Paper Number
SG6 

Session
Solids, Composites & Granular Materials

Title
Unveiling the effects of heterogeneity and sub-entanglement chain stretching on the mechanical response of deformed polymeric glass

Presentation Date and Time
October 17, 2018 (Wednesday) 1:30

Track / Room
Track 4 / Post Oak

Authors (Click on name to view author profile)

  1. Zou, Weizhong (University of Michigan)
  2. Moghadam, Soroush (University of Michigan, Department of Mechanical Engineering)
  3. Hoy, Robert S. (University of South Florida, Department of Physics)
  4. Larson, Ronald G. (University of Michigan, Department of Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Weizhong Zou1, Soroush Moghadam2, Robert S. Hoy3, and Ronald G. Larson4
1University of Michigan, Ann Arbor, MI; 2Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI; 3Department of Physics, University of South Florida, Tampa, FL 33620; 4Department of Chemical Engineering, University of Michigan, Ann Arbor, MI

Speaker / Presenter 
Larson, Ronald G.

Text of Abstract
We present a coarse-grained picture of polymeric glass that divides the stress into a series of distinct segmental modes governing the monomer friction, and a polymer mode contributed by the configuration of polymer chains. In this hybrid model, the discrete relaxation time spectrum of individual segmental modes is derived by fitting the KWW function whose time constant and non-exponentiality are obtained from dye reorientation experiments (Lee, et al. J. Polym. Sci. B, 47, 1713, 2009; Bending & Ediger J. Polym. Sci. B, 54, 1957, 2016); while the polymer mode is represented by finitely extensible bead-spring chains whose conformational changes are solved by Brownian dynamics simulation with the bead drag coefficient proportional to the averaged viscosity of segmental modes. The model is shown to successfully reproduce both the pre-yielding and the post-yielding behaviors of PMMA (poly-methymethacrylate) glass under constant rate of extension and uniaxial tensile creep. The obtained parameters, such as predeformation aging time, rubbery and shear modulus, are all in good agreement with the properties of the glass and its preparation history. When compared to molecular dynamics simulations for entangled Kremer-Grest chains under uniaxial extension, the conformation of chains predicted from both MD and BD simulations turn out to be very similar at large deformation (1.8 Hencky strain) where strain hardening arises and individual molecules start to collapse into folded states (so-called kinks), analogous to that in fast extensional flows of dilute polymer chains. We show from the MD simulations that very few of the folds are entangled with other chains, and so the above similarity provides strong evidence that the high tensions in those folded sub-entangled polymer strands is the cause of the strain hardening rather than the deformation of entanglement networks.