Abstract
The goal of this work was to compare the barrier, mechanical, and thermal properties of two types of starch–polyvinyl alcohol (PVA) nanocomposites. Sodium montmorillonite (MMT) and nanocrystalline cellulose were chosen as nanoreinforcements. X-ray diffraction (XRD) test showed well-distributed MMT in the starch–PVA matrix, possibly implying that the clay nanolayers formed an exfoliated structure. The moisture sorption, solubility and water vapor permeability (WVP) studies revealed that the addition of MMT and nanocrystalline cellulose reduced the moisture affinity of starch–PVA biocomposite. At the level of 7 % MMT, the nanocomposite films showed the highest ultimate tensile strength (UTS) (4.93 MPa) and the lowest strain to break (SB) (57.65 %). The differential scanning calorimetry (DSC) results showed an improvement in thermal properties for the starch–PVA–MMT nanocomposites, but not for the starch–PVA–NCC nanocomposites. Results of this study demonstrated that the use of MMT in the fabrication of starch–PVA nanocomposites is more favorable than that of nanocrystalline cellulose to produce a desirable biodegradable film for food packaging applications.
1 Introduction
With increasing awareness on environmental problems and decrease of petroleum resources, the production of environment-friendly biodegradable plastic materials derived from natural resources is of significant attention in both academic and industrial fields [1]. Natural polymers from renewable resources have been considered to be excellent raw materials for replacement of petroleum resources. In this regard, starch due to its attractive combination of price, abundance and thermoplastic behavior as well as good biodegradability has been considered as one of the most promising candidates among the natural polymers. Nevertheless, starch exhibits several disadvantages such as a strong hydrophilic character (water sensitivity) and poor mechanical properties compared to conventional synthetic polymers, making it unsatisfactory for some applications such as packaging purposes [2]. One approach to improve the functional properties of the starch films is to blend starch with other polymers [3–5].
Polyvinyl alcohol (PVA) has excellent film forming [6], high tensile strength and flexibility, as well as high oxygen and aroma barrier properties [7, 8]. Since 1980s, starch–PVA has been studied primarily for producing biodegradable films. In general, blending starch with PVA improves the mechanical properties of starch-based materials [9–12]. However, a major problem with starch–PVA blends is their poor water barrier properties. Since starch and PVA molecules have a large number of hydroxyl groups, starch–PVA biocomposite plays a hydrophilic nature [8].
Preparation of nanocomposites has been considered as a promising method to improve the barrier and mechanical properties and thermal stability without affecting the transparency of biopolymers [13, 14]. Nanocomposites refer to multiphase materials where at least one of the constituent phases has one dimension less than 100 nm [15]. Nanoreinforcements have been classified as organic and inorganic nanomaterials. Today, inorganic layered silicates (Montmorillonite) (MMT) are the most commonly used nanomaterials in the plastic industry to improve flame retardancy, and mechanical and barrier properties [2]. Therefore, it is possible to improve the properties of starch–PVA polymer by the addition of small amounts of MMT [16–18]. When producing biopolymer-based nanocomposites, it is important to use nanoreinforcements, ensuring that the final material is biodegradable and solely based on renewable resources [19]. Layered silicates are naturally occurring materials; however, they are neither renewable nor biodegradable. Cellulosic nanofillers have attracted interests during the last decade as potential nanoreinforcing materials in different polymers due to their appealing intrinsic properties including nanoscale dimensions, high surface area, low density and high mechanical strength, renewability and biodegradability [20]. Cellulosic nanocrystals possess three unique molecular characteristics of significance that allow them to act as scaffolds for composite applications: cellulose nanocrystals are rigid molecular rods and can impart significant strength and directional rigidity to a composite; cellulose nanocrystals have an embedded polymeric directionality that can be preferentially exploited for building 3D network structured nanocomposites; and finally, cellulose nanocrystals have an etched molecular pattern on their surfaces composed of primary hydroxyl groups at the C6 position, which can also be exploited for grafting specific hydrophobes or hydrophiles [21].
Briefly, the effect of nanocrystalline cellulose (NCC) addition on the physicochemical properties of various polymers and biopolymers such as cellulose acetate [22], alginate [23]; pullulan [24], gelatin [25], poly(lactic acid) (PLA) [26] PLA/natural rubber [27, 28], nitrile rubber [29] carboxylated styrene-butadiene rubber [30], bacterial polyester poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [31] and Poly(ε-caprolactone) (PCL) [32] has been studied. Over the last few years, the production of the starch–NCC nanocomposites [19, 33, 34] and PVA–NCC nanocomposites [35, 36] have been reported by several research groups. Chen [37] compared the effect of NCC on the mechanical, thermal, barrier and structural properties of starch and PVA nanocomposites.
