Accepted Manuscript Title: Starch and cellulose nanocrystals together into thermoplastic starch bionanocomposites Author: Kizkitza Gonz´alez Alo˜na Retegi Alba Gonz´alez Arantxa Eceiza Nagore Gabilondo PII: DOI: Reference:
S0144-8617(14)00965-5 http://dx.doi.org/doi:10.1016/j.carbpol.2014.09.055 CARP 9313
To appear in: Received date: Revised date: Accepted date:
14-3-2014 11-9-2014 12-9-2014
Please cite this article as: Gonz´alez, K., Retegi, A., Gonz´alez, A., Eceiza, A., and Gabilondo, N.,Starch and cellulose nanocrystals together into thermoplastic starch bionanocomposites, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.09.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
STARCH AND CELLULOSE NANOCRYSTALS TOGETHER INTO
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THERMOPLASTIC STARCH BIONANOCOMPOSITES
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Kizkitza Gonzáleza, Aloña Retegia, Alba Gonzálezb, Arantxa Eceizaa, Nagore
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Gabilondoa*
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Polytechnic School, University of the Basque Country. Pza. Europa 1. 20018 Donostia-
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San Sebastián, Spain
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POLYMAT, Department of Polymer Science and Technology. Faculty of Chemistry,
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‘Materials + Technologies’ Group, Dept. of Chemical and Environmental Engineering,
University of the Basque Country. P.O. Box 1072, 20080 Donostia-San Sebastián,
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Spain
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*To whom all correspondence should be addressed. Tel.: (+34)943017231/7162; fax:
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(+34)943017200; e-mail address: [email protected] (Nagore Gabilondo)
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ABSTRACT
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In the present work, thermoplastic maize starch based bionanocomposites were prepared
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as transparent films, plasticised with 35% of glycerol and reinforced with both waxy
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starch (WSNC) and cellulose nanocrystals (CNC), previously extracted by acidic
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hydrolysis. The influence of the nanofiller content was evaluated at 1 wt.%, 2.5 wt.%
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and 5 wt.% of WSNC. The effect of adding the two different nanoparticles at 1 wt.%
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was also investigated. As determined by tensile measurements, mechanical properties
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were improved at any composition of WSNC. Water vapour permeance values
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maintained constant, whereas barrier properties to oxygen reduced in a 70%, indicating
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the effectiveness of hydrogen bonding at the interphase. The use of CNC or CNC and
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WSNC upgraded mechanical results, but no significant differences in barrier properties
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were obtained. A homogeneous distribution of the nanofillers was demonstrated by
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atomic force microscopy, and a shift of the two relaxation peaks to higher temperatures
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was detected by dynamic mechanical analysis.
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KEYWORDS
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Bionanocomposite; Thermoplastic starch; Polysaccharide nanocrystals; Morphology;
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Thermal, mechanical and barrier properties.
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1. INTRODUCTION
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Starch, in form of its thermoplastic derivate (TPS), has been revealed as an appropriate
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candidate to be employed as substitute of synthetic polymers traditionally used for
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packaging, due to its versatility, availability, biodegradability, processability and low
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cost. Starch is a hydrophilic semicrystalline biopolymer stored in form of granules in
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the native state that consists of a mixture of two glycosidic macromolecules, i.e.
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amylose and amylopectin (Carvalho, 2008; Le Corre, Bras, & Dufresne, 2010).
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Amylose is a linear polysaccharide made up of (1-4) -D-glycopyranose, whereas
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amylopectin is a highly branched macromolecule composed of both (1-4) and (1-6)
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glycopyranosyl linkages (García, Ribba, Dufresne, Aranguren, & Goyanes, 2011; Le
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Corre et al. 2010). The arrangement of the two polymers in form of alternant amorphous
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and crystalline rings lead to the final semicrystalline granules, being amylopectin the
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main responsible of the crystalline character (Le Corre et al. 2010).
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Actually, starch is not a real thermoplastic polymer, but can be processed after its
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gelatinization by mixing with enough water and/or plasticizers (Angellier, Molina-
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Boisseau, Dole, & Dufresne, 2006; Carvalho, 2008; García et al., 2011; Viguié, Molina-
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Boisseau, & Dufresne, 2007). The crystallinity of starch granules disappears during the
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gelatinization process, and at the end, an amorphous material is obtained (Carvalho,
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2008). In most investigations the plasticisation of the material is carried out by casting a
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dispersion of starch with glycerol (Angellier et al., 2006; Bertuzzi, Vidaurre, Armada, &
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Gottifredi, 2007; Carvalho, 2008; García, Ribba, Dufresne, Aranguren, & Goyanes,
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2009; Yan, Hou, Guo, & Dong, 2012), but other water soluble plasticizers such as
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sorbitol, xylitol, maltitol, urea or ethylene glycol have been also used (Abdorreza,
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Cheng, & Karim, 2011; Da Róz, Carvalho, Gandini, & Curvelo, 2006; Vigué et al.,
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2007; Wang, Cheng, & Zhu, 2014). Attempting to reduce the undesired well-known
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sensitivity to moisture and temperature of TPS, several strategies have been developed
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in the literature (Averous, Moro, Dole, & Fringant, 2000; Xie, Pollet, Halley, &
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Avérous, 2013).
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Bionanocomposites have already been identified as useful materials for achieving the
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mentioned objectives. The addition of different nanocrystals to the starch based matrix,
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would lead to both mechanical and barrier properties improvement of the material. In
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this context, nanoclays and polysaccharide derived nanocrystals have been tried in the
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most recent researches (Angellier et al., 2006; García et al., 2011; Xie et al., 2013).
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Actually, a large amount of work has been published mainly with cellulose whiskers
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and, in a less extent, chitin or starch nanocrystals (Wang, Tian, & Zhang, 2010).
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Disordered amorphous domains of natural polysaccharides can be removed by acid
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hydrolysis (Habibi, Lucia, & Rojas, 2010; Lin, Huang, Chang, Anderson, & Yu, 2011a;
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Lin, Huang, & Dufresne, 2012), under controlled conditions, leaving the crystalline
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regions intact (Lin et al., 2012), and thus, nanometrical high crystallinity particles are
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obtained. However, little work has been reported using different polysaccharide
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nanocrystals simultaneously (Wang et al., 2010). Nanocrystals extracted from different
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sources have a variety of geometrical structures (Wang et al., 2010), and thus, they are
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supposed to have their own properties and applications as reinforcing agent. While
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cellulose nanocrystals were found to be rod like or fibrillar (Angellier, Molina-
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Boisseau, Belgacem, & Dufresne, 2005; Lin et al., 2012; Wang et al., 2010), starch
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nanocrystals isolated from waxy maize presented a platelet like morphology (Habibi et
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al., 2010; Lin et al., 2012; Saralegi, et al. 2013) and would be preferred for barrier
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properties enhancement (LeCorre, Bras, & Dufresne, 2012). However, the expected
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improvement of mechanical, thermal or barrier properties by the addition of
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nanocrystals extracted from different origins would depend on both a correct dispersion
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and the generation of an active interphase nanoreinforcement/matrix.
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One of the most common applications of TPS based biocomposites is their use as films
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in food packaging products. A critical issue in food packaging is that of migration of
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certain components and vapour and gases permeability (Duncan, 2011). Many authors
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have investigated the improvement of water vapour and oxygen permeability in
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reinforced TPS materials (Bertuzzi et al., 2007; Chivrac, Angellier-Coussy, Guillard,
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Pollet, & Avérous, 2010; García et al., 2009; Kelnar, Kaprálková, Brozová,
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Hromádkova, 2013; Xie et al., 2013). The barriers to oxygen and water vapour are two
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essential properties to consider in starch-based material because these can deteriorate
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food properties. The gas transport trough the film depends hardly on the diffusivity.
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Tortuous pathways for gas molecules diffusion, created by dispersed nanocrystals,
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increase molecule migration and consequently limit permeability (Le Corre et al., 2010).
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It is important to take into account that permeability is also affected by solubility, which
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is related with the chemical nature and crystallinity of the polymer, and sample
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homogeneity. Polysaccharides, such as TPS, are well known to be effective barrier
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materials to oxygen transport, whereas they present high water permeability values
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(Bertuzzi et al., 2007).
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The main objective of the present work was to investigate the effect of different
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contents of waxy maize starch nanocrystals on a normal maize starch matrix plasticized
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by glycerol. In addition, the influence of using together nanocrystals from similar
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chemical nature with different geometries, i.e. waxy maize starch and cellulose
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nanocrystals, was studied in order to investigate possible synergistic effects. The
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thermoplastic starch was obtained by casting process and both polysaccharide
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nanocrystals were extracted by acid hydrolysis. The nanometrical dimensions and the
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degree of crystallinity of nanocrystals were investigated. Resulting nanobiocomposites
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were characterized in terms of thermal stability, mechanical properties and barrier
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properties.
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2. EXPERIMENTAL SECTION 2.1. Materials
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Waxy maize starch and normal corn starch were purchased from Sigma-Aldrich and
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glycerol (99% purity) from Panreac. The former, almost completely amylopectin, was
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employed for the preparation of starch nanocrystals (WSNC) and the latter, with higher
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content of amylose, nearly 25%, was used for the preparation of thermoplastic starch
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(TPS).
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2.2. Preparation of waxy starch nanocrystals (WSNC) The preparation of the waxy starch nanocrystals (WSNC) was carried out by acidic
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hydrolysis according to the method described elsewhere (Angellier, Choisnard, Molina-
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Boisseau, Ozil, & Dufresne, 2004; García et al., 2011). 36.725 g of waxy starch maize
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were mixed with 250 ml of H2SO4 solution (3.16M) during 5 days at 40 °C under
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continuous magnetic stirring. The suspension was washed and centrifuged with distilled
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water until neutralization. The obtained nanocrystals were stored at 4 °C after adding
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some drops of chloroform.
