Dynamics in entangled polyethylene melts using coarse-grained models

Brandon L. Peters (1), Gary S. Grest (1), K Michael Salerno (2), Anupriya Agrawal (3), Dvora Perahia (4)

1. Sandia National Laboratories, Albuquerque, New Mexico 87185, United States
2. U.S. Naval Research Laboratory, Washington, D.C. 20375, United States
3. Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, Missouri 63130, United States
4. Department of Chemistry, Clemson University, Clemson, South Carolina 29634, United States

The distinctive viscoelastic behavior of polymers results from a coupled interplay of motion on multiple length and time scales. Capturing the broad time and length scales of polymer motion remains a challenge. Using polyethylene as a model macromolecule, we construct coarse-grained (CG) models with λ = 3 to 6 methyl groups per CG bead and from a fully-atomistic polyethylene melt simulation and probe the effects of the degree of coarse graining on polymer dynamics. By rescaling time in the CG models by a temperature dependent factor α, the chain mobility for the atomistic and CG models match. We show that independent of the degree of coarse graining, all measured static and dynamic properties are essentially the same once the dynamic scaling factor α and a non-crossing constraint for the CG6 model are included. The speedup in the simulation for the CG4 model is 6-10 times that of the fully atomistic model, depending on temperature, enabling us to reach times of over 500 μs. This allows us to measure a number of quantities, including the stress relaxation function, plateau modulus and shear viscosity, and compare directly to experiment. Lastly, the transferability was investigated by the stress autocorrelation function and mean squared displacement at different temperatures.