Molecular Dynamics Simulations of Short-Chain Branched Bimodal Polyethylene: Topological Characteristics and Mechanical Behavior

It
has previously been shown that polyethylene (PE) with a bimodal
molar mass distribution has a high fracture toughness. Our approach
has been to use coarse-grained (CG) molecular dynamics (MD) simulations
to investigate the effects of including short-chain branches in the
high molar mass fraction of bimodal PE on topological features and
mechanical behavior of the material. The CG potentials were derived,
validated, and utilized to simulate melt equilibration, cooling, crystallization,
and mechanical deformation. Crystallinity, tie chain, and entanglement
concentrations were continuously monitored. During crystallization,
the branched bimodal systems disentangled to a lesser degree and ended
up with a higher entanglement density than the linear bimodal systems
simulated in our previous study. The increase in entanglement concentration
was proportional to the content of the branched high molar mass fraction.
A significantly higher tie chain concentration was obtained in the
short-chain branched bimodal systems than in the linear systems. The
increase in the number of ties was more pronounced than the increase
in the number of entanglements. The tie chain concentration was not
proportional to the content of the high molar mass fraction. Despite
a lower crystal thickness and content, the elastic modulus and yield
stress values were higher in the branched bimodal systems. A more
pronounced strain hardening region was observed in the branched systems.
It was suggested that the higher tie chain and entanglement concentration
prior to the deformation, the more extensive disentanglement during
the deformation, and the disappearance of formed voids prior to failure
point were the reasons for the observed higher toughness of the short-chain
branched bimodal PE compared with that of the linear bimodal systems.
The toughest system, which contained respectively 25 and 75 wt % low
molar mass and branched high molar mass fractions, had the highest
tie chain concentration and the second highest entanglement concentration
of the simulated systems.