U.S. patent application number 11/380776 was filed with the patent office on 2007-11-01 for reverse osmosis membrane with branched poly(alkylene oxide) modified antifouling surface.
Invention is credited to Q. Jason Niu.
Application Number | 20070251883 11/380776 |
Document ID | / |
Family ID | 38353405 |
Filed Date | 2007-11-01 |
United States Patent
Application |
20070251883 |
Kind Code |
A1 |
Niu; Q. Jason |
November 1, 2007 |
Reverse Osmosis Membrane with Branched Poly(Alkylene Oxide)
Modified Antifouling Surface
Abstract
Composite membranes that exhibit long-term resistance to
biofouling comprise a porous support and a crosslinked polyamide
discriminating layer having an external surface, the discriminating
layer comprising a branched poly(alkylene oxide) (PAO) polymer
attached to its external surface. The branched PAO polymer
typically has the structure of a molecular comb or brush, and is
made by polymerization of a PAO macromonomer of the following
formula: RO--[(CHR').sub.n--O].sub.m-V in which R is hydrogen or a
C.sub.1-20 aliphatic or aromatic group, V is any group containing a
polymerizable site, each R' is independently hydrogen or a short
chain alkyl group, n is an integer of 1-6, and m is an integer of 1
to about 200. The .alpha. end group can be either polymerized or
copolymerized.
Inventors: |
Niu; Q. Jason; (Chanhassen,
MN) |
Correspondence
Address: |
WHYTE HIRSCHBOECK DUDEK S.C.
555 EAST WELLS STREET
SUITE 1900
MILWAUKEE
WI
53202
US
|
Family ID: |
38353405 |
Appl. No.: |
11/380776 |
Filed: |
April 28, 2006 |
Current U.S.
Class: |
210/653 ;
210/500.27; 210/654; 264/41 |
Current CPC
Class: |
B01D 69/125 20130101;
B01D 71/56 20130101; B01D 67/0093 20130101; Y02A 20/131 20180101;
B01D 2323/30 20130101; B01D 61/025 20130101; B01D 2325/34
20130101 |
Class at
Publication: |
210/653 ;
210/654; 210/500.27; 264/041 |
International
Class: |
B01D 61/00 20060101
B01D061/00 |
Claims
1. A composite membrane comprising a porous support and a
crosslinked polyamide discriminating layer having an external
surface to which are attached crosslinkced, branched poly(alkylene
oxide) (PAO) polymers, the polymers having a relative weight
average molecular weight before crosslinking, as measured by size
exclusion chromatography against a linear PEO standard, of at least
about 5,000.
2. The composite membrane of claim 1 in which the branched PAO
polymers comprise derivatives of PAO macromonomers of the formula:
RO--[(CHR').sub.n--O].sub.mV (I) in which R is hydrogen or a
C.sub.1-20 aliphatic or aromatic group, V is any group containing a
polymerizable site, each R' is independently hydrogen or a short
chain alkyl group, n is an integer of 1-6, and m is an integer of 1
to about 200.
3. The composite membrane of claim 2 in which R is a C.sub.1-20
alkyl group, V is a derivative of at least one of p- and m-vinyl
benzene, p- and m-vinyl benzoic acid, methacryloyl chloride,
acryloyl chloride and isopropenyl oxazoline, R' is hydrogen or
methyl, n is 2 or 3, and m is an integer between about 3 and about
50.
4. The composite membrane of claim 3 in which R is a C.sub.1-12
alkyl group, V is a derivative of methacryloyl chloride, R' is
hydrogen, n is 2, and m is an integer between about 7 and about
25.
5. The composite membrane of claim 4 in which the relative weight
average molecular weight of the PEO brush is at least about
10,000.
6. A branched poly(alkylene oxide) polymer having a relative weight
average molecular weight, as measured by size exclusion
chromatography against a linear PEO standard, of at least about
5,000, the polymer prepared by the copolymerization of
poly(alkylene oxide) and glycidyl methacrylate under atom transfer
radical polymerization (ATRP) conditions.
7. The branched polymer of claim 6 in which the ATRP conditions
include an organic solvent as the polymerization medium.
8. The branched polymer of claim 6 in which the ATRP conditions
include water as the polymerization medium.
9. The branched polymer of claim 6 in which the ATRP conditions
include CuBr/2,2'-bipyridine/ethyl 2-bromoisobutyrate as a
polymerization initiator.
10. A branched poly(alkylene oxide) polymer having a relative
weight average molecular weight, as measured by size exclusion
chromatography against a linear PEO standard, of at least about
5,000, the polymer prepared by the copolymerization of
poly(alkylene oxide) and glycidyl methacrylate with a free radical
initiator.
11. The branched polymer of claim 10 in which the free radical
initiator is azobisisobutyronitrile.
12. A branched poly(alkylene oxide) polymer having a relative
weight average molecular weight, as measured by size exclusion
chromatography against a linear PEO standard, of at least about
5,000, the polymer prepared by the copolymerization of
poly(alkylene oxide) and 2-isopropenyl-2-oxazoline under ATRP or
free radical initiator conditions.
13. A method of desalting brine, the method comprising passing the
brine through a composite membrane comprising a porous support and
a crosslinked polyamide discriminating layer having an external
surface to which are attached branched, crosslinked poly(alkylene
oxide) polymers, the polymers having a relative weight average
molecular weight before crosslinking, as measured by size exclusion
chromatography against a linear PEO standard, of at least about
5,000.
14. A method of treating unpurified water to produce purified
water, the method comprising passing the unpurified water through a
composite membrane comprising a porous support and a crosslinked
polyamide discriminating layer having an external surface to which
are attached branched, crosslinked poly(alkylene oxide) polymers,
the polymers having a relative weight average molecular weight
before crosslinking, as measured by size exclusion chromatography
against a linear PEO standard, of at least about 5,000.
