Concentration-Dominated Orientation of Phenyl Groups at the Polystyrene/Graphene Interface

The interfacial orientation of aromatic groups plays a crucial role in determining the properties of graphene-based aromatic polymer nanocomposites. Here, the interfacial orientation of the polystyrene (PS) phenyl groups in contact with graphene is revealed by sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra showed that the orientation of the phenyl groups is closely related to the interfacial concentration as the chains reach the quasi-equilibrium state. The phenyl groups remain in a relatively unrestricted state at a low concentration of the PS phenyl groups, and they prefer to recline to more favorably interact with graphene via a face-to-face configuration. Densely stacked phenyl groups are too crowded to form multilayer face-to-face interactions with graphene, and they prefer to remain upright, while pi-pi interactions are formed among the phenyl groups themselves in addition to the edge-to-face interactions to maximize the bonding energy of the pi-pi interactions. This is enthalpically favorable and driven mainly by the pi-pi interactions. This work provides important knowledge for the design and optimization of functional graphene-based aromatic polymer nanocomposites.