U.S. patent application number 10/007687 was filed with the patent office on 2002-10-24 for polymeric membranes and other polymer articles having desired surface characteristics and method for their preparation.
Invention is credited to Master, Jonathan F., Mayes, Anne M., Walton, David G..
Application Number | 20020155311 10/007687 |
Document ID | / |
Family ID | 26698599 |
Filed Date | 2002-10-24 |
United States Patent
Application |
20020155311 |
Kind Code |
A1 |
Mayes, Anne M. ; et
al. |
October 24, 2002 |
Polymeric membranes and other polymer articles having desired
surface characteristics and method for their preparation
Abstract
Polymeric articles, including membranes, with surfaces having a
desired chemical functionality are created by surface segregation
of a branched component blended with a compatible, matrix base
component, the branched component having the desired chemical
functionality. In particular, hydrophilic surfaces are created via
surface segregation of a branched hydrophilic copolymer blended
into a polymer matrix. The use of branched molecular architecture
provides a thermodynamic mechanism for the segregation of the
hydrophilic species to the surface and a means for achieving a high
surface coverage of the hydrophilic moiety. The branched
hydrophilic copolymer can be defined by a random copolymer
including two or more methacrylate or acrylate monomers, at least
one of which features a short hydrophilic side chain, such as a
polyethylene glycol side chain. The branched hydrophilic copolymer
is compatible, and well-entangled, with the acrylate polymer
matrix.
Inventors: |
Mayes, Anne M.; (Waltham,
WA) ; Walton, David G.; (White Bear Lake, MN)
; Master, Jonathan F.; (Cambridge, MA) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, PC
FEDERAL RESERVE PLAZA
600 ATLANTIC AVENUE
BOSTON
MA
02210-2211
US
|
Family ID: |
26698599 |
Appl. No.: |
10/007687 |
Filed: |
December 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10007687 |
Dec 5, 2001 |
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09258526 |
Feb 26, 1999 |
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6413621 |
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09258526 |
Feb 26, 1999 |
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PCT/US97/16488 |
Aug 26, 1997 |
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08819610 |
Mar 17, 1997 |
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60024579 |
Aug 26, 1996 |
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Current U.S.
Class: |
428/522 ;
428/304.4; 428/310.5; 428/409 |
Current CPC
Class: |
Y10T 428/249961
20150401; B01D 2323/30 20130101; B01D 2323/12 20130101; Y10T
428/249978 20150401; B01D 67/0011 20130101; B01D 69/141 20130101;
B01D 71/34 20130101; B01J 45/00 20130101; C08J 3/005 20130101; B01D
71/82 20130101; Y10T 428/24942 20150115; Y10T 428/31 20150115; Y10T
428/31928 20150401; Y10T 428/31935 20150401; B01D 71/40 20130101;
B01D 2325/38 20130101; Y10T 428/249953 20150401 |
Class at
Publication: |
428/522 ;
428/409; 428/310.5; 428/304.4 |
International
Class: |
B32B 027/30; B32B
005/14 |
Claims
What is claimed is:
1. An article having a surface, comprising an entangled blend of a
first, relatively lower-cohesive-energy polymer component and a
second, relatively higher-cohesive-energy polymer component that is
compatible with the first polymer component at room temperature,
the second polymer component present at the surface of the article
in a ratio to the first polymer that is greater than the overall
ratio in the article of the second polymer component to the first
polymer component.
2. An article having a surface, comprising an entangled blend of a
first, essentially hydrophobic polymer component and a second
polymer component that is a random copolymer entangled with the
first polymer component that is more hydrophilic than the first
polymer component, the second polymer component having a molecular
weight of at least about 15,000 and being present at the surface of
the article in a ratio to the first polymer component that is
greater than the overall ratio in the article of the second polymer
component to the first polymer component.
3. An article having a surface, comprising an entangled blend of a
first polymer having an affinity to water and a second polymer
having an affinity to water, compatible with the first polymer at
room temperature, the surface of the article having an affinity to
water that is greater than the average water affinity of the total
of the first and second polymers in the article.
4. An article as any preceding claim, the article defining a porous
membrane.
5. An article as in any preceding claim, wherein the first polymer
component is a relatively lower-cohesive-energy polymer component
and the second polymer component is a relatively
higher-cohesive-energy polymer component.
6. An article as in any preceding claim, wherein the first polymer
component is essentially hydrophobic and the second polymer
component is more hydrophilic than that first polymer
component.
7. An article as in any preceding claim, wherein each of the first
polymer component and the second polymer component has an affinity
to water, and the surface of the article has an affinity to water
that is greater than the average water affinity of the total of the
first and second polymers in the article.
8. An article as in any preceding claim, wherein the second polymer
component is present at the surface of the article in a ratio to
the first polymer component that is greater than the overall ratio
in the article of the first polymer component to the second polymer
component.
9. An article as in any preceding claim, wherein at least the
second polymer component is an acrylic polymer.
10. An article as in any preceding claim, wherein each of the first
and second polymer components is an acrylic polymer.
11. An article as in any preceding claim, wherein each of the first
and second polymer components is insoluble in water.
12. A method comprising providing a miscible blend of at least
first and second polymer components each insoluble in water, the
first component being essentially hydrophobic and the second
component being more hydrophilic than the first component, and
allowing the components to phase segregate to form a porous
membrane having a core, and a surface of greater hydrophilicity
than the core.
13. A method comprising: providing a fluid blend of a first,
relatively lower-cohesive-energy polymer component and a second,
relatively higher-cohesive-energy polymer component that is
compatible with the first polymer component at room temperature;
and allowing the blend to harden to form a polymeric article having
a surface, the second polymer component present at the surface of
the article in a ratio to the first polymer component that is
greater than the overall ratio in the article of the second polymer
component to the first polymer component.
14. A method of making a polymer membrane having a particular
surface chemical functionality, comprising: providing a polymeric
fluid comprising a blend of a first polymer component and a second
polymer component that is compatible with the first polymer
component at room temperature and that includes a particular
chemical functionality; subjecting the polymeric fluid to phase
inversion and recovering an article comprising the blend of the
first and second polymer with the second polymer present at the
surface of the article in a ratio to the first polymer that is
greater than the overall ratio in the article of the second polymer
to the first polymer.
15. A method of making a polymer membrane having a particular
surface chemical functionality, comprising: providing a polymeric
fluid comprising a blend of a first polymer component and a second
polymer component that is compatible with the first polymer
component at room temperature; forming an emulsion by exposing the
polymeric fluid to a fluid incompatible with the first and second
components and allowing the incompatible fluid to form the emulsion
in the polymeric fluid; and recovering from the mixture a porous
article comprising the blend of the first and second polymer with
the second polymer present at the surface of the article in a ratio
to the first polymer that is greater than the overall ratio in the
article of the second polymer to the first polymer.
16. A method as in either of claims 14 or 15, involving recovering
a porous article comprising the blend of the first and second
polymer with the second polymer present at the surface of the
article in a ratio to the first polymer that is greater than the
overall ratio in the article of the first polymer to the second
polymer.
17. A method as in any of claims 12-16, wherein the article is a
porous membrane.
18. A method as in any of claims claim 12-17, involving allowing
the second polymer component to be entropically driven to the
surface.
19. A method as in any of claims claim 12-18, wherein the first
polymer component is a relatively lower-cohesive-energy polymer
component and the second polymer component is a relatively
higher-cohesive-energy polymer component.
20. A method as in any of claims claim 12-19, wherein the first
polymer component is essentially hydrophobic and the second polymer
component is more hydrophilic than that first polymer
component.
21. A method as in any of claims claim 12-20, wherein each of the
first polymer component and the second polymer component has an
affinity to water, and the surface of the article has an affinity
to water that is greater than the average water affinity of the
total of the first and second polymers in the article.
22. A method as in any of claims claim 12-21, wherein the second
polymer component is present at the surface of the article in a
ratio to the first polymer component that is greater than the
overall ratio in the article of the second polymer component to the
first polymer component.
23. A method as in any of claims claim 12-22, wherein at least the
second polymer component is an acrylic polymer.
24. A method as in any of claims claim 12-23, wherein each of the
first and second polymer components is an acrylic polymer.
25. A method as in any of claims claim 12-24, wherein each of the
first and second polymer components is insoluble in water.
26. An article or method as in any preceding claim, wherein the
second polymer component has a polyionic functionality.
27. An article or method as in any preceding claim, wherein the
second polymer component has a chelating functionality.
28. An article or method as in any preceding claim, wherein the
second polymer component is more highly branched than the first
polymer component.
29. An article or method as in any preceding claim, wherein the
first polymer component is PVDF.
30. An article or method as in any preceding claim, wherein the
first polymer component is PMMA.
31. An article or method as in any preceding claim, wherein the
second polymer component is P(MMA-r-MnG).
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/258,526, filed Feb. 26, 1999, which is a
continuation-in-part of International Patent Application Serial No.
PCT/US97/16488, filed Aug. 26, 1997, which is a PCT of U.S. patent
application Ser. No. 08/819,610, filed Mar. 17, 1997 and U.S.
Provisional Patent Application Serial No. 60/024,579.
FIELD OF THE INVENTION
[0002] The invention relates generally to polymer articles having
modified surfaces, such as an essentially hydrophobic polymer
article having a hydrophilic surface resulting from
entropically-enhanced migration of a miscible, hydrophilic
component to the surface of the article.
[0003] A membrane having a hydrophobic core and a hydrophilic
surface component is provided as well.
BACKGROUND OF THE INVENTION
[0004] Control of the surface chemistry of polymeric articles and
compositions has technological relevance to a variety of
commercially-important areas such as the medical devices industry,
personal products, coatings, membranes, etc. Many polymeric
articles and compositions that are useful in these areas are
defined by a particular type of material because of economic
considerations or mechanical requirements. For example, an
essentially hydrophobic material might be used for structural
reasons where it would be desirable to provide a different type of
surface, for example a hydrophilic surface, on the article. Other
examples involve imparting a chemical functionality to a surface
such as a chelating functionality or other functionality that can
selectively remove particular species from solution, or otherwise
expose a desired chemical characteristic. While many techniques
exist for modifying surface properties of polymers, many involve
multi-step processes and/or do not result in thermodynamically or
physically-stable incorporation of surface-modifying
components.
[0005] It is often a goal in polymer chemistry to create a polymer
article having a surface of high surface tension (surface energy)
relative to the article as a whole, since higher surface tension
typically corresponds to better wettability. However, in polymer
blends including a higher surface energy component and a lower
surface energy component the lower surface energy component (lower
wettability component) tends to be present disproportionately at
the surface since surface energy is characterized by
inter-molecular attraction. That is, thermodynamic considerations
result in the component with the higher inter-molecular attraction
residing below the surface where it can be surrounded by a higher
number of like molecules, while the lower surface energy component
resides at the surface where a molecule is inherently surrounded by
less like molecules. Techniques exist for creating polymeric
materials having higher surface tension components at the surface,
but a problem typically encountered with conventional methods is
the tendency of the surface to reconstruct over time through chain
reorientation where the lower surface tension component migrates to
the surface of the polymer. (e.g., Wu, Supra; Garbassi, et al.,
Supra). Such reconstruction is consequently accompanied by an
irrevocable loss of desired surface properties.
[0006] The control of surface properties of acrylate polymers has
technological relevance to areas including biomedical devices,
latex paints and other coatings, textiles, and recording media.
However, conventional techniques for modification of acrylate
polymer surface chemistry typically is achieved through
kinetically-governed processes that allow little control over the
final surface composition and structure. Plasma and flame
treatments, commonly employed to oxygenate surfaces in order to
improve wetting and/or adhesion, invoke reaction cascades of bond
scission, fragmentation, and crosslinking, yielding poorly-defined
surface compositions. Chemical oxidation by acid treatment
typically causes pitting and solubilization that modifies surface
morphology in an uncontrolled fashion (E.g., Wu, Polymer Interface
and Adhesion (Marcel Dekker, Inc., New York, 1982); Garbassi, et
al., Polymer Surfaces: From Physics to Technology (John Wiley &
Sons, West Sussex, 1994)). Grafting methods used to bond
hydrophilic species like heparin or poly(ethylene glycol) to
surfaces in order to improve biocompatibility typically yield low
surface coverages (E.g., Pekna, et al., Biomaterials, 14, 189
(1993); Harris, J. M., ed., Poly(ethylene glycol) Chemistry:
Biotechnical and Biomedical Applications (Plenum Press, New York,
1992)).
