Innovative PEEK membrane structure fabrication using non-solvent induced phase separation

Research on the fabrication of polymeric membranes for energy conversion, wastewater treatment, filtration, and gas separation applications has attracted attention for high-temperature and chemical-resistant polymeric materials such as poly ether ether ketone (PEEK). In this study, a non-solvent induced phase separation (NIPS) technique was introduced to prepare a porous PEEK membrane. 4-chlorophenol (4 CP) was used as a compatible solvent for the dissolution and precipitation of PEEK, which was achieved through solidification of the membrane by NIPS in three different nonsolvents: ethanol (EtOH), water, and isopropanol (IPA). The processing of PEEK in 4 CP had no effect on its chemical structure, and the achieved thermal properties were similar to virgin PEEK. Process parameters such as polymer concentration and the use of nonsolvents were studied to understand their effects on membrane morphology and performance. Changes in polymer concentration not only alter the casting solution rheology but also restrict the percentage of polymer, which affects the membrane morphology, whereas the use of nonsolvents affects the morphology through diffusion of solvent into nonsolvent. EtOH and IPA as nonsolvents showed sudden changes in the color of the membrane from transparent to white, which resulted in instantaneous de-mixing of solvent and nonsolvent, that formed finger-like structure at 7 wt% and 9 wt% polymer concentrations. Water as a nonsolvent showed a slow color change for phase separation, which resulted in delayed de-mixing, and formed sponge-like structure in the membranes with 7 wt%, 9 wt%, and 11 wt % polymer concentrations. Partial solidification before immersion of the membrane in nonsolvents was observed at 11 wt% polymer concentration owing to reduction in temperature. A dense PEEK membrane was observed after 9 wt% immersion in water, and a porous PEEK membrane was observed after 11 wt% immersion in IPA. Raising the polymer concentration increased the density and reduced membrane shrinkage. The proposed novel approach advances the fabrication of PEEK membranes through the dissolution of PEEK in 4 CP without the use of concentrated acids as well as the synthesis of the PEEK precursor and NIPS process.