15. A method of preparing a branched poly(alkylene oxide) polymer
having a relative weight average molecular weight, as measured by
size exclusion chromatography against a linear PEO standard, of at
least about 5,000, the method comprising copolymerizing
poly(alkylene oxide) and glycidyl methacrylate under atom transfer
radical polymerization (ATRP) conditions.
16. A method of preparing a composite membrane comprising a porous
support and a crosslinked polyamide discriminating layer having an
external surface to which are attached branched, crosslinked
poly(alkylene oxide) polymers, the method comprising coating the
external surface of the discriminating layer with a branched,
crosslinked PAO polymer having a relative weight average molecular
weight before crosslinking, as measured by size exclusion
chromatography against a linear PEO standard, of at least about
5,000.
17. A method of preparing a composite membrane comprising a porous
support and a crosslinked polyamide discriminating layer having an
external surface to which are attached branched, crosslinked
poly(alkylene oxide) polymers, the method comprising coating the
external surface of the discriminating layer with a branched PAO
polymer having a relative weight average molecular weight before
crosslinking, as measured by size exclusion chromatography against
a linear PEO standard, of at least about 5,000, and then subjecting
the branched PAO polymer attached to the external surface of the
discriminating layer to crosslinking.
18. A composite membrane that exhibits a performance level at least
as effective removal of debris accumulated during use in a
biofouling environment as before its use in such an environment,
the membrane comprising a porous support and a crosslinked
polyamide discriminating layer having an external surface to which
are attached crosslinked, branched poly(alkylene oxide) polymers,
the polymers having a relative weight average molecular weight
before crosslinking, as measured by size exclusion chromatography
against a linear PEO standard, of at least about 5,000.
19. The membrane of claim 18 in which the crosslinked, branched PAO
polymer is a copolymer of PEO and glycidal methacrylate.
Description
FIELD OF THE INVENTION
[0001] This invention relates to membranes. In one aspect, the
invention relates to reverse osmosis (RO) membranes while in
another aspect, the invention relates to thin-film-composite
(TFC)RO membranes. In still another aspect, the invention relates
to TFC RO membranes comprising a porous support and a
discriminating layer in which the exterior surface of the
discriminating layer is chemically modified to reduce or prevent
fouling of the membrane during operation. In yet other aspects, the
invention is a method of modifying the exterior surface of the
discriminating layer of the TFC RO, and a method of using the
modified TFC RO.
BACKGROUND OF THE INVENTION
[0002] Aromatic polyamide TFC RO membranes are ubiquitous in our
daily lives finding application in many industrial areas such as
desalting of brine, ultra-pure water production, environmental
pollution treatment, and the like. The trend for the next
generation of such membranes is to require more sophisticated and
specified functions of the polymeric materials from which they are
constructed to provide for an overall enhanced performance of the
membrane. This, in turn, drives the need for so-called "tailor fit"
materials whose functions and properties are precisely tuned for
the intended application of the membrane.
[0003] Tailor fit materials for RO TFC membranes are available
through either (i) design and synthesis of totally new polymers
forming the thin film discriminating layer of the RO membranes, or
(ii) the physical and/or chemical modification of the thin-film.
The former approach has produced TFC RO membranes of enhanced water
flux but with an accompanying considerable loss of salt rejection,
or vice versa. The latter approach results from one of two routes
that involve either (i) the post-treatment of the thin-film surface
of the membrane with various chemicals, or (ii) the use of
additives during the formation of the thin film.
[0004] Regarding posttreatment, a number of RO membranes have been
coated with either with polyvinyl alcohol (PVA) or a vinyl acetate
homopolymer with self-crosslinking functionality (e.g., Vinac.TM.
available from Air Products Polymers, L.P.). Regarding the use of
additives, a number of membranes, particularly nanofiltration
membranes, have been prepared with polymer additives that
presumably have been incorporated in the membrane. Important
improvements to the membrane resulting from modification of the
exterior surface of the discriminating layer include stabilizing
the discriminating layer during long-term operations, and balancing
the improvement of rejection against the loss of flow due to the
alteration of the membrane transport characteristics.
[0005] FIG. 1 is a schematic representation of a cross-section of a
commercially successful RO TFC membrane, e.g., an FT-30 TFC RO
membrane by FilmTec Corporation of Edina, Minn. The first or top
layer is an ultra-thin barrier or discriminating layer typically
comprising a crosslinked polyamide of 10-100 nanometers (nm) in
thickness. One method of preparing this layer is by the interfacial
polymerization of m-phenylenediamine (MPD) in the aqueous phase and
trimesoyl trichloride (TMC) in the organic phase.
[0006] The second or middle layer typically comprises an
engineering plastic, such as polysulfone, and it typically has a
thickness of about 40 microns (.mu.m). This second layer provides a
hard, smooth (relative to the third layer) surface for the top
layer, and it enables the top layer to perform under high operating
pressure, e.g., 10 to 2,000 psi.
[0007] The third or bottom layer is typically nonwoven polyester,
e.g., a polyethylene terephthalate (PET) web, with a thickness
typically of about 120 .mu.m. This third or bottom layer is
typically too porous and irregular to provide a proper, direct
support for the top layer, and thus the need for the second or
middle layer.
[0008] The RO TFC membrane is usually employed in one or two
different configurations, i.e., flat panel or spiral wound. The
flat panel configuration is simply the membrane, or more typically
a plurality of membranes separated from one another by a porous
spacer sheet, stacked upon one another and disposed as a panel
between a feed solution and a permeate discharge. The spiral wound
configuration is shown schematically in FIG. 2, and it is simply a
membrane/spacer stack coiled about a central feed tube. Both
configurations are well known in the art.
