Gas Permeability in a Polymer of Intrinsic Microporosity (PIM-1): Effect of Time and Heat Treatments

In this work we studied the permeability of different gases (N₂, CH₄, CO₂, O₂) in films of a polymer of intrinsic microporosity (PIM-1) at various temperatures (from 30 to 60°C) and upstream pressures (from 0.5 to 5 bar). The gas permeability decreases with pressure, in particular for the case of CO₂, and increases with temperature.

The experiments were carried out on samples subject to different prior history, as follows :

a)      Vacuum heating at 100°C  and fast cooling

b)      Vacuum heating at 80°C and slow cooling

c)      Ageing at room temperature over 135 days

The experiments allowed to detect that the gas permeability decreases rather markedly with both age and thermal treatment. The two phenomena seem to produce a similar effect on the gas transport rate of all penetrants investigated, so that the ideal CO₂/gas selectivities are much less affected than the single gas permeabilities. On the other hand, the physical ageing at room temperature alters the gas permeability mostly in the first 20 days of observation: subsequent measurements show that the gas permeability decreases much more slowly with time.

The determination of the rates of transport at different temperature allows the estimation of the activation energy, E_p, whose values depend on the history of the sample, likely due to the fact that in the course of the experimental investigation the polymer free volume decreases as time and/or temperature increase. As the different phenomena occurring in the polymer matrix are difficult to decouple, such results indicate that the E_p value must be calculated using values measured on samples of the same age and at a maximum temperature lower than the one experienced by the sample in its prior history, in order to avoid physically meaningless situations like negative E_p values.