To our knowledge, there is no definite report on the preparation of starch–PVA–NCC nanocomposites. Petersson and Oksman [13] compared layered silicates and microcrystalline cellulose as nanoreinforcement in PLA nanocomposites. Also, Bendahou et al. [38] investigated the synergism effect of MMT and NCC on the mechanical and barrier properties of natural rubber composites. However, the comparison of the effect of MMT and NCC on the physicochemical properties of starch–PVA films has not been investigated and so, current research is the first comparative study in this area. The goal of this study was to compare the mechanical, thermal and barrier properties of two types of starch-PVA–based nanocomposites containing two different nanoreinforcements, NCC and MMT.
2 Materials and methods
2.1 Materials
Potato starch (PS) (12 % moisture content) was provided from Alvand industry (Hamedan, Iran). Polyvinyl alcohol (PVA) (degree of polymerization: 145,000, degree of hydrolysis: 89–87 %, molecular weight: 23,000) was obtained from Merck (Darmstadt, Germany). Sodium montmorillonite (Cloisite® Na+) with a cation exchange capacity (CEC) of 92.6 mequiv./100 g clay was supplied by Nanocor Inc. (Arlington Heights, IL). Analytical grade glycerol, potassium sulfate, calcium sulfate and calcium nitrite were also purchased from Merck (Darmstadt, Germany).
2.2 Preparation of nanocrystalline cellulose (NCC)
Cotton linter was soaked in a 2 wt% NaOH aqueous solution for 12 h at room temperature under mechanical stirring to remove impurities and then washed with distilled water. Cotton fibers (11 g) were hydrolyzed with 64 wt% aqueous sulfuric acid solution (100 mL) under continuous stirring at 45 °C for 1 h. Then the mixture was diluted with cold distilled water to stop the reaction followed by centrifugation at 10,000 rpm for 15 min. The process of dilution and centrifugation was repeated 8 times until pH reached to 4. The resultant suspension was dialyzed against distilled water until approximate neutrality was achieved. The dispersion of NCC was treated with Ultra TuraxT25 homogenizer (Medisca, USA) at 24,000 rpm for 30 min followed by ultrasonic treatment (Branson sonifier, USA) for 15 min at room temperature. The whisker content was determined by weighting aliquot of the solution before and after drying [39].
2.3 Preparation of PS–PVA nanocomposite films
0.3 g of PVA powder was dissolved in 80 mL hot water at 98 °C and then 3 g starch powder was mixed and stirred well for about 15 min with constant stirring until complete gelatinization of starch was achieved. The desired amount of NCC suspension was weighed and sonified for 15 min to re-disperse any agglomerates possibly formed. Then suspension poured into starch–PVA blend and stirred on 80 °C water bath and then glycerol (50 wt% starch) was added and was stirred for 10 min. Finally, the solution was casted into a Teflon dish. The thickness of the films was controlled by pouring a known volume of the blend solution into the Teflon dish. The casted solution was dried at 55 °C for 16 h in an oven. The dried films were removed from the Teflon dish and used for further study.
In order to prepare starch–PVA–MMT nanocomposites, the desired amount of MMT was dispersed in 25 mL distilled water by sonication during 10 min at room temperature. The MMT dispersion was added to the aqueous dispersion of starch and PVA. Subsequently, glycerol (50 wt% starch) was added and the procedure continued as that of starch–PVA–NCC samples. All of the samples were prepared at room temperature and relative humidity in three days. The same conditions were used in the preparation of nanocomposites. The resultant starch–PVA–NCC and starch–PVA–MMT nanocomposites with 3, 5 and 7 wt% loading level of NCC and MMT, were coded as PS-PVA-NCC3, PS-PVA-NCC5, PS-PVA-NCC7, PS-PVA-MMT3, PS-PVA-MMT5 and PS-PVA-MMT7, respectively. A neat starch–PVA film was also prepared according to the aforementioned process without the addition of nanofillers (PS-PVA). Average thickness of the films measured by Alton M820-25 hand-held micrometer (China) was 0.08 ± 0.01 mm.