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2.3. Preparation of cellulose nanocrystals (CNC)
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The extraction of the crystalline part of cellulose was also performed by acidic
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hydrolysis as explained by Saralegi et al. (2013). 5 g of microcrystalline cellulose
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(MCC) were treated with a H2SO4 64 wt.% aqueous solution. The reaction was
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completed during 30 min at 45 °C with continuous stirring. After that, the acidity was
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reduced by centrifugation (twice, 4500 rpm, 20 min) and the precipitate was neutralized
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by dialysis (4-5 days). Finally, the suspension was ultrasonicated for 15 min.
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2.4. Preparation of thermoplastic starch (TPS) The thermoplastic starch and the subsequent nanocomposites were obtained by casting
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based on the method described by Angellier et al. (2006) with some modifications. A
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mixture of 3.58 g of normal maize starch, 1.93 g of glycerol and 35 g of distilled water
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was gelatinized at 90 °C with continuous stirring until viscosity increased (about 20
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min). After that, the material was spread into glass petri and dried in an oven at 55 °C
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for 2 hours (CSg35). For the preparation of nanocomposites, the desired amount of
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nanocrystals was added, after their previous dispersion in distilled water by
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ultrasonication. In order to avoid the gelatinization of starch nanocrystals, the mixture
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was cooled down below 50 °C before adding the nanoreinforcements. Thereafter, the
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processing was carried out as described above (CSg35+wt.%). All the samples were
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stored at 43% relative humidity (RH) with a K2CO3 saturated solution for two weeks
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before all characterization tests. A glycerol content of 35 wt.% was used for all samples,
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i.e. relative to the starch plus water mixture weight. The nanofiller content was relative
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to the starch plus glycerol weight and WSNC or CNC contents were relative to the total
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nanocrystals weight. The samples were named as CSg35 plus the nanocrystals content,
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followed by the WSNC and/or CNC proportion. Bionanocomposites are referred
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hereafter as shown in Table 1.
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Nanocrystal (wt.%) 0
WSNC (%) 0
CNC (%) 0
CSg35+1 (100/0)
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100
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CSg35+1 (50/50)
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CSg35+1 (0/100)
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CSg35
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CSg35+2.5 CSg35+5 161
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Table 1. Compositions of obtained bionanocomposites.
2.5. Characterization 2.5.1.
Atomic force microscopy (AFM)
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Atomic force microscopy (AFM) measurements were performed in tapping mode using
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a Veeco Multimode scanning probe microscope equipped with a Nanoscope IIIa
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Controller. All measurements were recorded using TESP type silicon tips having a
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resonance frequency of approximately 340 kHz and a cantilever spring constant about
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40 N/m.
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analyzed.
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Both nanocrystals and cryofractured surfaces of nanocomposites were
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Optical microscopy (OM)
The study of the gelatinization process, i.e. the transformation of starch into a
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thermoplastic and amorphous material, was performed by optical microscopy, analyzing
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the change of the crystalline structure and the evolution of the birefringence. The
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analyses were carried out with a Nicon Eclipse E600 instrument in reflectance and with
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a magnification of x500.
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2.5.3.
X-ray diffraction (XRD)
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All samples were submitted to X-ray radiation using an Xpert diffractometer, 40kV,
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40mA. Scattered radiation was detected in the angular range 1-40° (2). The XRD
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curves were used to determine the crystallinity percentage of WSNC and CNC. For
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WSNC an appropriate method is not so widely established, for this reason the
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crystallinity degree was determined by the ratio of the crystalline area to the total area
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WSNC, Crystallinity percentage (%) =
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Area under the peaks × 100 Total area
Crystallinity for CNC was calculated according to the method described in literature
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(Kargarzadeh et al., 2012; Segal, Creely, Martin, & Conrad, 1959).
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002 − × 100 002
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Where I002 is the peak intensity of the (002) crystallographic plane (Kargarzadeh et al.,
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2012), i.e. the intensity peak near 2= 22.5° (Lin, Huang, Chang, Feng, & Yu, 2011b)
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and Iam is the intensity scattered by the amorphous part of the sample measured around
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2= 18.0° (Kargarzadeh et al., 2012).
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Thermogravimetric analysis (TGA)
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Thermogravimetric analysis was carried out in order to investigate the degradation
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process of all nanocomposites using a Mettler Toledo TGA-SDTA 851 instrument. The
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samples were heated from room temperature to 800 °C, with a heating rate of 10 °C
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min-1 and under nitrogen.
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2.5.5.
Tensile tests
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The mechanical behaviour of the nanocomposites was analyzed using a MTS Insight 10
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instrument. Dumbbell shaped specimens 5 mm wide and 15 mm long (L0) were used.
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The initial gap between jaws was adjusted to 22 mm.
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2.5.6.
Dynamic mechanical analysis (DMA)
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Dynamic mechanical analysis was carried out using an Eplexor 100N instrument from
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Gabo Qualimeter. The specimen was a rectangular strip (25×3×0.2 mm3), it was
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subjected to a 0.05% constant strain and the setup measured the stored modulus E’ the -9Page 9 of 33
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loss modulus E’’ and the ratio between the two components (tan=E’’/E’).
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Measurements were performed at 1 Hz, working from -100 °C to 150 °C with a heating
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rate of 2 °C min-1.
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2.5.7.
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Water vapour permeability (WVP)
Water vapour permeation experiments were performed at 25 °C on a gravimetric cell in
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which a small amount of liquid water was sealed by a membrane. The cell was put on a
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balance with a readability of 10-5 g, and the weight loss of the cell, solely due to the
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permeation of the water vapour through the membrane, was followed by means of a
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computer connected to the balance.
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Oxygen permeability (OP)
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The measurements were carried out using a MOCON OX-TRAN Model 2/21 gas
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permeability tester in accordance with the ASTM standard D3985. The permeability of
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samples was tested at 760 mmHg, HR= 50% and 23 °C.
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3. RESULTS AND DISCUSSION
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3.1. Characterization of polysaccharide nanocrystals
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Geometrical structure and dimensions at the nanoscale of the nanocrystals were studied
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by Atomic force microscopy (AFM) (Figure 1a and 1b). Assuming the cylindrical shape
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of CNC (Saralegi et al., 2013) mean diameter values were obtained by the evaluation of
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AFM height profiles. For WSNC, since they trend to agglomerate, less single
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nanocrystals were possible to measure. Nanometrical dimensions of both waxy starch
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and cellulose nanocrystals were confirmed. Waxy starch nanoparticles were found to be
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platelet like, with 28.2 7.4 nm in width and 35 7.9 nm in length. Cellulose
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nanocrystals presented the typical fibril like geometry with 9.1 2.6 nm in diameter and -10Page 10 of 33
150.6 29.1 nm in length. These results agreed with those obtained in literature
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(Angellier et al., 2005; Wang et al., 2010; Saralegi et al., 2013).
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Figure 1. Characterization of polysaccharide nanoreinforcements: AFM images of extracted a) WSNC and b) CNC and c) X-ray diffraction patterns for nanocrystals.
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X-ray diffraction (XRD) measurements were carried out in order to observe the
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crystalline polymorphism for each type of nanocrystal (Figure 1c). XRD pattern of
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WSNC showed the typical A-type polymorphism with two peaks of low intensity
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located at 2=10.1° and 2=11.5°, a peak at 2=15.2° of higher intensity, a double peak
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near 2=17.3°/18.1° and an intense peak at 23.2°. Cellulose nanocrystals pattern
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exhibited strong peaks at 2=14.5°, 2=16.8° and 2=22.7° associated to the typical
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cellulose I crystalline structure. As explained by Lin et al. (2012), the crystallinity
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degree of polysaccharide nanocrystals should be 100%. However, disorder or
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amorphous domains cannot be completely removed by hydrolysis, leading to a lower
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crystallinity degree. Following the method described elsewhere (Jivan et al., 2013; Rosa
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et al., 2010), a crystallinity percentage of 22.8% for WSNC was established. Besides,
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according to the method reported previously (Kargarzadeh et al., 2012; Segal et al.,
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1959) a crystallinity percentage of 83.6% was calculated for CNC. Other authors
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(Buléon, Colonna, Planchot, & Ball, 1998; Lin et al., 2012) determined similar values
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for CNC obtained from MCC (54-88%), but higher crystallinity degree percentages for
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WSNC (38-48%). Differences could be related to the different extraction protocol
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employed, along with the method used for calculating crystallinity degrees (Jivan et al.,
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2013; Rosa et al., 2010). However, even if the crystallinity percent of the WSNC could
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be affected slightly by the calculation method, the results for CNC were, as expected,
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significantly higher. Attending to AFM and XRD results, it was concluded that the
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employed hydrolysis methods were satisfactory. 3.2. Characterization of bionanocomposites
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3.2.1. Influence of the WSNC content
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Prior to the preparation of the bionanocomposites, the gelatinization of the matrix was
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optimized following the disappearance of the birefringence by optical microscopy. The
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effect of the nanoreinforcement content on thermal, mechanical and barrier properties
258
was investigated for nanocomposites of TPS reinforced with WSNC. XRD results for
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the TPS/WSNC bionanocomposites showed that the A-type polymorphism pattern of
260
the native normal maize starch was replaced by a typical broad peak centred at
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2=19.6° in the case of the amorphous thermoplastic derivative. However, as WSNC
262
content increased, peaks associated to the nanoreinforcements were progressively
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detected, in concordance with results published previously in literature (Angellier et al.,
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2006).
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Thermogravimetric measurements were carried out to assess the thermal decomposition
266
of all nanocomposites (Figure 2). Un-plasticized normal maize starch showed a single
267
degradation step at 300 °C. On the contrary, the thermal decomposition process for as-
268
prepared nanocomposites occurs in three main steps (García et al., 2009, 2011). The
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first step (25-100 °C) related to the loss of sorbed humidity, the second one (100-200
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°C) associated to the decomposition of the glycerol-rich phase and the last one (below
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340 °C) due to the oxidation of the partially decomposed starch. The degradation
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stability decrease of the bionanocomposites may be maximized by the affinity between
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glycerol and waxy starch nanocrystals. Acidic sulphate ester groups attached to waxy
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starch nanocrystals surface during the extraction seemed to act as catalyzers of the
275
thermal decomposition, resulting in a clear decrease of degradation temperature for
276
nanocomposites compared with that for the unfilled matrix. At the same time, the final
277
char percentage increased with the nanoreinforcement content.
