Gopinath Subramanian

Preparation of SWNT-reinforced composites by a continuous mixing process

A continuous impingement mixing process is described that has been used to disperse unpurified single-walled carbon nanotubes (SWNTs) in a Shell EPON-862/EpiCURE system. Composites with up to 0.5 wt% of nanotubes were prepared by this process. Mechanical properties of these samples were compared with those of plain material and with samples prepared by hand-mixing, and they were found to be enhanced by up to 40%. More specifically, tensile stress was increased by 40% at 5% strain in composites prepared by impingement mixing. Recurrent mixing was also found to improve SWNT dispersion. The effect of stabilizing SWNTs with polyvinyl pyrrolidone (PVP) prior to composite preparation is also reported.

On the relationship between two popular lattice models for polymer melts.

A mapping between two well known lattice bond-fluctuation models for polymers [I. Carmesin and K. Kremer, Macromolecules 21, 2819 (1988); J. S. Shaffer, J. Chem. Phys. 101, 4205 (1994)] is investigated by performing primitive path analysis to identify the average number of monomers per entanglement. Simulations conducted using both models, and previously published data are compared in an attempt to establish relationships between molecular weight, lengthscale, and timescale. Using these relationships, an examination of the self-diffusion coefficient yields excellent agreement not only between the two models, but also with experimental data on polystyrene, polybutadiene, and polydimethylsiloxane. However, it is shown that even with the limited set of criteria examined in this paper, a true mapping between these two models is elusive. Nevertheless, a practical guide to convert between models is provided.

A topology preserving method for generating equilibrated polymer melts in computer simulations

A new method for generating equilibrated configurations of polymer melts is presented. In this method, the molecular weight of an equilibrated melt of polymers is successively doubled by affinely scaling the simulation box and adding beads along the contour of the chains. At each stage of molecular weight doubling, compressive deformations are produced on all length scales, while the random walk nature of the polymers is preserved, thereby requiring relaxation times significantly smaller than the reptation time to fully equilibrate the melt. This method preserves the topological state of individual polymers in the melt and its effectiveness is demonstrated for linear polymers with molecular weight N up to 1024, and cyclic polymers with N up to 8192. For the range of N studied, the method requires simulation time that scales as N(2) and is thought to be applicable to a variety of polymer architectures.

Numerical investigation of the annulus fibrosus

A coarse-grained bead-spring model of the annulus fibrosus is presented, and shown to be capable of qualitatively replicating experimental behavior. Stochastic elements built into this model enable the simulation of fatigue loading. Quantitative comparison with experiments is possible, after proper calibration has been accomplished.