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1815
Full Paper Macromolecular
Chemistry and Physics
wileyonlinelibrary.com DOI: 10.1002/macp.201200172
Macroporous Polymer Beads and Monoliths
From Pickering Simple, Double, and Triple
Emulsions
Amro K. F. Dyab*
Various polymeric materials based on simple, double, and triple emulsions stabilized solely by
dichlorodimethylsilane (DCDMS)-modifi ed amorphous silica nanoparticles of well-controlled
surface hydrophobicities or organo-modifi ed Laponite clay nanoparticles are described. Mag-
netic and fl uorescent polymer beads having different mor-
phologies are fabricated from double-emulsion templates.
Polymer foams based on Pickering emulsions are fabricated
and functionalized using different materials such as sporopol-
lenin and zinc metal particles. Suspension polymerization of
both oil phases of a novel Pickering triple emulsion results in
the formation of new porous hierarchical structures. Polymer
foams and beads based on novel nonaqueous Pickering simple
and double emulsions are fabricated.
Dr. A. K. F. Dyab
Chemistry Department, Faculty of Science, Minia University,
Minia 61519, Egypt
E-mail: amrokhalil@yahoo.com
Dr. A. K. F. Dyab
Surfactant Research Chair, Department of Chemistry,
College of Science, King Saud University, P.O. Box 2455,
Riyadh 11451, Saudi Arabia
of magnetic and fl uorescent properties into a single micro-
or nanocomposite material may greatly enhance the
applications of the material in the biomedical and biop-
harmaceutical fi elds.
Emulsion templating methods represent an attractive
route to highly porous and permeable polymeric materials
with a well-defi ned porosity. The process has been known
since at least the 1960s
[ 12 ] and was developed extensively
by workers at Unilever in the 1980s
[ 13 ] and in recent
years has seen increased interest from both academia
and industry. A high-internal-phase emulsion (HIPE) is a
concentrated emulsion in which the droplet phase com-
prises greater than 70% of the total volume of the emul-
sion. The external (non-droplet) phase is converted into
a solid polymer and the emulsion droplets are removed
yielding (in most cases) a highly interconnected network
of micron sized pores of quite well-defi ned diameter. The
resulting material is often termed a polymerized HIPE, or
poly(HIPE).
[ 14 ] In recent years, there has been increasing
interest in the use of solid particles as sole emulsifi er to
stabilize the internal phase (e.g., droplets) of the HIPEs.
However, one important limitation was raised but has
been recently overcomed. Kralchevsky et.al.
[ 15 ] have theo-
retically predicted that particle-stabilized emulsions will
phase invert above an internal-phase volume fraction of
1. Introduction
The synthesis of multifunctional polymeric microparticles
and diverse porous materials has been topics of increasing
interest in both academic and industrial fi elds. Several
functionalities are able to be simultaneously encapsulated
into polymeric microspheres, such as magnetism and fl uo-
rescence,
[ 1 ] and others.
[ 2–6 ] These functional microspheres
exhibit an ability to change their physicochemical proper-
ties in response to environmental stimuli, including mag-
netic or electric fi elds, light, temperature, pH, chemicals,
or mechanical stress. Such multifunctional microspheres
have found diverse applications in biomedical and biotech-
nological fi elds, such as controlled drug delivery,
[ 7 ] cellular
imaging,
[ 8 ] cell recognition,
[ 9 ] cancer-specifi c multimodal
imaging,
[ 10 ] and dual-protein delivery.
[ 11 ] The combination
Macromol. Chem. Phys. 2012, 213, 1815−1832
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1816
A. K. F. Dyab
www.mcp-journal.de
Macromolecular
Chemistry and Physics
www.MaterialsViews.com
Supporting Information). The motivation of the current
work was to fabricate different polymeric functionalized
materials ranging from porous polymer microparticles
to porous monoliths using different Pickering aqueous
and nonaqueous emulsion templates. We report here
straightforward methods for fabrication of polymeric
materials including dual function magnetic and fl uores-
cent hybrid microparticles with different morphologies,
namely the multicore or porous polymer beads (based
on double w/o/w emulsions). We show that the struc-
ture of the formed microparticles from double emulsion
templated can be tailored by the volume fraction of the
primary simple emulsion of the parent double emulsion.
Generally, the use of fl uorescent quantum dots showed
many advantages over the conventional organic dyes,
such as, high intensities and improved photostability.
In addition, incorporation of quantum dots in polymeric
matrix can effectively inhibit the release of quantum
dots and avoid their health side effects. Porous materials
based on aqueous or novel nonaqueous emulsions stabi-
lized solely by hydrophobic silica nanoparticles or organo-
modifi ed Laponite clay nanoparticles have been prepared.
Selected porous structures have been functionalized by
inclusion of natural sporopollenin microcapsules (derived
from Lycopodium clavatum ) for possible encapsulations
of inorganic or organic materials within the cavities of
these pores. Incorporation of zinc metal particles into
the polymer can provide an additional functionality that
can fi nd potential applications, for example, in catalysis.
In addition, we present here a novel hierarchical porous
structure based on polymerization of a triple Pickering
water-in-oil-in-water-in-oil (w/o/w/o) emulsion of dif-
ferent morphologies for the fi rst time.
