U.S. patent application number 10/346284 was filed with the patent office on 2003-08-07 for crystallization of constrained polymers.
This patent application is currently assigned to University of Massachusetts a Massachusetts corporation. Invention is credited to Gappert, Griffin, Winter, H. Henning.
Application Number | 20030146548 10/346284 |
Document ID | / |
Family ID | 26853906 |
Filed Date | 2003-08-07 |
United States Patent
Application |
20030146548 |
Kind Code |
A1 |
Winter, H. Henning ; et
al. |
August 7, 2003 |
Crystallization of constrained polymers
Abstract
The invention provides micro- and nano-porous materials made
from crosslinked polymers crystallized from supercritical fluids.
The resulting products show an open cell porous network, and can be
used for a variety of applications in medical fields, textiles,
separation science and others. The invention also provides methods
for obtaining such products.
Inventors: |
Winter, H. Henning;
(Amherst, MA) ; Gappert, Griffin; (Amherst,
MA) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Assignee: |
University of Massachusetts a
Massachusetts corporation
|
Family ID: |
26853906 |
Appl. No.: |
10/346284 |
Filed: |
January 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10346284 |
Jan 17, 2003 |
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09676145 |
Sep 29, 2000 |
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6558607 |
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60157201 |
Sep 30, 1999 |
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Current U.S.
Class: |
264/425 ;
264/232; 264/41; 264/621; 264/85; 521/64 |
Current CPC
Class: |
C08J 9/141 20130101;
Y10S 977/902 20130101; Y10S 977/788 20130101; C08J 2203/14
20130101; Y02P 20/54 20151101; H01M 50/491 20210101; H01M 50/403
20210101; H01M 50/411 20210101; H01M 50/426 20210101; Y02E 60/50
20130101; C08J 2203/08 20130101; H01M 8/106 20130101; H01M 50/417
20210101; H01M 2300/0082 20130101; H01M 50/423 20210101; Y10S
977/90 20130101; C08J 2323/02 20130101; C08J 2201/032 20130101;
H01M 50/406 20210101; Y02E 60/10 20130101 |
Class at
Publication: |
264/425 ; 264/41;
264/85; 264/232; 264/621; 521/64 |
International
Class: |
C04B 033/32; B29C
067/20 |
Goverment Interests
[0002] The work described herein has been partially funded by a
grant from National Environmental Technology for Waste Prevention
Institute (NETI) at the University of Massachusetts, Amherst. The
government may have certain rights in this invention.
Claims
What is claimed is:
1. A method for producing porous structure in a polymer, the method
comprising: a) shaping a polymer; b) constraining the structure of
at least a portion of the polymer; c) melting the polymer; d)
contacting the melted, constrained polymer with a solvent under
conditions, and for a time sufficient to cause at least partial
swelling of the polymer; e) crystallizing the swollen polymer; and
f) removing the solvent, to yield a porous polymer.
2. The method of claim 1, wherein the solvent is a supercritical
fluid.
3. The method of claim 2, wherein the supercritical fluid is
propane.
4. The method of claim 1, wherein steps a) and b) are performed
simultaneously.
5. The method of claim 4, wherein shaping is reactive
extrusion.
6. The method of claim 1, wherein the structure of at least a
portion of the polymer is constrained by crosslinking.
7. The method of claim 6, wherein the crosslinking is achieved by
radiation.
8. The method of claim 6, wherein the crosslinking is achieved by
reacting functional groups on the polymer.
9. The method of claim 6, wherein the crosslinking is achieved by
chemical radical-initiation.
10. The method of claim 6, wherein the crosslinking is achieved by
photochemical reaction.
11. The method of claim 6, further comprising extracting an
uncrosslinked portion of the polymer from the crosslinked portion
of the polymer with a solvent before crystallization to produce a
solution comprising an uncrosslinked portion of polymer.
12. The method of claim 11, further comprising extracting
substantially the entire uncrosslinked portion of the polymer from
the crosslinked polymer.
13. The method of claim 11, further comprising impregnating the
crosslinked portion of the polymer with a further material, wherein
the further material penetrates the interior of the crosslinked
portion of the polymer.
14. The method of claim 11, further comprising impregnating the
crosslinked portion of the polymer with a further material, wherein
the further material remains substantially on the exterior of the
crosslinked portion of the polymer.
15. The method of claim 13, wherein the further material comprises
a polymer.
16. The method of claim 14, wherein the further material comprises
a cell culture.
17. The method of claim 14, wherein the further material comprises
a pharmaceutically active material.
18. The method of claim 13, wherein the further material comprises
a lubricant.
19. The method of claim 13, wherein the further material comprises
a reactive crosslinking material.
20. The method of claim 11, further comprising replacing the
solution comprising uncrosslinked portion of polymer with solvent
containing substantially no uncrosslinked portion of polymer.
21. A method for making a shaped material, the method comprising:
allowing a solidifiable material to impregnate the interior of a
porous structure; solidifying the solidifiable material; and
removing the porous structure to produce a shaped material.
22. The method of claim 20, wherein the porous structure has pore
sizes between about 0.01 .mu.m and 100 .mu.m.
23. The method of claim 20, wherein the solidifiable material is an
inorganic sol.
24. The method of claim 22, wherein the inorganic sol is a metal
alkoxide or metalloid alkoxide.
25. A porous crosslinked polymer having pore diameters from about
0.01 .mu.m to about 100 .mu.m, and having a open-cell, bicontinuous
structure.
26. A tissue scaffold comprising the porous crosslinked polymer of
claim 25.
27. A catalyst substrate comprising the porous crosslinked polymer
of claim 25.
