Topological Control on the Structural Relaxation of Atomic Networks under Stress

Upon loading, atomic networks can feature delayed irreversible relaxation. However, the effect of composition and structure on relaxation remains poorly understood. Herein, relying on accelerated molecular dynamics simulations and topological constraint theory, we investigate the relationship between atomic topology and stress-induced structural relaxation, by taking the example of creep deformations in calcium silicate hydrates (C─S─H), the binding phase of concrete. Under constant shear stress, C─S─H is found to feature delayed logarithmic shear deformations. We demonstrate that the propensity for relaxation is minimum for isostatic atomic networks, which are characterized by the simultaneous absence of floppy internal modes of relaxation and eigenstress. This suggests that topological nanoengineering could lead to the discovery of nonaging materials.National Science Foundation (U.S.) (Grant 1562066)Schlumberger-Doll Research CenterMassachusetts Institute of Technology. Concrete Sustainability HubMassachusetts Institute of Technology. Interdisciplinary Center on MultiScale Material Science for Energy and Environment (Grant ANR-11-LABX-0053)Massachusetts Institute of Technology. Interdisciplinary Center on MultiScale Material Science for Energy and Environment (Grant ANR-11-IDEX-0001- 02

Tunable Porous Organic Crystals: Structural Scope and Adsorption Properties of Nanoporous Steroidal Ureas

Previous work has shown that certain steroidal bis-(N-phenyl)ureas, derived from cholic acid, form crystals in the P61 space group with unusually wide unidimensional pores. A key feature of the nanoporous steroidal urea (NPSU) structure is that groups at either end of the steroid are directed into the channels and may in principle be altered without disturbing the crystal packing. Herein we report an expanded study of this system, which increases the structural variety of NPSUs and also examines their inclusion properties. Nineteen new NPSU crystal structures are described, to add to the six which were previously reported. The materials show wide variations in channel size, shape, and chemical nature. Minimum pore diameters vary from ∼0 up to 13.1 Å, while some of the interior surfaces are markedly corrugated. Several variants possess functional groups positioned in the channels with potential to interact with guest molecules. Inclusion studies were performed using a relatively accessible tris-(N-phenyl)urea. Solvent removal was possible without crystal degradation, and gas adsorption could be demonstrated. Organic molecules ranging from simple aromatics (e.g., aniline and chlorobenzene) to the much larger squalene (Mw = 411) could be adsorbed from the liquid state, while several dyes were taken up from solutions in ether. Some dyes gave dichroic complexes, implying alignment of the chromophores in the NPSU channels. Notably, these complexes were formed by direct adsorption rather than cocrystallization, emphasizing the unusually robust nature of these organic molecular hosts

Engineering the bonding scheme in C-S-H: the iono-covalent framework

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Multi-scale Study of Desiccation Shrinkage in Granular Soils

International audienceThis paper aims to identify and evaluate various critical mechanisms associated with the processes of desiccation shrinkage in drying silty soils. A previously developed 1D bundle-of-tubes model is refined to simulate the various stages of drying shrinkage in 2D using the actual pore size distribution based on Mercury Intrusion Porosimetry (MIP) data. It is revealed that the resulting shrinkage evolution is affected by air entry that may occur in two possible scenarios: air incursion at the external surface and formation of vapor nucleus in the interior. The analysis of mechanical deformation is coupled with the numerical simulation of the drying process which can be often characterized as a two-stage development, consisting of a constant rate period and a falling rate period. Numerical simulation of the drying rate evolution suggests that it may be closely associated with the onset of air entry and/or the progress of desaturation. Further transition of solid-water structural configuration into funicular and pendular states from initially capillary state is simulated

Topological control on the structural relaxation of atomic networks under stress

Upon loading, atomic networks can feature delayed irreversible relaxation. However, the effect of composition and structure on relaxation remains poorly understood. Herein, relying on accelerated molecular dynamics simulations and topological constraint theory, we investigate the relationship between atomic topology and stress-induced structural relaxation, by taking the example of creep deformations in calcium silicate hydrates (C-S-H), the binding phase of concrete. Under constant shear stress, C-S-H is found to feature delayed logarithmic shear deformations. We demonstrate that the propensity for relaxation is minimum for isostatic atomic networks, which are characterized by the simultaneous absence of floppy internal modes of relaxation and eigenstress. This suggests that topological nanoengineering could lead to the discovery of nonaging materials