Pore size tailoring of poly(ether imide) membrane from UF to NF range by chemical post-treatment using aminated oligomers

An accurate control of the pore size has been demonstrated with poly(ether imide) (PEI) membranes by using a chemical post-treatment with amino oligomers. Jeffamine M-2070, a mono aminated poly(propylene oxide)-co-poly(ethylene oxide) copolymer (PEG-amine, 2 kDa), and poly(ethyleneimine) (Pei), with molecular weight ranging from 0.8 to 2 kDa, were used as molecular weight cut-off (MWCO) modifying agents. PEI membranes having a MWCO of 120±10 kDa (measured by PEG/PEO rejection) and a pure water permeability of 180±30 l h−1 m−2 bar were prepared from a 16 wt% PEI solution in NMP by coagulation into a water bath. Modification with the mono-functionalized PEG-amine leads to a pore opening owing to the dissolution of the overreacted PEI chains. PEGylated UF membranes with increasing MWCO up to 500 kDa were thus prepared by varying the reaction time. Beyond this value, the mechanical strength of self-standing membranes decreased. In contrast, use of a poly-amino reagent prevented the dissolution by the cross-linking of PEI chains. Positively charged membranes with decreasing MWCO down to the NF range were prepared by using Pei oligomers. A plateau was reached after ca 3 h of reaction. Longer reaction times only gave rise to a decline of the permeate rate due to an increase of the dense top layer thickness. A mechanism involving a dynamic rearrangement of the PEI chains was proposed to account for the experimental observations.