Carbohydrate Polymers 112 (2014) 494–501 Contents lists available at ScienceDirect Carbohydrate Polymers j ourna l ho me pa g e: www.elsevier.com/locate/carbpol Gas permeability and selectivity of cellulose nanocrystals films (layers) deposited by spin coating Martha A. Herrera, Aji P. Mathew, Kristiina Oksman ∗ Division of Materials Science, Luleå University of Technology, SE-97187 Luleå, Sweden a r t i c l e i n f o Article history: Received 10 November 2013 Received in revised form 5 June 2014 Accepted 6 June 2014 Available online 20 June 2014 Poly(allylamine hydrochloride) (PubChem CID: 82291) Keywords: Cellulose nanocrystal Surface charge Spin coating Gas permeability Gas selectivity a b s t r a c t Cellulose nanocrystals (CNC) were extracted from a cellulose residue using two different acid hydrol- ysis procedures. CNC extracted with sulfuric acid (CNC S ) showed higher surface charge (339 mol/g) compared with crystals extracted with hydrochloric acid (CNC HCl ). Spin-coated films with two different configurations were prepared; the first with alternate layers of poly(allylamine hydrochloride) (PAHCl) and CNC, and the second with a single layer of PAHCl coated with multilayers of CNC. Film characteris- tics such as roughness, thickness, contact angle, orientation, gas permeability and gas selectivity were studied. Optical microscopy showed more homogeneous films of CNC S compared to CNC HCl . The surface charge of the crystals impacted the films’ hydrophobicity, being highest for 25 alternate layers of PAHCl and CNC HCl . The gas permeability coefficient was different for each film, depending primarily on the sur- face charge of the crystals and secondly on the film configuration. The films made with CNC HCl displayed gas barriers with nitrogen and oxygen, and gas selectivity with some gas combinations. CNC S films did not show gas selectivity. These results indicate that CNC with low surface charge can be further developed for gas separation and barrier applications. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Polymer nanocomposites research has continued to advance since polymer nanocomposites were first prepared in 1990 (Kanatzidis, 1990). Polymer nanocomposites have been shown to have potential application in various sectors such as rein- forced materials (Favier, Chanzy, & Cavaille, 1995; Petersson & Oksman, 2006) and gas barrier selectors (Belbekhouche, Bras, Siqueira, Chappey, & Lebrun, 2011), among others. However, due to the increase in the residual problems associated with the use of petroleum-based polymers, the focus in the improvement of polymer nanocomposites is on the use of renewable and natural constituents, like cellulose. Cellulose is a structural polysaccha- ride found in the cell wall of green plants and some algae (Rånby, Banderet, & Sillén, 1949). This polysaccharide is composed of an amorphous part and a crystalline part known as cellulose nanocrys- tals (CNC) or nanowhiskers (CNW) (for its rod-like shape). The dimensions of these crystals vary depending on the source from which they are isolated and, in general, CNCs fromwood source ∗ Corresponding author. Tel.: +46 920 493371. E-mail addresses: martha.herrera@ltu.se (M.A. Herrera), aji.mathew@ltu.se (A.P. Mathew), kristiina.oksman@ltu.se (K. Oksman). have a diameter of around 5 nm and a length of around 200 nm (Belbekhouche et al., 2011; Bondeson, Mathew, & Oksman, 2006; Herrera, Mathew, & Oksman, 2012a). This crystalline cellulose is isolated using acid hydrolysis, which cuts the native cellulose chain, consuming the amorphous part and leaving behind the crystalline part (Rånby et al., 1949). Due to the natural abundance of cellulose, bioinert behavior, low weight, and high strength and stiffness, these nanocrystals have served as an additive in the manufacture of com- posite materials in recent years (Chen, Liu, Chang, Cao, & Anderson, 2009; Cranston & Gray, 2008; Favier et al., 1995; Gindl & Keckes, 2005). Self-assembly of cellulose nanocrystals has been researched during the last years to obtain films with a high degree of crystal orientation to study the physical properties and crystalline struc- ture of the cellulose (Belbekhouche et al., 2011; Cranston & Gray, 2008; Yoshiharu, Shigenori, Masahisa, & Takeshi, 1997). For a num- ber of studies, the main interest has been the interaction between the crystals on the surface of the cellulosic material and the envi- ronment (Belbekhouche et al., 2011; Mohan, Kargl, Doliska, Vesel, & Koestler, 2011; Yoshiharu et al., 1997). For this reason, it is nec- essary to have a well-defined film with thickness in the nanometric range. Several studies have reported water contact angle and gas permeability properties of cellulose or associated nanocomposites (Belbekhouche et al., 2011; Favier et al., 1995; Kontturi, Johansson, Kontturi, Ahonen, & Thune, 2007; Mohan et al., 2011; Petersson & Oksman, 2006). http://dx.doi.org/10.1016/j.carbpol.2014.06.036 0144-8617/© 2014 Elsevier Ltd. All rights reserved.