2. Experimental Section
2.1. Materials
Fumed silica nanoparticles were produced (by Wacker Chemie,
Germany) via the introduction of volatile chlorosilanes into an
oxyhydrogen fl ame and consist of ultrapure amorphous silicon
dioxide. The nanoparticles had a mean primary particle dia meter
of approximately 20 nm and were coated to different extents
with dichlorodimethylsilane (DCDMS). According to the supplier,
particle wettability was characterized in terms of the measured
percentage of unreacted SiOH groups remaining on the surface
using a base titration method.
[ 25 ] This ranged from 100% (most
hydrophilic) to 14% (most hydrophobic). The coated silica nano-
particles with measured values of percentage SiOH were kindly
supplied by Wacker-Chemie (Germany). Although the primary
particle diameter was 20 nm, the powder also contained fused
aggregates of multiple primary particles and larger agglomer-
ates of the fused aggregates. Laponite (RD) clay nanoparticles
were provided by (Southern Clay Products Texas, USA). Iron (III)
chloride (FeCl
3 , anhydrous, 97%), ethyl alcohol, and acetone were
0.5. Colver and Bon
[ 16 ] reported that, in practice, emul-
sions are stabilized by sub-micrometer microgel particles
with volume fractions of the dispersed phase of only 50%.
Binks and co-workers
[ 17 ] showed that particle-stabilized
water-in-oil (w/o) emulsions commonly phase invert
between volume fractions of 0.65–0.70. However, Arditty
et al.
[ 18 ] described oil-in-water (o/w) emulsions with up to
90% internal phase and w/o emulsions having internal
phase up to 75% by manual shaking and stabilized by as
received hydrophilic silica particles and silanized hydro-
phobic silica particles, respectively.
[ 18 ] Recently, a suc-
cessful preparation of polyHIPEs with closed pores struc-
ture stabilized solely by oleic acid-functionalized titania
or silica particles has been reported with 90% internal
phase volume.
[ 19 ] In addition, different polyHIPEs have
been prepared using laboratory modifi cation of silica nan-
oparticles with different silanizing agents.
[ 20 ] Surfactant-
stabilized polyHIPE monodisperse porous polymer beads
have been prepared via oil-in-water-in-oil (o/w/o) emul-
sion-templated sedimentation polymerization.
[ 21 ]
The knowledge that fi ne solid powders can stabilize
emulsions dates back to the turn of the last century. The
credit is usually given to Pickering,
[ 22 ] who originally rec-
ognized the role of fi nely divided insoluble solid particles
in stabilizing emulsions. It is now well established that
solid particles of colloidal dimensions can act as excellent
emulsifi es alone for both o/w and w/o emulsions.
[ 23–26 ]
Since the energy of attachment of particles to oil–water
interfaces is very high (thousands of kT per particle), such
particles are held very strongly in the interface giving rise
to extremely stable drops within emulsions.
[ 23 ] Multiple
emulsions were fi rst described by Seifriz
[ 27 ] in 1925, but
it is only in the last 20 years that they have been studied
in more detail. Common types of multiple emulsions may
be either of the water-in-oil-in-water type (w/o/w) or of
the (o/w/o). Increasing attention has been devoted to
these systems with the aim of taking advantage of their
multiple compartment structure.
[ 28 ] Multiple emulsions
are generally prepared with high concentrations (5–50%)
of two emulsifi ers (one lipophilic, one hydrophilic) of
oppositely curved interfaces. However, the most common
problem associated with multiple emulsions is their
inherent thermodynamic instability since the emulsifi ers
tend to mix at the interfaces and destabilize giving simple
o/w emulsions.
[ 29 , 30 ] This is why the use of multiple emul-
sions as commercial products is so restricted, although
much attention has been paid to their many potential
practical applications.
[ 28 ]
Extremely stable Pickering simple and multiple emul-
sions of the two common types stabilized solely by silica
nanoparticles for different liquids have been reported
in previous work.
[ 25 ] The formulated multiple emulsions
were completely stable against coalescence for around
10 years (see optical images of emulsions in Figure S1,
Macromol. Chem. Phys. 2012, 213, 1815−1832
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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2.4. Preparation of Double w/o/w Emulsions
Table 1 shows recipes of selected emulsion templates used in
fabrication of various polymeric systems studied here. Double
w/o/w emulsions were prepared using a two-stage process.
[ 25 ]
Stage 1 involved the preparation of a simple w/o emulsion, in
which the volume fraction of water (
φ
w ) equals 0.2. It was pre-
pared by adding 2.5 cm
3 of water into 10 cm
3 of monomer oil
containing a known mass of hydrophobic silica nanoparticles
of 50% SiOH. The mixture was then homogenized using a DI-25
basic Yellow-line, IKA (Germany) homogenizer (rotor/stator)
with an 18 mm head operating at 13 000 rpm for 2 min. Water
phase of the primary w/o emulsion can be loaded, as required,
with 1 mg mL
− 1 CdTe quantum dots and 1 wt% magnetite Fe
3 O 4
nanoparticles. The oil phase of the primary w/o emulsion con-
tains, as required, 2 wt% (based on oil mass) AIBN or Irgacure
184 and 1 wt% DVB (based on oil mass). In stage 2, the primary
w/o emulsion, just prepared in stage 1, is re-emulsifi ed into 10
cm
3 of an aqueous phase containing a known mass (expressed
in wt% of the continuous phase) of hydrophilic silica nanopar-
ticles of 80% SiOH (using the homogenizer at 8000 rpm for 10
s). The volume fraction of the w/o emulsion (
φ
w/o ) in the fi nal
double emulsion can be varied from 0.1 to 0.6. In some cases,
the second step of homogenization was carried out simply by
hand shaking for 10 s. The emulsion continuous phase was
determined by measurement by observation of what happened
when a drop of emulsion was added to a volume of each of the
pure liquid phases. The emulsions only dispersed in the liquid
when its continuous phase matched the liquid to which it was
added.
