1,718 research outputs found
Self-Consistent-Field Study of Adsorption and Desorption Kinetics of Polyethylene Melts on Graphite and Comparison with Atomistic Simulations
A method is formulated, based on combining self-consistent field theory with
dynamically corrected transition state theory, for estimating the rates of
adsorption and desorption of end-constrained chains (e.g. by crosslinks or
entanglements) from a polymer melt onto a solid substrate. This approach is
tested on a polyethylene/graphite system, where the whole methodology is
parametrized by atomistically detailed molecular simulations. For short-chain
melts, which can still be addressed by molecular dynamics simulations with
reasonable computational resources, the self-consistent field approach gives
predictions of the adsorption and desorption rate constants which are
gratifyingly close to molecular dynamics estimates.Comment: 18 pages, 10 figure
Equation of State Based Slip Spring Model for Entangled Polymer Dynamics
A mesoscopic, mixed particle- and field-based Brownian dynamics methodology
for the simulation of entangled polymer melts has been developed. Polymeric
beads consist of several Kuhn segments, and their motion is dictated by the
Helmholtz energy of the sample, which is a sum of the entropic elasticity of
chain strands between beads, slip springs, and nonbonded interactions. The
entanglement effect is introduced by the slip springs, which are springs
connecting either nonsuccessive beads on the same chain or beads on different
polymer chains. The terminal positions of slip springs are altered during the
simulation through a kinetic Monte Carlo hopping scheme, with rate-controlled
creation/destruction processes for the slip springs at chain ends. The rate
constants are consistent with the free energy function employed and satisfy
microscopic reversibility at equilibrium. The free energy of nonbonded
interactions is derived from an appropriate equation of state, and it is
computed as a functional of the local density by passing an orthogonal grid
through the simulation box; accounting for it is necessary for reproducing the
correct compressibility of the polymeric material. Parameters invoked by the
mesoscopic model are derived from experimental volumetric and viscosity data or
from atomistic molecular dynamics simulations, establishing a "bottom-up"
predictive framework for conducting slip spring simulations of polymeric
systems of specific chemistry. The mesoscopic simulation methodology is
implemented for the case of cis-1,4-polyisoprene, whose structure, dynamics,
thermodynamics, and linear rheology in the melt state are quantitatively
predicted and validated without a posteriori fitting the results to
experimental measurements.Comment: 80 pages, 17 figure
Thermal modeling environment for TMT
In a previous study we had presented a summary of the TMT Aero-Thermal modeling effort to support thermal seeing and dynamic loading estimates. In this paper a summary of the current status of Computational Fluid Dynamics (CFD) simulations for TMT is presented, with the focus shifted in particular towards the synergy between CFD and the TMT Finite Element Analysis (FEA) structural and optical models, so that the thermal and consequent optical deformations of the telescope can be calculated. To minimize thermal deformations and mirror seeing the TMT enclosure will be air conditioned during day-time to the expected night-time ambient temperature. Transient simulations with closed shutter were performed to investigate the optimum cooling configuration and power requirements for the standard telescope parking position. A complete model of the observatory on Mauna Kea was used to calculate night-time air temperature inside the enclosure (along with velocity and pressure) for a matrix of given telescope orientations and enclosure configurations. Generated records of temperature variations inside the air volume of the optical paths are also fed into the TMT thermal seeing model. The temperature and heat transfer coefficient outputs from both models are used as input surface boundary conditions in the telescope structure and optics FEA models. The results are parameterized so that sequential records several days long can be generated and used by the FEA model to estimate the observing spatial and temporal temperature range of the structure and optics
Is the European citizen’s initiative a serious threat for the community method?
This article proceeds to a normative claim that the potential of the European Citizens’ Initiative (ECI) – an instrument expected to increase democratic legitimacy in the EU – should be evaluated in the light of the post-Lisbon Community method and not as an additional ‘opportunity structure for citizens’ participation’. The first section explains why the Community method is primarily a mechanism of ‘output legitimacy’, even after the Lisbon Treaty. Furthermore, the legal framework of the ECI (notably the Regulation 211/2011 but also the Commission’s Green Paper preceding the adoption of the Regulation) is provided. The evaluation section concludes that the ECI’s legislative framework, far from an instrument of direct democracy, perhaps an additional ‘opportunity structure’, cannot affect the Community method nor seriously increase democratic legitimacy at the EU level due to the – simultaneous – presence of two thresholds: the intactness of the Commission’s legislative monopoly and the burdensome formalities imposed upon citizens and organisers
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