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Figure 2. TGA curves for a) Normal maize starch b) CSg35, c) CSg35+1 (100/0), d) CSg35+2.5 and e) CSg35+5.
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The mechanical properties of bionanocomposites reinforced with different contents of
282
WSNC were determined by means of tensile tests. Obtained results are collected in
283
Table 2. The addition of WSNC resulted in a significant improvement of the mechanical
284
properties of the TPS. Indeed, the Young’s Modulus increased from 2.5 MPa for the
285
plasticized starch to 7.5 MPa for the bionanocomposites, the tensile strength was
286
improved in a 70%, with only a slight variation in deformability at break. Due to the
287
identical chemical nature of the matrix and the nanofiller, a good affinity between both
288
components was expected, allowing strong TPS/WSNC hydrogen bonding at the
289
interfaces and the subsequent favourable reinforcing effect. However, it is worth noting
290
that mechanical properties of bionanocomposites were not further improved from 2.5
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wt.% to 5 wt.% of WSNC content. The reinforcing effect of starch nanocrystals is
292
generally ascribed to the formation of a hydrogen bonded percolating filler network
293
above a given starch content corresponding to the percolation threshold (Le Corre et al.,
294
2010). Our results seemed to indicate that starch nanocrystals were correctly dispersed
295
in an effective percolated network up to a 2.5 wt.% of reinforcement.
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Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Oxygen Permeance (cm3/m2 day atm)
66.7 4.5
26.9 1.6
Strain at break (%)
Tensile strength (MPa)
CSg35 CSg35+1 (100/0) CSg35+2.5
2.5 0.9
52.7 9.4
1.0 0.2
5.8 0.9
48.8 24.4
1.7 0.4
7.4 1.1
41.8 8.7
1.6 0.3
57.3 3.8
15.5 4.7
CSg35+5
7.5 1.1
47.9 10.3
1.6 0.1
57.3 1.4
7.7 1.5
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Young’s Modulus (MPa)
22.7 3.9
Table 2. Tensile properties and water vapour and oxygen permeance values for plasticized starch and TPS-WSNC nanocomposites.
298
Gas and vapour permeability of polymeric materials is independent of film thickness in
299
ideal systems. However, TPS based films present non-ideal behaviour with increasing
300
permeability values proportionally to the film thickness providing an increasing
301
resistance to mass transfer trough the film (Bertuzzi et al., 2007). In our study such
302
deviation was detected, being the relationship between thickness and water vapour
303
permeability almost linear (R2>0.98). For this reason, both for water vapour and oxygen
304
mass transfer, the calculation of permeance values was preferred. Obtained results are
305
collected in Table 2. Nanocomposites development respond to the hypothesis that
306
nanometric particles incorporated into a polymeric matrix would generate a tortuous
307
path for vapour or gas and, consequently, the diffusive component of the permeability
308
would decrease. Besides the diffusion, polymers barrier properties depends on other
309
factors as polarity and structural features of polymeric side chains, hydrogen bonding
310
characteristics, molecular weight and polydispersity, degree of branching or cross-
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linking, processing methodology and degree of crystallinity (Duncan, 2011). Platelet-
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like starch nanocrystals are supposed to have good attributes in the improvement of
313
barrier properties but, according to water vapour permeability, due to their hydrophilic
314
nature, low crystallinity and agglomeration opposite conclusions have been reported for
315
their nanocomposites (Le Corre et al., 2010). In our particular investigation, whereas
316
water permeance values were not reduced by the addition of starch nanocrystals, oxygen
317
permeance was significantly improved for 2.5 wt.% and 5 wt.% of WSNC contents.
318
Bertuzzi et al. (2007) and Forssell et al. (2002) correlated these results with sorbed
319
water-starch interactions that are closely related with water transport phenomena. These
320
interactions were not generated with oxygen molecules, and that is why oxygen
321
permeance values were improved. As it could be noted, whereas CSg35 and CSg35+1
322
(100/0) presented similar permeance to O2, it was reduced to 7.7 1.5 cm3/m2 day atm,
323
for 5 wt.% of WSNC content, suggesting that the presence of a minimum concentration
324
was necessary to create an actual tortuous pathway. This significant improvement for
325
higher WSNC contents suggests that those were effectively dispersed in the TPS matrix
326
and points out the close interface adhesion between nanocrystals and matrix.
327
The viscoelastic behaviour of the bionanocomposites was assessed by Dynamic
328
mechanical analysis (Figure 3). Two-step modulus drop relative to the tan peaks
329
temperatures near -60 °C (Tα1) and -20 °C (Tα2) was observed. García et al. (2009)
330
attributed them to the glycerol-rich phase and starch-rich phase relaxation temperatures,
331
respectively. As it can be noted, the addition of 1 wt.% of WSNC resulted in a
332
significant shift of Tα1 which is related to an increase in the rigidity of the glycerol-rich
333
phase. As the WSNC content increased, the peak shifted to temperatures close to -50 ºC.
334
At the same time, Tα2 also moved to higher temperatures mainly in bionanocomposites
335
filled with 2.5 wt.% and 5 wt.%, revealing that a good dispersion of the nanocrystals
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was achieved and the reinforcement effect was homogeneously distributed in both
337
phases. Thus, DMA results were in good agreement with results obtained from both
338
tensile and permeance measurements.
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Figure 3. Evolution of E’ and tan with temperature for a) CSg35, b) CSg35+1 (100/0), c) CSg35+2.5, d) CSg35+5.
M
340 341
3.2.2. Influence of nanocrystals’ origin on bionanocomposites
343
The influence of nanocrystal nature was analysed when WSNC and CNC were
344
combined together in TPS. 1 wt.% composition was chosen in order to better analysing
345
any synergistic effect resulting from the simultaneous action of both nanocrystals. This
346
assumption lays on the fact that the platelet shape of WSNC would enhance barrier
347
properties improvement (Duncan, 2010), whereas rod-like high crystalline CNC would
348
contribute in a higher extent to the mechanical reinforcement as they present higher
349
aspect ratio, reducing the percolation threshold. Four different WSNC-CNC nanocrystal
350
contents were studied (Table 1).
351
Mechanical behaviour was evaluated by means of tensile tests and obtained results are
352
shown in Table 3. Some remarkable changes were detected when cellulose or waxy
353
starch nanocrystals were added into the matrix. Firstly, higher values of Modulus were
354
obtained for CNC filled nanocomposites at the same composition. This finding agrees
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with the fact that higher mechanical reinforcement would be achieved by using high
356
aspect ratio rod-like nanocrystals (Flauzino Neto, Silvério, Dantas, & Pasquini, 2013;
357
Wang et al., 2010). The opposite effect was detected for deformation at break, as it was
358
reduced in a larger extent. It is worth noting that the Young’s Modulus was
359
considerably improved up to 12.0 MPa for the CSg35+1 (50/50) system, i.e. the one
360
where WSNC and CNC were combined. Wang et al. (2010) obtained similar results
361
using waterborne polyurethane as matrix. As they explained, the combination of both
362
nanometric reinforcements would lead to a much jammed hydrogen bonding network
363
between nanofiller and the matrix. However, taking into account the variability of the
364
measurements, in our opinion these findings should be corroborated in further
365
investigations.
366 367
M
Oxygen Permeance (cm3/m2 day atm)
1.0 0.2
Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Strain at break (%)
Tensile Strength (MPa)
2.5 0.9
52.7 9.4
te
d
Young’s Modulus (MPa)
22.7 3.9
5.8 0.9
48.8 24.4
1.7 0.4
66.7 4.5
26.9 1.6
12.0 2.0
41.7 4.2
1.6 0.1
54.2 1.1
26.6 1.9
8.5 2.2
41.9 8.5
1.5 0.2
57.9 2.7
22.1 1.1
Ac ce p
CSg35 CSg35+1 (100/0) CSg35+1 (50/50) CSg35+1 (0/100)
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Table 3. Tensile properties and water vapour and oxygen permeance values for TPS and TPS+WSNC/CNC.
368
Table 3 summarized the results of water vapour and oxygen permeance obtained for the
369
bionanocomposites containing WSNC and/or CNC. Large nanoparticle aspect ratios are
370
required to reduce the gas permeability by an appreciable degree (Duncan, 2011).
371
Attending to the results, it could be concluded that the incorporation of CNC to the
372
bionanocomposite did not result in any improvement of the barrier properties. On one
373
hand, water vapour permeability seemed to be again governed by the strong chemical
374
affinity of the material with water molecules. On the other hand, the higher aspect ratio -17Page 17 of 33
and crystallinity of CNC did not reduce the oxygen permeance values (only slight
376
decrease is noting for the CSg35+1 (0/100) sample), revealing that in studied
377
polysaccharide-based films, barrier properties improvement was more correlated to the
378
nanofiller content than to the origin of it.
379
Nanocomposites reinforced with waxy starch and cellulose nanocrystals showed the
380
same rheological behaviour in comparison with films reinforced only with WSNC
381
(Figure 4). Indeed, almost identical DMA curves were obtained with the characteristic
382
two-step modulus drop related to the two different phases. The addition of
383
polysaccharide nanocrystals resulted in higher relaxation temperatures (T) values
384
whatever its origin was. This effect could be ascribed to the fact that both starch and
385
cellulose nanocrystals interacted strongly with the glycerol-rich phase. However, a
386
remarkable higher shift of the Tα1 value to higher temperatures was observed for
387
bionanocompostes containing WSNC. On the contrary, Tα2 maintained almost constant.
