Structure and properties of the poly(vinyl alcohol-co-ethylene)/montmorillonite-phosphorylated soybean protein isolate barrier film

Abstract

The present work developed a novel poly(vinyl alcohol-co-ethylene)/montmorillonite-phosphorylated soybean isolate protein (EVOH/MMT–PSPI) nanocomposite barrier film via a two-step mixing process and solution casting, and estimated its properties for food packaging. PSPI was first used as an intercalating agent to achieve the highly intercalated MMT–PSPI nanocomposites. The structure of EVOH/MMT–PSPI was proposed and confirmed. The structure of MMT–PSPI was associated with the level of MMT loading. An appropriate incorporation of MMT–PSPI resulted in a remarkable enhancement in the mechanical (tensile strength/Young's modulus) and barrier properties of the EVOH/MMT–PSPI films. In comparison with the neat EVOH film, an incorporation of MMT–PSPI at 3 wt% could improve the oxygen and water vapor barrier properties of the nanocomposite films by 73.5 ± 3.1% and 61.3 ± 4.1%, respectively, meanwhile, the tensile strength and Young's modulus reached 48.2 ± 2.4 MPa and 192.4 ± 4.2 MPa, the elongation at break still kept high at 225 ± 9.25% and the light transmittance of the films didn't obviously change. However, excess MMT–PSPI loadings gave rise to a reduction in those packing properties due to the aggregation of MMT. Additionally, the intercalated MMT nanoplatelets from MMT–PSPI in the EVOH matrix could significantly suppress the moisture-derived deterioration in the oxygen barrier property.

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1. Introduction

An important property of packaging materials for food is the high resistance to oxygen and moisture to ensure that the products do not deteriorate during storage and handling.¹ Poly(vinyl alcohol-co-ethylene) (EVOH) is one of the highly impermeable polymers to oxygen because of the presence of polar –OH and excellent self-association.^2,3 It has been commercially used in various multilayered packaging films as an oxygen barrier. The barrier properties of these materials depend on the ethylene content,⁴ crystallinity,⁵ temperature⁶ and humidity.^7,8 However, in the presence of moisture, the interactions among H₂O molecules and –OH groups of EVOH may result in a plasticization of the chains of EVOH, which will lead to an increase in the permeability of oxygen.^6,9

Layered silicate–polymer nanocomposites have been the focus of academic and industrial attention because the final composites often exhibit a desired enhancement in barrier, thermal and mechanical properties in comparison to pure polymer, even at very low clay contents.^10–14 Among these layered silicates by the names kaolin, bentonite, montmorillonite (MMT), hectorite and LAPONITE®, MMT is commonly used as the most promising inorganic clay minerals in the modification of polymer, especially in barrier properties.^3,15 Some representative research work has been done to decrease the oxygen permeability and water sensitivity of EVOH by incorporating MMT.^3,16,17

In order to achieve highly intercalated or exfoliated polymer–MMT nanocomposites, organ–MMT is usually adopted. In general, this modification may be carried out via exchanging the original inter-layer cations by organic cations and significantly increase the basal spacing of the clay layers. The common used organic cations are quaternary ammonium ions,^18–20 however, some of the quaternary ammonium halides are quite expensive and even environmental toxic.²¹ Soy protein isolate (SPI), a biopolymer extracted from soy bean contains more than 65% protein and 18% carbohydrates, is a renewable and abundant biopolymer.²² Chen et al.²³ reported highly exfoliated and intercalated SPI/MMT nanocomposites; Echeverría et al.¹⁴ and Lee et al.²⁴ fabricated nanocomposites films based on soy proteins and montmorillonite.

The phosphorylated soybean protein isolate (PSPI) could be easily achieved by phosphorylating reaction of SPI with sodium tripolyphosphate (STP), which has been used as food additive in food industry and the changes of functional properties.^25,26 However, no literature was reported on MMT–PSPI nanocomposites with PSPI as intercalating agent.

Therefore, we attempt to use PSPI as intercalating agent to achieve highly intercalated MMT–PSPI nanocomposites, and then fabricate poly(vinyl alcohol-co-ethylene)/montmorillonite-phosphorylated soybean isolate protein (EVOH/MMT–PSPI) nanocomposite barrier film. The structures of MMT–PSPI nanocomposites and EVOH/MMT–PSPI film were characterized and analyzed, and effect of MMT–PSPI contents on light transmittance, mechanical, thermal and barrier properties of films were investigated.

