Research Article pubs.acs.org/journal/ascecg
Synergistic Effect of Halloysite and Cellulose Nanocrystals on the Functional Properties of PVA Based Nanocomposites Hajer Aloui,† Khaoula Khwaldia,† Moktar Hamdi,‡ Elena Fortunati,§ Jose M. Kenny,§ Giovanna G. Buonocore,*,∥ and Marino Lavorgna∥
ACS Sustainable Chem. Eng. 2016.4:794-800. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/28/19. For personal use only.
†
Laboratoire des Substances Naturelles (LSN, LR10 INRAP02), Institut National de Recherche et d’Analyse Physico-chimique (INRAP), Pôle Technologique de Sidi Thabet, 2020 Sidi Thabet, Tunisia ‡ Laboratoire d’Ecologie et de Technologie Microbienne, Institut National des Sciences Appliquées et de Technologie (INSAT), 2 Boulevard de la Terre, BP 676, 1080 Tunis, Tunisia § Civil and Environmental Engineering Department, UdR INSTM, University of Perugia, Strada di Pentima, 4, 05100 Terni, Italy ∥ CNR − Institute of Polymers, Composites and Biomaterials, P.le E. Fermi, 1 80055 Portici (Naples), Italy S Supporting Information *
ABSTRACT: Poly(vinyl alcohol) (PVA) based nanocomposites filled with different amounts of halloysites (HNTs) and/or cellulose nanocrystals (CNC) were produced and characterized in terms of mechanical and barrier properties, thermal stability, and transparency. A significant increase in tensile strength by more than 70% and an unexpected improvement in elongation at break were observed for all the PVA nanocomposites when compared to the pristine PVA. Moreover, the presence of both CNC and HNTs at the highest loadings of 5 and 3 wt %, respectively, improved the thermal stability of the PVA matrix and reduced its water vapor permeability (WVP) by more than 42%. All the developed PVA nanocomposites maintained their transparency due to the good and homogeneous dispersion of both the nanofillers in the PVA matrix. Results highlight the synergistic effect of HNT and CNC on the barrier and mechanical properties of PVA, mainly due to the establishment of specific interactions between the OH groups of HNT and CNC particles. KEYWORDS: PVA, Halloysite, Cellulose Nanocrystals, Nanocomposites, Barrier Properties
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high filler contents, the strong interparticle interactions can cause aggregation during the preparation of nanocomposites and limit the homogeneous dispersion of CNC, thus reducing the mechanical properties of the resulting films.6 In order to promote the dispersion of CNC and increase their compatibility with several matrices, different chemical surface modifications have been attempted, including the use of adsorbing surfactant and coupling agents, etherification, oxidation, silylation, amidation, and polymer grafting. 7 However, these approaches not only are difficult to carry out but also can affect the reinforcing performance of CNC.8 Recently, the combination of CNC with other nanostructured materials such as silver nanoparticles or nanosilica has been investigated as a promising strategy to further improve the performance of nanocomposites based on PVA, such as thermal, mechanical, and barrier properties.9−12 The present work focuses on a new approach to effectively disperse CNC in a PVA matrix making use of the fibrous clay
INTRODUCTION Poly(vinyl alcohol) (PVA) is a thermoplastic and biocompatible petroleum based polymer extensively studied because of its excellent film forming and emulsifying properties and its complete biodegradability.1 However, because of its high hydrophilic nature and relatively low strength and thermal stability, the use of this polymer in some application fields such as the food packaging sector has never been widely diffused. Recently several research teams have reported the great potential of cellulose nanostructures for enhancing PVA performances in terms of mechanical, thermal, and barrier properties, thus leading to the production of green nanocomposites.2,3 Cellulose nanocrystals (CNC) are promising candidates for nanocomposites production, owing to their large specific surface area, wide availability, biocompatibility, and biodegradability, which may offer great opportunities to develop environmentally friendly structural composites.4 Due to the strong hydrogen bonding interactions between their hydroxyl groups, CNC have a great tendency for selfassociation, leading to the formation of a three-dimensional rigid percolating network within the matrix, which can enhance the functional properties of nanocomposite films.5 However, at © 2015 American Chemical Society
