55 research outputs found
Percolation of Immobile Domains in Supercooled Thin Polymeric Films
We present an analysis of heterogeneous dynamics in molecular dynamics
simulations of a thin polymeric film, supported by an absorbing structured
surface. Near the glass transition "immobile" domains occur throughout the
film, yet the probability of their occurrence decreasing with larger distance
from the surface. Still, enough immobile domains are located near the free
surface to cause them to percolate in the direction perpendicular to surface,
at a temperature near the glass transition temperature. This result is in
agreement with a recent theoretical model of glass transition
Individual Entanglements in a Simulated Polymer Melt
We examine entanglements using monomer contacts between pairs of chains in a
Brownian-dynamics simulation of a polymer melt. A map of contact positions with
respect to the contacting monomer numbers (i,j) shows clustering in small
regions of (i,j) which persists in time, as expected for entanglements. Using
the ``space''-time correlation function of the aforementioned contacts, we show
that a pair of entangled chains exhibits a qualitatively different behavior
than a pair of distant chains when brought together. Quantitatively, about 50%
of the contacts between entangled chains are persistent contacts not present in
independently moving chains. In addition, we account for several observed
scaling properties of the contact correlation function.Comment: latex, 12 pages, 7 figures, postscript file available at
http://arnold.uchicago.edu/~ebn
Tensile Forces and Shape Entropy Explain Observed Crista Structure in Mitochondria
A model is presented from which the observed morphology of the inner
mitochondrial membrane can be inferred as minimizing the system's free energy.
Besides the usual energetic terms for bending, surface area, and pressure
difference, our free energy includes terms for tension that we believe to be
exerted by proteins and for an entropic contribution due to many dimensions
worth of shapes available at a given energy.
In order to test the model, we measured the structural features of
mitochondria in HeLa cells and mouse embryonic fibroblasts using 3D electron
tomography. Such tomograms reveal that the inner membrane self-assembles into a
complex structure that contains both tubular and flat lamellar crista
components. This structure, which contains one matrix compartment, is believed
to be essential to the proper functioning of mitochondria as the powerhouse of
the cell. We find that tensile forces of the order of 10 pN are required to
stabilize a stress-induced coexistence of tubular and flat lamellar cristae
phases. The model also predicts \Deltap = -0.036 \pm 0.004 atm and \sigma=0.09
\pm 0.04 pN/nm
Shear yielding of amorphous glassy solids: Effect of temperature and strain rate
We study shear yielding and steady state flow of glassy materials with
molecular dynamics simulations of two standard models: amorphous polymers and
bidisperse Lennard-Jones glasses. For a fixed strain rate, the maximum shear
yield stress and the steady state flow stress in simple shear both drop
linearly with increasing temperature. The dependence on strain rate can be
described by a either a logarithm or a power-law added to a constant. In marked
contrast to predictions of traditional thermal activation models, the rate
dependence is nearly independent of temperature. The relation to more recent
models of plastic deformation and glassy rheology is discussed, and the
dynamics of particles and stress in small regions is examined in light of these
findings
Glass Transition Behavior of Polymer Films of Nanoscopic Dimensions
Glass transition behavior of nanoscopically thin polymer films is
investigated by means of molecular dynamics simulations. A thin polymer film
that is composed of bead-spring model chains and supported on an idealized, fcc
lattice substrate surface is studied in this work.Comment: in review, macromolecule
Tensile Fracture of Welded Polymer Interfaces: Miscibility, Entanglements and Crazing
Large-scale molecular simulations are performed to investigate tensile
failure of polymer interfaces as a function of welding time . Changes in the
tensile stress, mode of failure and interfacial fracture energy are
correlated to changes in the interfacial entanglements as determined from
Primitive Path Analysis. Bulk polymers fail through craze formation, followed
by craze breakdown through chain scission. At small welded interfaces are
not strong enough to support craze formation and fail at small strains through
chain pullout at the interface. Once chains have formed an average of about one
entanglement across the interface, a stable craze is formed throughout the
sample. The failure stress of the craze rises with welding time and the mode of
craze breakdown changes from chain pullout to chain scission as the interface
approaches bulk strength. The interfacial fracture energy is calculated
by coupling the simulation results to a continuum fracture mechanics model. As
in experiment, increases as before saturating at the average
bulk fracture energy . As in previous simulations of shear strength,
saturation coincides with the recovery of the bulk entanglement density. Before
saturation, is proportional to the areal density of interfacial
entanglements. Immiscibiltiy limits interdiffusion and thus suppresses
entanglements at the interface. Even small degrees of immisciblity reduce
interfacial entanglements enough that failure occurs by chain pullout and
Simulations of the Static Friction Due to Adsorbed Molecules
The static friction between crystalline surfaces separated by a molecularly
thin layer of adsorbed molecules is calculated using molecular dynamics
simulations. These molecules naturally lead to a finite static friction that is
consistent with macroscopic friction laws. Crystalline alignment, sliding
direction, and the number of adsorbed molecules are not controlled in most
experiments and are shown to have little effect on the friction. Temperature,
molecular geometry and interaction potentials can have larger effects on
friction. The observed trends in friction can be understood in terms of a
simple hard sphere model.Comment: 13 pages, 13 figure
- …