[0009] From the viewpoint of performance efficiency, TFC membranes
are usually required to have dramatically enhanced water
permeability without sacrificing salt separability. Such aromatic
polyamide TFC membranes with excellent water flux and reasonable
salt rejection characteristics are formed by the interfacial
reaction of MPD/TMC that have been kinetically altered with
organo-metals and non-metals to form complexes of the TMC as taught
in U.S. Pat. No. 6,337,018. This (i) reduces the rate of the
reaction of the TMC by reducing the diffusion coefficient and use
steric hindrance to block MPD from the acid chloride sites, and
(ii) complexes the TMC to block water from hydrolyzing the acid
chlorides.
[0010] Unlike the chemically analogous FT-30 membrane, the
kinetically modified version based on MPD/TMC interfacial
polymerization results in modification in surface morphology and
variation in the polymer chain organization during formation of the
thin film. The combined effect is to increase the rejection of the
membrane and to allow the use of other process variables to
influence the rate of reaction and therefore the membrane
performance. This approach allows for an increase to the membrane
flux by over 100% in certain products, e.g., FilmTec's XLE
membranes, due to the reduced residual acid chloride after
interfacial polymerization and improved swelling ability of the
resulting thin film, and it gives the possibility of compensating
for flow loss in future post-treatment of thin-film surfaces.
[0011] Many applications using membrane processes could benefit
from the availability of a wide range of polymer chemistries, e.g.,
they could exhibit better performance, more robustness and less
fouling, and they could use less expense polymers. However, due to
the uncertainty of new chemistry and the reluctance of companies to
invest in the development of new polymers, alternate approaches
such as the surface modification of widely used polymers have
increased in importance.
[0012] One of the goals of research and industry in the RO membrane
field is to enhance, or at least maintain, water flux without
sacrificing salt rejection over a long period of time in order to
increase the efficiency and reduce the cost of the operation.
Nevertheless, the main difficulty in accomplishing this goal is
fouling that produces a serious flux decline over the operational
time of the membrane.
[0013] The principal types of fouling are crystalline fouling
(mineral scaling, or deposit of minerals due to an excess in the
solution product), organic fouling (deposition of dissolved humic
acid, oil, grease, etc.), particle and colloid fouling (deposition
of clay, silt, particulate humic substances, debris and silica),
and microbial fouling (biofouling, adhesion and accumulation of
microorganisms, and the formation of biofilms). Various approaches
to reducing fouling have been used, and these usually involve
pretreatment of the feed solution, modification of the membrane
surface properties (e.g., the attachment of hydrophobic or
hydrophilic, and/or electronegative or electropositive groups),
optimization of module arrangement and process conditions, and
periodic cleaning. However, these methods vary widely in
applicability and efficiency and this, in turn, has required
continuous, on-going efforts to solve these problems.
[0014] For polyamide RO TFC membranes, fouling from the formation
of biofilm on the surface caused by microorganisms has been
regarded as of the uppermost importance. Microorganisms, such as
bacteria and viruses, in the water to be filtered, as well as other
microscopic material, e.g., protein, adhere to membrane surfaces
and grow at the expense of nutrients accumulated from the water
phase. The attached microorganisms excrete an extra-cellular
polymeric substance (EPS), and this, in combination with the
microorganism and protein, form a biofilm. Biofilm formation is
believed related to the depletion of residual disinfectant
concentration, and that biofilm is not formed from
disinfectant-treated water, such as chlorinated water containing a
residual of 0.04-0.05 milligrams per liter (mg/L) of free chlorine.
However, chlorination, although effective for the destruction of
microorganisms, generates harmful byproducts such as
trihalomethanes and other carcinogens.
[0015] Protein, cell and bacterial fouling of the membrane surface
occur spontaneously upon exposure of the membrane surface, i.e.,
the external surface of the discriminating layer, to physiologic
fluids and tissues. In many cases, biofouling is an adverse event
that can impair the function of RO membranes. Common strategies for
inhibiting biofouling include grafting antifouling polymers or
self-assembled monolayers onto the membrane surfaces. Many
synthetic polymers have been investigated as antifouling coatings,
and these have met with variable success in antifouling tests.
[0016] One common and prominent example of a material used to
render a surface inert to nonspecific protein adsorption in medical
devices is poly(ethylene oxide) (PEO), a linear, flexible,
hydrophilic and water-soluble polyether. Self-assembled monolayers
(SAMS) presenting oligo(ethylene glycol) (OEG) groups (as in
HS(CH.sub.2).sub.11(EG).sub.nOH)) on a gold surface also prevent
the adsorption of proteins, even if the number of ethylene glycol
(EC) units present is as low as three. Anti-fouling membranes based
on grafted, linear polyalkylene oxide oligomers are known, and they
provide an improved resistance to fouling while offering excellent
flux and salt passage performance (U.S. Pat. No. 6,280,853).
SUMMARY OF THE INVENTION
[0017] The present invention provides improved reduced fouling
composite membranes and methods for their preparation. In one
embodiment, the present invention develops and characterizes new
branched poly(alkylene oxide) (PAO) modified TFC RO membranes
capable of preventing nonspecific protein adsorption as a means of
precluding the formation of biofilms and, hence, reduced fouling.
These branched, particularly the highly branched, PAO-modified TFC
RO membranes exhibit surprisingly improved stability in fouling,
particularly biofouling, environments. Moreover, the membranes of
this invention are more thoroughly cleaned under either basic or
acidic conditions than linear PAO-modified TFC RO membranes.
[0018] In another embodiment, the invention is a composite membrane
comprising a porous support and a crosslinked polyamide
discriminating layer having an external surface to which are
attached crosslinked and branched poly(alkylene oxide) polymers of
a relative weight average molecular weight (before crosslinking),
as measured by size exclusion chromatography against a linear PEO
standard, of at least about 5,000, preferably of at least about
10,000, more preferably between about 20,000 and about 1,000,000,
and even more preferably between about 100,000 and about
500,000.
