Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J4: Dynamics of Polymers on Multi-Length Scales: Melts |
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Sponsoring Units: DPOLY Chair: Anna Balazs, University of Pittsburgh Room: Oregon Ballroom 204 |
Tuesday, March 16, 2010 11:15AM - 11:51AM |
J4.00001: Making coarse grained polymer simulations quantitatively predictive for statics and dynamics Invited Speaker: By combining input from short simulation runs of rather small systems with all atomistic details together with properly adapted coarse grained models we are able quantitatively predict static and especially dynamical properties of both pure polymer melts of long fully entangled but also of systems with low molecular weight additives. Comparisons to rather different experiments such as diffusion constant measurements or NMR relaxation experiments show a remarkable quantitative agreement without any adjustable parameter. Reintroduction of chemical details into the coarse grained trajectories allows the study of long time trajectories in all atomistic detail providing the opportunity for rather different means of data analysis. References: V. Harmandaris, K. Kremer, Macromolecules, in press (2009) V. Harmandaris et al, Macromolecules, 40, 7026 (2007) B. Hess, S. Leon, N. van der Vegt, K. Kremer, Soft Matter 2, 409 (2006) D. Fritz et al, Soft Matter 5, 4556 (2009) [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:27PM |
J4.00002: Interdiffusion and Self-Healing of Entangled Polymer Melts Invited Speaker: The interdiffusion of two polymer films above the glass transition is the simplest way to weld many polymeric materials and is important for self-healing of polymer films which have been torn or damaged. To understand this process better we present molecular dynamics simulations of the interpenetration of two identical homopolymer films described by a bead spring model for the chains with $N=25$ to $500$ beads/chain. As the entanglement length $N_e ~ 70$, $N$ spans the range from unentangled to highly entangled polymers. For unentangled polymers the mass uptake and penetration depth increase with time $t$ as $t^{1/4}$ for early times crossing over to $t^{1/2}$ for $t>\tau_R$ where $\tau_R$ is the Rouse time. For $t>\tau_R$, the mass density profiles are well described by an erf function consistent with classical Fickian diffusion. Entangled polymers also show an extended, early time $t^{1/4}$ scaling for the mass uptake. Unlike the dynamics in a polymer melt which is dominated by the reptation motion of the monomers in the middle of the chain, interdiffusion is found to be dominated by motion of the chain ends, which are known to follow Rouse dynamics. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 1:03PM |
J4.00003: On the dynamics of polymers in nanocomposites and under confinement Invited Speaker: Confinement effects in polymer melts may lead to unusual properties. This concerns both, the chain conformation as well as chain dynamics that may be altered due to the surface interactions and changes of topology. Today microscopic studies on the chain level are rare and most of what we know comes from simulations, while experiments addressed mainly macroscopic phenomena. In my presentation I will display neutron scattering data, addressing length and time scales from the single monomer to the entanglement network and beyond. These experiments reveal the basic relaxation rates related to monomeric friction, the intermediate scale Rouse dynamics as well as the entanglement controlled dynamics. Polymer nanocomposites have been investigated at various compositions using filler particles smaller and larger than the polymer size. I will discuss the effects of the filler size and concentration on the polymer conformation as well as on the dynamics on the various important length scales. The effect of confinement was also studied on well defined porous alumina samples which were filled with polyethylene oxide (PEO). Thereby the chain dimensions were much larger or smaller than the lateral pore size $D$. While for the long chains an expanded entanglement network is observed, the confinement seems to have a weaker effect on the short chains. In particular we do not observe any corset effect as proposed by NMR relaxometry. As compared to bulk PEO a moderate slowing down in the intermediate time Rouse regime was noticed that other than suggested by simulation is not related to a general increase of the Rouse friction -- the local dynamics at short times was found to be largely unchanged. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:39PM |
J4.00004: From Structure to Dynamics of Polymer Melts Invited Speaker: |
Tuesday, March 16, 2010 1:39PM - 2:15PM |
J4.00005: Rheology of Entangled Polymer Melts: Recent Results from Molecular Dynamics Simulations Invited Speaker: Models for the rheology of entangled polymers, based on the ``tube" model are now open to investigation by molecular dynamics simulations using the Kremer-Grest ``pearl necklace" model of polymers. Here, we present extensive molecular dynamics simulations of the dynamics and stress in entangled melts of branched polymers and of ``binary blends" of diluted long probe chains entangled with a matrix of shorter chains. Direct evidence of ``hierarchical relaxation" is obtained in diffusion of asymmetric star polymers, wherein the rate of slow diffusion of the branch point is controlled by the much faster motion of the attached arm. In studies of binary blends, the ratio of their lengths is varied over a wide range to cover the crossover from the chain reptation regime to tube Rouse motion regime of the long probe chains. Reducing the matrix chain length results in a faster decay of the dynamic structure factor of the probe chains, in good agreement with recent Neutron Spin Echo experiments. The diffusion of the long chains, measured by the mean square displacements of the monomers and the centers of mass of the chains, demonstrates a systematic speed-up relative to the pure reptation behavior expected for monodisperse melts of sufficiently long polymers. On the other hand, the diffusion of the matrix chains is only weakly perturbed by the diluted long probe chains. The simulation results are qualitatively consistent with the theoretical predictions based on constraint release Rouse model, but a detailed comparison reveals the existence of a broad distribution of the disentanglement rates, which is partly confirmed by an analysis of the packing and diffusion of the matrix chains in the tube region of the probe chains. A coarse-grained simulation model based on the tube Rouse motion model with incorporation of the probability distribution of the tube segment jump rates is developed and shows results qualitatively consistent with the fine scale molecular dynamics simulations. [Preview Abstract] |
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