388
These results confirmed the interaction between glycerol-rich phase domains and the
389
reinforcing nanocrystals, especially with waxy starch nanocrystals.
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390 391 392
Figure 4. Evolution of the E’ and tan with temperature for a) CSg35, b) CSg35+1 (100/0), c) CSg35+1 (50/50), d) CSg35+1 (0/100).
-18Page 18 of 33
AFM was used for the morphological characterization of the cryofractured
394
nanocomposite samples. Figure 5a and 5b show the obtained images for CSg35+1
395
(100/0) and CSg35+1 (0/100), respectively. In case of the CSg35+1 (100/0) sample,
396
starch nanocrystals were observed as ill-defined nanoparticles due to their extremely
397
reduced dimensions. However, focusing on the amplified image where the dimensions
398
of the nanofillers were possible to evaluate individually, it could be concluded that
399
nanocrystals were correctly dispersed, in good agreement with the conclusions extracted
400
previously.
401 402 403
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Figure 5. AFM height images of a) CSg35+1 (100/0) and b) CSg35+1 (0/100).
404
4. CONCLUSIONS
405
AFM and XRD results demonstrate that the acid hydrolysis protocols used to obtain
406
both waxy starch and cellulose nanocrystals were satisfactory. Also TOM and XRD
407
corroborated that method employed in the gelatinization to produce thermoplastic starch -19Page 19 of 33
and resulting nanocomposites was successful. TGA results defined that the thermal
409
degradation of nanocomposites occurs in three main steps. Also was demonstrated that
410
the addition of NC reduces the thermal stability of all samples. The mechanical
411
properties and oxygen permeance values of the TPS-WSNC nanocomposites are clearly
412
higher than those of the unfilled matrix. The addition of 2.5 wt.% of WSNC to the
413
starch thermoplastic matrix represents a significant improvement of the mechanical and
414
oxygen permeance properties. Mechanical properties were also enhanced with the
415
addition of CNC for CSg35+1 (50/50) and CSg35+1 (0/100) samples, due to the
416
reinforcement ability of CNC. However, DMA curves show an increased of the
417
relaxation temperature since the addition of nanocrystals result in a stronger affinity
418
between glycerol and NC phases.
419
5. ACKNOWLEDGMENTS
420
Financial support from the Basque Country Government in the frame of Saiotek S-
421
PE12UN036 and Grupos Consolidados (IT-776-13) and from the University of the
422
Basque Country (UPV/EHU) in the frame of EHUA12/19 is gratefully acknowledged.
423
Technical and human support provided by SGIker (UPV/EHU, MINECO, GV/EJ,
424
ERDF and ESF) is also gratefully acknowledged.
425
6. REFERENCES
426
Abdorreza, M. N., Cheng, L. H., & Karim, A. A. (2011). Effects of plasticizers on
427
thermal properties and heat sealability of sago starch films. Food Hydrocolloids, 25, 56–
428
60.
429
Angellier, H., Choisnard, L., Molina-Boisseau, S., Ozil, P., & Dufresne, A. (2004)
430
Optimization of the preparation of aqueous suspensions of waxy maize starch
431
nanocrystals using a response surface methodology. Biomacromolecules, 5, 1545–1551.
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-20Page 20 of 33
Angellier, H., Molina-Boisseau, S., Belgacem, M. N., & Dufresne, A. (2005). Surface
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chemical modification of waxy maize starch nanocrystals. Langmuir, 21, 2425–2433.
434
Angellier, H., Molina-Boisseau, S., Dole, P., & Dufresne, A. (2006). Thermoplastic
435
starch-waxy maize starch nanocrystals nanocomposites. Biomacromolecules, 7(2), 531–
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539.
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Angellier-Coussy, H., Putaux, J.-L., Molina-Boisseau, S., Dufresne, A., Bertoft, E., &
438
Perez, S. (2009). The molecular structure of waxy maize starch nanocrystals.
439
Carbohydrate Research, 344, 1558-1566.
440
Averous, L., Moro, L., Dole, P., & Fringant, C. (2000). Properties of thermoplastic
441
blends: starch–polycaprolactone. Polymer, 41(11), 4157–4167.
442
Bertuzzi, M. A., Vidaurre, E. F. C., Armada, M., & Gottifredi, J. C. (2007). Water vapor
443
permeability of edible starch based films. Journal of Food Engineering, 80, 972–978.
444
Buléon, A., Colonna, P., Planchot, V., & Ball, S. (1998). Starch granules: structure and
445
biosynthesis. International Journal of Biological Macromolecules, 23, 85-112.
446
Carvalho, A. J. F. (2008). Starch: Major sources, properties and applications as
447
thermoplastic materials. In M. N. Belgacem & A. Gandini (Eds.), Monomers, polymers
448
and composites from renewable resources (pp. 321-342).
449
Chivrac, F., Angellier-Coussy, H., Guillard, V., Pollet, E., & Averous, L. (2010). How
450
does
451
Carbohydrate Polymers, 82(1), 128–135.
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water
diffuse
in
starch/montmorillonite
nano-biocomposite
materials?.
-21Page 21 of 33
Da Róz, A., Carvalho, A., Gandini, A., & Curvelo, A. (2006). The effects of plasticizers
453
on thermoplastic starch compositions obtained by melt processing. Carbohydrate
454
Polymers, 63, 417–424.
455
Duncan V. T. (2011). Applications of nanotechnology in food packaging and food
456
safety: Barrier materials, antimicrobials and sensors. Journal of Colloid and Interface
457
Science, 363, 1-24.
458
Flauzino Neto, W. P., Silvério, H. A., Dantas, N. O., & Pasquini, D. (2013). Extraction
459
and characterization of cellulose nanocrystals from agro-industrial residue-soy hulls.
460
Industrial Crops and Products, 42, 480- 488.
461
Forssell, P., Lahtinen, R., Lahelin, M., & Myllärinen, P. (2002). Oxygen permeability of
462
amylose and amylopectin films. Carbohydrate Polymers, 47, 125–129.
463
García, N. L., Ribba, L., Dufresne, A., Aranguren, M. & Goyanes, S. (2009). Physico-
464
Mechanical properties of biodegradable starch nanocomposites. Macromolecular
465
Materials and Engineering, 294, 169–177.
466
García, N. L., Ribba, L., Dufresne, A., Aranguren, M., & Goyanes, S. (2011). Effect of
467
glycerol on the morphology of nanocomposites made from thermoplastic starch and
468
starch nanocrystals. Carbohydrate Polymers, 84, 203–210.
469
Habibi, Y., Lucia, L. A., & Rojas, O. J. (2010). Cellulose nanocrystals: Chemistry, self-
470
assembly, and applications. Chemical Reviews, 110(6), 3479–3500.
471
Jivan, M. J., Madadlou, A., & Yarmand, M. (2013). An attempt to cast light into starch
472
nanocrystals preparation and cross-linking. Food Chemistry, 141, 1661-1666.
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Kargarzadeh, H., Ahmad, I., Abdullah, I., Dufresne, A., Zainudin, S. Y., & Sheltami, R.
474
M. (2012). Effects of hydrolysis conditions on the morphology, crystallinity, and
475
thermal stability of cellulose nanocrystals extracted from kenaf bast fibers. Cellulose,
476
19(3), 855–866.
477
Kelnar, I., Kaprálková, L., Brozová, L., Hromádková, J., Kotek, J. (2013). Effect of
478
chitosan on the behaviour of the wheat B-starch nanocomposite. Industrial Crops and
479
Products, 46, 186– 190.
480
Le Corre, D., Bras, J., & Dufresne, A. (2010). Starch nanoparticles: A review.
481
Biomacromolecules, 11, 1139-1153.
482
LeCorre, D., Bras, J., & Dufresne, A. (2012). Influence of native starch’s properties on
483
starch nanocrystals thermal properties. Carbohydrate Polymers, 87, 658–666.
484
Lin, N., Huang, J., Chang, P. R., Anderson D. P., & Yu, J. (2011a). Preparation,
485
Modification, and Application of Starch Nanocrystals in Nanomaterials: A Review.
486
Journal of Nanomaterials, doi: 10.1155/2011/573687.
487
Lin, N., Huang, J., Chang, P. R., Feng, L., & Yu, J. (2011b). Effect of polysaccharide
488
nanocrystals on structure, properties, and drug release kinetics of alginate-based
489
microspheres. Colloids and Surfaces B: Biointerfaces, 85, 270–279.
490
Lin, N., Huang, J., & Dufresne, A. (2012). Preparation, properties and applications of
491
polysaccharide nanocrystals in advanced functional nanomaterials: a review. Nanoscale,
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4(11), 3274–3294.
493
Rosa, M. F., Medeiros, E. S., Malmonge, J. A., Gregorski, K. S., Wood, D. F., Mattoso,
494
L. H. C., Glenn, G., Orts, W. J., & Imam, S. H. (2010). Cellulose nanowhiskers from
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-23Page 23 of 33
coconut husk fibers: Effect of preparation conditions on their thermal and
496
morphological behaviour. Carbohydrate Polymers, 81(1), 83–92.
497
Saralegi, A., Rueda, L., Martin, L., Arbelaiz, A., Eceiza, A., & Corcuera, M.A. (2013).
498
From elastomeric to rigid polyurethane/cellulose nanocrystal bionanocomposites.
499
Composites Science and Technology, 88, 39-47.
500
Segal, L., Creely, J. J., Martin, A. E., Jr., & Conrad, C. M. (1959). An empirical method
501
for estimating the degree of crystallinity of native cellulose using the X-ray
502
diffractometer. Textile Research Journal, 29(10), 786–794.
503
Viguié, J., Molina-Boisseau, S., & Dufresne, A. (2007). Processing and characterization
504
of waxy maize starch films plasticized by sorbitol and reinforced with starch
505
nanocrystals. Macromolecular Bioscience, 7(11), 1206-1216.