[0019] In certain preferred embodiments of the invention, the
branched PAO polymers used in the practice of this invention are
made from the polymerization of macromonomers of the following
formula: RO--[(CHR').sub.n--O].sub.m-V (1) in which V is the
.alpha. end group, R is the .omega. end group, each R' is
independently hydrogen or a short chain, e.g., C.sub.1-3, alkyl
group, n is an integer of 1-6, and m is an integer of 1 to about
200. Polymerization of the macromonomer occurs through the .alpha.
end group, and it can be either polymerized or copolymerized with
comonomer processes through either V or other a end groups. R is
typically a C.sub.1-20 aliphatic or aromatic group; V is a
derivative of any compound containing a polymerizable site, e.g., a
group containing a double bond such as a derivative of p- or
m-vinyl benzene, or p- or m-vinyl benzoic acid, or methacryloyl
chloride, or acryloyl chloride or isopropenyl oxazoline; R' is
preferably hydrogen or methyl; m is an integer preferably of 2 or
3; and n is an integer preferably between about 3 and about 50,
more preferably between about 7 and about 25. The macromonomers of
formula I include both homo- and copolymers and if a copolymer,
then random, block and mixed random/block polymers, e.g., PEO
macromonomers, poly(propylene oxide) (PPO) macromonomers, and
random and block macromonomers based on both ethylene oxide and
propylene oxide units. As here used, "copolymer" means a polymer
made from two or more monomers.
[0020] The branched PAO polymers exhibit three prominent structural
features that impart good protein resistance to the external
surface of the discriminating layer, i.e., (i) a hydrophilic
repeating unit, i.e., a unit that hydrogen bonds with water and is
thus water-soluble (it swells in water), (ii) an oligomer side
chain that is very flexible due to aliphatic ether bonds, and (iii)
a branched, preferably a highly branched, architecture that forms a
dense protective layer for the external surface. The "external
surface of the discriminating layer" is the surface of the
discriminating layer that is in contact with the material, e.g.,
solution, dispersion, etc., to be filtered and opposite the surface
of the discriminating layer that is in contact with the porous
support.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic cross-section of a thin film composite
membrane.
[0022] FIG. 2 is a schematic of a TFC membrane in a spiral wound
configuration.
[0023] FIG. 3 is a graph reporting the molecular weight
distribution of certain crosslinkable PEO brushes made by radical
polymerization of PEO macromers with AIBN.
[0024] FIG. 4 is a graph reporting a comparison of flux and salt
passage between a branched PEO-modified membrane (PEO Brush, MA2)
and two linear PEO-modified membranes (the PEO Macromers, MA1 and
MA2).
[0025] FIG. 5 is a graph reporting flux and salt passage of several
membranes prepared by surface modification with branched PLO
polymers.
[0026] FIG. 6 is a graph reporting the results of an oil/soap
fouling experiment comparing a crosslinked, branched PEO-modified
membrane of this invention (571-5 Brush) with four commercially
available membranes.
[0027] FIG. 7 is a graph reporting a comparison of the relative
productivity of an element made from a crosslinked, branched
PEO-modified membrane (571-5) with four commercially available
membranes.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] "Recovery" means the percentage of membrane system feedwater
that emerges from the system as product water or permeate. Membrane
system design is based on expected feedwater quality, and recovery
is defined through initial adjustment of valves on the concentrate
stream. Recovery is often fixed at the highest level that maximizes
permeate flow while preventing precipitation of super-saturated
salts within the membrane system.
[0029] "Rejection" means the percentage of solute concentration
removed from system feedwater by the membrane. In reverse osmosis,
a high rejection of total dissolved solids (TDS) is important;
while in nanofiltration the solutes of interest are specific, e.g.,
low rejection for hardness and high rejection for organic
matter.
[0030] "Passage" means the opposite of "rejection". Passage is the
percentage of dissolved constituents (contaminants) in the
feedwater allowed to pass through the membrane.
[0031] "Permeate" means the purified product water produced by a
membrane system.
[0032] "Flow" means the rate of feedwater introduced to the
membrane element or membrane system, usually measured in gallon per
minute (gpm) or cubic meters per hour (m.sup.3/h). Concentrate flow
is the rate of flow of non-permeated feedwater that exits the
membrane element or membrane system. This concentrate contains most
of the dissolved constituents originally carried into the element
or into the system from the feed source. It is usually measured in
gpm or m.sup.3/h.
[0033] "Flux" means the rate of permeate transported per unit of
membrane area, usually measured in gallons per square foot per day
(gfd) or liters per square meter per hour (l/m.sup.2h).
[0034] "Macromonomer" or "macromer" are abbreviations of
macromolecular monomer. Macromonomer generally refers to a linear
macromolecule possessing a polymerizable group at the chain end. In
most cases, the polymerizable group is of a vinyl type; typically a
styryl, (meth)acryl, or vinylester group.
[0035] "Branched polymer" and similar terms mean a nonlinear
polymacromonomers, i.e., a macromonomer comprising a core chain or
backbone with one or more side chains or arms extending from it.
Polymerization of macromonomers provides a series of model branched
polymers, e.g., comb, brush, star and dendritic. While some
fluidity in terms exist, comb polymers typically refer to polymers
comprising a backbone and one arm depending from each split point,
each arm extending in the same direction (assuming a straight,
i.e., untwisted, backbone). Brush polymers typically refer to
polymers comprising either a backbone with two arms depending from
each split point, or a backbone with one arm extending each from
each split point but all the arms not extending in the same general
direction. Star polymers typically refer to polymers comprising a
backbone with three arms extending from a split point, and
dendritic polymers (or dendrimers) typically refer to polymers
comprising a multifunctional core molecule with a branched wedge
attached to each functional site. The branched wedge is normally
built stepwise with a regular monomer leading to a mono-disperse,
tree-like or generation structure. Generally, homopolymerization of
macromonomers affords regular comb polymers of a well-defined
structure. Thus, a PEO macromonomer, e.g., C.sub.1-PEO-MA, (see
Scheme 1) readily polymerizes to give a polymethacrylate with PEO
side chains, which are regularly and densely spaced, each on every
repeating unit of methylacrylate backbone. Polymer brushes are more
fully described by Zhang, M. and Muller, A. H. E. in Cylindrical
Polymer Brushes, J. Polym. Sci. Part A: Polym. Chem.: 43 (2005),
pp. 3461-3481.
