PhD Public Lecture (Math) - Venkat Bala

A Molecular dynamics study of polymer chains in shear flows and nanocomposites

Abstract: In this work we study single chain polymers in shear flows and nanocomposite polymer melts extensively through the use of large scale molecular dynamics simulations through LAMMPS. In the single polymer chain shear flow study, we use the Lattice Boltzmann method to simulate fluid dynamics and also include thermal noise as per the \emph{fluctuation-dissipation} theorem in the system. When simulating the nanocomposite polymer melts, we simply use a Langevin thermostat to mimic a heat bath.

In the single polymer in shear flow study we investigated the margination of a single chain towards solid surfaces and how strongly the shear flow influences this effect. In particular we also tried to study the effect of the polymer's monomer size $a$ on its overall tendency to marginate. To this end, we studied polymer chains of length $N=16, 32$ in flows at multiple shear rates, $\dot{\gamma}$ and noted higher margination rates in the case of chains with larger radii monomers in comparison to smaller radii monomer chains. We quantified this behaviour and effect by considering various measures such as the distribution of the chain's maximum extent into the flow, the distribution of its centre of mass normal to the surface as well as its radius of gyration in directions parallel and normal to the surface i.e $R_{x}, R_{y}, R_{z}$.

In the second work, we looked at the effects of introducing nanorods into polymeric melts. We primarily focused on understanding the dispersion, orientation and conformational patterns exhibited by the nanorods and chains respectively. At lower concentrations, rods phase separated into distinct nematic clusters, while at higher concentrations they remained more isotropic and disordered. We noted that this behaviour is being driven by the system finding a trade-off between the entropic forces trying to create the isolated clusters and the enthalpic effects that work to improve mixing of the rods. We also noted that the rigid rods induced significant local conformational changes in the flexible chains in close proximity which in turn made the whole melt more ordered.