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The adhesion between a polymer melt and substrate is studied in the presence of chemically attached chains on the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of temperature, tethered chain areal density ($\Sigma$), tethered chain length ($N_{t}$), chain bending energy ($k_{\theta}$) and tensile pull velocity ($v$) 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 $N_{t}$ is increased. Below the glass transition, the value of $N_{t}$ for which this crossover begins approaches the bulk entanglement length $N_{e}$. For the values of $N_{t}$ and $\Sigma$ used here, no crossover to crazing is observed., Comment: To be published in Macromolecules. 18 pages of text and 12 GIF figures

We have performed a systematic, large-scale simulation study of granular media in two- and three-dimensions, investigating the rheology of cohesionless granular particles in inclined plane geometries, i.e., chute flows. We find that over a wide range of parameter space of interaction coefficients and inclination angles, a steady state flow regime exists in which the energy input from gravity balances that dissipated from friction and inelastic collisions. In this regime, the bulk packing fraction (away from the top free surface and the bottom plate boundary) remains constant as a function of depth z, of the pile. The velocity profile in the direction of flow v(z) scales with height of the pile H, according to v(z) proportional to H^a, with a=1.52. However, the behavior of the normal stresses indicates that existing simple theories of granular flow do not capture all of the features evidenced in the simulations., Comment: 17 pages, 20 figures, submitted to Phys. Rev. E

In this paper we investigate the relationship between glassy and ferromagnetic phases in disordered Ising ferromagnets in the presence of transverse magnetic fields, $\Gamma$. Iterative mean field simulations probe the free energy landscape and suggest the existence of a glass transition as a function of $\Gamma$ which is distinct from the Curie temperature. New experimental field-cooled and zero-field-cooled data on LiHo$_x$Y$_{1-x}$F$_4$ provide support for our theoretical picture., Comment: 4 pages RevTex; 5 figures

We report and analyze the results of numerical studies of dense granular flows in two and three dimensions, using both linear damped springs and Hertzian force laws between particles. Chute flow generically produces a constant density profile that satisfies scaling relations suggestive of a Bagnold grain inertia regime. The type of force law has little impact on the behavior of the system. Bulk and surface flows differ in their failure criteria and flow rheology, as evidenced by the change in principal stress directions near the surface. Surface-only flows are not observed in this geometry., Comment: 4 pages, RevTeX 3.0, 4 PostScript figures (5 files) embedded with epsf

The non-linear stress-strain relation for crosslinked polymer networks is studied using molecular dynamics simulations. Previously we demonstrated the importance of trapped entanglements in determining the elastic and relaxational properties of networks. Here we present new results for the stress versus strain for both dry and swollen networks. Models which limit the fluctuations of the network strands like the tube model are shown to describe the stress for both elongation and compression. For swollen networks, the total modulus is found to decrease like (V_0/V)^{2/3} and goes to the phantom model result only for short strand networks., Comment: 9 pages, 3 figures, RevTeX

Evidence for capillary waves at a liquid/vapor interface are presented from extensive molecular dynamics simulations of a system containing up to 1.24 million Lennard-Jones particles. Careful measurements show that the total interfacial width depends logarithmically on $L_\parallel$, the length of the simulation cell parallel to the interface, as predicted theoretically. The strength of the divergence of the interfacial width on $L_\parallel$ depends inversely on the surface tension $\gamma$. This allows us to measure $\gamma$ two ways since $\gamma$ can also be obtained from the difference in the pressure parallel and perpendicular to the interface. These two independent measures of $\gamma$ agree provided that the interfacial order parameter profile is fit to an error function and not a hyperbolic tangent, as often assumed. We explore why these two common fitting functions give different results for $\gamma$.

We present results of molecular dynamics simulations of very long model polymer chains analyzed by various experimentally relevant techniques. The segment motion of the chains is found to be in very good agreement with the repatation model. We also calculated the plateau-modulus G_N. The predicitions of the entanglement length N_e from G_N and from the mean square displacements of the chains segments disagree by a factor of about 2.2(2), indicating an error in the prefactor in the standard formula for G_N. We show that recent neutron spin echo measurements were carried out for chain lengths which are too small for a correct determination of N_e., Comment: 5 pages, 4 figures, RevTex

The crossover to Rouse-like behavior for the self-diffusion constant D, the viscosity $\eta$, and the equilibrium structural statistics of n-alkanes $(6 \le n \le 66)$ is studied numerically. For small n the chains are non-Gaussian and the mean squared end-to-end distance $R^2$ is greater than $R_G^2$, where $R_g^2$ is the mean squared radius of gyration. As n increases, $R^2/R_G^2 \to 6(1+b/n)$, where b depends on the interaction model. At constant density, the Rouse model is used to extract the monomeric friction coefficient $\zeta$ and the viscosity $\eta$ independently from the diffusion constant D and the longest relaxation time $\tau_R$. $\zeta_D$ extracted from D is nearly independent of chain length while $\zeta_\tau$ obtained from $\tau_R$ is much larger than $\zeta_D$ for small n. The viscosity measured in a non-equilibrium molecular dynamics simulation is closely approximated by the value of $\eta$ determined from $\tau_R$ while $\eta$ inferred from D is smaller for small n. For $n\agt 60$, the two estimates for both $\zeta$ and $\eta$ agree as predicted from the Rouse model. D calculated from three interaction models is studied for increasing $n$ and compared to experimental data., Comment: 21 pages, 8 postscript figures, uses revtex

