Gas transport property of polyallylamine-poly(vinyl alcohol)/polysulfone composite membranes

Polyallylamine (PAAm) was synthesized by free radical polymerization and characterized by Fourier transform infrared resonance (FT-IR) spectroscopy, hydrogen nuclear magnetic resonance (¹H NMR) spectroscopy and differential scanning calorimetry (DSC). The composite membranes were prepared by using PAAm-poly(vinyl alcohol) (PVA) blend polymer as the separation layer and polysulfone (PSF) ultrafiltration membranes as the support layer. The surface and cross-section morphology of the membrane was inspected by environmental scanning electron microscopy (ESEM). The gas transport property of the membranes, including gas permeance, flux and selectivity, were investigated by using pure CO₂, N₂, CH₄ gases and CO₂/N₂ gas mixture (20 vol% CO₂ and 80 vol% N₂) and CO₂/CH₄ gas mixture (10 vol% CO₂ and 90 vol% CH₄). The plots of gas permeance or flux versus feed gas pressure imply that CO₂ permeation through the membranes follows facilitated transport mechanism whereas N₂ and CH₄ permeation follows solution-diffusion mechanism. Effect of PAAm content in the separation layer on gas transport property was investigated by measuring the membranes with 0-50 wt% PAAm content. With increasing PAAm content, gas permeance increases initially, reaches a maximum, and then decreases gradually. For CO₂/N₂ gas mixture, the membranes with 10 wt% PAAm content show the highest CO₂ permeance of about 1.80 × 10^-5 cm³ (STP) cm^-2 s^-1 KPa^-1 and CO₂/N₂ selectivity of 80 at 0.1 MPa feed gas pressure. For CO₂/CH₄ gas mixture, the membranes with 20 wt% PAAm content display the highest CO₂ permeance of about 1.95 × 10^-5 cm³ (STP) cm^-2 s^-1 KPa^-1 and CO₂/CH₄ selectivity of 58 at 0.1 MPa feed gas pressure. In order to explore the possible reason of gas permeance varying with PAAm content, the crystallinity of PVA and PAAm-PVA blend polymers was measured by X-ray diffraction (XRD) spectra. The experimental results show an inverse relationship between crystallinity and gas permeance, e.g., a minimum crystallinity and a maximum CO₂ permeance are obtained at 20 wt% PAAm content, indicating that the possibility of increasing CO₂ permeance with PAAm content due to the increase of carrier concentration could be weakened by the increase of crystallinity.