Ionic-Liquid-Based Polyurethane Dispersions for Stabilizing Graphene in Water
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.328
Ionic-Liquid-Based Polyurethane Dispersions for Stabilizing Graphene in Water Harshit Gupta§ and John Texter* Coatings Research Institute, School of Engineering, Eastern Michigan University, Ypsilanti, MI 48197, USA § Present Address: Axalta Coating Systems, Global Innovation Center, 1050 Constitution Avenue, Philadelphia, PA 19112, USA
ABSTRACT
Aqueous auto-dispersing polyurethane dispersions (PUDs) have recently been reported to form nanoparticulate dispersions at up to 25% by weight. Their incorporation of an ionicliquid (IL) monomer, 1-hydroxyundecyl-3-methyl imidazolium bromide (HOC11C1ImBr) as a chain-terminating group appears to account for their auto-dispersing ability, and these PUD nanoparticles bear similarity to IL-based nanolatexes that have provided thermodynamically stable aqueous dispersions of nanocarbons. We demonstrate that these HOC 11C1ImBr-based PUDs stabilize aqueous graphene dispersions at 1% by weight graphene in ultrasonicated top-down liquid phase exfoliation. Their formation quantitatively follows an analytical model of exfoliation kinetics of layered materials and a stretched exponential kinetic model. Such dispersions are ideally formulated for making nanocomposites composed of similar or compatible PUDs and other condensation polymers.
INTRODUCTION Polyurethanes (PUs) are intrinsically biodegradable [1] and are key components in flexible and rigid foams, elastomers, lightweight composite materials, paints, coatings, and adhesives [2]. Polyurethane dispersions (PUDs) of microparticles and nanoparticles of PUs in various solvents, especially water, have been commercially available as protective coatings for almost 50 years [3,4]. PUDs typically are electrostatically stabilized with polymeric backbones having pendant anionic or cationic groups with accompanying counterions. Nonionic PUDs also have been studied. Ionic liquids (ILs) are organic salts that melt below 100 qC [5]. Polymerized ionic liquids (PIL) are revolutionizing polymer-based materials science and have been
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extensively reviewed [6-8]. IL-based PUs have been introduced and have been found to exhibit gel formation and stimuli responsiveness [9] very similar to properties obtained earlier by free radical chain polymerization. IL-based PUDs that spontaneously disperse in water as thermodynamically stable nanoparticles have recently been introduced [10,11].These dispersions differ from commercially available PUDs that require high shear mixing in order to obtain metastable dispersions. Liquid-phase exfoliation (LPE) of carbon nanotubes and graphene and of many different kinds of 2D materials is a burgeoning area of research. Methods of exfoliating SWCNT [12,13], MWCNT [14-16], and graphene [12,17-19] and
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