[0007] An alternative method of preparing a hydrophilic surface on
a hydrophobic polymer article might be through the addition of a
hydrophilic species to the polymer which selectively segregates to
the surface upon processing, providing the desired surface
hydrophilicity. This approach would be particularly useful if the
hydrophilic additive were miscible with the polymer, so as not to
adversely influence the bulk properties of the article, such as
mechanical behavior or optical clarity. One such candidate additive
might be poly(ethylene oxide), PEO, because of its high degree of
hydrophilicity and well-known resistance to protein adsorption. PEO
is known to be miscible in poly(methyl methacrylate) up to very
high concentrations. It is also known, however, that the surface
tension of PEO is somewhat higher than that of PMMA. From this, we
would assume that a surface of an article prepared from a PMMA/PEO
blend should be depleted with PEO, in order to reduce the surface
energy. It has been reported that neither component is enhanced at
the surface of such blends (Sakellariu, Polymer, 34, 3408, (1993)).
However, in this study samples were annealed for only three hours
at 170 C.
[0008] Membrane technology presents a particularly interesting
challenge in connection with surface functionalization. The use of
polymer membranes for water treatment has become increasingly
widespread in the past thirty years in such applications as
desalination of sea and brackish water, water softening, production
of ultrapure water, and purification of industrial wastewater.
Membrane processes have additionally been used to generate
ultrapure water sources for the electronics and pharmaceutical
industries, and to treat wastewater from such diverse industries as
textiles and laundry, electroplating and metal finishing, petroleum
and petrochemical, food and beverage, and pulp and paper.
[0009] Membrane processes offer significant advantages over
conventional water treatment technologies. They require no phase
change and are thus inherently less energy-intensive than
distillation methods used for desalination. They provide an
absolute filter for pollutants above a given pore size, and are
hence more reliable than flocculation methods that can leave
residuals in treated water if improperly performed. In addition,
the modular and compact design of membrane filtration units offers
great flexibility in the scale of operation. And because membranes
can separate pollutants without chemical alteration, they allow for
more cost-effective recovery of valuable components from
wastewater.
[0010] However, membrane technologies suffer from critical
materials-related drawbacks that limit their efficiency and
lifetime, and hence cost-effectiveness in water treatment
applications. In particular, membrane fouling is a major problem
which results in reduced efficiency due to flux decline, high
cleaning and maintenance costs, and low membrane lifetimes. The
cleaning and replacement costs for ultrafiltration processes are
estimated to account for 24% and 23%, respectively, of the total
process costs. While careful system operation and flow-pattern
design can reduce fouling by suspended particulates or precipitated
salts, the adsorption of proteins onto membrane surfaces is more
insidious, generating a monolayer film that provides a foothold for
slower deposition processes which deteriorate membrane performance
and lifetime substantially. Membranes used in reverse osmosis
processes have additional materials-related limitations. While the
cellulose acetate-based membranes most commonly found in this
application exhibit high flux and good salt rejection, these
polymers hydrolyze over time, generating physical holes in the
membrane which reduces its useful lifetime. Clearly, the need
exists for new membrane materials with improved fouling resistance
and longer service lifetimes. Moreover, membranes with improved
selectivity are sought for more cost-effective recovery of
wastewater constituents.
[0011] Methods to impart hydrophilicity to hydrophobic membrane
surfaces have primarily focused on the grafting or coating of
hydrophilic species directly onto membranes. In general, this
approach suffers from several drawbacks: 1) achievable grafting
densities are typically low due to kinetic limitations, 2) grafting
reactions require an additional processing step and are difficult
to scale up, 3) grated monolayers are susceptible to wear or
removal during membrane cleaning procedures. An appealing
alternative approach which might circumvent these problems is the
addition of a hydrophilic macromolecular component to the membrane
material that selectively segregates to the membrane surface during
processing. Membrane materials prepared by this approach can offer
important performance and processing advantages over commercial
membrane materials as well as coated and graft-modified membranes.
Unlike typical coated membranes, the surfaces of these membranes
present an additive which is intimately entangled with the matrix.
Furthermore, where segregation can be accomplished through a
thermodynamic driving force, "self-healing" membranes are possible,
whereby surface-active additive material removed from the membrane
surface can be replaced by further segregation of the branched
component, optionally during a periodic annealing operation.
Finally, surface localization of the branched component can occur
during a standard processing step, thus eliminating the need for
additional membrane fabrication steps.
[0012] A variety of surface-modification techniques have been
described in the patent literature. For example, Varady, et al., in
U.S. Pat. No. 5,030,352, describes modification of a hydrophobic
chromatography solid phase with a block copolymer including
hydrophobic domains and hydrophilic domains. The hydrophobic
domains associate with the solid phase via hydrophobic-hydrophobic
interaction, and the hydrophilic domains extend outwardly away from
the surface. The technique involves the step of crosslinking the
block copolymer in place to produce a hydrophilic surface coating
masking hydrophobic regions of the solid phase.
[0013] Stedronsky, in U.S. Pat. No. 5,098,569, describes a
surface-modified membrane including a modifying polymer adsorbed
onto a surface of the membrane and uniformly crosslinked
thereon.
[0014] Nohr, et al., in U.S. Pat. Nos. 4,923,914, 5,120,888,
5,344,862, 5,494,855, and 5,057,262, describe thermoplastic
compositions designed to expose a particular desired surface
characteristic. Typically, Nohr, et al. employ a hydrophilic
additive that is immiscible (incompatible) with the bulk polymeric
component under ambient conditions, and therefore is driven to the
surface of the blend upon solidification due to this
incompatibility (via enthalpy). In U.S. Pat. No. 5,494,855, Nohr,
et al. described blends including additives having good tensile
properties or surface wettability. Formulation of a blend having
good surface wettability involves an additive having a molecular
weight of as low as from about 350 to about 1,200. Low molecular
weight additives typically migrate more readily in blends and
articles, thus it would not be unreasonable to assume that in this
patent there is a teaching that advantageous mechanical properties
resulting from a higher molecular weight additive and advantageous
surface properties resulting from migration of a lower molecular
weight additive are mutually exclusive. Nohr, et al. use fumed
silica to aid segregation.
[0015] U.S. Pat. No. 4,698,388 (Ohmura, et al.) describes a block
copolymer additive for modifying the surface of polymeric material.
The block copolymer includes a matrix-compatible portion and a
portion having a characteristic desirably present at the surface
which is incompatible with the matrix. Due to the incompatibility
of the surface-modifying portion of the block copolymer, that
portion is segregated to the surface while the compatible portion
interacts with the polymer matrix to retain the additive in the
matrix. U.S. Pat. No. 4,578,414 (Sawyer, et al.) describes fine
denier, wettable fibers and/or filaments prepared from olefin
polymers including a relatively short, polymeric wetting agent
including a hydrophilic domain and a hydrophobic domain. The
additive segregates such that the hydrophilic domain modifies the
surface.
[0016] Allegrezza, et al., in U.S. Pat. Nos. 5,079,272 and
5,158,721, describe a porous membrane defined by an
interpenetrating polymer network of a hydrophobic polymer and an
in-situ-crosslinked, interpenetrating hydrophilic polymer. The
described technique includes the step of annealing the network,
whereby the hydrophobic component crystallizes, "excluding" the
hydrophilic component to the surface.
[0017] U.S. Pat. No. 5,190,989 (Himori) describes an AB-type block
copolymer having a hydrophilic group and a group having an affinity
for a resin. The block copolymer is oriented with the hydrophilic
component toward the surface or interface of the resin.
[0018] Meirowitz, et al., in U.S. Pat. No. 5,258,221, describe a
two-step process in which a surface of a hydrophobic polyolefin
article is modified by contacting the surface with a copolymeric
material above the glass transition temperature of the polyolefin
to fuse the copolymeric material to the polyolefin. The copolymeric
material includes a hydrophobic moiety compatible with the
polyolefin and a modifying moiety (e.g. hydrophilic) incompatible
with the polyolefin.
[0019] U.S. Pat. No. 5,328,951 (Gardiner) describes a technique for
increasing the surface energy of an organic polymeric article, in
particular a polyolefin article, by forming a blend including a
base polymer and an amphiphile having a molecular weight of from
about 150 to about 500 Daltons. The amphiphile has a lipophilic
component compatible with the base polymeric material, which is
thought to anchor the amphiphile in the base polymer, and a
hydrophilic component less compatible with the polymeric base which
resides at the surface of the article.
[0020] Membranes from miscible blends of PVDF with from 5% to 34%
poly(methyl methacrylate) (PMMA) are reported by Nunes, et al.,
"Ultrafiltration Membranes From PVDF/PMMA Blends", J. Mebm. Sci.,
73, 25-35, 1992; Ito, et al., "pH-Sensitive Gating by
Conformational Change of a Polypeptide Brush Grafted onto a Porous
Polymer Membrane", J. Am. Chem. Soc., 119, 1619-1623 (1997)
describe graft-polymerization of benzyl glutamate NCA onto a porous
PTFE membrane, and a study of the effects of pH and ionic strength
on permeation rate. The rate of water permeation through the
membrane was found to be slow under high-pH conditions and fast
under low-pH conditions since, under high-pH conditions, randomly
coiled graft chains extended to close the pores. Kojima, et al.,
"Selective Permeation of Metal Ions Through Cation Exchange
Membrane Carrying N-(8-quinolyl)-sulfonamide as a Chelating
Ligand", Journal of Membrane Science, 102, 49-54 (1995) describe
chemical attachment of a chelating reagent, selective for Cu.sub.2+
over Fe.sup.3+, to side chains of a polymer to create a cation
exchange membrane. This polymer was diluted in a solvent and
impregnated into a porous Teflon.TM. PTFE membrane and the solvent
was evaporated. Mika, et al., "A New Class of
Polyelectrolyte-Filled Microfiltration Membranes with
Environmentally Controlled Porosity", Journal of Membrane Science,
108, 37-56 (1995) describe grafting of 4-vinylpyridine onto
polyethylene and polypropylene microfiltration membranes. Grafting
is UV-induced and results in membranes showing a pH valve effect
and the capability of rejecting small inorganic ions in the
presence of reverse osmosis.
[0021] Iwata, et al. ("Preparation and Properties of Novel
Environmental-Sensitive Membranes Prepared by Graft Polymerization
Onto a Porus Membrane", J. Mem. Sci., 38, 185-199, 1988) report a
glow discharge technique to graft polyacrylamide and polyacrylic
acid chains onto polyvinylidene fluoride (PVDF) membrane. The
permeation rates and separation characteristics of membranes so
treated were found to vary significantly with pH and ionic strength
of the feed solution, both of which influence the configurations of
the grafted chains. Variations in the pH and ionic strength of the
feed solution vary the extent to which electrostatic forces between
the charges along the grafted polyion chains are screened. At low
pH, the negative charges along the grafted chains are heavily
screened by positive counterions, and the chains adopt random coil
like configurations. At high pH, the grafted chains are
dissociated, and they adopt extended configurations due to
electrostatic repulsion between the negative charges spaced along
them, effectively blocking the pores. Addition of methanol (a poor
solvent for PAAm and PAA) was shown to be another method of
collapsing the grafted chains. While significant, the variations in
permeation rate were not as pronounced as those demonstrated in the
system of Ito, et al., which were probably emphasized by the
helix-coil transition which occurs in that system. Hautojarvi, et
al., (J. Mem. Sci., 108, 37, 1995) published a similar study of
PVDF membranes graft-modified with poly(acrylic acid).
[0022] In many prior techniques for modifying surfaces, durability
of the modified surface and/or physical or optical characteristics
of the article may be compromised. In particular, where a
surface-modifying component is water-soluble, the component can
become disassociated from the polymer surface over time if the
article is used in an aqueous environment and the surface-modifying
component is not securely associated with the article. Polymer
blends that exploit the incompatibility of a surface-modifying
component run the risk of formation of micelles or other segregated
groupings within the polymer, which can render a polymer opaque
(disadvantageous in many circumstances). Since incompatibility is
the property necessary for segregation in many techniques, these
techniques inherently carry these potential drawbacks.
[0023] The academic literature describes studies involving surface
migration of components of a polymer blend based upon their
architecture. For example, Steiner, et al., Science, 258, 1126
(1992) and Sikka, et al., Phys. Rev. Lett., 70, 307 (1993),
describe experiments on polyolefin blends demonstrating that, where
components of the blends are similar in energy, more
highly-branched components tend to segregate to the surface of the
article. However, there is some controversy in the literature in
that Steiner, et al. (Supra) report that it is not clear that
surface migration of the more highly-branched polyolefin occurs due
to its architecture. Indeed, in these systems since the more
branched component is the lower surface tension component the more
branched component would be expected to reside at the surface
according to the reported technique.
