The Synthesis and Gas Separation Properties of Soluble Polyimides Using An Alicyclic Dianhydride (DOCDA)

In gas separation field, aromatic polyimides have received great attention as excellent membrane materials because they have excellent gas separation properties, good mechanical strength, heat resistance and chemical resistance. However, they have shown very poor solvent solubility, which have restricted their fabrication into hollow fiber type membranes. In general, the solubility in organic solvents  is influenced by chemical structure of polymer  backbones. The incorporation of  alicyclic structures into polyimide backbon can be one way to synthesize soluble aromatic polyimide.

In this study, aromatic soluble polyimides were synthesized and characterized from DOCDA and various diamines. All synthesized polyimides were characterized by ¹H-NMR and FT-IR. Thermo-mechanical properties and crystallinity of the polyimide membranes were observed with DSC, TGA and XRD. Physicochemical properties of separation membranes were confirmed with solubility and fractional free volumes. Synthesized three polyimides showed good solubility in various organic solvents. Glass transition temperatures(Tg) of prepared polyimides were in the range of 242∼251°C and the first decomposition started 305∼370°C. Intrinsic viscosities increased in the order of DOCDA-p-PDA < DOCDA-ODA < DOCDA-MDA. This showed the same results as order of tensile strength. FFV values of prepared polyimide membranes were calculated from the density and increased in order of DOCDA-p-pDA < DOCDA-MDA < DOCDA-ODA. The X-ray diffraction patterns showed that all polyimides were amorphrous. DOCDA-ODA showed the smallest d-spacing value of other polyimides. The gas permeability coefficients(P) and ideal selectivities for N₂, O₂, CO₂ and CH₄ of the prepared polyimides were investigated. Among three polyimides, DOCDA-ODA showed excellent permeability (6.10 barrers of CO₂, 1.17 barrers of O₂, 0.17 barrers of N₂, 0.091 barrers of CH₄) and the selectivity of CO₂/CH₄ and CO₂/N₂ and O₂/N₂ and CH₄/N₂ were 67.03, 35.90, 6.85, 1.87, respectively.