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Three bis(phenyl)fluorene-based cardo diamine monomers with different side groups (–CF3, –H, and –CH3) were synthesized and used to synthesize three 6FDA-based polyimides (6FDA-FBPF, 6FDA-BPF, and 6FDA-MBPF). The influence of bis(phenyl)fluorene cardo moiety and different side groups on the glass transition temperature (Tg), thermal stabilities, and chain packing of the synthesized polyimides were systematically studied. The gas permeabilities and selectivities of three bis(phenyl)fluorene-based polyimide membranes were studied and correlated with their fractional free volumes and chain packing conditions. As a result, the obtained polyimides showed promising performance for hydrogen separation. Among the three polyimides, 6FDA-FBPF had the highest gas permeabilities which exhibited H2 permeability of 151.12 Barrer and H2/CH4 selectivity of 62.19. The high gas separation performance of 6FDA-FBPF mainly attributed to the ineffective chain packing via the incorporation of bi(phenyl)fluorene cardo moiety and introduction of bulky CF3 side groups into the polyimide backbones that is favorable to formation hourglass-shaped pores. [ABSTRACT FROM AUTHOR]

This study aimed to establish a quantitative structure–property relationship (QSPR) model for predicting the gas separation performance of polyimide membranes using neural networks combined with the repeat unit structure of materials. Using a data bank based on 125 polyimides, we calculated a total of 20 descriptors for all polyimides using Yampolskii's group contribution method, which divides polyimides' repeat units into their smallest groups. The number of groups contained in each polyimide is taken as the network input, and the gas permeability as the network output. Two neural network models, back-propagation (BP) and genetic algorithm-optimized back-propagation (GABP) algorithms, were used as the prediction model, and the prediction results were compared. When compared with the previous models used to predict the gas separation performance for all polymers and other machine learning (ML) models, the prediction results obtained using the GABP model are encouraging, showing a root mean squared error (RMSE) of 0.44 for CO 2 , indicating that the model is applicable to polyimide. In addition, the GABP model is easy to operate, requires few parameters, it is also applicable to copolyimides. The GABP model based on the group contribution method can thus satisfactorily predict polyimides' gas separation. This is expected to be used to guide the synthesis and structure screening of polyimides for saving resources and commercialization. [Display omitted] • A quantitative structure–property relationship (QSPR) model is established. • Neural network models are proposed to predict the gas permeability of polyimides. • The results show that GABP model achieves better results. • GABP model is applicable to copolymers. • GABP model requires few parameters and is easy to operate and extend. [ABSTRACT FROM AUTHOR]

Abstract: Polymers with rigid and contorted structures are always preferred for use as gas separation membranes. Owing to the contorted nature of the spiro‐center, the spirobichroman structure is an effective scaffold for impeding polymer chain packing. Two spirobichroman‐based diamines, 6,6′‐bis(4‐amino‐2‐trifluoromethylphenoxy)‐4,4,4′,4′,7,7′‐hexamethyl‐2,2′‐spirobichroman (FSBC) and 6,6′‐bis(4‐aminophenoxy)‐4,4,4′,4′,7,7′‐hexamethyl‐2,2′‐spirobichroman (SBC), with or without CF3 side groups are prepared. The two diamines are separately reacted with 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA) and 3,3′,4,4′‐diphenylsulfonetetracarboxylic dianhydride (DSDA) to obtain four spirobichroman‐based polyimides, denoted as BTDA‐FSBC, BTDA‐SBC, DSDA‐FSBC, and DSDA‐SBC. The different molecular structures among the four monomers are expected to alter the fractional free volume of the corresponding polyimides by impacting chain‐packing conditions, and then controlling the gas transport properties. Chemical structures, thermal and physical properties of the polyimides are fully characterized. Two different methods, molecular dynamics simulation and Bondi's group contribution method are employed to characterize the fractional free volumes (FFVs) of these polyimides. In the end, it is found that FSBC‐based polyimides exhibit relatively higher gas permeability than SBC‐based polyimides. [ABSTRACT FROM AUTHOR]

