(755d) Unprecedented Improvement in Mechanical Property Achieved through Layer-By-Layer Assembly of Polymer and Oxidation-Free Graphene

Negatively charged graphene oxide (GO) can be combined with positively charged polymers to produce layer-by-layer assemblies with greatly improved mechanical and electrical properties, which are imparted by the almost parallel orientation of exfoliated GO platelets. However, desirable properties of these assemblies are not maximized unless GO is chemically reduced. It is possible to eliminate this extra reduction step by replacing GO with pristine (i.e., oxidation-free) graphene. But the lack of oxygen functional groups on pristine graphene makes its orientation and exfoliation within a polymer matrix extremely challenging. We have discovered a new way to prepare polymer/pristine-graphene multilayer thin films, which overcomes this problem. This method involves depositing polyvinylpyrrolidone-stabilized graphene platelets alternately with poly(acrylic acid) using hydrogen-bonding assisted layer-by-layer assembly. With this method, we can incorporate pristine graphene sheets into a polymer matrix with excellent orientation and exfoliation, resulting in an unprecedented improvement in mechanical property. A multilayer thin film containing just 3.9 vol% of highly exfoliated and structurally intact graphene increases the elastic modulus (E) of a polymeric multilayer thin film by more than 320% (from 1.41 to 4.81 GPa), while maintaining a visible light transmittance of â??90%. This magnitude of increase in elastic modulus exceeds all previously reported values for any graphene-filled polymer nanocomposite with a glassy (E > 1 GPa) matrix. This method provides a powerful new avenue for improving properties (mechanical, gas transport, etc.) of polymer nanocomposites by enabling the incorporation, exfoliation, and parallel orientation of 2D nanoparticles, which feature excellent intrinsic properties but are initially unsuitable for layer-by-layer assembly due to the lack of essential functional groups.