97 research outputs found
Self Assembly of Soft Matter Quasicrystals and Their Approximants
The surprising recent discoveries of quasicrystals and their approximants in
soft matter systems poses the intriguing possibility that these structures can
be realized in a broad range of nano- and micro-scale assemblies. It has been
theorized that soft matter quasicrystals and approximants are largely
entropically stabilized, but the thermodynamic mechanism underlying their
formation remains elusive. Here, we use computer simulation and free energy
calculations to demonstrate a simple design heuristic for assembling
quasicrystals and approximants in soft matter systems. Our study builds on
previous simulation studies of the self-assembly of dodecagonal quasicrystals
and approximants in minimal systems of spherical particles with complex,
highly-specific interaction potentials. We demonstrate an alternative
entropy-based approach for assembling dodecagonal quasicrystals and
approximants based solely on particle functionalization and shape, thereby
recasting the interaction-potential-based assembly strategy in terms of
simpler-to-achieve bonded and excluded-volume interactions. Here, spherical
building blocks are functionalized with mobile surface entities to encourage
the formation of structures with low surface contact area, including
non-close-packed and polytetrahedral structures. The building blocks also
possess shape polydispersity, where a subset of the building blocks deviate
from the ideal spherical shape, discouraging the formation of close-packed
crystals. We show that three different model systems with both of these
features -- mobile surface entities and shape polydispersity -- consistently
assemble quasicrystals and/or approximants. We argue that this design strategy
can be widely exploited to assemble quasicrystals and approximants on the nano-
and micro- scales. In addition, our results further elucidate the formation of
soft matter quasicrystals in experiment.Comment: 12 pages 6 figure
Derivation of coarse-grained potentials via multistate iterative Boltzmann inversion
In this work, an extension to the standard iterative Boltzmann inversion
(IBI) method to derive coarse-grained potentials is proposed. It is shown that
the inclusion of target data from multiple states yields a less state-dependent
potential, and is thus better suited to simulate systems over a range of
thermodynamic states than the standard IBI method. The inclusion of target data
from multiple states forces the algorithm to sample regions of potential phase
space that match the radial distribution function at multiple state points,
thus producing a derived potential that is more representative of the
underlying potential interactions. It is shown that the algorithm is able to
converge to the true potential for a system where the underlying potential is
known. It is also shown that potentials derived via the proposed method better
predict the behavior of n-alkane chains than those derived via the standard
method. Additionally, through the examination of alkane monolayers, it is shown
that the relative weight given to each state in the fitting procedure can
impact bulk system properties, allowing the potentials to be further tuned in
order to match the properties of reference atomistic and/or experimental
systems
Icosahedral packing of polymer-tethered nanospheres and stabilization of the gyroid phase
We present results of molecular simulations that predict the phases formed by
the self-assembly of model nanospheres functionalized with a single polymer
"tether", including double gyroid, perforated lamella and crystalline bilayer
phases. We show that microphase separation of the immiscible tethers and
nanospheres causes confinement of the nanoparticles, which promotes local
icosahedral packing that stabilizes the gyroid and perforated lamella phases.
We present a new metric for determining the local arrangement of particles
based on spherical harmonic "fingerprints", which we use to quantify the extent
of icosahedral ordering.Comment: 8 pages, 4 figure
Hydrodynamics and microphase ordering in block copolymers: Are hydrodynamics required for ordered phases with periodicity in more than one dimension?