A proper increase of d -spacing after crosslinking is an effective strategy to enhance gas separation performance and anti-plasticization property of polymeric membranes. One way to achieve high chain distance is to design and prepare polymer precursors that have large side groups with crosslinkable functionality. Our previous research proved that the decarboxylation crosslinked polyimides connected by a biphenyl group showed enlarged d -spacing. However, the 100% crosslinking temperature is 425–450 °C that is higher than the glass transition temperature ( T g ) of most polymers and may cause problem for preparing asymmetric crosslink membranes. Here we proposed a thermal oxidative crosslinking method which had a lower crosslinking temperature than the decarboxylation crosslinking method. Specifically, we synthesized two phenolphthalein-based cardo diamines, MPP and PP, with or without CH 3 substituted groups and prepared the 6FDA-MPP and 6FDA-PP polyimides. The lactone ring in the cardo moiety decomposed and the crosslinking reaction took place by heating the polymer in an air purge atmosphere. A 100% conversion of the crosslinking reaction was obtained at 400 °C. In addition, the crosslinked polyimides showed increased d -spacing, high CO 2 permeability and CO 2 /CH 4 selectivity than before. The best result for pure gas test was achieved for 6FDA-MPP-400, which had a CO 2 permeability of 193.8 Barrer with a CO 2 /CH 4 ideal selectivity of 39.2. No plasticization was observed for the 6FDA-MPP-400 and 6FDA-PP-400 polyimides at a CO 2 pressure up to 450 psia. In addition, both 6FDA-MPP-400 and 6FDA-PP-400 showed excellent mixed gas selectivity of 35 at a partial CO 2 pressure of 147 psia. [ABSTRACT FROM AUTHOR]

Three spirobichroman-based diamines, FSBC, SBC and MSBC, with different substituted groups (-CF 3, -H, and -CH 3 ) were synthesized. The diamines were reacted with 4,4′-hexafluoroisopropylidene bisphthalic dianhydride (6FDA) to form 6FDA-FSBC, 6FDA-SBC, and 6FDA-MSBC polyimides, respectively. The organosolubilities, molecular weights ( M n ), densities, glass transition temperature (T g ), thermal stabilities, fractional free volume (FFV), d -spacing and gas separation properties of the spirobichroman-based polyimides were systematically studied. The three spirobichroman-based polyimides had high M n and T g , excellent thermal stabilities, and good solubilities in a wide range of organic solvents. 6FDA-FSBC exhibited the highest gas permeability which could be attributed to its insufficient chain packing conditions rooting from the incorporation of spriobichroman backbone with the spiro and non-coplaner nature, and the bulky CF 3 substituting group that increased the polymer FFV and decreased the polymer chain flexibility. 6FDA-MSBC had the highest H 2 /CH 4 selectivity due to the present of the CH 3 group in the diamine moiety which had the turnstile-like rotary thermal motion that inhibited the diffusion of large molecules like CH 4 but had little effect on the diffusion of small molecules like H 2 . [ABSTRACT FROM AUTHOR]

Two carboxylic acid containing diamines, CADA1 and CADA2, with or without bulky CF 3 group were synthesized and polymerized with 6FDA, BTDA and DSDA to form four polyimides, denoted as 6FDA-CADA1, 6FDA-CADA2, BTDA-CADA1 and DSDA-CADA1. After the thermal induced decarboxylation crosslink reaction, polyimides with 100 times’ higher CO 2 permeabilities, similar CO 2 /CH 4 selectivities and much better plasticization resistant properties were obtained compared with the un-crosslinked polyimides. The polymer d-spacing increased with the heating temperature and those of the fully crosslinked polyimides, 6FDA-CADA1-450 and DSDA-CADA1-450, were 5.81 Å and 5.51 Å, respectively, which were very close to the estimated distance between the crosslinked polymer chains (5.57 Å, calculated by Material Studio). The CO 2 -induced plasticization phenomenon was not observed at a CO 2 pressure up to 30 atm for polyimides after thermally treated at temperatures above 425 °C. Moreover, polyimides crosslinked at different temperatures had almost the same selectivities, but their permeabilities gradually increased with the crosslinking temperature. For CO 2 /CH 4 gas pair, the separation performances of 6FDA-CADA1-425 and BTDA-CADA1-450 membranes surpassed the “2008 Robeson Upper Bound”, and other membranes thermally treated at 400 and 425 °C were all positioned near the “1991 Robeson Upper Bound”. [ABSTRACT FROM AUTHOR]

