Nonsolvent induced phase separation (NIPS) is the most common approach to produce polymeric membranes. Unfortunately, NIPS relies heavily on aprotic organic solvents like N-methyl-pyrrolidone. These solvents are unsustainable, repro-toxic for humans and are therefore becoming increasingly restricted within the European Union. A new and sustainable method, aqueous phase separation (APS), is reported that eliminates the use of organic solvents. A homogeneous solution of two polyelectrolytes, the strong polyanion poly(sodium 4-styrenesulfonate) (PSS) and the weak polycation poly(allylamine hydrochloride) (PAH), is prepared at high pH, where PAH is uncharged. Immersing a film of this solution in a low pH bath charges the PAH and results in a controlled precipitation, forming a porous water-insoluble polyelectrolyte complex, a membrane. Pore sizes can be tuned from micrometers to just a few nanometers, and even to dense films, simply by tuning the polyelectrolyte concentrations, molecular weights, and by changing the salinity of the bath. This leads to excellent examples of microfiltration, ultrafiltration, and nanofiltration membranes. Polyelectrolyte complexation induced APS is a viable and sustainable approach to membrane production that provides excellent control over membrane properties and even allows new types of separations.
Polymeric
membranes are used on very large scales for drinking
water production and kidney dialysis, but they are nearly always prepared
by using large quantities of unsustainable and toxic aprotic solvents.
In this study, a water-based, sustainable, and simple way of making
polymeric membranes is presented without the need for harmful solvents
or extreme pH conditions. Membranes were prepared from water-insoluble
polyelectrolyte complexes (PECs) via aqueous phase separation (APS).
Strong polyelectrolytes (PEs), poly(sodium 4-styrenesulfonate)
(PSS), and poly(diallyldimethylammonium chloride) (PDADMAC)
were mixed in the presence of excess of salt, thereby preventing complexation.
Immersing a thin film of this mixture into a low-salinity bath induces
complexation and consequently the precipitation of a solid PEC-based
membrane. This approach leads to asymmetric nanofiltration membranes,
with thin dense top layers and porous, macrovoid-free support layers.
While the PSS molecular weight and the total polymer concentrations
of the casting mixture did not significantly affect the membrane structure,
they did affect the film formation process, the resulting mechanical
stability of the films, and the membrane separation properties. The
salt concentration of the coagulation bath has a large effect on membrane
structure and allows for control over the thickness of the separation
layer. The nanofiltration membranes prepared by APS have a low molecular
weight cutoff (<300 Da), a high MgSO
4
retention (∼80%),
and good stability even at high pressures (10 bar). PE complexation
induced APS is a simple and sustainable way to prepare membranes where
membrane structure and performance can be tuned with molecular weight,
polymer concentration, and ionic strength.
The global society
is in a transition, where dealing with climate
change and water scarcity are important challenges. More efficient
separations of chemical species are essential to reduce energy consumption
and to provide more reliable access to clean water. Here, membranes
with advanced functionalities that go beyond standard separation properties
can play a key role. This includes relevant functionalities, such
as stimuli-responsiveness, fouling control, stability, specific selectivity,
sustainability, and antimicrobial activity. Polyelectrolytes and their
complexes are an especially promising system to provide advanced membrane
functionalities. Here, we have reviewed recent work where advanced
membrane properties stem directly from the material properties provided
by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based
membrane modifications, where polyelectrolytes are not only applied
as single layers, including brushes, but also as more complex polyelectrolyte
multilayers on both porous membrane supports and dense membranes.
Moreover, free-standing membranes can also be produced completely
from aqueous polyelectrolyte solutions allowing much more sustainable
approaches to membrane fabrication. The Review demonstrates the promise
that polyelectrolytes and their complexes hold for next-generation
membranes with advanced properties, while it also provides a clear
outlook on the future of this promising field.
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