470146 Nonequilibrium Molecular Dynamics Simulations of Entangled Polymer Melts and Solutions Undergoing Planar Elongational Flows
Tube based models predict that concentrated polymer solutions behave roughly the same as entangled polymer melts, if they have the same number of entanglements and if stress, strain rate and time are made dimensionless appropriately. But there are a lot of experimental works that show entangled polymer melts and solutions exhibit different behaviors when they are subject to extensional flows at high extension rates. Specifically there is an upturn in extensional viscosity of polymer solutions at extension rates higher than the inverse of the Rouse time which has not been observed in polymer melts. Recent experiments of Wingstrand et al., however, have demonstrated that using oligomers as solvent, polymer solutions could behave in a similar fashion as melts.
In this work we performed nonequilibrium molecular dynamics (NEMD) simulations of entangled C700H1402 and C1000H2002 linear polyethylene melts subject to planer elongational flows. We also simulated concentrated solutions of C1000H2002 in C16H32 as an oligomeric solvent with the same monomeric units in order to compare the behavior of polymer melts and solutions with roughly the same number of entanglements. Simulation results show that polymer solutions and melts exhibit roughly the same behavior at high extension rates. Specifically, similar to the melt, no upturn was observed in primary extensional viscosity of the solution, consistent with Wingstrand et al. experiments. Other system properties including chain orientation, chain stretch, and entanglement network properties and their potential connection to this finding will be discussed in this presentation. Preliminary results of extensional viscosity of concentrated solutions in small solvent molecules with various molecular architectures such as benzene will also be shown.
 Wingstrand, S.L., N.J. Alvarez, Q. Huang, O. Hassager, Phys. Rev. Lett. 115, 078302 (2015).
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