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

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

Home-based maintenance tele-rehabilitation reduces the risk for acute exacerbations of COPD, hospitalisations and emergency department visits

Pulmonary rehabilitation (PR) remains grossly underutilised by suitable patients worldwide. We investigated whether home-based maintenance tele-rehabilitation will be as effective as hospital-based maintenance rehabilitation and superior to usual care in reducing the risk for acute chronic obstructive pulmonary disease (COPD) exacerbations, hospitalisations and emergency department (ED) visits. Following completion of an initial 2-month PR programme this prospective, randomised controlled trial (between December 2013 and July 2015) compared 12 months of home-based maintenance tele-rehabilitation (n=47) with 12 months of hospital-based, outpatient, maintenance rehabilitation (n=50) and also to 12 months of usual care treatment (n=50) without initial PR. In a multivariate analysis during the 12-month follow-up, both home-based tele-rehabilitation and hospital-based PR remained independent predictors of a lower risk for 1) acute COPD exacerbation (incidence rate ratio (IRR) 0.517, 95% CI 0.389–0.687, and IRR 0.635, 95% CI 0.473–0.853), respectively, and 2) hospitalisations for acute COPD exacerbation (IRR 0.189, 95% CI 0.100–0.358, and IRR 0.375, 95% CI 0.207–0.681), respectively. However, only home-based maintenance tele-rehabilitation and not hospital-based, outpatient, maintenance PR was an independent predictor of ED visits (IRR 0.116, 95% CI 0.072–0.185). Home-based maintenance tele-rehabilitation is equally effective as hospital-based, outpatient, maintenance PR in reducing the risk for acute COPD exacerbation and hospitalisations. In addition, it encounters a lower risk for ED visits, thereby constituting a potentially effective alternative strategy to hospital-based, outpatient, maintenance PR