[0036] "Graft polymers" and like terms mean that a macromonomer was
polymerized or copolymerized with other comonomers to form a
homogeneous macromolecule. The macromonomers and comonomers are
linked via covalent bonds by a free radical mechanism instead of
simple blending without reaction.
[0037] "Linear polymers" and like terms mean macromers essentially
free of branching. As here used, "branching" and like terms mean an
arm or side chain attached to a backbone, and the minimum length of
the arm is at least as long as the longest monomer from which the
backbone is derived. In Scheme 1, the arm or branch is the
molecular segment derived from PEO, not the methyl group attached
to the backbone that forms a part of the methacrylate monomer from
which the backbone is derived.
[0038] The composite membranes of this invention include a porous
(sometimes called a microporous) support and a relatively thin
crosslinked polyamide discriminating layer. In some known
embodiments, e.g., the composite membranes of U.S. Pat. No.
6,280,853, linear PEO groups are grafted to the surface of the
crosslinked polyamide discriminating layer. In this invention,
these PAO groups are in the structure of a branched, preferably a
highly branched, polymer, e.g., a comb or brush, and are
crosslinked either with surface functional groups or through inter
or intra macromolecule reactions. The grafting can be accomplished
as a post-treatment on a pre-made membrane, e.g., FT-30 available
from FilmTec Corporation, or during membrane fabrication, e.g.,
just after the initiation of the interfacial polymerization of the
polyamine and polyfunctional acyl halide reaction that forms the
crosslinked discriminating layer. The manufacture of composite
membranes and the surface grafting with PAO groups are well known
and are described in, among other places, U.S. Pat. No.
6,280,853.
[0039] In one preferred embodiment of this invention, the PAO
macromonomer is a macromonomer of PEO. PEO macromonomers with a
number average molecular weight (Mn) of about 200 to about 10,000
g/mole are well-known, water-soluble, commercially available
nonionic oligomers with a variety of practical applications. They
have been the subject of a number of recent publications describing
the synthesis of well defined, graft copolymers/brushes by
copolymerization with one or more of any of a number of
conventional monomers, and regular comb polymers by
homopolymerization. One of the most important and interesting
features of PEO macromonomers, like other PAO macromonomers, is
their amphiphilic nature. These macromonomers are soluble in a very
wide range of solvents including water, alcohol, benzene, and even
petroleum, depending on the nature of their end groups, R and V,
and the PEO chain length m. Such amphiphilicity is not available
with conventional monomers, and this makes their polymerization
chemistry very facile.
[0040] Polymerization of macromonomers provides a series of model
branched polymers. In particular, homopolymerization affords
regular comb and brush polymers of a well-defined structure. For
example, a PEO methacrylate readily polymerizes with free radical
initiators, e.g., azobisisobutyronitrile (AIBN), or atom-transfer
radical initiators, e.g., (CuBr/2,2-bipyridine/ethyl
2-bromoisobutyrate) in an organic solvent, in water or in bulk to
give a poly(methacrylate) with PEO side chains which are regularly
and densely spaced, each on every repeating unit of the
methacrylate backbone, as shown in Scheme 1. ##STR1##
[0041] As indicated above, the branched architecture of the film
obtained from polymerization of PEO methacrylate is particularly
interesting in cases where protein adsorption is to be avoided
since this architecture combines both high-density grafting and
high PEO segment mobility. Table 1 summarizes the properties of
various PEO macromonomers useful for the synthesis of branched,
e.g., comb or brush, polymers. All the monomers are widely
available from commercial sources. Both the .alpha. and .omega.
groups as well as the polymer chain length can be modified to meet
various requirements and performance. TABLE-US-00001 TABLE 1 PEO
Macromonomers Useful in the Preparation of Branched PEO Polymers
.alpha. Group (Abbreviation) .omega. Group Mn (g/mole) Methacryloyl
(MA1) Methyl 475 Methacryloyl (MA2) Methyl 1,100 Acryloyl (AA1)
Methyl 454
[0042] Although Simple Coatings Such as Linear PVA have been
Applied to the External surface of the discriminating layer of TFC
RO membranes for the improvement of performance, experience has
shown that they often wash out and demonstrate a reduction in
performance over time. By incorporating crosslinkable groups into
the branched polymers, this deficiency can be reduced or
eliminated. One common method for incorporating a pendent
crosslinkable group into the polymer is by using dual functional
comonomers during polymerization. Monomers that contain dual
functionality allow for the preparation of unique polymeric
structures. One such monomer is 2-isopropenyl-2-oxazoline (IPO),
which readily copolymerizes with most commercially useful monomers
such as PEO methacrylate via the isopropenyl group, while the
oxazoline functionality is capable of both polymerization under
acid catalysis and facile coupling via a ring-opening reaction with
carboxylic acid. As a consequence, the oxazoline group serves as a
crosslinkable group on the external surface of the discriminating
layer of the membrane because many TFC RO membranes contain many
carboxylic acid functionalities on this surface.
