(217a) A Brownian Dynamics Study of the Effect of Solvent Quality On the Coil-Stretch Transition

When long chain, linear polymers in dilute solutions are subjected to purely elongation flows or elongation-dominated mixed flows, the solution properties show a sharp variation near a critical flow rate where the conformation of the polymer changes from a coiled to a stretched state or vice versa. Since the solution properties change dramatically at this so-called coil-stretch transition, it is important in many applications. This transition is primarily characterized by the critical flow rate for a given polymer molecular weight and solvent quality. While the effect of solvent quality on equilibrium properties of polymer solutions has been widely studied, there have been relatively few attempts at examining its effect on properties far from equilibrium. One such study is by Cifre and Torre1, where they examined the effect of solvent quality on the critical elongation rate for coil-stretch transition by Brownian dynamics simulations and found that there was no effect on the transition properties. However, certain important aspects of the newly developed method of successive fine graining (SFG2, 3) and new theories about coil-stretch hysteresis4-7 were not considered in their work. We revisit the specific problem of steady, homogeneous elongational flow, and attempt to accurately determine the dependence of the critical elongational rate on solvent quality and chain length,.We have rigorously included, effects such as fluctuating hydrodynamic interactions (HI) and excluded volume (EV) in our calculations. The solvent effect is captured systematically using the EV parameter ?z' which is the appropriate parameter in perturbation expansions for determining the effect of solvent quality about the ?theta' state. Exact results are obtained numerically using Brownian dynamics simulations, while the Successive Fine Graining (SFG) technique has been used to eliminate parameter dependence in the results of the bead-spring chain simulations. Contrary to earlier findings, it is observed that with improvement in solvent quality, even though the critical strain rate decreases, critical Deborah number (based on either longest relaxation time(λ1) or zero shear rate viscosity(λη,0)) increases and this is solely due to an increase in relaxation time. The approach is then further extended to study the effect of solvent quality on the phenomena associated with polymer conformational hysteresis, including the nature of transition states and the activation energy for the transition.

References:

1. Cifre; J. G. H. and Torre; J. G. D. L. J.Rheol. 1999, 43, 2, 339-358

2. Sunthar P. and Prakash J. R. Macromolecules 2005, 38, 617-640

3. Sunthar P.; Nguyen D. A.; Dubbelboer R.; Prakash J. R. and Sridhar T. Macromolecules 2005, 38, 10200-10209

4. Schroeder C. M.; Babcock H. P.; Shaqfeh E. S. G. and Chu S. Science 2003, 301, 5639, 1515-1519

5. Schroeder C. M.; Shaqfeh E. S. G. and Chu S. Macromolecules 2004, 37, 24, 9242-9256

6. Hsieh C. C. and Larson R. G. J. Rheol. 2005, 49, 5, 1081-1089

7. Beck, V.A. and E.S.G. Shaqfeh, J. Rheol., 2007 51(3), 561-574