synthesis and applications of emulsion-templated porous materials
TRANSCRIPT
Synthesis and applications of emulsion-templated porous materials
Haifei Zhang and Andrew I. Cooper*
Received 22nd February 2005, Accepted 19th April 2005
First published as an Advance Article on the web 27th May 2005
DOI: 10.1039/b502551f
This review describes the use of macroemulsions as templates for the production of porous
materials. We focus on the use of high internal phase emulsions in order to produce
interconnected open porous structures. The review encompasses porous hydrophobic polymers,
hydrophilic polymers, composites, silica, metal oxides, and metals. The potential applications of
these materials are also discussed.
1 Introduction
Emulsions are heterogeneous mixtures of at least one
immiscible liquid dispersed in another in the form of droplets.1
In most cases, at least one of the liquids will be water or an
aqueous solution. An emulsion is often described as either oil-
in-water (O/W) or water-in-oil (W/O) where the first phase
mentioned refers to the internal (or dispersed) phase.
Generally, emulsions have average droplet sizes of at least
several micrometres and the droplets have a rather broad
size distribution unless special procedures are adopted (e.g.,
fractionation of the emulsion). Emulsions have been investi-
gated extensively for decades and are used in a range of
common practical applications.
In the context of polymer synthesis, emulsions can be used
in three ways (Fig. 1) as templates for the preparation of
colloids, porous materials, and composite materials, respec-
tively. In order to produce colloids, polymerisation takes
place in the dispersed monomer phase, although initiation
tends to occur in monomer-swollen micelles rather than
the larger monomer droplets.2,3 In the case of an O/W
emulsion polymerisation, this process leads to an aqueous
polymer latex. By contrast, polymerisation of the continuous
phase of a concentrated emulsion followed by removal of the
internal phase leads to the formation of emulsion-templated
porous materials.4 Polymerisation of both the droplet phase
and the continuous phase results in the production of
composites.3,5
Dr Haifei Zhang
Haifei Zhang was born inHebei Province, China andreceived his PhD in Chemistryin 2001 from the Institute ofChemistry, Chinese Academyof Sciences. He is currentlya Research Associate atLiverpool University. Hisresearch has involved supercri-tical fluids, emulsion templat-ing, and various porousmaterials.
Professor Andrew Cooper
Andrew Cooper received hisPhD in Chemistry fromNottingham University in1994. His research involvespolymer synthesis, porousmaterials, nanostructures, andsupercritical fluids. This workhas been supported by EPSRC,BBSRC, the Royal Society,the ACS Petroleum ResearchFund, and by industry.Professor Cooper is currentlya Royal Society UniversityResearch Fellow at theUniversity of Liverpool.
Fig. 1 Schematic representation of polymerisation of an emulsion in
the dispersed phase, continuous phase, and both phases for the
preparation of colloids, porous materials, and composites, respectively.
This review focuses on the porous materials.
REVIEW www.rsc.org/softmatter | Soft Matter
This journal is � The Royal Society of Chemistry 2005 Soft Matter, 2005, 1, 107–113 | 107
There are a number ways to fabricate interconnected porous
structures, for example, by using supercritical fluids,6 gas
blowing,7 self-assembled colloidal templates,8 and sacrificial
polymer templating.9 Emulsion templating is a flexible and
easily controlled method for the fabrication of macroporous
materials (pore size range 5–100 mm) by polymerising the
continuous phase of a high internal phase emulsion (HIPE)
(internal phase volume . 74.05%).4 If a less concentrated
emulsion is used (internal phase volume , 60%), a more
closed-cell porous structure will be obtained. Materials with
closed pores may be useful for sound absorption and as
lightweight alloys. Highly interconnected macroporous mate-
rials are popular in applications such as separation, catalyst
support, tissue engineering, and membrane support. A key
potential advantage of introducing large macropores is that it
enhances the mass transport of large molecules, especially in
viscous systems.
This review focuses on the preparation of various emulsion-
templated porous materials and their potential applications.
Table 1 summarizes the range of emulsion-templated materials
that has been prepared. Table 2 lists some applications in
which emulsion-templated materials have been used.