2. Experimental

2.1. Materials

Soybean protein isolate (SPI, containing 90% protein) was purchased from Anyang Detianli Food Co., Ltd. (Henan, China). Na⁺-montmorillonite (MMT, cation exchange capacity 100 meq./100 g) was supplied by Fenghong Clay Chemical Corporation (Zhejiang, China). Poly(vinyl alcohol-co-ethylene) (EVOH, Soarnol®, containing 32 mol% of ethylene) was provided by The Nippon Synthetic Chemical Industry Co., Ltd. (Nippon Gohsei) (Osaka, Japan). Methanoic acid (88%) was purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used to dissolve EVOH without further purification. All the other chemical reagents were of analytical grade and were available from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).

2.2. Preparation of samples

Achieving a high degree of intercalation and/or delamination of organoclay in the polymer matrix has been considered to be prerequisite for the preparation of high performance polymer nanocomposites.²⁷ In this study, two-step mixing process was employed to induce intercalated MMT platelets with a homogeneous dispersion within the EVOH matrix.

2.2.1. MMT–PSPI nanocomposites. The phosphorylating reaction of SPI was according to a slightly modified method as described by Jiang et al.²⁶ Assisted with magnetic stirring, 3 g of SPI was dispersed in 100 g Milli-Q water and then 0.09 g sodium tripolyphosphate (STP) was added into. After the phosphorylating reaction was performed at pH 8 and 35 °C for 3.5 h, the required amount of MMT was added. 2 h later, the suspension was centrifugated at 10 000 rpm for 5 min. Subsequently, the precipitate was suspended in 10 mL acetone and then filtered. Finally, the products were collected and vacuum dried at 60 °C for 24 h to obtain a yellow montmorillonite-phosphorylated soybean protein isolate (MMT–PSPI) nanocomposite powders. By controlling MMT contents to be 5, 10, 15, 20 and 25 wt% of SPI, the MMT–PSPI powders were coded as MS-5, MS-10, MS-15, MS-20, and MS-25, respectively.

2.2.2. EVOH/MMT–PSPI films. EVOH particles were dissolved in an 88% methanoic acid solvent at 80 °C for 1 h to obtain 10 wt% solution. Then as-prepared MMT–PSPI powders were dispersed in the solution by ultrasound for 30 min. After magnetic stirring for another 2 h, the resultant viscous liquid was cast onto a glass substrate. After drying at 40 °C for 24 h, the EVOH/MMT–PSPI films were peeled from the substrate and vacuum dried at 80 °C for another 24 h. By controlling MMT–PSPI contents to be 1, 3, 5, and 7 wt% of EVOH, the EVOH/MMT–PSPI films were designated as EM-1, EM-3, EM-5, and EM-7, respectively. All of the dried samples were kept in a dessicator to prevent the moisture influence prior to performing characterization.

2.3. Structures of MMT–PSPI nanocomposites and EVOH/MMT–PSPI films

2.3.1. X-ray diffraction. The gallery distance between the stacked nanoclay layers was determined using an X-ray diffractometer (D/MAX2500V diffractometer, Rigaku, Japan) with Cu Kα radiation (λ = 0.15418 nm). Samples were scanned in the range of 1.5–10° at a scanning rate of 1.0° min⁻¹.¹⁴ The basal spacing of the clay was determined from the position of d₀₀₁ peak in the X-ray diffraction (XRD) pattern using Bragg equation:

λ = 2d sin θ (1)

where θ is the diffraction position and λ is the wavelength.

2.3.2. Fourier transform infrared spectroscopy. Fourier transform infrared spectroscopy (FTIR) spectra were used to investigate the structure of samples and were conducted on a Nicolet 6700 spectrometer (Thermo Nicolet, Madison, WI, USA). The powders were subjected to FTIR spectroscopy in the range of 4000–400 cm⁻¹ using KBr pellets. The membranes were subjected to attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR).

2.3.3. Transmission electron microscopy. The microstructures and morphologies of MMT and MMT–PSPI (MS-20) powders were characterized by a transmission electron microscopy (TEM, JEM-2100, JEOL, Japan) at an operating voltage of 200 kV. Powder samples (1.0 mg) were suspended in absolute alcohol (10 mL) under the condition of ultrasonic power (150 W, 2 min) at room temperature. The suspensions were dropped onto a Formbar-backed carbon-coated copper grid for TEM observation.

