  Motivated by recent experiments showing that stiff biopolymer gels exhibit
highly unusual negative normal elastic stresses, we develop a computational
model for stiff polymer networks subject to large strains. In all cases, we
find that such networks develop normal stresses that are both negative and of
magnitude comparable to the corresponding shear stress. We find that these
normal stresses coincide with other nonlinearities in our networks, and
specifically with compressive bucking of the individual filaments. Our results
suggest that negative normal stresses are a characteristic feature of stiff
(bio)polymer gels that have been shown to exhibit strong nonlinear elastic
properties.