2. Synthesis of emulsion-templated porous materials
2.1. Hydrophobic polymers
In 1982, researchers at Unilever patented a process for the
preparation of highly porous, cross-linked polymeric materials
with a dry density of less than 0.1 g cm23 by polymerising the
continuous phase of a HIPE (internal phase at least 90%).10
These materials were branded polyHIPE. The system which
has been most extensively studied involves a W/O emulsion
containing styrene (ST) and divinylbenzene (DVB) (plus
comonomers and/or porogenic solvents in some cases) as the
continuous phase. Fig. 2a shows a typical scanning electron
microscope (SEM) image of a ST/DVB polyHIPE sample
illustrating the emulsion-templated, cellular pore structure.
Williams et al.11 studied the effect of varying the monomer
concentration and surfactant level on the openness of the cells
in the resulting materials. They found that the variation in cell
openness was strongly related to the surfactant-to-oil ratio.
This research was extended to include oil phases comprised
of 100% styrene or 100% divinylbenzene.12 The effects of
initiator, salt concentration, and the degree of cross-linking
were also investigated. It was found that the highest salt
concentrations gave the smallest cells, while the cell sizes were
insensitive to an increase in monomer concentration. Eight
surfactants and 22 cosurfactants were further studied for their
influence on emulsion stability and pore structure.13 By adding
4-vinylbenzyl chloride (VBC) to the ST/DVB system, the cell
Table 1 Summary of the synthesized emulsion-templated materials
Category Materialsa References
Hydrophobic polymer ST/DVB 10–13,25–27VBC-ST/DVB 14ST/DVB with high surface areas 15–18ST/DVB-based poly(aryl ether
sulfone)19
poly(styrene-co-alkylmaleimide) 20ST/DVB containing PCL, PLA,
acrylate, organotin21–23,61,62
Hydrophilic polymer Cross-linked polyacrylamide 30–34Cross-linked PHEA and PHMA 32
Composites Silica-containing composite 35–38Inorganic oxide/polymer 37,38
Inorganic oxides Silica 36–44,46Titania and zirconia 37–39,43Alumina 38,39,45
Metal and carbon Gold 47Nickel 63,64Carbon 49,50,52
a ST/DVB indicates a polyHIPE polymer composed of the mono-mers ST and DVB, and similarly for other polyHIPEs. Abbrevia-tions: ST 5 styrene; DVB 5 divinylbenzene; VBC; 5 4-vinylbenzenechloride; PCL 5 poly(e-caprolactone); PLA 5 poly(lactic acid).
Table 2 Applications of the synthesized emulsion-templated materials
Category Materialsa Applications References
Scaffolds for tissue engineering PCL-ST/DVB Cell growth 22PLA-ST/DVB Cell growth 23HA-ST/DVB Osteoblast growth, bone formation 53Poly-D-lysine-ST/DVB Growth of neurons 54Emulsion coating Protein release 55
Separation medium ST/DVB sulfonation Modules, filtration test 56Supports for reactions ST/DVB sulfonation Hydration of cyclohexene 57
ST/DVB sulfonation Ion exchange 58VBC/DVB amine functionalization CBC scavenger 60Organotin chloride-ST/DVB Dehalogenation and radical cyclisation 61Aryl acrylate based ST/DVB Reactive supports 62
Electrodes Porous nickel coating on the cathode 63,64a The same abbreviations apply as used in Table 1, and HA 5 hydroxyapatite; CBC 5 4-chlorobenzoyl chloride.
Fig. 2 (a) A typical SEM image of styrene–divinylbenzene polyHIPE,
scale bar 5 100 mm. Reprinted with permission from ref. 14,E The
Royal Society of Chemistry 2000. (b) An optical microscope image of
monodisperse emulsion-templated polyacrylamide beads, scale bar 5
10 mm. Reprinted with permission from ref. 33, E 2002 American
Chemical Society.
