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Vlasis G Mavrantzas - One of the best experts on this subject based on the ideXlab platform.

  • detailed atomistic simulation of a Polymer Melt solid interface structure density and conformation of a thin film of polyethylene Melt adsorbed on graphite
    Macromolecules, 2005
    Co-Authors: Kostas Ch Daoulas, Vagelis Harmandaris, Vlasis G Mavrantzas
    Abstract:

    An atomistic modeling approach is presented for simulating the interface between a Polymer Melt and a crystalline solid substrate. As a test case, a thin film of polyethylene (PE) Melt confined between a semiinfinite graphite phase on the one side and vacuum on the other is considered. The simulation is carried out in the NPT statistical ensemble with an efficient Monte Carlo (MC) algorithm based on state-of-the-art variable connectivity moves. The atomistic simulations are conducted by describing the PE chains with a united atom model, which considers each methylene (CH2) and methyl (CH3) group along the chain backbone as single interaction sites. To calculate the potential energy of interaction between Polymer atoms and the semiinfinite graphite substrate, the method designed by Steele was implemented, capable of incorporating the exact crystallographic structure of graphite. The new approach has allowed us to analyze structural and conformational properties on the length scale of just a few angstroms f...

  • molecular dynamics simulation of a Polymer Melt solid interface local dynamics and chain mobility in a thin film of polyethylene Melt adsorbed on graphite
    Macromolecules, 2005
    Co-Authors: Vagelis Harmandaris, And Kostas Ch Daoulas, Vlasis G Mavrantzas
    Abstract:

    Molecular dynamics (MD) simulations have been performed on a dense Polymer Melt adsorbed on a solid substrate on the one side and exposed to vacuum on the other. As a model system, a thin film of polyethylene (PE) Melt supported by a crystalline graphite phase on its one side (the other surface of the film is free) has been examined. Most simulations have been carried out with unentangled PE Melt systems, such as C78 and C156, in the NPT statistical ensemble at T = 450 K and P = 0 atm for times up to 100 ns, using a multiple-time step MD algorithm and by incorporating the correct dependence of the long-range contribution to the energy and stress tensor on the density profile. To increase the statistical accuracy of the results, large systems have been employed in the MD simulations, such as a 200-chain C78 Melt consisting of 15 600 carbon atoms. The MD simulation data have been analyzed to provide information about the spatial dependence of the short-time dynamical properties (conformational relaxation) o...

Emmanuel P Giannelis - One of the best experts on this subject based on the ideXlab platform.

  • Polymer Melt intercalation in organically modified layered silicates model predictions and experiment
    Macromolecules, 1997
    Co-Authors: Richard A Vaia, Emmanuel P Giannelis
    Abstract:

    The effect of silicate functionalization, anneal temperature, Polymer molecular weight, and constituent interactions on Polymer Melt intercalation of a variety of styrene-derivative Polymers in alkylammonium-functionalized silicates is examined. Hybrid formation requires an optimal interlayer structure for the organically-modified layered silicate (OLS), with respect to the number per host area and size of the alkylammonium chains, as well as the presence of polar interactions between the OLS and Polymer. From these observations and the qualitative predictions of the mean-field lattice-based model of Polymer Melt intercalation (preceding paper in this issue), general guidelines may be established for selecting potentially compatible Polymer−OLS systems. The interlayer structure of the OLS should be optimized to maximize the configurational freedom of the functionalizing chains upon layer separation while maximizing potential interaction sites with the surface. The most successful Polymers for intercalatio...

  • lattice model of Polymer Melt intercalation in organically modified layered silicates
    Macromolecules, 1997
    Co-Authors: Richard A Vaia, Emmanuel P Giannelis
    Abstract:

    A mean-field, lattice-based model of Polymer Melt intercalation in organically-modified mica-type silicates (OLS) has been developed. In general, an interplay of entropic and energetic factors determines the outcome of Polymer intercalation. Free energy curves and their dependence on energetic and entropic factors suggest three possible equilibrium statesimmiscible, intercalated, and exfoliatedall of which have been experimentally observed. The entropic penalty of Polymer confinement may be compensated for by the increased conformational freedom of the surfactant chain as the layers separate. When the total entropy change is small, small changes in the system's internal energy will determine if intercalation is thermodynamically possible. Complete layer separation, though, depends on the establishment of very favorable Polymer−OLS interactions to overcome the penalty of Polymer confinement. For alkylammonium-modified layered silicates, a favorable energy change is accentuated by maximizing the magnitude a...