2.5. Preparation of Polymeric Magnetic and Fluorescent
Microparticles
The as-prepared Pickering double w/o/w emulsions of either
HDDA or styrene were either photo or thermally polymerized
without stirring after bubbling in pure N
2 gas for 5 min. Poly-
merization was started by placing the double emulsion in a UV
cell under a 12 W UV lamp (UVP, Upland, USA) for 3.5 h. Thermal
polymerization was started by heating the system to 75 ° C for
24 h in glass vessels (diameter around 25 mm) to complete the
polymerization reaction without stirring. Finally, microparticles
were recovered by washing three times with ethanol and then
with Milli-Q water to remove any unreacted oil and excess of
stabilizers.
Images and videos were recorded by optical and fl uorescence
microscopy using (Olympus BX-41 fi tted with DP70 digital
camera) and applying the Rhodamine TRITC fi lter set. Images
and videos were processed using Corel Paintshop pro X4
software. In some experiments, we applied a permanent magnet
close to the sample on the microscope stage while capturing
still images or videos to examine the response of the magnetite
inside the polymeric microparticles to the external magnetic
fi eld. Mean emulsion drop diameters were obtained from the
images by measuring a minimum of 20 drops from each slide.
Digital images were taken by personal Samsung galaxy smart
phone S1. Polymeric materials were characterized by scanning
electron microscopy (SEM) and energy dispersive X-ray (EDX)
using the JEOL SEM (JSM-6380 LA). Around 0.5 cm
3 or solid dry
purchased from Fisher Chemicals. Iron (II) chloride tetrahydrate
(FeCl
2 .4H 2 O, 98%), zinc powder, and ammonia solution (33 wt%)
were purchased from BDH Chemicals. 1,6 hexanediol diacrylate
(HDDA, 90%), styrene (99.5%), thermal initiator 2,2 ′ -azobis(2-
methylpropionitrile) (AIBN, 98%), toluene, divinylbenzene (DVB),
CaCl
2 · 2H 2 O, Cetyltrimethylammonium bromide (CTAB, 99.99%),
Glycerine ( > 95.5%), and Rhodamine B (95%) were purchased from
Sigma–Aldrich. Formamide ( > 98.5%) was purchased from Merck.
The photoinitiator (Irgacure 184) was supplied by Ciba Specialty
Chemicals (Switzerland). Lycopodium clavatum pollen powder
was purchased from Fagron, UK. HDDA and styrene were dein-
hibited by passing through neutral alumina and then washed
with 10% NaOH solution three times prior to use. Cadmium tel-
luride (CdTe) quantum dots (PlasmaChem GmbH, Germany) with
λ
max = 560 ± 5 nm, M. W. = 240.01, and theoretically calculated
diameter of 3.02 nm was used. The particles of CdTe are coated
by thiocarbonic acid and have a negative charge in aqueous solu-
tions. We have verifi ed the charge of these nanoparticles using
Marvlen Zeta-Sizer and the zeta-potential of dilute sample in pure
water was found to be 77.5 ± 2 mV. All the experiments were car-
ried out at room temperature unless otherwise stated. Water was
passed through a reverse osmosis unit and then a Milli-Q reagent
system. The CdTe nanoparticles were supplied as 50 mg powder
sample and a stock solution of 10 mg mL
− 1 was formulated with
pure water. The typical concentration of the CdTe used in most
of the formulations was 1 mg mL
− 1 . The water-based QD sample
was prepared by adding 50 mL of CdTe (10 mg mL
− 1 ) into 0.5 mL
pure water.
2.2. Modifi cation of Laponite (RD) Nanoparticles
Hydrophobic Laponite nanoparticles were obtained according
to the cation-exchange method described elsewhere. In a round
fl ask, 20 g of Laponite clay was dispersed in 500 mL distilled
water containing 6 g, 65 × 10 − 4 M of CTAB, which caused com-
plete cation exchange at room temperature, then the temper-
ature was increased to 80 ° C under vigorous stirring for 6–8
h with a condenser. The resulting modifi ed Laponite clay was
separated by fi ltration, washed several times with distilled
water to remove any free surfactant (checked by AgNO
3 solu-
tion), then vacuum dried at 60 ° C for 24 h, and kept in a sealed
container until use.
2.3. Preparation of Magnetite Nanoparticles
We have used a classical coprecipitation of Fe
3 +
and Fe
2 +
with
NH
4 OH to prepare the magnetite (Fe
3 O 4 ) nanoparticles. The
method involves coprecipitation from Fe
2 +
and Fe
3 +
aqueous
salt solutions by addition of a base. An initial molar ratio
of Fe
3 +
:Fe 2 +
= 2:1 is needed for the production of Fe
3 O 4 . The
required amount of FeCl
3 and FeCl
4 · 4H 2 O was dissolved in
40 mL of Milli-Q water. The solution was heated at 80 ° C for 1 h
while being stirred. Then, 12 mL of NH
4 OH (33 wt%) was added.
The resulting suspension is vigorously stirred for another 1 h
at the same temperature and then cooled to room temperature.
