(625f) Ultrathin Coatings Tethered to Commercial Membranes Via Vapor Phase Treatment for Fouling and Chlorine-Resistance

Division: membrane based separation?membrane surface
engineering

Ultrathin coatings tethered to commercial
membranes

via vapor phase treatment for fouling and chlorine-resistance

Rong Yang, Hongchul Jang, Roman
Stocker, Karen K. Gleason

Fouling
refers to the unintended accumulation of organic matters, biomolecules or
organisms on wetted structures, and the subsequent formation of biofilms. It is
very prevalent in membrane-based separation processes and is extremely
undesirable as fouling leads to impaired membrane performance and shortened
membrane lifetime. It has become the bottle-neck to improve the efficiency of membrane-based
separation processes, especially for reverse osmosis (RO) operations.¹ The current antifouling methods in
industrial practice are insufficient, costly and time consuming; methods such
as periodic cleaning also shortens the membrane lifetime. Membrane surface engineering
such as physical adsorption and chemical bond formation are potential solutions
to the membrane fouling problem.¹
However, surface modification is no longer effective in preventing fouling once
the first layer of foulants has adsorbed to the membrane
surface. Therefore, stable coatings that impart ultralow-fouling
characteristics to a surface are highly desired for membrane-based separations.
Zwitterionic coatings are promising candidates to
reduce fouling during membrane separations given the ultra-low absorption of
protein and high resistance to bacteria adhesion and biofilm formation in
long-term tests.²

Most
membranes are made of polymeric materials for the low cost and ease of
fabrication. However, such membranes usually lack solvent- or
thermal-resistance, which makes the existing coating methods for zwitterionic materials inapplicable. A vapor phase
deposition method at low temperature avoids the potential for such damages.
Here, we report on the synthesis of zwitterionic
polymers through vapor phase treatment. A copolymer containing sulfobetaine zwitterionic groups
was synthesized through an initiated chemical vapor deposition (iCVD) technique and was grown directly on various membrane
substrates including commercial membranes as well as electrospun nanofiber mats. Thin film composite (TFC) RO
membranes were chosen because of the significant impact of fouling on the
membrane performance. This also demonstrates the benignness
of the iCVD conditions; although TFC RO membranes are
extremely sensitivity to heat and solvent,¹ full membrane function was
retained after iCVD without compromising the delicate
polyamide layer. It is worth noting that the
coating procedure is substrate-independent and no re-optimization is required
for other membrane substrates.

The iCVD zwitterionic coating is
unique because the antifouling zwitterionic groups
are highly concentrated on the surface, precisely where the antifouling
properties of the zwitterionic groups can be
exploited. Angle-resolved photoelectron spectroscopy (ARXPS) measurements
revealed that 97% of the total nitrogen was in the zwitterionic
form within the top 3 nm, whereas when we probed the top 9 nm, only 30% was
zwitterion (Fig. 1). It is important
to note that the currently observed high concentration of zwitterionic
groups at the surface is exactly opposite to that for films formed by
solution-phase methods.³Coated surfaces exhibited 93% and 98% reduction in
surface absorption of bovine serum albumin (BSA) and sodium alginate (SA), as
measured with the quartz crystal microbalance with dissipation monitoring
(QCM-D). Cell adhesion tests were carried out using Escherichia coli and Vibrio cyclitrophicus, and the coated RO membranes showed greatly
reduced bacteria attachment while on the bare RO membrane surface, significant
bacteria adhesion and formation of a confluent biofilm were observed.

The in situ monitor of polymer coating
growth inside an iCVD chamber enabled the precise
control over coating thickness; ultrathin and pin-hole free coating with the
thickness of ~30 nm was deposited on commercial RO membranes. The inherent
crosslinking in the iCVD copolymer made the coating
insoluble in water. A strategy to covalently attach the coating to RO membranes
was developed to increase the stability of the coating and adapt the coated
membrane for long-term usage in water processing. This is the first time that
the library of iCVD functional groups has been
extended to charged zwitterionic moieties, and the zwitterionic coatings have been applied on delicate
substrates, such as RO membranes.

The iCVD zwitterionic coatings were
then improved to possess chlorine-resistance. Chlorine-resistance is of
interest for membrane-based separation because chlorine bleach is the most
economical and prevalent disinfection technique in water industry. To be
compatible with the disinfection procedure and to further reduce surface
fouling by using chlorine in addition to surface engineering, the resistance to
oxidizing agents is highly desirable for antifouling coatings. However, currently
available zwitterionic coatings are fabricated almost
exclusively with acrylate monomers containing ester and/or amide linkages and
do not stand up to oxidizing agents.^{4, 5}
For example, continuous exposure to water containing even a few parts per
billion (ppb) chlorine degrades the amide chemistry significantly.⁶ The highly desirable chlorine-resistant
zwitterionic coatings have not been fabricated to the
best of our knowledge. Here, we
report for the first time, a novel zwitterionic
chemistry that is also chlorine-resistant. The coating derives from a polymer
that contains no labile linkages susceptible to oxidative damages by chlorine.

The
scale-up of the iCVD process has been demonstrated in
a roll-to-roll system,⁷
which is compatible with the existing infrastructures in membrane industries.
Combining the significant antifouling performance with high stability in strong
oxidizing agent, as well as the scalability of the iCVD
process, this surface modification scheme can reduce the energy usage, improve
the reliability and lower the environmental impact of RO and potentially other
membrane-based separation processes.

Reference

1.         Elimelech,
M.; Phillip, W. A. Science 2011, 333, (6043), 712-717.

2.         Jiang, S.;
Cao, Z. Adv. Mat. 2010, 22, (9), 920-932.

3.         Yang, S.;
Zhang, S.-P.; Winnik, F. M.; Mwale, F.; Gong, Y.-K. J. Biomed. Mater. Res., Part A 2008,
84A, (3), 837-841.

4.         Chiantore,
O.; Trossarelli, L.; Lazzari, M. Polymer 2000, 41, (5), 1657-1668.

5.         Butt, C. M.;
Young, C. J.; Mabury, S. A.; Hurley, M. D.; Wallington, T. J. The Journal of Physical Chemistry A 2009, 113, (13), 3155-3161.

6.         Park, H. B.;
Freeman, B. D.; Zhang, Z.-B.; Sankir, M.; McGrath, J. E. Angewandte Chemie International Edition 2008, 47, (32), 6019-6024.

7.         Gupta, M.; Gleason, K. K. Thin Solid Films 2006, 515, (4), 1579-1584.

Figure 1. The
concentrations of quaternary ammonium and protonated amine along the coating
cross-section and ARXPS spectra. a) Composition of the zwitterionic component (quaternary ammonium) at different
depth in the film, obtained by depth-profiling XPS. b) High resolution ARXPS N
(1s) scans with photoelectron takeoff angles of 19.5^o and c) 90^o.