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Claims  |
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What we claim is:
1. A method for producing a microporous, absorbent foam, comprising the
steps of:
(a) mixing a cross-linkable polymer and a first solvent to form a stable
solution, wherein said stable solution can be induced to phase separate;
(b) inducing said stable solution to phase separate into a
polymer-concentrated phase and a polymer-dilute phase;
(c) inducing chemical crosslinking of said polymer, so that said polymer
will crosslink in said concentrated phase during said phase separation to
thereby form a microporous material; and
(d) drying said microporous material to produce said absorbent foam.
2. The method of claim 1, wherein said stable solution is a substantially
homogeneous, single-phase solution.
3. The method of claim 2, wherein said phase separation is induced by
changing the temperature of said stable solution.
4. The method of claim 2, wherein said phase separation is induced by
adding a phase separation enhancer to said stable solution.
5. The method of claim 4, wherein said phase separation enhancer is either
another solute or an additional solvent, and wherein said additional
solvent is a non-solvent for said polymer and is miscible with said first
solvent.
6. The method of claim 3, wherein said crosslinking is induced prior to
said phase separtion, such that crosslinking will occur prior to said
phase separation.
7. The method of claim 3, wherein said single-phase solution exhibits a
lower consolute solution temperature, and wherein said phase separation is
induced by increasing the temperature of said single-phase solution to a
point above said lower consolute solution temperature.
8. The method of claim 3, wherein said single-phase solution exhibits an
upper consolute solution temperature, and wherein said phase separation is
induced by decreasing the temperature of said single-phase solution to a
point below said upper consolute solution temperature.
9. The method of claim 3 further comprising the step of adding a
phase-separation enhancer to said single-phase solution prior to inducing
said phase separation.
10. The method of claim 3, further comprising the step of removing the
uncrosslinked sol fraction present in said microporous material prior to
said drying step.
11. The method of claim 3, wherein said drying step is accomplished by
air-drying.
12. The method of claim 3, wherein said drying step is accomplished by
freeze-drying.
13. The method of claim 3, wherein said drying step is accomplished by
placing said microporous material in a second solvent, thereby swelling
said material with said second solvent and replacing any of said first
solvent which is present in said material, followed by air-drying said
material so that said second solvent will evaporate.
14. The method of claim 13, wherein said drying step further comprises the
step of placing said microporous material swollen with said second solvent
in a third solvent, thereby replacing said second solvent with said third
solvent, and thereafter air-drying said material so that said third
solvent will evaporate.
15. The method of claim 3, wherein said polymer is chosen from the group
consisting of: hydrophobically modified carbohydrate polymers, poly(vinyl
alcohol-co-vinyl acetate), poly(methacrylic acid), cyanoethylated or
partially formalized poly(vinyl alcohol), poly-N-vinyl-2-oxazolidone,
polypeptides, acrylate (or analogous methacrylate) copolymers, and
N-alkylacrylamide (or analogous N-alkylmethacrylamide) derivatives.
16. The method of claim 15, wherein said hydrophobically modified
carbohydrate polymers include: hydroxypropyl dextran, hydroxypropyl guar,
hydroxypropyl starch, hydroxypropyl cellulose (HPC), hydroxyethyl
cellulose (HEC), methyl cellulose, hydroxypropylmethyl cellulose, and
ethylhydroxyethyl cellulose; wherein said polypeptides include:
poly(L-proline), and poly(valine-proline-glycine-X-glycine), wherein X=any
amino acid; wherein said acrylate (or analogous methacrylate) copolymers
include: hydroxypropyl acrylate-co-acrylamide, diacetone
acrylamide-co-hydroxyethyl acrylate, and hydroxypropyl
acrylate-co-hydroxyethyl acrylate; and wherein said N-alkylacrylamide (or
analogous N-alkylmethacrylamide) derivatives include: ethylacrylamide,
cyclopropylacrylamide, n-propylacrylamide, and isopropylacrylamide.
17. The method of claim 16, wherein said polymer is either HPC or HEC.
18. The method of claim 3, wherein said crosslinking is further
accomplished by physical means.
19. The method of claim 2, wherein crosslinking is induced by adding a
suitable crosslinking agent to said solution.
20. The method of claim 2, wherein said solution is photo-crosslinked.
21. The method of claim 19, wherein said physical crosslinking is
accomplished by employing a polymer having hydrophobic polymer side chains
capable of interacting with one another.
