40,659 research outputs found
PEO/CHCl3: Crystallinity of the polymer and vapor pressure of the solvent - Equilibrium and non-equilibrium phenomena -
Vapor pressures were measured for the system chloroform/polyethylene oxide
(peo, weight average molar mass = 1000 kg/mol) at 25 degrees centigrade as a
function of the weight fraction w of the polymer by means of a combination of
head space sampling and gas chromatography. The establishment of thermodynamic
equilibria was assisted by employing thin polymer films. The degrees of
crystallinity alpha of the pure peo and of the solid polymer contained in the
mixtures were determined via dsc. An analogous degree of polymer insolubility,
beta, was calculated from the vapor pressures measured in this composition
range. The experiments demonstrate that both quantities and their concentration
dependence are markedly affected by the particular mode of film preparation.
These non-equilibrium phenomena are discussed in terms of frozen local and
temporal equilibria, where differences between alpha and beta are attributed to
the occlusion of amorphous material within crystalline domains. Equilibrium
information was obtained from two sources, namely from the vapor pressures in
the absence of crystalline material (gas/liquid) and from the saturation
concentration of peo (liquid/solid). The thermodynamic consistency of these
data is demonstrated using a new approach that enables the modeling of
composition dependent interaction parameters by means of two adjustable
parameters only
Action Potential Onset Dynamics and the Response Speed of Neuronal Populations
The result of computational operations performed at the single cell level are
coded into sequences of action potentials (APs). In the cerebral cortex, due to
its columnar organization, large number of neurons are involved in any
individual processing task. It is therefore important to understand how the
properties of coding at the level of neuronal populations are determined by the
dynamics of single neuron AP generation. Here we analyze how the AP generating
mechanism determines the speed with which an ensemble of neurons can represent
transient stochastic input signals. We analyze a generalization of the
-neuron, the normal form of the dynamics of Type-I excitable membranes.
Using a novel sparse matrix representation of the Fokker-Planck equation, which
describes the ensemble dynamics, we calculate the transmission functions for
small modulations of the mean current and noise noise amplitude. In the
high-frequency limit the transmission function decays as ,
where surprisingly depends on the phase at which APs are
emitted. In a physiologically plausible regime up to 1kHz the typical response
speed is, however, independent of the high-frequency limit and is set by the
rapidness of the AP onset, as revealed by the full transmission function. In
this regime modulations of the noise amplitude can be transmitted faithfully up
to much higher frequencies than modulations in the mean input current. We
finally show that the linear response approach used is valid for a large regime
of stimulus amplitudes.Comment: Submitted to the Journal of Computational Neuroscienc
Towards time-dependent, non-equilibrium charge-transfer force fields: Contact electrification and history-dependent dissociation limits
Force fields uniquely assign interatomic forces for a given set of atomic
coordinates. The underlying assumption is that electrons are in their
quantum-mechanical ground state or in thermal equilibrium. However, there is an
abundance of cases where this is unjustified because the system is only locally
in equilibrium. In particular, the fractional charges of atoms, clusters, or
solids tend to not only depend on atomic positions but also on how the system
reached its state. For example, the charge of an isolated solid -- and thus the
forces between atoms in that solid -- usually depends on the counterbody with
which it has last formed contact. Similarly, the charge of an atom, resulting
from the dissociation of a molecule, can differ for different solvents in which
the dissociation took place. In this paper we demonstrate that such
charge-transfer history effects can be accounted for by assigning discrete
oxidation states to atoms. With our method, an atom can donate an integer
charge to another, nearby atom to change its oxidation state as in a redox
reaction. In addition to integer charges, atoms can exchange "partial charges"
which are determined with the split charge equilibration method.Comment: 11 pages, 7 figure
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