Amorphous Polymers and Deformation Behavior

Thermoplastic polymers are one of the most widely consumed man-made materials. Their mechanical property is of great importance for their practical applications. Some polymers are amorphous under their glass transition temperature, whose properties may be affected by, at a molecular level:

  • Chain length
  • Number of chains
  • Strain rate
  • Temperature

They demonstrate typical mechanical responses:

  • Elastic regime
  • Yield
  • Strain softening
  • Strain hardening

Molecular dynamics (MD) is well-suited to extract the mechanical properties of them due to:

  • Well defined system and energy component
  • Deformation dependent energy contribution
  • Molecular insights on deformation behavior

Main Points

Below are the observations in the tension tests of polyethylene (PE) found in a MD paper [1], which used the Dreiding force field and the unite-atom model:

  • Interchain non-bonded interaction dominate in elastic and yield regions
  • Intrachain dihedral motion dominate in strain hardening region

Chain length & number of chains:

  • Stress-strain curve is sensitive to chain length (entanglements) while not sensitive to number of chains
  • Longer chains (entanglements) have stiffer elastic regimes, higher yield stress and more predominant softening
  • With more chains, the stress-strain curve is much smoother while both capture the same behavior

Temperature (brittle ductile transition, measured Tg of PE is about 300K):

  • Material stiffness decreases with increasing temperature
  • The yield, and softening regime is much more pronounced for 100K (than 250K)
  • Higher than glass transition temperature (e.g. 400K), no significant yield, softening, and hardening are observed

Strain rate:

  • Elastic modulus and peak yield stress increase with increasing strain rate
  • The strain hardening modulus are the same for different loading rate

Chain orientation (tension-induced crystallization):

  • Chain segment tends to align with loading direction
  • The larger the strain, the more extend of the alignment of the chain to the loading direction
  • Strain rate has positive effect on the alignment (the higher strain rate, the smaller increase of orientational parameter P2)

Chain entanglement:

  • In initial stages, entanglements (geometric definition) are not sensitive to strain
  • At higher strains (strain hardening region), the entanglement parameter decreases in a linear fashion w.r.t. strain
  • Entanglement parameter decreases more at a lower strain rate (a lower strain rate would allow more time for chains to disentangle)

References

[1] “Molecular dynamics simulations of deformation mechanisms of amorphous polyethylene.” Polymer 51.25 (2010): 6071-6083.