28. A liquid or gas filter comprising the porous crosslinked
polymer of claim 25.
29. A method for growing cells comprising: providing a porous
crosslinked polymeric scaffold; at least a portion of the surface
of which is coated with cells; and allowing the cells to grow for a
time, and under conditions, sufficient to produce new cell.
30. The method of claim 29, wherein the cells produce a material
excreted into an extracellular matrix.
31. The method of claim 29, wherein the cells and new cells form
tissue.
32. A battery separator comprising a porous crosslinked
polymer.
33. A porous polymer having pore sizes between about 0.1 .mu.m and
100 .mu.m.
34. The porous polymer of claim 33, having a volume porosity of
from about 1% to about 90%.
35. The porous polymer of claim 33, having an open-celled,
bicontinuous pore structure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Serial No. 60/157,201, filed Sep. 30, 1999,
which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0003] The invention relates to porous polymers and solid-state
expansion processes using solvents. The processes can be used to
make porous films, fibers, tubes, and coatings for use in filters,
chromatography and numerous other applications.
BACKGROUND OF THE INVENTION
[0004] Porous semicrystalline polymers have a range of important
and useful applications. In typical applications, the control of
pore structure and purity of the product, bulk mechanical
properties, and macroscopic shape are of fundamental
importance.
[0005] Porous semicrystalline polymers can be produced by
crystallization from solution. In the well-known process of
thermally induced phase separation (TIPS), the porous material is
formed from homogeneous solution by lowering the temperature,
inducing crystallization, and/or liquid-liquid phase separation.
The TIPS method involves dissolving a polymer in a solvent. The
solid product forms from solution, either assuming the shape of the
crystallization vessel, or becoming film or sediment at the bottom
of the vessel. This method cannot be used to create complicated
shapes (e.g. complicated injection molded parts). Complete removal
of solvent (e.g., drying) is generally difficult (often a second
solvent is used to extract the first solvent) and the surface
forces of the solvent can lead to pore collapse during removal.
Problems associated with current methods include: inability to
control fine pore structure and pore size distribution, lack of
mechanical coherency in the product, reliance on hazardous
processing solvents, and solvent removal and recovery from the
final product. Other methods such as foaming, sintering,
stretching, and leaching have also been developed over the years to
create porous materials with desired properties.
[0006] Increasingly strict environmental legislation has forced
many industries to reevaluate their use of hazardous solvents.
International agreements such as the Montreal Protocol (1987), the
Clean Air Act Amendments (1990), and the Kyoto Summit (1997) have
all had as their focus the reduction or elimination of volatile
organic compound (VOC) emissions as a way to stop ozone depletion
and greenhouse warming. The polymer industry in particular is
notorious for its reliance on VOCs, which have been used as
monomers, solvents, plasticizers, and cleaning agents in polymer
synthesis and processing.
SUMMARY OF THE INVENTION
[0007] The invention is based on the discovery that crystallizing
constrained polymers from swollen states can lead to porous
structure, including open celled, bicontinuous porous structure.
Solvents can include supercritical fluids (SCF). After
crystallization, from the swollen state the polymers show an
increase in volume, a decrease in density, and the overall shape is
controlled by the shape before swelling and the processing history.
Scanning electron micrographs of the samples show an open cell
porous network.
[0008] In one aspect, the invention provides a new process for
creating porous polymers, the pore structure and distribution of
which can be controlled through material properties and processing
parameters. The process is applicable to many different types of
polymers. The final shape of the porous polymer is determined by
shaping methods such as extrusion, blow molding, fiber spinning,
and injection molding applied prior to the process, as well as by
material properties, and further processing history.
[0009] In another aspect, the invention provides porous polymeric
materials with open pore structures having new morphologies,
improved pore size distribution, and improved mechanical strength.
These porous polymers are produced in such a way that all interior
surfaces are extremely clean, and do not contain residual materials
(such as residual solvents, for example) which are typically
introduced by previously used processes. This property can reduce
or eliminate the need for post-processing cleaning, and can make
the porous polymers amenable to further processing such as surface
modification, surface functionalization, or biological and medical
applications.
[0010] The invention, in some embodiments, further provides porous
materials of increased strength, by virtue of a crosslinked
structure. The shaping of polymers before processing is also
substantially maintained during processing, which results in porous
materials having a wide variety of shapes that were previously
unavailable.
[0011] In one aspect the invention provides a method for producing
porous structure in a polymer. The method includes shaping a
polymer; constraining the structure of at least a portion of the
polymer; melting the polymer; contacting the melted, constrained
polymer with a solvent under conditions, and for a time sufficient
to cause at least partial swelling of the polymer; crystallizing
the swollen polymer; and removing the solvent, to yield a porous
polymer. The solvent can be a supercritical fluid, such as propane.
Some of the steps can be performed simultaneously. The shaping can
be by reactive extrusion. The structure of at least a portion of
the polymer can be constrained by crosslinking, for example, as
achieved by radiation, by reacting functional groups on the
polymer, by chemical radical-initiation, or by photochemical
reaction. The method can also include extracting an uncrosslinked
portion of the polymer from the crosslinked portion of the polymer
with a solvent before crystallization to produce a solution
comprising an uncrosslinked portion of polymer. This method can
also include extracting substantially the entire uncrosslinked
portion of the polymer from the crosslinked polymer, and can also
include impregnating the crosslinked portion of the polymer with a
further material, wherein the further material penetrates the
interior of the crosslinked portion of the polymer, and can also
include impregnating the crosslinked portion of the polymer with a
further material, wherein the further material remains
substantially on the exterior of the crosslinked portion of the
polymer. The further material can include a polymer, a cell
culture, a pharmaceutically active material, a lubricant, or a
reactive crosslinking material. The method can also include
replacing the solution comprising uncrosslinked portion of polymer
with solvent containing substantially no uncrosslinked portion of
polymer.