[0043] Another useful crosslinking monomer is glycidyl methacrylate
(GMA) that can polymerize with a PAO methacrylate, e.g., PEO
methacrylate, under free radical conditions to form a branched PAO
polymer with pendent epoxy groups (much in the same manner as the
formation of epoxy resins). The reactions between oxirane
(glycidyl, epoxy) groups and residual amines (MPD) on the surface
of membrane form the basis for crosslinked coatings. Still another
useful crosslinking monomer is maleic anhydride (MAH). The
anhydride group can react with residual amine groups on the
external surface of the discriminating layer to form a crosslinked
polymer on the surface of membrane. ##STR2##
Structure of Glycidyl and MAH Comonomers
[0044] Free radical copolymerization of PAO, e.g., PEO, methyl
ether methacrylate with either IPO or glycidyl monomers using AIBN
as a radical initiator in dioxane (50% wt of monomers) affords high
molecular weight, branched polymers which are still soluble in
water. Since IPO and MAH decompose in an aqueous solution, water is
not used as polymerization solvent when IPO or MAH are used as the
comonomers. The relative molecular weights of branched polymers
were measured by size exclusion chromatography (SEC) using narrow
poly(ethylene oxide) as a standard, and these are reported in Table
2. The high polydispersity (Table 2 and FIG. 3) of these polymers
indicates that they are highly branched. The absolute molecular
weight of these macromers could be much higher than reported in
Table 2 because such highly branched PEO macromers are more compact
in solution than linear PEO macromers. TABLE-US-00002 TABLE 2
Characterization of Branched PEO Made by AIBN Initiator
Macromonomer Comonomer Mn(g/mole) Mw(g/mole) Mw/Mn MA1 (100% wt)
None 13,200 112,000 8.48 MA1 (92% wt) GMA (8%) 13,600 87,500 6.43
MA1 (86% wt) GMA (14%) 15,800 113,000 7.15 MA1 (80% wt) GMA (20%)
14,000 70,500 5.04 MA1 (74% wt) GMA (26%) 17,300 109,000 6.30 MA2
(86% wt) GMA (14%) 26,500 221,000 8.34 AA1 (85% wt) GMA (14%)
13,100 123,000 9.39 MA1 (86% wt) IPO (14%) 7,860 17,500 2.23 MA1
(86% wt) MAH (14%) 32,700 317,000 9.69
[0045] On the other hand, atom transfer radical polymerization
(ATKP) of a PAO, e.g., PEO, methyl ether methacrylate can be done
in water or in bulk with or without glycidyl methacrylate
comonomer. For example, an ATRP formulation in which the transition
metal catalyst was CuBr and the ligand was 2,2'-bipyridine was
prepared. The initiator 2-bromoisobutyrate is insoluble in water
but dissolves in an aqueous PEO methyl ether methacrylate solution
at 20 C. Various conditions were examined for this polymerization,
and the results are report in Table 3. Compared to AIBN
polymerization, ATRP provided good control of the molecular weight
and polydispersities (less than 2.0 in most cases). The rate of
polymerization in aqueous ATRP at 20 C is markedly faster than
conventional ATRP (bulk or in organic solvent) at elevated
temperature (65 C). Moreover, under ATRP conditions, high
conversion is achieved while residual glycidyl methacrylate at less
than 0.05% in most cases after polymerization. TABLE-US-00003 TABLE
3 Preparation of Branched PEO by Atom Transfer Radical
Polymerization Using MA1 (89 wt %) and GMA (11 wt %) Mn Mw
Temperature/Solvent [M]/[I]* (g/mole)** (g/mole) Mw/Mn 65 C./Bulk
212 41,000 87,700 2.14 65 C./Dioxane (33%) 180 34,900 56,800 1.63
65 C./Water (33%), 197 38,900 84,200 2.16 crude 65 C./Water (33%),
197 43,900 86,300 1.97 purified 20 C./Water (33%) 197 41,200 78,700
1.91 20 C./Water (50%) 212 43,900 102,000 2.32 20 C./Water (33%)
115 24,100 37,600 1.56 *[M]/[I] means monomer/initiator. **Measured
by SEC in dimethylformamide using linear narrow molecular weight
PEO as the standard
[0046] In the examples reported below, size exclusion
chromatography (SEC) was used to provide relative weight average
molecular weight data for the branched PEO polymers. The
experimental procedure was as follows: [0047] Sample Prep: The
solutions were prepared by placing approximately 0.04 grams of
sample in 10 ml of the N,N-Dimethylformamide containing 0.4 w/v %
LiNO.sub.3. The target polymer concentration in the final DMF
sample solution was 2 mg/mL. The solutions were shaken for about 4
hours and filtered through an Alltech 0.2 micron (.mu.m) Nylon
filter with a syringe prior to injection. [0048] Pump: Waters model
2695 separations module at a nominal flow rate of 1.0 mL/min.
[0049] Eluent: Fisher ACS certified dimethylformamide containing
0.4 w/v % LiNO.sub.3, vacuum degassed in line. [0050] Injector:
Waters model 2695 separations module set to inject 50 microliters
of sample. [0051] Columns: Two Polymer Laboratories 10 .mu.m
Mixed-B at 50.degree. C. [0052] Detection: Waters 410 DR1 with a
sensitivity of 128, scale factor of 1, and temperature of
50.degree. C. [0053] Data system: Polymer Laboratories Calibre
GPC/SEC, acquisition version 6.0 and re-analysis version 7.04.
[0054] Calibration: The calibration was determined using narrow
molecular weight polyethylene oxide standards from Polymer
Laboratories over the range of 960 to 1,169,000 g/mole. Yau, W. W.,
Kirkland, J. J., and Bly, D. D., Modern Size Exclusion Liquid
Chromatography, John Wiley &Sons, NY, 1979 provide a general
description of the SEC method.
Specific Embodiments
[0054] Preparation of Membranes
[0055] FT-30 reverse osmosis composite membranes were prepared on a
FilmTec Corporation pilot coater in a continuous process. First the
MPD was applied in water to the pre-made microporous polysulfone
support including the backing non-woven fabric, and then the
support was drained and nip rolled to remove the excess aqueous
solution. The top surface of the support was sprayed with a
solution of TMC in Isopar L (available from ExxonMobil Corp.).
[0056] At the oil water interface the polyamide was formed. The
first coating was made with a MPD solution of 2.0 to 4.0%, and the
second coating was made with a TMC concentration of 0.13% (5 mM).