[0024] Accordingly, it is an object of the present invention to
provide a simple, inexpensive technique for generating
thermodynamically-stable polymeric articles having a desired
surface property. In particular, it is an object to provide a
technique for generating thermodynamically-stable, relatively
high-surface-energy surfaces on polymeric articles for a variety of
purposes. It is another object of the invention to provide stable
hydrophilic surfaces on various acrylate polymers to improve
emulsification in latex paints, impart resistance to static charge
build-up on compact discs and textiles, improve anti-fouling
properties of intraocular lenses and dental composites, and
increase the wettability of acrylates to inks, glues, and paints.
It is another object of the invention to provide straightforward
techniques for creation of membranes of a variety of polymers
having desired surface properties, and robust membranes having
desired exposed functionality.
SUMMARY OF THE INVENTION
[0025] The present invention provides a technique for imparting, to
a surface of a polymeric article, a desired chemical functionality
that differs from the article as a whole. The technique makes use
of the discovery that, in a compatible blend of different polymeric
components, migration of the more highly-branched component can be
enhanced by entropy. The molecule having the greatest number of
chain ends can be present at the surface with the least
configurational entropy penalty.
[0026] Also provided is a technique for imparting a desired
chemical functionality to a surface of a polymeric article
involving subjecting a blend of at least of a first and a second
polymer component to phase inversion and allowing the second
component to migrate, disproportionately, to the surface of the
blend. The phase inversion technique can be driven completely by
enthalpy, or by entropy, or a combination. That is, the above-noted
technique involving allowing migration of a more highly-branched
component to a surface of a blend can be combined with a phase
inversion technique.
[0027] In one embodiment the invention provides an article having a
surface, comprising an entangled blend of a first, relatively
lower-cohesive-energy polymer component and a second, relatively
higher-cohesive-energy polymer component. The first and second
polymer components are compatible with each other at room
temperature, that is, are miscible. The second polymer component is
present at the surface of the article in a ratio to the first
polymer that is greater than the overall ratio in the article of
the second polymer component to the first polymer component.
[0028] In another embodiment the invention provides an article,
having a surface, comprising an entangled blend of a first,
essentially hydrophobic polymer component and a second polymer
component that is a random co-polymer entangled with the first
polymer component. The second polymer component is more hydrophilic
than the first polymer component. The second polymer component has
a molecular weight of at least about 15,000 and is present at the
surface of the article in a ratio to the first polymer component
that is greater than the overall ratio in the article of the second
polymer component to the first polymer component. The second
polymer component can be a random co-polymer.
[0029] Also provided is an article having a surface, comprising an
entangled blend of a first polymer having an affinity to water and
a second polymer having an affinity to water. The first and second
polymers are compatible at room temperature. The surface of the
article has an affinity to water that is greater than the average
water affinity of the total of the first and second polymers in the
article.
[0030] All of the articles of the invention can be porous membranes
having a desired surface chemical functionality.
[0031] In another aspect the invention provides a series of
methods. In one embodiment a method involves providing a miscible
blend of at least first and second polymer components each
insoluble in water. The first component is essentially hydrophobic
and the second component is more hydrophilic than the first
component. The components are allowed to phase segregate to form a
porous membrane having a core and a surface of greater
hydrophilicity than the core.
[0032] In another embodiment, a method involves providing a fluid
blend of a first, relatively lower-cohesive-energy polymer
component and second, relatively higher-cohesive-energy polymer
component that is compatible with the first polymer component at
room temperature. The blend is allowed to harden to form a
polymeric article having a surface. The second polymer component is
present at the surface of the article in a ratio to the first
polymer component that is greater than the overall ratio in the
article of the second polymer component to the first polymer
component.
[0033] In another embodiment, a method is provided in which a
polymer membrane is fabricated having a particular surface chemical
functionality. A polymeric fluid is provided that includes a blend
of a first polymer component and a second polymer component that is
compatible with the first polymer component at room temperature
that includes a particular chemical functionality. The polymeric
fluid is subjected to phase inversion and an article is recovered
that includes the blend of the first and second polymers. The
second polymer is present at the surface of the article in a ratio
to the first polymer that is greater than the overall ratio in the
article of the second polymer to the first polymer.
[0034] In another embodiment a method of making a polymer membrane
is provided that involves providing a polymeric fluid including a
blend of a first polymer component and a second polymer component
compatible with the first polymer component at room temperature. An
emulsion is formed by exposing the polymeric fluid to a fluid
incompatible with the first and second components and allowing the
incompatible fluid to form the emulsion in the polymeric fluid. A
porous article is recovered from the mixture that includes a blend
of the first and second polymers with the second polymer present at
the surface of the article in a ratio to the first polymer that is
greater than the overall ratio in the article of the second polymer
to the first polymer.
[0035] In some embodiments the second polymer component is more
branched than is the first polymer component. In all cases the
second and first polymer components can have different
functionalities, with the surface enhancement of the second polymer
component allowing formation of a polymeric article, such as a
membrane, having a desired surface chemical functionality. The
first and second components can be thermodynamically compatible at
room and use temperatures, in addition to being compatible as a
melt, therefore a thermodynamically stable, surface-segregated
article results. In one embodiment, each component has a molecular
weight of at least about 5,000, and a very well-entangled
combination of first and second components results in the
article.
[0036] Any of the articles described herein can be acrylic, for
example the above-described polymers can be acrylic polymers. One
set of methods involves blending a first, essentially hydrophobic
acrylic polymer with a second, more-hydrophilic acrylic polymer and
allowing the more-hydrophilic polymer to be driven to the surface
of the article. In another method, a fluid blend of an essentially
hydrophobic acrylic polymer and a more-hydrophilic acrylic polymer
is provided, the two polymers being compatible. The blend is
hardened to form an article in which the more-hydrophilic acrylic
polymer is present at the surface disproportionately. The same
method can be carried out where the first and second acrylic
polymers are not necessarily hydrophobic and more-hydrophilic, but
differ in chemical functionality of another type.
[0037] Other advantages, novel features, and objects of the
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings, which are schematic and which are not
intended to be drawn to scale. In the figures, each identical or
nearly identical component that is illustrated in various figures
is represented by a single numeral. For purposes of clarity, not
every component is labeled in every figure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a schematic illustration of a cross section of a
surface of an essentially hydrophobic, prior art polymeric article
modified with a low molecular weight component having a hydrophobic
portion anchored in the article and a hydrophilic portion facing
the surface of the article;
[0039] FIG. 2 is a schematic illustration of an essentially
hydrophobic polymeric article of the invention having a compatible,
less-hydrophobic, branched component residing at the surface of the
article and entangled with the polymer chains of the article;
[0040] FIG. 3 illustrates schematically a branched component of the
invention;
[0041] FIG. 4 shows neutron reflectivity (NR) data for blends of
polymethyl methacrylate (PMMA) with a branched, hydrophilic
additive prepared by random copolymerization of methyl methacrylate
with methoxy polyethylene glycol methacrylate(P(MMA-r-MnG));
[0042] FIG. 5 shows volume fraction profiles extracted from data
shown in FIG. 4;
[0043] FIG. 6 shows experimental and theoretical reflectivities for
thick blends of 2, 5, 10, and 20% P(MMA-r-MnG) in PMMA-d.sub.8;
[0044] FIG. 7 shows volume fraction of P(MMA-r-MnG) in thick 2, 5,
10, and 20% blends with PMMA-d.sub.8;
[0045] FIG. 8 shows volume fraction profiles for
P(MMA-r-MnG)/PMMA-d.sub.8 or P(MMA-r-MnG)/PMMA blends hydrated with
H.sub.2O or D.sub.2O;
[0046] FIG. 9 shows protein adsorption on PMMA, P(MMA-r-MnG), and a
blend of the two, respectively;
[0047] FIG. 10 shows amount of HSA irreversibly adsorbed onto
surfaces of thick polymer films of P(MMA-r-MnG), P(MMA-r-MnG)/PMMA
blends, and PMMA;
[0048] FIG. 11 shows amount of ECC irreversibly adsorbed onto
surfaces of thick polymer films of P(MMA-r-MnG), P(MMA-r-MnG)/PMMA
blends, and PMMA;
[0049] FIG. 12 is a photocopy of a photomicrograph of CHO LA cells
on PMMA;
[0050] FIG. 13 is a photocopy of a photomicrograph of CHO LA cells
on P(MMA-r-MnG);
[0051] FIG. 14 is a photocopy of a photomicrograph of CHO LA cells
on a 20% blend of P(MMA-r-MnG) in PMMA;
[0052] FIG. 15 is a scheme illustrating a first portion of a
chemical root to the synthesis of a branched polymeric component
capable of chelating a metal ion;
[0053] FIG. 16 is a scheme of a second portion of a chemical root
to the synthesis of a branched component capable of chelating a
metal ion;
[0054] FIG. 17 is a scheme of a first portion of a chemical
synthesis of a branched polyacid component for environment
sensitive pore gating;
[0055] FIG. 18 is a scheme of a second portion of a chemical
synthesis of a branched polyacid component for environment
sensitive pore gating;
[0056] FIG. 19 is a photocopy of a scanning electron micrograph
(SEM) image of a membrane (comparative) prepared by phase
inversion, from polyvinylidene fluoride (PVDF); and
[0057] FIG. 20 is a photocopy of an SEM image of a membrane,
prepared by phase inversion, from an entangled blend of 80% PVDF
and 20% (by weight) P(MMA-r-MnG).
DETAILED DESCRIPTION OF THE INVENTION
[0058] U.S. Provisional Pat. Appln. Ser. No. 60/024,570, filed Aug.
26, 1996 by Mayes et al., U.S. patent application Ser. No.
08/819,610, filed Mar. 17, 1997 by Mayes et al. and PCT Appln. No.
PCT/US97/16488, filed Aug. 26, 1997, and all are incorporated
herein by reference.
[0059] The present invention provides surface-modification
techniques for polymeric articles that involve selective
segregation of a component of a polymer blend to a surface of the
blend to provide desired chemical functionality at the surface. One
aspect of the invention makes use of entropic segregation to create
a desired surface property. Another aspect involves segregation
driven at least in part by entropy. One embodiment of the invention
involves surface-functionalized membranes.
[0060] Specifically, the present invention involves techniques for
surface segregation, from a polymer blend of a plurality of
miscible components, of a relatively higher-cohesive-energy
component including a chemical functionality that is desired at the
surface. In one set of embodiments, control of the surface
chemistry of a polymer can be achieved by designing a
surface-modifying polymer component that will be
entropically-driven to the surface of the article because of its
branched molecular architecture, providing the surface-modifying
component with a chemical functionality desired at the surface, and
designing the component so that it will be compatible with the base
component of the polymer matrix. As used herein,
"entropically-driven" is meant to define driven by a force enhanced
at least in part by entropy. That is, the surface-modifying polymer
component is driven to the surface essentially exclusively by
entropic forces, or by a combination of forces at least one of
which is entropic. In particular, use of a branched molecular
architecture in a component having a chemical functionality
desirable at the surface provides a thermodynamic mechanism for the
segregation of the component to the surface and a means to achieve
a high surface coverage of the component.
[0061] In another set of embodiments, a phase inversion technique
is used to enhance migration of a desired component of a miscible
blend of a plurality of components to a surface. In this set of
embodiments migration of a selected component to a surface can be
driven by enthalpy, entropy, or a combination.
[0062] Manufacture, or surface modification, of a variety of
articles can be enhanced by the techniques of the invention. For
example, articles having an essentially hydrophobic core can be
modified to have a hydrophilic surface which is useful for
applications requiring low static-charge build-up or improved
wettability to glues, paints, inks, or the like. Surfaces of
articles of the invention can be modified to resist protein
adsorption, providing new candidate materials for biomedical uses.
Optical devices, such as intraocular lenses and the like can be
enhanced, as well as membranes for water purification and other
separation.
[0063] Where a branched component is entropically driven to a
surface from a blend, the additive is thermodynamically favored at
the surface despite higher surface tension, thus the surface does
not tend to reconstruct over time, in contrast to high energy
surfaces prepared on polymers by more traditional routes.
[0064] The technique of the invention results in surfaces that are
essentially hydrophilic according to one set of embodiments, the
surface being more hydrophilic than the bulk of the articles.
"Essentially hydrophilic", as used herein, means that the contact
angle formed with water at the surface is less than about
70.degree. C., preferably less than about 65.degree. C., and more
preferably still 59.degree.-62.degree. C. In another embodiment the
articles are essentially hydrophilic when the articles, upon
exposure to water, develop a water content of at least 15% within
the first 50 .ANG. of the surface, preferably at least 20%, more
preferably at least 25%, and more preferably still at least 30%.