2.3.4. Scanning electron microscopy. The morphologies of fracture surfaces of specimens were observed using scanning electron microscopy (SEM, SU8020, Hitachi, Japan) under an accelerating voltage of 15 kV. Prior to examination, the fracture surfaces were coated with sputter-gold to improve conductivity. Cryogenic fracturing method was used to prepare the fracture surfaces of specimens by placing them in liquid nitrogen for 5 min.

2.3.5. Zeta potential. Zeta potential was determined using a Zetasizer Nano-ZS90 apparatus (Malvern Instruments, Worcestershire, UK) to assess the surface charge SPI, PSPI and MMT–PSPI nanocomposite particles. Assisted with magnetic stirring, the testing suspensions (1%, w/v) were prepared by dispersing the powders into Milli-Q water (Millipore, Bedford, MA, USA) at 35 °C for 3.5 h, and then cooled to room temperature. The suspensions (1 mL) were injected into a plug-type electrode using a needle tubing, and then the electrode was placed in the test cell. Triplicate measurements were taken for each sample at room temperature.

2.4. Film surface color and opacity

Thickness of the films was measured at nine different randomly selected locations using a hand-held micrometer (BC Ames Co., Waltham, MA, USA). The average value of the film thickness was used in determining mechanical and barrier properties.

A Minolta Chroma meter (CR300, Minolta Chroma Co., Osaka, Japan) was used to measure the film surface color, and a Hunter Lab color scale was used to measure the parameters [L (lightness), +a/−a (redness/greenness) and +b/−b (yellowness/blueness)]. The films were measured on the surface of the white standard plate (L = 97.48, a = −0.48 and b = 2.15). Total color difference (ΔE) was calculated from:

ΔE = [(ΔL)² + (Δa)² + (Δb)²]^0.5 (2)

where ΔL, Δa and Δb are the difference between color value of standard color plate and films. Values were expressed as the means of nine measurements on different area of each film.

Opacity values of the films were determined using a UV-vis spectrophotometer (754PC, Shanghai Jinghua Technology Instruments Co., Ltd., Shanghai, China). Each film specimen was cut into a rectangular piece (4.5 cm × 1 cm) and placed directly in a spectrophotometer cell, and measurement was performed with air as the reference for transparency. The area under the absorption curve from 400 to 800 nm was recorded and the opacity of film (arbitrary units per mm) calculated by dividing the absorbance at 500 nm by the film's thickness (mm).^14,28 All determinations were performed in triplicate.

2.5. Mechanical properties of EVOH/MMT–PSPI films

The tensile strength, Young's modulus, and elongation at break of the films were determined following the procedures outlined in the ASTM methods D882-91 with an average of five measurements taken for each film and with at least two films per formulation. All the specimens were cut into rectangular strips (about 1 cm × 10 cm in width and length) that were mounted between the grips of the TA-XTPlus Texture Analyser (Stable Micro Systems, Co., UK). The initial grip separation was set at 50 mm and the crosshead speed at 0.5 mm s⁻¹. The tensile strength (σ = force per initial cross-sectional area) and elongation at break (ε) were determined directly from the stress–strain curves through the use of OriginPro 8 software (Origin Lab Corporation, USA), and Young's modulus (E) was obtained from the slope of the initial linear portion of that curve.

2.6. Thermal properties of EVOH/MMT–PSPI films

Differential scanning calorimetry (DSC) was conducted on a DSC Q2000 (TA Instruments, New Castle, USA). Film samples by weight of 5–10 mg were weighed in aluminum pans and hermetically sealed. The sample was initially heated from 10 °C to 210 °C at a heating rate of 30 °C min⁻¹ and the temperature was held at 210 °C for 5 min to eliminate the previous thermal history. Subsequently, the sample was cooled down to 10 °C at a cooling rate of 10 °C min⁻¹ to obtain cooling thermograms exhibiting crystallization temperature, and second heated to 210 °C at a rate of 10 °C min⁻¹ to obtain heating thermograms.