  • kinetics of Polymer Melt intercalation
    Macromolecules, 1995
    Co-Authors: Richard A Vaia, Klaus D Jandt, Edward J Kramer, Emmanuel P Giannelis
    Abstract:

    the kinetics of polystyrene Melt intercalation in organically modified mica-type silicates were studied using X-ray diffraction and transmission electron microscopy. By monitoring the change in the integrated intensity of the basal reflection of the silicate host, the rate of conversion from unintercalated to intercalated silicate was determined at various temperatures and for various molecular weights of polystyrene. Hybrid formation is limited by mass transport into the primary particles of the host silicate and not specifically by diffusion of the Polymer chains within the silicate galleries. The activation energy of hybrid formation is similar to that previously measured for polystyrene self-diffusion in the Melt, implying that the mobility of the Polymer chains within the host galleries is at least comparable to that in the Melt.

Liliane Leger - One of the best experts on this subject based on the ideXlab platform.

  • slip transition of a Polymer Melt under shear stress
    Physical Review Letters, 1993
    Co-Authors: Kalman B Migler, H Hervet, Liliane Leger
    Abstract:

    We present the first direct measurements of the local velocity of a sheared Polymer Melt within the first 100 nm from the solid-liquid interface. For high enough shear rates we observe a sharp transition between weak and strong slip (i.e., a nonzero boundary fluid velocity) in the case of weak Polymer-surface interactions [polydimethylsiloxane (PDMS) on silanated silica surfaces]. For strong Polymersurface interactions the slip is strongly reduced. These results are compared to a theoretical model recently proposed by Brochard and de Gennes

W. Paul - One of the best experts on this subject based on the ideXlab platform.

  • Temperature dependent micro-rheology of a glass-forming Polymer Melt studied by molecular dynamics simulation
    Journal of Chemical Physics, 2014
    Co-Authors: A. Kuhnhold, W. Paul
    Abstract:

    We present a Molecular Dynamics simulation study of a micro-rheological probing of the glass transition in a Polymer Melt. Our model system consists of short bead-spring chains and the temperature ranges from well above the glass transition temperature to about 10% above it. The nano-particle clearly couples to the slowing down of the Polymer segments and the calculated storage and loss moduli reveal the approach to the glass transition. At temperatures close to the mode coupling T c of the Polymer Melt, the micro-rheological moduli measure the local viscoelastic response of the cage of monomers surrounding the nano-particle and no longer reveal the true Melt moduli. The incoherent scattering function of the nano-particle exhibits a stretched exponential decay, typical for the α-process in glass forming systems. We find no indication of a strong superdiffusive regime as has been deduced from a recent experiment in the same temperature range but for smaller momentum transfers.

  • molecular dynamics simulation of a glassy Polymer Melt rouse model and cage effect
    Computational and Theoretical Polymer Science, 1999
    Co-Authors: C Bennemann, Jorg Baschnagel, W. Paul, K Binder
    Abstract:

    Abstract We report results of molecular-dynamics simulations for a glassy Polymer Melt consisting of short, linear bead-spring chains. The model does not crystallize upon cooling, but exhibits a glassy slowing down. The onset of this slowing down is brought about by the dense packing in the Melt. It was shown in an earlier work that this onset is compatible with the predictions of the mode coupling theory of the glass transition. The physical process of “caging” of a monomer by its spatial neighbors leads to a distinct two step behavior in scattering functions and particle mean square displacements. In this work, we analyze the effects of this caging process on the Rouse description of Polymer Melt dynamics. The Rouse model is known, both from experimental and simulational work, to be a reasonable description of the dynamics of short chains in the Melt. We show that the Rouse description is applicable for length and time scales above the typical scales for the caging process, and that the typical time scale of the Rouse model reflects the onset of freezing as described by mode coupling theory. The Rouse modes are eigenmodes of the chains in the supercooled state, and the relaxation times of the modes exhibit the same temperature dependence as the diffusion coefficient of the chains. The decay of the mode correlation functions is stretched and depends on the mode index. Therefore, there is no time-mode superposition of the correlation functions. However, they exhibit a time–temperature superposition at late times. At intermediate times, they decay in two steps for temperatures close to the dynamical critical temperature of mode coupling theory. The monomer displacement is compared with simulation results for a binary LJ-mixture to illustrate the differences which are introduced by the connectivity of the particles.