These values of percent water at the surface can also extend to
within at least 100 .ANG. from the surface.
[0065] The invention provides selective surface segregation of a
component of a blend where both the surface and bulk of the article
can comprise the polymer component that is found disproportionately
at the surface. In this embodiment, both the surface and the bulk
each comprise a first polymer component and a second polymer
component where a higher ratio of the second component to the first
component is present in the surface than in the bulk. Thus, both
polymer components are present in significant quantities throughout
the entire article. An advantageous feature of this arrangement is
that the article can function as a self-healing article, i.e., upon
loss of the surface-segregated component from the surface via,
e.g., normal wear, a sufficient amount of that component can
migrate from the bulk of the article to restore the surface to a
desired state. This self-healing segregation takes place at room
temperature in many arrangements or, if desired, an article can be
heated to render it slightly fluent to aid segregation.
[0066] In one embodiment, the second, surface segregated component
is more hydrophilic than the first component, and the article is
self-healing with respect to hydrophilicity at the surface. A
self-healing hydrophilic membrane is one example.
[0067] In one embodiment, the invention provides a polymer matrix
base component defined by a first, essentially hydrophobic acrylate
polymer that is not highly branched, and preferably is linear. A
second, more-hydrophilic acrylate polymer having non-linear
architecture, preferably a branched architecture, is added. The
non-linear component preferably is a random copolymer comprised of
two or more acrylate or methacrylate species, at least one of which
features a hydrophilic side chain that imparts hydrophilicity to
the copolymer. The side chain can be essentially any hydrophilic
moiety, preferably a polyalkylene oxide such as polyethylene
glycol. The side chain should be sufficiently short to inhibit
crystallinity (see Sakellariou, Polymer, 34, 3408 (1993)). The
second polymer should also be insoluble in water and compatible
with the matrix base component. The second polymer also should have
a molecular weight large enough that it remains entangled with the
matrix base component. As used herein, the term "entangled" means
that portions of polymer chains of separate, non-crystalline
components wrap about each other, creating physical barriers to
motion. The term is well-known to those of ordinary skill in the
art. Generally, this means that the second polymer should have a
molecular weight of at least about 5,000, preferably at least about
10,000, more preferably at least about 15,000, and more preferably
still at least about 20,000. The higher molecular weight of the
additive of the invention facilitates greater entanglement because
of longer chain length.
[0068] The second polymer also should, as noted, be compatible with
the matrix base component. As used herein, "compatible" means that
the additive does not phase separate from the matrix base
component. For example, the additive does not phase separate
because of inter-component repulsion due to differences in energy
of the components, such as hydrophobic-hydrophilic repulsion, which
has been exploited in the prior art for providing
surface-segregated polymers. While several prior art arrangements
involve an additive that is miscible with a matrix base component
at melt temperatures, the additive and matrix base component
typically are not compatible at room temperature and this property
is used to cause the additive to migrate to the surface upon
cooling and hardening of a melted blend. In contrast, in the
present invention the matrix base component and additive not only
are miscible to form a homogeneous solution as a melt but, upon
cooling and hardening, the additive and matrix base component are
compatible and remain well-entangled rather than separating into
distinct phases. One advantage of a well-entangled article rather
than a phase-separated article is that phase separation can lead to
delamination and other mechanical failures.
[0069] In some embodiments, in which materials remain amorphous,
one advantageous characteristic of this room-temperature,
solid-phase compatibility is that a blend of the invention
typically will remain clear when solidified, rather than running
the danger of becoming opaque when solidified due to phase
separation or to the formation of micelles resulting from phase
incompatibility. These embodiments are particularly suited for use
with devices and articles that must remain optically clear, such as
intraocular lenses. That is, components of the present invention,
in certain embodiments, are selected not to crystallize, and this
lack of crystallization provides clarity, rather than opacity, to
articles of the invention. Those of ordinary skill in the art, with
reference to standard polymer handbooks and texts such as Young, et
al., "Introduction to Polymers" (Second Edition), Chapman &
Hall, London (1991) and Stevens, "Polymer Chemistry--An
Introduction" can select a matrix base component and additive,
neither of which will crystallize. Lack of crystallization in the
additive of the invention can be facilitated according to these
embodiments by providing the additive in random copolymer form, for
example, synthesized via anionic or free radical polymerization.
Additionally, in general acrylates do not crystallize.
[0070] Thus, one simple screening test to determine suitable matrix
base component/additive combinations for use in accordance with
non-crystalline embodiments of the invention involves blending a
matrix base component with an additive, allowing the blend to
harden, and observing optical properties of the resulting article.
Where the resulting article is clear, rather than opaque, the
matrix base component and additive are good candidates for use in
this embodiment. Prior to a screening test to determine optical
clarity of a blend of candidate components, those of ordinary skill
in the art can select components that are good candidates for
miscibility by considering cohesive energy density which can be
calculated through group contribution methods. Two components that
differ drastically in cohesive energy density may not be
miscible.
[0071] FIG. 1 illustrates schematically, in cross section, one
essentially hydrophilic prior art polymeric article designed to
have a hydrophobic surface. Although FIG. 1 is representative of
certain known prior art systems, the applicants are not aware of
membranes prepared in this way in the prior art. The article
includes a polymeric matrix base component 10 which, as
illustrated, is a long-chain, linear, hydrophobic polymer, and an
additive 12 designed to impart hydrophilicity to the surface of the
article. In the figure, the dotted line is representative of the
surface of the article. Of course, the surface is defined by the
boundary of polymer molecules of which the article is
comprised.
[0072] Additive 12 is a relatively low-molecular weight component,
typically in a molecular weight range of from about 500 to about
5000, and includes a hydrophilic portion 14 at a first end and a
hydrophobic portion 16 at a second end. This can be achieved by
synthesizing an A-B block copolymer in which segment A is
hydrophilic and segment B is hydrophobic.
[0073] Such an article is fabricated typically by blending
component 10 with additive 12 in a melt (in which condition
component 10 and additive 12 may be miscible) and allowing the
blend to cool and harden. Additive 12 typically is incompatible
with base component 10 at room temperature and, when the blend
cools and hardens, it segregates to the surface. Segregation of
additive 12 to the surface of the article, driven by enthalpy, is
facilitated by the low molecular weight of additive 12. Additive 12
can remain anchored to some extent in the article because its
hydrophobic portion 16 is compatible with the hydrophobic polymeric
matrix base component 10.
[0074] While hydrophilic portion 14 creates some degree of
hydrophilicity at the surface of the article, the overall
incompatibility of additive 12 with polymer matrix base component
10 can render the overall arrangement thermodynamically unstable.
Additionally, in the article of FIG. 1, because additive 12 can
have one or more of the characteristics of water-solubility, low
molecular weight, lack of entanglement with base component 10, and
incompatibility with base component 10, component 12 can be
scavenged from the polymeric article and dissolved in water to
which the article is brought into contact. This is illustrated
schematically in FIG. 1 by removal of one of components 12 by water
molecule 18.
[0075] Another disadvantage in the incompatibility of components 10
and 12 of the prior art article of FIG. 1 is that micelles 20,
formed of a plurality of additives 12 with their hydrophobic
portions facing outward, can form. Such micelles can render the
material opaque, and can compromise mechanical properties.
[0076] Not shown is a prior art polymeric article in which
crystallization of the base polymeric component is the primary
driving force behind segregation of a surface-modifying additive to
the surface of the article. For example, with reference to FIG. 1,
if matrix base component 10 was a polymer forming regions of
crystallinity and additive 12 was selected to be chemically
compatible with individual units of matrix base component 10 but
incompatible with crystallinity, the blend could be formed and
annealed to form regions of crystallinity driving additive 12 to
the surface. However, this type of arrangement typically relies on
low-molecular-weight, highly mobile additives which therefore are
not well entangled with the base component, and surface segregation
relies upon the phase incompatibility of the additive with the base
component.
[0077] Referring now to FIG. 2, a polymeric article of the present
invention is illustrated schematically. Like the article of FIG. 1,
the article of FIG. 2 includes a matrix base component 22 that is a
first long-chain polymer that is essentially hydrophobic in one set
of embodiments. The article of the invention includes an additive
24 that is a second, branched polymer including branch points 26
and relatively short side chains 28 emanating from branch points
26, which have a surface-modifying characteristic such as a
hydrophilic characteristic. As discussed, surface-modifying side
groups such as hydrophilic side groups can be provided by random
copolymerization of units including the surface-modifying side
chain and units that do not include the hydrophilic side chain.
[0078] Second polymer component 24 is of relatively high molecular
weight and is compatible with first, base component 22 and,
therefore, component 22 and component 24 are entangled, as
illustrated schematically at entanglement locations 30. This
entanglement, which can be due to one or more of compatibility,
non-crystallinity, and threshold molecular weight, secures
component 24 in the article and water molecule 18, brought into
contact with the surface of the article, does not dislodge the
additive where the additive is hydrophilic (where the additive has
a different chemical functionality, exposure of the article to a
solvent attracted to that functionality will not dislodge the
additive). Second polymer component 24 typically is insoluble in
water, due to backbone polymer insolubility, rendering the
component more securely anchored to the surface of the article.
[0079] Migration of the second component 24 to the surface, where
the second component is hydrophilic is surprising since, as the
higher surface tension (higher-cohesive-energy) component, those of
ordinary skill in the art would expect, due to enthalpic
considerations, that the second component would be favored to
remain away from the surface in the bulk of the polymer, while the
first, lower-cohesive-energy matrix base component 22 would be
found predominantly at the surface (see Example 1, below).
[0080] Although FIG. 2 illustrates a second polymeric component 24
that is highly branched and a first, matrix base component 22 that
is entirely linear, this is for purposes of clarity only,
illustrating a preferred embodiment of the invention. A branched
second component and first, linear matrix base component will
satisfy selection criteria of the invention, in which the second
component and matrix base component are selected in conjunction
with each other such that the second component is entropically
favored at the surface of an article made of these components. This
involves selecting a second component 24 having more chain ends
than the first, matrix base component 22, since a surface of an
article introduces a reflecting boundary condition on a random walk
that characterizes spatial distribution of the polymer chain, which
lowers the number of total configurations available to a chain, and
hence the entropy of the system. To minimize the number of
reflections required at the material boundary, chain ends
preferentially segregate to the surface of a polymer melt, in the
absence of strong interactions.
[0081] Accordingly, second component 24 includes more chain ends
than does matrix base component 22. The number of chain ends of
each can vary widely, especially in the case of the second
component, so long as there is a difference in number of chain ends
that causes the second component to surface segregate. Architecture
of the components of the invention will be discussed with reference
to FIG. 3, showing a branched species in which circles represent
units such as mer units or atoms (e.g. carbon atoms). The species
of Scheme 1 includes 5 branches, each of length 1=6, and 2 branch
connections, each of length d=4. For clarity, branch segments and
branch-connection segments are depicted as white- and black-filled
circles, respectively. Chain ends are shown as white circles with
black dots, and branch points are shown as black circles with white
dots.
[0082] The branches represent side groups on a polymer chain over
and above those that exist inherently on the polymer backbone. For
example, where the matrix base component and the second component
are both methylmethacrylates, the --COOMe side group of each mer is
not considered a branch.
[0083] The matrix base component 22 of the invention should be
linear or, if branched, should include no more than about 4 units
per branch, and less branching than the additive. For example, the
matrix base component can be polybutylmethacrylate.
[0084] The second component 24 should include branches that are not
long enough to form regions of crystallinity at use temperatures if
selected of material that can crystallize, in embodiments where
crystallinity is not desired. Generally, the branches should be of
length no longer than about 1=25 segments, preferably no longer
than about 1=20 segments, more preferably no longer than about 1=15
segments. Where the second component is a random copolymer, of
course, the spacing d between branches varies. Therefore, the
second component is best described as one having at least about 4%
branched segments in the backbone (5% of the chain units along the
backbone are branch points as illustrated in FIG. 3), preferably at
least about 7%, more preferably at least about 12%, more preferably
at least about 15%, and more preferably still at least about 18%.
Where branches are hydrophilic, the percent of branched segments is
preferably less than an amount that renders the additive
water-soluble. In a different embodiment in which the branches are
of a chemical functionality that is attracted to a surrounding
environment (such as a hydrophobic functionality when used in the
presence of a hydrophobic solvent), the branches preferably are
less than amount that renders the second component soluble in the
hydrophobic solvent.
