Session W18 - Computer Simulation II.
ORAL session, Thursday morning, March 15
Room 308-308, Washington State Convention Center

[W18.001] Effects of tethered chains on adhesion

We study adhesion between a polymer melt and substrate due to chemically attached polymer chains on the substrate surface. We have performed extensive molecular dynamics simulations to study the effect of temperature, crosslink density, tethered chain density (\Sigma), tethered chain length (N_t), tensile pull velocity (v) and chain stiffness on the adhesive failure mechanisms of pullout and/or scission of the tethered chains. We observe a crossover from pure chain pullout to chain scission as N_t and v are increased. The value of N_t at which this crossover occurs is comparable to the chain entanglement length for the coarse-grained model used. Experiments and simulations have shown that the energy required to separate a polymer melt from a substrate increases considerably if the formation of large voids, or crazing can be initiated in the melt. The onset of crazing depends on the temperature and the interaction strength of the substrate with the melt. We also present data illustrating the additional effects of tethered chains on crazing mechanisms.

[W18.002] Stretching and Relaxation of Polymer Molecules: A Molecular Dynamics Study with an Explicit Fluid

We have studied a variety of polymer stretching problems using equilibrium and non-equilibrium Molecular Dynamics (MD) simulations with explicit solvent. These problems include the stretching of polymer chains in strong flows, the effects of confinement on hydrodynamic drag, the difference between the stress and the strain ensemble when pulling the two ends of a linear chain apart, and the transition from the freely-jointed chain (FJC) to the worm-like chain. Our results disagree with the predictions of the FJC models for flow-induced stretching, but not for mechanical stretching. We also demonstrate that an important delay in hydrodynamic interactions exists that is not considered by any current model. Finally, we observe a short-time scale ballistic regime for the free relaxation of stretched chains.

[W18.003] Theoretical Analysis of Hydrogen Bonding and Behavior of PEO in Aqueous Solutions

Hydrogen bonding in aqueous solutions of polyethylene oxide (PEO) is studied analytically in the framework of a statistical mean-field model. PEO-water and water-water hydrogen bonding turn out to compete with each other as they both involve water hydrogen as a donor. The temperature and composition dependence of the average degree of association and the second virial coefficient are analyzed and compared with experimental and computer simulation data. In the most cases the results agree reasonably well with the experimental observations. Being dependent on the average degree of association, the phase behavior of aqueous solutions of PEO exhibits unusual (for non-hydrogen bonding polymers) properties including both upper and lower critical solution temperature phenomena (UCST and LCST).

[W18.004] Free Energy Self-Averaging in Protein-Sized Random Heteropolymers

Current theories of heteropolymers are inherently macroscopic, but are applied to folding proteins which are only mesoscopic. In particular, theoretically one computes the averaged free energy over sequences, always assuming that it is self-averaging -- a property well-established only if a system with quenched disorder is macroscopic. By enumerating the states and energies of compact 18mers, 27mers, and 36mers on a simplified lattice model with an ensemble of random sequences, we test the validity of the self-averaging approximation. We find that fluctuations in the free energy between sequences are weak, and that self-averaging is a valid approximation at the length scale of real proteins. These results validate simple sequence design methods which rely on self-averaging, exponentially reducing the time needed for computational design as well as greatly simplifying experimental realizations.

[W18.005] Cluster Lifetime and Heterogeneity in a Glass-forming Liquids

Cluster behavior and dynamic heterogeneity in glass-forming liquids have been reported in the past. One of the key properties characterizing the cluster and dynamic heterogeneity is the cluster lifetime. Using extensive molecular dynamics simulation, we investigated the dynamic properties of the clusters in a binary Lennard-Jones glass-forming liquid. The cluster lifetime, t(s), where s is the cluster size, is found to increase with time as the liquid is cooled progressively toward the glass transition temperature Tg. The solidlike, amorphous clusters also show percolating characteristics close to Tg, indicating the spatial inhomogeneity of the liquid at the glass transition.

[W18.006] Calculating the Toughness of Glassy Polymers from Atomic Scale Simulations

The toughness G is the energy per unit area needed to rupture an adhesive bond, and represents the resistance to crack propagation. Most of the toughness of polymer glasses comes from growth of a micron thick craze zone around the crack tip. We present molecular dynamics simulations with a simple bead-spring model of entangled polymers that allow the toughness to be calculated using a fracture model proposed by H. R. Brown (Macromol. 24, 2752 (1990)). Polymer is deformed from a dense initial state into a craze network at a constant plateau stress. Calculations of the anisotropic elastic moduli of the craze are used to determine the stress intensity factor at the crack tip. The craze stops thickening when the stress at the crack tip reaches the maximum stress that the craze can withstand before chain scission or disentanglement causes the crack to advance. The competition between these processes is studied as a function of chain length. Combining our results in Brown's model gives values for the craze thickness and G that are consistent with experimental results.

