Zwitterionic Copolymers for Fouling Resistant and Responsive Membranes
Abstract: Fouling is
likely the most important obstacle to the use of membranes in many applications,
especially in those that the feed contains high concentrations of organics such as oil
and biomacromolecules. This thesis focuses on the use of zwitterionic copolymers, which
are known to show excellent fouling resistance and hydrophilicity, to enhance the
fouling resistance of ultrafiltration ... read more(UF) membranes and/or incorporate responsive
features into these systems that can enable self-cleaning mechanisms. One approach to
incorporate these features into UF membranes is during their manufacture, by blending a
commodity polymer with a zwitterion containing copolymer. We synthesized random
copolymers incorporating two different zwitterionic groups, sulfobetaine methacrylate
(SBMA) and sulfobetaine-2-vinylpyridine (SB2VP). We showed that, with the appropriate
choice of parameters, zwitterionic additive content as low as 10 wt.% can lead to
membranes with high flux and complete fouling resistance to oil suspensions and protein
solutions. We also studied the effect of the skeletal architecture of zwitterionic
additives in membrane properties, by comparing zwitterionic copolymers with two
different architectural structures, random and comb-shaped. We showed that both
additives lead to a water permeance five times that of the additive free membrane.
However, comb additive containing membrane exhibits a more prominent ionic strength
responsive change in its surface nanostructure and permeance, in addition to its better
resistance to oil fouling and protein adhesion. As another approach, we developed a
novel self-cleaning, photoresponsive membrane that is capable of removing pre-deposited
foulant layers upon changes in surface morphology in response to UV or visible light
irradiation. This membrane was prepared by coating a porous support membrane with a thin
layer of a comb-shaped graft copolymer with photoreactive side-chains. Before any
photo-treatment, the as-coated membrane allows the adsorption of organic solutes on its
surface, reducing flow rate. Once exposed to UV light, conversion of hydrophobic
spiropyran groups to hydrophilic merocyanine groups leads to the release of adsorbed
molecules and full recovery of the initial water flux. Lastly, we developed selective
thin films whose morphology and function arise from the self-assembled nanostructure of
the zwitterionic comb copolymers. Thin film morphology showed hierarchical features
including 17 ± 5 nm spherical micelles along with larger nanopores of 85 ± 45
nm, which arose from the self-assembly of the copolymer in this bilayer system. In
addition, the film proved ionic strength responsive permeation in ultrafiltration range
with a pore density as high as 1013 pores/m2.
Thesis (Ph.D.)--Tufts University, 2018.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Ayse Asatekin.
Committee: Matthew Panzer, Samuel Thomas III, and Scott Husson.
Keywords: Chemical engineering, Polymer chemistry, and Materials Science.read less