Proximity to Graphene Dramatically Alters Polymer Dynamics

Screening of hydrogen bonding interactions by a single layer graphene

A single layer of graphene when transferred to a solid substrate has the ability to screen or transmit interactions from the underlying substrate, which has direct consequences in applications of this 2D material to flexible electronics and sensors. Previous reports using a multitude of techniques present contradictory views on graphene's ability to screen or transmit van der Waals (vdW) and polar interactions. In the present study, we use interface-sensitive spectroscopy to demonstrate that a single layer graphene is opaque to hydrogen bonding interactions (a subset of acid-base interactions), answering a question that has remained unresolved for a decade. Similar frequency shifts of sapphire hydroxyl (OH) peak for graphene-coated sapphire in contact with air and polydimethylsiloxane (PDMS) demonstrate the insensitivity of sapphire OH to PDMS. The screening ability of graphene is also evident in the smaller magnitude of this frequency shift for graphene-coated sapphire in comparison to that for bare sapphire. The screening of acid-base interactions by a single layer graphene results in the significant reduction of adhesion hysteresis for PDMS lens on graphene-coated substrates (sapphire and silicon wafer, SiO₂/Si) than bare substrates. Our results have implications in the use of PDMS stamps to transfer graphene to other substrates eliminating the need for a wet-transfer process.

Large T g Shift in Hybrid Bragg Stacks through Interfacial Slowdown

Studies of glass transition under confinement frequently employ supported polymer thin films, which are known to exhibit different transition temperature T g close to and far from the interface. Various techniques can selectively probe interfaces, however, often at the expense of sample designs very specific to a single experiment. Here, we show how to translate results on confined thin film T g to a "nacre-mimetic" clay/polymer Bragg stack, where periodicity allows to limit and tune the number of polymer layers to either one or two. Exceptional lattice coherence multiplies signal manifold, allowing for interface studies with both standard T g and broadband dynamic measurements. For the monolayer, we not only observe a dramatic increase in T g (∼ 100 K) but also use X-ray photon correlation spectroscopy (XPCS) to probe platelet dynamics, originating from interfacial slowdown. This is confirmed from the bilayer, which comprises both "bulk-like" and clay/polymer interface contributions, as manifested in two distinct T g processes. Because the platelet dynamics of monolayers and bilayers are similar, while the segmental dynamics of the latter are found to be much faster, we conclude that XPCS is sensitive to the clay/polymer interface. Thus, large T g shifts can be engineered and studied once lattice spacing approaches interfacial layer dimensions.

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 π-π interactions are formed among the phenyl groups themselves in addition to the edge-to-face interactions to maximize the bonding energy of the π-π interactions. This is enthalpically favorable and driven mainly by the π-π interactions. This work provides important knowledge for the design and optimization of functional graphene-based aromatic polymer nanocomposites.