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Two kinds of polyimide membranes 6F-AP and 6F-AP/DA (APAF:DABA=3:2) were synthesized and heated between 300–450 °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 CO 2 , respectively, after increasing the heating temperature from 300 to 450 °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 °C exhibits a H 2 permeability of 196.4 barrer and a H 2 /CH 4 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. [Display omitted] • Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) membranes were prepared by heating at varying temperatures. • Thermal rearrangement and thermal crosslinking had been analyzed to understand the change of free volume of polymers. • The free volume of membranes was accurately measured by positron annihilation. • Cross-Linked TR membrane treated at 400 °C exhibited excellent H 2 /CH 4 separation performance exceeding the 2008 upper bound. [ABSTRACT FROM AUTHOR]

In this work, a series of hydrolyzed polymers of intrinsic microporosity (PIMs) membranes were successfully synthesized under alkaline conditions. Positron annihilation lifetime measurement is employed to analyze the pore structure of the hydrolyzed PIMs. In the original PIM-1, there are two kinds of pores, i.e. micropores with radius of 4.32 Å and ultramicropores with radius of 2.58 Å. The size of all the pores shows a continuous decrease with increasing hydrolysis time, which is confirmed by the reduced chain-to-chain spacing in the hydrolyzed PIMs measured by X-ray diffraction. Meanwhile, the fractional free volume also shows decrease after the hydrolysis process, while the relative number of ultramicropores increases. Benefiting from the smaller pore size in the hydrolyzed PIMs membrane, the gas molecules with smaller size (such as CO 2) can be separated more efficiently. The selectivity of CO 2 /CH 4 of hydrolyzed PIMs reaches up to 30.05 with a relatively high CO 2 permeability of 140.58 Barrer, which exceeds the Robeson's 1991 upper bound. Our results indicate that the hydrolyzed polymers of intrinsic microporosity membranes are promising candidates for gas separation membrane in the future. [Display omitted] • A series of hydrolyzed PIMs membranes were synthesized under alkaline conditions. • The process shortens the reaction time and improves the gas separation performance. • The logarithm of P shows a linear relationship with 1/FFV. • Positron lifetime measurements indicate the free volume adjusts the selectivity. [ABSTRACT FROM AUTHOR]

Two series of 6FDA-based co-polyimide membranes were synthesized by adding different diamine monomer containing special functional groups: APAF (containing OH) and DABA (containing -COOH). The ODA/APAF and ODA/DABA series were heat treated at 450 °C and 350 °C, respectively. Thermal rearrangement of polymer chain segments is confirmed in ODA/APAF membranes by infrared spectroscopy, while addition of DABA in ODA/DABA membranes causes decarboxylation crosslinking. Positron lifetime measurements reveal existence of ultramicropores in polyimides. The thermal rearrangement in ODA/APAF membranes induces remarkable increase in both size and number of ultramicropores and therefore increase in fractional free volume (FFV), but the crosslinking in ODA/DABA causes significant decrease in both size and number of ultramicropores. The increase in FFV is favorable for gas permeation, but too large size of the micropore drastically deteriorates the gas selectivity. Finally, the ODA/APAF(3:7) shows a high CO 2 permeability of 127.4 Barrer with CO 2 /CH 4 selectivity of 34.2, which surpasses the 1991 Robeson upper bound. Besides, the ODA/DABA(3:7) exhibits a H 2 permeability of 22.6 Barrer with H 2 /CH 4 selectivity of 256.8, which also exceeds the 1991 Robeson upper bound. Our results indicate that the gas permeability and selectivity of polymer membranes can be effectively adjusted by tailoring the ultramicropore structures, and positron can provide precise information about micropores. • Positron annihilation results show that the adding of APAF and DABA can change the FFV by changing both size and number of ultramicropores. • The positron annihilation results prove the increase in FFV is favorable for gas permeation, but too large size of the micropore drastically deteriorates the gas selectivity. • The positron annihilation results confirm gas selectivity depends on the difference in the diameter of the gas pairs to be separated as proposed by Robeson et al. • ODA/APAF(3:7) surpasses the 1991 CO 2 /CH 4 Robeson upper bound, while ODA/DABA(3:7) surpassing the 1991 H 2 /CH 4 Robeson upper bound. [ABSTRACT FROM AUTHOR]