Advanced Membrane Synthesis Methods: Exploration of 3D Printed Membranes for Oil/Water Separation and Development of Novel Polymers for Organic Solvent Nanofiltration

Various fabrication methods and different polymers have been applied in recent years to synthesize membranes. Combined with current membrane fabrication methods and commercial polymers, almost all kinds of polymeric membrane ranging from porous microfiltration membranes to dense reverse osmosis membranes have been fabricated in academic and commercial environments. However, membranes synthesized by currently used methods and polymers cannot solve all the problems encountered in industry. Novel fabrication methods with ability to control the structure and surface roughness of a membrane for oil/water separation are still urgently demanded. New polymers with good solvent stability are also highly needed for the fabrication of organic solvent nanofiltration (OSN) membranes. In this thesis, selective laser sintering as 3D printing technique was explored as a novel membrane fabrication method, to synthesize a porous membrane. In addition, two high-performance polymers, poly (arylene sulfide sulfone) (PASS) and Kevlar, were used to fabricate OSN membranes.

First, a preliminary study was conducted by applying selective laser sintering to produce a membrane from polyamide-12 powder. The obtained membrane turned out to be a porous microfiltration membrane. Results show that processing parameters including laser power, hatch space and scanning count have a significant influence on the structure and performance of a membrane. A membrane fabricated with higher energy density, like a larger laser power, smaller hatch space and 2 counts, would have a denser structure and a lower water flux. Following the same procedure, polysulfone porous membranes were also obtained.

Unlike the smooth flat membrane fabricated from phase inversion, a 3D printed membrane has a rough surface, a 3-dimensional structure and a strong mechanical strength, enabling it to be the perfect substrate to construct a superhydrophobic membrane for oil/water separation. Therefore, two methods were proposed to modify the 3D printed membranes with superhydrophobic surface. In the first approach, a 3D printed PSU membrane was immersed in a candle soot/hexane solution for 40 mins to obtain an ordered candle soot loose network on the surface. In the second method, the 3D printed PA membrane was coated with a layer of micro/nano-structural zeolitic imidazolate framework (ZIF-L) and then chemically modified by PDMS. Both modified membranes demonstrated superhydrophobic and super-oleophilic properties at the same time and exhibited a high separation efficiency of oil from oil/water mixtures.

Apart from the exploration of novel membrane fabrication methods, two high-performance polymers, PASS and Kevlar, were adopted to synthesize OSN membranes.  The oxidized PASS OSN membrane was fabricated by a post oxidation treatment of the pristine PASS membrane. This membrane showed an excellent chemical stability in aggressive organic solvents even in 98% H2SO4 due to the significantly improved intermolecular interactions. During organic solvent filtration, it exhibited a high rejection of RB, EB, RO16 and SBB, and a high DMF permeability compared to other reported membranes. The Kevlar OSN membrane was synthesized by a simple thermal treatment of a hydrogel ultrafiltration membrane via the regeneration of hydrogen bonds. This membrane exhibited an excellent solvent resistance in commonly used solvents. They maintain a relatively high dye rejection during the filtration of dye solutions for 6 hours.

In conclusion, this thesis showed that membranes fabricated from selective laser sintering can be porous and in the range of microfiltration. Besides, the superhydrophobic modified membranes demonstrate a high separation efficiency of oil from oil/water mixture. In addition, both PASS and Kevlar can be fabricated to OSN membrane. These OSN membranes exhibit an excellent solvent resistance and a high dye rejection in solvents.