Transient and steady-state effective diffusivity in high free volume glassy polymers

The transport properties of high free volume glassy polymers such as poly(1-trimethylsilyl-1-propyne) (PTMSP) were analyzed by comparing the steady-state permeation behavior with the transient diffusion of light hydrocarbons. It has been observed that the apparent diffusivity measured in the two different modes are indeed quite different from one another, even by nearly an order of magnitude. The behavior can be explained by considering the high free volume glassy polymer as a two-phase system formed by a continuous solid phase and a discontinuous nano-void phase, with a suitable degree of interconnection. The influence of pore distribution and pore connectivity on the differences observed between transient sorption and steady permeation behavior has been analyzed by performing various mathematical simulations of transient and steady-state diffusion for different morphological structures of the two-phase system, used to model the polymer matrix. Qualitative comparison indicates that the difference experimentally observed is associated to a pore interconnectivity effect and to the corresponding anisotropy in pore distribution.