average θ/°

Wa/mJ

95.4 98.3 101.8 104.5 107 111.6

65.95 62.30 57.91 54.57 51.51 46

γsv/mN m−1 25.86 24.08 21.94 20.32 18.83 16.16

3.2.3. RMS 3.2.3.1. Steady 6

rheological

characteristics

of

EVA-NDs

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Fig. 8. Cross-sectional images of EVAND-0, EVAND-0.5, and EVAND-2.

could be attributed to the reduction of the kinetic energy of polymer chains as a result of an increase in nanoparticle concentration, and hindering chains slippage on each other by nanoparticles. It can be seen that shear stress applied to the samples increases almost linearly by an increase in the shear rate, indicating that the rheological behavior of samples could be estimated with the power-law rheological model. As mentioned previously, the shear stress and viscosity increase with an increase in the NDs contents, which is more pronounced at the low NDs concentration. However, at the highest NDs concentration viscosity increment is not pronounced which could be probably due to nanoparticle inducing slipping effects.

3.2.3.2. Dynamic rheological characteristics of the EVA-NDs solutions. Fig. 10 illustrates the effect of modified nanoparticles on the complex viscosity (η∗ = η ″ − iη′) and storage modulus (G′) as a function of angular frequency (ω) for various prepared nanocomposites. From graphs of complex viscosity versus frequency, it can be seen that by increasing in the concentration of nanoparticles, complex shear viscosity at the low frequencies increases considerably, especially for the sample with minimum ND concentration. This suggests that the strong interaction between filler and polymer chains could exist. Furthermore, by increasing in nanoparticle content, the zero complex shear viscosity regions of nanocomposites have shortened and shear thinning starting frequency decreases in comparison with that of pure EVA. The emergence of shear-thinning behavior implies the existence of a layer or sheet structure in the nanocomposites. Fig. 11 presents the complex modulus (G⁎ = G′ + iG″) as a function of the angular

Fig. 9. Effects of NDs concentration on the shear viscosity and shear stress of EVA vs. shear rate at 75 °C.

solutions. Obtained nanocomposites similar to many other polymeric materials show a shear-dependent rheological behavior that the shear dependency increases by an increase in nanoparticle concentration. Steady shear viscosity of different samples is measured using RMS instrument [8]. To do the test, a circular plate with a diameter of 20 mm and a thickness of 1 mm was placed inside the device and temperature was fixed at 75 °C. Fig. 9 illustrates the shear viscosity and shear stress of the different samples as a function of shear rate (γ̇) [43]. As can be seen from Fig. 9, the apparent zero shear viscosity of samples increase with increasing in nanoparticle concentration, which 7

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containing different contents of surface-modified NDs illustrated that the tendency of shear-thinning over the Pseudoplastic region decrease in the presence of more ND nanoparticles. 3.2.4. Tensile test EVA matrix has a low modulus; therefore, the aim of this study is to compensate for this problem by the addition of the surface-modified NDs. Effect of the surface-modified NDs on the mechanical properties of EVA/ND nanocomposite samples was evaluated using tensile test. The mechanical parameters of the EVA samples including Young's modulus, yield stress, tensile strength and elongation at break can be obtained from the stress-strain curves. Fig. 12 shows the typical stress-strain curves for the pure EVA and EVA containing different contents of the surface-modified NDs. The mechanical parameters extracted from the typical stress-strain curves of the EVA/NDs samples are illustrated in Fig. 13. The plastic region for EVAND-0 is broad indicating the chain alignment under the applied stresses during the tensile test, but in the nanocomposite samples with addition of NDs, the length of the plastic zone decreased. This trend was amplified with increasing in the ND content. In fact, increasing in the ND content changes the tensile behavior of the nanocomposite samples from ductile to brittle. This phenomenon could be due to a decrease in the mobility of the EVA chains in the presence of the modified NDs. In fact, the presence of NDs among the EVA chains reduces the segmental motion and the free volume between the chains. This result can be also inferred from an increase in Young's modulus about 70% and decrement of the elongation at the break about 120% for EVAND-2 compared to EVAND-0 shown in Fig. 13a and b. It can be concluded from Fig. 13 that increase in ND contents led to the increment of Young's modulus, yield stress and tensile strength and a decrease in the elongation at break compared to the neat EVA. The same trends of yield stress and tensile strength showed these factors were enhanced with the addition of NDs to the EVA matrix in comparison with the pure EVA. Accordingly, the maximum improvement in yield stress and tensile strength were found for EVAND-2 which are about 121% and 82%, respectively. These improvements in mechanical properties should be due to the improvement of NDs dispersion and interaction between the modified NDs and EVA matrix. Surface modification of NDs promotes the dispersion and interaction between NDs and EVA matrix and provides the greater interfacial area between them. Therefore, the mechanical stresses could easily transfer from the EVA matrix to NDs.