22. The method of claim 19, wherein said physical crosslinking is
accomplished through hydrogen-bonding, van der Waals interactions, ionic
bonding, hydrogen bonding, coordination interactions, or salt bridges.
23. The method of claim 2, wherein said crosslinking agent is chosen from
the group consisting of: acetaldehyde, formaldehyde, glutaraldehyde,
diglycidyl ether, divinyl sulfone, diisocyanates, dimethyl urea,
epichlorohydrin, oxalic acid, phosphoryl chloride, trimetaphosphate,
trimethylomelamine, polyacrolein, and ceric ion redox systems.
24. The method of claim 23, wherein said crosslinking agent is divinyl
sulfone.
25. A method for producing a microporous, open-celled foam, comprising the
steps of:
(a) mixing hydroxypropyl cellulose (HPC) and water to form a substantially
homogeneous, single-phase solution:
(b) inducing crosslinking of said HPC by adding a suitable crosslinking
agent to said single-phase solution;
(c) inducing phase separation of said single-phase solution into a
polymer-concentrated phase and a polymer-dilute phase, wherein said phase
separation is induced by increasing the temperature of said single-phase
solution to above the lower consolute solution temperature of said
single-phase solution;
(d) permitting said crosslinking to continue, so that said HPC will
crosslink in said concentrated phase during said phase separation to
thereby form a microporous material; and
(e) drying said microporous material to produce said foam.
26. The method of claim 25, wherein said crosslinking agent is divinyl
sulfone (DVS).
27. The method of claim 26, wherein the concentration of HPC is between
about 1.9 and about 25 weight percent of said single-phase solution, and
wherein the pH of said single-phase solution is above about 11.
28. The method of claim 27, wherein the concentration of DVS is between
about 0.2 and about 5.5 weight percent of said single-phase solution.
29. The method of claim 28, wherein said phase separation is induced by
increasing the temperature of said solution to above about 40.degree. C.
30. The method of claim 29, wherein said phase separation is induced after
said crosslinking has proceeded for between about 1 and about 45 minutes,
and said crosslinking is permitted to continue after inducing said phase
separation for between about 0.3 and about 100 hours.
31. A method for producing a microporous, open-celled foam, comprising the
steps of:
(a) mixing hydroxyethyl cellulose (HEC), water and a phase-separation
enhancer to form a substantially homogeneous, single-phase solution,
wherein said phase-separation enhancer is chosen from the group consisting
of: a salt, a water-soluble organic solvent, and a combination of a salt
and a water-soluble organic solvent;
(b) inducing crosslinking of said HEC by adding a suitable crosslinking
agent to said single-phase solution;
(c) inducing phase separation of said single-phase solution into a
polymer-concentrated phase and a polymer-dilute phase, wherein said phase
separation is induced by increasing the temperature of said single-phase
solution to above the lower consolute solution temperature of said
single-phase solution; and
(d) permitting said crosslinking to continue, so that said HEC will
crosslink in said concentrated phase during said phase separation to
thereby form a microporous material; and
(e) drying said microporous material to produce said foam.
32. The method of claim 31, wherein said crosslinking agent is divinyl
sulfone (DVS).
33. The method of claim 32, wherein said phase separation enhancer is
sodium chloride.
34. The method of claim 32, wherein said phase separation is induced by
increasing the temperature of said solution to above about 94.degree. C.
35. The method of claim 34, wherein the pH of said single-phase solution is
above about 11, and wherein the concentration of HEC is between about 1.3
and about 18 weight percent of said single-phase solution.
36. The method of claim 35, wherein the concentration of DVS is between
about 0.4 and about 2 weight percent.
37. The method of claim 36, wherein said phase separation is induced after
said crosslinking has proceeded for between about 1 and about 3 minutes,
and said crosslinking is permitted to continue after said inducing for
between about 20 and about 60 minutes. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to microcellular, open-celled, superabsorbent
polymer foams, and a method for producing the same. The foams thus
produced have exceptionally rapid sorption rates, as they absorb and
retain liquid by a combination of capillary action and pore wall swelling.
DESCRIPTION OF RELATED ART
Microporous, open-celled foams have garnered much interest recently due to
their potential for numerous and varied applications. For example, these
materials are useful in multishell fusion target experiments, as
filtration media, in controlled release systems, and as artificial skin
and blood vessels. Microporous, open-celled foams can also be employed in
much simpler consumer applications such as reusable diapers and other
personal hygiene devices. These latter uses often depend upon the
absorbent capabilities of the foam and rate of sorption, as well as its
strength.
