Phase separation behaviors of polyethersulfone/ionic liquid blends and the correlation with the physical properties

Abstract

Polyethersulfone (PES) is a widely used membrane material which shows excellent comprehensive performance, but the inherent hydrophobicity greatly limits its applications. Herein, we modified the PES with a hydrophilic ionic liquid, 1-vinyl-3-butylimidazole tetrafluoroborate, IL(BF4), by melt blending. The phase behavior and properties of the PES/IL(BF4) binary blends were investigated systematically. It was found that the sulfone group on PES can form a hydrogen bond with the H atom at the C-2 position of the imidazole ring of ionic liquid, which makes the good compatibility between PES and IL(BF4). Nuclear magnetic resonance (NMR) confirmed the existence of hydrogen bonding between PES and IL(BF4). It was further found that the homogenous PES/IL(BF4) blends undergo phase separation with increasing temperature and the blends have the typical lower critical solution temperature (LCST) phase behaviors. Small angle X-ray scattering analysis confirmed the microphase separation of the blends during the annealing at elevated temperature and the formation of the IL(BF4) enriched domains dispersed in PES matrix. In situ Fourier transform infrared spectroscopy (FTIR) revealed that such phase separation was attributed to the weakening of the hydrogen bonding with increasing temperature. Moreover, the addition of IL(BF4) endows the homogeneous PES/IL(BF4) blends with hydrophilicity and electrical conductivity; while the phase separation leads to the enhanced hydrophilicity and decreased electrical conductivity because of the enrichment of the IL(BF4).