Computer vision-based measurement of pure- and mixed-gas swelling in polymer membranes: Individual and synergistic effects of configurational free volume and thermal rearrangement

Although polymer swelling (i.e., dilation) data are crucial to assess the long-term stability of polymer membranes and correct sorption isotherms for highly sorbing penetrants, the swelling propensity of new generation membranes exhibiting configurational free volume (i.e, triptycene units) has never been investigated. To fill this gap, in this paper we study pure and mixed-gas swelling of a series of thermally rearranged polybenzoxazoles (TPBOs) and their poly(o-hydroxyimide) precursors exhibiting systematically varied amount of configurational free volume. The individual and synergistic effects of configurational free volume and thermal rearrangement on the polymer swelling propensity are presented and discussed. To do so, we use a newly developed experimental approach based on computer vision, which is capable of correcting most of the limitations of traditional methods used to measure swelling and achieve superior accuracy. We show that thermally rearranged glassy polymers exhibiting configurational free volume exhibit better swelling resistance relative to conventional polymers. The fundamental origin of this behavior resides, as explained by the dual mode theory, on the fact that the Langmuir's mode swelling propensity in thermally rearranged triptycene-based polymers is much lower than in glassy polymers exhibiting only conformational free volume. Differences between pure-gas and mixed-gas induced swelling are also highlighted and discussed. Finally, a thorough investigation of the swelling measurement uncertainty and precision is provided and discussed, including an analysis of the major factors affecting experimental uncertainty.