2.7. Barrier properties of EVOH/MMT–PSPI films

2.7.1. Water vapor permeability. Water vapor permeability (WVP) were measured using a modified method as described by Limpan et al.²⁹ The films, sealed on beakers containing silica gel (0% RH), were placed in an incubator, which equipped with a psychrometer for relative humidity and a big cup containing distilled water. The temperature of incubator was adjusted to 25 °C. The moisture absorbed was estimated by periodical weighing of beakers at 6 h intervals during 3 days. WVP (g m m⁻² d⁻¹ bar⁻¹) was determined for three replicate specimens each type, as follows:

WVP = (w × x)/(A × t × ΔP) (3)

where w is the weight gain of beaker (g), x is the film thickness (m), A is the area of exposed film (m²), t is the time of weight gain (s), and ΔP is the water vapor partial pressure difference (Pa) across the two sides of film calculated on the basis of relative humidity.

2.7.2. Oxygen permeability. Oxygen transmission rate (OTR, according to ASTMD1434) of films was determined at 23 °C and 0% RH on a N500 gas permeameter (Guangzhou Biaoji packaging equipment Co., Ltd., Guangzhou, China). Oxygen permeability (OP, cm³ m m⁻² d⁻¹ bar⁻¹) was calculated from OTR by the following equation:

OP = OTR × thickness (4)

The thickness and open testing area of each sample in three parallel measurements were approximately 100 μm and 50 cm². Two types of dried and moisture absorbed samples were used in the OTR measurement. The samples with different amounts of absorbed water were prepared by storing them in a container maintaining 100% relative humidity for different periods.

2.8. Statistical analysis

Each experiment was repeated three times. Statistical analysis was performed using the unpaired Student's t-test, and the results were expressed as the means ± standard deviation (SD). A value of p < 0.05 was considered to be statistically significant.

3. Results and discussion

3.1. Structure analysis

3.1.1. Structure of MMT–PSPI nanocomposites. Fig. 1A illustrates XRD patterns of SPI, MMT and MS-20. Pure SPI showed no transition in the pattern within the range analyzed, while MMT pattern reflected its characteristic crystallographic structure with a peak of diffraction at 2θ = 6.94°, which was associated with the interlaminar space of d₀₀₁ = 1.274 nm. In sharp contrast to MMT, the peak at 2θ = 6.94° had shifted to lower degree at 2θ = 2.94° in the pattern of MS-20, correspondingly, the value of d₀₀₁ was enlarged from 1.274 nm to 3.005 nm based on Bragg relationship (eqn (1)). The intercalation of polymer chains into interlayers of clay usually increases the interlaminar space and causes a shift of the diffraction peak towards a lower angle. Therefore, the SPI after being phosphorylated (PSPI) was confirmed to be intercalated into the interlayers of MMT successfully. For further investigating the structures of MMT interlayers in MMT–PSPI nanocomposites, XRD patterns of nanocomposite powers with different MMT contents were performed as shown in Fig. 1B. It was noteworthy that no peak associated with MMT was presented in the pattern of MS-5, indicating the exfoliated structure of MMT in MS-5 matrix. In addition, the values of 2θ were dependent on MMT contents. MS-10, MS-15 and MS-20 showed peaks at 2θ = 2.52°, 2.60° and 2.94° (corresponding to d₀₀₁ = 3.506, 3.398 and 3.005 nm), respectively. These data indicated that the MMT–PSPI nanocomposites could be transferred from exfoliated structure to intercalated structure and the values of d₀₀₁ decreased with the increase of MMT contents in a certain degree. However, as MMT content was raised up to 25 wt% (MS-25), the intercalated peak disappeared and only a single peak at around 2θ = 7 (characteristic of MMT) was observed, indicated the agglomeration of MMT clay in nanocomposites. Furthermore, Fig. 2 illustrates the TEM images of MMT and MS-20. It could be seen that the slice layers of MMT stacked tightly together and the margin of each slice layer was blurred (Fig. 2a). In sharp contrast to MMT, the MS-20 showed a clearly margin of each slice layer, indicating that the interlayer spacing was enlarged owing to the intercalation of PSPI (Fig. 2b). Therefore, the intercalated structure of MS-20 nanocomposites was further confirmed. According to our experimental results, MS-20 nanocomposites were chosen to prepare EVOH/MMT–PSPI barrier films.

Fig. 1 XRD patterns of (A) SPI, MMT and MS-20; (B) MMT–PSPI nanocomposites with different MMT contents and (C) MS-20, EVOH and EM-3.

Fig. 2 TEM images of (a) MMT, (b) MS-20.