506
Wang, J.-L., Cheng, F., Zhu, P-X. (2014). Structure and properties of urea-plasticized
507
starch films with different urea contents. Carbohydrate Polymers, 101, 1109-1115.
508
Wang, Y. X., Tian, H., & Zhang, L. (2010). Role of starch nanocrystals and cellulose
509
whiskers in synergistic reinforcement of waterborne polyurethane. Carbohydrate
510
Polymers, 80, 665–671.
511
Xie, F., Pollet, E., Halley, P. J., Avérous, L. (2013) Starch-based nano-biocomposites.
512
Progress in Polymer Science, 38, 1590– 1628.
513
Yan, Q., Hou, H., Guo, P., & Dong H (2012). Effects of extrusion and glycerol content
514
on properties of oxidized and acetylated corn starch-based films. Carbohydrate
515
Polymers, 87, 707– 712.
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516
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516
WSNC (wt.%) 0
CNC (wt.%) 0
CSg35+1 (100/0)
1
100
0
CSg35+1 (50/50)
1
50
50
CSg35+1 (0/100)
1
0
100
2.5
100
0
5
100
CSg35+2.5 CSg35+5
0
Table 1. Compositions of obtained bionanocomposites.
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517
cr
CSg35
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Nanocrystal (wt.%) 0
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Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Oxygen Permeance (cm3/m2 day atm) 22.7 3.9
48.8 24.4
1.7 0.4
66.7 4.5
26.9 1.6
7.4 1.1
41.8 8.7
1.6 0.3
57.3 3.8
7.5 1.1
47.9 10.3
1.6 0.1
57.3 1.4
7.7 1.5
Strain at break (%)
Tensile strength (MPa)
CSg35 CSg35+1 (100/0) CSg35+2.5
2.5 0.9
52.7 9.4
5.8 0.9
CSg35+5
15.5 4.7
cr
Young’s Modulus (MPa)
Table 2. Tensile properties and water vapour and oxygen permeance values for plasticized starch and TPS-WSNC nanocomposites.
us
519 520
1.0 0.2
ip t
518
521
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522
523 524
48.8 24.4
1.7 0.4
66.7 4.5
26.9 1.6
12.0 2.0
41.7 4.2
1.6 0.1
54.2 1.1
8.5 2.2
41.9 8.5
1.5 0.2
57.9 2.7
2.5 0.9
52.7 9.4
5.8 0.9
26.6 1.9
cr
Tensile Strength (MPa)
ip t
1.0 0.2
Oxygen Permeance (cm3/m2 day atm) 22.7 3.9
Strain at break (%)
22.1 1.1
us
CSg35 CSg35+1 (100/0) CSg35+1 (50/50) CSg35+1 (0/100)
Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Young’s Modulus (MPa)
Table 3. Tensile properties and water vapour and oxygen permeance values for TPS and TPS+WSNC/CNC.
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Highlights (for review)
STARCH AND CELLULOSE NANOCRYSTALS TOGETHER INTO THERMOPLASTIC STARCH BIONANOCOMPOSITES K. González et al.
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HIGHLIGHTS Bioanocomposites were reinforced with combined polysaccharide nanocrystals.
−
Increasing waxy starch nanocrystals content mechanical properties were enhanced.
−
An important improvement of oxygen permeance was obtained at certain
us
cr
−
percentages.
Barrier to water vapour was maintained almost insensitive to nanoreinforcement.
−
Mechanical properties were improved by the incorporation of cellulose
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−
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nanocrystals.
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Figure 2
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Figure 3
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Figure 4
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S0144-8617(14)00965-5 http://dx.doi.org/doi:10.1016/j.carbpol.2014.09.055 CARP 9313
To appear in: Received date: Revised date: Accepted date:
14-3-2014 11-9-2014 12-9-2014
Please cite this article as: Gonz´alez, K., Retegi, A., Gonz´alez, A., Eceiza, A., and Gabilondo, N.,Starch and cellulose nanocrystals together into thermoplastic starch bionanocomposites, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.09.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
STARCH AND CELLULOSE NANOCRYSTALS TOGETHER INTO
2
THERMOPLASTIC STARCH BIONANOCOMPOSITES
3
Kizkitza Gonzáleza, Aloña Retegia, Alba Gonzálezb, Arantxa Eceizaa, Nagore
4
Gabilondoa*
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1
5
a
6
Polytechnic School, University of the Basque Country. Pza. Europa 1. 20018 Donostia-
7
San Sebastián, Spain
cr
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b
POLYMAT, Department of Polymer Science and Technology. Faculty of Chemistry,
an
8
‘Materials + Technologies’ Group, Dept. of Chemical and Environmental Engineering,
University of the Basque Country. P.O. Box 1072, 20080 Donostia-San Sebastián,
10
Spain
11 12
*To whom all correspondence should be addressed. Tel.: (+34)943017231/7162; fax:
13
(+34)943017200; e-mail address: [email protected] (Nagore Gabilondo)
15 16 17 18
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ABSTRACT
23
In the present work, thermoplastic maize starch based bionanocomposites were prepared
24
as transparent films, plasticised with 35% of glycerol and reinforced with both waxy
25
starch (WSNC) and cellulose nanocrystals (CNC), previously extracted by acidic
26
hydrolysis. The influence of the nanofiller content was evaluated at 1 wt.%, 2.5 wt.%
27
and 5 wt.% of WSNC. The effect of adding the two different nanoparticles at 1 wt.%
28
was also investigated. As determined by tensile measurements, mechanical properties
29
were improved at any composition of WSNC. Water vapour permeance values
30
maintained constant, whereas barrier properties to oxygen reduced in a 70%, indicating
31
the effectiveness of hydrogen bonding at the interphase. The use of CNC or CNC and
32
WSNC upgraded mechanical results, but no significant differences in barrier properties
33
were obtained. A homogeneous distribution of the nanofillers was demonstrated by
34
atomic force microscopy, and a shift of the two relaxation peaks to higher temperatures
35
was detected by dynamic mechanical analysis.
36
KEYWORDS
37
Bionanocomposite; Thermoplastic starch; Polysaccharide nanocrystals; Morphology;
38
Thermal, mechanical and barrier properties.
40
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1. INTRODUCTION
45
Starch, in form of its thermoplastic derivate (TPS), has been revealed as an appropriate
46
candidate to be employed as substitute of synthetic polymers traditionally used for
47
packaging, due to its versatility, availability, biodegradability, processability and low
48
cost. Starch is a hydrophilic semicrystalline biopolymer stored in form of granules in
49
the native state that consists of a mixture of two glycosidic macromolecules, i.e.
50
amylose and amylopectin (Carvalho, 2008; Le Corre, Bras, & Dufresne, 2010).
51
Amylose is a linear polysaccharide made up of (1-4) -D-glycopyranose, whereas
52
amylopectin is a highly branched macromolecule composed of both (1-4) and (1-6)
53
glycopyranosyl linkages (García, Ribba, Dufresne, Aranguren, & Goyanes, 2011; Le
54
Corre et al. 2010). The arrangement of the two polymers in form of alternant amorphous
55
and crystalline rings lead to the final semicrystalline granules, being amylopectin the
56
main responsible of the crystalline character (Le Corre et al. 2010).
57
Actually, starch is not a real thermoplastic polymer, but can be processed after its
58
gelatinization by mixing with enough water and/or plasticizers (Angellier, Molina-
59
Boisseau, Dole, & Dufresne, 2006; Carvalho, 2008; García et al., 2011; Viguié, Molina-
60
Boisseau, & Dufresne, 2007). The crystallinity of starch granules disappears during the
61
gelatinization process, and at the end, an amorphous material is obtained (Carvalho,
62
2008). In most investigations the plasticisation of the material is carried out by casting a
63
dispersion of starch with glycerol (Angellier et al., 2006; Bertuzzi, Vidaurre, Armada, &
64
Gottifredi, 2007; Carvalho, 2008; García, Ribba, Dufresne, Aranguren, & Goyanes,
65
2009; Yan, Hou, Guo, & Dong, 2012), but other water soluble plasticizers such as
66
sorbitol, xylitol, maltitol, urea or ethylene glycol have been also used (Abdorreza,
67
Cheng, & Karim, 2011; Da Róz, Carvalho, Gandini, & Curvelo, 2006; Vigué et al.,
68
2007; Wang, Cheng, & Zhu, 2014). Attempting to reduce the undesired well-known
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sensitivity to moisture and temperature of TPS, several strategies have been developed
70
in the literature (Averous, Moro, Dole, & Fringant, 2000; Xie, Pollet, Halley, &
71
Avérous, 2013).
72
Bionanocomposites have already been identified as useful materials for achieving the
73
mentioned objectives. The addition of different nanocrystals to the starch based matrix,
74
would lead to both mechanical and barrier properties improvement of the material. In
75
this context, nanoclays and polysaccharide derived nanocrystals have been tried in the
76
most recent researches (Angellier et al., 2006; García et al., 2011; Xie et al., 2013).
77
Actually, a large amount of work has been published mainly with cellulose whiskers
78
and, in a less extent, chitin or starch nanocrystals (Wang, Tian, & Zhang, 2010).
79
Disordered amorphous domains of natural polysaccharides can be removed by acid
80
hydrolysis (Habibi, Lucia, & Rojas, 2010; Lin, Huang, Chang, Anderson, & Yu, 2011a;
81
Lin, Huang, & Dufresne, 2012), under controlled conditions, leaving the crystalline
82
regions intact (Lin et al., 2012), and thus, nanometrical high crystallinity particles are
83
obtained. However, little work has been reported using different polysaccharide
84
nanocrystals simultaneously (Wang et al., 2010). Nanocrystals extracted from different
85
sources have a variety of geometrical structures (Wang et al., 2010), and thus, they are
86
supposed to have their own properties and applications as reinforcing agent. While
87
cellulose nanocrystals were found to be rod like or fibrillar (Angellier, Molina-
88
Boisseau, Belgacem, & Dufresne, 2005; Lin et al., 2012; Wang et al., 2010), starch
89
nanocrystals isolated from waxy maize presented a platelet like morphology (Habibi et
90
al., 2010; Lin et al., 2012; Saralegi, et al. 2013) and would be preferred for barrier
91
properties enhancement (LeCorre, Bras, & Dufresne, 2012). However, the expected
92
improvement of mechanical, thermal or barrier properties by the addition of
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nanocrystals extracted from different origins would depend on both a correct dispersion
94
and the generation of an active interphase nanoreinforcement/matrix.