Atomistic simulation of polystyrene - fullerene nanocomposites

53 σ.Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Μικροσυστήματα και Νανοδιατάξεις”Μικρές συγκεντρώσεις νανοσωματιδίων, της τάξης του 1%, μπορούν να τροποποιήσουν σημαντικά τη φασική συμπεριφορά και τα μηχανικά και ηλεκτρικά χαρακτηριστικά των πολυμερικών υλικών. Ο περίπλοκος χαρακτήρας των διαμοριακών αλληλεπιδράσεων στα συστήματα πολυμερούς - νανοσωματιδίων μπορεί να οδηγήσει σε χωρική διαφοροποίηση της δομής και της δυναμικής της μήτρας σε μεσοσκοπικό, όσο και σε νανοσκοπικό επίπεδο. Κατά μέγα μέρος, η συμπεριφορά αυτή εκδηλώνεται στην τοπική δυναμική του πολυμερούς και το μακροσκοπικά παρατηρούμενο ιξώδες. Στην παρούσα εργασία μελετάται υπολογιστικά ένα κοινό και καλά μελετημένο νανοσύνθετο σύστημα: πολυμερική μήτρα ατακτικού πολυστυρενίου υψηλού μοριακού βάρους με διεσπαρμένα φουλερένια C60 σε κλάσμα βάρους φουλερενίων 1%. Καλή διασπορά φουλερενίων μπορεί να επηρεάσει τη δυναμική του πολυμερούς, όπως αποδεικνύεται από πρόσφατες αναφορές για σημαντικές αλλαγές του ιξώδους και της θερμοκρασίας υαλώδους μετάπτωσης. Στη βιβλιογραφία έχει αναφερθεί ότι το ιξώδες ισχυρά διαπλεγμένου πολυστυρενίου αναμεμειγμένου με νανοσωματίδια C60 μειώνεται κατά δύο μέχρι πέντε φορές, σε πλήρη αντίθεση με την αναμενόμενη αύξηση του ιξώδους για συμβατικά υλικά πλήρωσης, την ίδια στιγμή που η θερμοκρασία υαλώδους μετάπτωσης της ίδιας τάξης υλικών βρέθηκε να αυξάνται κατά έναν με δύο βαθμούς. Με σκοπό να μελετηθεί η τοπική δυναμική των συστημάτων PS-C60, πραγματοποιήθηκαν προσομοιώσεις σε δύο συνδεδεμένα επίπεδα αναπαράστασης των νανοσύνθετων πολυστυρενίου: (α) μία αδροποιημένη αναπαράσταση, στην οποία κάθε δομική μονάδα του πολυστυρενίου αναπαρίσταται ως ένα και μοναδικό "υπερ-άτομο" και κάθε φουλερένιο ως σφαιρικό κέλυφος. Με χρήση κινήσεων Monte Carlo (MC) μεταβλητής συνδετικότητας επιτεύχθηκε εξισορρόπηση των αδροποιημένων συστημάτων σε όλες τις κλίμακες μήκους. (β) Μία ατομιστική αναπαράσταση, όπου τόσο τα νανοσωματίδια, όσο και η πολυμερική μήτρα αναπαρίστανται σε επίπεδο ενοποιημένων ατόμων (τα άτομα άνθρακα απορροφούν τα συνδεδεμένα σε αυτά υδρογόνα). Αρχικές απεικονίσεις για ατομιστικές προσομοιώσεις Μοριακής Δυναμικής (MD) λαμβάνονται με αντίστροφη απεικόνιση καλά εξισορροπημένων αδροποιημένων απεικονίσεων. Η διαδικασία της αντίστροφης απεικόνισης διατηρεί την τακτικότητα η οποία ενυπήρχε στην αδροποιημένη αναπαράσταση, κατά την οικοδόμηση των ατομιστικών συντεταγμένων με μία οιονεί-Metropolis διαδικασία η οποία αποφεύγει μη φυσικές διαμορφώσεις. Προσομοιώσεις MD διεξάγονται σε ατμοσφαιρική πίεση στο εύρος θερμοκρασιών 400 - 500 K με σκοπό να επιτευχθεί εξισορρόπηση της πυκνότητας. Αναλύοντας τις τροχιές προσομοιώσεων MD υπό σταθερή ενέργεια, η τοπική δυναμική του πολυστυρενίου (για τα καθαρά και τα σύνθετα συστήματα) μπορεί να χαρακτηρισθεί σε όρος χρονικών συναρτήσεων αποσυσχέτισης προσανατολισμού δεσμών, και παρατηρήσιμων μεγεθών πειραμάτων ανελαστικής σκέδασης νετρονίων. Τα νανοσύνθετα συστήματα βρέθηκαν να παρουσιάζουν ελαφρά βραδύτερη τοπική δυναμική σε σχέση με το καθαρό πολυμερές, σε καλή συμφωνία με πειραματικές παρατηρήσεις.Very small concentrations of nanoparticles, on the order of a percent, can significantly alter the phase behavior and the mechanical and electrical characteristics of polymeric materials. The complexity of intermolecular interactions in polymer-nanoparticle systems leads to spatial variations in structure and dynamics at both the meso- and nanoscale. Much of this behavior is manifested in polymer segmental dynamics and the viscosity. A well characterized and common nanocomposite system of fullerene C60 nanoparticles (whose diameter is approximately 1 nm) dispersed in high molecular weight monodisperse atactic polystyrene (PS) is investigated. Well dispersed fullerenes can affect polymer dynamics, as evidenced by recent reports of significant changes in viscosity and glass transition temperature. The viscosity of highly entangled polystyrene blended with C60 nanoparticles was reported to decrease by a factor of 2 to 5, in contrast with the expected viscosity increase for conventional fillers, while the glass transition temperature of the same class of systems was found to increase by 1 or 2 degrees. In order to shed some light into segmental dynamics of PS-C60 systems, molecular simulations have been conducted using two interconnected levels of representation for polystyrene nanocomposites: (a) a coarse-grained representation, in which each polystyrene repeat unit is mapped into a single “superatom” and each fullerene is viewed as a spherical shell. Equilibration of coarse-grained polymer-nanoparticle systems at all length scales is achieved via connectivity-altering Monte Carlo (MC) simulations. (b) An atomistic representation, where both nanoparticles and polymer chains are represented in terms of united-atom forcefields. Initial configurations for atomistic Molecular Dynamics (MD) simulations are obtained by reverse mapping well-equilibrated coarse-grained configurations. The reverse mapping procedure retains the tactility which is implicit in the coarse-grained representation, while regrowing atomistic sites by a quasi-Metropolis procedure that avoids unphysical configurations. MD simulations are performed under atmospheric pressure in the temperature range 400 - 500 K for the density to equilibrate. By analyzing MD trajectories under constant energy, segmental dynamics of polystyrene (for neat and filled systems) can then be characterized in terms of bond orientation time autocorrelation functions and Inelastic Neutron Scattering (IENS) observables. Nanocomposite systems were found to exhibit slightly slower segmental dynamics than the unfilled ones, in good agreement with IENS data. Moreover, the scattering spectrum of the melt suggests that the influence of C60 on polymer dynamics is limited to the nanosecond time scales, which can account for changes in bulk dynamics resolved with mechanical measurements.Γεώργιος Γ. Βογιατζή

