Microstructure, residual stress, and intermolecular force distribution maps of graphene/polymer hybrid composites: Nanoscale morphology-promoted synergistic effects

Intrinsically conducting (or π-conjugated) polymers and graphene related materials are attractive for manufacturing hybrid nanocomposite materials and devices owing to their multifunction at the constituents' interface i.e. graphene/conducting polymer. To utilize these materials, understanding of and relating their microscopic internal structure, surface morphology, and physical properties become indispensable. In this work, we report on the electrochemically synthesized conducting polymers with electrochemically processed graphene nanosheets forming hybrid films and the enhancement of mechanical properties of polymers arising due to synergistic effects promoted by nanostructured morphology and vicinal polymeric chain ordering induced by embedded or impregnated graphene nanosheets. We investigated surface topography, chain ordering, residual stress distribution, force curves and force volume imaging using micro-Raman spectroscopy, Raman mapping, atomic force microscopy and force spectroscopy gaining insights into structural and interfacial bonding, conjugation length distribution, and intermolecular forces. Traditional force curves measure the force felt by the tip as it approaches and retracts from a point on the sample surface (i.e. tip-sample adhesion force), whereas force volume is an array of force curves over an extended sample area. Moreover, detailed structural studies are able to demonstrate that the bonding configurations and structural conformations intertwined with crystallinity and surface chemistry in both the polymers and graphene derivatives nanosheets have a strong effect on nanoscale intermolecular forces and surface elasticity maps. Meanwhile the spring constant (k) estimated from the force curves using elastic contact model was synchronously enhanced for hybrid composites with interspersed graphene sheets on polymer chains forming a kind of ordered stacked structure. Alternatively, the mechanical properties of polymers are improved following in the order: PAni/ErGO > PPy/ErGO > PAni/GO > PPy/GO > PAni ≥ PPy. The electrostatic force between the tip and residual charged polymers surface, are also discussed in terms of multilayer model consisting of three layers analogous to electrostatic double-layer. Furthermore, the information in the force volume measurement was decoupled from topographic data offering new insights into the materials' surface and mechanical properties of hybrid nanocomposites. These measurements are complemented with electron microscopy and X-ray diffraction revealing their surface morphology, intrinsic structure and crystallinity having far reaching implications and impacts on alternative renewable electrochemical energy, photovoltaic and aerospace device applications.