Fig. 3A depicts the FT-IR spectra of SPI, PSPI, MMT and MS-20 nanocomposite powders. The spectrum of SPI powders shows a broad band at about at 3300 cm⁻¹ assigned to the vibration of N–H and –OH. The three absorption bands at 2956, 2927, and 2870 cm⁻¹ related to the protein antisymmetric and symmetric C–H stretching vibrations. The peaks at 1660 and 1527 cm⁻¹ were assigned to the amide I (C=O stretching) and amide II (N–H bending and C–N stretching modes) vibrations. In sharp contrast to SPI, a new absorption band at 928 cm⁻¹ associated with N-terminal PO₃²⁻ at SPI molecular chains appeared in the spectra of PSPI, which was different from the absorption of PO₄³⁻ ions at 1000–1100 cm⁻¹.³⁰ The phosphorylation reaction might be described as follow:

Fig. 3 FTIR spectra of (A) SPI, PSPI, MMT and MS-20 powders and (B) MS-20 powders, EVOH and EM-3 films.

From spectra of MMT, the absorption band at 3630 cm⁻¹ was assigned to the stretching vibration of –OH in AlOH and SiOH; the bands at 3450 and 1648 cm⁻¹ were ascribed to the stretching and bending vibration of –OH in H₂O; the band at about 1440 cm⁻¹ was speculated to be associated with the characteristic absorption of Na⁺-MMT, which was not found in spectra of Ca²⁺-MMT,³¹ Ni²⁺-MMT³² and Hg²⁺-MMT;³³ the strong broad band at about 1035 cm⁻¹ was associated with the stretching vibration of Si–O, and a very weak band at 1100 cm⁻¹ represented the out-of-plane Si–O vibration absorbance; the band at about 916 cm⁻¹ was ascribed to the bending vibration of Al–O; the bands at 523 and 463 cm⁻¹ indicated the stretching vibration of Al–O and bending vibration of Si–O, respectively. As compared to the spectra of PSPI and MMT, the characteristic absorption bands of PSPI and MMT were presented in the spectra of MS-20, however, the typical bands of 3630 cm⁻¹ and 3450 cm⁻¹ in MMT were almost transformed to broad band centered at around 3348 cm⁻¹, meanwhile, the band at 1100 cm⁻¹ was strengthen significantly. Moreover, the band at 928 cm⁻¹ associated with N-terminal PO₃²⁻ in PSPI shifted lower to 918 cm⁻¹, at the same time, the band at 1527 cm⁻¹ (C–N stretching) shifted higher to 1538 cm⁻¹. This distinct change meant the orientation structure of MMT was strongly perturbed by the PSPI molecules, indicating that PSPI molecules had successfully been intercalated into the interlayers of MMT, meanwhile, hydrogen bonds had been formed between MMT platelets and PSPI molecules (mainly in N⋯H–O or O⋯H–O).

As shown in Fig. 4, the zeta potential values of SPI and MMT were −10.7 ± 0.24 and −34.8 ± 1.74 mV, respectively. When phosphorylation reaction was taken on SPI molecules, the zeta potential value turned to be −13.8 ± 0.49 mV for PSPI. The more negative PSPI was owing to the negative ionization of N-terminal PO₃²⁻ as shown in eqn (5). In addition, the zeta potential of MMT–PSPI nanocomposites decreased gradually from −22.4 ± 0.73 mV (MS-5) to −27.3 ± 1.065 mV (MS-20) with increasing MMT contents, and then increased to −21.1 ± 0.855 mV (MS-25). It could be concluded that the decreasing zeta potential data derived from the addition of appropriate MMT could favor the stability of the blending MMT–PSPI aqueous system; however, as the MMT content reached 25%, the zeta potential increased owing to the aggregation MMT, which was in accordance with the XRD result (Fig. 1B).

Fig. 4 Zeta potential of SPI, PSPI and MMT–PSPI nanocomposites with varying MMT contents. The data (mean ± SD) are results from three independent experiments. In the graph, the p values are reported with respected to PSPI (*p < 0.05) and MMT (#p < 0.05).

3.1.2. Structure of EVOH/MMT–PSPI films. Fig. 1C illustrates the XRD spectra of MS-20 particles, EVOH and representative EVOH/MMT–PSPI films (MMT–PSPI 3%, EM-3, according to mechanical and barrier properties). EVOH film showed no diffraction peak in the 2θ range from 1 to 10°, indicating that EVOH has no ordered structure in this dimension range. Moreover, the 2θ of EM-3 shifted lower from 2.94° to 1.92° as compared to MS-20, correspondingly, the d₀₀₁ value was enlarged from 3.005 nm to 4.601 nm. The enlarged d₀₀₁ value indicated that some EVOH molecules had intercalated into the interlayers of MMT during dispersion of MMT–PSPI powders in EVOH matrix.