The precipitated particles were washed fi ve times with water
and ethanol, separated by magnetic decantation, and dried in
oven at 80 ° C.
Macromol. Chem. Phys. 2012, 213, 1815−1832
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1818
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www.mcp-journal.de
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Chemistry and Physics
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2.7. Preparation of Pickering Triple w/o/w/o Emulsions
Triple Pickering w/o/w/o emulsions were simply prepared by
re-homogenize by hand shaking for 10 s the double w/o/w
emulsions described above in the outermost oil (styrene) phase
containing 2 wt% AIBN and a known mass of solid nanoparticles.
Triple emulsions were polymerized in the same way as described
for PolyHIPEs above.
3. Results and Discussion
3.1. Preparation and Characterizations of Pickering
Double w/o/w Emulsion Templates
We have used two types of silica nanoparticles differing
only in their hydrophobicites to stabilize the double
and triple emulsions. Dispersing the hydrophilic silica
particles (80% SiOH) in water using ultrasound pro-
duced stable bluish dispersions at particle concentra-
tions between 1 and 4 wt% with no apparent change
in solution viscosity. For partially hydrophobic silica
particles (50% SiOH) dispersed in HDDA or styrene, the
dispersions were found to have similar viscosity of the
monomer oil up to around 1 wt%. Between 2 and 4 wt%,
powder of each sample was fi xed in the SEM sample holder by
double-sided carbon black sticker. All samples were Pt coated
before examined by SEM. Pose size was measured as an average
of around 30 pores in the sample. Brunauer-Emmett-Teller (BET)
surface areas were measured using a Micromeritics instrument.
Thermogravimetric analysis (TGA) was carried out using
Mettler Toledo TGA (Switzerland) under nitrogen atmosphere
in the temperature range of 25–600 ° C with heating rate of
10 ° C min − 1 .
2.6. Preparation of Pickering polyMIPE/HIPEs
The HIPEs were prepared by adding the required volume of
aqueous phase (containing 0.3
M CaCl 2 · 2H 2 O) or nonaqueous
phase (without salts) to (50:50) styrene:DVB mixture (con-
taining 2 wt% AIBN initiator and a known mass of the desired
solid nano particles (based on monomer oil mass). The mixture
was stirred by a standard magnetic stirrer at fi xed speed of
400 rpm for 10 min. Viscous MIPE and HIPE were obtained and
a drop test was performed to ensure the oil is the continuous
phase. The resulted MIPE or HIPE were transferred into glass
vessels and heated in a water bath at 75 ° C for 24 h to produce
polyMIPE–HIPEs. The formed monolith was released from vessels
by breaking the glass and then purifi ed in acetone using Soxhlet
extraction for 3 h and eventually dried at 100 ° C for 24 h. Porous
microstructure was studied by SEM.
Ta b l e 1 . Recipes for selected Pickering emulsion templates used in the given polymeric systems.
System Phase components
Inner (W
1 ) a) Middle (O 1 ) b) Outer (W 2 ) Outermost
(O
2 )
polyLIPDE-1 1 mg mL
− 1 CdTe,
1 wt% Fe
3 O 4
HDDA, 1 wt% DVB, 3 wt% (50% SiOH)
silica, 2 wt% Irgacure 184,
φ
w = 0.2
2 wt% (80% SiOH) silica,
φ
w/o = 0.2
–
polyLIPDE-2 1 wt% Rhodamine B,
1 wt% Fe
3 O 4
Styrene, 1 wt% DVB, 3 wt%
(50% SiOH) silica, 2 wt% AIBN,
φ
w = 0.2
2 wt% (80% SiOH) silica,
φ
w/o = 0.2
–
polyMIPDE-3 1 wt% Rhodamine B,
1 wt% Fe
3 O 4
Styrene, 1 wt% DVB, 3 wt%
(50% SiOH) silica, 2 wt% AIBN,
φ
w = 0.2
2 wt% (80% SiOH) silica,
φ
w/o = 0.5
–
polyHIPE-4 0.3
M CaCl 2 .2H 2 O Styrene/DVB (50:50) vol%, 20 vol%
toluene, 3 wt% (50% SiOH) silica,
2 wt% AIBN,
φ
w = 0.8
––
polyHIPE-5/6 0.3 M CaCl 2 .2H 2 O Styrene/DVB (50:50) vol%, 20 vol%
toluene, 1.5 (or 3) wt% modifi ed
Laponite, 2 wt% AIBN,
φ
w = 0.8
––
polyHIPE-7 0.3
M CaCl 2 .2H 2 O Styrene/DVB (50:50) vol%,
20 vol% toluene, 3 wt% (50% SiOH)
silica, 2 wt% zinc powder,
2 wt% AIBN,
φ
w = 0.8
––
polyMIPTE-8 1 wt% Rhodamine B,
1 wt% Fe
3 O 4
Styrene, 1 wt% DVB,
3 wt% (50% SiOH) silica,
2 wt% AIBN,
φ
w = 0.2
2 wt% (80% SiOH) silica,
φ
w/o = 0.5
Styrene, 2
wt% (50%
SiOH) silica,
2 wt% AIBN
φ
w/o/w = 0.5
a) (W) refers to the aqueous phase;
b) (O) refers to the oil phase.
