Jake Song

Designing multi-layer graphene-based assemblies for enhanced toughness in nacre-inspired nanocomposites

Polymers reinforced with multi-layer graphene (MLG) phases are promising candidates for new materials with high modulus, strength and toughness. Drawing inspiration from nacres layered “brick and mortar” structure, here we propose molecular scale design strategies to improve the mechanical performance of MLG–polymer layer-by-layer nanocomposites. We present a coarse-grained molecular dynamics (CG-MD) study of interfacial failure mechanisms of MLG domains embedded in a poly(methyl methacrylate) (PMMA) matrix through pull-out simulations. Our simulations reveal two distinct deformation and failure mechanisms that greatly influence the toughness and energy dissipation of the system: pull-out failure, which occurs along the MLG–PMMA interface, and yielding failure, which occurs within the graphitic phase through the sliding of staggered graphene sheets. For any length of the graphitic assembly, the energy dissipated per layer from MLG yielding is greater than that of MLG pull-out. Theoretical continuum analysis further reveals that there exists a critical number of layers of graphene, beyond which the failure mode changes from yielding to pull-out. Our modeling framework provides effective strategies to design graphene–polymer layered nanocomposites with optimal toughness, and advance the mechanical performance of nanomaterials.

Side-group size effects on interfaces and glass formation in supported polymer thin films

Recent studies on glass-forming polymers near interfaces have emphasized the importance of molecular features such as chain stiffness, side-groups, molecular packing, and associated changes in fragility as key factors that govern the magnitude of Tg changes with respect to the bulk in polymer thin films. However, how such molecular features are coupled with substrate and free surface effects on Tg in thin films remains to be fully understood. Here, we employ a chemically specific coarse-grained polymer model for methacrylates to investigate the role of side-group volume on glass formation in bulk polymers and supported thin films. Our results show that bulkier side-groups lead to higher bulk Tg and fragility and are associated with a pronounced free surface effect on overall Tg depression. By probing local Tg within the films, however, we find that the polymers with bulkier side-groups experience a reduced confinement-induced increase in local Tg near a strongly interacting substrate. Further analyses indicate that this is due to the packing frustration of chains near the substrate interface, which lowers the attractive interactions with the substrate and thus lessens the surface-induced reduction in segmental mobility. Our results reveal that the size of the polymer side-group may be a design element that controls the confinement effects induced by the free surface and substrates in supported polymer thin films. Our analyses provide new insights into the factors governing polymer dynamics in bulk and confined environments.

Temperature Effects on the Nanoindentation Characterization of Stiffness Gradients in Confined Polymers

The stiffening of polymers near inorganic fillers plays an important role in strengthening polymer nanocomposites, and recent advances in metrology have allowed us to sample such effects using local mechanical measurement techniques such as nanoindentation and atomic force microscopy. A general understanding of temperature and confinement effects on the measured stiffness gradient length-scale ξint is lacking however, which convolutes molecular interpretation of local property measurements. Using coarse-grained molecular dynamics and finite element nanoindentation simulations, we show that the measured ξint increases with temperature in highly confined polymer systems, a dependence which acts in the opposite direction in systems with low confinement. These disparate trends are closely related to the polymers viscoelastic state and the resulting changes in incompressibility and dissipative ability as the polymer transitions from glassy to rubbery. At high temperatures above the glass transition temperature, a geometrically confined system restricts the viscous dissipation of the applied load by the increasingly incompressible polymer. The indentation causes a dramatic build-up of hydrostatic pressure near the confining surface, which contributes to an enlarged measurement of ξint. By contrast, a less-confined system allows the pressure to dissipate via intermolecular motion, thus lowering the measured ξint with increased temperature above the glass transition temperature. These findings suggest that the well-established thin film-nancomposite analogy for polymer mobility near interfaces can be convoluted when measuring local mechanical properties, as the viscoelastic state and geometric confinement of the polymer can affect the nanomechanical response during indentation purely from continuum effects.