Correlating the microstructure of novel polyamide thin-film composite membranes with ethanol dehydration performances

Polyamide thin-film composite (TFC) membranes were prepared by interfacial polymerization. The purpose is to improve the pervaporation performance of polyamide membranes in separating an aqueous ethanol solution. A novel amine monomer 2,2′-dimethylbenzidine hydrochloride (m-tolidine-H) was reacted with an acyl chloride monomer trimesoyl chloride (TMC) on the surface of a modified polyacrylonitrile (mPAN) membrane. The effects of the following interfacial polymerization conditions on the TFC membrane pervaporation performance were investigated: monomer concentration, immersion time of mPAN in aqueous m-tolidine-H solution, and interfacial polymerization (IP) time for reacting m-tolidine-H with TMC. Attenuated total reflectance Fourier transform infrared spectroscopy and scanning electron microscopy were used to characterize the chemical structure and the morphology of the membranes, respectively. To probe the variation in the fine-structure of the polyamide active layer and the free volume depth profile in the TFC membranes, positron annihilation lifetime spectroscopy (PALS) experiments coupled to a variable monoenergy slow positron beam were conducted. The densest portion of the polyamide layer was at the interface of the two immiscible monomer solutions, as detected based on the smallest o-Ps annihilation lifetime at positron incident energies of 1 and 2 keV with 0.05 and 0.5 wt% TMC, respectively. A high pervaporation performance of 2191 g/m2h permeation flux and 99 wt% water content of permeate was delivered by the polyamide TFC membrane that was prepared by immersing the mPAN membrane in a 1.5 wt% aqueous m-tolidine-H solution for 10 s, followed by contacting it with a 0.05 wt% organic TMC solution for 10 s.