Do bending and torsional potentials affect the unraveling dynamics of flexible polymer chains in extensional or shear flows?

Abstract

We compare the flow-induced unraveling of a bead-spring model chain in which each stiff spring corresponds to a single Kuhn step to that of an atomistic representation of the backbone of the same polymer chain with realistic bending and torsional potentials. In our earlier work [Jain, S. and Larson, R.G. 2008. Effect of bending and torsional potentials on high-frequency viscoelasticity of dilute polymer solutions. Macromolecules 41(10), 3692–3700], we showed that in the linear viscoelastic regime, bending and torsional potentials suppress fast local diffusive modes. Now, in strong shear and extensional flows, we observe that bending and torsional potentials have only a slight effect on unraveling dynamics. However, in shear flow, we find that for relatively short coarse-grained chains having less than 20 or so Kuhn steps—representing polymers with fewer than around 150 backbone bonds—coarse-graining introduces periodic peaks in the probability distribution of steady-state stretch. We believe this occurs at high shear because of the dominance of a discrete set of stretch values each of which corresponds to an integral number of bonds that are nearly fully aligned between back-folds that are created during the chain's tumbling orbit. Even this difference between coarse-grained and realistic fine-grained chains disappears for more typical long chain lengths.