Foams can generally be characterized as materials which have numerous
fluid-filled cells distributed throughout their mass. The properties of
these materials vary greatly, and depend in large part on the degree of
interconnectivity of the cells. For example, should it be desired to use
the foam as an absorbent, a greater degree of interconnectivity is
desired. If the cells in this two phase fluid-solid system are
interconnected, the material is termed an "open-celled" foam. Ideally, a
foam used for absorbent purposes should have 100% interconnectivity, in
which case the material is termed "bicontinuous" or "open-celled." In
contrast, closed cell foams have cells which are discrete, having fluid
phases which are independent of that of the other cells.
Another characteristic which greatly affects the properties of a foam is
the size of its pores. For example, while natural sponge is a well-known
absorbent, it cannot be used in products such as diapers because its
large, macroscopic pores cannot hold fluids under even the slightest
pressure. For a foam to be useful in a diaper, fluid must be retained
under a pressure of about 0.5 psi. In order to achieve this level of
retention, the pores must be microscopic, since only then will the
capillary forces responsible for fluid retention be sufficient to
withstand applied pressures at the desired levels. In addition, only
microscopic pores will retain fluid in competition with other absorbent
materials such as clothing ("wicking"). Thus, microporous foams (0.1-100
.mu.m pores) are desired for absorbent purposes.
Conventional, macroporous (>100 .mu.m pores) polymeric foams can be
produced by a number of methods, the most common being a gas dispersion
process whereby a gaseous phase is dispersed throughout a liquid polymer
phase. The resultant gas-solid state is then preserved either by physical
means such as vitrification, or by polymerization and/or crosslinking of
the liquid phase. The cell size in these foams, however, is generally
100-200 .mu.m or larger, and thus their usefulness as absorbents is
limited. These products do find use as insulation and packaging material.
Microporous (i.e., 0.1-100 .mu.m pores) polymeric foams have generally been
produced by phase separation techniques, however these methods are
generally only suitable for hydrophobic polymers. For example, polystyrene
foams having densities of 0.02 to 0.20 g/cm.sup.3 and cell sizes of 1-20
.mu.m have been produced. Typically, a homogeneous polymer/solvent
solution is first prepared. This solution is then permitted to phase
separate by either dissolving a nonsolvent for the polymer in the
solution, decreasing the temperature to a point below the upper consolute
solution temperature (UCST), or both. Most non-aqueous polymer/solvent
systems capable of phase separating exhibit an UCST, and these polymers
are typically hydrophobic. After phase separation, the temperature is
further reduced to either below the freezing point of the solvent or below
the glass transition temperature in order to lock in the desired
structure. The solvent can then be removed from the porous, polymer
structure either by freeze drying or supercritical drying to produce a
microcellular foam. Unfortunately, simple evaporation of the solvent may
not be employed for these products because large capillary forces at the
liquid-vapor interface will cause the structure to shrink or crack,
resulting in the destruction of the cells. In addition, although the
expensive and tedious procedures of freeze-drying or supercritical drying
may be employed, the resulting microporous foam will redissolve when
brought into contact with a good solvent and melt when subjected to
elevated temperatures.
Thus, there is a need for microcellular, open-celled foams which exhibit
superabsorbency and can be readily synthesized from numerous
polymer/solvent systems, particularly hydrophilic polymers.
SUMMARY OF THE INVENTION
While not exclusive, the following describes some of the important features
and objects of the present invention.
It is an object of the present invention to provide a method for producing
microporous, open-celled foam.
It is another object of the present invention to provide a method for
producing microporous, open-celled foam which can be employed with
numerous types of polymer/solvent systems.
It is yet another object of the present invention to provide microporous,
open-celled foams, as well as a method for producing the same, wherein
these foams which exhibit superabsorbancy, can be dried by a number of
different methods, and which retain a significant amount of liquid even
under pressure. These foams will absorb and retain liquid by a combination
of capillary action and pore wall swelling.
It is still another object of the present invention to provide a method for
producing microporous, open-celled foam, wherein the properties of the
foam can be regulated by the choice of synthesis parameters.
The foregoing objects can be accomplished by providing a method for
producing a microporous, open-celled foam, comprising the steps of: (a)
mixing a cross-linkable polymer and a first solvent to form a stable
solution, preferably a substantially homogeneous, single-phase solution,
wherein the stable solution can be induced to phase separate (preferably
upon a change in temperature of the solution); (b) inducing the stable
solution to phase separate by into a polymer-concentrated phase and a
polymer-dilute phase after a predetermined period of time; (c) inducing
crosslinking of said polymer, so that the polymer will crosslink in said
concentrated phase for a predetermined period of time during the phase
separation to thereby form a microporous material; and (d) drying the
microporous material to produce the absorbent foam. Preferably,
crosslinking is induced prior to the phase separation, and is permitted to
continue for a predetermined period of time prior to phase separation.
