382 research outputs found
Novel Porous Polymorphs of Zinc Cyanide with Rich Thermal and Mechanical Behavior
We investigate the feasibility of four-connected nets as hypothetical zinc
cyanide polymorphs, as well as their thermal and mechanical properties, through
quantum chemical calculations and molecular dynamics simulations. We confirm
the metastability of the two porous phases recently discovered experimentally
(Lapidus, S. H.; et al. J. Am. Chem. Soc. 2013, 135, 7621-7628), suggest the
existence of seven novel porous phases of Zn(CN)2, and show that isotropic
negative thermal expansion is a common occurrence among all members of this
family of materials, with thermal expansion coefficients close to that of the
dense dia-c phase. In constrast, we find a wide variety in the mechanical
behavior of these porous structures with framework-dependent anisotropic
compressibilities. All porous structures, however, show pressure-induced
softening leading to a structural transition at modest pressure.Comment: Chem. Mater. 201
Structure and Dynamics of Solvated Polymers near a Silica Surface: On the Different Roles Played by Solvent
Whereas it is experimentally known that the inclusion of nanoparticles in
hydrogels can lead to a mechanical reinforcement, a detailed molecular
understanding of the adhesion mechanism is still lacking. Here we use
coarse-grained molecular dynamics simulations to investigate the nature of the
interface between silica surfaces and solvated polymers. We show how
differences in the nature of the polymer and the polymer--solvent interactions
can lead to drastically different behavior of the polymer--surface adhesion.
Comparing explicit and implicit solvent models, we conclude that this effect
cannot be fully described in an implicit solvent. We highlight the crucial role
of polymer solvation for the adsorption of the polymer chain on the silica
surface, the significant dynamics of polymer chains on the surface, and details
of the modifications in the structure solvated polymer close to the interface
On the use of the IAST method for gas separation studies in porous materials with gate-opening behavior
Highly flexible nanoporous materials, exhibiting for instance gate opening or
breathing behavior, are often presented as candidates for separation processes
due to their supposed high adsorption selectivity. But this view, based on
"classical" considerations of rigid materials and the use of the Ideal Adsorbed
Solution Theory (IAST), does not necessarily hold in the presence of framework
deformations. Here, we revisit some results from the published literature and
show how proper inclusion of framework flexibility in the osmotic thermodynamic
ensemble drastically changes the conclusions, in contrast to what intuition and
standard IAST would yield. In all cases, the IAST method does not reproduce the
gate-opening behavior in the adsorption of mixtures, and may overestimates the
selectivity by up to two orders of magnitude
Challenges in first-principles NPT molecular dynamics of soft porous crystals: A case study on MIL-53(Ga)
Soft porous crystals present a challenge to molecular dynamics simulations
with flexible size and shape of the simulation cell (i.e., in the NPT
ensemble), since their framework responds very sensitively to small external
stimuli. Hence, all interactions have to be described very accurately in order
to obtain correct equilibrium structures. Here, we report a methodological
study on the nanoporous metal-organic framework MIL-53(Ga), which undergoes a
large-amplitude transition between a narrow- and a large-pore phase upon a
change in temperature. Since this system has not been investigated by density
functional theory (DFT)-based NPT simulations so far, we carefully check the
convergence of the stress tensor with respect to computational parameters.
Furthermore, we demonstrate the importance of dispersion interactions and test
two different ways of incorporating them into the DFT framework. As a result,
we propose two computational schemes which describe accurately the narrow- and
the large-pore phase of the material, respectively. These schemes can be used
in future work on the delicate interplay between adsorption in the nanopores
and structural flexibility of the host material
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