[0085] In one embodiment, the invention involves a first, acrylate
matrix base component 22 and second acrylate component 24. That is,
the matrix base component 22 and second component each are the
polymerization product of one or more monomers having the formula
CH.sub.2.dbd.C(R.sub.1)(COOR.sub.2), where R.sub.1 and R.sub.2 are
each selected from the group consisting of hydrogen, hydrocarbon
groups, and alcohol groups and R.sub.1 and R.sub.2 can be the same
or different. Hydrocarbon groups such as hydrogen, alkyl, alkenyl,
alkynyl, cycloalkyl, aryl, alkaryl, aralkyl, and the like may be
selected. As used herein, the terms "hydrocarbon", "alkyl",
"cycloalkyl" and similar hydrocarbon terminology is meant to
include alcohols and hydrogen, although specific reference to the
inclusion of hydrogen and/or alcohols is frequently made herein.
Examples of such groups are methyl, propenyl, ethynyl, cyclohexyl,
phenyl, tolyl, benzyl, hydroxyethyl and the like. R.sub.1 is
preferably selected from groups including hydrogen and the general
class of lower alkyl compounds such as methyl, ethyl, or the
like.
[0086] R.sub.2 can be an alkyl group, preferably having 1 to 24
carbon atoms, most preferably 1 to 18 carbon atoms; an alkenyl
group, preferably having 2 to 4 carbon atoms; an aminoalkyl group,
preferably having 1 to 8 carbon atoms, and optionally substituted
on the nitrogen atom with one or, preferably two alkyl groups,
preferably having 1 to 4 carbon atoms; an alkyl group, preferably
having 1 to 4 carbon atoms, having a five or six-membered
heterocyclic ring as a substituent; an allyloxyalkyl group,
preferably having up to 12 carbon atoms; an alkoxyalkyl group,
preferably having a total of 2 to 12 carbon atoms; an aryloxyalkyl
group, preferably having 7 to 12 carbon atoms; an aralkyl group,
preferably having up to 10 carbon atoms; or a similar alkyl or
aralkyl group having substituents which will not interfere with the
polymerization of the ester. That is, the matrix base component and
additive, so long as each is selected according to other criteria
described, can include esters selected from the group consisting of
(C.sub.1-C.sub.24)alkyl esters of acrylic acid, preferably a
(C.sub.1-C.sub.4)alkyl acrylate, di(C.sub.1-C.sub.4)alkylami-
no(C.sub.2-C.sub.4)alkyl esters of acrylic acid,
(C.sub.1-C.sub.8)alkoxyal- kyl esters of acrylic acid,
(C.sub.6-C.sub.10)aryloxyalkyl esters of acrylic acid,
(C.sub.7-C.sub.10)ar alkoxyalkyl esters of acrylic acid, and
(C.sub.7-C.sub.10)aralkyl esters of acrylic acid. Copolymers can
include polymers in which more than one monomer is selected from a
given group, for instance, the case where the polymer is a
copolymer of at least two (C.sub.1-C.sub.24)alkyl acrylates. Other
copolymers of the invention comprise monomers which may or may not
be acrylates, such as copolymers of at least one
(C.sub.1-C.sub.24)alkyl acrylate and at least one other
copolymerizable ethylenically-unsaturated monomer. This
copolymerizable monomer may be acrylonitrile or dimethylaminoethyl
acrylate, preferably when the alkyl acrylate is a
(C.sub.1-C.sub.4)alkyl acrylate.
[0087] Among the esters embraced by the formula
CH.sub.2.dbd.C(R.sub.1)(CO- OR.sub.2) which are suitable monomers
are unsubstituted alkyl acrylates, in which the alkyl group can
have branched or straight-chain, cyclic or acyclic spatial
configurations, such as methyl acrylate, ethyl acrylate, propyl,
isopropyl and cyclopropyl acrylates, isobutyl, t-butyl, n-butyl and
cyclobutyl acrylates, pentyl and cyclopentyl acrylates, hexyl and
cyclohexyl acrylates, heptyl and cycloheptyl acrylates, octyl,
acrylates, including 2-ethylhexyl acrylate, nonyl acrylates, decyl
acrylates, undecyl acrylates, lauryl acrylates, myristyl acrylates,
cetyl acrylates, stearyl acrylates, and the like; aralkyl
acrylates, such as phenylethyl acrylates, phenylpropyl acrylates,
and the like; aralkyl acrylates, in which the aryl group is
substituted with alkyl groups, halogen atoms, alkoxy groups, nitro
groups, or similar substituents which will not interfere with the
polymerization reaction; alkenyl acrylates, such as allyl acrylate,
and the like; aminoalkyl acrylates, such as dimethylaminoethyl
acrylate, phenylaminoethyl acrylates, t-butylaminoethyl acrylates,
dimethylaminobutyl acrylates, diethylaminoethyl acrylate, and the
like; alkyl acrylates having a heterocyclic group as a substituent
on the alkyl group, such as morpholinoalkyl acrylates,
oxazolidinylalkyl acrylates, piperidinodalkyl acrylates,
dioxolanylalkyl acrylates, i.e., ketals and acetals of glyceryl
acrylate, and the like; iminoalkyl acrylates, such as ketiminoalkyl
acrylates and aldiminoalkyl acrylates; alkoxyalkyl, aryloxyalkyl,
and aralkoxyalkyl acrylates, such as methoxyethyl acrylate,
ethoxyethyl acrylate, butoxyethyl acrylates, hexyloxypropyl
acrylates, ethoxypropyl acrylates, propoxybutyl acrylates,
hexloxyhexyl acrylates, phenoxyethyl acrylates, benzyloxyethyl
acrylates, and the like; and allyloxyalkyl acrylates, such as
allyloxyethyl acrylate, allyloxyethoxyethyl acrylate,
allyloxypropyl acrylate, and the like. Bis acrylate esters of
diols, such as the diester of 1,4-butanediol and acrylic acid, can
also be used. Other esters of acrylic acid which do not contain
substituents which would interfere with the polymerization of these
esters are also suitable. Methacrylates of the above acrylates also
are suitable.
[0088] The matrix base component can be the polymerization product
of a monomer having the formula
CH.sub.2.dbd.C(R.sub.1)(COOR.sub.2), where R.sub.1 is H or
CH.sub.3, and R.sub.2 is H or C1-C8 alkyl. The matrix base
component can be a random copolymer of a species such as this with
a species in which R.sub.2 is larger but, as discussed above,
preferably with no more than about 4 additional units in
R.sub.2.
[0089] Although the matrix base component is exemplified, in the
discussion above, as an acrylate, this is for purposes of
illustration only and any of a wide variety of base components can
be used, so long as the first, base polymeric component and the
second polymeric component meet the criteria set forth herein. For
example, the matrix base component can be a fluorinated polymer
such as polyvinylidene fluoride (PVDF), or other polymer components
such as those described below in connection with phase inversion
techniques. In connection with the description above, the most
important consideration is architecture of the specific polymer
components.
[0090] The second component of the invention, in a preferred
embodiment, is as described above for the matrix component and, in
addition, includes hydrophilic branches via incorporation of a
monomer in copolymerization where R.sub.2 is hydrophilic, such as a
polyalkylene oxide. As noted above, the second component should be
of molecular weight sufficient to be well-entangled with the matrix
base component, and added side chains should be selected to impart
to the second component a different chemical functionality than
that of the base component and in particular a chemical
functionality desired at the surface. In one, preferred embodiment,
the second component is made by a copolymerization reaction
including a monomer that constitutes the monomer of the matrix base
component and a monomer in which R.sub.2 is a polyethylene glycol.
Specific examples of monomers suitable for polymerization to form a
copolymer composition according to this embodiment of the present
invention include, but are not limited to: acrylonitrile,
2-ethylhexylmethacrylate, methylmethacrylate, dodecylmethacrylate,
vinylacetate, cyclohexylmethacrylate, 2-hydroxypropylmethacrylate,
and acrylamide.
[0091] Acrylates are advantageous in that they can be synthesized
economically. For example, free radical polymerization or anionic
polymerization of a random copolymer additive from acrylates
including an acrylate macromonomer having a side chain with a
desired chemical functionality results in a random, branched
copolymer having characteristics of the chemical functionality of
the side chain to an extent related to the relative amount of the
acrylate monomer including the side chain.
[0092] A random copolymer can be advantageous since a block
copolymer that includes hydrophilic blocks and hydrophobic blocks
can be water soluble in that it can form a micelle-like structure
in which blocks of hydrophilic polymer units segregate to the
exterior of the micelle, while an analogous random copolymer will
not form micelles and therefore may not be water soluble.
[0093] The technique of the invention involving allowing a
particular component of a miscible polymer blend to segregate to
the surface can be applied to injection molding polymer processes.
For example, intraocular lenses, other optical devices, and
essentially any polymeric article can be injection molded using a
miscible blend of a plurality of components at a temperature at
which one component can be preferentially entropically-driven to
the surface. In injection molding, or other techniques, a polymer
blend can be fabricated and allowed to harden under conditions at
which entropically-driven segregation does not occur, or occurs
only to a limited extent, but the article can be further heated for
a period of time and at a temperature sufficient to allow further
segregation to take place.
[0094] In another aspect of the invention a surface of an article
is modified to expose a desired chemical functionality by
delivering to the surface, in a solvent in which the article is at
least partially soluble, a surface-functionalizing branched polymer
that can entangle with molecules of the article. The solvent
dissolves a thin layer of molecules at the surface and, after
delivery of the modifying branched polymer, entanglement occurs,
followed by evaporation of the solvent. One set of articles that
can be created in accordance with the invention are
surface-functionalized intraocular lenses. Intraocular lenses are
surgically inserted into a patient's eye to replace a clouded or
otherwise damaged lens. These lenses typically are made of
synthetic polymeric materials and, desirably, would have a
physiologically-compatib- le surface coating. The lenses can be
made from PMMA, which can be lathed from a PMMA rod. In accordance
with one aspect of the invention, the surface of a PMMA intraocular
lens can be coated with a PMMA/P(MMA-r-MnG) blend in a solvent
highly compatible with the lens, such as tetrahydrofuran (THF),
which solvent evaporates to leave a hydrophilic coating. The
technique results in a compatiblized coating. In another technique
a PMMA lens can be coated with the P(MMA-r-MnG) alone in THF. In
either case, the solvent solubilizes a thin film of polymer at the
surface of the article, resulting in entanglement of PMMA in the
lens itself with PMMA in the additive that is entangled with
P(MMA-r-MnG) and/or the P(MMA-r-MnG) itself, or with P(MMA-r-MnG)
alone where the additive is solely P(MMA-r-MnG). Thus, according to
one embodiment the invention involves an optical device, such as an
intraocular lens, surface functionalized as described by the
coating method above.
[0095] Although rendering a hydrophobic bulk polymer blend
hydrophilic is discussed predominantly above, the invention allows
for tailoring the surface characteristic of a polymer article in a
wide variety of ways. For example a second, branched component that
resides at the surface of an article formed from a blend can
include polyion side chains made rigid by electrostatic repulsion
localized at a surface of the article. Where the article is a
membrane, this creates a technique for tuning the pore size of the
membrane by varying water pH levels. For example, benzyl glutamate
NCA can define an exposed functionality of a branched component in
a membrane created in accordance with the invention, creating a
membrane with tuneable pores. Ito, et al., J. Am. Chem. Soc., 119,
1619-1623 (1997), incorporated herein by reference, describes graft
polymerization of such a species onto a membrane after the membrane
is formed. Mika, et al., Journal of Membrane Science, 108, 37-56
(1995) also is incorporated herein by reference for the disclosure
of ion-selectivity in membranes. Mika, et al. use graft
polymerization. In connection with the present invention this
functionality is provided on a branched second component that
segregates from a blend of the first, bulk material and the second
component. An ion-exchange membrane also can be created by
providing a branched component, as a second component, including a
chelating functionality as described by Kojima, et al., Journal of
Membrane Science, 49-54 (1995), incorporated herein by reference.
Bidentate, tridentate, and quadradentate chelating agents can be
used, for example.
[0096] Functionalization of a branched component that resides
preferentially at the surface of polymeric articles in accordance
with the invention can be modified once the article is made when
the branched component includes a modifiable functionality. For
example, with reference to Example 2, below, MnG could be
substituted with PEG methacrylate which can be used in free-radical
copolymerization with MMA resulting in a branched component having
functionalizable terminal groups, such as -OH at terminal ends of
the branches. This allows post-segregation functionalization easily
in a variety of ways. This aspect of the invention can be used in
combination with any other aspect of the invention. That is, for
example, an article of the invention can include a first polymer
and a second polymer that is compatible with the first polymer, and
is more highly-branched than the first polymer, and includes
readily-functionalizable groups such as -OH groups at the ends of
branches, and is allowed to migrate disproportionately to the
surface of the article.