Received: August 3, 2015 Revised: November 18, 2015 Published: November 30, 2015 794
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to obtain nanocomposite films containing single filler. PVA and several nanocomposites films were respectively named as follows: • PVA for the neat poly(vinyl alcohol); • PVA/1CNC and PVA/5CNC for binary poly(vinyl alcohol) nanocomposites containing cellulose nanocrystals at 1 and 5 wt %, respectively; • PVA/1HNTs and PVA/3HNTs for binary poly(vinyl alcohol) nanocomposites containing halloysite nanotubes at 1 and 3 wt %, respectively; • PVA/1CNC/1HNTs, PVA/1CNC/3HNTs, PVA/5CNC/ 1HNTs and PVA/5CNC/3HNTs for ternary PVA nanocomposites containing both cellulose nanocrystals and halloysite nanotubes. X-ray analysis. Wide-angle X-ray diffraction (WAXD) was used to evaluate the PVA crystallinity in the several nanocomposites. X-ray spectra were collected in transmission mode by using an Anton Paar SAXSess diffractomer (40 kV, 50 mA) equipped with a Cu Kα radiation (λ = 0.1542 nm) source and an image plate detector. All scattering data were corrected for background and normalized for the primary beam intensity. Thermogravimetric analysis (TGA). Thermal stability of the different nanocomposites films was evaluated by thermogravimetric analysis (TGA 2950, TA Instruments). Experiments were carried out in nitrogen atmosphere and heating scans from 30 to 600 °C at 10 °C min−1 were performed for each sample. SEM analysis. The surface morphology of the nanocomposites was studied using a Scanning Electron Microscope (SEM) (FEI QUANTA 200F). Before examination, samples were coated with gold in argon by means of a Sputter Coater S150 to avoid charging under electron beam. All samples were examined using an accelerating voltage of 5 kV. FTIR analysis. The chemical interactions between halloysite and cellulose nanocrystals were evaluated by FTIR spectroscopy. HNT and CNC powders were mixed in the ratio 37.5/62.5 wt/wt and the mixture was submitted to the same steps followed for the preparation of PVA based nanocomposites, such as sonication of the water dispersion, casting, solvent evaporation. For comparison the HNT powder was submitted to the same procedure. The resulting powder mixture was used for the preparation of KBr disk (1 wt %) for FTIR analysis in order to detect specific interactions between filler. FTIR spectra were collected at room temperature by using a Nicolet apparatus (Thermo Scientific, Italy) from 4000 to 600 cm−1 with a wavenumber resolution of 4 cm−1. Water uptake. Water absorption capacity of pristine PVA and PVA nanocomposites was evaluated according to the procedure described by Lavorgna et al. with some modifications.21 Briefly, samples with rectangular shaped dimensions (1 × 1 cm) were desiccated overnight under vacuum and weighed to determine their dry mass. Films were then immersed in 30 mL of water (pH = 7) at 25 °C. At prescribed intervals, samples were removed from water and, after gently bottling the surface with a tissue, weighted and monitored until equilibrium was reached. The water gain (WG) was calculated as follows: m wetfilm − mdryfilm WG (%) = × 100 mdryfilm
mineral halloysite. Halloysite nanotubes (HNTs), owing to their unique cylindrical shape and low density of hydroxyl functional groups, can be easily dispersed in polymer matrices even at high loading contents, and they do not require exfoliation like CNC and other nanoclays.13 HNTs have drawn much attention also in the field of packaging due to their use as nanocarriers for controlled release of active compounds31−33 as well as innovative reinforcing fillers due to their potential to enhance the functional properties of many polymers.14−16,24,25,34 Although many studies have demonstrated the reinforcement potential of HNTs and CNC when incorporated individually, until recently there has been no reported research on the use of halloysite as codispersing filler alongside with more interacting fillers in the improvement of functional properties of polymeric matrices. In this study, HNTs have been incorporated as the second nanofiller into a PVA matrix in order to assess their ability to provide a better dispersion of CNC and to confer better performance to the reinforced PVA material. PVA nanocomposites were produced by solvent casting in water, and the effects of different amounts of HNTs and/or CNC on the functional and physical properties of the developed films were evaluated in terms of crystallinity, thermal stability, mechanical properties, water vapor permeability, water absorption, microstructure, and transparency.
