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Synergistic Effect of Halloysite and Cellulose Nanocrystals on Functional Properties of PVA Based Nanocomposites Hajer ALOUI, Khaoula Khwaldia, Moktar Hamdi, Elena Fortunati, Jose M Kenny, Giovanna Giuliana Buonocore, and Marino Lavorgna ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00806 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on December 29, 2015

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ACS Sustainable Chemistry & Engineering

Synergistic Effect of Halloysite and Cellulose Nanocrystals on Functional Properties of PVA Based Nanocomposites Hajer Aloui,† Khaoula Khwaldia,† Moktar Hamdi,‡ Elena Fortunati,§ Jose M. Kenny,§ Giovanna G. Buonocore,*,ǁ, Marino Lavorgna,ǁ †

Laboratoire des Substances Naturelles (LSN, LR10 INRAP02), Institut National de Recherche et d'Analyse Physicochimique (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 ǁ

CNR – Institute of Polymers, Composites and Biomaterials, P.le E. Fermi, 1 80055 Portici (Naples), Italy Email Corresponding Author: [email protected]

ABSTRACT Polyvinyl 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 wt % and 3 wt %, respectively, improved thermal stability of PVA matrix and reduced its water vapour 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 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 ACS Paragon Plus Environment

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INTRODUCTION Polyvinyl 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 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 self-association 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 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 ACS Paragon Plus Environment

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The present work focuses on a new approach to effectively disperse CNC in PVA matrix making use of fibrous clay 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 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 nanocarrier 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 second nanofiller into 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 effect of different amounts of HNTs and/or CNC on the functional and physical properties of the developed films were evaluated in terms of cristallinity, thermal stability, mechanical properties, water vapor permeability, water absorption, microstructure and transparency.

EXPERIMENTAL SECTION Materials. Polyvinyl alcohol (PVA) in powder form (average Mw (89.000-98.000 Da), 99.2-99.7% hydrolysed) 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.

ACS Paragon Plus Environment

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Preparation of CNC nanocrystals. MCC was hydrolyzed by sulphuric 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 ion exchange 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 0.5wt%, 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


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casted into plastic moulds to obtain nanocomposite films containing single filler. PVA and several nanocomposites films were respectively named as follows: •

PVA for the neat polyvinyl alcohol;

•

PVA/1CNC and PVA/5CNC for binary polyvinyl alcohol nanocomposites containing cellulose nanocrystals at 1 and 5 wt %, respectively;

•

PVA/1HNTs and PVA/3HNTs for binary polyvinyl 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 cristallinity 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-K α radiation (Ȟ = 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.


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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: WG (%) =

mwetfilm − mdryfilm mdryfilm

× 100

where mwetFilm and mdryFilm are the weight of the wet and dry film, 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.


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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 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% (p0.05). A significant improvement in the water uptake properties of PVA matrix has been noticed only in the case of the simultaneous presence of CNC and HNTs at the highest loadings of 5 wt % and 3 wt %, respectively, compared with the pristine PVA (p0.05). 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

Synergistic Effect of Halloysite and Cellulose ... - ACS Publications

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