108 | Soft Matter, 2005, 1, 107–113 This journal is � The Royal Society of Chemistry 2005
size could be changed significantly and it was found that the
average cell size decreased with increasing VBC content.14
The cell size in polyHIPE polymers is typically in the range
5–100 mm with the interconnecting pore size constituting
around 20–50% of the cell diameter.4 PolyHIPE polymers have
high pore volumes (up to 10 cm3 g21) but low surface areas
due to their large pore sizes. By adjusting the level of cross-
linker in the ST/DVB system and by using an inert diluent
(porogen) solvent, a secondary pore structure was generated
within the cell walls of a polyHIPE material, thus leading to a
hierarchical, interconnected pore structure.15 A much higher
surface area of 350 m2 g21 was achieved using this approach.
By using mixtures of porogenic solvents (e.g., chlorobenzene
and 2-chloroethylbenzene), a cellular material with a surface
area of 554 m2 g21 was produced.16 Recently, the morphology
and surface area of ST/DVB polyHIPE foams prepared with
an oil-phase soluble porogen was investigated in a single
surfactant (Span 80) system17 and in a three-component
surfactant (Span 20/dodecyl-benzenesulfonic acid sodium
salt/cetyltrimethylammonium bromide (CTAB)) system.18
Other hydrophobic polyHIPEs have been prepared by the
copolymerisation of certain active monomers with the ST/
DVB system. For example, the synthesis of poly(aryl ether
sulfone)19 and poly(styrene-co-alkylmaleimide)20 polyHIPEs
were reported. The addition of another component or
modification to the normal ST/DVB system can endow the
materials with specific properties for various applications (see
also Section 3). Elastomeric polyHIPEs were prepared by
polymerisation of an emulsion continuous phase containing
ST, DVB, and varying amounts of 2-ethylhexyl acrylate or the
corresponding methacrylate comonomer.21 Because of the
highly interconnected macroporous structure, polyHIPEs
show promise as scaffolds for tissue engineering. PolyHIPE
foams containing poly(e-caprolactone) (PCL) were prepared
from PCL macromonomers and either styrene, methyl
methacrylate, or toluene used as an organic diluent.22 A
similar procedure was adopted to produce polyHIPE foams
containing poly(lactic acid) (PLA).23 Deleuze et al. prepared
polyHIPE foams by ring opening metathesis polymerisation
of a norbornene derivative using Grubb’s catalyst.24 It was
claimed that the polymerisation had a living character, such
that the metal–carbene chain end remained active at the end of
the preparation and could thus be modified further.
Most hydrophobic polyHIPE polymers have been prepared
as monolithic blocks which conform to the shape of the
reaction vessel in which the material is prepared. It is possible
to convert these porous monoliths into a particulate form
by grinding, for example, but this can be an inefficient
and laborious process. In an alternative approach, a W/O/W
suspension polymerisation was performed using a concen-
trated W/O emulsion to prepare discrete spheroidal particles
of interconnected cellular foams (Fig. 3b). This approach
was disclosed in a series of patents25 and the resulting
materials were commercialized by Biopore.26 To achieve this,
ST/DVB W/O emulsions were injected into a stirred aqueous
suspension medium which contained a hydrophilic polymer
as a stabilizer in order to carry out the suspension
polymerization and to produce beaded materials. The yield
of beads recovered was claimed to be in the range of 65–95%
and the materials appeared to exhibit quite broad particle size
distributions.27
2.2. Hydrophilic polymers
Emulsion-templated hydrophilic polymers can be obtained by
sulfonation of the corresponding hydrophobic materials28 or
by the production of highly porous polymers with ionic or
polar groups.29 However, there have been comparatively few
reports on the preparation of hydrophilic polyHIPE foams
from hydrophilic monomers. From an applications point of
view this is perhaps surprising, especially given the broad range
of porous biomaterials that could be produced from an
aqueous continuous-phase environment. In order to obtain
emulsion-templated hydrophilic polymers, a concentrated oil
phase must be emulsified in a continuous aqueous monomer
solution. For example, in a patent filed by Katagawa,30 porous
hydrophilic polymeric materials having cavities joined by
interconnecting pores were produced. To achieve this, an
emulsion was suspended in an oil suspension medium (Fig. 3b),
and the emulsion droplets were polymerised to produce
polymeric microbeads.
