Tunable Free Volume
Structure on the Gas Separation
Performance of Thermally Rearranged Poly(benzoxazole-co-imide) Membranes Studied by Positron Annihilation
posted on 2023-11-02, 14:40authored byQian Liu, Junjie Liu, Peiyu Jia, Tian Yu, Ning Qi, Wei Zhou, Nanwen Li, Zhiquan Chen
In this work, two series of poly(benzoxazole-co-imide) membranes were synthesized through thermal rearrangement
(TR) reaction at 450 °C. Different kinds and proportions of the
nonrearrangeable (non-TR-able) diamines were incorporated into the
rearrangeable (TR-able) precursors to modify the free volume structure
of the resulting membranes. Results of positron annihilation measurements
exhibit that the increase of TR-able content or number of methyl groups
leads to an increase of both free volume size and fractional free
volume, while the incorporation of non-TR-able diamines obviously
improves the mechanical properties of the membranes. The pure gas
permeability shows a perfect linear relationship with the fractional
free volume of the samples, confirming the decisive effect of the
free volume structure on gas transport. Meanwhile, the gas selectivity
is also found to depend on the fractional free volume. In addition,
the high fractional free volume obtained by TR reaction leads to exceptionally
high H2/CH4 separation performance, with majority
of the samples exceeding the 2008 Robeson upper bound. The plasticization
resistivity of the membranes is also enhanced by the TR reaction.
Our study exhibits that the free volume structure of the poly(benzoxazole-co-imide) membranes can be tailored for appropriate gas
separation performance. Furthermore, our results show that positron
annihilation technique is an effective tool for exploring the microstructural
information on gas separation membranes.