[0097] The particular type of polymerization used to form
components of the invention is not strictly important. In one
embodiment anionic polymerization is used, and in another set of
embodiments free-radical polymerization is used. In yet another set
of embodiments cationic polymerization is used.
[0098] As noted above, the invention also provides techniques for
membrane fabrication where the membrane includes desired surface
characteristics. A dense layer of relatively short chains can be
provided at the surface of an article which can allow for a high
degree of tailoring of surface characteristics, such as fouling
resistance, which can be provided with hydrophilic branching. In
membranes, control of pore structure can be provided by providing
branches that will straighten or coil depending upon the pH of the
environment. Where many branches exist at the surface the chain
ends can be functionalized, as described above, where the chains
terminate in a linking functionality such as --OH. Chelating agents
can be provided at the surface of membranes to selectively absorb
metals or other species. Other surface characteristics can be
selected.
[0099] Another aspect of the present invention is a technique
involving subjecting a blend of the first polymer component and
second polymer component, described above, to phase inversion. In
one embodiment the blend is first provided as a polymeric fluid,
typically including the blend dissolved in a solvent such as
dimethyl fornamide (DMF), and then exposing the polymeric fluid to
a second, incompatible fluid (nonsolvent; such as water) to form an
emulsion. The second, incompatible fluid forms a porous structure
in the polymeric fluid, and from the emulsion is recovered a porous
article. Any of a variety of articles, including membranes, can be
fabricated in this manner. In the article, the second polymer is
present at the surface in a ratio greater than the overall ratio of
the second polymer to the first polymer. In accordance with the
invention membranes can be formed having very small pores, thus
reverse osmosis membranes, ultrafiltration membranes, and the like
can be made. The invention includes membranes, which can be formed
according to methods of the invention, having pores smaller than 10
microns in diameter, more commonly smaller than 1 micron in
diameter, more commonly smaller than 0.5 microns in diameter, and
can include membranes having pores on the order of 0.1 micron
diameter average pore size. Membranes can be formed having pores of
even smaller size, for example as small as 10 Angstroms. These
values can define maximum pore sizes of membranes of the invention,
average pore sizes of membranes of the invention, or a combination.
That is, in one embodiment the invention includes a membrane having
average pore size of less than 0.1 micron and maximum pore size of
less than 0.2 micron, etc. Various combinations are possible.
[0100] The phase inversion technique can be used to fabricate a
variety of articles, from a variety of different polymer blends.
All that is required is that at least two miscible, polymer
components be mixed and allowed to undergo phase inversion
segregation. The segregation can be entropically-driven,
enthalpically-driven, or a combination. For example, blends of PVDF
with P(MMA-r-MnG), blends of PVDF with polyethyleneimine branched
(polyion) components, blends of PVDF with a branched component
having poly(acrylic acid) side chains (see Example 13, below), or a
blend of a polysulfone base with a second, miscible component such
as a branched additive can be used.
[0101] A simple, initial screening test can be carried out to
determine whether miscible blends might be suitable for phase
inversion in accordance with the invention. In one test blends are
first determined to be miscible, and then the blends are provided
as a viscous fluid in a minimum amount of solvent. A non-solvent
(precipitating incompatible fluids such as water) is used to test
the phase inversion potential of the fluid blend. In a multi-well
laboratory plate, a series of polymer blends are provided in the
bottoms of wells, and a non-solvent is gently added on top of each
blend. Alternatively, a non-solvent can be provided in the bottom
of each well and the viscous polymer blends can be provided gently
atop the non-solvent. After evaporation of the non-solvent and the
solvent carrier in which the blend is provided, a film of polymer
blend exists. Contact angle measurements of the surface of the
film, against which the non-solvent had been placed, can indicate
whether segregation has taken place. That is, the contact angle at
the surface in contact with the non-solvent can be measured and
compared with the contact angle of a similar, homogeneous blend,
which can be prepared by spin coating. If the contact angle is
different, surface segregation likely has occurred and the
particular blend is a good candidate for surface modification via
phase inversion. Alternatively, XPS can be used to characterize
surfaces of the films.
[0102] As noted, one set of articles that can be created in
accordance with the invention are membranes formed by phase
inversion. Polymer membranes for water treatment can be formed by
phase inversion and the resulting membranes are categorized
according to their pore sizes. Reverse osmosis membranes used for
water desalination typically contain pores of about 5-20 .ANG. in
diameter. Colloids and macromolecules are separated from water
using ultrafiltration membranes typically having pore sizes from
about 10 to about 1,000 .ANG.. Reverse osmosis and ultrafiltration
membranes are prepared essentially exclusively by phase inversion
processes (see, for example, Loeb, et al., Advan. Chem. Ser., 38,
117, 1962; Kesting, et al., Synthetic Polymeric Membranes, New
York: McGraw-Hill Book Company, 1971, pages 116-157; Strathmann, et
al., "A Rationale for the Preparation of Loeb-Sourirajan-Type
Cellulose Acetate Membranes", J. Appl. Poly. Chem., 15, 811-28,
1971; Strathmann, et al., "The Formation Mechanism of Phase
Inversion Membranes", Desalination, 21, 241-55, 1977; Strathmann,
et al., "The Formation Mechanism of Asymmetric Membranes",
Desalination, 16, 179-203, 1975). Membranes produced using this
process typically have an asymmetric porous structure consisting of
a dense, 0.1 to 1 micron surface layer overlaying a highly porous,
100 to 200 micron sublayer (Strathmann, in Synthetic Membranes:
Science Engineering and Applications, Bungay, P. M., et al., eds.
Dordrecht, The Netherlands: Kluwer Academic Publishers, 1983, page
1). The separation characteristics of the membrane are determined
by the pore size distribution in the surface, or "active" layer.
The porous sublayer provides mechanical support.
[0103] Apparatus for the continuous fabrication of polymer
membranes by phase inversion is known. The steps, in general,
involve dissolving a polymer in a solvent to form a solution
containing from about 10 to about 30 weight percent polymer. Small
quantities of nonsolvent and organic or inorganic salts are
sometimes, but not always, added to the solution. The solution then
is cast under a doctor blade onto a moving, nonwoven polyester or
Mylar.TM. belt. Often, this belt will serve as a permanent support
for the finished membrane. The thickness of the cast film is
typically between 100 and 500 microns. Partial evaporation of the
solvent may or may not be allowed to occur. The film then is
immersed in a nonsolvent, that is, a fluid that is incompatible
with the polymer (usually water) resulting in gelation of the
polymer to form an asymmetric, porous structure. The nonsolvent
temperature is typically between about -10 and about 20.degree. C.
The membrane can be heat treated in a second water bath to promote
pore shrinkage. The heat treatment temperature is usually between
about 50 and about 90.degree. C. The membrane then is rinsed and
taken up on a roll.
[0104] Several mechanistic theories of polymer membrane formation
exist in the literature. A particular comprehensive theory is that
presented by Strathmann, et al., (Desalination, 21, 241-55;
Desalination, 16, 179-203, both referenced above). According to
this theory, the formation of the initial membrane morphology
during the coagulation step in the membrane fabrication process is
fundamentally a phase separation process. An initially homogeneous
casting solution becomes unstable as nonsolvent is imbibed from the
gelation bath. When the local concentration of nonsolvent exceeds a
critical value, the homogeneous casting solution separates into a
polymer-rich phase and a polymer-poor phase, the polymer-poor phase
eventually becoming the fluid-filled pores of the membrane.
[0105] The function and advantage of these and other embodiments of
the present invention will be more fully understood from the
examples below. The following examples are intended to illustrate
the benefits of the present invention, but do not exemplify the
full scope of the invention.
EXAMPLE 1 (COMPARATIVE)
Synthesis and Characterization of PEO/PMMA Polymeric Article
[0106] PEO/PMMA blends were made, specifically, 50,100 g/mol PEO in
222,000 g/mol PMMA-d.sub.8, with 2, 5, 10, and 20% by weight of
PEO. The samples were annealed at 190.degree. C. for two weeks.
Following this treatment, it was shown via neutron reflectivity
(NR) data that the surface of each sample was substantially
depleted of PEO within approximately 50 .ANG. from each surface.
This shows that the samples of Sakellariou, et al. (referenced
above) were not fully annealed, and that the lower-energy polymeric
component will segregate to the surface when driven by enthalpic
energy.
EXAMPLE 2
Synthesis of Branched, Hydrophilic Acrylate Additive
[0107] Methyl methacrylate (MMA) monomer,
CH.sub.2C(CH.sub.3)(CO.sub.2CH.s- ub.3), was purchased from Aldrich
Chemical, and methoxy poly(ethylene glycol) monomethacrylate (MnG)
macromonomer, CH.sub.2C(CH.sub.3)[CO.sub.2-
(CH.sub.2CH.sub.2O).sub.nCH.sub.3], having approximately n=9
ethylene oxide units (number average molecular weight, {overscore
(M.sub.n)} g/mol) per mer, was purchased from Polysciences, Inc.
The branched hydrophilic additive was prepared by random
copolymerization of MMA with MnG using anionic polymerization
techniques. The resulting copolymer, P(MMA-r-MnG), had
approximately 40 poly(ethylene oxide) (PEO) side chains
statistically distributed along the 200-unit long methacrylate
backbone, with molecular weight {overscore (M.sub.n)}=40,700 and
polydispersity {overscore (M.sub.w)}/{overscore (M.sub.n)}=1.26
(where w denotes weight average) as measured by a combination of
gel permeation chromatography (GPC) and light scattering.
Incorporating MMA into the backbone (the MMA fraction is
f.about.0.5 by mass from nuclear magnetic resonance (NMR)
spectroscopy) helps anchor the copolymer to the poly(methyl
methacrylate) (PMMA) matrix and renders the copolymer water
insoluble.
[0108] From initial contact angles with water and diidomethane, the
surface tension for the random copolymer was found to be
46.0.+-.0.9 mJ/m.sup.2, higher than PMMA (43.6.+-.1.1 mJ/m.sup.2,
purchased from Polysciences, Inc. with {overscore
(M.sub.n)}=330,000 g/mol and {overscore (M.sub.w)}/{overscore
(M.sub.n)}=1.11) and PMMA-d.sub.8 (43.9.+-.0.9 mJ/m.sup.2,
purchased from Polymer Laboratories with {overscore
(M.sub.n)}=314,000 g/mol and {overscore (M.sub.w)}/{overscore
(M.sub.n)}=1.06; deuteration was used for contrast in neutron
scattering experiments) used in this study, but lower than pure
PEO. Since the copolymer is the higher-energy component, any
surface enrichment of this species when blended with PMMA is
expected to have entropic rather than enthalpic origins.
EXAMPLE 3
Creation of Polymeric Article Including Entangled Blend of
Compatible High-Energy Additive with Low-Energy Matrix Base
Component
[0109] Samples of polymeric articles including entangled blends of
a compatible high-energy additive P(MMA-r-MnG) with a low-energy
matrix base component (PMMA-d.sub.8; deuteration was used for
contrast in neutron scattering experiments) were prepared by spin
coating various blend compositions from toluene onto 10 cm diameter
polished silicon wafers, creating films approximately 1000 .ANG.
thick. Films were subsequently annealed in vacuo at 190.degree. C.
for 7 days to achieve equilibrium. Small angle neutron scattering
(SANS) measurements were performed on a 50% blend of 314,000 g/mol
PMMA-d.sub.8 and 40,700 g/mol P(MMA-r-MnG) in order to directly
measure their miscibility. The resultant interaction parameter was
higher than the interaction parameter between PEO and PMMA-d.sub.8,
but lower than that between PMMA and PMMA-d.sub.8, confirming that
the blend is miscible.
EXAMPLE 4
Characterization of Polymeric Articles of Example 3
[0110] Neutron reflectivity (NR) was used to characterize the
degree of surface segregation in the miscible blends of Example 3
(shown by cloud point measurements and SANS). FIG. 4 shows NR data
(measured on a neutron reflectometer with monochromated neutrons of
wavelength .lambda.=2.35 .ANG.) for sample films containing 2, 5,
10, and 20 wt % branched P(MMA-r-MnG) additive. NR data is shown as
circles as a function of wave vector k.sub.z perpendicular to
sample surfaces. Data is fit (lines) with a model scattering length
density (b/V) profile (illustrated in the inset for the 20% sample)
which depends on the type and amount of material present at any
distance z into the sample. The inset depicts the expected
configuration of the branched additive at the interfaces with the
chain ends localized at the material boundary.
