We demonstrate here that microporous
materials can exhibit softening
upon adsorption of guest molecules, at low to intermediate pore loading,
in parallel to the pore shrinking that is well-known in this regime.
This novel and counterintuitive mechanical response was observed through
molecular simulations of both model pore systems (such as slit pore)
and real metal–organic frameworks. It is contrary to common
belief that adsorption of guest molecules necessarily leads to stiffening
due to increased density, a fact that we show is the high-loading
limit of a more complex behavior: a nonmonotonic softening-then-stiffening
