Increased functional properties and thermal stability of flexible cellulose nanocrystal/ZnO films
Graphical abstract
AFM image showing surface topography of flexible cellulose nanocrystal/ZnO films with improved functional properties upon ZnO loading.
Introduction
Nowadays, most of the widely utilized polymers such as polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) or polyethylene (PE) are derived from petroleum feedstocks. The use of those synthetic materials represents serious environmental issues since they are not compostable/biodegradable (Pramila, Fulekar, & Bhawana, 2012), which makes imperative the development of environmentally friendly materials with lower energy consumption and increased biodegradability/biocompatibility. In this framework, the aim of this work is to develop a biopolymer-based systems where sustainable materials would be fabricated solely from renewable feedstocks, which would serve to overcome the burnout of fossil resources that deals our society.
Cellulose is a naturally available carbohydrate polymer based on β-d-glucose (Mark, 1980) and is the most abundant natural polymer in the biosphere (Xiao, Gao, Lu, Li, & Sun, 2015). Some examples highlighting the enormous relevance to develop smart cellulosic materials are filter paper (Lindqvist et al., 2008), nanofibrillated cellulose (Kiziltas, Kiziltas, Bollin, & Gardner, 2015) and cellulose nanocrystals (CNCs) (Filpponen and Argyropoulos, 2010, Fortunati et al., 2014). Among all the cellulosic materials, cellulose nanocrystals (CNC) are of especial interest for the development of novel bio-based materials because their rod-like structure with average diameter and length of ∼10 nm and 100–200 nm respectively (Goffin et al., 2011), high mechanical stiffness, low density, good biocompatibility, low cost and enormous possibilities for chemical modification (Dufresne, 2008, Habibi et al., 2010).
Currently, papermaking approach based on nanocellulose is being successfully employed to develop novel materials with improved functional properties (Gutierrez et al., 2012, Kubacka et al., 2012, Liu et al., 2011, Sehaqui et al., 2010). The composition of CNC-based nanopaper is rather similar to traditionally fabricated paper, being the first-one made up by nanosized crystalline cellulose. In comparison to traditional paper excellent optical transmittances, enhanced printabilities and higher tensile strengths are achieved in cellulose nanopapers (Kubacka et al., 2012, Liu et al., 2011, Sehaqui et al., 2010). Another noteworthy advantage represents the low coefficient of thermal expansion (CTE) of CNC-based papers in comparison with metals and polymers, making the especially suitable for high-temperature (150–200 °C) applications (Nishino, Matsuda, & Hirao, 2004).
Unfortunately, some of the functional properties of cellulose nanopapers such as ultraviolet (UV) permeability (Sehaqui et al., 2010), stability against moisture (Lindström, Karabulut, Kulachenko, Sehaqui, & Wågberg, 2012) and thermal stability (Mtibe et al., 2015) still remain below the required standards for their commercialization in industrial applications. Accordingly, it would be expected that the combination of CNC with other nanostructured material should further improve the functional properties of cellulose nanopapers, i.e. tensile strength, thermal stability and surface hydrophobicity. Several authors have been working on cellulose nanopaper/carbon nanotube hybrids for sensing and actuating applications (Lu, Liu, Gou, Leng, & Du, 2011), vanadium oxide/nanopaper structures with conductive photoswitchable properties (Gutierrez et al., 2012), clay-based nanopaper for the development of oxygen barrier layers (Liu et al., 2011) and so on. Among all the available metallic nanoparticles, zinc oxide (ZnO) nanoparticles present an especially bright future because of their naturally available nature, high elastic modulus, their ability to increase the antimicrobial resistance of its hosting matrix (Kanmani & Rhim, 2014), photostability, safety as denoted by American Food and Drug Administration (FDA) approval (Sambandan & Ratner, 2011) and so on.
In this work we develop ecologically friendly renewable materials based on CNC and zinc oxide in agreement with the 12 Principles of Green Chemistry, which raise awareness of the use environmentally friendly substances by energetically efficient processes which minimize wasted materials (Anastas & Warner, 1998). Hybrid CNC/ZnO films with concentrations up to 5 wt.% have been fabricated by a simple and environmentally friendly method using solely water as a dispersing agent. Morphological investigations carried out by transmission electron microscopy, scanning electron microscopy and scanning probe microscopy reveal the formation of highly homogeneous closely packed hybrids. Functional properties of nanocomposites have been investigated by UV–Vis spectroscopy, water contact angle measurements and dynamic mechanical analysis (DMA). Thermogravimetric analysis (TGA) has been carried out to determine the maximum operating temperature of synthesized materials. In overall, this work highlights the potential for environmentally friendly processing of sustainable nanostructured functional materials based on cellulose.
Section snippets
Starting materials
Microcrystalline cellulose with a particle size of 20 μm (310697-500G), sulfuric acid and sodium hydroxide have been supplied by Sigma Aldrich. The acetone (HPLC grade, assay >99.8%) was purchased from LabScan, while methanol (reagent ≥99.5%) was purchased by Panreac. ZnO nanoparticles (Supplementary data, Fig. S1a) have been kindly purchased by L’Urederra Technological Centre (Spain). For the determination of water vapour permeability (WVP), poly (l-lactide) (PLLA) of number-average molecular
Physicochemical characterization
Both transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) have been carried out to analyze the morphology of synthesized CNC and CNC-based films. TEM micrograph shown in Fig. 1(a) shows the well-individualized cellulose nanocrystals, having 72 ± 25 nm long and ∼10 nm wide rod-shaped structure (please, see particle size distribution in Supporting Information, Fig. S1d). The formation of a highly homogeneous closely packed CNC network is confirmed in Fig. 1
Conclusions
In this work highly homogeneous and smooth CNC/ZnO films have been prepared by a simple and environmentally friendly process. Results reveal a strong UV-shielding behaviour of transparent CNC/ZnO hybrids with UV transmission of only 1.5% for the nanopaper containing 1 wt.% ZnO. It is found that WCA values dramatically increase from 56 ± 2° for neat nanopaper to 112° for the CNC/ZnO 5 wt.% hybrid. This marked decrease in wettability would result in slower water diffusion through the film, allowing
Acknowledgements
E.L. thanks the University of the Basque Country (UPV/EHU) for a postdoctoral fellowship. We gratefully acknowledge Corbion-Purac for the kind donation of PLLA. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, EGEF and ESF) is gratefully acknowledged. Authors thank the Basque Country Government for financial support (Ayudas para apoyar las actividades de los grupos de investigación del sistema universitario vasco, IT718-13).
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