Carbohydrate Polymers 92 (2013) 2128–2134 Contents lists available at SciVerse ScienceDirect Carbohydrate Polymers jou rn al hom epa ge: www.elsevier.com/locate/carbpol Tunable green oxygen barrier through layer-by-layer self-assembly of chitosan and cellulose nanocrystals Fei Li a , Paolo Biagioni b , Marco Finazzi b , Silvia Tavazzi c , Luciano Piergiovanni a,∗ a DeFENS – Department of Food, Environmental and Nutritional Sciences, Packaging Division, Università degli Studi di Milano, Via Celoria, 2, 20133 Milano, Italy b Dipartimento di Fisica and CNISM, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy c Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, Via Cozzi, 53, I-20125 Milano, Italy a r t i c l e i n f o Article history: Received 12 September 2012 Received in revised form 16 November 2012 Accepted 17 November 2012 Available online xxx Keywords: Oxygen barrier Flexible packaging Layer-by-layer coating Cellulose nanocrystals Chitosan Packaging sustainability a b s t r a c t We address the oxygen-barrier properties of a nanocomposite created by layer-by-layer assembly of two biopolymers, chitosan (CS) and cellulose, in nanocrystals form (CNs), on an amorphous PET substrate. We systematically investigated the oxygen permeability, morphology, and thickness of the nanocomposite grown under two different pH combinations and with different number of deposition cycles, up to 30 bilayers. Noticeably, the thickness of each deposited bilayer can be largely tuned by the pH value of the solution, from ∼7 up to ∼26 nm in the tested conditions. By our analysis, it is reliably concluded that such CS/CNs nanocomposite holds promises for gas barrier applications in food and drug packaging as a clear coating on plastic films and tridimensional objects, improving performance and sustainability of the final packages. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction High gas-barrier materials are crucial components in critical applications, such as food and drug packaging (Cooper, Douglas, & Perchonok, 2011; Priolo, Gamboa, Holder, & Grunlan, 2010; Svagan et al., 2012). In particular, due to their oxygen permeability, con- ventional plastic films are very often not suitable to assure the long shelf lives required for a wide variety of foods and expected by the market (Cooper et al., 2011). The solution of such a tough problem, in any case, cannot neglect environmental issues: films should efficiently prevent oxygen from penetrating to food, whilst it is recommended that the barrier materials or coatings should be sustainable and environmentally friendly (Svagan et al., 2012). So far, the main strategies to increase the barrier properties of flexible and transparent materials for food packaging applica- tions have been limited to vacuum metallization, silicon oxide (SiO x ) coatings (Jang, Rawson, & Grunlan, 2008; Leterrier, 2003), manufacturing of multi-layers (by co-extrusion and/or laminat- ing) (Affinito et al., 1996), or development of clay-nanocomposites (Chang, An, & Sur, 2003; Donadi, Modesti, Lorenzetti, & Besco, 2011; Ghasemi, Carreau, Kamal, & Tabatabaei, 2012). However, ∗ Corresponding author at: Via Celoria, 2 20133 Milan, Italy. Tel.: +39 02 50316638; fax: +39 02 50316672. E-mail address: Luciano.Piergiovanni@unimi.it (L. Piergiovanni). these solutions do not completely fulfill the sustainability expec- tations and the needs of transparency, while some technological drawbacks still exist. For instance, SiO x vapor-deposited thin films are prone to cracking when flexed and show poor adhe- sion to plastic substrates (Leterrier, 2003); multilayer materials can also present adhesion problems denoting delaminating occur- rences; nanoclay-reinforced polymer composites and metallized films suffer from low transparency and relatively high values of the oxygen transmission rate (Osman, Rupp, & Suter, 2005; Sánchez- Valdes, López-Quintanilla, Ramírez-Vargas, Medellín-Rodríguez, & Gutierrez-Rodriguez, 2006). Also, although the most claimed option to increase gas barrier properties of flexible packaging materials is currently represented by nanoclays inclusions (Jang et al., 2008; Priolo et al., 2010; Svagan et al., 2012), yet it was demonstrated that various nanoclays are highly cytotoxic, posing a possible risk to human health (Lordan, Kennedy, & Higginbotham, 2011). Ideally, a barrier coating made of natural bio-polymers could achieve all the goals of high transparency, low gas per- meability, bio-compatibility, sustainability and good adhesion. Layer-by-layer (LbL) assembly is a basic technique for the fabri- cation of multicomponent films on solid supports by controlled adsorption from solutions or dispersions (Decher, 1997) and is also considered as a potential means for implementing new and versatile surface applications including gas barrier coatings (Jang et al., 2008; Priolo et al., 2010; Svagan et al., 2012; Yang, Haile, Park, Malek, & Grunlan, 2011), anti-fog and super-hydrophobic 0144-8617/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbpol.2012.11.091