Optionally, the solution may be returned to a single phase condition, and
further crosslinked in this state to produce the desired foam. The
single-phase solution may exhibit a lower consolute solution temperature
or an upper consolute solution temperature, and phase separation is
preferably induced by increasing or decreasing the temperature of the
single-phase solution to a point above or below the lower consolute
solution temperature or the upper consolute solution temperature,
respectively. If necessary, a phase-separation enhancer may be added to
the single-phase solution prior to inducing phase separation so as to
assist the stable solution to phase-separate, either in conjunction with a
temperature change or at a constant temperature. Suitable phase separation
enhancers include other solutes such as a salt, other solvents, or even
additional polymer. The foams may be dried by a number of different
methods, and it is preferred that any uncrosslinked sol fraction be
removed from the foam prior to drying.
Drying of the microporous materials produced by the methods of the present
invention to produce the desired foams may be accomplished by air-drying,
freeze-drying, or a solvent-exchange method. This latter method of drying
is accomplished by placing the material in a second solvent, thereby
replacing any of the synthesis (or "first") solvent which is present in
the material with the second solvent. The microporous material may then be
air-dried to evaporate the second solvent, or the solvent-exchange method
repeated using a third solvent. It is preferable that if second and third
solvents are employed that the second solvent be miscible with the
synthesis solvent, and that the third solvent be miscible with the second
solvent and a non-solvent for the polymer itself. In this fashion the
third solvent will not be absorbed by the cell walls, and the evaporation
process will not exert as great a force on the pores. It is also
preferably that the second and third solvents (if employed) exhibit a high
degree of volatility.
The synthesis method of the present invention may be employed with any
polymer/solvent system which can be induced to phase separate, and wherein
the polymer is crosslinkable. The polymer, for example, may be chosen from
the following:
hydrophobically modified carbohydrate polymers, including: hydroxypropyl
dextran, hydroxypropyl guar, hydroxypropyl starch, hydroxypropyl cellulose
(HPC), hydroxyethyl cellulose (HEC), methyl cellulose, hydroxypropylmethyl
cellulose, and ethylhydroxyethyl cellulose
poly(vinyl alcohol-co-vinyl acetate) poly(methacrylic acid) cyanoethylated
or partially formalized poly(vinyl alcohol) poly-N-vinyl-2-oxazolidone
polypeptides, including: poly(L-proline), and
poly(valine-proline-glycine-X-glycine), wherein X=any amino acid
acrylate (or analogous methacrylate) copolymers, including: hydroxypropyl
acrylate-co-acrylamide, diacetone acrylamide-co-hydroxyethyl acrylate, and
hydroxypropyl acrylate-co-hydroxyethyl acrylate
N-alkylacrylamide (or analogous N-alkylmethacrylamide) derivatives
including: ethylacrylamide, cyclopropylacrylamide, n-propylacrylamide, and
isopropylacrylamide.
The polymer is preferably HEC.
The crosslinker may be chosen from the following: acetaldehyde,
formaldehyde, glutaraldehyde, diglycidyl ether, divinyl sulfone,
diisocyanates, dimethyl urea, epichlorohydrin, oxalic acid, phosphoryl
chloride, trimetaphosphate, trimethylomelamine, polyacrolein, and ceric
ion redox systems Preferably, the crosslinker is divinyl sulfone, when the
polymer is either HPC or HEC.