2.4 Morphological characterization of NCC
The morphology of NCC was investigated using transmission electron microscope (TEM) with a LEO 906 (Germany) microscope at an acceleration voltage of 120 kV. A drop of diluted NCC aqueous suspension was deposited on a carbon-coated grid and then stained with a 2 % (w/v) aqueous solution of uranyl acetate. Also the atomic force microscopy (AFM) measurements were performed with a Dimension V (Veeco) atomic force microscope (AFM, Nanotec Electronica S.L., Madrid, Spain). A drop of diluted nanocrystal aqueous suspension was allowed to dry on optical glass substrate at room temperature and analyzed subsequently. The film samples were placed on the surface of the steel substrate then were analyzed.
2.5 Characterization of starch–PVA nanocomposites
2.5.1 X-ray diffraction (XRD)
X-ray diffraction (XRD) studies of the samples were carried out using a Bruker D5000 Advance X-ray diffractometer (Siemens, Germany) operating at wavelength of 0.1539 nm. The samples were exposed to the X-ray beam with the X-ray generator running at 40 kV and 30 mA. Reflection mode was used as geometry type of test and scattered radiation was detected at ambient temperature in the angular region (2θ) of 4–40° at a rate of 1°/min and a step size of 0.05°.
2.5.2 Solubility in water
Film specimens were kept in a desiccator containing dry calcium sulfate until they reached constant weight. Afterward about 500 mg of each film was immersed in beakers containing 50 mL of distilled water at 23 °C for 24 h with periodical gentle manual agitation. The films were removed from the water and were placed back in the desiccator until they reached a constant weight to obtain the final dry weight of the film. The percentage of the total soluble matter (%TSM) of the films was calculated using the following equation:
TSM tests for each type of film were carried out in three replicates.
2.5.3 Moisture absorption
Moisture absorption was measured according to the method of Almasi et al. [2]. The dried sheets of 20 mm × 20 mm were first conditioned at 0 % RH (calcium sulfate) for 24 h. After weighing, they were conditioned in a desiccator containing potassium sulfate saturated solution at 20–25 °C to ensure a relative humidity of 97.3 %. The samples were weighed at desired intervals until the equilibrium state was reached. The moisture absorption of the samples was calculated as follows:
where W0 is the initial weight of the samples and Wt is the weights of the samples after time at 97.3 % RH. All measurements were performed in three replicates.
2.5.4 Water vapor permeability (WVP)
WVP tests were carried out using the standard ASTM method E96 [40] with some modifications. Special cups, with an average diameter of 2 cm and a depth of 4.5 cm, were utilized to determine WVP of films. Films were cut into discs with a diameter slightly larger than that of the cup. After placing 3 g of anhydrous CaSO4 in each cup, they were covered with starch–PVA films of varying compositions. Relative humidity (RH) of 0 was maintained using anhydrous CaSO4 in the cup. Each cup was placed in a desiccators containing saturated K2SO4 solution in a small beaker at the bottom. A small amount of solid K2SO4 was left at the bottom of the saturated solution to ensure that the solution remained saturated at all times. Saturated K2SO4 solution in the desiccator provides a constant RH of 97.3 % at 25 °C. The desiccator was kept in an incubator at 25.0 ± 0.1 °C. Cups were weighed every 24 h and water vapor transport was determined by the weight gain of the cup. Changes in the weight of the cup were recorded as a function of time. Slopes were calculated by linear regression (weight change vs. time). The water vapor transmission rate (WVTR) was defined as the slope (g/h) divided by the transfer area (m2). WVP (g m−1 h−1 Pa−1) was calculated as:
where P is the saturation vapor pressure of water (Pa) at the test temperature (25 °C), R1 is the RH in the desiccator, R2, the RH in the cup and X is the film thickness (m). Under these conditions, the driving force [P (R1 − R2)] is 3,073.93 Pa. All measurements were performed in three replicates.
2.5.5 Mechanical properties
Ultimate tensile strength (UTS) and strain to break (SB) of the films were determined at 25 ± 1 °C using a tensile tester (Zwick/Roell model FR010, Germany) according to ASTM D882-91 [41]. After conditioning in RH = 55 % for 24 h, three film specimens, 8 cm × 0.5 cm dumbbelly forms, were cut from each of film samples and were mounted between the grips of the machine. The initial grip separation and cross-head speed were set to 50 mm and 5 mm/min, respectively.