Fig. 10. Complex viscosity and storage modulus as a function of angular frequency at 75 °C.

Fig. 11. Complex modulus as a function of angular frequency at 75 °C.

frequency of the samples. It can be concluded from Fig. 11 that storage modulus was increased because of the existence of hard nanoparticle into a soft matrix with the addition of filler into the matrix. In Fig. 11, complex modulus increases as the filler content increases which is due to the increase in storage modulus based on mentioned reason. Another reason is the loss modulus increase by the addition of nanofillers. As a result, vacant space was reduced and this leads to slippage of chains on each other and increases the loss modulus. Another reason for this improvement was an increase in the viscosity of the EVA matrix in the presence of NDs. Increase in the rate of shear-thinning by increasing NDs contents demonstrated an increase in nanocomposites solid-like behavior. Furthermore, in dynamic mode, the rheological parameters of EVA samples containing different contents of surface-modified NDs were calculated using adopted Carreau-Yasuda model (Eq. (5)) as follows [44]: (n − 1)

η∗ = η0∗ [1 + (λw )a]

a

3.2.5. DSC The effect of surface-modified NDs on the melting and crystallization characteristics of EVA was studied using DSC. The degree of crystallinity (Xc) for both EVA and EVA/NDs samples was calculated by Eq. (6):

∆Hf ⎞ X% = ⎜⎛ × 100% 0⎟ ∆ ⎝ Hf ⎠

(5)

η0∗

is the zero-complex shear rate viscosity (Pa.s), λ is the rewhere laxation time (s) which related to reciprocal of frequency for the onset of shear thinning, a represents the width of the transition region between η0∗ and Power-law region and n represents the line slope over the shear thinning region in the logarithmic plot. The calculated rheological parameters based on adopted Carreau-Yasuda model are tabulated in Table 4. According to the data shown in Table 4, the adopted Carreau-Yasuda model fits the data with a correlation coefficient (R2) being > 0.998. Also, the incorporation of EVA with NDs led to the increment of η0 and decrement of λ. It can be inferred that the addition of more NDs into EVA could decrease relaxation time and chain mobility of EVA chains due to the steric hindrance of the ND nanoparticles in the EVA matrix. Furthermore, a considerable decrease in a and n values in EVA matrix

(6)

ΔHf: Heat of fusion determined by DSC ΔHf0: Heat of fusion of 100% crystallinity (283 J/g) Fig. 14 depicts the endothermic and exothermic sections of DSC thermograms for EVA/NDs samples. The melting and crystallization parameters including the melting enthalpy (ΔHm), crystallization temperature (Tc), the total area under the crystallization peak (ΔHc), and Xc are tabulated in Table 5. As can be seen in Fig. 14, the intensity and width of the endothermic peaks decreased by the addition of NDs. This could be due to the lower segmental motion of the EVA chains in the presence of the modified NDs restricting the relative sliding of the chains. In addition, another reason can be attributed to the steric hindrance caused by placing of NDs among the free volume between the EVA chains trapping the volatile small molecules such as water, solvent and their decrease the 8