[0111] Fits to this data yield the volume fraction profiles shown
in FIG. 5. FIG. 5 provides volume fraction profiles normalized by
the sample thickness, L ca. 1000 Angstroms, extracted directly from
fits to the NR data, with coverage of the exposed surface of the
film by the additive on the left axis and coverage of the substrate
by the additive on the right axis. The film containing 20% additive
shows approximately 60% coverage of the surface and 100% coverage
of the substrate.
[0112] The data of FIG. 5 demonstrates the branched copolymer
additive's propensity to segregate to both the surface and
substrate. In all cases, complete segregation of the P(MMA-r-MnG)
to the interfaces occurs.
[0113] Entropically-driven segregation of the higher-energy
branched component is indicated also by the fact that the
P(MMA-r-MnG) additive has a higher surface tension than
PMMA-d.sub.8, but slightly less than that of PMnG. Table 2 shows
contact angles of various liquids (W=water, DIM=diidomethane,
TP=tricresyl phosphate, respectively) on 330,000 g/mol PMMA,
314,000 g/mol PMMA-d.sub.8, 19,900 g/mol PMnG, and 40,700 g/mol
P(MMA-r-MnG).
1TABLE 1 Contact Angle (.degree.) Sample Initial After 1 Hour After
4 Days PMMA 70.7 .+-. 1.0 69.5 .+-. 1.0 70.0 .+-. 1.0 P(MMA-r-MnG)
70.8 .+-. 1.0 61.5 .+-. 1.0 61.8 .+-. 1.2 P(MMA-r-MnG)/PMMA blend
68.1 .+-. 1.9 65.1 .+-. 1.7 60.8 .+-. 1.6
[0114] Surface tensions calculated from the contact angle
information of Table 1 for each combination of two liquids is
presented in Table 2.
2 TABLE 2 Contact Angle [.degree.] Liquid PMMA d-PMMA PMnG
P(MMA-r-MnG) W 70.7 .+-. 1.0 70.2 .+-. 1.0 -- 70.8 .+-. 1.0 DIM
42.8 .+-. 1.0 42.6 .+-. 1.2 35.4 .+-. 1.2 35.6 .+-. 1.0 TP 41.8
.+-. 1.0 42.1 .+-. 1.0 34.9 .+-. 1.6 33.5 .+-. 1.3
EXAMPLE 5
Demonstration of Hydrophilicity and Durability of Samples of
Example 3
[0115] To test for hydrophilicity, advancing contact angles were
measured following immersion in water for a specified time (Table
1). For PMMA the contact angle remains constant as a function
of
3 TABLE 3 Surface Tension [dyne/cm] Liquid Pair PMMA d-PMMA PMnG
P(MMA-r-MnG) W DIM 43.9 .+-. 0.7 44.2 .+-. 0.7 -- 46.1 .+-. 0.7 W
TP 44.3 .+-. 0.7 44.4 .+-. 0.7 -- 46.2 .+-. 0.8 DIM TP 42.5 .+-.
1.8 43.1 .+-. 1.4 47.0 .+-. 1.9 45.6 .+-. 1.2 Average 43.6 .+-. 1.1
43.9 .+-. 0.9 47.0 .+-. 1.9 46.0 .+-. 0.9
[0116] exposure time to water. The contact angle for pure
P(MMA-r-MnG) initially resembles that of PMMA, but drops sharply,
initially, then stabilizes about 10.degree. lower than the initial
value as the surface absorbs water. These values stabilized at
about 80 minutes, remaining unchanged after four days of immersion.
When fully hydrated, films with 60% surface coverage of the
branched material also exhibit a contact angle roughly 10.degree.
lower than that of pure PMMA.
[0117] After drying, the contact angles for all samples return to
their initial values and the measurements are repeatable
demonstrating the physical stability of the surface.
EXAMPLE 6
Synthesis and Characterization of Thick Polymeric Articles
[0118] Polymeric articles were synthesized from blends as described
in the examples above, but were spun to a final thickness of about
2,000 .ANG.. FIG. 6 shows experimental and theoretical
reflectivities for thick blends of 2, 5, 10, and 20% P(MMA-r-MnG)
in PMMA-d.sub.8, and FIG. 7 shows volume fractions of P(MMA-r-MnG)
in these blends, showing complete coverage of both interfaces of
the sample, at all blend compositions, by the more hydrophilic,
branched component, without significant depletion of this component
from the bulk of the article.
[0119] In these films, complete surface coverage of the branched,
high-energy, random copolymer additive was observed, and surface
segregation was limited to a monolayer of the additive independent
of the amount of additive in the article. This demonstrates that
the additive is very compatible with the base PMMA component, since
at levels of additive loading greater than that needed to create a
monolayer, excess additive necessarily resided in the bulk, thus it
is bulk-miscible.
EXAMPLE 7
Hydration of Entropically-Driven Surface-Segregated Polymer
Articles
[0120] P(MMA-r-MnG)/PMMA-d.sub.8 and P(MMA-r-MnG)/PMMA blends were
hydrated with H.sub.2O and D.sub.2O, respectively. Films were
formed as described above to thicknesses of about 800 .ANG.. In
particular, a 20% blend of 40,700 g/mol P(MMA-r-MnG) in 314,000
g/mol PMMA-d.sub.8 was formed, as well as similar blends using
330,000 g/mol PMMA. Reflectivity and scattering length density
profiles were obtained before and after hydration as well as after
re-drying. The volume fraction profile of FIG. 8 describes both
systems. A 27% equilibrium water content is seen at the surface,
roughly corresponding to three water molecules per ethylene oxide
unit. This shows that P(MMA-r-MnG) copolymers are hydrophilic
additives, as are surfaces enriched with these additives in blends
with PMMA. The bulk properties remain essentially intact, that is,
a glassy, transparent, polymeric material.
[0121] These results compare to poly(hydroxy ethyl methacrylate)
(PHEMA), the main constituent of soft contact lenses, which also
has a contact angle 10.degree. less than pure PMMA, with an
equilibrium water content of approximately 40% (Garbassi, et al.,
Polymer Surfaces: From Physics to Technology, John Wiley &
Sons, West Sussex, 1994).
EXAMPLE 8
Protein Adsorption Study of Polymeric Articles of Example 3
[0122] Protein adsorption studies were performed with .apprxeq.800
.ANG. thick samples prepared as in Example 3 on 1 cm diameter
double-sided polished silicon wafers. These films were exposed to a
mixture of unlabeled and .sup.14C-labeled bovine serum albumin
(BSA) (obtained from Sigma Chemical Co. and American Radiolabeled
Chemical Inc., respectively) in a buffered water solution (0.01 M
phosphate and 0.15 M NaCl with pH=7.0) and equilibrated by shaking
for 6 hours. By comparing a treated sample's degree of
radioactivity with that of the solution, the amount of BSA
irreversibly adsorbed to the surface was determined (FIG. 9). The
surface of the blend of the invention including branched random
copolymer additive with PMMA demonstrates a reduction in protein
adsorption compared with pure PMMA (for comparison, a demonstration
of protein absorption of the pure copolymer additive, which
effectively inhibits all protein adsorption, is shown). The degree
of BSA adsorption for the film with 20% P(MMA-r-MnG) is consistent
with a 60% surface coverage of this material as shown by NR.
EXAMPLE 9
Protein Adsorption Study of Thick Polymeric Article
[0123] Protein adsorption studies were performed on articles as
described above but of thickness of approximately 2000 .ANG..
Specifically, adsorption studies were performed on double-sided
polished silicon wafers (Semiconductor Processing Co.) using HSA
(68,000 g/mol) and equine cytochrome-c (ECC), 12,000 g/mol) (both
from Sigma). The protein adsorption resistance was examined using
two proteins with different molecular weights in order to compare
results with a recent study reporting that with grafted PEO
surfaces, lower-molecular-weight proteins need higher grafting
densities to effectively resist a protein adsorption ([133]).
Samples were exposed to a mixture of .sup.125I-labeled and
unlabeled proteins in 0.01M phosphate buffer solution (PBS;
pH=7.4). After 3.5 hours, the samples were rinsed with saline
solution and the degree of radioactivity of each sample was
measured. FIGS. 10 and 11 show the results for HSA and ECC,
respectively. In both cases, irrespective of molecular weight, both
pure P(MMA-r-MnG) and P(MMA-r-MnG)-enriched PMMA surfaces almost
completely inhibit protein adsorption compared to the pure PMMA
samples, indicating that the surface of a P(MMA-r-MnG)/PMMA blend
does indeed, as shown by neutron reflectivity, resemble pure
P(MMA-r-MnG).
EXAMPLE 10
Cell Adhesion Studies
[0124] Films were prepared as described above and were exposed to
Chinese hamster ovary cells (CHO LA). Cells were incubated in a 10%
CO.sub.2 environment at 37.degree. C., trypsinized, and collected
by centrifugation. After further incubation for four hours, the
cells on PMMA strongly adsorbed and spread (FIG. 12) while CHO LA
adsorption on pure P(MMA-r-MnG) (FIG. 13) and the P(MMA-r-MnG)/PMMA
blend (FIG. 14) was limited and spreading was negligible.
[0125] These cell studies and the protein adsorption experiments
concur with the NR and contact angle measurements, showing that the
surface of the blend is completely covered with the branched
additive.
EXAMPLE 11
Preparation of a Membrane Having a Hydrophilic Surface via
Selective Segregation of a Branched Hydrophilic Additive to the
Surface
[0126] A branched hydrophilic additive, P(MMA-r-MnG), as described
above, was synthesized having approximately 40 PEG side chains
statistically distributed along the 200-unit long methacrylate
backbone. Polymer membranes were prepared from this material and
poly(vinylidene fluoride) (PVDF) by phase inversion casting.
P(MMA-r-MnG) and PVDF were co-dissolved in the ratio of 10:90 by
mass in N,N-dimethyl formamide. This solution was cast onto a glass
plate and coagulated in water at room temperature, resulting in a
membrane containing 10% P(MMA-r-MnG) by mass, corresponding to 3.8
mol % P(MMA-r-MnG). Membranes were subsequently dehydrated by
freeze-drying. The freeze-dried membranes were shown by scanning
electron microscopy to be highly porous. X-ray photoelectron
spectroscopy measurements conducted on freeze-dried membranes
showed a near-surface composition of 16 mol % P(MMA-r-MnG). This
near-surface composition corresponds to approximately 25% by volume
of hydrophilic PEG segments.
EXAMPLE 12
(Prophetic) Synthesis of Chelating Functionality for Polymer
Blend
[0127] FIGS. 15 and 16 are schemata illustrating a chemical route
to the synthesis of a second, branched component capable of
chelating mercury(II). This synthesis takes advantage of the
vulnerability of the styrene aromatic ring to electrophilic
substitution. The branched component III (FIG. 16) has a methyl
methacrylate backbone, for compatibility with PVDF. Each side chain
has approximately 10 styrene repeat units, to which are attached
thiazoline groups which are highly selective for mercury(II).
[0128] The methyl methacrylate-terminated polystyrene macromonomer
I (FIG. 15) is prepared using the anionic procedure of Schulz, et
al., "Graph Polymers With Macromonomers, I, Synthesis From
Methacrylate-Terminating Polystyrene", J. Appl. Poly. Sci. 27,
4773-86, (1982). Styrene monomer is purified by distillation over
excess calcium hydride. The initiator is sec-butyllithium, ligated
with 1,1-diphenylethylene. The reaction is carried out in benzene,
out of which the impurities are titrated by adding sec-butyllithium
slowly via syringe until a light tan-orange color is maintained.
Purified styrene monomer and sec-butyllithium are then charged into
the reactor according to the relationship:
Degree of Polymerization=[mol Monomer]/[mol Initiator]
[0129] The polymerization reaction is carried out for 30 minutes at
40.degree. C., under nitrogen. The reactor temperature is then
reduced to 20.degree. C., and the living styryl anions are capped
by the introduction into the reactor of excess liquid ethylene
oxide. The formation of the ethylene oxide anion is indicated by a
disappearance of the orange color of the styryl anion. The reactor
temperature is then increased to 40.degree. C., and the chains are
terminated by the additions of excess methacroyl chloride. The
methyl methacrylate-terminated polystyrene macromonomer I is
recovered by dropwise addition of the benzene solution to
methanol.
[0130] While it is possible to terminate the living styryl anion
directly with methacroyl chloride, Schulz, et al. found that this
led to side reactions due to attack of the extremely basic styryl
anion on the carbonyl or alpha hydrogen of methacroyl chloride.
These reactions were avoided by first endcapping the stryl anion
with ethylene oxide, which presents a less basic alkoxy anion.