There is also provided a method for producing a microporous, open-celled
foam, comprising the steps of: (a) mixing hydroxypropyl cellulose (HPC)
and water to form a substantially homogeneous, single-phase solution; (b)
inducing crosslinking of the HPC by adding a suitable crosslinking agent
to the single-phase solution; (c) inducing phase separation of the
single-phase solution into a polymer-concentrated phase and a
polymer-dilute phase after a predetermined period of time, wherein phase
separation is induced by increasing the temperature of the single-phase
solution to above the lower consolute solution temperature of the
single-phase solution; and (d) permitting crosslinking to continue in the
concentrated phase after inducing phase separation to thereby form an
open-celled foam. The crosslinker is preferably divinyl sulfone (DVS). The
concentration of HPC may be between about 1.9 and about 25 weight percent
of the total weight of the single-phase solution, and is preferably about
4 weight percent. The pH of the single-phase solution should preferably be
above about 11, and more preferably about 12. The molecular weight of the
HPC employed is between about 100,000 and about 1,000,000, preferably
about 400,000. The concentration of DVS is preferably between about 0.2
and about 5.5 weight percent of the single phase solution, and more
preferably about 2 weight percent. Phase separation of these aqueous
polymer solution can be induced by increasing the temperature of the
solution to above about 40.degree. C., preferably to a temperature of
about 50.degree. C. Phase separation may be induced after the crosslinking
has proceeded for between about 1 and about 45 minutes (preferably about 5
minutes), and the crosslinking may be permitted to continue after inducing
phase separation for between about 0.3 and about 100 hours (preferably
about 24 hours). Excessive incubation at a high temperature and pH should
be avoided, as it may cause polymer degradation.
A method for producing a microporous, open-celled foam, is provided,
wherein this method comprises the steps of: (a) mixing hydroxyethyl
cellulose (HEC), water and a phase-separation enhancer to form a
substantially homogeneous, single-phase solution, wherein the
phase-separation enhancer is chosen from the group consisting of: a salt,
a water-soluble organic solvent, and a combination of a salt and a
water-soluble organic solvent; (b) inducing crosslinking of the HEC by
adding a suitable crosslinking agent to the single-phase solution; (c)
inducing phase separation of the single-phase solution into a
polymer-concentrated phase and a polymer-dilute phase after a
predetermined period of time, wherein phase separation is induced by
increasing the temperature of the single-phase solution to a point above
the lower consolute solution temperature of the single-phase solution; and
(d) permitting crosslinking to continue in the concentrated phase after
phase separation is induced to thereby form an open-celled foam. The
crosslinking agent is preferably divinyl sulfone (DVS), and the phase
separation enhancer is preferably sodium chloride. Phase separation may be
induced by increasing the temperature of the solution to above about
94.degree. C., preferably to about 95.degree. C. The pH of the
single-phase solution is preferably above about 11, and more preferably
about 12. The concentration of HEC is preferably between about 1.3 and
about 8 weight percent of the total weight of the single-phase solution
and the DVS, more preferably about 3 weight percent. The concentration of
DVS is preferably between about 0.4 and about 2 weight percent, and more
preferably about 1.6 weight percent. Phase separation may be induced after
crosslinking has proceeded for between about 1 and about 3 minutes,
preferably about 2.5 minutes. Crosslinking may then be permitted to
continue, after phase separation is induced, for between about 20 and
about 240 minutes, and preferably for about 60 minutes.
The HPC and HEC foams produced by the methods described above may also be
dried by the three methods previously described. If the solvent-exchange
method is employed, these foams may be dried using any of a number of
second solvents including: methanol, ethanol, 1-propanol, 2-propanol,
1-butanol, sec-butanol, and acetone. If both a second and third solvent
are employed in the drying process, the third solvent may, for example, be
pentane, hexane or heptane. HPC and HEC foams are preferably dried by the
solvent-exchange process using an ethanol-pentane solvent system. Other
solvents in addition to those specifically enumerated may also be
employed, and the choice of solvent(s) do not appreciably affect the foam
properties. It is preferred, however, that the final solvent employed
prior to drying be a non-solvent for the polymer.
There is also provided an absorbent (preferably superabsorbent),
microporous foam comprising a crosslinked polymer having interconnected
fluid cells distributed throughout its mass, wherein the fluid cells have
a diameter of between about 0.1 and about 100 .mu.m, and wherein the foam
can rapidly absorb at least about twice its dry weight in fluid. These
foams preferably absorb and retain fluid by a combination of capillary
action and pore wall swelling. The foams also do not lose a significant
amount of resorption capacity upon repeated swellin/drying cycles, and the
polymers employed are preferably hydrophilic. The polymers employed also
preferably exhibit a lower consolute solution temperature (LCST) in an
aqueous solution and phase separate from an aqueous solution as the
temperature of the solution is increased to above said LCST. The polymer
employed is preferably HPC or HEC.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Applicants have discovered a method for producing microcellular,
open-celled foams, which preferably exhibit superabsorbency. As used
herein, an absorbent foam is one which absorbs at least twice its dry
weight in fluid, and a superabsorbent foam is one which absorbs at least
ten times its dry weight in fluid. Additionally, the resulting foams will
swell, rather than dissolve, when placed in good solvents. The foams can
be produced in any size or shape, and the method is readily applicable to
numerous polymer/solvent systems which exhibit certain characteristics.