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EXPERIMENTAL SECTION
Materials. Poly(vinyl alcohol) (PVA) in powder form (average Mw (89.000−98.000 Da), 99.2−99.7% hydrolyzed) used as matrix for the nanocomposite formulations, was purchased from Sigma-Aldrich (Steinheim, Germany). Halloysite nanotubes (HNTs, Imerys, Cornwall, England) and cellulose nanocrystals (CNC) extracted from microcrystalline cellulose (MCC dimensions of 10−15 μm, Aldrich, Steinheim, Germany) were used as nanofillers. Preparation of CNC nanocrystals. MCC was hydrolyzed by sulfuric acid (64 wt %) at 45 °C for 30 min, following what has been done in previous works.17,18 A centrifugation and a dialysis procedure were applied in order to remove the acid excess while a mixed bed ionexchange resin (Dowex Marathon MR-3 hydrogen and hydroxide form) was added to the cellulose suspension for 48 h and then removed by filtration. The resultant cellulose based aqueous suspension was sonicated (Vibracell 750) for 5 min in an ice-bath. Additionally, the CNC was thermally neutralized by addition of 1.0% (v/v) of 0.25 M NaOH. The final CNC water suspension had approximately 0.5 wt %, with a calculated hydrolysis yield of approximately 20%. The obtained CNC showed the typical acicular structure with dimensions ranging from 100 to 200 nm in length and 5−10 nm in width, as previously reported by Fortunati et al.18,19 Preparation of PVA based nanocomposites. PVA nanocomposite films reinforced with CNC and HNTs were prepared by solvent casting method in water. Pure PVA film forming solution (10 wt %/v) was prepared by dissolving 10 g of PVA powder in 100 mL of distilled water at 80 °C for 2 h, under mechanical stirring trough sonication.20 Nanocomposite formulations were obtained by dispersing selected amounts of HNTs (1 and 3 wt %/wt, based on PVA content) in 100 mL of distilled water for 2 h. This dispersion was added to the PVA solution, and the mixture was continuously stirred for 1 h at room temperature and then for 2 h at 25 °C in an ultrasonic bath. Nanocomposite samples were then sonicated using a probe sonicator for 1 h to obtain a good dispersion of HNTs. Specific amounts of the aqueous dispersion of cellulose nanocrystals were subsequently added to PVA/HNTs nanocomposite solutions in order to obtain a 1 and 5 wt % of CNC content respect to the PVA matrix. The resulting mixtures were sonicated for 5 min before being poured into plastic Petri dishes and dried under room temperature to form uniform films with an average thickness of 100 ± 7 μm. The aqueous dispersions of HNTs or CNC filler were also casted into plastic molds
where mwetfilm and mdryfilm are the weights of the wet and dry films, respectively. Water vapor permeability (WVP). WVP measurements were determined by means of a Permatran W3/31 (Mocon, Germany). Samples with a surface area of 5 cm2 were tested at 25 °C. Permeation tests were performed by setting the relative humidity at the downstream and upstream sides of the film to 0% and 50% respectively. A flow rate of 10 mL/min of a nitrogen stream was used. Each test was carried out in triplicate. Mechanical behavior of PVA nanocomposites. Mechanical behavior of pristine PVA and nanocomposites, binary and ternary systems, was evaluated by tensile tests, performed on rectangular strips (100 mm × 10 mm) according to a standard method of UNE-EN ISO 795
DOI: 10.1021/acssuschemeng.5b00806 ACS Sustainable Chem. Eng. 2016, 4, 794−800
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ACS Sustainable Chemistry & Engineering 527-5, with a crosshead speed of 20 mm min−1, a load cell of 5 KN and an initial gauge length of 50 mm. Elongation at break (%E) and tensile strength (TS) were calculated from the resulting stress−strain curves. Ten replicates of each film type were tested. Statistical analysis. Data were subjected to analysis of variance (ANOVA) with a 95% significance level using Statgraphics Plus 5.1(Manugistics Inc., Rockville, MD, USA). Means comparisons were performed through 95% Fisher’s LSD intervals. Differences between means were considered significant when the confidence interval is smaller than 5% (p < 0.05).