One significant drawback to this approach (and a possible
reason for the relatively small number of examples) is the large
volume of organic solvent that must be used to produce porous
hydrophilic polymers from an O/W HIPE. Indeed, the volume
required in the synthesis may amount to 20 kg organic solvent
per 1 kg of porous polymer produced, even if one disregards
any additional solvents that may be used post-synthesis to
wash out any residual internal oil phase. We have developed a
method which avoids this problem by using high internal
phase supercritical fluid emulsions.31 For example, super-
critical CO2 (scCO2) was emulsified in an aqueous solution of
acrylamide (AM) and N,N9-methylene bisacrylamide (MBAM)
using a perfluoropolyether ammonium carboxylate (PFPE)
surfactant and poly(vinyl alcohol) (PVA) as a cosurfactant
to form a concentrated CO2-in-water (C/W) emulsion.31 The
Fig. 3 A schematic representation of two methods for preparing
porous emulsion-templated beads. (a) O/W/O sedimentation poly-
merisation to produce large uniform beads (diameter y 2 mm). A
concentrated O/W emulsion is injected into a hot oil medium and
polymerised during sedimentation. (b) O/W/O or W/O/W suspension
polymerisation for the production of porous emulsion-templated micro-
spheres. An emulsion (O/W or W/O) is dispersed and polymerised in a
continuous phase (oil or water) as micrometre-sized droplets.
This journal is � The Royal Society of Chemistry 2005 Soft Matter, 2005, 1, 107–113 | 109
ratio of CO2 to the aqueous phase was typically 80 : 20 v/v. In
the presence of water-soluble initiator, K2S2O8, polymeriza-
tion occurred at 60 uC and gelation of the aqueous phase was
observed. The complete removal of internal phase CO2 was
achieved by simple pressure reduction to produce a highly
porous poly(acrylamide) (PAM) material. This process avoids
the use of any organic solvents either in the synthesis or
purification steps. No solvent residues are left in the materials
which could be advantageous for biological or medical
applications. As first reported,31 however, the process has a
number of disadvantages: in particular, the PFPE surfactant is
expensive and non-degradable; the monomer (AM) is toxic;
and the reaction pressure is rather high (300 bar). In a more
extensive study,32 we addressed these issues by using a redox
co-initiator and inexpensive commercial surfactants (e.g.,
Tween 40) to produce porous polymers at modest tempera-
tures and pressures (20 uC, 65 bar). In addition to PAM,
poly(hydroxylethyl acrylate) (PHEA) and poly(hydroxyethyl
methacrylate) (PHMA) were also synthesised.32
Most emulsion-templated materials have been produced as
monolithic blocks11–24,28–32 or in the form of microspheres.30
We have developed an oil-in-water-in-oil (O/W/O) sedimenta-
tion polymerization method to prepare monodisperse, highly
porous polymer beads (Fig. 3a).33 The yield of beads was 100%
and the diameter of the beads was typically about 2 mm. These
materials have several advantages over other morphologies
such as ease of handling, simple separation from reaction
mixtures by filtration, and relatively easy removal of the
internal phase of the original emulsions as a result of the
beaded form. To produce these materials, an O/W emulsion
(light mineral oil emulsified in an aqueous solution of AM and
MBAM) was injected into a hot oil medium (60 uC) using a
syringe pump.33 The droplets of the emulsion were poly-
merised during sedimentation in order to lock in the structure
of the bead. Sedimentation lasted about 10 s: in order to
achieve sufficiently rapid polymerisation, a redox co-initiator
tetramethylethylenediamine (TMEDA) was added to the
internal oil phase in addition to the initiator ammonium
persulfate (APS) which was dissolved in the aqueous phase.