[W18.007] Plastic deformation and yielding of amorphous polymer glasses

The mechanical properties of polymer glasses up to and beyond the yield point are studied with molecular dynamics simulations. A simple bead-spring model is employed, in which entangled flexible polymer chains form an isotropic glassy state upon cooling from the melt. Uni-, bi- and triaxial stress states are imposed on the solid, thereby exploring the effect of stress mixity. The mechanical response of the material is monitored for a wide range of strain rates and temperatures below the glass transition temperature. The yield stress under uniaxial shear deformation may be described with conventional transition state (Eyring) theory. For biaxial loading, a generalized von Mises yield criterion applies. Triaxial stress states change the mode of failure from shear deformation to cavitation and subsequent craze formation. Simulation results are compared to existing models and typical experimental results.

[W18.008] Local friction in polyolefins - a small-scale simulation approach

Processes on different length scales affect the dynamics of chain molecules. A convenient measure for small-scale effects is the (monomeric) friction coefficient, which is inversely proportional to the mobility of individual chain segments. Local friction in polymers depends on small-scale chain properties such as local architecture and flexibility and on the local environment of the chain segments. In polymer melts, the density is the important environmental variable. In mixtures of polymers, the local concentration will also play a role. In this work, we investigate local friction in dense polymeric fluids with the aid of a small-scale simulation approach. By evaluating exact enumeration results for two short chain sections on a lattice in conjunction with an equation of state, we are able to make predictions about the variation of segmental mobility with local chain architecture, flexibility, and thermodynamic state (temperature, pressure, composition). We apply the approach here to polyolefins and compare our predictions with experimental data.

[W18.009] A Crossover Behavior between the Diffusion Coefficients of Linear and Cyclic Alkanes

Monte Carlo simulations of linear and cyclic alkanes were performed on a coarse-grained high coordination lattice. The simulations were performed at 473 K for carbon numbers of 60, 100, and 316. The results indicated: (i) at low molecular weights (M), cyclic alkanes have lower diffusion coefficients (D) than linear alkanes, and (ii) at high M, they have higher D than linear alkanes. The lower D of the small cyclic alkanes was attributed to the high local density due within the volume defined by the smaller mean square radius of gyration of the cyclic alkanes. The high local density of cyclic alkane segments resulted in a decrease in the mobility of the beads. The crossover in D was observed around the entanglement weight of linear alkanes, which suggests that the linear alkanes are more susceptible to the effects of entanglements than are the cyclic alkanes.

[W18.010] Local structure of a polymer melt and the glass transition

The absence of significant changes in structural properties of glass forming liquids and polymer melts on cooling, while dynamic properties change by many orders of magnitude, is one underlying mystery of the glass transition. In this talk, we aim to better elucidate the structural changes of a simulated polymer melt on cooling using a Voronoi analysis of the local structure. Preliminary results indicate that the local structural changes on cooling can be related to the glass transition temperature. Additionally, we show that the local structure plays an important role for single monomer motion. This local structural heterogeneity is also reflected in the potential energy, as anticipated by previous works.

[W18.011] Spatially correlated dynamics in a simulated glass-forming polymer melt

We present a detailed analysis of cooperative motion and dynamical heterogeneity in a simulated bead-spring model of a low molecular weight polymer melt. We investigate the transient nature and size distribution of clusters of mobile monomers at temperatures T above and approaching the glass transition. We show that the mean cluster size exhibits a time dependent behavior with a peak at intermediate time. The timescale of the peak corresponds to the timescale of the end of the ``caging'' regime. The mean cluster size at the peak time grows with decreasing T. The growing size of clusters underlies the growing range of correlated motion previously reported for this same system (Bennemann, et al., Nature 399, 246 (1999)). We quantify the range of correlation by investigating the time and temperature dependence of the characteristic size and radius of the clusters. The distribution of cluster sizes is found to approach a power law as T decreases with an exponent near 2, similar to behavior reported for a dense colloidal suspension and a simulated binary mixture, demonstrating a potentially universal feature of the dynamically heterogeneous nature of glass-forming liquids.

[W18.012] Molecular Dynamics Simulations of Polymer Bulk and Surface Properties

Simulations of more than a dozen polymer chemistries have been performed using the COMPASS force field (Molecular Simulations, Inc.). Both glassy and molten polymers were included in the study. Simulated densities match experimental values within 4containing fluorine (polytetrafluroethylene and polyvinylidenefluoride). Cohesive energy densities agree with the range of reported experimental values. Interfaces with vacuum were created and the surface energies of the models were calculated from the components of the pressure tensor. For the polymers which are well above their glass transitions, surface tensions are in good agreement with experimental results; however, large fluctuations in the pressure tensor prohibit estimation of the surface energies of glassy polymers. Some alternate routes to calculating the surface energies of glassy polymers will be discussed. Some simulation results will be briefly compared with contact angle measurements.

Part W of program listing