[0012] In another aspect, the invention provides a method for
making a shaped material. The method includes allowing a
solidifiable material to impregnate the interior of a porous
structure; solidifying the solidifiable material; and removing the
porous structure to produce a shaped material. The porous structure
can have pore sizes between about 0.01 .mu.m and 100 .mu.m. The
solidifiable material can be an inorganic sol, such as a metal
alkoxide or metalloid alkoxide.
[0013] In another aspect, the invention provides a porous
crosslinked polymer having pore diameters from about 0.01 .mu.m to
about 100 .mu.m, and having a open-cell, bicontinuous structure.
This porous crosslinked polymer can form part of a tissue scaffold,
a catalyst substrate, a liquid or gas filter.
[0014] In another aspect, the invention provides a method for
growing cells including providing a porous crosslinked polymeric
scaffold; at least a portion of the surface of which is coated with
cells; and allowing the cells to grow for a time, and under
conditions, sufficient to produce new cell. The cells produce a
material excreted into an extracellular matrix, or the cells and
new cells form tissue.
[0015] In another aspect, the invention provides a battery
separator comprising a porous crosslinked polymer.
[0016] In another aspect, the invention provides a porous polymer
having pore sizes between about 0.1 .mu.m and 100 .mu.m. The volume
porosity can be of from about 1% to about 90%, and can have an
open-celled, bicontinuous pore structure.
[0017] The invention provides a number of advantages. The process
is markedly simpler and more cost efficient than previous methods.
Standard polymers can be employed in the process, rather than only
high cost specialty polymers required of prior processes. The
shaping of the polymer can be carried out in steps separate from
pore generation. The variety of shapes and relative dimensions
possible (thin, thick, round, flat, surface coating, bulk material)
is greater than that enabled by prior processes. The product is
substantially clean, and can be readily used in medical
applications. The new process avoids the use of hazardous or
environmentally damaging solvents, for example those with volatile
organic components. The solvent can be recovered in a useable form
after use, and solvent use is thereby reduced. The new processes
described herein are ideal for high yield and large scale
production, such as reactive extrusion. The processes are useful
for inexpensive commodity polymers, and can replace foaming or
TIPS.
[0018] The polymeric products have a regular pore structure, and
can be used in separations as filters, membranes, or chromatography
support. The high surface areas of open cell networks make them
ideal candidates for catalyst supports where high surface
area-to-volume ratios are crucial. Biomedical applications of open
cell networks include scaffolding for tissue growth and
controlled-release drug delivery methods. Other applications
include textiles having "breathable" laminates or fibers, porous
nonwoven materials, thermal insulation material able to pass water
vapor, porous precursors for forging (prosthetic devices) or fiber
spinning (with pore modifications possible through stretching, with
high modulus final products available by ultradrawing), low
dielectric coatings for electronic parts and wires, filter
applications for liquid purification, membrane separators for gas
and liquid separations, support for transport media (such as
battery separators, and fuel cell membranes), super absorbing
linings for diapers and the like, semi-permeable vesicles or
bottles (for controlled release), introducing porosity into closed
cell porous materials such as foams, templates for producing
inorganic porous materials, and other applications. The articles
produced according to these methods take on a wide variety of
shapes including films, sheets, tubes, fiber, and bulk forms.
[0019] As used herein, "bicontinuous porous structure" refers to
pore structures in materials in which a continuous path can be
traced through either the pore voids or the pore walls across the
material.
[0020] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, suitable methods and materials are described below. All
publications, patent applications, patents, and other references
mentioned herein are incorporated by reference in their entirety.
In case of conflict, the present specification, including
definitions, will control. In addition, the materials, methods, and
examples are illustrative only and not intended to be limiting.
[0021] Other features and advantages of the invention will be
apparent from the following detailed description, and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic diagram of the crystallization of
polymer in a supercritical fluid (SCF).
[0023] FIG. 2 is a scanning electron micrograph of a 3 surface of
an LLDPE sample crystallized at 85.degree. C., recorded at
2000.times..
[0024] FIG. 3 is a scanning electron micrograph of a 3 surface of
an LLDPE sample crystallized at 105.degree. C., recorded at
300.times..
[0025] FIG. 4 is a scanning electron micrograph of a 3 surface of
an LLDPE sample crystallized at 105.degree. C., recorded at
2000.times..
[0026] FIG. 5 is a scanning electron micrograph of a surface of a
LLDPE sample crystallized at 85.degree. C., recorded at
2000.times..
[0027] FIG. 6 is a scanning electron micrograph of a high density
polyethylene (HDPE) sample prepared by peroxide cross-linking,
crystallized at 85.degree. C., recorded at 200.times..
[0028] FIG. 7 is a scanning electron micrograph of an HDPE sample
prepared by silane-grafting, crystallized at 85.degree. C.,
recorded at 2000.times..
DETAILED DESCRIPTION OF THE INVENTION
[0029] The crystallization of constrained polymers, which are
swollen by solvents, preferably supercritical fluids (SCF),
produces a porous structure in a sample which maintains its
macroscopic shape. The process leads to larger volumes, reduced
densities, and, in some cases, reduced mass. Porous structure is
visible in cross-sections of the samples after exposure. The
methods described herein produce stronger porous polymers than
previously available, and porous polymers which are produced in a
wider range of geometries than previously available.