A continuum-space bead-spring model is used to study the phase behavior of binary blends of homopolymers and the structure of the interface between the two immiscible phases. The structure of the interface is investigated as a function of immiscibility, chain length, and system size. Capillary waves are observed and their measurement allows us to determine the surface tension $\gamma$. We propose a more universal method of measuring the interfacial width in terms of second moments of the different contributions to the first derivative of the interfacial profile. Predictions of this method are directly verified. The effect of chain length on the surface tension is also studied., Comment: 5 pages, 5 figures

Diffusion of symmetric diblock copolymer chains in macroscopically oriented lamellar block copolymers are studied in a molecular dynamics simulation. Results for diffusion constant both parallel $D_\parallel$ and perpendicular $D_\perp$ to the lamellar planes are compared to results in the disordered one phase region. For diblocks of length N=40 (1.1 N_e) and $100 (3N_e)$, where $N_e$ is the entanglement length of a homopolymer melt at the same density, $D_\parallel$ is nearly independent of $\chi$, while $D_\perp$ is strongly suppressed as $\chi$ is increased. These results are in agreement with theoretical predictions based on the Rouse model. The isotropic diffusion constant of quenched disordered systems is approximated fairly well by $(2D_\parallel+D_\perp)/3$ of the corresponding lamellar system., Comment: Revtex 12 pages, 4 postscript figures, uses psfig

We investigate the elastic properties of small droplets under compression. The compression of a bubble by two parallel plates is solved exactly and it is shown that a lowest-order expansion of the solution reduces to a form similar to that obtained by Morse and Witten. Other systems are studied numerically and results for configurations involving between 2 and 20 compressing planes are presented. It is found that the response to compression depends on the number of planes. The shear modulus is also calculated for common lattices and the stability crossover between f.c.c.\ and b.c.c.\ is discussed., Comment: RevTeX with psfig-included figures and a galley macro

We present a new model to describe the unusual elastic properties of compressed emulsions. The response of a single droplet under compression is investigated numerically for different Wigner-Seitz cells. The response is softer than harmonic, and depends on the coordination number of the droplet. Using these results, we propose a new effective inter-droplet potential which is used to determine the elastic response of a monodisperse collection of disordered droplets as a function of volume fraction. Our results are in excellent agreement with recent experiments. This suggests that anharmonicity, together with disorder, are responsible for the quasi-linear increase of $G$ and $\Pi$ observed at $\varphi_c$., Comment: RevTeX with psfig-included figures and a galley macro

We present a Monte Carlo simulation of a polymer nematic for varying volume fractions, concentrating on the structure function of the sample. We achieve nematic ordering with stiff polymers made of spherical monomers that would otherwise not form a nematic state. Our results are in good qualitative agreement with theoretical and experimental predictions, most notably the bowtie pattern in the static structure function., Comment: 10 pages, plain TeX, macros included, 3 figures available from archive. Published version

We examine entanglements using monomer contacts between pairs of chains in a Brownian-dynamics simulation of a polymer melt. A map of contact positions with respect to the contacting monomer numbers (i,j) shows clustering in small regions of (i,j) which persists in time, as expected for entanglements. Using the ``space''-time correlation function of the aforementioned contacts, we show that a pair of entangled chains exhibits a qualitatively different behavior than a pair of distant chains when brought together. Quantitatively, about 50% of the contacts between entangled chains are persistent contacts not present in independently moving chains. In addition, we account for several observed scaling properties of the contact correlation function., Comment: latex, 12 pages, 7 figures, postscript file available at http://arnold.uchicago.edu/~ebn/

Simulations of the structure and dynamics of fluid films confined to a thickness of a few molecular diameters are described. Confining walls introduce layering and in-plane order in the adjacent fluid. The latter is essential to transfer of shear stress. As the film thickness is decreased, by increasing pressure or decreasing the number of molecular layers, the entire film may undergo a phase transition. Spherical molecules tend to crystallize, while short chain molecules enter a glassy state with strong local orientational and translational order. These phase transitions lead to dramatic changes in the response of the film to imposed shear velocities $v$. Spherical molecules show an abrupt transition from Newtonian response to a yield stress as they crystallize. Chain molecules exhibit a continuously growing regime of non-Newtonian behavior where the shear viscosity drops as $v^{-2/3}$ at constant normal load. The same power law is found for a wide range of parameters, and extends to lower and lower velocities as a glass transition is approached. Once in the glassy state, chain molecules exhibit a finite yield stress. Shear may occur either within the film or at the film/wall interface. Interfacial shear dominates when films become glassy and when the film viscosity is increased by increasing the chain length., Comment: Latex, 17 figures, 30 pages Israel Journal of Chemistry (1995)

We examine two aspects of Stockmayer fluids which consists of point dipoles that additionally interact via an attractive Lennard-Jones potential. We perform Monte Carlo simulations to examine the effect of an applied field on the liquid-gas phase coexistence and show that a magnetic fluid phase does exist in the absence of an applied field. As part of the search for the magnetic fluid phase, we perform Gibbs ensemble simulations to determine phase coexistence curves at large dipole moments, $\mu$. The critical temperature is found to depend linearly on $\mu^2$ for intermediate values of $\mu$ beyond the initial nonlinear behavior near $\mu=0$ and less than the $\mu$ where no liquid-gas phase coexistence has been found. For phase coexistence in an applied field, the critical temperatures as a function of the applied field for two different $\mu$ are mapped onto a single curve. The critical densities hardly change as a function of applied field. We also verify that in an applied field the liquid droplets within the two phase coexistence region become elongated in the direction of the field., Comment: 23 pages, ReVTeX, 7 figures