On the basis of these observations and considering the possible effect of AG 494 on PA absorption, the mass swelling and volume variation of PVDF/BMIMPF6 membrane immersing into PA were monitored within 48 h at room temperature. Unlike the increment on the mass and volume while doping PA into SPEEK/BMIMPF6/PA membrane, the phenomenon of an increase in mass but a decrease in volume was observed in PVDF/BMIMPF6/PA membrane. For example, the mass swelling and volume variation were 43.1% and − 0.8%, respectively, when PVDF/60%BMIMPF6 membrane was immersed into PA for 48 h in equilibrium. Due to the lack of strong interaction between BMIMPF6 and PVDF polymeric chains, BMIMPF6 could partially leak from the membranes while immersing into PA. The absorbed PA molecules occupied the vacancy once belonged to BMIMPF6 in polymeric skeleton, which caused the volume decreasing even the mass increasing. According to the remarkable discrepancy observed here, it is further confirmed that the sulfonated polymers favor capturing imidazolium ionic liquids to form the stable composite structure.