Hyperbranched-polymer functionalization of graphene sheets for enhanced mechanical and dielectric properties of polyurethane composites

The incorporation of graphene sheets (GSs) into polymer matrices affords engineers an opportunity to synthesize polymer composites with excellent physical performances. However, the development of high performance GS-based composites is difficult because of the easy aggregation of GSs in a polymer matrix as well as the weak interfacial adhesion between GSs and the host polymer. Herein, we present a simple and effective route to hyperbranched aromatic polyamide functionalized graphene sheets (GS-HBA). The resulting GS-HBA exhibits uniform dispersion in a thermoplastic polyurethane (TPU) matrix and strong adhesion with the matrix by hydrogen-bond coupling, which improve the load transfer efficiency from the matrix to the GSs. Thus, the GS-HBA-TPU composites possess excellent mechanical performance and high dielectric performance. It has been demonstrated that the GS-HBA composite has higher modulus, higher tensile strength and higher yield strength, and remains at nearly the same strain at break when compared with the composites with graphene oxide, ethylene diamine-modified graphene, and hydrazine reduced graphene. In addition, the hyperbranched polymer chains allow construction of a large number of microcapacitors and suppress the leakage current by isolating the GSs in a TPU matrix, resulting in a higher permittivity and lower loss tangent for the GS-HBA composite in comparison with ethylene diamine-modified graphene, or hydrazine reduced-graphene composites.