44618-AC7
Molecular Simulation Study of Flow-Induced Chain Scission in Polymer Systems
Work is being carried out to address the effects of stress on polymer systems. The relevance is in regard to flowing polymer melts, and deformed polymer glasses. Two students have been working on the project: Michael-Paul Robinson and Gallia Painter -- Mike is an undergraduate student who was a PRF SUMR Fellow in 2007, and Gallia is working on a combined BS-MS degree.
The work has two thrusts:
First, the stress-induced chain scission is addressed. This is a very difficult problem to address, because of the relevance of a range of time and length scales. In particular, quantum mechanics is necessary to treat the covalent bonds which will broken, but such methods cannot be used to treat the many atoms in the condensed phase system that are relevant to causing the stress that will break the bond. To treat this problem, we are using the Quantum Mechanics/Molecular Mechanics (QM/MM) approach, in which a subsystem is treated with a quantum mechanical electronic structure method, while the rest of the system is treated with a more approximate molecular mechanics method. We are using the Charmm software for this purpose.
Second, we are investigating the scaling of free energy barriers (to conformation transions) with applied stress. These simulations are based on classical force fields, and evaluate the free energy using umbrella sampling and histogram reweighting methods. Our previous work on potential energy barriers showed that applied stress causes fold catastrophes to occur (i.e., barriers decrease to zero), which implies a particular scaling of the barrier height with stress. We will investigate the relevance of these ideas to free energy barriers.
