Zwitterionic copolymer self-assembly for high flux, fouling resistant membranes with small molecule selectivity
with ~1 nanometer size-based selectivity have numerous applications in the biochemical
and pharmaceutical industries, as well as in wastewater treatment processes. However,
most commercial membranes show poor selectivity in this size range due to wide pore size
distributions, and charged surfaces that complicate the separation mechanisms involved.
There is a need for new ... read moremembrane materials that enable these separations while
exhibiting high flux and fouling resistance, and are readily fabricated into large-area
membranes. Zwitterionic groups strongly resist biomacromolecular fouling due to their
high degree of hydration, which makes them promising materials for membrane
applications. Zwitterions are also documented to self-assemble into channel-type
clusters 0.6-2 nm in size. We introduce a new class of membranes whose selective layers
are made of zwitterionic amphiphilic random copolymers. These membranes derive their
permeability, selectivity and fouling resistance from this self-assembled nanostructure.
We synthesized random copolymers of hydrophobic monomers (acrylonitrile, methyl
methacrylate, trifluoroethyl methacrylate) and zwitterionic monomers (sulfobetaine
methacrylate, sulfobetaine vinylpyridines, and phosphorylcholine) by free radical
polymerization. These copolymers self-assemble into bicontinuous networks of ~1 nm
nanochannels, driven by the strong dipoles of the zwitterionic groups, as documented by
transmission electron microscopy (TEM) imaging. Membranes prepared by coating these
polymers on porous supports exhibit high flux, size-based selectivity with a ~1 nm size
cut-off, and excellent fouling resistance to common foulants. We have systematically
investigated how copolymer composition affects nanostructure (self-assembled domain
size) and the membrane performance. We have also used ionic liquid co-solvents and other
additives in the coating solutions to boost and alter the performance of these
membranes. Membranes prepared by coating copolymer solutions with sufficient amounts of
selected additives on commercial ultrafiltration membrane supports exhibit permeances as
high as 50 L/m2.hr.bar, up to 10 times higher than membranes formed without additives.
These membranes also exhibit a narrow pore size distribution, retaining the same
size-based selectivity with a ~1 nm size cut-off. Performance of these membranes depends
on the amount of additive as well as the membrane manufacturing method (non-solvent,
drying time etc.). We have also applied two of the best performing membranes to real
world applications such as treating municipal, oily and textile wastewaters. In single-
and mixed-solute fouling tests with common components of effluent organic matter in
municipal wastewater feeds (proteins, polysaccharides, natural organic matter, and fatty
acids), these membranes showed complete resistance to irreversible fouling. Excellent
fouling resistance was also observed with oil emulsions common in industrial wastewater.
These membranes also exhibited no irreversible flux decline after filtering real
wastewater samples from a textile dying plant in Turkey. High rejections of dyes and
colored substances were achieved while salts were allowed to permeate, allowing low
pressure operation and potential reuse of effluent. Thus, zwitterionic copolymer
membranes are very promising for municipal and industrial wastewater treatment and
Thesis (Ph.D.)--Tufts University, 2017.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Ayse Asatekin.
Committee: Jerry Meldon, Peggy Cebe, Matthew Panzer, and Meagan Mauter.
Keyword: Chemical engineering.read less