Fig. 2b shows an optical microscope image of emulsion-
templated PAM beads prepared by this method. The beads
are very uniform with a standard deviation in diameter of
just 2%. Highly interconnected emulsion-templated pores
were formed which were also open to the bead surface
(average pore size 5 2–15 mm). The beads showed high
macropore volumes (.8 cm3 g21 as characterized by mercury
intrusion porosimetry).33 We have also developed a com-
pressed fluid sedimentation polymerization method to prepare
uniform polymer beads by injecting an aqueous monomer
solution into a high-pressure fluid medium (e.g., a mixture
of tetrafluoroethane and CO2).34 This process avoids the use
of a non-volatile oil as the sedimentation medium and thus
greatly simplifies production separation and purification. The
materials produced by this route do not exhibit emulsion-
templated porosity,34 although in principle one could envisage
a C/W/C sedimentation polymerisation method by which it
would be possible to synthesise porous emulsion-templated
beads in the complete absence of any organic solvents or non-
volatile oils.
2.3. Organic–inorganic composite materials
Organic–inorganic composites often exhibit enhanced
thermal stability or additional functionality with respect to
the equivalent organic materials. It is possible to produce
porous, emulsion-templated organic–inorganic composites by
a number of methods. For example, methacryloxypropyl-
trimethoxysilane (MPS) was copolymerised with ST and DVB
using free radical initiation in the organic phase of a W/O
emulsion. Subsequent hydrolytic condensation of the tri-
methoxysilyl groups formed an inorganic polysilsesquioxane
network.35 In another method,36 tetraethyl orthosilicate
(TEOS) was first hydrolyzed to obtain TEOS sol. This sol
was then mixed with an aqueous monomer solution which was
used to form an O/W emulsion. Emulsion-templated silica–
polymer composite beads were produced by a one-step O/W/O
sedimentation polymerization at 60 uC.36 We have also
reported a more general two-step method whereby emulsion-
templated PAM beads are used as scaffolds to produce
inorganic–polymer composite materials by chemical post-
modification of the organic material with inorganic reagents.
For example, emulsion-templated polymer–silica37,38 and
polymer–metal oxide37,38 beads were produced in this way.
2.4. Inorganic oxide porous materials
In terms of inorganic materials, emulsion-templated silica
structures have been most widely studied. For example, O/W
emulsions with an aqueous solution or sol of an inorganic
precursor in the continuous phase have been gelled to produce
porous three-dimensional inorganic oxides.39 Recently, a
cationic surfactant CTAB was dissolved in acidified water
and a mixture of 1,3,5-trimethylbenzene (TMB) and TEOS
was added slowly with stirring.40,41 A white solid was obtained
after the sol-gel process which was washed, dried, and then
calcined at 650 uC in air. The templated macroporosity was
ascribed to the natural creaming process that occurred in the
O/W emulsion.40 The effect of various factors such as the
degree of hydrolysis of TEOS, the condensation of the silica,
and the creaming or coalescence of oil or polystyrene particles
was also investigated.41 In another case, TEOS with surfactant
tetradecyltrimethyl ammonium bromide (TTAB) were mixed
in an O/W emulsion to produced a hierarchically textured
silica monolith.42 No organic polymers were involved although
a large quantity of surfactant was used to stabilise the
emulsion (TTAB : TEOS # 3 : 1 by mass).
Imhof and Pine used an oil-in-formamide emulsion to
produce ordered macroporous silica.43 Silica sols were
prepared by vigorously mixing silicon tetramethoxide with a
mixture of water and formamide acidified to pH 5 2. The
emulsion was then fractioned and centrifuged to produce a
monodisperse concentrated emulsion which was templated to
produce ordered macroporous silica. The microstructures of
porous silica prepared from aqueous and nonaqueous emul-
sions were studied in more detail.44 In the case of silicon oil-
in-water emulsions, it was found that the emulsion droplets
migrated and self-assembled in relatively mobile local regions,
which deteriorated the uniformity of the macropore struc-
ture.44 We have employed polymers to produce macroporous
silica with quite uniform mesopores by using O/W emulsions
110 | Soft Matter, 2005, 1, 107–113 This journal is � The Royal Society of Chemistry 2005
as templates.36 First, polymer–silica composites were formed,
as described above, by O/W/O sedimentation polymerisation.
