Understanding the transport enhancement of poly (vinyl alcohol) based hybrid membranes with dispersed nanochannels for pervaporation application

Abstract

Hybrid membranes, featuring combinations of inorganic and organic materials at the nanometre or molecular level, have been widely reported as providing superior separation performance compared with the traditional polymeric membranes. However, for rational design of membranes, there remains a major doubt as to how each constituent functions in the separation process. Herein, carbon nanotube (CNT) incorporated poly (vinyl alcohol) (PVA) membranes were analysed using pervaporation (PV) process for aqueous mixture separation. The impacts of CNT on the functional properties, morphologies and microscale structures of the PVA/CNT hybrid membranes was investigated by ATR-FTIR, AFM, SEM and positron annihilation lifetime spectroscopy (PALS). Further, a comparison of the interactions between the membrane and solvents (water and ethanol) were identified by swelling test and XRD. The resultant PVA/CNT hybrid membranes were then subject to both desalination and dehydration of ethanol. The results showed that PVA exhibited preferential adsorption of water over ethanol. The addition of CNT enlarged the fractional free volume (FFV) and enhanced water diffusivity (up to 185%), which indicated a diffusion-dominated type of the PVA/CNT hybrid membranes with a synergistic effect of CNT on water transport. The transport of Na+ ions through the membrane was examined to be with larger activation energy than that of ethanol during the separation process. This work investigated the state of polymer as well as the effect of nanofillers in the separation of water vs. non-volatile or volatile component for the first time, which can provide in-depth understanding of the polymer-based hybrid membranes for practical applications.