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an increase in 50% SiOH particle concentration in oil
resulted in an increase in oil globule size, whereas an
increase in 80% SiOH par
ticles concentration caused a
continuous decrease in oil globule size. The oil globule
sizes of emulsions ranged from 25 to 65 μ m in diameter.
In addition, the oil globule sizes were larger than those
of corresponding simple o/w emulsions stabilized by
80% SiOH silica particles alone, indicative of the ability
of oil globules to entrap a high number of small water
drops and swell as a result. We noticed that no signifi -
cant growth of oil globules occurred as a result of ageing
(35 μ m after preparation and 37 μ m after 3 months) with
no evidence of coalescence of either inner water drops
or outer oil globules indicative of an excellent stability
of these emulsions and the feasibility to be used as tem-
plates for preparing different polymeric materials.
the dispersions became more viscous and bluish in
color. This was attributed to the formation of a three-
dimensional network of the silica agglomerates.
[ 17 ] As
this network forms, the viscosity of the liquid increases
greatly. Pickering double emulsion of the type w/o/w
was fi rst prepared with HDDA or styrene as a monomer
oil phase with 1 wt% DVB as a crosslinker. The inner
water phase, having a volume fraction of
φ
w = 0.2, of
the double emulsion was loaded with 1 wt% Fe
3 O 4 mag-
netite nanoparticles and 1 mg mL
− 1 CdTe quantum dots.
The water drops in the primary w/o emulsion were
stabilized by 3 wt% of partially hydrophobic 50% SiOH
silica nanoparticles (based in oil phase). The volume
fraction of the primary w/o emulsion in the fi nal double
w/o/w emulsion was
φ
w/o = 0.2. The oil globules of the
w/o/w emulsions was stabilized by 2 wt% hydrophilic
80% SiOH silica nanoparticles based in the outer water
phase. We have designated the obtained double emul-
sion as a low-internal-phase double emulsion (LIPDE),
since we used
φ
w/o = 0.2. An optical micrograph of the
resulted double emulsion 2 months after preparation is
presented in Figure 1 a. Oil globules with diameters in
the range of 20–50 μ m can be clearly seen entrapping
a number inner water drops. The emulsion exhibited
creaming without coalescence and remained stable for
more than 2 months. The problems associated with sur-
factant-stabilized double emulsions are numerous and
remain mostly unsolved. The emulsifi er combination
as well as the nature of the oil phase and the volume
ratio of the different media are basic and essential
parameters for double emulsion stability. The resulting
instability of the double emulsions is of crucial impor-
tance since it governs the storage and the potential use
of such systems. In order to study the stability of the
double emulsion templates, a set of LIPDEs has been
prepared and stabilized by different concentrations of
both 50% SiOH silica particles (0.5–4 wt%) in oil and 80%
SiOH silica particles (1–4 wt%) in water. The freshly pre-
pared double emulsions exhibited creaming, without
coalescence, at concentrations of 80% SiOH particles
between 1 and 3 wt% for all concentrations of 50% SiOH
particles, but were stable to creaming at 4 wt% of 80%
SiOH particles above 1 wt% of 50% SiOH particles. This
was thought to be due to a combination of a smaller
globule size and an increase in the viscosity of the con-
tinuous aqueous phase with 80% SiOH particles con-
centration, both of which reduced the rate and extent
of creaming. We can easily control the size and the
number of both the inner water drops and the oil glob-
ules of the resulted double emulsions by adjusting the
concentration of the silica particles used in stabilizing
both water and oil phases. Effects of silica contents on
the average globule diameters immediately after prepa-
ration are given in Figure 1 b where it can be seen that
Figure 1 . (a) An optical image of HDDA double emulsion 2 months
after preparation. The emulsion was stabilized by 3 wt% of 50%
SiOH silica in the HDDA and 2 wt% of 80% SiOH silica in the outer
water phase. The inner water phase was loaded with 1 wt% Fe
3 O 4
and 1 mg mL
− 1 CdTe. (b) Average diameter of HDDA globules in
freshly prepared LIPDEs as a function of concentration of both
50% SiOH silica particles in HDDA (
φ
w = 0.2), in wt% given, and
80% SiOH silica particles in the outer water phase (
φ
w/o = 0.2).
The scale bar is 50 μ m.
Macromol. Chem. Phys. 2012, 213, 1815−1832
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1820
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www.mcp-journal.de
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inside the polymeric microparticles, indicative of the
existence of these drops totally within the polymerized
oil globules. We noticed also the fl uorescence behavior
of the multicores inside these microparticles, which fur-
ther confi rms the integrity of the structure before and
after polymerization. In similar way, we have prepared
another dual function microparticles (polyLIPDE-2) using
styrene as the monomer oil globules, which were poly-
merized thermally at 75 ° C using AIBN for 24 h. We have
examined the morphology of the formed emulsion and
organic–inorganic hybrid microspheres using fl uorescent
and the SEM microscopy as shown in Figure 3 a–c. The
fi rst notice was that the resulted stable w/o/w emulsion
exhibited high loading capacity of the inner water drops
compared with that prepared with HDDA. We found that
the microparticles (25–65 μ m) were also spherical having
multicore compartment and silica aggregates were seen
covering signifi cant parts of their surfaces. The inset of
Figure 3 c shows a closer look at the surface morphology
of one microparticle where the existence of silica aggre-
gates that stabilized the outer oil–water interface of the
precursor w/o/w double emulsion is evident.