2.5.6 Thermal properties
The thermal properties of the films were determined by differential scanning calorimetry (DSC) (Model F3 200 DSC Netzsch, Germany). After conditioning in RH = 55 % for 24 h, a 5 ± 3 mg film sample was cut as small pieces and was placed into a sample pan. The reference was an empty pan. The glass transition temperatures (Tg) of the different films were measured at a heating scan rate of 10 °C/min from 30 to 300 °C, and they were identified as the midpoint temperature of a step-down shift in baseline due to change in heat capacity upon glass transition. Also the melting points (Tm) of the films were determined. The Tg and Tm of each film were determined in duplicate and the results were averaged.
2.6 Statistical analysis
Statistics on a completely randomized design were performed with the analysis of variance (ANOVA) procedure in SPSS (Version 11.5, SPSS Inc., Chicago, IL) software. Duncan’s multiple range test (P ≤ 0.05) was used to detect differences among mean values of films properties.
3 Results and discussion
3.1 Morphological characterization of NCC
The morphologies and dimensions of prepared NCC were determined by TEM and AFM analyses (Figure 1). As shown in Figure 1(A), the obtained hydrolyzed suspension of cellulose contains both long and curved elongated nanofibers and short fragments. It is obvious that the width of the whiskers is less than 100 nm. Thus the fibers appear to be separated into individual fibril structures. In spite of indistinguishable individual nanoparticles in TEM images, AFM phase images (Figure 1(B)) show well-isolated and dispersed rod-like whiskers in the nanometer scale. The hydrolyzed suspension contains NCC fragments consisting of both individual and aggregated nanocrystals. The diameters of the whiskers are less than 100 nm. According to the size calculations on AFM images, NCC possesses average diameter of 50–80 nm, length of 600 nm to 1.5 μm and aspect ratio of 15.4.
TEM (A) and AFM (B) images of cotton cellulose nanowhiskers.
3.2 Characterization of starch–PVA nanocomposites containing MMT and NCC
3.2.1 X-ray diffraction (XRD)
In order to investigate the dispersion of both MMT layers and NCC nanocrystals in polymer matrix, X-ray diffraction analyses were performed on the nanocomposites. Figure 2(A) shows the XRD patterns for the pristine MMT and starch–PVA–MMT nanocomposites.
A characteristic diffraction peak is appeared at 2θ = 7° for the natural MMT [2], corresponding to gallery spacing of 1.28 nm. In the nanocomposite films, the 001 diffraction peak of the MMT (2θ = 7) was disappeared. These results indicate that either the starch or the PVA polymer chains or both entered into the silicate layers, forming exfoliated structure. It could possibly be due to the strong polar interactions between the hydroxyl groups of both biopolymer chains and also glycerol and silicate layers [2, 42]. In order to approve this statement, a control PS–PVA film containing 5 % neat MMT powder on its surface (laminated by using a glue) was prepared and analyzed by XRD. As shown in Figure 2(A), the peak corresponding to MMT layers has been revealed in this control sample that approves the intercalated structure in unfilled film.
There was no difference between the nanocomposites containing various amounts of MMT which indicates that clay content did not have any significant effect on the occurrence of intercalated or exfoliated structure. The hydroxyl groups of the starch and/or PVA are able to interact directly with the sodium ion of the MMT gallery (as the water molecules do) or with the edge hydroxyl groups of the MMT layers, making a very compatible system [42]. Similar results were reported by Ali et al. [17] for starch–PVA–MMT nanocomposites for loading levels of less than 10 wt%. However, Almasi et al. [2] observed the intercalated structure in starch–CMC–MMT nanocomposites even at MMT contents of 1 to 5 %.
XRD patterns of NCC suspension and starch–PVA–NCC nanocomposites are shown in Figure 2(B). Dried NCC suspension displays typical cellulose I diffraction spectrum [43, 44] with a sharp peak centered at about 22.7° and two overlapped weaker peaks at about 14.7ᵒ and 16.5ᵒ. Therefore, it could be noted that the crystalline structure of cellulose was unaltered by acid hydrolysis in this work. Starch–PVA film shows no peak, by the addition of NCC to the starch–PVA matrix, the diffraction peaks of NCC were disappeared. Also, Arrieta et al. [45] showed that at low contents of cellulose nanocrystal (up to 5 wt%), the expected diffraction peaks of cellulose I were not observed in poly(lactic acid) films contain cellulose nanocrystals. In another study, Cao et al. [46] showed that for polyurethane films contains 0–30 % NCC, under 10 % contents of NCCs, XRD patterns showed amorphous nature. With an addition of NCC in the polymer matrix, some diffraction peaks appear in the diffractograms. With increasing the NCCs content in the films, the peaks become more significant. When it increases up to 20 wt %, three well-defined peaks at for pure NCC, 2θ=14.7°, 16.5°, and 22.7° are observed for nanocomposites, too.