diffraction pattern of PVA/5CNC binary systems (Figure 1). Likewise, three additional peaks at around 2θ = 12°, 21°, and 23°, corresponding to the crystalline structure of halloysite, were observed for binary PVA systems loaded with the highest content of HNTs (Figure 1). The results confirm that the addition of a single filler as well as the simultaneous addition of HNTs and CNC at the highest loadings (5 and 3 wt %, respectively) does not change significantly the crystallinity of the PVA matrix, in agreement with crystallinity data from DSC analysis which revealed a low increment of about 5% in the degree of crystallization of PVA/ 5CNC/3HNTs nanocomposites, compared with pristine PVA (Table S1, Supporting Information). Likewise, Fortunati et al. reported a similar behavior for the combined effect of silver nanoparticles and CNC added to the PVA matrix.23 Thermogravimetric analysis (TGA) in a nitrogen atmosphere was carried out to investigate the effect of CNC and HNTs nanofillers on the degradation behavior and the thermal stability of the different nanocomposite systems. Derivative TGA curves (DTG) of pristine PVA and nanocomposites are shown in Figure 2, while degradation temperatures and residual mass percentages calculated at 600 °C are summarized in Table 1.
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RESULTS AND DISCUSSION X-ray diffraction analysis was carried out to evaluate the effect of the presence of different amounts of HNTs and/or CNC on the crystallinity of a PVA matrix (Figure 1). The pristine PVA
Table 1. TGA Data of Pristine PVA and Nanocomposites
PVA PVA/1CNC PVA/5CNC PVA/1HNTs PVA/3HNTs PVA/1CN/1HNTs PVA/1CNC/3HNTs PVA/5CNC/1HNTs PVA/5CNC/3HNTs
Figure 1. Wide angle X-ray sScattering patterns of neat PVA and of binary and ternary PVA based nanocomposites and HNT and CNC fillers (inset graph).
sample shows a strong peak at approximately 2θ = 19.33°, resulting from the strong interaction between PVA chains through intermolecular hydrogen bonding.22 When the higher amount of CNC (5 wt %) is added into the PVA matrix, a distinctive peak at about 2θ = 22.5°, assigned to the cellulose crystalline structure (Figure 1−inset graph), appeared in the
Tfirst peak (°C)
Tadditional peak (°C)
Tsecond peak (°C)
Residual mass (%) at 600 °C
268 264 267 262 261 261 268 270 279
322 325 325 323 323 324 339 318 332
425 423 425 432 428 424 427 424 432
11.00 11.39 13.31 12.19 12.73 11.71 12.78 13.05 13.03
Figure 2. DTG curves of neat PVA and PVA nanocomposites. 796
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ACS Sustainable Chemistry & Engineering PVA/HNTs binary nanocomposites show a similar pattern of thermal behavior as the pristine PVA film, with a small weight loss due to the removal of moisture in the range of 50−150 °C, and two main thermal decomposition steps, which were consistent with the generally accepted mechanism for the degradation of PVA.2 The second weight loss (T ∼ 200−360 °C) is a complex peak with an evident shoulder at about 300 °C which corresponds to the side chain decomposition of PVA, whereas the third degradation between 410 and 600 °C corresponds to decomposition of the main chain. The addition of HNTs shows an increase in the thermal stability of the PVA/HNTs binary nanocomposites, most likely, due to its ability to prevent heat transfer inside polymers.24 Similar behavior has been previously observed with other polymer nanocomposites, when a small amount of halloysite (less than 8% wt) was added.25 The DTG curves of PVA/CNC binary nanocomposites show that no evident shifts occur in the maximum degradation temperatures with the presence of CNC even at the highest loadings of 5 wt % (Figure 2b). For ternary nanocomposites, the addition of HNTs at the highest loadings brings about a marked change in the degradation mechanism (Figure 2c and d). PVA/5CNC/3H and PVA/1CNC/3H ternary systems show a more evident peak at around 325 °C, which may be tentatively ascribed to the pyrolysis of cellulose, which is expected to occur around 337 °C, as previously reported by Fortunati et al. for the same CNC powder used in this study.2 Overall, the combination of HNTs with CNC in the PVA matrix leads to an enhancement in the thermal stability of the ternary nanocomposites, when compared with both the neat PVA matrix and the binary