posted on 2021-12-27, 19:39authored byAkash Arora, Tzyy-Shyang Lin, Bradley D. Olsen
Fracture of an unfilled elastomer
occurs primarily due to mechanical
scission of polymer chains, which is inherently related to the topology
of the underlying long-range-connected polymer network. This work
presents a coarse-grained simulation framework to compute the fracture
strength of elastomers by performing a tensile test at a strain rate
of approximately 1/s, which is comparable to those employed in experiments
and at least 106 times smaller than those possible in conventional
molecular dynamics simulations. The simulation framework incorporates
key aspects of polymer fracture: nonlinear force-extension behavior,
the mechanochemistry of polymer chains, and the stochastic nature
of bond breaking. The developed framework is then used to understand
the role of topological defects, such as primary, secondary, and higher-order
loops, in the fracture of elastomers. It is observed that the modulus
decreases while the ultimate extension increases with the increase
in the concentration of primary loops in the network, in accord with
the recently developed theory of network fracture and several experimental
systems: bimodal elastic networks, poly(ethylene glycol) gels, and
thiol–ene elastomers. However, the increase in ultimate extension
with the primary-loop fraction is not as appreciable as predicted
by earlier theories and experiments. This is due to the correlation
and stress redistribution among different defects; as the chains break
during deformation, the stresses on defect-free linear strands are
transferred to the primary-loop-containing strands, essentially creating
a homogeneous stress distribution, which leads to synchronous breaking
of the linear strands and the loop-containing strands. Overall, this
work highlights the role of different topological defects in controlling
the fracture properties of networks and thereby provides opportunities
to design materials with improved mechanical performance.