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A series of fluorinated co-polyimide films based on the 6FDA-FTPPA/BAFL backbone were prepared from 4-(3-fluoro-4- (trifluoromethyl)phenyl)-2,6-bis(4-aminophenyl)pyridine (FTPPA), commercial 9,9-bis(4-aminophenyl)fluorene (BAFL), and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), with varying ratios of FTPPA:BAFL units (3:1, 2:1, 1:1, 1: 2, and 1:3). Co-polyimides not only exhibit high thermal stability (Tg > 357°C), good optical properties (λcutoff > 327 nm) and hydrophobicity (contact angle >91.4°), but also favorable solubility properties in both high and low boiling organic solvents. The fractional free volume of the polymers was simulated using molecular mechanics and molecular dynamics and correlated with experimental gas separation data. The presence of bulky groups led to high FFVs (>17.8%) and the formation of large microcavities with diameters ranging from 5.48 to 5.72 A°, which efficiently increased gas permeability. In pure gas permeation experiments, the permeability coefficients of CO2 and He for the 6FDA-FTPPA/BAFL (3:1) film were 41.48 and 79.38 bar, respectively, which were superior to those of commercial Matrimid and Kapton in terms of gas permeability. In especial, the 6FDA-FTPPA/BAFL (1:2) and 6FDA-FTPPA/BAFL (1:3) in copolyimide membranes exhibited the most attractive separation performance for O2/N2 gas pairs, approaching the 1991 Roberson upper limit. [ABSTRACT FROM AUTHOR]

The deep surrounding rock is usually in the true triaxial stress state, and previous constitutive models based on the understanding of uniaxial and conventional triaxial test results have difficulty characterizing the degradation and fracture process of rock ductile--brittle failure under true triaxial stress state. Therefore, this study conducted a series of true triaxial tests to obtain the understanding of the ductile--brittle behaviour of rock, and then combined the test results and the Mogi--Coulomb strength criterion, and proposed calculation methods for the elastic modulus E, cohesion c and internal friction angle u and the evolution functions of E, c and ф of rock under true triaxial stresses. With the decreasing of the minimum principal stress σ3 or increasing of the intermediate principal stress σ2, the marble post-peak stress drop rate gradually increases, the ductility gradually weakens, and the brittleness significantly strengthens. The calculation method and evolution function of rock E, c and ф under true triaxial stress were proposed. E decreased at first and then tended to remain stable with the increasing of equivalent plastic strain increment dep. c and u slowly increased at first and then rapidly decreased. With a method of parameter degradation rate to realize post-peak stress drop rate to reflect the ductile--brittle characteristics, a new three-dimensional ductile--brittle deterioration mechanical model (3DBDM) was established. The proposed model can accurately characterize the influence of rσ and rσ on mechanical parameters, the ductile--brittle behaviour of rock under true triaxial stresses, and the asymmetric failure characteristics of surrounding rock after excavation of deep underground engineering. The proposed model can be reduced to elastic--perfectly plastic, elastic--brittle, cohesion weakening friction strengthening (CWFS), Mohr--Coulomb, and Drucker--Prager models. [ABSTRACT FROM AUTHOR]