  • molecular dynamics simulation of a glassy Polymer Melt incoherent scattering function
    European Physical Journal B, 1999
    Co-Authors: C Bennemann, Jorg Baschnagel, W. Paul
    Abstract:

    We present simulation results for a model Polymer Melt, consisting of short, nonentangled chains, in the supercooled state. The analysis focuses on the monomer dynamics, which is monitored by the incoherent intermediate scattering function. The scattering function is recorded over six decades in time and for many different wave-vectors which range from the size of a chain to about three times the maximum position of the static structure factor. The lowest temperatures studied are slightly above Tc, the critical temperature of mode-coupling theory (MCT), where Tc was determined from a quantitative analysis of the β- and α-relaxations. We find evidence for the space-time factorization theorem in the β-relaxation regime, and for the time-temperature superposition principle in the α-regime, if the temperature is not too close to Tc. The wave-vector (q-) dependence of the nonergodicity parameter, of the critical amplitude, and the α-relaxation time are in qualitative agreement with calculations for hard spheres. For q larger than the maximum of the structure factor the α-relaxation time τq already agrees fairly well with the asymptotic MCT-prediction τq ∼ q -1/b. The behavior of the relaxation time at small q can be rationalized by the validity of the Gaussian approximation and the value of the Kohlrausch stretching exponent, as suggested in neutron-scattering experiments.

  • molecular dynamics simulation of a glassy Polymer Melt incoherent scattering function
    arXiv: Soft Condensed Matter, 1998
    Co-Authors: C Bennemann, Jorg Baschnagel, W. Paul
    Abstract:

    We present simulation results for a model Polymer Melt, consisting of short, nonentangled chains, in the supercooled state. The analysis focuses on the monomer dynamics, which is monitored by the incoherent intermediate scattering function. The scattering function is recorded over six decades in time and for many different wave-vectors. The lowest temperatures studied are slightly above the critical temperature of mode-coupling theory (MCT), which was determined from a quantitative analysis of the beta- and alpha-relaxations. We find evidence for the space-time factorization theorem in the beta-relaxation regime, and for the time-temperature superposition principle in the alpha-regime, if the temperature is not too close to the critical temperature. The wave-vector dependence of the nonergodicity parameter, of the critical amplitude, and the alpha-relaxation time are in qualitative agreement with calculations for hard spheres. For wave-vectors larger than the maximum of the structure factor the alpha-relaxation time already agrees fairly well with the asymptotic MCT-prediction. The behavior of the relaxation time at small wave-vectors can be rationalized by the validity of the Gaussian approximation and the value of the Kohlrausch stretching exponent.

Huilin Li - One of the best experts on this subject based on the ideXlab platform.

  • physical and chemical effects of ultrasound vibration on Polymer Melt in extrusion
    Ultrasonics Sonochemistry, 2010
    Co-Authors: Jinyao Chen, Yingzi Chen, Huilin Li
    Abstract:

    The physical and chemical effects of ultrasound on polypropylene (PP) Melts in extrusion were investigated. By applying ultrasound vibration to the entrance of the die, apparent pressure and viscosity of PP can be obviously decreased under the appropriate ultrasound power. Ultrasound has both physical and chemical effects on the Polymer Melt. In our study with specific Polymer and ultrasound system, we determined that the chemical effect makes up 35-40% of the total effect of ultrasound on the apparent viscosity reduction of PP Melts at most of the studied intensities. The physical effect plays a more important role in the ultrasound-applied extrusion than the chemical effect. This chemical effect is an irreversible and permanent change in molecule weight and the molecular-weight distribution due to ultrasound. As the ultrasound intensity increases, the molecular weight of PP reduces and its molecular-weight distribution becomes narrower; the orientation of PP molecules along the flow direction reduces (in Melt state) and the crystallinity of PP samples (in solid state) decreases by applying the ultrasound vibration. Ultrasound vibration increases the motion of molecular chains and makes them more disorder; it also affects the relaxation process of Polymer Melts by shortening the relaxation time of chain segments, leading to weakening the elastic effect and decreasing the extruding swell ratios. All the factors discussed above reduce the non-Newtonian flow characteristics of the Polymer Melt and result in the viscosity drop of the Polymer Melt in extrusion.

  • Mechanism for effect of ultrasound on Polymer Melt in extrusion
    Journal of Polymer Science Part B, 2007
    Co-Authors: Yingzi Chen, Huilin Li
    Abstract:

    This paper explores the rheological characteristic and molecular mechanism of the Polymer Melts during extrusion when ultrasound is applied to them at the entrance of the die. Applying of ultrasound disturbs the convergent flow of Polymer Melt in the entry zone and changes the stream patterns, which leads to lesser elastic tensile strains happen. It also activates the molecular chains, so that the elastic tensile strains can be recovered very quickly. That is why the ΔP measured is reduced. These conclusions are made based not only on rheological calculations but also on looking-inside characterizations of the molecular conformations. Ultrasound irradiation increases the free volume of PS, reduces the molecular entanglements of Polymer Melt. All these effects of ultrasound contribute to the productivity raising and product quality improvement of extrusion processing. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1226–1233, 2007