Hydrophilic porous polyimide/β-cyclodextrin composite membranes with enhanced gas separation performance and low dielectric constant

A series of co-polyimide (PI)/modified β-cyclodextrin (β-CD) composites were successfully fabricated from anhydride-terminated PI and (3-aminopropyl)triethoxysilane-modified β-CD (β-ACD). Co-PI was prepared from 4,4′-oxydianiline, 4,4′-(hexafluoroisopropylidene) diphthalic anhydride, and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride by chemical imidization. Different amounts of β-ACD (0, 1, 3, 5, and 7 wt%) were introduced into co-PI via strong covalent interactions between the terminal anhydride and amino groups. The structures and properties of the composites were characterized by means of Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, thermal gravimetric analysis, differential scanning calorimetry, dynamic thermomechanical analysis, mechanical properties tests, and contact angle tests. The results showed that β-ACD was successfully grafted on the PI segment. The composite films showed good thermal stability, glass transition temperatures between 244°C and 254°C, and 10% weight loss at temperatures of 514°C–545°C and 506°C–538°C in nitrogen and air atmosphere, respectively. They also exhibited excellent mechanical properties with tensile strength, tensile modulus, and elongation at break values of 78–111 MPa, 1.14–2.05 GPa, and 8–17%, respectively. All of these values were maximized at a β-ACD content of 1 wt%. The water uptake of the composites films was more than 1%, indicating that the addition of β-ACD can enhance the water absorption of PI films. All of these composite films are porous, and the contact angle indicated that the addition of β-ACD increased the hydrophilicity of the composite film. When the β-ACD doping content reached 7 wt%, the contact angle reached a minimum of 63°. All of the membranes were thermally annealed at 300°C for 1 h, after which gas adsorption tests showed that the composite films have enhanced CO2/CH4 selectivity, which can reach 12.7 (308 K).