Positron annihilation spectroscopic study on fine-structure characterization and ethanol dehydration by pervaporation for interfacially polymerized composite membranes

Abstract

In this study, polyamide (PA) composite membranes (DMDPA–TMC/CA, NTEA–TMC/CA, DETA–TMC/CA) were prepared through the interfacial polymerization reaction between a trimesoyl chloride (TMC) and each of the following three amines: 3,3-diamino-N-methyl-dipropylamine (DMDPA), nitrilotriethylamine (NTEA), and diethylenetriamine (DETA), on the surface of an asymmetric cellulose acetate (CA) membrane. The effects of the amine chemical structure and the interfacial polymerization conditions on the dehydration of an aqueous ethanol solution by pervaporation were investigated. To characterize the swelling behavior and the resulting free volume variation in the interfacially polymerized PA layers, the PA membranes in dry and wet (with aqueous ethanol solution) conditions were probed by positron annihilation lifetime spectroscopy (PALS). The difference between the o-Ps lifetime in the wet and that in the dry condition (Δτ3=τ3w–τ3d) of the interfacially polymerized PA membranes increased in the following order: DETA–TMC<DMDPA–TMC<NTEA–TMC. The swelling effect induced by the aqueous ethanol solution was restricted by the crosslinking structure in the PA layer. This result corresponded well with that of the XPS analysis. The DETA–TMC/CA composite membrane prepared using the desirable interfacial polymerization conditions (immersion in the aqueous solution of 0.5wt% DETA for 10s, and contact with the organic solution of 0.5wt% TMC for 5s) exhibited the following desirable pervaporation performance in dehydrating an aqueous solution of 90wt% ethanol at 25°C: a permeation flux of 1220g/m2h and a water concentration in permeate of 98.4wt%. It was also found that the DETA–TMC/CA composite membrane exhibited superior performance compared to the composite membranes discussed in the literature.