These results for two nanoreinforcements are in agreement with Kvien et al. [47] who studied the effects of cellulose nanowiskers and layered silicates (synthetic hectorite) as reinforcements in modified starch-based nanocomposites.
XRD patterns for pristine MMT and starch–PVA–MMT nanocomposites (A), and, starch–PVA biocomposite and starch–PVA–NCC nanocomposites (B).
3.2.2 Morphology of bionanocomposites by AFM
In order to investigate the dispersion of two nanoreinforcements in the matrix phase, AFM studies were done. The AFM images for PS–PVA film and two nanocomposites contain 7 % MMT and NCC are shown in Figure 3. Comparing with the PS–PVA, two types of nanocomposites show more dense structure. The images of the two nanocomposites show that the second phase (nanoreinforcement) embedded in the continuous phase. The brown regions in images could be attributed to the nanoreinforcement (NCC and MMT). The images for NCC nanocomposite show that NCC is distributed uniformly in the matrix, it can be seen the formation of network in the film matrix, but some aggregates can be seen, too and it could be attributed to NCC aggregates in this area. Etang Ayuk et al. [48] showed the similar images for cellulose acetate butyrate nanocomposites contain NCC. In starch–PVA–MMT bionanocomposite films, the linear and brown regions could be attributed to MMT and the homogenous and parallel distribution of MMT in the starch matrix indicate the presence of partly exfoliation clay layers dispersed into the matrix. Cao et al. [29] attributed this phenomenon to the similarity of chemical structure between matrix and filler, implying hydrogen bonding interaction between two phases. Also, Ghanbarzadeh et al. [49] showed homogenous distribution of MMT in starch/CMC/MMT nanocomposites too.
AFM images for starch–PVA film (A), starch–PVA–NCC 7 % (B), starch–PVA–MMT 7 % (C).
3.2.3 Solubility in water
The water solubility of the starch–PVA films with and without MMT and NCC is shown in Table 1. Addition of both of the nanoreinforcements decreased the solubility of starch–PVA films. Similarly, significant decreases (p≤0.05) in solubility observed in high levels of MMT and NCC. The lowest amount of %TSM was observed for the starch–PVA films containing 5 and 7 % MMT, which was significantly lower than those of starch–PVA films with 5 and 7 % NCC.
The hydroxyl groups of MMT can form strong hydrogen bonds with the hydroxyl groups on starch and PVA, thus improving the interactions between the molecules, improving the cohesiveness of biocomposite matrix and decreasing the water sensitivity. Decreasing effect of NCC on solubility of starch–PVA nanocomposites can be ascribed to the formation of a three-dimensional cellulosic network that strongly restricts the dissolution of the polymeric matrix in water [15]. Therefore, a similar mechanism can be suggested for decreasing effect of NCC and MMT on solubility of starch–PVA nanocomposites. However, the significant variation in solubility results, especially in higher loading levels, may be due to the morphological differences between two nanofillers. The surface area of MMT is approximately 750 m2/g. Besides, the aspect ratio of Cloisite® Na+ platelets had a wide distribution with values ranging from 180 to 2,900. But more than 40 % of the Cloisite® Na+ platelets have aspect ratios higher than 1,000 [50]. Thus, it is more obvious that the number of hydrogen bonds between such broad surfaces of MMT platelets and biopolymer chains would be notably more than those between NCC with aspect ratio of 15.4 and starch or PVA macromolecules.
3.2.4 Water vapor permeability (WVP)
Table 1 shows the effects of MMT and NCC on WVP of starch–PVA bionanocomposites. It can be seen that the WVP of films has decreased with addition of two nanoreinforcements. Comparison of the effects of two reinforcements reveals that the MMT is more effective than NCC in decreasing WVP, and PS-PVA-MMT7 had the lowest WVP (6.19 × 10−8 g/m.h.Pa). Generally, water vapor permeation through a hydrophilic film depends on both diffusivity and solubility of water molecules in the film matrix [2]. When the biopolymer-MMT nanocomposite structure is formed in a well-exfoliated and dispersed state, individual clay platelets have high aspect ratio, which are believed to increase the barrier properties by creating a maze or “tortuous pathway” for water molecules to traverse the film matrix, thereby increasing the effective path length for diffusion and retarding the progress of gas and vapor molecules through the polymer matrix [16, 51, 52]. The decreased diffusivity due to the formation of exfoliated nanostructure, in the case of starch–PVA–MMT composites, reduces the WVP. Also the reduction in OH groups, due to the formation of hydrogen interactions between matrix and MMT layers decreases the solubility of water molecules in the matrix.
