Praveen Depa

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Duration in group: 2003 - 2007

Research Summary

Multiscale Simulation of Polymers

Introduction
Many processes in polymeric materials occur during a large span of spatial and temporal scales. Figure 1 shows various simulation techniques at the relevant scales. In Molecular Dynamics [MD] Simulations, the positions of united atoms, (a carbon along with its bonded hydrogens e.g. CH 4, CH3 etc.) are simulated with force fields that are available in the literature. To reach larger length and longer times, coarse-grained [CG] simulations are performed. CG simulations are similar to MD, except that the united atoms are replaced by a collection of united atoms called a coarse-grained bead. The problem with CG simulations is the absence of accurate potentials or force fields to describe the interactions. One solution to this is reverse mapping of the distributions from MD simulations to obtain the force fields. The main objective of the research is to perform simulations at atomistic and coarse-grained scales concurrently with the exchange of information between them. This information is generally system specific and examples of it are structure factors, energies etc.

Research objectives
My research involved the development of coarse-grained models for a polyethylene melt. At present, four successive united atoms are grouped to form a coarse-grained bead. Thus, every fifth united atom is considered as a coarse-grained centre in a CG simulation. Figure 2 shows the probability distribution function [PDF] of distance between successive coarse-grained centers from the MD simulation. This corresponds to a coarse-grained bond length in the CG simulation. CG bond length potential is then obtained by Boltzmann inverting this PDF. Similarly, CG bond angle potential is obtained by Boltzmann inverting the probability distribution of angle between 1-5-9 united atoms, corresponding to a coarse-grained angle. Finally, inter-molecular interactions are optimized to match the pair distribution function of coarse-grain centers from the MD simulation. A CG simulation with these force fields gave static properties that are comparable with those obtained from the MD simulation.

Simulations with higher degree of coarse graining (mapping more than four united atoms on to a coarse-grain bead) are also being performed. Future work involves extending this procedure to branched polymers and polymer blends.