Synthesis of amphiphilic PCZ-r-PEG nanostructural copolymers and their use in CO₂/N₂ separation membranes

Poly(ethylene glycol) (PEG) is considered a good candidate material for CO₂ separation membranes due to its high affinity with CO₂, but the preparation of a robust, free-standing PEG membrane is challenging. Although a crosslinking method is often used to prepare PEG membranes, it has limitations in processibility, in that it is only applicable to the coating layer. We report the synthesis of amphiphilic polycarbonate Z-r-poly(ethylene glycol) (PCZ-r-PEG) nanostructural copolymers with improved processibility and their application to CO₂/N₂ separation membranes. The copolymers were synthesized using bisphenol Z, triphosgene and PEG (2000g/mol) via solution polycondensation polymerization, as confirmed by Fourier transform infra-red spectroscopy (FTIR), nuclear magnetic resonance (¹H-NMR) and gel permeation chromatography (GPC) analysis. Structural and morphological analysis was studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). TEM images revealed a self-assembled, microphase-separated morphology in the hydrophilic rubbery PEG domains and the hydrophobic glassy PCZ domains, which is also supported by the existence of two glass transition temperatures (T_gs) in the DSC curves. The thermal stability of the PCZ-r-PEG copolymers was less than that of the PCZ homopolymer, but was quite stable up to around 350°C. The mechanical properties of all the membranes were significant, with reduced elongation at break while the Young's modulus increased with PEG content. The PCZ-r-PEG copolymer exhibited a CO₂ permeability of 50.7 Barrer (1 Barrer=1×10^-10cm³(STP)cmcm^-2s^-1cmHg^-1) and CO₂/N₂ selectivity of 42.25 at 35°C and 2atm, which was much greater than those of the PCZ homopolymer membrane (CO₂ permeability of 2.7 Barrer, selectivity of 27).