Interestingly, different nonspherical morphologies,
namely multi-hollow or porous structure, of the micropar-
ticles (polyMIPDE-3) were observed when the precursor
w/o/w emulsion was prepared with volume fraction of the
primary w/o emulsion of
φ
w/o = 0.5 as depicted in optical
image in Figure 3 d and the SEM images in Figure 3 e–i. It is
obvious from Figure 3 d that the fabricated multiple emul-
sion exhibited a unique morphology where the internal
water drops, entrapped in oil globules, are remarkably
swollen and existed in few numbers. These arrangements
provided a nonspherical character of the oil globules
where a few large water drops can be seen going out of the
oil globules. As seen in Figure 3 e–g, each polymer micro-
particle contains a number of pores having sizes ranging
from 20–40 μ m in diameter, while the microparticles
sizes range was 30–90 μ m. Figure 3 g shows an evidence
of formation of few other shapes of the polymer particles
such as pot and cylindrical-shaped microparticles having
single or multiple large pores, respectively. Close exami-
nations of the pores of the resulted porous beads revealed
that most of these pores were empty and few were fi lled
with apparent silica aggregates as can be seen in the SEM
image shown in Figure 3 h. In addition, silica aggregates
were also seen armoring the surface of polymer beads
indicative of the formation of hybrid porous beads as
evident from Figure 3 i. We have analyzed the area inside
and outside these pores in the polymer beads for iron and
silica using EDX as shown in Figure S2 (Supporting Infor-
mation). Iron was detected only within the pores while
silica was found in larger amount either within the pores
or on the bead surfaces, which was expected during the
formulation of the parent double emulsions. The TGA
3.2. Fabrication of Dual-Function and Macroporous
Polymer Beads Templated from Double w/o/w
Emulsions
The as-prepared LIPDE was photopolymerized for 3.5 h
using 2 wt% Irgacure 184 (based on monomer weight) at
room temperature. PolyLIPDE-1 multi-aqueous core spher-
ical microparticles with size range of 20–50 μ m in diameter
were obtained with magnetic and fl uorescent properties
as can be seen in the optical and fl orescence images pre-
sented in Figure 2 a and b, respectively.
The resulted dual function microparticles showed a
response to an external magnetic fi eld generated by using
a Neodymium magnet as evident by the recorded videos
(see Supporting Information). One can see, from dif-
ferent angles, the internal aqueous drops are embedded
Figure 2 . (a) Optical and (b) fl uorescence images of multi-core
polymer microparticles (polyLIPDE-1) formed after UV polymeri-
zation of a w/o/w double emulsion of HDDA stabilized by 3 wt%
of 50% SiOH silica particles in the inner phase,
φ
w = 0.2, containing
1 mg mL
− 1 CdTe quantum dots and 1 wt% Fe
3 O 4 . The oil globules
were stabilized by 2 wt% of 80% SiOH silica particles,
φ
w/o = 0.2.
The scale bars are 50 μ m.
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Figure 3 . Fluorescent, optical, and the SEM images of emulsions and
hybrid polymeric microparticles with different morphologies. (a) A
fl uorescence image of a double w/o/w emulsion of styrene/DVB sta-
bilized by 3 wt% of 50% SiOH silica particles in the oil phase,
φ
w = 0.2.
The inner water phase contains Rhodamin B and 1 wt% Fe
3 O 4 . The
outer aqueous phase contains 2 wt% of 80% SiOH silica particles,
φ
w/o = 0.2. (b,c) SEM images of multicore polymeric spherical micro-
particles (polyLIPDE-2) formed from a w/o/w double emulsion of
styrene as in “a” after thermal poly merization. (d) An optical image
for a double w/o/w emulsion prepared as in “a” but with volume
fraction of primary emulsion of
φ
w/o = 0.5, showing large inner water
drops and the nonspherical morphology. The arrows indicate pos-
sible colloidosomes formed from the released water drops from
inside the multiple emulsion globules. (e–i) The SEM images of the
porous microparticles (polyMIPDE-3) formed from polymerization
of the precursor double w/o/w emulsion shown in “d.”
range of 430–510 ° C by the thermal decomposition of pol-
ystyrene, indicative of enhancement of thermal stability
of the porous microparticles as a result of incorporation
of silica nanoparticles. The initial weight loss of 5.8% at
25–130 ° C can be attributed to the evaporation of physi-
cally adsorbed water. After the complete decomposition
of polystyrene above 510 ° C, the residual weight of 3.57%
is considered to be the amount of incorporated silica and
magnetite nanoparticles in the polymer microparticles,
which is close to the initial content of silica and magnetite
nanoparticles loaded into the parent double emulsion
(2.3 wt%). The BET surface area of these porous micropar-
ticles was 9.6 m
2 g − 1 , which is in the range of some of the
reported surfactant-stabilized millimetre-sized porous
beads.
[ 21 ] In addition, we noticed the existence of a number
of spherical drops in the formed emulsion that resemble
water colloidosomes, which were indicated by arrows in
the optical image shown in Figure 4 a. These drops showed
an apparent rough surface morphology compared to that
Figure 4 . (a) Optical image of spherical drops (indicated by arrows)
formed in the double w/o/w emulsion shown in Figure 3 d. (b) The
SEM image of one of these drops after polymerization showing
the surface morphology, the inset image is a closer look at the
aggregated silica on the surface.
analysis of polyMIPDE-3 under inert conditions is shown
in Figure S3 (Supporting Information). The TGA revealed
that the main weight loss was recorded in temperature
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of all oil globules. We assume that these possible “water
colloidosomes” have been released from inside the swollen
oil globules where they already been covered with a dense
layers of hydrophobic silica particles used in stabilizing
them inside the oil globules. When they came out to the
outer water phase they did not merge since they covered
with hydrophobic silica and also they became dispersed in
water phase containing hydrophilic silica. Figure 4 b shows
the SEM image of the surface morphology of one of these
drops after polymerization where a dense layer of silica
aggregates (the inset image) was seen covering the par-
ticle. Further investigations of the structure and possible
formation mechanisms of these drops are required.
