Polyurethane nanocomposites prepared from solvent-free stable dispersions of functionalized graphene nanosheets in polyols

Abstract

Novel families of elastomeric polyurethane (PU) nanocomposites are prepared by in-situ polymerization of diisocyanate with functionalized graphene nanosheets dispersions in polyether polyols. The influences of graphene dispersion, interfacial coupling and PU hard segment content on PU nanocomposite morphologies, mechanical and electrical properties are investigated. High shear mechanical dispensing (UT process) and high pressure homogenization (HPH process) enable very effective dispersion of functionalized graphene, prepared by thermally reducing graphite oxide (TRGO), in polyols. The addition of dispersing agents and organophilic TRGO surface modification, reacting TRGO hydroxyl groups with phenyl isocyanate (TRGO-Phi), is not required. The TRGO dispersions in a 6/1 wt.-% blend of polypropylene oxide diol (Mn = 2000 g/mol) with polypropylene oxide triol (Mn = 6000 g/mol) are cured with methylene-diphenyl-4,4′-diisocyanate (MDI) at 60 °C. The content of PU hard segments is varied from 23 to 33 wt.-% using 1,4-butanediol (BD) as chain extender. According to the transmission electron microscopic (TEM) analysis of the PU nanocomposite morphologies, the TRGO nanosheets are exclusively allocated in the PU hard phases, thus causing a change of PU morphology. Upon increasing the hard segment content to 33 wt.-%, skeleton-like co-continuous superstructures are formed, paralleled by the simultaneous improvement of Young's modulus (300%) and the tensile strength (350%) without sacrificing high elongation at break. This behavior is not observed for conventional nanofillers such as nanometer scaled carbon black (CB) and multiwalled carbon nanotubes (CNT), when processed under the same condition. The comparison of TRGO with TRGO-Phi dispersions reveals that covalent coupling of TRGO and copolymerization of hydroxyl-functional TRGO with PU is essential.