Paper Number
PM14 

Session
Polymer Melts: From Molecular Rheology to Processing

Title
Chain length dispersity effects on viscoelastic response of entangled polymers

Presentation Date and Time
October 17, 2018 (Wednesday) 9:50

Track / Room
Track 2 / Plaza I

Authors (Click on name to view author profile)

  1. Grest, Gary S. (Sandia National Laboratories)
  2. Peters, Brandon L. (Sandia National Laboratories)
  3. Salerno, K. Michael (U.S. Army Research Laboratory)
  4. Ge, Ting (Duke University, Mechanical Engineering and Materials Science)
  5. Perahia, Dvora (Clemson University, Department of Chemistry)

Author and Affiliation Lines (in printed abstract book)
Gary S. Grest1, Brandon L. Peters1, K. Michael Salerno2, Ting Ge3, and Dvora Perahia4
1Sandia National Laboratories, Albuquerque, NM 87185; 2U.S. Army Research Laboratory, Aberdeen, MD; 3Mechanical Engineering and Materials Science, Duke University, Durham, NC 27710; 4Department of Chemistry, Clemson University, Clemson, SC

Speaker / Presenter 
Grest, Gary S.

Text of Abstract
While essentially all theoretical and computational studies of entangled polymer melts have focused on uniform samples, polymer synthesis routes always result in some dispersity, albeit narrow, of distribution of molecular weights (ÐM = Mw/Mn ~ 1.02-1.04). Here the effects of dispersity on chain mobility and viscoelastic response are studied for entangled, disperse melts using a coarse-grained model for polyethylene. Polymer melts with chain lengths set to follow a Schulz-Zimm distribution for the same average Mw = 36 kg/mol with ÐM = 1.0 to 1.16, were studied for times of 600 - 800 µs using molecular dynamics simulations. This time frame is longer than the time required to reach the diffusive regime. We find that dispersity in this range does not affect the entanglement time or tube diameter. However, while there is negligible difference in the average mobility of chains for the uniform distribution ÐM = 1.0 and 1.02, the shortest chains move significantly faster that the longest ones offering a constraint release pathway for the melts for larger ÐM. The stress autocorrelation function was fit to the theoretical expression proposed by Likhtman and McLeish. The resulting plateau modulus, terminal time and viscosity all decrease with increasing dispersity, corresponding to an apparent increase in the entanglement molecular weight.