13,673 research outputs found
Influence of solvent quality on effective pair potentials between polymers in solution
Solutions of interacting linear polymers are mapped onto a system of ``soft''
spherical particles interacting via an effective pair potential. This
coarse-graining reduces the individual monomer-level description to a problem
involving only the centers of mass (CM) of the polymer coils. The effective
pair potentials are derived by inverting the CM pair distribution function,
generated in Monte Carlo simulations, using the hypernetted chain (HNC)
closure. The method, previously devised for the self-avoiding walk model of
polymers in good solvent, is extended to the case of polymers in solvents of
variable quality by adding a finite nearest-neighbor monomer-monomer attraction
to the previous model and varying the temperature. The resulting effective pair
potential is found to depend strongly on temperature and polymer concentration.
At low concentration the effective interaction becomes increasingly attractive
as the temperature decreases, eventually violating thermodynamic stability
criteria. However, as polymer concentration is increased at fixed temperature,
the effective interaction reverts to mostly repulsive behavior. These issues
help illustrate some fundamental difficulties encountered when coarse-graining
complex systems via effective pair potentials.Comment: 15 pages, 12 figures (one added in revised version), revTeX
Relating monomer to centre-of-mass distribution functions in polymer solutions
A relationship between the measurable monomer-monomer structure factor, and
the centre-of-mass (CM) structure factor of dilute or semi-dilute polymer
solutions is derived from Ornstein-Zernike relations within the ``polymer
reference interaction site model'' (PRISM) formalism, by considering the CM of
each polymer as an auxiliary site and neglecting direct correlations between
the latter and the CM and monomers of neighbouring polymers. The predictions
agree well with Monte Carlo data for self-avoiding walk polymers, and are
considerably more accurate than the predictions of simple factorization
approximations.Comment: uses eps.cls, v2 is close to final published versio
Large angle transient dynamics (LATDYN) documentation. Post-processor manual
The post processor program was developed to view Large Angle Transient Dynamics (LATDYN) output data in predefined, predetermined formats. The post processor is used for plotting data, creating and maintaining a data base of plotting requests, comparing and manipulating data sets in the data base, and the preparing plots for documentation
Defining Technology for Learning: Cognitive and Physical Tools of Inquiry
This essay explores definitions of technology and educational technology. The authors argue the following points: 1. Educational stakeholders, and the public at large, use the term technology as though it has a universally agreed upon definition. It does not, and how technology is defined matters. 2. For technology in schools to support student learning, it must to be defined in a way that describes technology as a tool for problem-solving. 3. Integration of technology, particularly when paired with teacher-centered practices, has the potential of reinforcing and heightening the negative consequences of a conception of learning that positions students as recipients of knowledge instead constructors of knowledge. Essay concludes with a call for leaders in the field of educational technology to provide guidance by adopting a definition that encapsulates the third point above
Extension of Classical Nucleation Theory for Uniformly Sheared Systems
Nucleation is an out-of-equilibrium process, which can be strongly affected
by the presence of external fields. In this letter, we report a simple
extension of classical nucleation theory to systems submitted to an homogeneous
shear flow. The theory involves accounting for the anisotropy of the critical
nucleus formation, and introduces a shear rate dependent effective temperature.
This extended theory is used to analyze the results of extensive molecular
dynamics simulations, which explore a broad range of shear rates and
undercoolings. At fixed temperature, a maximum in the nucleation rate is
observed, when the relaxation time of the system is comparable to the inverse
shear rate. In contrast to previous studies, our approach does not require a
modification of the thermodynamic description, as the effect of shear is mainly
embodied into a modification of the kinetic prefactor and of the temperature.Comment: 6 pages, 4 figure
Crystal nucleation and cluster-growth kinetics in a model glass under shear
Crystal nucleation and growth processes induced by an externally applied
shear strain in a model metallic glass are studied by means of nonequilibrium
molecular dynamics simulations, in a range of temperatures. We observe that the
nucleation-growth process takes place after a transient, induction regime. The
critical cluster size and the lag-time associated with this induction period
are determined from a mean first-passage time analysis. The laws that describe
the cluster growth process are studied as a function of temperature and strain
rate. A theoretical model for crystallization kinetics that includes the time
dependence for nucleation and cluster growth is developed within the framework
of the Kolmogorov-Johnson-Mehl-Avrami scenario and is compared with the
molecular dynamics data. Scalings for the cluster growth laws and for the
crystallization kinetics are also proposed and tested. The observed nucleation
rates are found to display a nonmonotonic strain rate dependency
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