[0030] Without being limited by any particular mechanism, it is
believed that crosslink density and swelling conditions control a
mechanism in which chain segments between crosslinks crystallize,
independently or in association with other crystallizable materials
present, while being anchored into a sample-spanning matrix.
Crosslink density and process conditions can be varied to control
pore structure.
[0031] Supercritical fluids are preferred as swelling fluids
because they allow for complete removal of the solvent and preserve
the fine pore structure as the supercritical fluid exerts minimal
surface tension on the polymer. The critical temperature of the
supercritical fluid should be less than the melting temperature of
the polymer. A schematic of the process is shown in FIG. 1. The
initial state of preshaped, crosslinked polymer is shown as 1. The
material is also in contact with a solvent. The polymer and solvent
are first brought to thermodynamic conditions (for example,
pressure and temperature) in which the polymer crystals are molten
(shown as 2) and the constrained network is swollen (shown as 3),
then cooled below the melting temperature of the swollen polymer
(shown as 4), and finally, the solvent is vented above its critical
point, to give the material shown as 5, which is then cooled to
ambient conditions. The cooling from state 3 to state 4 need not be
isobaric, as is shown in FIG. 1, but the process path should cross
the melting curve of the polymer/solvent system above the pressure
which determines the volume-phase transition of the polymer/solvent
system. Additionally, the charge in temperature shown between
states 4 and 5 need not be carried out, rather the temperature may
remain significantly constant between these states. Further swollen
states may exist within the region containing state 3, which can be
explored using variable temperature and pressure within this region
of the phase diagram. Such varying swollen states can yield varying
polymer morphologies, and varying pore structures. The
crystallization temperature can be less than the final venting
temperature.
[0032] Crystallization from supercritical fluid is now possible for
constrained systems, for example, crosslinked linear low density
polyethylene.
[0033] The method generally involves the following steps. First a
polymer is selected. The polymer should be desirably constrainable,
for example, crosslinkable. Multi-component polymers such as
copolymers and blends, of which only some components are
crosslinkable, can also be used in the invention. Polymers that
contain dispersed particles can also be used. For example polymers
with from about 10% to about 90% crystallinity at room temperature,
or from about 20 to about 70% crystallinity at room temperature,
can be used successfully. For example, polyalkylenes such as
polyethylene and polypropylene; polyoxyalkylenes such as
polyoxyethylene and polyethyleneglycol; polylactones, such as
polycaprolactones and polylactones of other ring sizes; polyarenes,
such as polystyrene and substituted polystyrenes; polyvinyls,
including polyvinyl chlorides; fluoropolymers; polyamides such as
nylon-6,6, and nylon-6; polycarbonates; polyesters such as
poly(butylene terephthalate) and poly(ethylene terephthalate);
polyethers such as poly(phenylene oxides), polyethersulfones,
polyetheramides, and polyetherpolyketones; alkylene oxide polymers
such as poly(ethylene oxide), propylene oxide polymers and higher
1,2-epoxide polymers; polyvinyls such as polyvinyl chloride;
phenolic; and polyimides. Biocompatible or biodegradable polymers
such as ethylene vinyl acetate, poly(lactic acid coglycolic acid),
poly(lactic acid coglycine acid) can be used, as long as the
crystallinity of these materials is sufficiently high. The polymers
for use in the processes described herein can also be block
copolymers with at least one crystallizable phase, polymer blends
with at least one crystallizable component, blends of homopolymers
and copolymers, and crystallizable suspensions.
[0034] The polymers are to be provided in a constrained
configuration, which is stable or metastable. This can be achieved
by a number of methods, for example, by crosslinking of the
polymer, or by establishing an electromagnetic field which would
substantially immobilize and/or orient the polymer, or by
controlling ion concentration in polyelectrolytic materials.
Constraint by the application of an electromagnetic field can be
achieved by providing a polymer which is intrinsically ionic, or
intrinsically polarizable, or by providing a polymer having ionic,
or polarizable groups. The polymers can be crosslinked by any of
numerous generally known mechanisms including radiation-induced
crosslinking, chemical radical-type crosslinking, grafting,
including silane grafting, and other methods of crosslinking, for
example those based on the presence of functional groups (for
example, ionic and counterionic groups which can interact with each
other) on the polymer. Photochemical crosslinking with visible,
ultraviolet or infrared radiation is envisoned, carried out by
irradiation of a separately added or pendant chromophore that can
generate species which achieve crosslinking between polymer chains.
Such chromophores are known in the art and include dyes, aromatic
molecules and substituents, heteroaromatic molecules and
substituents, and other commonly used crosslinking chromophores and
moieties. The chemical radical-type crosslinking can require
impregnation of radical-generating species into the polymer to
generate radicals. For example, peroxide can be used as a
radical-generating species. Grafting-type crosslinking can use
reactive extrusion with radical generating species (for example,
peroxide) to graft functional groups (such as silanes) to the
polymer, which is subsequently crosslinked by hydrolysis and
condensation.
[0035] Second, the polymer is shaped or processed into a desired
geometry by conventional means, including extrusion of a sheet,
tube (hollow fiber), or fiber, injection molding, surface coating
onto a substrate, impregnation of a porous substrate, small
particle formation, conventional foaming, and other processes known
in the art. Concurrently, the creation of a chemically crosslinked
or otherwise constrained network can be carried out, or this can be
accomplished in the next step of the inventive method.
[0036] Third, the polymer is typically crosslinked, or otherwise
constrained. For example, chemical crosslinking can be carried out
by the addition of chemical crosslinking agents, for example those
which allow radical-generating species to generate radicals.