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Table 4 The calculated rheological parameters of the prepared nanocomposite samples based on the adapted Carreau-Yasuda model for complex viscosity. Sample η0∗

(Pa.s) λ (s) a n R2

EVAND-0

EVAND-0.1

EVAND-0.5

EVAND-1

EVAND-1.5

EVAND-2

2713.6 0.2125 0.7271 0.5734 0.9999

3088 0.1221 0.5310 0.4965 0.9998

3747.7 0.0820 0.4548 0.4019 0.9999

4461.3 0.0499 0.3930 0.2945 0.9998

5060.2 0.0098 0.3352 0.1001 0.9983

5698.6 0.0069 0.2837 0.0008 0.9992

mentioned volatile small molecules in form of hydrogen bonds decrease the evaporation rate of these species from the nanocomposite matrix leading to the decrement of the melting enthalpy [39,45]. It is crystal clear that NDs are non-volatile and the presence of them in the different percentages at the EVA matrix reduces the percentage of volatile compounds in the nanocomposite compared to the neat EVA. As a result, the required amount of heat for the fusion of the nanocomposites is decreased. After the dispersion of the modified NDs in the EVA matrix, the viscosity of nanocomposite samples increases substantially and leads to the decrement of the release of small molecules and other volatile components from the matrix. As illustrated in Table 5, with the loading of the modified NDs into the EVA matrix, Tc increased. One reason for an increment of Tc could be due to the steric hindrance and the lower mobility of the EVA chains in the presence of the modified NDs. Also, the increment of tendency between NDs and the EVA chains because of the surface modification led to the higher interaction and interfacial area between them. Consequently, the placing of the chains into the crystalline structures carries out at higher temperatures. Furthermore, the percentage of the chains perched in the crystalline structures is also reduced. That is why the degree of crystallinity diminished for the EVA samples containing the modified NDs shown in Table 5. It has been reported by Karami et al. [46] that the changing in surface chemistry of NDs could alter the crystallization parameters of polyamide 6 (PA 6).

Fig. 12. Typical stress-strain curves of the prepared EVA/NDs samples.

evaporation rate of them through the EVA bulk [45]. That is why the melting enthalpy of the neat EVA declined from 16.54 to 13.06 J/g in the presence of 2 wt% of NDs. Haddadi et al. [39] showed that the addition of silica nanoparticles into the polyvinylidene fluoride (PVDF) matrix leads to a decrease in melting enthalpy due to the chain hindrance of silica nanoparticles. In addition, the molecular interactions between the functional groups of modified NDs, EVA chains and

Fig. 13. The values of Young's modulus (a), yield stress (b), tensile strength (c), and elongation at break (d) for the samples. The error bar corresponds to standard deviations. 9

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Fig. 14. Endothermic (a) and exothermic (b) sections of DSC thermograms of the prepared EVA/NDs samples.

polymer chains with the higher segmental motion and free volume are thermally degraded at the lower temperatures. Addition of the modified NDs in the EVA matrix improves the matrix stiffness and reduces the segmental mobility of the chains. That is why the EVA containing the higher contents of NDs have a higher T0.5 than that of the pure EVA. The enhanced thermal stabilities in the samples containing different contents of modified NDs are likely due to the presence of dispersed NDs with the superior thermal stability among the EVA chains leads to increase in the thermal stability enhancement of EVA/NDs nanocomposites. Also, the incorporation of NDs in the EVA matrix acts as a barrier to N2 and insulating the underlying EVA chains and declining the thermal decomposition rate. Moreover, the presence of dispersed NDs among EVA chains hinders the release and diffusion of the volatile decomposed components within the nanocomposite samples [47]. In addition, at the nanoscale, the thermal conductivity of NDs is very low due to the phonon scattering. Then, they are used as thermal insulators and the dispersion of NDs in the EVA matrix may protect the EVA chains to decompose thermally at the higher temperatures. Consequently, the EVA samples containing the higher contents of NDs were decomposed at the higher temperatures. As shown in Fig. 15, the content of the remained char at 600 °C for EVAND-2 was about 4% while the content of the added NDs was 2 wt% that was 3.8% higher than that of EVAND-0. This result clearly illustrates that the thermal decomposition and degradation kinetic of the EVA matrix can be changed in the presence of NDs. This behavior suggests that the modified NDs has a higher capability to improve the thermal stability of the EVA/NDs nanocomposites.

