Enhanced polyphosphazene membranes for CO2/CH4 separation via molecular design

Developing new robust membrane materials for separating carbon dioxide are getting more and more attention today. In this work, Poly[bis(n-propylamine)phosphazene] (PPA), poly[bis(n-amylamine)phosphazene] (PAA) and poly[bis(n-octylamine)phosphazene] (POA) were synthesized and the corresponding composite membranes with polyvinylidene fluoride (PVDF) as supporting membrane were prepared. FT-IR, SEM, NMR, XPS, TGA and positron annihilation lifetime spectroscope (PALS) were used to characterize PPA/PVDF, PAA/PVDF and POA/ PVDF composite membranes. The characterization results showed that the prepared membranes had desired structure and the free volume of membranes were increased with increasing the length of side chain of polyphosphazene (PPO). The gas permeation test indicated that PPA/PVDF, PAA/PVDF and POA/PVDF membranes all had good gas permeability and selectivity for CO2, and as expected, the POA/PVDF membrane had the highest permeability owing to the biggest free volume, meanwhile, the selectivity was basically unchanged.