Hydrolysis or other chemical activation of functional groups can be
carried out. Alternatively or additionally, crosslinking can be
induced by processes involving radiation. Vulcanization,
conventional end-linking processes, or reactive extrusion are also
useful means of crosslinking the polymer. Crosslinking locks the
polymer into a network so that exposure of the polymer to solvent
does not result in complete disruption of the structure of the
polymer. The extent of crosslinking can be used to tune the
resulting structure. The extent of crosslinking can be defined by
the "gel fraction," or mass fraction remaining after
non-crosslinked molecules are extracted. Polymer chains which are
not crosslinked (that is, "loose chains") form what is known as the
sol fraction, and can be removed by exposure of the partially
crosslinked polymer to solvent, under certain conditions. In some
embodiments, the crosslinking is deliberately carried out to a
degree less than completion (for example, from about 10% to about
80% gel fraction). The sol fraction is an important processing
parameter, particularly during the next step, and it can optionally
be partially or completely removed at this stage. The extent of
crosslinking which can give open celled, bicontinuous pore
structure varies according to the material and the method of
crosslinking used. For example, for peroxide crosslinked HDPE, gel
content of between about 50 and about 80%, or between about 60% and
about 70% will give open celled, bicontinuous pore structure. For
example, for radiation crosslinked linear low density polyethylene,
gel content of between about 10 and about 40%, or between about 15%
and about 30% will give open celled, bicontinuous pore structure.
For example, for silane grafted HDPE, gel content of between about
15 and about 60%, or between about 25% and about 45% will give open
celled, bicontinuous pore structure. Gel contents significantly
above the given ranges can tend to give non bicontinuous structure,
and gel contents significantly below the given ranges can tend to
produce dissolution of the polymer in solvent.
[0037] In alternative methods, the polymer could be prepared by
constraint in the molten, or partially molten, state. If
crosslinking constrains the polymer, this would change the location
of the crosslinks, and thus affect the final pore structure. For
radiation-induced crosslinking of crystallized polyethylene, for
example, it is known that crosslinking occurs preferably in the
amorphous regions. If crosslinking occurs in the melt, the
crystallizable regions are not excluded.
[0038] According to the general process, the polymer is next
swollen by exposure to a solvent which swells the network
structure, while maintaining the integrity of the network. The
uncrosslinked polymer is at least partially soluble in the solvent
under these conditions. The crystal phase of the polymer is melted
in the presence of solvent at this stage, for example, by exposure
to heat or increased pressure, to facilitate swelling of the
polymer by the solvent. Solvents useful for this purpose can be
liquid, gas, or supercritical fluid. Conventional solvents,
including benzene, xylene, toluene, and other solvents in which the
uncrosslinked polymer precursor is at least partially soluble can
also be useful for these purposes. The solubility of the polymer in
solvent is typically increased by increasing temperature or
pressure. For example, the second critical endpoint for a high
molecular weight fraction of linear polyethylene in propane lies at
about 118.degree. C. and 640 bar, and the cloud point decreases
slightly with increasing temperature. Polymers and solvents which
can be used together include polyethylene or polypropylene and
propane, polyethylene or polypropylene and xylene, aliphatic
polyester and supercritical carbon dioxide, fluoropolymers and
supercritical carbon dioxide.
[0039] The amount of swelling is an important processing parameter
and, under equilibrium conditions, depends on solvent quality and
network structure. For example, xylene used at relatively low
temperature (such as 110.degree. C. for polyethylene) can swell
polyethylene or polypropylene to a relatively large degree, while
supercritical propane swells these polymers to a more moderate
degree. Other supercritical hydrocarbons such as methane and ethane
also induce less than maximal swelling of polyethylene and
polypropylene. The degree of swelling induced by a given solvent
can be controlled by tuning with temperature, pressure, with the
connectivity of the polymer, or with the presence of co-solvents,
as are commonly used in conjunction with fluoropolymers. The
swelling induced by a solvent is resisted by the network structure
introduced in the previous step. If too few crosslinks are created,
the polymer will tend to dissolve and if too many crosslinks are
created, the network will be unable to swell. Variations in the
intermediate range of crosslinking lead to different resultant pore
structure. The optimum crosslink density depends on the material
used, and the pore structure desired.
[0040] Partial swelling (any swelling less than a maximum swelling)
can be used to create products according to the invention. The
extent of swelling can also be controlled by constraining the
polymer to be in a vessel of a given dimension. Additionally, by
limiting the amount of solvent available, the degree of swelling
can be limited. By limiting the amount of time available for
swelling, partial swelling and gradients in the degree of swelling
across the sample may be achieved.
[0041] During swelling, any sol fraction tends to be extracted from
the polymer. This diffusion process depends on swelling time,
sample geometry, and solvent concentration, and reduces the mass of
the polymer. The reduction in mass can range from about 10% to
about 75%, or from about 20% to about 50%. Gradients of sol
fraction may be introduced across the sample. Sol fractions
remaining in the polymer participate with the gel fraction during
the following crystallization step. The degree of sol extraction is
expected to have significant influence on the final morphology,
that is, the structure of crystal lamellae, specific surface area,
pore size distribution, as well as the flexibility of the final
product, and the like. Therefore, partial or complete sol fraction
extraction may be desired. Sol fractions removed from the sample
can accumulate in the surrounding solvent and precipitate upon
return to ambient conditions. The accumulation of sol fraction in
the surrounding solvent can be limited by solvent replacement
during swelling. Additionally, by the same mechanism, materials
(e.g., co-crystallizing species, immiscible species, reacting
species, additives, surface modifiers) dissolved in the solvent can
be impregnated into the polymer, or onto its surface, during the
swelling step to modify final properties. The distribution between
surface localized and interior localized material can be determined
by the amount of time allowed for penetration. Additionally, the
materials present in the polymer (additives, stabilizers,
plasticizers and the like) which are introduced during polymer
processing can be extracted by the swelling procedure. These
materials can be reintroduced into the polymer, or onto its
surface, by allowing such materials, added to solvent, to contact
the polymer. Material properties can also be modified during the
swelling phase by the application of mechanical stresses (as in
bi-axial stretching, or drawing) or by secondary processes (such as
using a third component to foam the polymer within the swollen gel
region).