Thermal oxidative crosslinking results in the generation of oxygen bridges among polymer chains and helps regulate the membrane microstructure. Simultaneously, any strategy to increase the crosslinking sites of polyimide (PI) can improve the crosslinking efficiency and allow precise control of the pore structure of membranes. Phenolphthalein-based PIs are potentially suitable precursors for thermal oxidative crosslinking of membranes, since thermal treatment will result in the degradation of lactone ring leading to the generation of a pair of phenyl crosslinking sites. In this work, three phenolphthalein-based diamines with different structures were designed and synthesized. The three diamines were subsequently reacted with 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) to prepare phenolphthalein-based PIs. The original membranes were post-treated by thermal oxidation crosslinking at 400 °C and 425 °C, respectively; the lactone ring underwent decomposition, and the imide ring opened at the same time, resulting in the generation of multiple crosslinking sites and correspondingly improving the crosslinking efficiency. The effect of the side groups (−CH 3 , –CF 3 or unsubstituted) of phenolphthalein-based diamine and thermal treatment temperatures on the gas separation performance of phenolphthalein-based PI membranes were investigated. Wide-angle X-ray diffractometry (WAXD) and wide-angle X-ray scattering (WAXS) analysis demonstrated that the π-π stacking distance were improved. BTDA-FPP-425 had a CO 2 /CH 4 selectivity of 37.68. This thermal degradation along with thermal oxidative crosslinking strategy provides a promising approach for fabrication of CO 2 /CH 4 separation membrane. [Display omitted] • Phenolphthalein-based polyimide is a suitable precursor for thermal crosslinking. • BTDA-based polyimide has low cost and good mechanical properties. • The lactone ring and the imide ring open, generating multiple crosslinking sites. • Thermal degradation and oxidative crosslinking improved the π-π stacking values. • A strategy for making CO 2 /CH 4 separation membrane with anti-plastic was achieved. [ABSTRACT FROM AUTHOR]

This article reports synthesis and structure property studies of block copolyimides synthesized using diamino room temperature ionic liquids (RTIL) as diamine monomers. Specifically, polyimide oligomers of different lengths were synthesized using 2,2- bis (3,4-carboxylphenyl) hexafluoropropane dianhydride (6FDA) and diamino RTIL (1,3-di(3-aminopropyl) imidazolium bis[(trifluoromethyl) sulfonyl] imide). These oligomers were copolymerized with 6FDA and m-phenylenediamine (MDA) using in situ polymerization to form (6FDA-RTIL)-(6FDA-MDA) block copolyimides. The impact of the length and relative concentration of 6FDA-RTIL oligomer in the copolymer on the resulting thermal, physical, and gas transport properties was monitored. As the concentration of the 6FDA-RTIL segments increased, the backbone of the block copolyimides became more flexible resulting in a decrease in the glass transition temperature (Tg) and an increase in the density. The permeabilities of the RTIL containing copolyimides were consistently lower than those of the base polyimide, 6FDA-MDA, with some increase in selectivities. Interestingly, the permeabilities of films produced with the low molecular weight oligomers were very different than those produced with same composition of the high molecular weight oligomers. This may be indicative of very different morphologies within these copolyimides. [ABSTRACT FROM AUTHOR]

Polyimides with intrinsic microporosity (PIM‐PIs) are widely regarded as one of the most promising next‐generation membrane materials to simultaneously achieve high permeability and selectivity. Despite the fact that tremendous microporous polyimides have been synthesized, only cost‐efficient PIM‐PIs have the potential to be used in industrial applications. In this work, a PIM‐PI is prepared by using the commercial and inexpensive planar pyromellitic dianhydride (PMDA) as the dianhydride monomer, and 2,6‐diaminotriptycene as the diamine monomer. The CO2 permeability of PMDA‐DAT is ≈23‐fold higher than that of Kapton, one of the commercially available polyimide membranes also made from PMDA, and with almost the same CO2/CH4 selectivity. In addition, no plasticization phenomenon is observed for PMDA‐DAT membrane even at a CO2 pressure up to 15 atm. The good plasticization resistance performance of PMDA‐DAT can be mainly attributed to the formation of pseudo‐physical crosslinking by interlocking and π–π interactions in triptycene moieties. [ABSTRACT FROM AUTHOR]