  It is widely known that hydrogels, a class of soft materials made of a
polymer chain network, are prone to fatigue failure. To understand the
underlying mechanism, here we simulate polymer scission and fatigue initiation
in the vicinity of a crack tip in a two-dimensional chain network. For a
network without pores, our findings reveal that polymer scission can take place
across multiple layers of chains, rather than just a single layer as assumed in
the classical Lake-Thomas theory, in consistency with previus studies. For a
network with a high density of micropores, our results demonstrate that the
pores can substantially enhance the intrinsic fracture energy of the network in
direct proportion to the pore size. The underlying mechanism is attributed to
pore-pore interactions which lead to a relatively uniform distribution of
cohesive energy ahead of the crack tip. Our model suggests that micropores
could be a promising strategy for improving the intrinsic fracture energy of
hydrogels and that natural porous tissues may have evolved for enhanced fatigue
resistance.