Table 5 DSC results of the prepared nanocomposite samples. Sample EVAND-0 EVAND-0.1 EVAND-0.5 EVAND-1 EVAND-1.5 EVAND-2

ΔHm (J/g)

Tc (°C)

ΔHc (J/g)

X (%)

16.54 16.08 15.71 14.45 13.94 13.06

48.29 48.37 49.01 49.36 49.45 50.76

17.58 16.54 15.04 13.69 12.11 10.44

6.21 5.85 5.34 4.88 4.34 3.76

They demonstrated for PA 6 containing NDs, Tc shifts to the higher temperatures and Xc decreases due to the surface modification of NDs with ethylenediamine (EDA). 3.2.6. TGA TGA analysis was conducted to evaluate the thermal stability of the prepared EVA/NDs samples. TGA thermograms of the EVA/NDs samples are shown in Fig. 15. As can be seen in Fig. 15, the thermal decomposition of nanocomposites includes two main steps. The first step, which was happened at a temperature range of 305–400 °C, can be attributed to the thermal decomposition of acetic acid and the formation of vinyl acetate and double bonds of the EVA skeletons. The second weight loss happened at the thermal range of 410–500 °C was related to the decomposition of ethylene. As it can be inferred from Fig. 15, the decomposition of EVA in the presence of modified NDs was shifted to the higher temperatures with NDs loading. T0.5 is the onset temperature at which 50% of the thermal degradation occurs. According to Fig. 15, the values of T0.5 for EVA containing 0, 0.1, 0.5, 1, 1.5 and 2 wt% of the modified NDs are 281.65, 292.3, 305, 315.45, 327.7 and 347.15 °C, respectively. The

3.2.7. Oxygen permeability coefficients of the EVA/NDs films Results and specifications of prepared films with EVA as matrix and NDs as filler obtained from the oxygen permeability analysis in accordance with Eq. (2) are given in Table 6. As can be seen from Table 6, in the entire EVA/NDs samples, the oxygen permeability is lower than that of EVAND-0. These considerable reductions could be attributed to the obstruction of gas flow paths through the EVA films in the presence of NDs. This could be related to Table 6 Values of oxygen permeability coefficients for the prepared EVA/NDs samples; the values are the mean of three replicates and ( ± ) corresponds to the standard deviations. NDs content (wt %) 0 0.1 0.5 1 1.5 2

Fig. 15. TGA thermograms of the EVA/NDs samples. 10

Thickness (μm)

496 500 505 503 499 497

± ± ± ± ± ±

10 15 10 10 15 15

r (mm)

40 40 40 40 40 40

± ± ± ± ± ±

1 1 1 1 1 1

Oxygen permeability coefficient (Barrier) 23.12 ± 0.65 19.71 ± 0.43 15.47 ± 0.38 10.60 ± 0.31 7.86 ± 0.13 4.96 ± 0.015

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NDs films and free volume among the EVA chains due to the barrier properties of the fillers into the polymer matrix.