We use Brownian dynamics (BD), molecular dynamics, and dissipative particle dynamics to study the phase behavior of diblock copolymer melts and to determine if hydrodynamics is required in the formation of phases with greater than one-dimensional periodicity. We present a phase diagram for diblock copolymers predicted by BD and provide a relationship between the inverse dimensionless temperature ϵ/kBTϵ/kBT and the Flory–Huggins χ parameter, allowing for a quantitative comparison between methods and to mean field predictions. Our results concerning phase behavior are in good qualitative agreement with the theoretical predictions of Matsen and Bates [M. W. Matsen and F. S. Bates, Macromolecules 29, 1091 (1996)]; however, fluctuation effects arising from finite polymer lengths substantially alter the phase boundaries. Our results pertaining to the hydrodynamics are in contrast to earlier work by Groot et al. [R. D. Groot, T. J. Madden, and D. J. Tildesley, J. Chem. Phys. 110, 9739 (1999); D. Frenkel and B. Smit, Understanding Molecular Simulation, 2nd ed. (Academic, New York, 2001)]. In particular, we obtain the hexagonal ordered cylinder phase with BD, a method that does not include hydrodynamics. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69378/2/JCPSA6-121-22-11455-1.pd
Large-Scale Atomistic Simulations of Environmental Effects on the Formation and Properties of Molecular Junctions
Using an updated simulation tool, we examine molecular junctions comprised of
benzene-1,4-dithiolate bonded between gold nanotips, focusing on the importance
of environmental factors and inter-electrode distance on the formation and
structure of bridged molecules. We investigate the complex relationship between
monolayer density and tip separation, finding that the formation of
multi-molecule junctions is favored at low monolayer density, while
single-molecule junctions are favored at high density. We demonstrate that tip
geometry and monolayer interactions, two factors that are often neglected in
simulation, affect the bonding geometry and tilt angle of bridged molecules. We
further show that the structures of bridged molecules at 298 and 77 K are
similar.Comment: To appear in ACS Nano, 30 pages, 5 figure
Perfluoropolyethers: Development of an All-Atom Force Field for Molecular Simulations and Validation with New Experimental Vapor Pressures and Liquid Densities
A force field for perfluoropolyethers (PFPEs) based on the
general optimized potentials for liquid simulations all-atom (OPLS-AA) force
field has been derived in conjunction with experiments and ab initio quantum
mechanical calculations. Vapor pressures and densities of two liquid PFPEs,
perfluorodiglyme (CF3−O−(CF2−CF2−O)2−CF3) and perfluorotriglyme
(CF3−O−(CF2−CF2−O)3−CF3), have been measured experimentally to
validate the force field and increase our understanding of the physical
properties of PFPEs. Force field parameters build upon those for related
molecules (e.g., ethers and perfluoroalkanes) in the OPLS-AA force field, with
new parameters introduced for interactions specific to PFPEs. Molecular
dynamics simulations using the new force field demonstrate excellent
agreement with ab initio calculations at the RHF/6-31G* level for gas-phase
torsional energies (<0.5 kcal mol−1 error) and molecular structures for several
PFPEs, and also accurately reproduce experimentally determined densities (<0.02 g cm−3 error) and enthalpies of vaporization
derived from experimental vapor pressures (<0.3 kcal mol−1). Additional comparisons between experiment and simulation show
that polyethers demonstrate a significant decrease in enthalpy of vaporization upon fluorination unlike related molecules (e.g.,
alkanes and alcohols). Simulation suggests this phenomenon is a result of reduced cohesion in liquid PFPEs due to a reduction in
localized associations between backbone oxygen atoms and neighboring molecules
Complex crystal structures formed by the self assembly of di-tethered nanospheres
We report the results from a computational study of the self-assembly of
amphiphilic di-tethered nanospheres using molecular simulation. As a function
of the interaction strength and directionality of the tether-tether
interactions, we predict the formation of four highly ordered phases not
previously reported for nanoparticle systems. We find a double diamond
structure comprised of a zincblende (binary diamond) arrangement of spherical
micelles with a complementary diamond network of nanoparticles (ZnS/D); a phase
of alternating spherical micelles in a NaCl structure with a complementary
simple cubic network of nanoparticles to form an overall crystal structure
identical to that of AlCu_2Mn (NaCl/SC); an alternating tetragonal ordered
cylinder phase with a tetragonal mesh of nanoparticles described by the [8,8,4]
Archimedean tiling (TC/T); and an alternating diamond phase in which both
diamond networks are formed by the tethers (AD) within a nanoparticle matrix.
We compare these structures with those observed in linear and star triblock
copolymer systems
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