The barrier properties are enhanced if the filler has both good dispersion in the matrix and a high aspect ratio [44]. Therefore, NCC nanofiller cannot be more effective than MMT in decreasing WVP, since its aspect ratio is lower than that of MMT layers in the exfoliated state. NCC cannot have significant effect on tortuosity of film matrix and diffusivity of water molecules. The reduction in WVP in starch–PVA–NCC nanocomposites can be justified by the fact that the water resistance of NCC was better than film matrix. This can be attributed to the highly crystalline structure of the cellulose fibers in comparison to starch or PVA biopolymers [39]. Also the reduction in free OH groups of polymers can be another reason for decreasing the WVP of NCC contained nanocomposites. Accordingly, MMT in a fully exfoliated state with higher aspect ratio is more effective than NCC in decreasing WVP.
The total soluble matter (TSM) and water vapor permeability (WVP) of starch–PVA films as a function of MMT and NCC content.
| Sample | TSM (%) | WVP (×10−8g/m.h.Pa) |
| PS-PVA | 23.89 ± 3.12a | 7.41 ± 0.41a |
| PS-PVA-MMT3 | 20.69 ± 1.68a,b | 6.76 ± 0.65c |
| PS-PVA-MMT5 | 13.69 ± 1.61c | 6.37 ± 1.45d |
| PS-PVA-MMT7 | 11.75 ± 1.61c | 6.19 ± 1.85d |
| PS-PVA-NCC3 | 20.85 ± 2.93a,b | 7.26 ± 0.24a |
| PS-PVA-NCC5 | 20.39 ± 2.33a,b | 7.17 ± 1.5a,b |
| PS-PVA-NCC7 | 17.87 ± 1.24b | 7.05 ± 1.94b,c |
3.2.5 Moisture absorption
Figure 4 shows the moisture absorption of the bionanocomposites as a function of MMT and NCC contents at an environmental relative humidity of 97.3 %. The effect of MMT on the reduction of moisture absorption was more than that of NCC. Total moisture absorption was 58.65 % for PS-PVA-NCC5 sample; however, it was reduced to 49.78 % when the MMT content reached to 5 %. At the level of 7 % MMT, the films showed the lowest moisture absorption values (14.78 %). Generally it was observed that the moisture absorption decreased in starch–PVA bionanocomposite films compared to the unreinforced one. This could be due to lowering in the amount of free hydroxyl groups of the matrix which took part in hydrogen bonding with the MMT and NCC. The equilibrium moisture uptake value depends on hydrophilic character as well as morphology (macro-voids, free volume, crystal size and degree of crystallinity). NCC provided the matrix with a stabilization effect by forming a three-dimensional cellulosic network which restricted the chain mobility and reduced the number of hydroxyl groups and thus reduced water sorption [44]. The lower moisture absorption of starch–PVA–MMT nanocomposites can be related to the nanocomposite morphology, more specifically to the filler dispersion state. MMT layers in the exfoliated state are able to form more hydrogen bonds with the hydroxyl groups of starch and PVA. Accordingly, this strong structure could reduce the diffusion of water molecules in the material, more efficiently than NCC nanofiller. On the other hand, in comparison to NCC, the exfoliated clay layers produce a tortuous pathway and also a diminution of the length of free way for water uptake.
Moisture absorption of starch–PVA nanocomposites as a function of MMT and NCC content.