The TMC solution also included a molar stoichiometric ratio of TBP
(tributyl phosphate) to TMC of 1:1. The membrane traveled first
through a room temperature water bath after application of the
second coating, then through a 98 C bath that contained 3.5% of
glycerin. At this stage, a layer of PEO brush was coated onto the
surface of membrane by contact with a coating roller, and the
membranes were dried through an air floatation dryer at a
temperature of 95 C. The test was done according to standard test
conditions of 150 psi and 2000 ppm NaCl for the BW membranes.
Post-Treatment of XLE Membranes
[0057] XLE BW RO membranes were obtained from FilmTec Corporation.
Aqueous treatment solutions were prepared by heating the
appropriate quantity of water at 75 C unless otherwise noted,
followed by the addition of an appropriate quantity of either
poly(ethylene oxide) (PEO) brushes with different weight average
molecular weights (Mw). The membranes were submerged in the PEO
brush solution for a given time. The membranes were then tested
utilizing an aqueous test solution containing approximately 2,000
ppm at a cross-membrane pressure of 150 psi.
Synthesis of PEO Brushes by AIBN Initiator
[0058] In a 250 ml round flask was added 34.4 g of poly(ethylene
glycol) methyl ether methacrylate (average Mn .about.475), 5.6 g of
glycidyl methacrylate, 40 g of dioxane and 1.0 g of AIBN. The
resulting mixture was purged with argon for 15 minutes, and then
heated under argon for 8 hours at 75 C. SEC analysis confirmed the
formation of polymer brushes with an Mw of 113,000 g/mole
(Mw/Mn=7.15) based on narrow molecular weight polyethylene glycol
standards. This polymer solution was used without purification.
Synthesis of PEO Brush by ATRP
[0059] In a small ACE Diels-Alder reaction tube was added 29.2 g of
poly(ethylene glycol) methyl ether methacrylate (Mn .about.475
g/mole), and 3.6 g of glycidyl methacrylate. The mixture was purged
with argon for 5 minutes before 104 mg of CuBr, 226 mg of
2,2'-bipyridine (also know as .alpha.,.alpha.'-dipyridyl from
Aldrich) were added. The solution immediately became brown,
indicating formation of Cu(I)-2,2'-dipyridyl complex. With
continuing purging of argon, 80 mg of ethyl 2-bromoisobutyrate
initiator was added and the solution was sealed with a Teflon cap.
After heating the mixture at 65 C for 7 hours in an oil bath, a
viscous polymer was obtained. SEC analysis showed that this polymer
had a Mn of about 41000 g/mole with a polydispersity of 2.1.
Dissolving it in THF and then precipitating into ether isolated the
resulting polymer.
Performance of PEO Brush Modified Membranes:
[0060] The performance of various polyamide membranes based on
surface modification from PEO brushes are shown in FIGS. 4 and 5,
and all the membranes were made using FilmTec's pilot plant
technology. FIG. 4 shows the performance (flux and NaCl passage at
0.2, 0.4 and 0.6% aqueous solution concentrations) of an uncoated
membrane (XLE Control), a standard brackish water membrane coated
with PVA (BW Standard), two membranes coated with linear PEO
macromers (PEO Macromers with Mn of 475 and 1100, respectively),
and a membrane coated with a brush made by the copolymerization of
a PEO macromer (Mn of 1100) with an IPO monomer (14 wt %). The
water flux decreased while the salt passage remained essentially
level as the PEO macromonomer weight increased. However, when the
PEO brush was coated on the membrane surface, the salt passage was
greatly improved with a decrease in water flux. This was probably
due to the surface coverage of PEO because a high molecular weight
polymer tends to stay on the surface longer than does a low
molecular weight polymer. Moreover, viscosity plays an important
role in the application of the PEO because an increased amount of
PEO can be applied to the surface if it has a relatively high
viscosity, and this reduces the fraction between the coating roller
and the membrane. In addition, PEO macromonomers contain
methacrylate as pendent reactive groups while PEO brushes contain
IPO as crosslinkable groups, and such a structural difference
influences the coverage of the membrane surface. For all the
membranes, the BW Standard gave the worst salt passage while the
PEO brush membrane gave the best salt passage.
[0061] FIG. 5 shows the flux and NaCl passage at 0.2, 0.4 and 0.6%
aqueous solution concentrations of several other membranes prepared
by surface modification of PEO brushes. High flux is observed in
the case of the XLE control, as expected. When the XLE control
membrane is coated with a PEO brush, the flux decreased
dramatically, and it reached the flux level of a standard BW
membrane. However, the salt passage of these coated membranes was
at the level of 0.3% at 150 psi tested pressure and 2000 ppm NaCl
that is only 1/3 of the BW membrane. Overall, the SW Standard
membrane has essentially similar flux but a much lower salt passage
based on the surface modification of high flux XLE membranes. Given
the similar repeat units of PEO brushes, longer chain PEO brushes
offer improvement of salt passage but decrease of flux.
[0062] In addition, the effect of glycidal methacrylate (GMA)
concentration on the performance of PEO brush modified BW membranes
was evaluated. The results are shown in Table 4 and as reported
there, PEO brush with less GMA during polymerization gives better
salt passage. More GMA increases the salt passage and reduces the
flux. The optimum is around 10% GMA. At the same concentration of
comonomers, IPO (oxazoline) containing PEO give worse salt passage
than that of GMA containing PEO brush. Compared to standard BW
control and XLE control, membranes coated with PEO brushes cut the
salt passage by 2 to 5 fold. In order to obtain efficient surface
modification, the concentration of PEO brushes has to be around
0.3%. This is also a dramatic decrease given that the concentration
of PVA in the surface modification for the preparation of BW
membranes is around 1%. TABLE-US-00004 TABLE 4 Effect of GMA on the
Performance of PEO Brush Modified Membranes* Sample Name Flux SP
(NaCl), % XLE control 32.58 .+-. 2.59 0.517 .+-. 0.052 XLE control
34.51 .+-. 0.81 0.489 .+-. 0.039 0.3% PEO brush (no GMA) 18.97 .+-.