Fig. 3B shows the FT-IR spectra of MS-20 particles, EVOH and EM-3 films. In the spectrum of EVOH, the characteristic peaks for EVOH were present at: 3340 cm⁻¹ (–OH stretching); 2945 (–CH₂ anti-symmetric stretching) and 2912 cm⁻¹ (–CH anti-symmetric stretching); 1417 cm⁻¹ (–CH₂ asymmetric stretching) and 1336 cm⁻¹ (–CH asymmetric stretching); 1092 cm⁻¹ (C–O stretching); 850 cm⁻¹ (–CH out-of-plane deformation). Additionally, the peaks at 1650, and 850 cm⁻¹ might be associated with vibrations of the residual C=O or C=C groups in EVOH. In contrast to the spectra of MS-20 and EVOH, the characteristic absorption bands of MS-20 and EVOH were presented in the spectra of EM-3, meanwhile, the band at 3340 cm⁻¹ of EVOH shifted lower to 3320 cm⁻¹ and broadened significantly. It could be speculated that there existed intermolecular hydrogen bonds (mainly in N⋯H–O or O⋯H–O) between EVOH and MMT–PSPI in EVOH/MMT–PSPI composite film. Correspondingly, the fracture surfaces of the EVOH and EM-3 films were shown in Fig. 5. More patterns of cracks were observed on the fracture surface of EM-3 (Fig. 5b) as compared to neat EVOH film (Fig. 5a). The cracks on the fracture surface may imply the intensity of interactions between the polymer chains.³ Herein, the increase of cracks could mainly be attributed to the interfacial hydrogen bonding interactions between MMT–PSPI and EVOH.

Fig. 5 SEM images of the fracture surfaces for (a) EVOH and (b) EM-3 films.

On the basis of the analyzed results, the proposed structure of EVOH/MMT–PSPI was given in Fig. 6. The intercalated structure was formed in EVOH/MMT–PSPI film, and the interfacial interactions (MMT and PSPI, EVOH and MMT–PSPI) were attributed to hydrogen bonds.

Fig. 6 Proposed structure of EVOH/MMT–PSPI.

3.2. Thermal properties of EVOH/MMT–PSPI films

Fig. 7 shows the DSC second-heating curves of neat EVOH and EVOH/MMT–PSPI films, correspondingly, the glass transition temperatures (T_g) and melting temperatures (T_m) of each film were summarized in Table 2. With increasing the MMT–PSPI content, the values of T_g increased from 57.3 °C (neat EVOH) to the maximum value of 65.6 °C (EM-3), and decreased afterwards; on the contrary, the values of T_m decreased from 186.7 °C (neat EVOH) to the minimum value of 179.1 °C, and then increased at the same time. When a small amount of MMT–PSPI was added, the high interaction level with EVOH restricted chain mobility and caused a increase in T_g, meanwhile, the restriction hindered the crystallization process and resulted in a smaller crystallite size, correspondingly, T_m decreased.³⁴ However, as MMT–PSPI contents were beyond 3%, a decrease of T_g and an increase of T_m could be assumed to the aggregation and poor dispersion of MMT–PSPI in the matrix, which reduced the restriction to EVOH chains. Similar results were observed by Jiang et al.³⁵ they attributed the decrease of T_m in thermoplastic acetylated starch/poly(ethylene-co-vinyl alcohol) blends to the interactions between TPAS and EVOH which interrupt the crystallization of EVOH.

Fig. 7 DSC thermograms of EVOH film and EVOH/MMT–PSPI films.

