Rigidity of disordered networks with bond-bending forces.

At zero temperature, the elastic constants of a disordered network with bond-bending forces vanish at the geometric percolation point p(c), in contrast to networks with only central forces which lose the ability to withstand shear at a rigidity percolation point p(r). Moreover, the critical behavior of the modulus is different in the two cases. I report on extensive molecular dynamics simulations on a model system with central and bond-bending forces between the monomers over a range of temperatures T.

Critical behavior of entropic shear rigidity.

We report on extensive molecular dynamics (MD) simulations of a model for gels in both two and three dimensions. The model consists of randomly cross-linked monomers with a concentration p of cross-links above the percolation concentration so that the system is in the amorphous solid phase. As the concentration of cross-links approaches the percolation concentration, the entropic shear modulus vanishes as G approximately(p-p(c))(t) with t approximately 1.

Single-polymer Brownian motor: a simulation study.

Numerical simulation is used to study a single polymer chain in a flashing ratchet potential to determine how the mechanism of this Brownian motor system is affected by the presence of internal degrees of freedom. The polymer is modeled by a freely jointed chain with N monomers in which the monomers interact via a repulsive Lennard-Jones potential and neighboring monomers on the chain are connected by finite extensible nonlinear elastic bonds. Each monomer is acted upon by a 1D asymmetric, piecewise linear potential of spatial period L comparable to the radius of gyration of the polymer.

Rigidity transition in polymer melts with van der Waals interaction.

We study the onset of rigidity near the glass transition (GT) in a short-chain polymer melt modelled by a bead-spring model, where all beads interact with Lennard-Jones potentials. The properties of the system are examined above and below the GT. In order to minimize high-cooling-rate effects and computational times, equilibrium configurations are reached via isothermal compression.

Transport properties of incipient gels.

We investigate the behavior of the shear viscosity eta(p) and the mass-dependent diffusion coefficient D(m,p) in the context of a simple model that, as the cross link density p is increased, undergoes a continuous transition from a fluid to a gel. The shear viscosity diverges at the gel point according to eta(p) approximately (p(c)-p)(-s) with s approximately 0.65.

Model for gelation with explicit solvent effects: structure and dynamics.

We study a two-component model for gelation consisting of f-functional monomers (the gel) and inert particles (the solvent). After equilibration as a simple liquid, the gel particles are gradually cross linked to each other until the desired number of cross links have been attained. At a critical cross-link density, the largest gel cluster percolates and an amorphous solid forms.