Calcination of organic polymer led to a hierarchically porous
silica structure with a high intrusion volume (5.81 cm3 g21)
and a high surface area (421.9 m2 g21).36 The latter arose from
the mesopore structure in the material. Fig. 4a shows an SEM
image of an emulsion-templated silica material produced by
this route.
In general, many inorganic precursors such as metal
alkoxides are highly reactive to water, even towards moisture
in the air. Silica precursors such as TEOS can be hydrolyzed
under acidic or basic conditions and are relatively tolerant to
water.36,40–44 Other alkoxide precursors are highly reactive
and this essentially precludes the use of O/W or W/O
emulsions. One method to circumvent the problem of
precursor reactivity is to use inorganic particles instead of
molecular precursors. For example, porous alumina bodies
were produced by polymerization and drying polymerizable
W/O emulsions containing alumina powders.45 The total
porosity of the alumina bodies increased markedly with the
water content of the W/O emulsion. Binks has reported a
simple and effective method for preparing porous silica from
emulsions stabilized by silica particles alone.46 O/W or W/O
emulsions were stabilized entirely by nanosized silica particles
of controlled wettability with excess particles present in the
continuous phase. Macroporous silica materials with average
pore sizes in the range 5–50 mm were prepared by evaporation
of the two liquids in the emulsions. Another approach is to
pretreat the inorganic precursors in order to reduce their
reactivity. For example, titanium tetraisopropoxide can be
treated with an equimolar amount of the chelating agent 2,4-
pentanedione.43 This method was employed to prepare
ordered macroporous titania and zirconia materials using
isooctane-formamide emulsions as templates.43
An alternative two-step method for producing porous
inorganic materials is to use a sacrificial, preformed
emulsion-templated scaffold, thus avoiding direct contact of
the inorganic precursors with the liquid–liquid emulsion.
Typically, an inorganic precursor sol (or solution) is imbibed
into the scaffold and the targeted inorganic materials are
then obtained by removing the organic scaffold, usually
by extraction or by calcination. Chmelka et al. used such
a method to prepare meso/macroporous inorganic oxide
(silica, zirconia, titania) monoliths from polymer foams.37
Macroporous polystyrene foam monoliths were prepared from
highly concentrated W/O or O/W emulsions. A self-assembling
block copolymer/sol–gel mixture was then imbibed into the
preformed macroporous foam. Elimination of the organic
agents by solvent extraction or by calcination resulted in a
dual meso/macroporous inorganic material.37 We have used
uniform emulsion-templated porous polymer beads33 as tem-
plates to produce various inorganic structures.38 Uniform,
hierarchically porous inorganic beads of silica, alumina, titania
and zirconia were prepared by simply immersing the polymer
scaffold beads in an appropriate inorganic precursor solution,
followed by sol-gel condensation in air and subsequent
calcination of the polymer phase. Fig. 4b–d shows the
emulsion-templated porous structures of alumina, zirconia,
and titania materials prepared by this method. These
hierarchical structures are composed of mesopores (2–5 nm),
micropores (in the case of silica), and large emulsion-templated
macropores of around 5–10 mm. All of the pores in the
structures are highly interconnected.