systems incorporating either HNTs or CNC. However, the best result in terms of thermal stability is obtained in the case of PVA/5CNC/3H nanocomposite films, for which an increase of about 11 and 7 °C in the temperature values related to the first and the second peaks has been recorded, respectively, with respect to the neat PVA matrix (Table 1). The improvement in the thermal stability of the ternary systems can be explained by the reduced mobility of the PVA chains in the nanocomposites induced by the combined presence of halloysites and cellulose nanostructures. With respect to char residue values at 600 °C, higher residual masses were observed in the case of ternary systems (Table 1), most likely due to the different degradation mechanism occurring in the case of ternary systems. Water absorption capacity is one of the most important properties of biodegradable films intended for food packaging applications, which usually require the use of materials with low water sorption. In fact, the water absorbed on the materials may promote the growth of microorganisms, which, under specific conditions, may use the materials as energy source, leading to an increased degradation capability.2 Figure 3 shows the percentage of water absorbed by the neat PVA and PVA nanocomposites as a function of immersion time. The water uptake occurs rapidly during the first 2 h of samples exposure to water (p < 0.05), after which the absorption rate slows down until reaching the equilibrium point (p > 0.05). In the case of binary systems, the presence of CNC at the highest loading of 5 wt % leads to a slight increase in the water uptake with respect to pristine PVA, probably due to the hydrophilic nature of cellulose fibers, as previously reported by Fortunati et al.2 On the contrary, a decrease of water sorption properties of binary systems has been noticed when HNTs is added at the highest content (3 wt %), in
Figure 3. Water sorption profiles as a function of immersion time of pristine PVA and PVA nanocomposites at 25 °C and pH = 7.
agreement with the findings of Alhuthali and Low, who reported a reduction by more than 20% in the water uptake of vinylester films reinforced with 3 wt % HNTs.26 Overall, the ANOVA results reveal that the incorporation of both HNTs and CNC in PVA leads to no significant reduction in the water absorption capacity of the ternary nanocomposites when compared with the pristine PVA and the binary systems (p > 0.05). A significant improvement in the water uptake properties of the PVA matrix has been noticed only in the case of the simultaneous presence of CNC and HNTs at the highest loadings of 5 and 3 wt %, respectively, compared with the pristine PVA (p < 0.05). This is likely due to the presence of HNTs at the highest loading of 3 wt %, which allows a better dispersion of CNC in the polymeric matrix, thus increasing the film tortuosity and leading to a slower diffusion of water molecules through the film matrix. This hypothesis is supported by SEM images of nanocomposite surfaces (Figure 4) which show, in binary systems, that HNTs is well and homogeneously dispersed in the PVA matrix (Figure 4b) whereas CNC aggregate during the evaporation of the solvent, forming on the surface sample typical wrinkle features, ascribed to the assembling of cellulose nanocrystals (Figure 4c). In agreement with our results, Fortunati et al.2 reported that incorporation of CNC into the PVA matrix increased the surface roughness of PVA/CNC binary systems. According to these authors, CNC have a tendency to aggregate due to their strong hydrogen bonding property, which leads to the formation of a percolating network that is responsible for the increased roughness and appearance of wrinkle features. On the contrary, SEM micrographs of ternary PVA nanocomposites reveal that the presence of HNTs allows a better dispersion of CNC in the PVA matrix. In particular, only the presence of a 3 wt % of halloysite filler allows the homogeneous dispersion of cellulosic nanocrystals, without the formation of surface wrinkles (Figure 4e). This can be tentatively attributed to specific interactions which occur between the two investigated fillers, as proved by FTIR analysis (Figure 5) of both the HNT and HNT-CNC mixtures. The FTIR spectrum of the HNT exhibits two Al2OH stretching bands at 3693 and 3621 cm−1 attributed to the OH bending which connects the two Al atoms, alongside with other bands characteristic of the inorganic alumino-silicate structure of halloysite.27 The mixture of HNT and CNC exhibits all characteristic absorbance bands of CNC,28 alongside with the bands of HNT. Moreover it is possible to observe that the two 797
