Huan Wang

Towards High Water Permeability in Triazine-Framework-Based Microporous Membranes for Dehydration of Ethanol

The microstructural evolution of a series of triazine framework-based microporous (TFM) membranes under different conditions has been explored in this work. The pristine TFM membrane is inu2005situ fabricated in the course of polymer synthesis via a facile Brønsted-acid-catalyzed cyclotrimerizaiton reaction. The as-synthesized polymer exhibits a microporous network with high thermal stability. The free volume size of the TFM membranes gradually evolved from a unimodal distribution to a bimodal distribution under annealing, as analyzed by positron annihilation lifetime spectroscopy (PALS). The emergence of the bimodal distribution is probably ascribed to the synergetic effect of quenching and thermal cyclization reaction. In addition, the fractional free volume (FFV) of the membranes presents a concave trend with increasing annealing temperature. Vapor sorption tests reveal that the mass transport properties are closely associated with the free volume evolution, which provides an optimal condition for dehydration of biofuels. A promising separation performance with extremely high water permeability has been attained for dehydration of an 85u2005wtu2009% ethanol aqueous solution at 45u2009°C. The study on the free volume evolution of the TFM membranes may provide useful insights about the microstructure and mass transport behavior of the microporous polymeric materials.

Characterizing free volumes and layer structures in polymeric membranes using slow positron annihilation spectroscopy

Positron annihilation spectroscopy coupled with a newly built slow positron beam at National University of Singapore has been used to study the free volume, pore, and depth profile (0 – 10 μm) in cellulose acetate polymeric membrane at the bottom and top sides of membranes for ionic separation in water purification applications. The S and R parameters from Doppler broadening energy of annihilation radiation representing free volumes (0.1–1 nm size) and pores (>1 nm–μm) as a function of depth have been analyzed into multilayers, i.e. skin dense, transition, and porous layers, respectively. The top side of membrane has large free volumes and pores and the bottom side has a skin dense layer, which plays a key role in membrane performance. Positron annihilation lifetime results provide additional information about free-volume size and distribution at the atomic and molecular scale in polymeric membrane systems. Doppler broadening energy and lifetime spectroscopies coupled with a variable mono-energy slow positron beam are sensitive and novel techniques for characterization of polymeric membrane in separation applications.