2.5. Porous metals and carbon
In principle, emulsion-templated metal or carbon structures
could find a range of applications, for example as porous
sensors or electrodes. We have found that polyacrylamide
beads can irreversibly adsorb gold nanoparticles from aqueous
gold sols.47 Based on this finding, emulsion-templated gold
beads were prepared using gold nanoparticles as building
blocks.47 First, emulsion-templated porous polyacrylamide
beads33 were soaked in a gold nanoparticle sol. The adsorption
of the gold nanoparticles on the polymer surface was con-
firmed by the fact that the red sol was decolourised and the
beads assumed a deep red colour. The polymer–gold compo-
site beads were then soaked in isopropanol, dried in air, and
calcined to produce uniform porous gold beads. Fig. 4e
shows the macroporous structure of a gold bead prepared
by this method. Surprisingly, the emulsion-templated structure
is completely retained, despite the high mass loss that occurs
during calcination. This method is quite versatile; for example,
preliminary results have shown that emulsion-templated
palladium beads can also be produced.48 It is also possible
to control the distribution of the gold nanoparticles in the
metal-polymer composite such that hollow gold beads can be
produced after calcination with an emulsion-templated shell
structure.47
Porous carbon materials have been prepared by carbonizing
a carbonizable emulsion-templated polymer structure by
heating in an inert atmosphere.49 For example, high internal
phase emulsions of ST and DVB with 0–40% vinylbenzene
chloride (VBC) in the oil continuous phase were polymerised
to give porous polymers.50 These materials were then
Fig. 4 Images of PolyHIPE structures of inorganic materials. (a)
silica, scale bar 5 50 mm, reprinted with permission from ref. 36, E
2003 WILEY-VCH Verlag GmbH. (b–d) the structures are, respec-
tively, alumina, zirconia, and titania. Scale bar 5 25 mm. Reprinted
with permission from ref. 38, E 2004 American Chemical Society. (e)
gold, scale bar 5 5 mm, reprinted with permission from ref. 47, E 2004
WILEY-VCH Verlag GmbH. (f) carbon, scale bar 5 10 mm, reprinted
with permission from ref. 50, E 2005 Society of Chemical Industry.
This journal is � The Royal Society of Chemistry 2005 Soft Matter, 2005, 1, 107–113 | 111
sulfonated and carbonized at up to 700 uC to produce
macroporous carbon monoliths, as shown in Fig. 4f. Porous
carbon can also be prepared via a silica templating method,
that is, by carbonization of a carbon precursor in the voids of
porous silica and subsequent removal of the silica using HF
or NaOH solution.51,52 Similarly, an interconnected porous
silica with mesopores and macropores was impregnated with
furfuryl alcohol (FA) by incipient wetness. FA polymerization,
carbonization and subsequent removal of the silica component
were carried out to produce a carbon monolith possessing a
hierarchical and interconnected porous structure.52
3. Applications of emulsion-templated porousmaterials
3.1. Scaffolds and tissue engineering
The highly interconnected macroporosity of emulsion-
templated polymers makes them good candidates as scaffolds
for biological applications such as tissue engineering, provid-
ing that an appropriate material is chosen. For example,
PolyHIPE foams containing poly(e-caprolactone) (PCL)22 or
poly(lactic acid) (PLA)23 were tested for tissue growth. These
studies used whole chicken embryo explants, rat skin explants,
or individual human skin cells. The results showed excellent
biocompatibility for both types of foam for the duration of
each experiment. Comparative studies indicated that cells
adhered more rapidly to PCL-based materials than the PLA-
containing counterparts.23 Three groups of ST/DVB
polyHIPE polymers containing hydroxyapatite (HA), with
pore sizes of 40, 60 and 100 mm, were evaluated as materials
for osteoblast growth and bone formation.53 The results
demonstrated in vitro cell-polymer compatibility, with osteo-
blasts forming multicellular layers on the polymer surface and
also migrating to a maximum depth of 1.4 mm inside the
porous matrix. This study showed that osteoblasts seeded onto
the polymer demonstrated cellular attachment, proliferation
and in-growth, leading to the support of an osteoblastic
phenotype. In another study, porous ST/DVB polyHIPE
matrices were coated with aqueous-based solutions including
poly-D-lysine and laminin.54 The growth of human stem cell-
derived neurons on these porous matrices was investigated.