95
One of the most common applications of TPS based biocomposites is their use as films
96
in food packaging products. A critical issue in food packaging is that of migration of
97
certain components and vapour and gases permeability (Duncan, 2011). Many authors
98
have investigated the improvement of water vapour and oxygen permeability in
99
reinforced TPS materials (Bertuzzi et al., 2007; Chivrac, Angellier-Coussy, Guillard,
100
Pollet, & Avérous, 2010; García et al., 2009; Kelnar, Kaprálková, Brozová,
101
Hromádkova, 2013; Xie et al., 2013). The barriers to oxygen and water vapour are two
102
essential properties to consider in starch-based material because these can deteriorate
103
food properties. The gas transport trough the film depends hardly on the diffusivity.
104
Tortuous pathways for gas molecules diffusion, created by dispersed nanocrystals,
105
increase molecule migration and consequently limit permeability (Le Corre et al., 2010).
106
It is important to take into account that permeability is also affected by solubility, which
107
is related with the chemical nature and crystallinity of the polymer, and sample
108
homogeneity. Polysaccharides, such as TPS, are well known to be effective barrier
109
materials to oxygen transport, whereas they present high water permeability values
110
(Bertuzzi et al., 2007).
111
The main objective of the present work was to investigate the effect of different
112
contents of waxy maize starch nanocrystals on a normal maize starch matrix plasticized
113
by glycerol. In addition, the influence of using together nanocrystals from similar
114
chemical nature with different geometries, i.e. waxy maize starch and cellulose
115
nanocrystals, was studied in order to investigate possible synergistic effects. The
116
thermoplastic starch was obtained by casting process and both polysaccharide
117
nanocrystals were extracted by acid hydrolysis. The nanometrical dimensions and the
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degree of crystallinity of nanocrystals were investigated. Resulting nanobiocomposites
119
were characterized in terms of thermal stability, mechanical properties and barrier
120
properties.
121
2. EXPERIMENTAL SECTION 2.1. Materials
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118
Waxy maize starch and normal corn starch were purchased from Sigma-Aldrich and
124
glycerol (99% purity) from Panreac. The former, almost completely amylopectin, was
125
employed for the preparation of starch nanocrystals (WSNC) and the latter, with higher
126
content of amylose, nearly 25%, was used for the preparation of thermoplastic starch
127
(TPS).
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2.2. Preparation of waxy starch nanocrystals (WSNC) The preparation of the waxy starch nanocrystals (WSNC) was carried out by acidic
130
hydrolysis according to the method described elsewhere (Angellier, Choisnard, Molina-
131
Boisseau, Ozil, & Dufresne, 2004; García et al., 2011). 36.725 g of waxy starch maize
132
were mixed with 250 ml of H2SO4 solution (3.16M) during 5 days at 40 °C under
133
continuous magnetic stirring. The suspension was washed and centrifuged with distilled
134
water until neutralization. The obtained nanocrystals were stored at 4 °C after adding
135
some drops of chloroform.
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2.3. Preparation of cellulose nanocrystals (CNC)
137
The extraction of the crystalline part of cellulose was also performed by acidic
138
hydrolysis as explained by Saralegi et al. (2013). 5 g of microcrystalline cellulose
139
(MCC) were treated with a H2SO4 64 wt.% aqueous solution. The reaction was
140
completed during 30 min at 45 °C with continuous stirring. After that, the acidity was
-6Page 6 of 33
141
reduced by centrifugation (twice, 4500 rpm, 20 min) and the precipitate was neutralized
142
by dialysis (4-5 days). Finally, the suspension was ultrasonicated for 15 min.
143
2.4. Preparation of thermoplastic starch (TPS) The thermoplastic starch and the subsequent nanocomposites were obtained by casting
145
based on the method described by Angellier et al. (2006) with some modifications. A
146
mixture of 3.58 g of normal maize starch, 1.93 g of glycerol and 35 g of distilled water
147
was gelatinized at 90 °C with continuous stirring until viscosity increased (about 20
148
min). After that, the material was spread into glass petri and dried in an oven at 55 °C
149
for 2 hours (CSg35). For the preparation of nanocomposites, the desired amount of
150
nanocrystals was added, after their previous dispersion in distilled water by
151
ultrasonication. In order to avoid the gelatinization of starch nanocrystals, the mixture
152
was cooled down below 50 °C before adding the nanoreinforcements. Thereafter, the
153
processing was carried out as described above (CSg35+wt.%). All the samples were
154
stored at 43% relative humidity (RH) with a K2CO3 saturated solution for two weeks
155
before all characterization tests. A glycerol content of 35 wt.% was used for all samples,
156
i.e. relative to the starch plus water mixture weight. The nanofiller content was relative
157
to the starch plus glycerol weight and WSNC or CNC contents were relative to the total
158
nanocrystals weight. The samples were named as CSg35 plus the nanocrystals content,
159
followed by the WSNC and/or CNC proportion. Bionanocomposites are referred
160
hereafter as shown in Table 1.
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Nanocrystal (wt.%) 0
WSNC (%) 0
CNC (%) 0
CSg35+1 (100/0)
1
100
0
CSg35+1 (50/50)
1
50
50
CSg35+1 (0/100)
1
0
100
CSg35
-7Page 7 of 33
CSg35+2.5 CSg35+5 161
163
100
0
5
100
0
Table 1. Compositions of obtained bionanocomposites.
2.5. Characterization 2.5.1.
Atomic force microscopy (AFM)
ip t
162
2.5
Atomic force microscopy (AFM) measurements were performed in tapping mode using
165
a Veeco Multimode scanning probe microscope equipped with a Nanoscope IIIa
166
Controller. All measurements were recorded using TESP type silicon tips having a
167
resonance frequency of approximately 340 kHz and a cantilever spring constant about
168
40 N/m.
169
analyzed.
an
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2.5.2.
Both nanocrystals and cryofractured surfaces of nanocomposites were
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170
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164
Optical microscopy (OM)
The study of the gelatinization process, i.e. the transformation of starch into a
172
thermoplastic and amorphous material, was performed by optical microscopy, analyzing
173
the change of the crystalline structure and the evolution of the birefringence. The
174
analyses were carried out with a Nicon Eclipse E600 instrument in reflectance and with
175
a magnification of x500.
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2.5.3.
X-ray diffraction (XRD)
177
All samples were submitted to X-ray radiation using an Xpert diffractometer, 40kV,
178
40mA. Scattered radiation was detected in the angular range 1-40° (2). The XRD
179
curves were used to determine the crystallinity percentage of WSNC and CNC. For
180
WSNC an appropriate method is not so widely established, for this reason the
181
crystallinity degree was determined by the ratio of the crystalline area to the total area
182
(Jivan, Madadlou, &Yarmand, 2013; Rosa et al., 2010). -8Page 8 of 33
WSNC, Crystallinity percentage (%) =
183
Area under the peaks × 100 Total area
Crystallinity for CNC was calculated according to the method described in literature
185
(Kargarzadeh et al., 2012; Segal, Creely, Martin, & Conrad, 1959).
186
002 − × 100 002
cr
CNC, Crystallinity percentage (%) =
ip t
184
Where I002 is the peak intensity of the (002) crystallographic plane (Kargarzadeh et al.,
188
2012), i.e. the intensity peak near 2= 22.5° (Lin, Huang, Chang, Feng, & Yu, 2011b)
189
and Iam is the intensity scattered by the amorphous part of the sample measured around
190
2= 18.0° (Kargarzadeh et al., 2012).
an
2.5.4.
Thermogravimetric analysis (TGA)
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191
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187
Thermogravimetric analysis was carried out in order to investigate the degradation
193
process of all nanocomposites using a Mettler Toledo TGA-SDTA 851 instrument. The
194
samples were heated from room temperature to 800 °C, with a heating rate of 10 °C
195
min-1 and under nitrogen.
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2.5.5.
Tensile tests
197
The mechanical behaviour of the nanocomposites was analyzed using a MTS Insight 10
198
instrument. Dumbbell shaped specimens 5 mm wide and 15 mm long (L0) were used.
199
The initial gap between jaws was adjusted to 22 mm.
200
2.5.6.
Dynamic mechanical analysis (DMA)
201
Dynamic mechanical analysis was carried out using an Eplexor 100N instrument from
202
Gabo Qualimeter. The specimen was a rectangular strip (25×3×0.2 mm3), it was
203
subjected to a 0.05% constant strain and the setup measured the stored modulus E’ the -9Page 9 of 33
204
loss modulus E’’ and the ratio between the two components (tan=E’’/E’).
205
Measurements were performed at 1 Hz, working from -100 °C to 150 °C with a heating
206
rate of 2 °C min-1.
207
2.5.7.
ip t
Water vapour permeability (WVP)
Water vapour permeation experiments were performed at 25 °C on a gravimetric cell in
209
which a small amount of liquid water was sealed by a membrane. The cell was put on a
210
balance with a readability of 10-5 g, and the weight loss of the cell, solely due to the
211
permeation of the water vapour through the membrane, was followed by means of a
212
computer connected to the balance.
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2.5.8.
Oxygen permeability (OP)
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213
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208
The measurements were carried out using a MOCON OX-TRAN Model 2/21 gas
215
permeability tester in accordance with the ASTM standard D3985. The permeability of
216
samples was tested at 760 mmHg, HR= 50% and 23 °C.