3.2.6 Mechanical properties
Mechanical properties resulted from the tensile test are shown in Figure 5(A) and (B), and Table 2. The results showed that by addition of lower amounts (3 wt%) of NCC and MMT, UTS of starch–PVA films decreased that can be attributed to the fact that the fillers might play the role of impurities, raising the stress concentration points in the matrix and initiating fracture from these points [53]. Also, Salehudin et al. [54] observed a decrease in tensile strength in some percentages (4 and 8 %) in starch-NCC films, and they attributed this to the agglomeration of NCC in the matrix of polymer, due to the heterogeneous size distribution of cellulose nanocrystals. By increasing contents of both fillers from 3 % to 7 %, UTS increased, significantly (p ≤ 0.05). PS-PVA-MMT7 and PS-PVA-NCC7 bionanocomposites showed the highest UTS values at 4.93 and 4.80 MPa, respectively. During the processing and drying of the composites, the original hydrogen bonds formed between the starch and PVA molecules were replaced by the new hydrogen bonds formed between the hydroxyl groups in starch, PVA matrix and the hydroxyl groups in MMT and NCC fillers. On the other hand, strong interactions through hydrogen bonds between cellulose nanocrystals inducing a rigid network through the film’s matrix. The existence of these new hydrogen bonds would improve the tensile strength [55]. Also, Liu et al. [19] showed increasing the tensile strength and decreasing the elongation with increasing the NCC content in starch–NCC nanocomposites. They attributed this phenomenon to the higher content of modulus, up to 134 GPa, and strong tensile strength of more than 4 GPa for cellulose nanocrystal. Also, the results of XRD showed the higher UTS values of MMT-contained nanocomposites were attributed to the resistance exerted by the clay, and the orientation and aspect ratio of the exfoliated silicate layers.
Table 2 shows that two nanoreinforcements have different effects on the SB. SB increased by adding 3 % and 5 % MMT. Probably due to the low content of nanofiller, only a small fraction of PS and PVA chains could see their mobility modified by the presence of nanofiller. Thus, in these concentrations, the amount of MMT is low to act as reinforcing agent in PS–PVA composite matrix. Intermolecular bondings of starch and PVA chains probably decrease their affinity to form strong interactions with MMT layers. The SB was drastically reduced by adding 7 % MMT to starch–PVA film, while NCC was able to maintain the high level of elongation of nanocomposites. The difference in the SB between two nanoreinforcements can be due to a difference in the nature of nanoreinforcement and polymer/nanoreinforcement interactions. It is well known that the addition of stiff reinforcements (such as MMT) can reduce the strain to break of the matrix since the reinforcements will cause stress concentrations. Moreover, if the interaction between the nanofiller and the polymer is weak, the nanofiller can act as voids. Voids will allow a larger volume of the matrix to be drawn into the deformation zone, thereby increasing the strain to break. This is believed to occur in the PS–PVA–NCC nanocomposites. A lower and weaker PS/PVA/NCC interaction can also explain why the NCC is unable to show the same improvements as MMT in mechanical properties. Also it should be noted that the layered silicates with higher aspect ratios and more bonding capacities might provide some new nucleation sites, thus contributing in the growth of crystallites. The crystallization process not only causes the brittleness of nanocomposites but also reduces their strain at break [2]. Similar results were reported by Petersson and Oksman [13] who compared the effects of bentonite and microcrystalline cellulose on the mechanical properties of PLA nanocomposites.
Typical strain–stress curves of starch-PVA films contain (0, 3, 5 and 7) MMT (A) and, NCC (B).
The ultimate tensile strength (UTS), strain to break (SB), glass transition temperature (Tg) and melting temperature (Tm) of starch–PVA films as a function of MMT and NCC content.
| Sample | UTS (MPa) | SB (%) | Tg(°C) | Tm(°C) |
| PS-PVA | 4.20 ± 0.37a | 64.73 ± 9.8b | 148.2 ± 2.0a | 214.2 ± 2.2a |
| PS-PVA-MMT3 | 3.62 ± 0b | 83.17 ± 4.27a | 157.5 ± 4.2c | 254.4 ± 3.2c |
| PS-PVA-MMT5 | 3.73 ± 0.07b | 78.33 ± 13.56a | 172.6 ± 3.0e | 260.4 ± 1.4e |
| PS-PVA-MMT7 | 4.93 ± 0.08 c | 57.56 ± 5.59c | 188.1 ± 2.1g | 262.7 ± 3.0f |
| PS-PVA-NCC3 | 2.89 ± 0.14d | 67.04 ± 6.47b | 154.5 ±1.1b | 244.2 ± 2.1b |
| PS-PVA-NCC5 | 4.18 ± 0.43a | 65.94 ± 4.67b | 166.4 ± 3.1d | 253.4 ± 1.2c |
| PS-PVA-NCC7 | 4.80 ± 0.34c | 61.1 ± 1.48b,c | 174.0 ± 0.9f | 259.2 ± 1.0d |
3.2.7 Thermal properties
Glass transition temperature (Tg) and melting temperature (Tm) of PS–PVA film and its nanocomposites are shown in Table 2, and also in Figure 6(A) and (B). As can be seen in Figure 6(A) and (B), there is only one Tg and Tm for two nanocomposites. Observation of single Tg and Tm, due to Jiang et al. [55] study, indicates that two phase of nanofiller and matrix, are compatible which confirm the results of XRD and AFM. Tg observed in PS-PVA biocomposite film was around 148.2 °C. By addition of 3 % MMT, the Tg increased to 157.5 °C. In general, with the increase in MMT content, the Tg becomes broader and shifted toward a higher temperature (Table 2). The glass transition temperatures of polymer nanocomposites are affected by the extent of interaction between nanofillers and polymer chains. Since starch, PVA and MMT are hydrophilic in nature, a strong physical interaction between polymer phase and reinforcing filler in exfoliated state is quite possible. The formation of strong hydrogen bonds between MMT layers and starch–PVA matrix can restrict the segmental mobility of polymer chains and thereby can increase Tg [25].