[0042] In preferred embodiments of the invention, supercritical
fluid is used as a solvent. The unique physicochemical properties
of supercritical fluids, for example, their very low surface
tension and densities in the liquid phase, along with the high
molecular mobilities of the gas phase, make them useful for the
processes described herein. Such solvents could potentially
dissolve the polymer if crosslinks were not in place to restrict
the solvent uptake. Supercritical fluids useful for purposes of the
invention include carbon dioxide, sulfur hexafluoride, freons
(e.g., fluoroform, monofluoromethane, dichloromethane, chloroform,
chlorotrifluoromethane, and chlorodifluoromethane), C.sub.1-4
alkanes such as ethane and butane, C.sub.1-4 alcohols, C.sub.2-4
alkenes such as ethylene, and C.sub.2-4 alkynes such as acetylene,
and other similar compounds. Combinations are also useful,
including the above materials used in combination as
cosolvents.
[0043] In a further step, the polymer is supercooled below its
melting temperature. This step results in crystallization and phase
separation of the polymer from the solvent. Physical gelation
constrains the overall shape of the swollen polymer, and a porous
physical morphology is created. Crystallization temperature and
cooling rate can affect the size and structure of the physical
network. Volume changes can occur at this stage, but tend to be
relatively small.
[0044] Lastly, the solvent is removed by venting as a supercritical
fluid or as a gas. Fine pore structure is preserved, as there is
very low surface tension in these states. Excessive surface tension
can degrade the structure of porous polymers, and can prevent
complete removal of solvent or penetration of externally introduced
materials because of a loss of open cell structure. If a solvent
other than SCF is being used, it is preferably exchanged with an
SCF or a gas before venting. The venting step is desirably carried
out at a temperature above the critical temperature of the fluid to
avoid the formation of a liquid phase and the associated capillary
effect in the pores. Complete removal may be assisted by the
application of vacuum.
[0045] SCFs are easily recovered by controlled venting, reducing
waste generation and the necessity for post-processing cleaning.
Before reuse as a solvent, the SCF is cooled below its critical
point (to transfer it into a phase which is a relatively poor
solvent) to shed impurities that might have dissolved in it while
in the supercritical state. The low temperature processing
conditions and complete solvent recovery typical of SCF technology
are ideal for temperature-sensitive medicinal and biological
product handling, which demand high purity and may be easily
degraded by heat. SCF can also be used to infuse polymer substrates
with additives such as dyes, perfumes, and physiologically active
materials. The rapid expansion of supercritical fluids can be used
to create novel morphologies in foams, powders, films, fibers, and
biodegradable microspheres used for drug delivery. Because the
solubility of polymers in SCF tends to be highly dependent on
molecular weight, SCF can be used to fractionate polydisperse
polymers.
[0046] This process causes an expansion in the solid state. The
increase in volume of the final product can range from about 10% to
about 800%, or from about 50% to about 700%, of from about 100% to
about 600%. The transformation of a solid piece of polymer
(crosslinked and crystallized) into a porous, solid piece (once
again crosslinked and crystallized) is carried out without
completely liquefying and without external mechanical forces. The
forces are supplied completely internally, by the action of
solvent, which attempts to separate the polymer molecules from each
other until they are held back by the network bonds
(crosslinks).
[0047] The polymer thus created is solidified in independent ways:
by chemical crosslinking by bonds between molecules, and by
crystallization of molecules themselves. The chemical crosslinks,
once they are formed, remain intact throughout the process. That
is, the polymer is a solid at all stages in the expansion phase of
the process. Only the crystals are molten prior to reformation in
the swollen state. Additional embodiments utilize other types of
constraints on molecules (that is, networks) which are stable
through the process such as ionic, magnetic, or electric networks
and other types of physical structures such as field-induced
structures, microphase structures, and other types of
intermolecular interactions.
[0048] Swelling is associated with the gel fraction in the
crosslinked polymer. Gel fractions of crosslinked polymers useful
in the processes described herein can be from about 15% or more.
Loose molecules (sol fraction) are extracted as the exposure time
is increased. This behavior has been observed when SCF was used
with crosslinked polydimethylsiloxane (PDMS) as a model crosslinked
polymer. The degree of sol extraction is expected to have
significant influence on the final morphology, that is, the
structure of crystal lamallae, specific surface area, pore size
distribution, and the like. The external shape of the final product
and its pore structure are mostly determined by the crosslinking
and phase separation in connection with the phase separation due to
crystallization.
[0049] The properties of the resulting polymers are highly
advantageous for a large number of applications. The pore sizes can
range from about 10 nm to about 100 .mu.m, or from about 200 nm to
about 50 .mu.m, or from about 250 nm to about 10 .mu.m, with
distribution in pore sizes ranging from about 10 nm to about 1
.mu.m. The volume porosity can range from about 1% to about 90%, or
from about 5% to about 80%.
[0050] Applications of Porous Polymers
[0051] Porous polymers find utility in a great number of fields.