The superabsorbent foams produced by the method of the present invention
can be employed in any application wherein the foams of the prior art are
used. In addition, the foams of the present invention can also be employed
in biomedical applications (e.g., artificial blood vessels, tissue
supports, implants, artificial skin, and controlled release drug delivery
systems), packing for chromatography columns, reusable diapers, personal
hygiene products such as tampons, and generally as a substitute for
conventional superabsorbents. These foams can also be produced from
hydrophilic polymers, thereby providing a product which is particularly
suitable for biomedical applications since such products generally display
good biocompatibility. Such hydrophilic foams will absorb and retain water
by a combination of capillary action and swelling of the pore walls. The
porosity and pore size of the foams can also be precisely controlled by
varying the synthesis parameters.
Many homogeneous polymer/solvent solutions can be induced to phase separate
into polymer-concentrated and polymer-dilute phases merely by a change in
temperature. After the polymer is dissolved in a solvent, phase separation
can usually be induced by increasing the temperature to a point above the
lower consolute solution temperature (LCST). Some polymer/solvent systems
(particularly non-aqueous solvents) exhibit an upper consolute solution
temperature (UCST), and thus in these systems phase separation is induced
by decreasing the temperature to a point below the UCST. In addition, the
LCST or UCST can be modified when needed by the addition of other solutes
or solvents ("phase separation enhancer").
During the early stages of phase separation an interconnected morphology
exists, and the applicants have found that open-celled, superabsorbent
foams can be produced if this interconnected morphology can be preserved
in the final product. Phase separation can also be induced by a number of
other means while still producing the interconnected or bicontinuous
structure during the phase separation. Typically, this interconnected
structure is achieved merely by moving the polymer/solvent solution from a
thermodynamically stable phase to a thermodynamically unstable condition.
One skilled in the art can quite readily accomplish the phase separation
merely by employing the polymer/solvent phase diagram for the particular
polymer/solvent system employed. In addition to inducing phase separation
by raising or lowering the temperature, the addition of a phase separation
enhancer (with or without a change in temperature) may also induce phase
separation. Suitable phase separation enhancers include: solutes such as
salts, other solvents, additional polymer of the type used in the
synthesis. If a solvent phase separation enhancer employed is a
non-solvent for the polymer but is miscible with the solvent of the
homogeneous polymer/solvent system, phase separation can be induced by
spreading the enhancer over the surface of the polymer/solvent solution to
form sheets or by dispersing the polymer/solvent solution in the enhancer
to form particles. The phase separation enhancer can even be merely mixed
with the polymer/solvent solution in order to induce phase separation. The
use of a phase separation enhancer may also be combined with a change in
solution temperature in order to induce phase separation, particularly
when the phase separation is additional polymer.
The initial thermodynamically-stable state may even be in the form of a
suspension or emulsion. The suspension or emulsion can then be induced to
phase separate into a thermodynamically unstable condition by any of the
methods described above, thereby providing a polymer-concentrated and a
polymer-dilute phase.
In general, the foams of the present invention can be prepared from any
polymer having reactive functional groups (i.e., can be crosslinked). The
polymer is first solvated in order to form a stable, preferably
homogeneous solution. The polymer is then preferably crosslinked in this
stable solution for a predetermined period of time in order to form a
limited crosslinked network, which can help provide a macroscopic
structure for the final foam product. It should be noted, however, that
crosslinking prior to phase separation may be omitted in some instances.
The partially cross-linked, stable polymer solution is then induced to
phase separate, usually by quickly changing the solution temperature,
thereby resulting in interconnected polymer-concentrated and
polymer-dilute phases. The polymer-concentrated phase is crosslinked into
dense regions as the phase separation is occurring, thereby forming an
open-celled network of crosslinked polymer with sol fraction occupying the
cells. The crosslinked polymer-concentrated phase forms the cell walls,
while the uncrosslinked, polymer-dilute phase is contained within the
cells, thereby preserving the desired interconnected or bicontinuous
structure. Complete phase separation cannot occur, however, because the
crosslinking reaction freezes the desired microstructure. The extent of
crosslinking in the phase-separated state can be varied, and the solution
can even be returned to the homogenous state and the crosslinking
continued if desired. The sol fraction is then removed, and the product
dried, to produce the desired absorbent foam.
