Search results for "dynamics."
showing 10 items of 9637 documents
Reduction of the glass transition temperature in polymer films: A molecular-dynamics study
2001
We present results of molecular dynamics (MD) simulations for a non-entangled polymer melt confined between two completely smooth and repulsive walls, interacting with inner particles via the potential $U_{\rm wall}\myeq (\sigma/z)^9$, where $z \myeq |z_{\rm particle}-z_{\rm wall}|$ and $\sigma$ is (roughly) the monomer diameter. The influence of this confinement on the dynamic behavior of the melt is studied for various film thicknesses (wall-to-wall separations) $D$, ranging from about 3 to about 14 times the bulk radius of gyration. A comparison of the mean-square displacements in the film and in the bulk shows an acceleration of the dynamics due to the presence of the walls. %Consistent…
Relaxation processes and glass transition of confined polymer melts: A molecular dynamics simulation of 1,4-polybutadiene between graphite walls.
2017
Molecular dynamics simulations of a chemically realistic model for 1,4-polybutadiene in a thin film geometry confined by two graphite walls are presented. Previous work on melts in the bulk has shown that the model faithfully reproduces static and dynamic properties of the real material over a wide temperature range. The present work studies how these properties change due to nano-confinement. The focus is on orientational correlations observable in nuclear magnetic resonance experiments and on the local intermediate incoherent neutron scattering function, Fs(qz, z, t), for distances z from the graphite walls in the range of a few nanometers. Temperatures from about 2Tg down to about 1.15Tg…
Monte Carlo simulation of the glass transition in polymer melts: An application of MCT
1995
Abstract This paper reviews the results of a large scale Monte Carlo simulation for the dynamics of a supercooled polymer melt. The dynamics of the melt was studied by means of the time evolution of the incoherent intermediate scattering function φs q(t), which was monitored over seven decades in time. In an intermediate time window it is possible to describe the decay of φs q(t) quantitatively in the framework of mode-coupling theory, provided the extended version of the theory is used.
Pressure-induced formation of diblock copolymer "micelles" in supercritical fluids. A combined study by small angle scattering experiments and mean-f…
2004
We developed a simple time-dependent mean-field theory to describe the phase separation kinetics of either homopolymers or AB-diblock copolymers in supercritical (SC) fluids. The model, previously used to describe the phase behavior of AB-block copolymers under the assumption of strong solvent selectivity for just one copolymer chain, has been extended to study the kinetics of the phase separation process. Time resolved small angle x-ray scattering (TR-SAXS) measurements have been performed on different AB-diblock copolymers containing a perfluorinated chain and dissolved in SC-CO2. The data obtained over a wide range of pressure and temperature confirm our theoretical predictions. Particul…
Low-temperature specific heat of orientational glasses
1992
This review summarizes specific heat data measured at low temperatures (T<1 K) on orientational glasses. Three species of mixed molecular crystals exhibiting orientational disorder are considered, namely (KBr)1−x (KCN) x , (NaCN)1−x (KCN) x (Rb)1−x (NH4) x H2PO4. For intermediate concentrations of the anisotropic components, glass-like excitations have been observed. It is demonstrated that with respect to thermal properties, orientational disorder leads to the same “universal” behaviours than for structural disorder, i.e. a specific heat which varies below 1 K and for times 10−4 s–10 s as:C p(T,t)∞T 1×ln(t). The variation of the glass-like anomaly with compositional disorder is also discus…
Structure of bottle-brush brushes under good solvent conditions: a molecular dynamics study.
2011
We report a simulation study for bottle-brush polymers grafted on a rigid backbone. Using a standard coarse-grained bead-spring model extensive molecular dynamics simulations for such macromolecules under good solvent conditions are performed. We consider a broad range of parameters and present numerical results for the monomer density profile, density of the untethered ends of the grafted flexible backbones and the correlation function describing the range that neighboring grafted bottle-brushes are affected by the presence of the others due to the excluded volume interactions. The end beads of the flexible backbones of the grafted bottle-brushes do not access the region close to the rigid…
Friction between Polymer Brushes in Good Solvent Conditions: Steady-State Sliding versus Transient Behavior
2003
Previous molecular dynamics simulations of friction between polymer brushes in relative sliding motion [Kreer, T.; Muser, M. H.; Binder, K.; Klein, J. Langmuir 2001, 17, 7804] are extended beyond steady-state conditions. We study two different protocols: (i) stop and return and (ii) stop and go. In protocol (i), the relative, lateral motion between the two surfaces is stopped abruptly and reimposed opposite to the initial direction after the system could relax for some time. Protocol (ii) is similar except that the sliding direction is maintained. In the constant-velocity steady state, the average lateral extension lc of the polymers is found to be a power law of the sliding velocity v, na…
Correlations of the nonexponentiality and state dependence of mechanical relaxations with bond connectivity in Ge-As-Se supercooled liquids
1992
We have studied the mechanical responses of supercooled Ge-As-Se liquids to flexural strains and temperature steps. The departures from exponential relaxation correlate well with the variations in connectivity. The structural state dependence of the mechanical relaxation, detected in pure and weakly cross-linked Se, is suppressed completely at the rigidity percolation threshold {l angle}{ital r}{sub {ital c}}{r angle}, where the liquid fragility is a minimum. The shapes of the decay functions of samples with the same {l angle}{ital r}{sub {ital c}}{r angle} but different compositions are not universal at {ital T}{sub {ital g}} probably because of chemical effects near the binary edges of th…
Coil-Globule Collapse of Polystyrene Chains in Tetrahydrofuran-Water Mixtures.
2018
We study the coil and globule states of a single polymer chain in solution by performing molecular dynamics simulations with a united atom model. Specifically, we characterize the structural properties of atactic polystyrene chains with N = 20–150 monomers in tetrahydrofuran–water mixtures at varying mixing ratios. We find that the hydrophobic polymers form rather open coils when the mole fraction of water, XW, is roughly below 0.25, whereas the chains collapse into globules when XW ≳ 0.75. We confirm the theoretically expected scaling laws for the radius of gyration, Rg, in these regimes, i.e., Rg ∝ N3/5 and Rg ∝ N1/3 for good and poor solvent conditions, respectively. For poor solvent con…
Solidification behavior of the theta system 2-propanol/poly(n-butyl methacrylate)
1995
Thermoreversible gelation of the system 2-propanol/poly (n-butyl methacrylate) — as detected by D'SC or dielectric experiments — does not manifest itself in a straightforward manner in the dynamic-mechanical properties. Its occurrence can, however, be seen in many ways: i) For constant composition of the system and a reference temperature lower than Tgel, the storage modulus G′ is larger than the loss modulus G″ in the glass transition zone of the master curve and both vary in an almost parallel manner with the angular frequency ω over almost two decades (whereas this feature is normally found for other gelling systems within the rubber plateau or the flow region). ii) The entanglement mole…