For example, many applications in separation technology will find
use for porous polymers described herein. These separation
technology applications include filtration of gases, filtration of
liquids, chromatographic applications, and transport applications
such as ion transport and fuel cell transport media.
[0052] For example, battery separators can be fabricated from
porous polymers as produced herein. Battery separators are thin
membranes that physically divide the battery cathode from the anode
to prevent short circuiting. The requirements are thicknesses of
from about 1 to about 2 mil (or, for example, down to about 0.125
mm), pliability, porosity, hydrophilicity, tensile strength, and
conformability. The total porosity volume of the separator, the
structure of the pores in the separator, and the tortuosity of the
pore paths through the separator are particularly important
parameters to control for optimum performance. The performance of
alkaline battery cells can be improved by either decreasing the
tortousity of the path through the separator, or by decreasing the
thickness of the separator. Porous polyethylene including highly
crosslinked polyethylene, prepared as described herein, can be a
useful material for applications as battery separators.
[0053] Porous polymers are also useful in catalysis and other
fields that rely on high surface areas of the materials produced
according to the processes described herein.
[0054] Porous polymers are also useful in fields that utilize the
structural properties of the bulk material. For example, the porous
polymers described herein can be used as a scaffold for tissue
growth, for controlled release applications, or for encapsulated
cell culture applications. The fact that the porous polymers are
prepared by a process involving steps that effectively clean the
interior surfaces of the pores makes them particularly useful for
such biological applications.
[0055] The porous polymers described herein can also be used to
serve as a scaffold in a method to prepare other materials. In some
embodiments, this method is performed by creating a porous polymer
structure according to methods described above, impregnating the
structure with another material, and subsequently removing the
porous polymer. The porous polymer can be removed by a number of
known methods, including chemical or physical reactions, such as
reactions with an acid or base, by burning, or by phase
transformations of the porous polymer. It is understood that such
methods of porous polymer removal will be effective as long as the
methods have differential effects on the porous polymer and
impregnating material. These processes result in a cast of the
impregnating material, which has an external shape with high
surface area corresponding to the high surface area of the inner
walls of the pore structure of the discarded porous polymer. The
impregnating material can have its own pore structure, intrinsic to
the material and the method in which it is formed. In this way, the
surface area of the impregnating material can be made to be far
higher than the surface area of the porous polymer. For example,
sol gel chemistry (with metal oxides, or metal alkoxides, or
metalloid oxides, or metalloid alkoxides) carried out inside a
template of porous polymer described herein typically involves the
generation of water and methanol, which result in pore structure of
the resulting gel.
EXAMPLES
[0056] The invention will be further described in the following
examples, which do not limit the scope of the invention described
in the claims.
Example 1
[0057] Preparation of Polymer in Supercritical Fluid with Radiation
Crosslinking
[0058] Experiments were conducted with LLDPE synthesized with
metallocene catalyst. The sample was pressed into 100 mm.times.2 mm
sheets at 160.degree. C. under vacuum. Chemical crosslinking was
induced by exposure to 5 Mrad of radiation.
[0059] After crosslinking, some samples were extracted in a Soxhlet
apparatus with p-xylene for 24 hours to remove non-crosslinked
material.
[0060] The samples of crosslinked material were exposed to propane
at 9000 psi and 175.degree. C. for 1 hour and then cooled at the
crystallization temperature, (T.sub.cryst=80.degree. C.
-105.degree. C.) for 30 minutes. After this crystallization period,
the vessel was vented at 105.degree. C.
[0061] The mass of each sample was determined before and after
exposure with an analytical balance. Volume increase was determined
by measuring the linear dimensions of the rectangular samples.
Density was determined as the quotient of mass and volume. However,
the shape of the samples became somewhat irregular after exposure,
making volume measurements imprecise. Therefore, density was also
determined by Archimedes Method. The samples were coated with epoxy
and weighed in a solvent of known density to determine sample
density. The gel fraction of the cross-linked sample determined by
the weight loss after Soxhlet extraction was 20%. The amount of
swelling and mass loss after exposure for several different
crystallization temperatures (T.sub.cyst) are shown in Table 1.
[0062] In Table 1, "Mass" and "Volume" are expressed as percentage
of the original values. Density 1 is the mass/volume. Density 2 is
the Archimedes density. Density 3 is the percentage of the original
density (the density of the sample before exposure is 0.84
g/cm.sup.3). Entries "-" were not determined. The results indicate
that mass loss is accompanied by volume increase.
1TABLE 1 Properties as a Function of Crystallization Temperature
Crystallization Density 1 Density 2 Temp. (.degree. C.) Mass Volume
(g/cm.sup.3) (g/cm.sup.3) Density 3 80 78.5 536 0.12 -- 15 85 85.1
-- -- 0.11 13 105 84.8 -- -- -- --
Example 2
[0063] Scanning Electron Microscopy
[0064] Scanning electron microscopy (SEM) was performed on a JEOL
brand 20 kV scanning electron microscope. Samples were coated with
epoxy, cut with a razor and sputter coated with gold. Top surfaces,
1 surfaces, and 3 surfaces were studied. The surfaces are defined
according to an axis system in which the 1 axis is vertical, and
the 2 and 3 axes form a plane normal to the 1 axis.
[0065] Scanning electron micrographs (SEM) are shown in FIGS. 2-4.
Porous structures are visible in the cross-sections shown in these
Figs.
