  We have used coarse-grained simulation methods to investigate the effect of
stretching-induced structure orientation on the proton conductivity of
Nafion-like polyelectrolyte membranes. Recent experimental data on the
morphology of ionomers describe Nafion as an aggregation of polymeric backbone
chains forming elongated objects embedded in a continuous ionic medium.
Uniaxial stretching of a recast Nafion film causes a preferential orientation
of these objects in the direction of stretching. Our simulations of humid
Nafion show that this has a strong effect on the proton conductivity, which is
enhanced along the stretching direction, while the conductivity perpendicular
to the stretched polymer backbone is reduced. Stretching also causes the
perfluorinated side chains to orient perpendicular to the stretching axis.
  This in turn affects the distribution of water at low water contents. The
water forms a continuous network with narrow bridges between small water
clusters absorbed in head-group multiplets.