The process of the present invention can be performed using any
polymer/solvent system which can be induced to phase separate, as long as
the polymer is crosslinkable (i.e., has reactive functional groups).
Typical functional groups which are easily reacted include hydroxyl,
amine, carboxylic acid and amino, however the present invention is not
limited to these examples. Polymers which may be employed in the method of
the present invention include:
1. Hydrophobically modified carbohydrate polymers, including:
hydroxypropyl dextran
hydroxypropyl guar
hydroxypropyl starch
hydroxypropyl cellulose
hydroxyethyl cellulose
methyl cellulose
hydroxypropylmethyl cellulose
ethylhydroxyethyl cellulose
2. Poly(vinyl alcohol-co-vinyl acetate)
3. Poly(methacrylic acid)
4. cyanoethylated or partially formalized poly(vinyl alcohol)
5. Poly-N-vinyl-2-oxazolidone
6. Polypeptides, including:
poly(L-proline)
poly(valine-proline-glycine-X-glycine), wherein X=any amino acid
7. Acrylate (or analogous methacrylate) copolymers, including:
hydroxypropyl acrylate-co-acrylamide
diacetone acrylamide-co-hydroxyethyl acrylate
hydroxypropyl acrylate-co-hydroxyethyl acrylate
8. N-alkylacrylamide (or analogous N-alkylmethacrylamide) derivatives,
including:
ethylacrylamide
cyclopropylacrylamide
n-propylacrylamide
isopropylacrylamide
All of the polymers listed above can be readily crosslinked, and exhibit
LCST behavior in aqueous solutions. Thus, these polymers can be used to
prepare superabsorbent, microporous foams which rapidly absorb fluid by a
combination of capillary force and pore wall swelling.
Any crosslinker suitable for the particular polymer/solvent system employed
can be used, particularly the types used to produce conventional
superabsorbents from polymers such as starch (e.g., divinyl sulfone).
These crosslinking agents are generally di- or multi-functional
crosslinking agents which react with the pendant polymer functional
groups. Suitable crosslinking agents include:
acetaldehyde
formaldehyde
glutaraldehyde
diglycidyl ether
divinyl sulfone
diisocyanates
dimethyl urea
epichlorohydrin
oxalic acid
phosphoryl chloride
trimetaphosphate
trimethylomelamine
polyacrolein
ceric ion redox systems
Other known crosslinking means may be employed, including
photo-crosslinking, as well as other "physical" crosslinking means. By
physical crosslinking it is meant that the crosslinking occurs by
non-covalent bonding, whereas chemical crosslinking (e.g., by the list of
crosslinkers set forth above) results in the formation of new covalent
bonds within the product. Physical crosslinking can occur due to
non-covalent hydrophobic interactions between hydrophobic polymer side
chains of a polymer. This effect can often be enhanced by the addition of
a surfactant, and the term crosslinker in the context of the present
application is considered to include such surfactants (a physical
crosslinking agent). One such polymer which can be crosslinked in this
manner is hydrophobically modified hydroxyethyl cellulose (HMHEC)
(available from Aqualon Co., Wilmington, Del., as Natrosol Plus.RTM.).
Other types of physical crosslinking include hydrogen-bonding, van der
Waals interactions, ionic bonding, hydrogen bonding, coordination
interactions, and salt bridges. The present invention is considered to
include crosslinking by any of these physical methods, and these types of
crosslinking are set forth in further detail in Absorbent Polymer
Technology, edited by L. Brannon-Peppas and R. S. Harland, Elsevier
Science Publishing Co. Inc., N.Y. (1990).
After the product has been permitted to phase separate while crosslinking
for a predetermined period of time (a time sufficient to provide strength
to the final foam product), the product must be dried in order to produce
a microporous material suitable for use as an absorbent foam. The sol
fraction may be removed from the product by any of a number of means, but
preferably merely by leaching the sol from the microporous material using
the same solvent employed in the synthesis reaction (e.g., water). The
product may then be air-dried at room temperature, or even in a
conventional or microwave oven, in order to evaporate the solvent and
produce a foam. Freeze-drying (any conventional means) or solvent-exchange
may also be utilized.