Example 3
[0066] A Further Preparation of Polymer in Supercritical Fluid with
Radiation Crosslinking
[0067] A metallocene linear low density polyethylene (LLDPE;
polyethylene co-hexane, 2.6 mol % C-6, M.sub.w of 110,000,
M.sub.w/M.sub.n of 2.4) was molded from pellet form into 100
mm.times.2 mm discs at 160.degree. C. under vacuum. Samples were
exposed to 3 to 16 Mrad of electron beam radiation at room
temperature under inert atmosphere to induce crosslinking. The
relatively even distribution of branching content in metallocene
LLDPEs make them amenable to radiation crosslinking in the solid
state, because any given chain is likely to occupy both crystalline
and amorphous domains, the latter being where the crosslinking
preferentially occurs. Gel content was determined by mass loss
after Soxhlet extraction with p-xylene.
[0068] Crosslinked polyethylene does not form a solution with
supercritical propane, but rather a swollen gel. To ensure thorough
swelling, a square sheet (5 mm.times.5 mm.times.2 mm) of the
crosslinked sample was placed on a small metal tray in a 25 ml high
pressure vessel. Vacuum was then applied to the vessel followed by
an initial charge of propane to 10 bar. The temperature was
increased to 175.degree. C., followed by the addition of propane to
620 bar. After equilibration of 2 to 24 hours, the temperature was
reduced to 85.degree. C. for 0.5 hours to crystallize the polymer.
The propane was then vented at 105.degree. C., which is above its
critical point, so that the pore structure would be maintained.
[0069] Cross-sections of the samples were prepared by saturating
the porous samples with methanol, freezing in liquid nitrogen and
then cutting with a razor blade. Cross-sections were sputter coated
with gold and then scanning electron microscopy was performed on a
JEOL brand 20 kV SEM. FIG. 5 shows the SEM of this sample.
[0070] The mass of samples was determined before and after exposure
to supercritical fluid with an analytical balance. Density was
determined by Archimedes' method, in which the samples were coated
with epoxy and weighed while submerged in a fluid of known density.
Volume increase was also determined by measuring the linear
dimensions of the samples when possible.
2TABLE 2 Mass, Volume, and Density Changes After Processing Mass
Volume Density Gel Fraction Remaining Change Change (% mass) (% of
original) (% of original) (% of original) 4.1 41.4 NA NA 15.9 54.9
329 16.7 56.8 70.5 92.5 76.2
[0071] The entries marked "NA" were for samples which melted
completely and did not retain the pre-processing shape.
[0072] The results show that a minimum gel fraction is required,
and density reduction is decreased as gel content increases.
Example 4
[0073] Preparation of Polymer in Supercritical Fluid with Peroxide
Crosslinking
[0074] The peroxide initiator, dicumylperoxide, was dissolved in
methanol (initiator concentration of 0.07 wt %). Purified high
density polyethylene HDPE was poured into the methanol-initiator
solution. After stirring and evaporation of solvent at ambient
temperature, the powder was placed in a vacuum press to be
crosslinked at 200.degree. C. The HPDE had molecular weight of
80,000, with M.sub.w/M.sub.n of between 2.5 to 3.0.
[0075] The sample was extracted in a Soxhlet extractor with
p-xylene for 24 hours. The mass remaining after extraction (that
is, the gel content), was 70.3%. The sample was then swollen in
propane at 9000 psi, 172.degree. C. for 2 hours, and then
crystallized at 85.degree. C. for 0.5 hours. Venting of solvent was
carried out at 105.degree. C.
[0076] The sample was filled with methanol, frozen in liquid
nitrogen, cracked and sputter coated with gold for scanning
electron microscopy. FIG. 6 shows a scanning electron micrograph of
this sample at magnification of 200.times..
Example 5
[0077] Preparation of Polymer in Supercritical Fluid with
Silane-Grafting Crosslinking
[0078] High density polyethylene (HDPE) was mixed with
vinyltrimethoxysilane (VTOMS; 1.5 parts per hundred of
polyethylene), 2,5-dimethylhexane-2,5-di-t-butyl peroxide (DHBP;
0.05 parts per hundred of polyethylene), and dibutyltindilaurate
(DBTL; 2 parts per hundred of polyethylene) in a reactive extruder.
The sample was crosslinked in warm water until the gel content was
27.4%. The sol fraction was not extracted prior to swelling. The
sample was swollen in propane at 9000 psi, at 172.degree. C. for 2
hours, crystallized at 85.degree. C. for 0.5 hours, and the solvent
was vented at 105.degree. C. After processing, the mass remaining
was 76.3% of the original mass, and the volume had increased by
483%.
[0079] The sample was filled with methanol, frozen in liquid
nitrogen, cracked and sputter coated with gold for scanning
electron microscopy. FIG. 7 shows a scanning electron micrograph of
this sample at magnification of 2000.times..
Example 6
[0080] Preparation of a Battery Separator from Porous Polymers
[0081] A battery separator is made from high molecular weight
polyethylene prepared as described in Example 1, except that the
polymer is extruded into thin film, or film blown, for example, to
a thickness of 0.1 mm. The polymer is crosslinked by radiation, and
swollen as described in Example 1, except that the polymer is
impregnated with silica particles during the swelling process. The
product has pore sizes on the order of 1 micron, and has an overall
thickness of about 0.1 mm. In an alternate procedure, the polymer
is coated onto a surface, for example, an electrode surface, before
crosslinking, then swollen with impregnated silica particles, and
further processed as in Example 1 to create a final product with
the properties described above.
Other Embodiments
[0082] It is to be understood that while the invention has been
described in conjunction with the detailed description thereof, the
foregoing description is intended to illustrate and not limit the
scope of the invention, which is defined by the scope of the
appended claims. Other aspects, advantages, and modifications are
within the scope of the following claims.
* * * * *