Surface skin layer formation and molecular separation properties of asymmetric PEEKWC membranes

Asymmetric membranes based on a poly(ether ether ketone) with a phthalido group along the polymeric backbone (PEEKWC) were prepared by non-solvent induced phase separation. Three chlorinated solvents (i.e. chloroform, dichloromethane and 1,2-dichloroethane - TCM, DCM and DCE in the following), at fixed evaporation times between casting and coagulation in methanol, have been used to investigate the effect of different evaporation rates on the formation of the skin layer structure. The surface morphology of the membranes has been determined by AFM. Membranes prepared by using DCM and TCM as solvents are characterized by the presence of a dense skin layer with different grain size, whereas a porous skin layer is formed with DCE. These differences arise from the volatility (DCm > TCM > DCE) and the solvation ability (DCM < DCE <TCM) of the solvents, which in turn affect the thickness of the skin and the packing of PEEKWC chains in it. The morphology of the porous substructure. instead, can be rationalized in terms of rate of phase separation during the coagulation step, which are probably caused by the differences in the solvation properties of the solvent and affinity between the solvent and the external non-solvent. From PEEKWC solutions in DCM and with butanol as internal non-solvent, membranes were obtained with an effective dense skin of 86 nm, exhibiting CO2/N-2, O-2/N-2 and He/N-2 selectivities of 32, 7.0 and 113. These results compete very well with those of typical commercial membranes, with selectivities in the range of about 5-7 for O-2/N-2 and about 30-40 for CO2/N-2.