These results demonstrated the potential use of solid porous
scaffolds to create three-dimensional environments for cell
growth and differentiation. An emulsion-coating method was
developed to release proteins in a controlled way.55 In this
process, a W/O emulsion generated from an aqueous protein
solution and an organic polymer solution was forced through a
prefabricated scaffold by applying a vacuum. After solvent
evaporation, a polymer film, containing the protein, was then
deposited on the porous scaffold surface. A model protein
(lysozyme) could be released in a controlled fashion from
these scaffolds.55
3.2. Reactions and separations
Because of their highly interconnected porous structures,
emulsion-templated materials can be used as separation
media56 and as supports for heterogeneous reagents or
catalysts. In the latter case, the porous structure tends to
require chemical modification or functionalization in order
to fulfil a particular application. For example, a ST/DVB
polyHIPE was functionalized with sulfonic groups and
then used as an immobilized catalyst for the hydration of
cyclohexene.57 The reaction mixture was forced through the
pores of the monolith, and flow was enabled without high back
pressures due to the large pores in the structure. The kinetics
of the liquid-phase hydration was measured both in a three-
phase system and in a two-phase system. This type of
polyHIPE was also modified on its surface by sulfonation.58
By incorporating Na+ ions onto the sulfonated surfaces, a
modular form of ion exchange resin was produced which was
capable of effective exchange with metal ions in solution. In
single-pass dynamic adsorption tests, the material showed
better column utilization than the equivalent commercial
resins.58 Polystyrene polyHIPEs were modified with a wide
variety of functional groups both in a batch method and in a
continuous flow method.59 In the presence of a free-radical
initiator, compounds such as HBr and thiols underwent an
anti-Markovnikov addition to the residual vinyl groups in the
polystyrene matrix. Other reactions such as hydroboration
with BH3 followed by hydrolysis with H2O2 were also been
investigated.59 Monolithic polymer supports and scavengers
were prepared by amine functionalization of poly(4-vinyl-
benzyl chloride (VBC)-co-divinylbenzene (DVB)) polyHIPE
materials.60 The results showed that 4-chlorobenzoyl chloride
(CBC) was efficiently and rapidly scavenged from solution at
ambient temperature.
An organotin chloride catalyst supported on a highly porous
polymer was prepared by polymerisation of a highly concen-
trated reverse emulsion.61 The polymer-supported organotin
chloride showed good activity and good stability towards
dehalogenation and radical cyclisation. W/O HIPEs contain-
ing 4-nitrophenyl acrylate and 2,4,6-trichlorophenyl acrylate
as reactive monomers were prepared and polymerized to
produce highly porous monoliths.62 The monoliths proved
to be very active toward nucleophiles, and possibilities of
functionalizing this polymer support were demonstrated via
reactions with amines bearing additional functional groups
and via the synthesis of an acid chloride derivative.62
3.3. Porous electrodes
The preparation of highly porous Ni electrode materials is
relevant to a number of industrial applications of electro-
chemical technology. A polyHIPE polymer matrix was used
as a template for the preparation of porous nickel elecro-
deposits.63 Ni-mesh electrodes were embedded into a
polyHIPE by immersion in the precursor emulsion and
subsequent entrapment into the solid porous matrix produced
by polymerization and drying.63 Immersing the resulted
composites into a Ni electroplating bath and passing a direct
current through the two electrodes resulted in the growth of Ni
electrodeposits on the cathode and throughout the polymer
cells and pores. Calcination of the polymer matrix led to a
granular porous Ni coating on the cathode. This method could
be used to grow Ni through the polymer cells and pores.64 The
Ni coating showed an interconnected porous structure with a
specific surface area of 49.5 m2 g21. The application of
112 | Soft Matter, 2005, 1, 107–113 This journal is � The Royal Society of Chemistry 2005
emulsion-templating to the preparation of porous electrodes
and sensors could be extended in the future by implementing
the new methods described in section 2.5.47–50
4. Summary
This review has focused on the preparation and application of
various emulsion-templated materials including hydrophobic
polymers, hydrophilic polymers, inorganic–polymer compo-
sites, inorganic materials, metals, and carbon. Most of these
materials were produced by polymerisation of the continuous
phase of a concentrated emulsion. These materials have a
number of potential applications such as scaffolds for tissue
engineering and as catalyst supports. Future studies may be
on the preparation of novel emulsion-templated materials
and the development of potential applications especially of
emulsion-templated inorganic, metal, and nanoparticle-
dispersed materials.
Haifei Zhang and Andrew I. Cooper*Donnan and Robert Robinson Laboratories, Department of Chemistry,University of Liverpool, Crown Street, Liverpool, UK, L69 3BX.E-mail: [email protected]; Fax: +44 151 7943588; Tel: +44 151 7943548
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