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interactions between the hydroxyls of CNC with the more accessible hydroxyls of HNT particles, which avoid aggregations and allow a good dispersion within the polymeric matrix. To the best of our knowledge, this is the first documented specific interaction which testifies the synergistic action of HNT coupled with CNC filler in the improvement of reciprocal dispersion within polymeric nanocomposites. This result is confirmed also by WVP data (Figure 6) studied with the aim to determine the effect of HNTs and/or CNC at
Figure 4. Scanning electron microscopy images (×12000) of (a) neat PVA, (b) PVA/3HNTs, (c) PVA/5CNC, (d) PVA/1CNC/1HNTs, and (e) PVA/3HNTs/5CNC nanocomposites.
Figure 6. Water vapor permeability (a), tensile strength (b), and elongation at break (%E) (c) of pristine PVA and PVA nanocomposites. Means values and LSD intervals.
Figure 5. FTIR spectra of neat HNT and of a mixture HNT-CNC prepared following the same steps used for the preparation of PVA based nanocomposites.
different loading contents on the WVP of the PVA matrix. Films with low WVP, in fact, are usually required for food packaging applications, where efficient barrier properties are desired to impede, or at least to reduce, moisture transfer between the outside packaging environment and packaged food.
peaks of HNT ascribed to the bending of OH groups connecting Al atoms shift about 4 cm−1 to higher wavenumbers when compared to the pristine HNT (see inset graph in Figure 5). This shift may be ascribed to the establishment of specific interactions between CNC and HNT, i.e. H-bonding 798
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ACS Sustainable Chemistry & Engineering ANOVA results reveal that the addition of CNC and/or HNTs leads to no significant reduction in the WVP of the resulting PVA binary and ternary systems (p > 0.05). A significant improvement in water barrier properties has been noticed only in the case of PVA/5CNC/3HNTs ternary systems, for which a WVP reduction by more than 42% was obtained with respect to the neat PVA (p < 0.05). As previously reported in SEM and FTIR analysis, this is attributed to the better dispersion of CNC in the polymeric matrix in the presence of the higher amount of HNTs, which by improving the CNC dispersion increases the film tortuosity and leads to a slower diffusion of water vapor molecules through the film matrix. The importance of the tortuous paths is also supported by the evidence that the crystallinity of ternary systems does not depend on the fillers presence, as shown by XRD and DSC analysis. In agreement with our findings, Fortunati et al. report a reduction by more than 63% in the WVP of PLA films as a result of the simultaneous presence of CNC and silver nanoparticles at 1 wt %,11 highlighting the positive effect of the synergistic effect of silver and cellulose nanostructures. Films with high mechanical properties are usually required for food packaging applications to withstand the normal stress encountered during food handling, shipping, and transportation.29 Tensile strength (TS) and elongation at break (%E) are the key parameters to describe the mechanical properties of packaging materials. The mechanical behavior of the pristine PVA and nanocomposites binary and ternary systems has been investigated in order to evaluate the effect of different amounts of HNTs and/or CNC on the tensile response of the PVA matrix (Figure 6a and 6b). As can be inferred from Figure 6a, the addition of CNC and/or HNTs leads to a significant increase in the TS of binary and ternary systems when compared with the pristine PVA sample (p < 0.05). This increase has been estimated by more than 70% for all the PVA nanocomposites films regardless of the type and the amount of added nanofillers (p > 0.05). Such improvement in mechanical resistance of the produced PVA nanocomposites could be attributed to the good interfacial adhesion between the incorporated