  In the popular solution-diffusion picture, the membrane permeability is
defined as the product of the partition ratio and the diffusivity of
penetrating solutes inside the membrane in the linear response regime, i.e., in
equilibrium. However, of practical importance is the penetrants' flux across
the membrane driven by external forces. Here, we study nonequilibrium membrane
permeation orchestrated by a uniform external driving field using molecular
computer simulations and continuum (Smoluchowski) theory in the stationary
state. In the simulations, we explicitly resolve the penetrants' transport
across a finite monomer-resolved polymer network, addressing one-component
penetrant systems and mixtures. We introduce and discuss possible definitions
of nonequilibrium, force-dependent permeability, representing `system' and
`membrane' permeability. In particular, we present for the first time a
definition of the differential permeability response to the force. We
demonstrate that the latter turns out to be significantly nonlinear for
low-permeable systems, leading to a high amount of selectiveness in
permeability, called `permselectivity', and is tunable by the driving force.
Our continuum-level analytical solutions exhibit remarkable qualitative
agreement with the penetrant- and polymer-resolved simulations, thereby
allowing us to characterize the underlying mechanism of permeabilities and
steady-state transport beyond the linear response level.