217
3. RESULTS AND DISCUSSION
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3.1. Characterization of polysaccharide nanocrystals
219
Geometrical structure and dimensions at the nanoscale of the nanocrystals were studied
220
by Atomic force microscopy (AFM) (Figure 1a and 1b). Assuming the cylindrical shape
221
of CNC (Saralegi et al., 2013) mean diameter values were obtained by the evaluation of
222
AFM height profiles. For WSNC, since they trend to agglomerate, less single
223
nanocrystals were possible to measure. Nanometrical dimensions of both waxy starch
224
and cellulose nanocrystals were confirmed. Waxy starch nanoparticles were found to be
225
platelet like, with 28.2 7.4 nm in width and 35 7.9 nm in length. Cellulose
226
nanocrystals presented the typical fibril like geometry with 9.1 2.6 nm in diameter and -10Page 10 of 33
150.6 29.1 nm in length. These results agreed with those obtained in literature
228
(Angellier et al., 2005; Wang et al., 2010; Saralegi et al., 2013).
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Figure 1. Characterization of polysaccharide nanoreinforcements: AFM images of extracted a) WSNC and b) CNC and c) X-ray diffraction patterns for nanocrystals.
232
X-ray diffraction (XRD) measurements were carried out in order to observe the
233
crystalline polymorphism for each type of nanocrystal (Figure 1c). XRD pattern of
234
WSNC showed the typical A-type polymorphism with two peaks of low intensity
235
located at 2=10.1° and 2=11.5°, a peak at 2=15.2° of higher intensity, a double peak
236
near 2=17.3°/18.1° and an intense peak at 23.2°. Cellulose nanocrystals pattern
237
exhibited strong peaks at 2=14.5°, 2=16.8° and 2=22.7° associated to the typical
238
cellulose I crystalline structure. As explained by Lin et al. (2012), the crystallinity
239
degree of polysaccharide nanocrystals should be 100%. However, disorder or
240
amorphous domains cannot be completely removed by hydrolysis, leading to a lower
241
crystallinity degree. Following the method described elsewhere (Jivan et al., 2013; Rosa
242
et al., 2010), a crystallinity percentage of 22.8% for WSNC was established. Besides,
243
according to the method reported previously (Kargarzadeh et al., 2012; Segal et al.,
244
1959) a crystallinity percentage of 83.6% was calculated for CNC. Other authors
245
(Buléon, Colonna, Planchot, & Ball, 1998; Lin et al., 2012) determined similar values
246
for CNC obtained from MCC (54-88%), but higher crystallinity degree percentages for
247
WSNC (38-48%). Differences could be related to the different extraction protocol
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employed, along with the method used for calculating crystallinity degrees (Jivan et al.,
249
2013; Rosa et al., 2010). However, even if the crystallinity percent of the WSNC could
250
be affected slightly by the calculation method, the results for CNC were, as expected,
251
significantly higher. Attending to AFM and XRD results, it was concluded that the
252
employed hydrolysis methods were satisfactory. 3.2. Characterization of bionanocomposites
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253
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3.2.1. Influence of the WSNC content
255
Prior to the preparation of the bionanocomposites, the gelatinization of the matrix was
256
optimized following the disappearance of the birefringence by optical microscopy. The
257
effect of the nanoreinforcement content on thermal, mechanical and barrier properties
258
was investigated for nanocomposites of TPS reinforced with WSNC. XRD results for
259
the TPS/WSNC bionanocomposites showed that the A-type polymorphism pattern of
260
the native normal maize starch was replaced by a typical broad peak centred at
261
2=19.6° in the case of the amorphous thermoplastic derivative. However, as WSNC
262
content increased, peaks associated to the nanoreinforcements were progressively
263
detected, in concordance with results published previously in literature (Angellier et al.,
264
2006).
265
Thermogravimetric measurements were carried out to assess the thermal decomposition
266
of all nanocomposites (Figure 2). Un-plasticized normal maize starch showed a single
267
degradation step at 300 °C. On the contrary, the thermal decomposition process for as-
268
prepared nanocomposites occurs in three main steps (García et al., 2009, 2011). The
269
first step (25-100 °C) related to the loss of sorbed humidity, the second one (100-200
270
°C) associated to the decomposition of the glycerol-rich phase and the last one (below
271
340 °C) due to the oxidation of the partially decomposed starch. The degradation
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stability decrease of the bionanocomposites may be maximized by the affinity between
273
glycerol and waxy starch nanocrystals. Acidic sulphate ester groups attached to waxy
274
starch nanocrystals surface during the extraction seemed to act as catalyzers of the
275
thermal decomposition, resulting in a clear decrease of degradation temperature for
276
nanocomposites compared with that for the unfilled matrix. At the same time, the final
277
char percentage increased with the nanoreinforcement content.
278
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Figure 2. TGA curves for a) Normal maize starch b) CSg35, c) CSg35+1 (100/0), d) CSg35+2.5 and e) CSg35+5.
281
The mechanical properties of bionanocomposites reinforced with different contents of
282
WSNC were determined by means of tensile tests. Obtained results are collected in
283
Table 2. The addition of WSNC resulted in a significant improvement of the mechanical
284
properties of the TPS. Indeed, the Young’s Modulus increased from 2.5 MPa for the
285
plasticized starch to 7.5 MPa for the bionanocomposites, the tensile strength was
286
improved in a 70%, with only a slight variation in deformability at break. Due to the
287
identical chemical nature of the matrix and the nanofiller, a good affinity between both
288
components was expected, allowing strong TPS/WSNC hydrogen bonding at the
289
interfaces and the subsequent favourable reinforcing effect. However, it is worth noting
290
that mechanical properties of bionanocomposites were not further improved from 2.5
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wt.% to 5 wt.% of WSNC content. The reinforcing effect of starch nanocrystals is
292
generally ascribed to the formation of a hydrogen bonded percolating filler network
293
above a given starch content corresponding to the percolation threshold (Le Corre et al.,
294
2010). Our results seemed to indicate that starch nanocrystals were correctly dispersed
295
in an effective percolated network up to a 2.5 wt.% of reinforcement.
ip t
291
Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Oxygen Permeance (cm3/m2 day atm)
66.7 4.5
26.9 1.6
Strain at break (%)
Tensile strength (MPa)
CSg35 CSg35+1 (100/0) CSg35+2.5
2.5 0.9
52.7 9.4
1.0 0.2
5.8 0.9
48.8 24.4
1.7 0.4
7.4 1.1
41.8 8.7
1.6 0.3
57.3 3.8
15.5 4.7
CSg35+5
7.5 1.1
47.9 10.3
1.6 0.1
57.3 1.4
7.7 1.5
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Young’s Modulus (MPa)
22.7 3.9
Table 2. Tensile properties and water vapour and oxygen permeance values for plasticized starch and TPS-WSNC nanocomposites.
298
Gas and vapour permeability of polymeric materials is independent of film thickness in
299
ideal systems. However, TPS based films present non-ideal behaviour with increasing
300
permeability values proportionally to the film thickness providing an increasing
301
resistance to mass transfer trough the film (Bertuzzi et al., 2007). In our study such
302
deviation was detected, being the relationship between thickness and water vapour
303
permeability almost linear (R2>0.98). For this reason, both for water vapour and oxygen
304
mass transfer, the calculation of permeance values was preferred. Obtained results are
305
collected in Table 2. Nanocomposites development respond to the hypothesis that
306
nanometric particles incorporated into a polymeric matrix would generate a tortuous
307
path for vapour or gas and, consequently, the diffusive component of the permeability
308
would decrease. Besides the diffusion, polymers barrier properties depends on other
309
factors as polarity and structural features of polymeric side chains, hydrogen bonding
310
characteristics, molecular weight and polydispersity, degree of branching or cross-
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linking, processing methodology and degree of crystallinity (Duncan, 2011). Platelet-
312
like starch nanocrystals are supposed to have good attributes in the improvement of
313
barrier properties but, according to water vapour permeability, due to their hydrophilic
314
nature, low crystallinity and agglomeration opposite conclusions have been reported for
315
their nanocomposites (Le Corre et al., 2010). In our particular investigation, whereas
316
water permeance values were not reduced by the addition of starch nanocrystals, oxygen
317
permeance was significantly improved for 2.5 wt.% and 5 wt.% of WSNC contents.
318
Bertuzzi et al. (2007) and Forssell et al. (2002) correlated these results with sorbed
319
water-starch interactions that are closely related with water transport phenomena. These
320
interactions were not generated with oxygen molecules, and that is why oxygen
321
permeance values were improved. As it could be noted, whereas CSg35 and CSg35+1
322
(100/0) presented similar permeance to O2, it was reduced to 7.7 1.5 cm3/m2 day atm,
323
for 5 wt.% of WSNC content, suggesting that the presence of a minimum concentration
324
was necessary to create an actual tortuous pathway. This significant improvement for
325
higher WSNC contents suggests that those were effectively dispersed in the TPS matrix
326
and points out the close interface adhesion between nanocrystals and matrix.
327
The viscoelastic behaviour of the bionanocomposites was assessed by Dynamic
328
mechanical analysis (Figure 3). Two-step modulus drop relative to the tan peaks
329
temperatures near -60 °C (Tα1) and -20 °C (Tα2) was observed. García et al. (2009)
330
attributed them to the glycerol-rich phase and starch-rich phase relaxation temperatures,
331
respectively. As it can be noted, the addition of 1 wt.% of WSNC resulted in a
332
significant shift of Tα1 which is related to an increase in the rigidity of the glycerol-rich
333
phase. As the WSNC content increased, the peak shifted to temperatures close to -50 ºC.
334
At the same time, Tα2 also moved to higher temperatures mainly in bionanocomposites
335
filled with 2.5 wt.% and 5 wt.%, revealing that a good dispersion of the nanocrystals
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was achieved and the reinforcement effect was homogeneously distributed in both
337
phases. Thus, DMA results were in good agreement with results obtained from both
338
tensile and permeance measurements.