Friedmann-like universes with non-metricity

We study the potential effects of spacetime non-metricity in cosmology. In the spirit of Einstein–Cartan gravity, but with non-metricity replacing torsion, we consider the Einstein–Hilbert action and assume zero torsion. Adopting certain hyperfluid models, with non-vanishing hypermomentum that can source spacetime non-metricity, we add a matter component into the action and derive the field equations, along with the conservation laws. Applying our formulae to cosmology, we generalize the Friedmann and the Raychaudhuri equations in the presence of non-metricity. Our results show that, in a number of cases, non-metricity can mimic the effects of matter with unconventional equation of state. For instance, specific types of hypermomentum are found to act as an effective stiff fluid, thus opening the possibility that non-metricity could have played a significant role in the early stages of the universe’s evolution. Alternative forms of hypermomentum can dominate the universal dynamics at late times. In either case, the equilibrium moment depends on the initial conditions and it is determined by a simple relation between the matter component and the hyperfluid

Multiscale simulations of PS-SiO\u3csub\u3e2\u3c/sub\u3e nanocomposites:From melt to glassy state

\u3cp\u3eThe interaction energetics, molecular packing, entanglement network properties, segmental dynamics, and elastic constants of atactic polystyrene-amorphous silica nanocomposites in the molten and the glassy state are studied via molecular simulations using two interconnected levels of representation: (a) a coarse-grained one, wherein each polystyrene repeat unit is mapped onto a single superatom and the silica nanoparticle is viewed as a solid sphere. Equilibration at all length scales at this level is achieved via connectivity-altering Monte Carlo simulations. (b) A united-atom (UA) level, wherein the polymer chains are represented in terms of a united-atom forcefield and the silica nanoparticle is represented in terms of a simplified, fully atomistic model. Initial configurations for UA molecular dynamics (MD) simulations are obtained by reverse mapping well-equilibrated coarse-grained configurations. By analysing microcanonical UA MD trajectories, the polymer density profile is studied and the polymer is found to exhibit layering in the vicinity of the nanoparticle surface. An estimate of the enthalpy of mixing between polymer and nanoparticles, derived from the UA simulations, compares favourably against available experimental values. The dynamical behaviour of polystyrene (in neat and filled melt systems) is characterized in terms of bond orientation and dihedral angle time autocorrelation functions. At low concentration in the molten polymer matrix, silica nanoparticles are found to cause a slight deceleration of the segmental dynamics close to their surface compared to the bulk polymer. Well-equilibrated coarse-grained long-chain configurations are reduced to entanglement networks via topological analysis with the CReTA algorithm, yielding a slightly lower density of entanglements in the filled than in the neat systems. UA melt configurations are glassified by MD cooling. The elastic moduli of the resulting glassy nanocomposites are computed through an analysis of strain fluctuations in the undeformed state and through explicit mechanical deformation by MD, showing a stiffening of the polymer in the presence of nanoparticles. UA simulation results for the elastic constants are compared to continuum micromechanical calculations invoked in homogenization models of the overall mechanical behaviour of heterogeneous materials. They can be interpreted in terms of the presence of an interphase of approximate thickness 2 nm around the nanoparticles, with elastic constants intermediate between those of the filler and the matrix.\u3c/p\u3