3.3. Film surface color and opacity

Table 1 summarizes the tristimuli a, b and L values. The color parameters showed no obvious change when MMT–PSPI contents were lower at 3% and below. Whereas, as MMT–PSPI contents were beyond 3%, the lower L-values (p < 0.05) implied a decline in luminosity, the more negative a-values (p < 0.05) suggested the shift towards a greenish cast, and the more positive b-values (p < 0.05) gave an overall tone tending towards the greenish-yellow. Correspondingly, these shifts resulted in a significant increase in the ΔE-values (p < 0.05, calculated by eqn (2)). Herein, higher MMT–PSPI contents (beyond 3%) caused a larger change in film surface color, which might be associated with the aggregation of MMT–PSPI in matrix. The transmission of visible light was very important to preserve and protect products until they reach the consumer as well as to get an attractive transparent package.³⁶ In accordance with the change of surface color, the opacities of films showed no obvious change in values when MMT–PSPI contents were lower at 3% and below, indicating that there was not a large difference in the amount of light being transmitted through the nanocomposite films compared to EVOH owing to the better dispersed MMT–PSPI in matrix. As MMT–PSPI contents were beyond 3%, the opacity of film increased significantly (p < 0.05), for example, the EM-7 showed more than 2-fold increase in opacity value as compared to EVOH, because the aggregation of MMT–PSPI in matrix seriously blocked the transmission of visible light. Similarly, Tunç et al. observed a decrease in the transmittance values of nanocomposite films as the concentration of MMT in the methyl cellulose film matrix increased.³⁷

Table 1 Surface color and opacity of samples^a

Sample	Thickness (mm)	Hunter-lab color parameters				Opacity (UA mm^−1)
		L	a	b	ΔE
a Data with the same superscript letter in the same column indicate that they are not statistically different (p > 0.05). The data (mean ± SD) are results from six independent experiments.
EVOH	0.098 ± 0.007^a	96.06 ± 2.05^a	−0.78 ± 0.05^a	3.47 ± 0.19^a	1.96 ± 0.04^a	1.335 ± 0.124^a
EM-1	0.101 ± 0.007^a	96.22 ± 1.93^a	−0.82 ± 0.07^a	3.94 ± 0.32^ab	2.22 ± 0.06^a	1.321 ± 0.118^a
EM-3	0.102 ± 0.006^a	96.78 ± 2.2^ab	−0.87 ± 0.07^ab	4.10 ± 0.29^b	2.11 ± 0.06^a	1.302 ± 0.107^a
EM-5	0.105 ± 0.008^a	95.60 ± 2.21^b	−1.08 ± 0.10^b	5.55 ± 0.47^c	3.93 ± 0.09^b	1.782 ± 0.165^b
EM-7	0.107 ± 0.009^a	94.55 ± 1.89^c	−1.29 ± 0.09^c	7.82 ± 0.43^d	6.43 ± 0.11^c	2.813 ± 0.256^c

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Table 2 Glass transition and melting temperatures of samples

Sample	EVOH	EM-1	EM-3	EM-5	EM-7
Tg (°C)	57.3	63.1	65.6	64.4	63.4
Tm (°C)	186.7	179.9	179.1	180.6	182.4

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3.4. Mechanical properties of EVOH/MMT–PSPI films

Mechanical strength of nanocomposite films is described in terms of tensile strength, and the results are shown as a function of MMT–PSPI content in Fig. 8. As it could be seen from Fig. 8A, both tensile strength and Young's modulus showed a similar trend. With increasing the MMT–PSPI content, the tensile strength and Young's modulus increased from 21.6 ± 1.2 MPa and 58.8 ± 2.9 MPa (neat EVOH) to the maximum values of 48.2 ± 2.4 MPa and 192.4 ± 4.2 MPa (EM-3), respectively, and then decreased, while the elongation at break showed a decrease in values (Fig. 8B). When MMT–PSPI contents were at 3% and below, the higher the MMT–PSPI content was, the stronger the hydrogen bonds formed between MMT–PSPI and EVOH. This hydrogen interaction improved the tensile strength and Young's modulus of EVOH/MMT–PSPI film. As MMT–PSPI contents were beyond 3%, the decrease in the tensile strength and Young's modulus might be associated with the aggregation of MMT–PSPI in matrix (Section 3.2.). In addition, the obstructive action of MMT–PSPI worsened the flexibility of EVOH molecular chains, and caused the decreasing of elongation at break, however, the value still kept high at 225 ± 9.25% for EM3. Similar results were observed in EVOH/clay cast film by Kim et al. (the used clay was Na⁺-MMT modified with methyl tallow bis-2-hydroxyethyl quaternary ammonium).¹⁶

Fig. 8 Mechanical properties of EVOH/MMT–PSPI films as a function of MMT–PSPI content: (A) tensile strength and Young's modulus; (B) elongation at break.

