Article pubs.acs.org/Biomac
Multifunctional Coating Films by Layer-by-Layer Deposition of Cellulose and Chitin Nanofibrils Zi-Dong Qi, Tsuguyuki Saito, Yimin Fan, and Akira Isogai* Graduate School of Agricultural and Life Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan S Supporting Information *
ABSTRACT: An environmentally benign surface modification process for plastic films was demonstrated by fabricating composite coatings through layer-by-layer assembly with green solid materials: aqueous dispersions of two kinds of crystalline polysaccharide nanofibrils. Anionic cellulose nanofibrils were obtained by the TEMPO (2,2,6,6-tetramethylpiperidine1-oxyl radical)-mediated oxidation of native cellulose, while cationic βchitin nanofibrils were prepared by the protonation of squid pen chitin. Uniform layer depositions, driven by electrostatic attraction and enhanced by hydrogen bonding, were observed on silicon wafers and then reproduced onto poly(ethylene terephthalate) films. Contact angle measurements and dyeing tests on the resulting films revealed their hydrophilic nature and the sorption of both charged and uncharged substances. Antireflection properties were also confirmed via the light transmittance measurements. As might be presumed from all these properties, this composite coating exhibited its unique behavior largely due to its structure, which was distinct from both those of nanofibril cast films and polymer films.
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INTRODUCTION Our lives have been surrounded and facilitated by various types of commodity plastic films that are characterized by their low prices, material properties and convenience in production and shaping. However, because they are petrochemical products, most are lacking in a number of properties, such as hydrophilicity, thermal stability, absorption, and adhesion. To extend the uses of such plastic films, numerous surface modification methods have been developed, according to different demands. First suggested theoretically in 1966 and realized in practice in 1992,1,2 the layer-by-layer (LbL) technique is one of the most recent approaches in this area, which is highly ubiquitous and effective and through which ultrathin organic multilayered films can be assembled on a substrate. Generally speaking, an LbL fabricated film consists of alternating layers of oppositely charged polyelectrolytes. Such layers are accumulated spontaneously due to electrostatic attraction from aqueous solutions or dispersions. In comparison with other coating methods, highly defect-free structures can be formed in these ultrathin multilayered coatings without severe limitations of the components while offering many advantages, such as outstanding nanolevel controllability, synergistic effects between different components and the possibility of direct control over surface functionality. As a result, a wide variety of applications3,4 have been explored for LbL coatings from optical and electrical devices,5,6 superhydrophobic surfaces7 to biosensors and nanoreactors.8,9 Noteworthy is that the whole process of LbL assembly is environmentally benign, relatively fast and simple, without demands for expensive equipment or organic solvents. These favorable features are becoming increasingly important for © 2012 American Chemical Society
industrial production and could be amplified when combined with the biobased materials utilized in the present study, cellulose, and chitin nanofibrils. Both of these two types of nanofibril consist of crystalline polysaccharides: linear polymers of β-(1→4)-linked anhydroglucose and N-acetyl anhydroglucosamine for cellulose and chitin, respectively. In nature, these compounds form hierarchical structures with high chemical resistance to provide physical support to the living body of green plants and some marine animals. The annual accumulations of both cellulose and chitin rank the top among all organic compounds and thus their effective utilization has been regarded as a promising substitute for petrochemistry and has recently been strongly encouraged. Raw cellulose materials have been mass-consumed in paper, cloth, and other areas but advanced applications of these polysaccharides have long been restricted by poor dispersibility and solubility due to the numerous hydrogen bonds between nanofibrils and molecules. In 2006, a novel process to generate homogeneous aqueous dispersions of individual cellulose nanofibrils was developed in our laboratory using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)mediated oxidation under moderate aqueous conditions.10 A significant amount of carboxyl groups can be introduced to the anhydroglucose units at C6 on the crystal surfaces so that TEMPO-oxidized cellulose can be easily disintegrated into individual nanofibrils (TEMPO-oxidized cellulose nanofibrils, abbr. TOCN) after mild mechanical treatments due to osmotic Received: November 23, 2011 Revised: January 15, 2012 Published: January 18, 2012 553
dx.doi.org/10.1021/bm201659b | Biomacromolecules 2012, 13, 553−558
Biomacromolecules
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using an ultrasonic homogenizer equipped with a 7 mm probe tip (US300T, Nihon Seiki). The unfibrillated fraction (