The properties of the foams produced by the methods of the present
invention can be readily tailored to one's needs, and one skilled in the
art would be able to readily prepare suitable foams from any
polymer/solvent system wherein said system can be induced to phase
separate and wherein said polymer is crosslinkable. Polymer/solvent phase
diagrams are readily available in the literature, or can be easily
prepared in the laboratory. Suitable crosslinkers for polymers are also
well-known, and thus one skilled in the art could readily identify a
crosslinker suitable for the polymer/solvent solution employed. For the
preferred polymers which exhibit a LCST in aqueous solutions, selection
criteria are set forth in the work by L. D. Taylor and L. D. Cerankowski,
J. Polymer Science: Polymer Chemistry Edition, Vol. 13, pp. 2351-2570
(1975). In fact, the authors of this work stated that "the LCST
phenomenon, rather than being a rare curiosity, is quite predictable and
easy to achieve." Even polymers which are extremely hydrophilic (totally
miscible in water at all temperatures) can be modified to the point that
they exhibit LCST behavior. This can be accomplished, for example, by
merely copolymerizing the precursor monomer with a more hydrophobic
monomer (e.g., acrylamide with hydroxypropyl acrylate) to produce a
crosslinkable polymer which exhibits the desired LCST behavior. It should
be pointed out that, even though the copolymer exhibits LCST behavior, it
is still relatively hydrophilic and therefore the resultant foam will
absorb and retain water by a combination of capillary forces and pore wall
swelling.
The properties of the superabsorbent foams will depend upon a number of
factors, including: precursor polymer type, molecular weight of the
polymer, initial polymer concentration, crosslinker concentration, pH of
the polymer/solvent solution, reaction time prior to phase separation and
reaction time during and after phase separation. By varying these
parameters, the properties of the foams produced can be tailored to one's
needs. The most significant properties of these foams include: porosity,
sorption capacity, sorption rate, pore size, pore wall thickness, and
compression strength. The synthesis parameters of the method of the
present invention can be readily adjusted by one skilled in the art in
order to produce a foam of the desired properties, particularly the
desired rigidity. For example, increasing the initial polymer
concentration will decrease the porosity of the foam while increasing the
strength of the foam. Pore sizes can be reduced by increasing the amount
of crosslinking which occurs prior to phase separation. Pore size (as well
as pore wall thickness) will also decrease with corresponding increases in
the initial polymer concentration, molecular weight of the polymer, or
crosslinker concentration. Crosslinking during phase separation should,
however, proceed for a time sufficient to ensure that the foam product
will not collapse significantly under a modest load.
In general, the foams of the present invention are superabsorbent, and
their improved properties over that of the prior art are due, in part, to
the fact that these foams absorb and retain liquid not only by capillary
action, but also by a swelling of the pore walls. Unlike prior an foams,
foams can be produced by the method of the present invention which can be
air-dried while still producing a foam with good structural properties.
EXAMPLE 1
Hydroxypropyl cellulose (HPC) (available from Aldrich Chemical Co.) was
dissolved in an aqueous NaOH solution. An alkaline pH was maintained in
order to catalyze the crosslinking reaction. This solution was then
maintained in a glass vial at room temperature for at least 24 hours in
order to ensure complete and uniform hydration of the polymer. A
predetermined amount of divinyl sulfone (DVS) was then added, and the
solution was mixed thoroughly for approximately 30 seconds.
The polymer/crosslinker solution was next poured onto a glass plate
(6".times.6".times.0.12") between silicone rubber gaskets (1.6 mm thick),
and then covered with a second glass plate. The plates were secured to one
another using spring-loaded clamps, thereby forming a sealed mold
containing the polymer/crosslinker solution. The crosslinking reaction was
then permitted to proceed at room temperature for a predetermined period
of time (reaction time before phase separation). The mold was next
immersed in a constant temperature bath which was maintained above the
LCST of the polymer/solvent solution. The polymer/solvent solution then
began to phase separate, and the polymer-dense phase was crosslinked as
the solution phase separated and thereafter.
After a predetermined period of time, the mold was removed from the bath
and opened, and the sheet of microporous material was removed. The sheet
was then soaked in water in order to leach out the sol fraction which
contained small amounts of polymer and crosslinker which were not
incorporated into the foam network. The sheets were next dried in air
either at room temperature or at high temperature in an oven (either
conventional or microwave) to produce the microporous foams. At room
temperature the water-swollen sheets took approximately 10-20 hours to dry
completely. Table 1 provides the synthesis parameters for the HPC foams
produced according to this procedure.
The sheets can be air-dried more quickly by first replacing the water held
within the water-swollen foam with a more volatile solvent such as
heptane. Heptane, however, is immiscible in water, and thus the
water-swollen sheets were first soaked in ethanol to replace the water.
The ethanol-soaked sheet | | |