3.5. Barrier properties of EVOH/MMT–PSPI films

The oxygen permeability (OP) and water vapor permeability (WVP) are generally considered in food packaging design. Fig. 9A shows that the OP and WVP of EVOH/MMT–PSPI nanocomposite films are strongly dependent on MMT–PSPI contents. A similar trend was seen in OP and WVP curves with increasing MMT–PSPI content. The OP and WVP increased significantly (p < 0.05) from (3.25 ± 0.08) × 10⁻⁴ cm³ m m⁻² d⁻¹ bar⁻¹ and (4.13 ± 0.12) × 10⁻⁶ g m m⁻² d⁻¹ bar⁻¹ (neat EVOH) to the minimum values of (0.86 ± 0.02) × 10⁻⁴ cm³ m m⁻² d⁻¹ bar⁻¹ and (1.60 ± 0.05) × 10⁻⁶ g m m⁻² d⁻¹ bar⁻¹ (EM-3), respectively, after then, OP and WVP increased again. It was noteworthy that the OP and WVP barrier properties of the nanocomposite films were improved by 73.5 ± 3.1% and 61.3 ± 4.1% in the presence of MMT–PSPI at 3 wt%. These strong enhancements in barrier properties may be due to the highly intercalated MMT nanoplatelets in the EVOH matrix which prolong the tortuous path for penetrating gases.^38–40 However, a further increase in MMT–PSPI content beyond 3 wt% had a negative effect on the improvement in the barrier properties, which might due to poor interfacial compatibility associated with the aggregation of MMT–PSPI in matrix.

Fig. 9 (A) Barrier properties of EVOH/MMT–PSPI films as a function of MMT–PSPI content; (B) OP of neat EVOH film and EVOH/MMT–PSPI nanocomposite film (EM-3) as a function of storage time under humid atmosphere of 100% RH. The thickness of each sample was approximately 100 μm.

In general, barrier properties of films will be deteriorated gradually in the presence of moisture during storage. Herein, the representative EM-3 was chosen to examine the humidity effect on the OP according to Fig. 9A, and the OP of neat EVOH and EM-3 were measured as a function of storage time under wet condition of 100% RH (Fig. 9B). As expected, both EM-3 and neat EVOH film showed an increase in OP as storage time prolonged, because moisture would weaken the force between molecular chains. However, the OP values of EM-3 increased much more slowly as compared to neat EVOH film. This demonstrated the less moisture sensitive of EM-3 than that of neat EVOH film. That was to say that the intercalated MMT nanoplatelets from MMT–PSPI in the EVOH matrix could significantly suppress the moisture-derived deterioration in the oxygen barrier property owing to the strong interfacial interactions of hydrogen bonds, which efficiently prevented water molecules through EVOH matrix.

4. Conclusions

In the present work, PSPI was first used as an intercalating agent to achieve the highly intercalated MMT–PSPI nanocomposites, and then incorporated into EVOH matrix to prepare EVOH/MMT–PSPI nanocomposite films with improved physical and gas barrier properties. Structure scheme of EVOH/MMT–PSPI was proposed and the interfacial interactions were confirmed to be hydrogen bonds.

The structure of MMT–PSPI depended on the level of MMT loadings. The addition of appropriate MMT caused a decrease in zeta potential data, which enhanced the stability of the MMT–PSPI aqueous system. With increasing MMT–PSPI at lower contents (3% and below), the tensile strength and Young's modulus of EVOH/MMT–PSPI films were increased, meanwhile, the elongation at break decreased but still kept a relatively high value at 225 ± 9.25% for EM3; moreover, the light transmittance of films changed unobviously; however, excess MMT–PSPI loadings (beyond 3%) resulted in significant reductions in the tensile strength/modulus, elongation at break and light transmittance. The T_g and T_m of EVOH/MMT–PSPI films showed a reverse variation trend with MMT–PSPI contents. As compared with neat EVOH film, the EVOH/MMT–PSPI films exhibited substantially improved oxygen and water vapor barrier properties, and the intercalated MMT nanoplatelets in the polymeric matrix could significantly suppress the moisture-derived deterioration in the oxygen barrier property. The EVOH/MMT–PSPI film may have potential as a good food packaging and the results can provide some valuable information for the design of barrier film based layered silicates.

Acknowledgements

Financial support from National Natural Science Foundation of China (31371859) is gratefully acknowledged.

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