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336
339
Figure 3. Evolution of E’ and tan with temperature for a) CSg35, b) CSg35+1 (100/0), c) CSg35+2.5, d) CSg35+5.
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340 341
3.2.2. Influence of nanocrystals’ origin on bionanocomposites
343
The influence of nanocrystal nature was analysed when WSNC and CNC were
344
combined together in TPS. 1 wt.% composition was chosen in order to better analysing
345
any synergistic effect resulting from the simultaneous action of both nanocrystals. This
346
assumption lays on the fact that the platelet shape of WSNC would enhance barrier
347
properties improvement (Duncan, 2010), whereas rod-like high crystalline CNC would
348
contribute in a higher extent to the mechanical reinforcement as they present higher
349
aspect ratio, reducing the percolation threshold. Four different WSNC-CNC nanocrystal
350
contents were studied (Table 1).
351
Mechanical behaviour was evaluated by means of tensile tests and obtained results are
352
shown in Table 3. Some remarkable changes were detected when cellulose or waxy
353
starch nanocrystals were added into the matrix. Firstly, higher values of Modulus were
354
obtained for CNC filled nanocomposites at the same composition. This finding agrees
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with the fact that higher mechanical reinforcement would be achieved by using high
356
aspect ratio rod-like nanocrystals (Flauzino Neto, Silvério, Dantas, & Pasquini, 2013;
357
Wang et al., 2010). The opposite effect was detected for deformation at break, as it was
358
reduced in a larger extent. It is worth noting that the Young’s Modulus was
359
considerably improved up to 12.0 MPa for the CSg35+1 (50/50) system, i.e. the one
360
where WSNC and CNC were combined. Wang et al. (2010) obtained similar results
361
using waterborne polyurethane as matrix. As they explained, the combination of both
362
nanometric reinforcements would lead to a much jammed hydrogen bonding network
363
between nanofiller and the matrix. However, taking into account the variability of the
364
measurements, in our opinion these findings should be corroborated in further
365
investigations.
366 367
M
Oxygen Permeance (cm3/m2 day atm)
1.0 0.2
Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Strain at break (%)
Tensile Strength (MPa)
2.5 0.9
52.7 9.4
te
d
Young’s Modulus (MPa)
22.7 3.9
5.8 0.9
48.8 24.4
1.7 0.4
66.7 4.5
26.9 1.6
12.0 2.0
41.7 4.2
1.6 0.1
54.2 1.1
26.6 1.9
8.5 2.2
41.9 8.5
1.5 0.2
57.9 2.7
22.1 1.1
Ac ce p
CSg35 CSg35+1 (100/0) CSg35+1 (50/50) CSg35+1 (0/100)
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355
Table 3. Tensile properties and water vapour and oxygen permeance values for TPS and TPS+WSNC/CNC.
368
Table 3 summarized the results of water vapour and oxygen permeance obtained for the
369
bionanocomposites containing WSNC and/or CNC. Large nanoparticle aspect ratios are
370
required to reduce the gas permeability by an appreciable degree (Duncan, 2011).
371
Attending to the results, it could be concluded that the incorporation of CNC to the
372
bionanocomposite did not result in any improvement of the barrier properties. On one
373
hand, water vapour permeability seemed to be again governed by the strong chemical
374
affinity of the material with water molecules. On the other hand, the higher aspect ratio -17Page 17 of 33
and crystallinity of CNC did not reduce the oxygen permeance values (only slight
376
decrease is noting for the CSg35+1 (0/100) sample), revealing that in studied
377
polysaccharide-based films, barrier properties improvement was more correlated to the
378
nanofiller content than to the origin of it.
379
Nanocomposites reinforced with waxy starch and cellulose nanocrystals showed the
380
same rheological behaviour in comparison with films reinforced only with WSNC
381
(Figure 4). Indeed, almost identical DMA curves were obtained with the characteristic
382
two-step modulus drop related to the two different phases. The addition of
383
polysaccharide nanocrystals resulted in higher relaxation temperatures (T) values
384
whatever its origin was. This effect could be ascribed to the fact that both starch and
385
cellulose nanocrystals interacted strongly with the glycerol-rich phase. However, a
386
remarkable higher shift of the Tα1 value to higher temperatures was observed for
387
bionanocompostes containing WSNC. On the contrary, Tα2 maintained almost constant.
388
These results confirmed the interaction between glycerol-rich phase domains and the
389
reinforcing nanocrystals, especially with waxy starch nanocrystals.
Ac ce p
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375
390 391 392
Figure 4. Evolution of the E’ and tan with temperature for a) CSg35, b) CSg35+1 (100/0), c) CSg35+1 (50/50), d) CSg35+1 (0/100).
-18Page 18 of 33
AFM was used for the morphological characterization of the cryofractured
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nanocomposite samples. Figure 5a and 5b show the obtained images for CSg35+1
395
(100/0) and CSg35+1 (0/100), respectively. In case of the CSg35+1 (100/0) sample,
396
starch nanocrystals were observed as ill-defined nanoparticles due to their extremely
397
reduced dimensions. However, focusing on the amplified image where the dimensions
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of the nanofillers were possible to evaluate individually, it could be concluded that
399
nanocrystals were correctly dispersed, in good agreement with the conclusions extracted
400
previously.
401 402 403
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Figure 5. AFM height images of a) CSg35+1 (100/0) and b) CSg35+1 (0/100).
404
4. CONCLUSIONS
405
AFM and XRD results demonstrate that the acid hydrolysis protocols used to obtain
406
both waxy starch and cellulose nanocrystals were satisfactory. Also TOM and XRD
407
corroborated that method employed in the gelatinization to produce thermoplastic starch -19Page 19 of 33
and resulting nanocomposites was successful. TGA results defined that the thermal
409
degradation of nanocomposites occurs in three main steps. Also was demonstrated that
410
the addition of NC reduces the thermal stability of all samples. The mechanical
411
properties and oxygen permeance values of the TPS-WSNC nanocomposites are clearly
412
higher than those of the unfilled matrix. The addition of 2.5 wt.% of WSNC to the
413
starch thermoplastic matrix represents a significant improvement of the mechanical and
414
oxygen permeance properties. Mechanical properties were also enhanced with the
415
addition of CNC for CSg35+1 (50/50) and CSg35+1 (0/100) samples, due to the
416
reinforcement ability of CNC. However, DMA curves show an increased of the
417
relaxation temperature since the addition of nanocrystals result in a stronger affinity
418
between glycerol and NC phases.
419
5. ACKNOWLEDGMENTS
420
Financial support from the Basque Country Government in the frame of Saiotek S-
421
PE12UN036 and Grupos Consolidados (IT-776-13) and from the University of the
422
Basque Country (UPV/EHU) in the frame of EHUA12/19 is gratefully acknowledged.
423
Technical and human support provided by SGIker (UPV/EHU, MINECO, GV/EJ,
424
ERDF and ESF) is also gratefully acknowledged.
425
6. REFERENCES
426
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of waxy maize starch films plasticized by sorbitol and reinforced with starch
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nanocrystals. Macromolecular Bioscience, 7(11), 1206-1216.
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starch films with different urea contents. Carbohydrate Polymers, 101, 1109-1115.
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Wang, Y. X., Tian, H., & Zhang, L. (2010). Role of starch nanocrystals and cellulose
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Xie, F., Pollet, E., Halley, P. J., Avérous, L. (2013) Starch-based nano-biocomposites.
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WSNC (wt.%) 0
CNC (wt.%) 0
CSg35+1 (100/0)
1
100
0
CSg35+1 (50/50)
1
50
50
CSg35+1 (0/100)
1
0
100
2.5
100
0
5
100
CSg35+2.5 CSg35+5
0
Table 1. Compositions of obtained bionanocomposites.
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cr
CSg35
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Nanocrystal (wt.%) 0
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Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Oxygen Permeance (cm3/m2 day atm) 22.7 3.9
48.8 24.4
1.7 0.4
66.7 4.5
26.9 1.6
7.4 1.1
41.8 8.7
1.6 0.3
57.3 3.8
7.5 1.1
47.9 10.3
1.6 0.1
57.3 1.4
7.7 1.5
Strain at break (%)
Tensile strength (MPa)
CSg35 CSg35+1 (100/0) CSg35+2.5
2.5 0.9
52.7 9.4
5.8 0.9
CSg35+5
15.5 4.7
cr
Young’s Modulus (MPa)
Table 2. Tensile properties and water vapour and oxygen permeance values for plasticized starch and TPS-WSNC nanocomposites.
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519 520
1.0 0.2
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521
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522
523 524
48.8 24.4
1.7 0.4
66.7 4.5
26.9 1.6
12.0 2.0
41.7 4.2
1.6 0.1
54.2 1.1
8.5 2.2
41.9 8.5
1.5 0.2
57.9 2.7
2.5 0.9
52.7 9.4
5.8 0.9
26.6 1.9
cr
Tensile Strength (MPa)
ip t
1.0 0.2
Oxygen Permeance (cm3/m2 day atm) 22.7 3.9
Strain at break (%)
22.1 1.1
us
CSg35 CSg35+1 (100/0) CSg35+1 (50/50) CSg35+1 (0/100)
Water Vapour Permeance (kg/m2 s Pa)1010 56.2 2.0
Young’s Modulus (MPa)
Table 3. Tensile properties and water vapour and oxygen permeance values for TPS and TPS+WSNC/CNC.
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Highlights (for review)
STARCH AND CELLULOSE NANOCRYSTALS TOGETHER INTO THERMOPLASTIC STARCH BIONANOCOMPOSITES K. González et al.
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HIGHLIGHTS Bioanocomposites were reinforced with combined polysaccharide nanocrystals.
−
Increasing waxy starch nanocrystals content mechanical properties were enhanced.
−
An important improvement of oxygen permeance was obtained at certain
us
cr
−
percentages.
Barrier to water vapour was maintained almost insensitive to nanoreinforcement.
−
Mechanical properties were improved by the incorporation of cellulose
an
−
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nanocrystals.
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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