The Tm was raised by the increase of MMT contents. The temperature position of the melting peak increased from 254.4 to 262.7 °C as MMT content increased from 3 to 7 wt%. This may be attributed to ability of MMT in modifying starch–PVA chain orientation and increasing of regularity in polymer matrix, which in turn cause to formation of dense crystals in the starch matrix. Meanwhile, adding MMT could potentially increase nucleation in starch matrix which in turn promotes crystallization process. This type of behavior was reported in the case of starch [2], PVA [56] and starch–PVA [17] nanocomposites reinforced by MMT.
As shown in Table 2 and Figure 6(B), addition of NCC led to increase in Tg and Tm, too. The effect of NCC content on Tg and Tm was significant (p ≤ 0.05) and an increase in the NCC level from 3 to 7 % led to increase in Tg and Tm. It may be due to the presence of immobilized chains on the nanocrystals surface. Increased Tm shows the faster crystallization induced by NCC which act as nucleating agents for PS/PVA system. NCC allows heterogeneous nucleation mechanism which induces a decrease of the free energy barrier and fastens the crystallization. Due to the study of George et al. [57], because of the similarity of chemical structure between matrix and filler, they are highly miscible, and this lead to strong hydrogen bond formation between them and restriction of the mobility of polymer chains and thereby increase in Tg could occur in the nanocomposites. However, NCC has lower effect on thermal properties compare to MMT. This may attribute to higher aspect ratio for MMT compare to NCC, which leads to well dispersion of MMT in matrix phase, as well as higher interactions between filler and matrix compare to NCC. Also as seen in AFM images, observation of some aggregations in nanocomposite based on NCC, has negative effects on thermal properties by decrease the content of interactions between matrix and filler.
DSC thermograms of starch–PVA films contain (0, 3, 5 and 7) MMT (A) and, NCC (B).
4 Conclusions
The XRD patterns showed that the two nanoreinforcements (MMT and NCC) have good dispersion in matrix phase, also, in PS–PVA–MMT nanocomposites, the MMT peaks disappear completely, which can be led to a conclusion that the structure of PS–PVA–MMT is close to exfoliated structure. Highly crystalline cellulose nanocrystals and exfoliated MMT layers reduced the moisture sorption, water vapor permeability, and solubility of starch–PVA by interacting with hydrophilic sites of biopolymers and making it lose its effectiveness in moisture uptake. The mechanical properties were significantly improved upon filler addition. The significant increase in UTS and decrease in SB of nanocomposites upon addition of MMT and NCC was attributed to strong adhesion between the filler and the matrix. A lower effect was observed for NCC-rich nanocomposite films, which displays the lower aspect ratio of the filler. The addition of nanoreinforcements also affected the segmental mobility of starch chains, which in turn resulted in an increased glass transition temperature. By comparison of MMT and NCC, it was revealed that, the MMT is more effective than NCC in improvement of thermal properties. From all these results, in spite of desirable compatibility of both nanoreinforcements with starch–PVA matrix, MMT was found to be a better reinforcing agent in improving the properties of starch–PVA films. It shows the importance of consideration the morphology, aspect ratio and surface area while choosing a suitable nanofiller for a nanocomposite.
Acknowledgements
The authors wish to express their gratitude to the University of Tabriz for financial support and Sahand University of Technology, especially Dr. Abbasi, for technical assistance in this research.
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