fillers and the PVA matrix, along with their homogeneous dispersion in the polymeric matrix, which may result in an efficient load transfer between polymer chains and the percolating network of nanofillers.2 On the other hand, the incorporation of CNC and/or HNTs significantly enhances the %E of binary and ternary systems when compared with the neat PVA (p < 0.05), outlining the ability of these two nanofillers, either alone or in combination, to improve the plastic response of the reinforced material (Figure 6b). However, neither the type nor the amount of the added fillers affect significantly the %E of the PVA matrix (p > 0.05). In agreement with our findings, Fortunati et al. reported the ability of CNC from different sources to enhance the %E of PVA binary systems in the range of 4−36% as compared with the pure PVA matrix,2 while no significant decrease in the %E values of PVA films has been noticed, by the same research team, as a result of the simultaneous presence of CNC and silver nanoparticles,23 underlining the ability of both fillers to maintain the ductility of the resulting ternary systems. Such improvement in the plastic response of the reinforced PVA matrix was unexpected, since most of the cited literature reports a decrease in the %E of the reinforced materials due to the ability of the added nanofillers to interact with hydrophilic polymers, which may restrict the polymer matrix motion and thus reduce its flexibility.3,12,15,30
Finally, due to its direct impact on the appearance of packaged product, transparency, one of the most relevant physical properties of films designed for packaging, has also been investigated. Results from UV−vis measurements of neat PVA and PVA nanocomposites reinforced with different amounts of HNTs and/or CNC (Figure S1, Supporting Information) show that the incorporation of HNts and/or CNC leads only to a slight reduction in the percentage of transmitted light at a wavelength of 700 nm. In summary, PVA nanocomposites reinforced with different amounts of CNC and/or HNTs have been successfully produced by solvent casting. HNTs and/or CNC incorporation improve significantly the mechanical response and also the thermal properties of the PVA matrix without affecting to a high extent its transparency, attesting to the good interfacial adhesion between the reinforcement phase and the matrix and to the homogeneous dispersion of both the incorporated fillers. Furthermore, a significant reduction of water sorption and increase in the water barrier properties of the PVA matrix has been noticed as a result of the simultaneous presence of CNC and HNTs at the highest loadings. This improvement is due to the ability of HNTs to allow a better dispersion of CNC in the polymeric matrix, thus increasing the film tortuosity and leading to a slower diffusion of water molecules through the film matrix. This result is particularly interesting, since materials with low water absorption capacity and low permeability are usually required for food packaging applications. The synergistic effect of CNC and HNTs on the water sorption and water barrier properties of PVA, mainly due to the establishment of specific interactions between their OH groups, proves the success of the proposed codispersion approach and can open an interesting pathway for the use of halloysite and CNC in the improvement of functional properties of polymeric matrices.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00806.
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DSC and transmittance behavior of pristine PVA and PVA nanocomposites. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +390817758837. Fax: +390817758850. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS H.A. was supported by the Tunisian Ministry of Higher Education and Scientific Research. Financial support from the program PON Ricerca e Competitività 2007−2013, cofinanced by the European Regional Development Fund (ERDF), within the Research Project PON01 00636 FINGERIMBALL, is also gratefully acknowledged. The authors would like to acknowledge Alessandra Aldi and Mario De Angioletti for their technical support in the characterization of the materials. 799
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DOI: 10.1021/acssuschemeng.5b00806 ACS Sustainable Chem. Eng. 2016, 4, 794−800