1.18 0.264 .+-. 0.019 0.3% PEO brush (8% GMA) 19.85 .+-. 1.08 0.256
.+-. 0.049 0.3% PEO brush (14% GMA) 18.10 .+-. 1.87 0.252 .+-.
0.034 0.1% PEO brush (14% GMA) 19.84 .+-. 0.96 0.318 .+-. 0.040
0.3% PEO brush (20% GMA) 17.80 .+-. 1.60 0.304 .+-. 0.055 0.3% PEO
brush (26% GMA) 15.91 .+-. 0.72 0.307 .+-. 0.055 0.3% PEO brush
(14% IPO) 18.69 .+-. 1.49 0.315 .+-. 0.057 BW control 22.93 .+-.
0.42 1.080 .+-. 0.027 *The test conditions were 150 psi with 2000
ppm NaCl.
[0063] As discussed above, crosslinked aromatic polyamides made
from the in situ interfacial polymerization of MPD in the aqueous
phase and TMC in the organic phase are of considerable importance
in the development of commercial composite membranes. The salt
passage and flux of such XLE membranes can be adjusted by
controlling the MPD and TMC concentrations and the ratio of TMC to
TBP. This can reduce or eliminate the flow effect on fouling
evaluation. For example, by increasing the MPD concentration from
the standard 2.4% to 5.0%, the flux can be adjusted from the
standard XLE level to half of that level, a level very close to the
level of PEO modified membranes. In FIGS. 6 and 7, both the
commercial LE and 517-LE membranes were without an extra layer of
coating. However, the fluxes were different since 5.0% of MPD was
used during the preparation of 517-LE, thus making 517-LE a
non-coating standard for direct comparison.
[0064] Interactions between the membranes and components in the raw
water cause a rapid and often irreversible loss of flux through the
membrane. Many studies suggest that natural organic matter (NOM) is
the most important foulant. FIG. 6 shows the performance of
selected membranes fouled with sodium lauryl sulfate (SLS) and
dodecane (a C.sub.12 hydrocarbon). The tests were run with real
elements made from PEO modified surfaces and some commercial
elements. As seen from FIG. 6, the percentage of flux retained for
PEO brush-modified membranes is much higher than that of the
commercial membranes, a clear indication that a PEO
surface-modified membrane is capable of resisting NOM fouling. In
addition, PEO brush-modified membranes show outstanding performance
toward oil/soap fouling and flow recovery after cleaning, while the
conventional elements showed very poor performance toward oil/soap
fouling.
[0065] The ability of PEO modified surfaces to resist
bacterial/cell attachment over a long period of time was determined
by running the commercial membrane tests under tap water using
sodium acetate as bacterial food. The elements were specially
designed and fabricated so that all the membranes had similar flux
(around 30 gfd at 150 psi and 2000 ppm sodium chloride (NaCl)) thus
minimizing the effect of flux.
[0066] As shown in FIG. 7, the PEO brush-modified surface exhibited
remarkably low levels of cell attachment for over two weeks and
thus the flow loss was the lowest among the membranes tested. In
contrast, commercial BW membranes show high loss of flow. The flow
recovery after membrane cleaning indicates the same trend, i.e.,
PEO brush membranes perform best due to their antifouling
characteristics. Since cell attachment to surfaces is typically
mediated by adsorbed extra cellular polysaccharide, the membrane
coated with PEO brush has very low extra cellular polysaccharide
adsorption throughout the course of experiment. This excellent
extra cellular polysaccharide resistance is maintained for several
weeks, and it can be directly attributed to the chemical
composition of the anchoring (surface-active group (epoxy) and
antifouling domains (PEO chain). The epoxy groups are believed to
react with the residual amino groups from MPD, forming a robust
anchor for the antifouling (PEO chain) portion of the polymer.
These groups are stable to strong acid (e.g., a pH of 2.0) and
strong base (e.g., pH of 13) cleaning.
[0067] Moreover, FIG. 7 shows that the membranes of this invention
can be readily cleaned. The comparison membranes, particularly the
membrane surfaced modified with a linear PEG oligomer (571-4, PEG
Olig), gave an inferior performance from the start of the test.
[0068] The design of the PEO side chain and methacrylate backbone
adheres to general principle that effective antifouling surfaces
require the presence of hydrogen bond acceptors, lake of hydrogen
bond donors, a neutral charge, and water solubility. Additional
benefits of PEO brushes include readily available starting PEO
macromonomers, easy polymerization or copolymerization using AIBN,
and virtually unlimited compositional versatility obtained through
both methacrylate functional comonomers and modification of
resulting copolymers. These new synthetic PEO brush based
antifouling polymers provide long-term control of surface
biofouling of membranes in the physiologic, marine and industrial
environments.
[0069] Polymer design for the surface modification of RO membranes
for reduced membrane fouling is important. The synthesis of PEO
brushes from PEO methacrylate and a functional comonomer (epoxy,
maleic anhydride, oxazoline, etc.) is a technique that is very well
suited for making crosslinkable macromolecules for the hydrophilic
coating on FT-30 type membranes. These PEO brushes, which have a
comb or brush like architecture, are very efficient in preventing
the formation of biofilms, and such novel PEO-based antifouling
polymers can provide long-term control of surface biofouling in the
physiologic, marine and industrial environments.
[0070] Although the invention has been described in considerable
detail, this detail is for the purpose of illustration. Many
variations and modifications can be made on the invention as
described above without departing from the spirit and scope of the
invention as it is described in the appended claims. All U.S.
patents and allowed U.S. patent applications are incorporated
herein by reference.
* * * * *