118 research outputs found
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
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
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Assessing plantar sensation in the foot using the FOot Roughness Discrimination Test (FoRDT™): a reliability and validity study in stroke
BACKGROUND: The foot sole represents a sensory dynamometric map and is essential for balance and gait control. Sensory impairments are common, yet often difficult to quantify in neurological conditions, particularly stroke. A functionally oriented and quantifiable assessment, the Foot Roughness Discrimination Test (FoRDT™), was developed to address these shortcomings. OBJECTIVE: To evaluate inter- and intra-rater reliability, convergent and discriminant validity of the Foot Roughness Discrimination Test (FoRDT™). DESIGN: Test-retest design. SETTING: Hospital Outpatient. PARTICIPANTS: Thirty-two people with stroke (mean age 70) at least 3 months after stroke, and 32 healthy, age-matched controls (mean age 70). MAIN OUTCOME MEASURES: Roughness discrimination thresholds were quantified utilising acrylic foot plates, laser-cut to produce graded spatial gratings. Stroke participants were tested on three occasions, and by two different raters. Inter- and intra-rater reliability and agreement were evaluated with Intraclass Correlation Coefficients and Bland-Altman plots. Convergent validity was evaluated through Spearman rank correlation coefficients (rho) between the FoRDT™ and the Erasmus modified Nottingham Sensory Assessment (EmNSA). RESULTS: Intra- and inter rater reliability and agreement were excellent (ICC =.86 (95% CI .72-.92) and .90 (95% CI .76 -.96)). Discriminant validity was demonstrated through significant differences in FoRDT™ between stroke and control participants (p.05). CONCLUSIONS: This simple and functionally oriented test of plantar sensation is reliable, valid and clinically feasible for use in an ambulatory, chronic stroke and elderly population. It offers clinicians and researchers a sensitive and robust sensory measure and may further support the evaluation of rehabilitation targeting foot sensation. This article is protected by copyright. All rights reserved
Network dynamics:a computational framework for the simulation of the glassy state
An out-of-equilibrium simulation method for tracking the time evolution of glassy systems (or any other systems that can be described by hopping dynamics over a network of discrete states) is presented. Graph theory and complexity concepts are utilised, alongside the method of the dynamical integration of a Markovian web (G. C. Boulougouris and D. N. Theodorou, J. Chem. Phys., 2007, 127, 084903) in order to provide a unified framework for dealing with the long time-scales of non-ergodic systems. Within the developed formalism, the network of states accessible to the system is considered a finite part of the overall universe, communicating with it through well-defined boundary states. The analytical solution of the probability balance equation proceeds without the need for assuming the existence of an equilibrium distribution among the states of the network and the corresponding survival and escape probabilities (as functions of time) are defined. More importantly, the study of the probability flux through the dividing surface separating the system and its environment reveals the relaxation mechanisms of the system. We apply our approach to the network of states obtained by exploring the energy landscape of an atomistically detailed glassy specimen of atactic polystyrene. The rate constants connecting different basins of the landscape are evaluated by multi-dimensional transition-state-theory. We are able to accurately probe the appearance of the δ- and γ-subglass relaxation mechanisms and their relevant time-scales, out of atomistic simulations. The proposed approach can fill a gap in the rational molecular design toolbox, by providing an alternative to molecular dynamics for structural relaxation in glasses and/or other slow molecular processes (e.g., adsorption or desorption) that involve very distant time-scales.</p
Molecular Dynamics Test of the Stress-Thermal Rule in Polyethylene and Polystyrene Entangled Melts
Anisotropic thermal transport induced by deformation
and the linear relation between the thermal conductivity and
stress tensors, also known as the stress-thermal rule (STR), are
tested via molecular dynamics simulations in well-entangled linear
polyethylene (PE) and polystyrene (PS) melts subjected to
extensional flow. We propose a method to determine the stress
in deformed molecular melts, a key component missing in prior
simulation studies on thermal transport in polymers that prevented
verification of the STR. We compare our results with available data
from previous experimental and simulation studies. Thermal
conductivity (TC) is found to increase (decrease) in the direction
parallel (perpendicular) to the imposed stretch. We find that the STR is valid for both PE and PS over a wide range of deformation
rates and stress levels. In direct agreement with experimental evidence and the STR, we observe that for a given strain, the anisotropy
in TC increases with the strain rate. Surprisingly, our results for PE question the universal behavior with respect to polymer
chemistry suggested by experiments by showing a significantly higher proportionality constant (the stress-thermal coefficient)
between stress and anisotropy in TC. We argue that this discrepancy can be explained by the high degree of entanglement
interactions in PE affecting the transport of energy at the molecular level. Our conjecture is tested by studying an entangled linear PS
melt, a polymer with a much lower entanglement plateau, for which thermal transport experimental results are available. For PS, the
normalized stress-thermal coefficient is found to be commensurate with the experimental value. Finally, we test the fundamental
molecular hypothesis of preferential energy transport along the backbone of polymer chains used to formulate the STR, which was
prompted by early experimental evidence showing an increase in TC with chain length. We are able to establish that the increase in
TC with chain length in PE melts fades as the system becomes entangled (i.e., TC remains constant beyond the critical
entanglement chain length that marks the transition to entanglement-dominated rheological behavior). Our findings are of key
importance in developing robust molecular-to-continuum methodologies for the study of nonisothermal macroscopic flows that are
extremely relevant to polymer manufacturing processes.European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie IF MTCIATTP 750985
MARS therapy, the bridging to liver retransplantation-Three cases from the Hungarian liver transplant program
Besides orthotopic liver transplantation (OLT) there is no long-term and effective replacement therapy for severe liver failure. Artificial extracorporeal liver supply devices are able to reduce blood toxin levels, but do not replace any synthetic function of the liver. Molecular adsorbent recirculating system (MARS) is one of the methods that can be used to treat fulminant acute liver failure (ALF) or acute on chronic liver failure (AoCLF). The primary non-function (PNF) of the newly transplanted liver manifests in the clinical settings exactly like acute liver failure. MARS treatment can reduce the severity of complications by eliminating blood toxins, so that it can help hepatic encephalopathy (HE), hepatorenal syndrome (HRS), and the high rate mortality of cerebral herniation. This might serve as a bridging therapy before orthotopic liver retransplantation (reOLT). Three patients after a first liver transplantation became candidate for urgent MARS treatment as a bridging solution prior to reOLT in our center. Authors report these three cases, focusing on indications, MARS sessions, clinical courses, and final outcomes. © 2013 Akadémiai Kiadó, Budapest
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