Synergistic effect of thermal crosslinking and thermal rearrangement on free volume and gas separation properties of 6FDA based polyimide membranes studied by positron annihilation

Two kinds of polyimide membranes 6F-AP and 6F-AP/DA (APAF:DABA=3:2) were synthesized and heated between 300-450 degrees C to study the effect of thermal treatment on the chain structure and gas transport performance of membranes. Fourier Transform Infrared (FI-IR) and X-ray diffraction (XRD) measurements show that heating causes rearrangement of polyimide into rigid polybenzoxazole which prevents the effective stacking of polymer chains leading to the increase of d-spacing and fractional free volume. Positron annihilation results confirm the increase of both size and number of free volume due to the thermal rearrangement. The addition of DABA monomer into 6F-AP causes crosslinking during heating, which results in tightening of the rigid chain spacing of benzoxazole, thus leading to slight decrease in the size and number of free volume. The increase in fraction free volume induced by thermal rearrangement can significantly improve the gas permeability of 6F-AP series membranes, which increases from 16.2 and 3.9 barrer to 271.4 and 200.3 barrer for H-2 and CO2, respectively, after increasing the heating temperature from 300 to 450 degrees C. But this is accompanied by a decrease in gas selectivity due to the increase in free volume size. Finally, by careful tailoring the free volume structure through thermal rearrangement and crosslinking, the 6F-AP/DA polyimide treated at 400 degrees C exhibits a H-2 permeability of 196.4 barrer and a H-2/CH4 ideal selectivity of 90, which is above the 2008 Robeson upper bound. Our results indicate that by designing the functional groups of the reaction monomer plus appropriate thermal treatment, the microstructure of polyimide membrane can be effectively adjusted for high-performance gas separation.

Automated summary

This study investigates the effect of thermal treatment on the chain structure and gas transport performance of polyimide membranes. The results show that heating leads to the rearrangement of polyimide into rigid polybenzoxazole, increasing the size and quantity of free volume and enhancing gas permeability. Crosslinking during heating reduces the size and quantity of free volume by tightening the chain spacing. By designing the functional groups of the reaction monomer and applying appropriate thermal treatment, the microstructure of polyimide membranes can be adjusted to achieve high-performance gas separation.