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Coupling Molecules and Cooperative Molecular Systems

Steven Cranford (Northeastern University)

Materials Design and Biomimetic Material Concepts

Tue 2:40 - 4:00

Barus-Holley 158

Whether biological or synthetic, molecular composites (e.g., the intentional combination of different chemical species or structures) can be considered a fundamental hierarchical system, in terms of the structure formed by individual components. Upon hierarchical assembly, it is presumed that distinct material elements behave in synergetic units. Indeed, DNA is well-known to be constituted of double strands of nucleobases, but it is the highly cross-linked helical structure that allows potential application as a structural material at the nanoscale. Beyond Nature, success in cooperativity has been demonstrated in the complexation of polyelectrolytes or hydrogen bonded graphene oxides. Integration of multiple components within advanced functional materials (including graphene-based flexible electronics or energy storage materials, for example) requires knowledge of mechanical cooperativity between material interfaces, optimizing contact, adhesion, and deformation. The concept of mechanical coupling or cooperativity can be thought of as the cross-over from considering two (or more) components independently and relying on the combined behavior of a single composite element, or “equivalent mechanical behavior between components and complex”, or, colloquially speaking, when “1+1=1”. Here we consider two systems: the cross-linking of polyelectrolytes and a multi-layer composite graphene system. For the polyelectrolytes, through a simple elastic model, we determine general conditions for mechanical coupling of flexible molecules of finite rigidity via discrete cross-links with finite stiffness. We demonstrate the atomistic mechanism of cooperative deformation (or mechanical coupling) by applying an ideal structural model. For graphene, we study buckling induced delamination of mono- and bilayer systems, utilizing a hybrid full atomistic and coarse-grained molecular dynamics approach and derive a critical delamination condition.