Polyurethane hybrid membranes with confined mass transfer channels: The effect of functionalized multi-walled carbon nanotubes on permeation properties

Abstract

Aiming to prepare pervaporation membranes with confined mass transfer channels, multi-walled carbon nanotubes (MWCNTs) with and without functionalization were incorporated into polyurethane (PU) polymer matrices. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), Fourier-transform infrared (FTIR), and Raman spectroscopy were employed to explore the effect of functionalization on the morphologies of MWCNTs. The changes in the morphologies of the resulting PU hybrid membrane were then characterized by SEM, TEM, and XRD. Swelling-sorption and pervaporation experiments of benzene and cyclohexane were employed to evaluate the separation performances of various hybrid membranes. The results showed that the introduction of many oxygenated functional groups onto the MWCNT surfaces through functionalization not only efficiently inhibited the MWCNT aggregation but also improved the distributions of the MWCNTs in solvents and polymer materials. Due to the 1D tube structures of the MWCNTs with conjugated π bonds, the addition of well-distributed MWCNTs into the hybrid membranes not only increased the benzene sorption abilities of the membranes but also enhanced the benzene diffusion selectivities due to the strong driving force for the molecules in the confined mass transfer channels. A higher content of well-distributed MWCNTs in the membrane yielded a higher separation performance for benzene. Additionally, increasing the operating temperature in the pervaporation experiments accelerated the molecular motion and weakened the affinity between the benzene molecules and the walls of the MWCNTs, increasing the permeation fluxes of the membranes while decreasing the benzene separation factors.