The mechanism of the formation of such porous mor-
phology shown in Figure 3 d–i is not exactly known. How-
ever, we performed a series of experiments where we
changed the volume fraction of the primary w/o emul-
sion (from
φ
w/o = 0.1–0.6) in the double w/o/w emulsions.
It transpired that the double w/o/w emulsions were cata-
strophically phase inverted between
φ
w/o = 0.5 and
φ
w/o =
0.6 to simple w/o emulsions. This novel phase inversion
has not been explored before for Pickering multiple emul-
sions. Upon polymerization of the inverted emulsion,
formed at
φ
w/o = 0.6, a solid polymer monolith material
was obtained, confi rming that the monomer oil is the
continuous phase of the simple emulsion. In this case, one
can say that the inversion of the w/o/w emulsion leads to
formation of a medium internal phase simple emulsion
(MIPE), which upon polymerization gives polyMIPE. For
the resulted w/o/w emulsion with
φ
w/o = 0.5, the effective
water volume fraction would be 0.6, which is within the
region of the phase inversion process. Based on the early
concept of Ostwald
[ 31 ] who stated that phase inversion
occurs in simple emulsions when the system reaches a
critical close packing condition, Brooks and Richmond
[ 32 ]
argued that, by multiple emulsion formation, the effec-
tive volume fraction of dispersed phase required to induce
inversion can be obtained by enclosing the continuous
phase into drops of the dispersed phase. This occurs as a
result of the coalescence of drops of the dispersed phase.
The effective volume fraction of dispersed phase (made up
of droplets inside globules), thus, increases as long as this
inclusion dominates over loss of such enclosed droplets
by, say, dissolution back into the continuous phase. Inver-
sion is thus governed by the balance between the breakup
of bulk liquid/drops and the coalescence of drops. This
can shed some light about the mechanism of the forma-
tion of such porous microparticles shown in Figure 3 d–i.
We hypothesis that this could be as a result of inclusion
of part of the continuous water phase into the oil glob-
ules, which, in turn, enhances coalescence and eventually
lead to swelling of the inner water droplets of the w/o/w
emulsion during the start of the phase inversion process
as evident from optical image shown in Figure 3 d. It is
worth mentioning that the number of pores in each single
microparticle was few ( ≈ 1–10). In addition, the average
size of these porous microparticles was relatively larger,
by about 25 μ m, than that prepared with low volume
fraction (
φ
w/o = 0.2) of the primary emulsion (Figure 2 and
Figure 3 a–c). This trend was expected since globule sizes
normally increase upon increasing the volume fraction of
the inner phase of silica-stabilized multiple emulsions.
[ 25 ]
Another possible factor that could help increasing the
globule sizes is the coalescence of small water drops
inside the globules, forming larger few ones. Moreover,
based on our previous studies on the release from Pick-
ering multiple emulsions, the swelling of water drops can
also be driven by changing the osmotic pressure across
the middle oil fi lm in a controlled way using simple salt
and glucose solutions.
[ 25 ] The latter factor is currently
under investigation.
3.3. Functionalized Monoliths From Pickering w/o HIPE
Emulsion Templates
The fi ndings described above have motivated us to use
either the fumed silica or Laponite nanoparticles (Figure 5 )
to prepare porous monolith materials with added func-
tionalities from suspension polymerization of Pickering
emulsions. Preliminary experiments were conducted
where we prepared a w/o emulsion having water volume
fraction of
φ
w = 0.8 using 3 wt% of the very hydrophobic
23% SiOH silica nanoparticles initially dispersed in the oil
phase. The monomer oil phase was a mixture of styrene/
DVB in (50:50) volume ratio containing 20 vol% toluene (as
pores generator) and 2 wt% AIBN (based on total oil mass).
The aqueous phase was added dropwise to the oil phase
with constant stirring speed at 400 rpm. The resulted
emulsion type was w/o, confi rmed by the drop test. The
emulsion exhibited fast coalescence, which resulted in a
signifi cant release of most of the initial oil volume after
30 min. After polymerization at 75 ° C for 24 h, a mono-
lith was obtained with millimeter-sized cells that can be
seen by naked eyes as shown in Figure S4a (Supporting
Information). The structure of the formed monolith can be
explained by the signifi cant destabilization of the resulted
w/o emulsion when using very hydrophobic silica parti-
cles. However, stable w/o HIPE containing internal water
volume fraction of 0.8 was successfully prepared with
3 wt% of 50% SiOH silica nanoparticles initially dispersed
in the oil phase. The formed HIPE with 50% SiOH silica
did not show sedimentation or coalescence over a period
of 48 h indicative of the effectiveness of this type of silica
in stabilizing the water drops and the closeness values of
porosity to the initial internal volume fraction (80%). After
polymerization of the stable HIPE, porous polyHIPE-4 with
mostly closed cells was obtained as depicted in Figure 5 a
and f and Figure S4 b,c (Supporting Information). Pore
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evaporation of physically adsorbed water and the decom-
position of the remaining CaCl
2 · 2H 2 O that was initially
used as the inner aqueous phase. The main weight loss
occurred between 400 and 530 ° C, which can be attributed
to the decomposition of the crosslinked polystyrene–DVB
polymer. The residual weight of 18.1% can be attributed to
the amount of silica particles used in the emulsion (3 wt%)
and the amount of remaining weight after the decomposi-
tion of CaCl
2 · 2H 2 O (13.2 wt% relative to the weight of the
formed polyHIPE-4). The results obtained above suggested
that the presence of silica particle reinforced the thermal
stability of the polymer foam compared with polyHIPE
prepared using magnetite nanoparticles.
