Conversion of hydroxyl-containing polyimides into polybenzoxazole can be achieved by thermal rearrangement of the aromatic polymer chain with decarboxylation at elevated temperature. Synthetic methods to prepare polyimide precursors are important for the resulting thermally rearranged (TR) polymer membranes. Here, we report on the effect of several imidization methods on the properties of TR polymer membranes. Thermal and chemical imidizations are the most common routes to prepare polyimides, and solution thermal imidization using an azeotrope is also widely used, especially to obtain soluble polyimide-containing functional groups. We report here on the syntheses of ortho-functional polyimides from 4,4′-hexafluoroisopropylidene diphthalic anhydrides and 2,2′-bis(3-amino-4-hydroxyphenyl)hexafluoropropane by three different imidization methods. Acetate-containing polyimides by chemical imidization and further silylation treatment as well as hydroxyl-containing polyimides by thermal and azeotropic imidization are characterized using thermogravimetric analysis, density, positron annihilation lifetime spectroscopy, and gas permeation property measurements. Comparison between the precursor polyimides and the resulting thermally rearranged polybenzoxazole (TR-PBO) membranes exhibited significant increase in fractional free volumes and cavity sizes followed by enhanced gas permeation properties.

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Two‐dimensional nanosheets have shown great potential for separation applications because of their exceptional molecular transport properties. Nanosheet materials such as graphene oxides, metal–organic frameworks, and covalent organic frameworks display unique, precise, and fast molecular transport through nanopores and/or nanochannels. However, the dimensional instability of nanosheets in harsh environments diminishes the membrane performance and hinders their long‐term operation in various applications such as gas separation, water desalination, and ion separation. Recent progress in nanosheet membranes has included modification by crosslinking and functionalization that has improved the stability of the membranes, their separation functionality, and the scalability of membrane formation while the membranes’ excellent molecular transport properties are retained. These improvements have enhanced the potential of nanosheet membranes in practical applications such as separation processes.

Microporous materials have a great importance in catalysis, delivery, storage and separation in terms of their performance and efficiency. Most microporous materials are comprised of inorganic frameworks, while thermally rearranged (TR) polymers are a microporous organic polymer which is tuned to optimize the cavity sizes and distribution for difficult separation applications. The sub-nano sized microcavities are controlled by in situ thermal treatment conditions which have been investigated by positron annihilation lifetime spectroscopy (PALS). The size and relative number of cavities increased from room temperature to 230 °C resulting in improvements in both permeabilities and selectivities for H(2)/CO(2) separation due to the significant increase of gas diffusion and decrease of CO(2) solubility. The highest performance of the well-tuned TR-polymer membrane was 206 Barrer for H(2) permeability and 6.2 of H(2)/CO(2) selectivity, exceeding the polymeric upper bound for gas separation membranes.

Regenerable, high-efficiency salt sorption materials are highly desirable for water treatment. Here, a thermoresponsive, amphoteric metal-organic framework (MOF) material is reported that can adsorb multiple salts from saline water at room temperature and effectively release the adsorbed salts into water at elevated temperature (e.g., 80 °C). The amphoteric MOF, integrated with both cation-binding carboxylic groups and anion-binding tertiary amine groups, is synthesized by introducing a polymer with tertiary amine groups into the cavities of a water-stable MOF such as MIL-121 with carboxylic groups inside its frameworks. The amphoterized MIL-121 exhibits excellent salt adsorption properties, showing stable adsorption-desorption cycling performances and high LiCl, NaCl, MgCl , and CaCl adsorption capacities of 0.56, 0.92, 0.25, and 0.39 mmol g , respectively. This work provides a novel, effective strategy for synthesizing new-generation, environmental-friendly, and responsive salt adsorption materials for efficient water desalination and purification.

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