the barrier effects of NDs in the EVA films due to the relatively uniform distribution of NDs in the EVA matrix providing the entire surface impenetrable. In addition, the gas permeability reduction of the EVA nanocomposite sheets in the presence of NDs could be partially attributed to the reduction of polymer matrix free volume affecting significantly the barrier properties and gas permeability of the nanocomposite sheets [48–51]. That is why the oxygen permeability coefficient of EVAND-2 sheet reduced about 78.5% in comparison with EVAND-0 sheet. According to the literature [26,52,53], the barrier properties of the polymer films and films can be significantly improved by the incorporation of a wide variety of the nanoparticles into the polymer matrices. In fact, apart from the effects of the nanoparticles on the overall characteristics of the polymer matrices, the incorporation of the nanoparticles into the polymer matrix restricts the penetration and permeation of the gas molecules, aggressive ions such as hydroxyls and chlorines, water and other components through the polymer matrices [54,55]. This phenomenon highlights the potential of the protective polymer films with the remarkable barrier properties to use for many applications such as packaging, corrosion protection coatings [11,54], biomedical sheets and so on. Shafiee et al. [13] evaluated the gas barrier properties of EVA/polypropylene/nanoclay nanocomposites. They reported that the addition of 3 wt% organo-clay (Cloisite 15A) declined the gas barrier properties of the prepared nanocomposite about 26.5% in comparison with the control sample. Also, Dadfar et al. [7] reported 58.6% improvement in the oxygen permeability coefficient for the EVA films containing 3 wt% organ-oclay (Cloisite 15A). While in the presence of only 2 wt% NDs, the oxygen permeability coefficient of EVA/NDs nanocomposite films reduced about 78.8% compared to that of the neat EVA film. In another research [2], 55.8% improvement in the gas barrier properties of the EVA film was observed in the presence of 1.5 wt% graphene nanosheets. These results illustrate that the gas barrier properties of the polymer films can be decline sharply in comparison with other types of fillers in the same concentration. Consequently, NDs can be used as the appropriate filler with the good gas barrier properties in the polymer matrices for packaging applications due to their smaller particle size compared to other fillers providing a larger contact surface area with the polymer matrix. One of the major usages of these EVA nanocomposite films is for the packaging of the various food products in food industries. These types of polymeric packaging materials could prevent oxygen penetration reducing the food spoilage by aerobic bacteria and improve their shelf life.

Acknowledgment The Authors would like to thank the Iran National Science Foundation (INSF) for support of this project under, Proposal number 94027859. References [1] M. Amini, A. Ramazani SA, A.A. Varjouy, M. Faghihi, Effect of exfoliated molybdenum disulfide oxide on friction and wear properties of ultra high molecular weight polyethylene, Polym. Adv. Technol. 29 (2018) 3085–3096. [2] M. Tayebi, A. Ramazani SA, M.T. Hamed Mosavian, A. Tayyebi, LDPE/EVA/graphene nanocomposites with enhanced mechanical and gas permeability properties, Polym. Adv. Technol. 26 (2015) 1083–1090. [3] G.M. da Olyveira, L.M.M. Costa, A.J.F. da Carvalho, P. Basmaji, L.A. Pessan, Novel LDPE/EVA nanocomposites with silver/titanium dioxide particles for biomedical applications, J. Mater. Sci. Eng. B. 1 (2011) 516. [4] A. Mojtabaei, M. Otadi, V. Goodarzi, H.A. Khonakdar, S.H. Jafari, U. Reuter, U. 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4. Conclusions Comprehensive characterization of EVA nanocomposites containing different contents of sol-gel surface-modified NDs for the packaging applications was investigated. The grafting of VTS onto the surface of NDs was evaluated properly by FTIR and TGA analyses. Also, the addition of the modified NDs into the EVA matrix increased the hydrophobicity of the EVA/NDs films and makes them proper for the packaging applications. RMS results revealed that the addition of the modified NDs into the EVA matrix amplify the shear-thinning behavior of the EVA matrix. Also, the shear viscosity, complex viscosity, storage modulus as well as complex modulus increase with the increment of NDs content. Tensile test results revealed that the mechanical properties of the EVA matrix enhanced considerably in the presence of NDs. The degree of crystallinity decreases with increase in NDs contents due to the lower segmental motions of the EVA chains in the presence of NDs restricting the placements of the chains into the crystalline structures. Thermal stability and ash contents of the EVA/NDs samples enhance with an increment of the NDs contents due to the better heat distribution in the nanocomposite samples and the higher thermal stability of NDs compared to the EVA matrix. Addition of NDs into EVA matrix decreased the oxygen permeability through the prepared EVA/ 11

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