Paper Number
SM32 

Session
Polymer Solutions and Melts

Title
Molecular scale simulations of rheological behavior – Importance of polydispersity

Presentation Date and Time
October 8, 2014 (Wednesday) 10:25

Track / Room
Track 3 / Commonwealth C

Authors (Click on name to view author profile)

1. Rorrer, Nichoals A. (Colorado School of Mines)
2. Dorgan, John R. (Colorado School of Mines)

Author and Affiliation Lines (in printed abstract book)
Nichoals A. Rorrer and John R. Dorgan
Colorado School of Mines, Golden, CO 80401

Speaker / Presenter 
Rorrer, Nichoals A.

Text of Abstract
Polydispersity plays an important role in polymer physics influencing both processing approaches and final properties. Despite the obvious physical importance of polydispersity, studies usually simulate monodisperse chains or occasionally, a few different chain lengths. This work explores the consequences of polydispersity on dynamics and rheology. Polymer melts are simulated under static conditions, in confined geometries, under shear flow, and undergoing parabolic flow. Monodisperse and polydisperse melts demonstrate a transition from Rouse to Reptation dynamics based on chain length; however, the polydisperse melts possess lower characteristic times and a broader transition between the dynamic scaling regimes. Polymer melts possessing the same length average chain length (analogous to weight average molecular weight) exhibit the same zero shear viscosity regardless of their polydispersity index (PDI). As the PDI is increased the transition from the zero shear viscosity regime to the shear thinning region becomes broader. Depending on the distribution that is mapped into the simulation, there can be an emergence of an infinite shear viscosity, that is an upper Newtonian plateau. Under confinement it is again found that all melts demonstrate the same zero shear viscosity. However, the critical shear rate for shear thinning is decreased. For quiescent conditions at plate spacings above six radii of gyration, there is a slight preference of shorter chains at the wall. However, when the melts are subject to extreme degrees of confinement, migration behavior is present with the shortest chains being preferentially present at the walls. For shear flow, no enhanced migration is evident whereas for parabolic flow this excess of shorter chains becomes exacerbated. Results are in post facto agreement with many experiments and help explain important processing effects including die drool. The present study illuminates the need for including polydispersity in realistic simulations of polymer flows.