[0131] The comb polymer II (FIG. 15) is obtained by random
copolymerization of the polystyrene macromonomer with methyl
methacrylate, using standard anionic or free radical methods. It is
desirable to have as many polystyrene branches as possible, while
retaining compatibility with PVDF. The first comb polymers are made
with approximately 20 mole percent polystyrene macromonomers.
Compatibility with PVDF is checked by spin-coating thin films of
blends of II with PVDF. Any phase separation will result in
cloudiness in the thin films. The thin films may also be examined
for phase separation under the optical microscope.
[0132] Functionalization of II with mercury-selective thiazoline
groups (FIG. 16) is achieved using a two-step procedure following
Sugii, et al. ("Preparation and Properties of Macroreticular
Resins-Containing Thiazole and Thiazoline Group" Talanta, 27,
627-31, 1980), who prepared crosslinked polystyrene beads
functionalized with this group. The comb polymer II is first
dissolved in a suitable solvent at 30-40.degree. C., to which
finely ground anhydrous aluminum chloride is added. Chloroacetyl
chloride then is added, and the reactor is stirred at 30-40.degree.
C. for 6 h. Electrophilic aromatic substitution of chloroacetyl
chloride takes place by the well-known Friedel-Crafts acylation
reaction. The product is isolated by pouring the reaction mixture
into ice water, which precipitates the polymer and liberates the Al
Cl.sub.3 catalyst from a complex which it forms with the
substituted acyl group. Completion of the reaction is verified by
infrared (IR) spectroscopy in potassium bromide discs. The comb
polymer II will have characteristic adsorption bands at 1670
cm.sup.-1 (vc=o) and 650 cm.sup.-1 (.lambda..sup.C-C1) [105].
[0133] For the second step in the functionalization procedure, the
polymer is redissolved in N,N-dimethylformamide (DMF). Excess
N,N'-dimethylthiourea is then added, and the mixture is stirred at
80.degree. C. for 6 h. The functionalized comb polymer III is then
precipitated by dropwise addition of the reaction mixture to
methanol. Completion of the reaction may be confirmed by
observation of characteristic IR adsorption bands at 2760 cm
(v.sub.N--CH3) and 1620 cm.sup.-1 (c.sub.C.dbd.N).
[0134] Polymer III can be used as a mercury-selective membrane
additive. Sugii, et al. (above) found that crosslinked polystyrene
functionalized with the thiazoline group and used as a column
adsorbent was very effective in isolating mercury from highly
saline solutions. The adsorbent was highly selective for mercury.
The presence of other metal ions, such as cobalt, nickel, copper,
zinc, strontium, barium, lead, and uranium (IV) did not interfere
with the chelation of mercury. Similarly, neutral salts, such as
sodium chloride, sodium nitrate, sodium sulfate, and sodium
thiocyanate, had no effect on the adsorption of mercury. Although
mercury was effectively chelated at all pH's, the presence of
hydrochloric acid in the feed solution was found to be desirable
when rapid adsorption was required. Mercury was effectively
recovered from the adsorbent by washing with a solution of 0.1M
hydrochloric acid or perchloric acid containing 5% thiourea. The
adsorbent was found to be stable in 1-5M solutions of hydrochloric
acid, perchloric acid, nitric acid, and sodium hydroxide, and its
adsorption capacity for mercury was not affected by treatment with
these solutions.
[0135] Moreover, the thiazoline functionalized polystyrenes were
found to be hydrophilic, compared to unfunctionalized polystyrenes.
The polystyrene adsorbent material was immersed in water, kept
under reduced pressure for a time, then allowed to stand under
atmospheric pressure for 24 h. The material was then centrifuged
and weighed, dried at 100.degree. C., and weighed again. The water
regain, calculated as the difference in weight, was 1.14 g/g for
the thiazoline functionalized polymer, while that for the
unfunctionalized polymer was below the detectable limit of weighing
apparatus.
[0136] Thus, the branched polymer component III can
surface-segregate in polymer membranes as a result of both the
enthalpic driving force due to the hydrophilicity of its side
chains and the entropic driving forces due to its branched nature
and the stiffness of the thiazoline groups. Where these driving
forces are not sufficient to promote significant surface
segregation, poly(ethylene glycol) side chains are incorporated
into the methacrylate backbone by the addition of methoxy
poly(ethylene glycol) methacrylate macromonomer during the
polymerization of II.
EXAMPLE 13
(Prophetic) Synthesis of Gating Functionality for Polymer Blend
[0137] FIGS. 17 and 18 are synthesis schemata for a branched
polyacid second component for environment sensitive pore gating.
The component VI has a methyl methacrylate backbone, for
compatibility with PVDF and insolubility in water. The weakly
charged poly(acrylic acid) (PAA) side chains localize at the
membrane surface, providing a means for pore gating much like that
of the grafted PAA chains reported in the literature. Surface
segregation of this component can occur as a result of both an
enthalpic driving force due to the solubility of PAA in water and
an entropic one due to the branched architecture of the
component.
[0138] The synthesis strategy for branched component VI is much
like that for the mercury chelating component III, in that the side
branches are prepared ionically as macromonomers, which are reacted
with methyl methacrylate to obtain the comb structure. Because of
its ionic nature, however, direct anionic synthesis of acrylic acid
to yield PAA is not possible. Thus, the side branches are initially
synthesized as poly(tert-butyl acrylate) (PtBA). Once the comb
polymer has been obtained, the tert-butyl esters on the side chains
are hydrolyzed to yield the hydrophilic PAA analog.
[0139] The methyl methacrylate terminated PtBA macromonomer IV is
prepared using a procedure following Kubo, et al. ("Solubilization
of Peptides in Water and Hexane: Synthesis of Peptide-Terminated
Poly(tert-butyl acrylate) and Poly (acrylic acid) via Living
Anionic Polymerization", Macromolecules, 28, 838-43, 1995). The
initiator, sec-butyllithium, is ligated with 1,1-diphenylethylene
to prevent back-biting and termination reactions. The
sec-butyllithium is added via syringe to distilled THF containing
0.7% LiCl. 1,1-diphenylethylene is then added, resulting in a deep
red color. The reaction mixture is then cooled to -78.degree. C.,
and tert-butyl acrylate is added dropwise using a syringe. Addition
of the monomer is indicated by a color change to light yellow.
After 30 minutes, the reaction is terminated by the addition of
excess methyacroyl chloride. The reaction mixture is warmed to room
temperature, and the PtBA macromonomer IV is recovered by dropwise
addition of the THF solution to a mixture of methanol and water
(1:1 by volume).
[0140] The comb polymer V is obtained by random copolymerization of
the PtBA macromonomer IV with methyl methacrylate in DMF, using
standard anionic or free-radical methods. The PtBA side chains then
are deprotected to yield PAA by hydrolysis of the tert-butyl group
in formic acid at 60.degree. C. Conversion of PtBA into the
polyacid may be confirmed using .sup.1H NMR, by observing the
disappearance of the tert-butyl peak at 1.4 ppm. Although
hydrolysis of the tert-butyl group of PtBA is expected to occur
more readily than hydrolysis of the methyl group of PMMA, some
hydrolysis of the backbone may occur. This is checked by using
.sup.1H NMR, and, if necessary, different hydrolysis conditions are
used to prevent conversion of the backbone to PAA.
[0141] An initial selection of the side chain length can be
accomplished as follows. Pore gating will be accomplished through
uncoiling of the PAA side chains at high pH to nearly their
full-extended configurations (Iwata, et al. "Preparation Properties
of Novel Environmental-Sensitive Membranes Prepared by Graft
Polymerization Onto a Porous Membrane", J. Memb. Sci., 38, 185-99,
1988), such that they block the membrane pores. The membrane pores
in reverse osmosis are typically distributed between 10 and 100
.ANG.. A fully-extended chain length of about 25 .ANG., then, can
produce significant pore restriction. The theoretical length of a
fully-extended, all-trans PAA chain is 2.5 .ANG. per repeat unit.
Thus, an initial degree of polymerization of roughly 10 acrylic
acid repeat units is used. This initial selection is refined as the
pore size distributions of other membranes are characterized. The
side chain length and frequency is limited by the requirement that
the comb component VI remain insoluble in water.
EXAMPLE 14
Preparation of Membrane with Hydrophilic Surface via Phase
Inversion
[0142] Membranes were prepared from solutions of 10% polymer in
N,N-dimethylformamide (DMF). The solutions were mixed by stirring
at room temperature. They were then cast by pipette onto a glass
plate having a raised lip around its edge, after which the glass
plate was immediately immersed in a bath of deionized water (dW) at
room temperature. Membranes were allowed to remain in the
coagulation bath for approximately 10 min. after separation from
the glass plate was observed. They were then removed and rinsed in
a second bath of dW. Some membranes were heat treated while
immersed in dW in an apparatus which has been constructed for that
purpose. The apparatus is capable of maintaining a constant
temperature (.+-.1.degree. C.) in the range from room temperature
to boiling. Membranes for electron microscopy, XPS, or gas
adsorption measurements were freeze-dried.
[0143] Photocopies of scanning electron microscopy (SEM) studies of
the membranes which have been prepared are shown in FIGS. 19 and
20. FIG. 19 is a photocopy of an SEM micrograph of a membrane
prepared from 100% PVDF, and FIG. 20 is a photocopy of an SEM
micrograph of a membrane prepared from a blend of 80% PVDF and 20%
(by weight) P(MMA-r-MnG). It can be seen that the membrane of FIG.
20, including the blend of the invention, is much more porous, thus
will facilitate a higher flow rate at the same pore size.
EXAMPLE 15
(Prophetic) Preparation of Membranes Having Hydrophilic
Surfaces
[0144] A synthesis technique is carried out as described in example
14, with the following modifications.
[0145] Higher polymer concentrations (ca. 20%) are used in casting
solutions. The solubility of PVDF in DMF is improved by the
addition of 3% LiCl (Munari, et al., "Casting and Performance of
Polyvinylidene Fluoride Based Membranes", J. Memb. Sci., 16,
181-93, 1983). As an alternative, a volatile cosolvent in the
casting solution is used to optimize membrane properties.
Evaporation of this cosolvent prior to immersion of the cast film
in the coagulant results in densification of the surface layer. In
addition, the membranes are cast using a doctor blade, to better
control film thickness and uniformity.
EXAMPLE 16
Characterization of Membranes
[0146] X-ray photoelectron spectroscopy (XPS) studies were carried
out to investigate "water side" and "glass side" of membranes
fabricated as described above. That is, to determine surface
segregation of the second component at the water side, compared to
the glass side.
[0147] The XPS results make it clear that significant surface
segregation of the branched component P(MMA-r-MnG) occurs on the
"water side" of the membrane, even without heat treatment. This
result may be rationalized if we consider the effect of the steep
concentration gradient of water which exists in the active layer of
the membrane during its precipitation and gelation. This
concentration gradient brings about a macroscopic flux of polymer
directed into the casting solution just prior to gelation.
According to the Onsager relation for diffusion, this flux is
proportional to the gradient in chemical potential of the polymer,
which is in turn proportional to the concentration gradient of
water. It is this flux, in fact, which is responsible for the
formation of the dense surface layer.
[0148] In the case of a polymer blend, however, we have two
different polymeric components, each of which will experience a
different gradient in its chemical potential and thus a different
flux into the casting solution. In the case of the blend under
consideration, we expect P(MMA-r-MnG) to experience a chemical
potential gradient less steep than that of PVDF, due to the
solubility of the PEG side chains in water and the extreme
hydrophobicity of PVDF. Thus, we expect a comparatively slow
macroscopic flux of P(MMA-r-Mng) into the casting solution prior to
gelation, resulting in an enrichment of (PMMA-r-MnG) in the dense
surface layer of the coagulated film.
[0149] On the "glass side" of the membrane, we have a different
situation, in that there is virtually no macroscopic concentration
gradient of water upon precipitation of the polymer. Thus, we
expect only microscopic fluxes of polymer prior to coagulation, as
a result of microscopic gradients in the concentration of water.
The XPS results indicate only a moderate surface enrichment of the
hydrophilic additive component on the "glass side", which is
exactly what we might expect based on the absence of the
macroscopic gradients in chemical potential upon precipitation.
[0150] Heat treatment of the membrane in 90.degree. C. water
results in an increase in the degree of surface segregation of the
branched component as indicated by XPS.
[0151] Those skilled in the art would readily appreciate that all
parameters listed herein are meant to be exemplary and that actual
parameters will depend upon the specific application for which the
methods and apparatus of the present invention are used. It is,
therefore, to be understood that the foregoing embodiments are
presented by way of example only and that, within the scope of the
appended claims and equivalents thereto, the invention may be
practiced otherwise than as specifically described.
* * * * *