[ 1 ]
sizes were found to be in the range of 150–500 μ m in
diameter. The cells appeared as fused solid hollow spheres
with few cracks on their surfaces as shown in Figure S4b
(Supporting Information). Close examination of one of
these closed pores revealed that it has a smooth inner sur-
face morphology without the appearance of signifi cant
amount of silica aggregates. As summarized in Table 2 ,
BET surface area of the formed monolith was very low,
0.02 m
2 g − 1 , indicative of the formation of mostly closed
cell structure of the polymer foam. The polyHIPE-4 sample
showed excellent thermal stability as can be seen in the
TGA curve in Figure S3 (Supporting Information). The ini-
tial weight loss of 6.5% until 200 ° C can be attributed to the
Figure 5 . (a) SEM image for polyHIPE-4 prepared from Pickering HIPE stabilized by 3 wt% of 50% SiOH silica and
φ
w = 0.8. (b) polyHIPE-5
prepared with 1.5 wt% organo-modifi ed Laponite and
φ
w = 0.8. (c) polyHIPE-6 prepared with 3 wt% organo-modifi ed laponite and
φ
w = 0.8 showing interconnected pores by arrows. (d) polyHIPEs prepared with 3 wt% organo-modifi ed Laponite with 1 wt% sporopollenin
and
φ
w = 0.8. (e) A close look at cells wall, indicating sporopollenin by arrows. (f) A monolith of polyHIPE-4. (g,h) Monoliths of polyHIPEs
shown in c and d, respectively.
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according to the method described elsewhere.
[ 36 ] Pick-
ering HIPE was initially formed with
φ
w = 0.8 (Figure 6 a
and b) and showed stability against coalescence and sed-
imentation for more than 48 h. Upon polymerization,
polyHIPE-5 having large closed cells were formed when
using 1.5 wt% modifi ed Laponite as stabilizer. Pore sizes
were found to be in the range of 200–1000 μ m and the
formed monolith was very brittle. Open, closed, and
some interconnected cells were observed when the HIPE
was stabilized by 3 wt% of the modifi ed Laponite as can
be seen in Figure 5 c. The resulted polyHIPE-6 exhibited
more mechanical stability (tested by hand) compared to
that obtained with 1.5 wt% Laponite. As noted in Table 2 ,
both polyHIPE-5/6 showed different average pore sizes,
but their surface areas were comparable. To test the fea-
sibility of Laponite modifi cation on emulsion stability,
an attempt was made to prepare w/o HIPE with 3 wt%
unmodifi ed Laponite particles, but complete phase sepa-
ration has occurred immediately since these particles
prefer to be completely wetted by the aqueous phase
(Figure 6 c).
In order to widen the applications of the formed poly-
HIPEs and increase their functionalities, natural sporopol-
lenin empty microcapsules were incorporated in the oil
phase of the HIPE stabilized by 3 wt% modifi ed Laponite
before polymerization. Sporopollenin has long been rec-
ognized as suitable for microencapsulation of a variety of
inorganic and organic materials and was used and treated
as described by Paunov and co-workers.
[ 37 ] Figure 5 d, e, g
It is worth mentioning here that when HIPEs of silica
were prepared using a high-shear rotor/stator homog-
enizer with 18 mm head working at 13 000 rpm for
2 min, catastrophic phase inversion occurred resulted in
the formation of o/w emulsions. This was in agreement
with the previous results obtained with the same type of
silica nanoparticles.
[ 17 ] Therefore, our results suggested
that there is a relation between the amount of energy
introduced during the homogenization step and the
state of phase inversion of the emulsions. For surfactant-
stabilized emulsions, it was suggested that the position
of the phase inversion (locus) was strongly different for
two emulsifi cation routes, namely the direct emulsifi ca-
tion and the wash-out routes.
[ 33 ] For particle-stabilized
emulsions, Ikem et.al.
[ 19 ] and Gurevitch and Silverstein
[ 20 ]
reported that HIPEs can be prepared using a less energetic
emulsifi cation process whereas using too high energy
input resulted in emulsion phase inversion. However, it
was concluded that the presence of mechanical barrier of
solid particles prevent the phase inversion.
[ 19 ] This sug-
gestion seems inadequate as the formation of such a bar-
rier is confi rmed in Pickering emulsions prepared either
with high shear
[ 34 , 35 ] or, as we show in this study, with
low-energy homogenization.
We have extended our investigation to prepare poly-
HIPEs using organo-modifi ed disk-shaped Laponite nan-
oparticles with different concentrations (1.5 and 3 wt%)
as shown in Figures 5 b and c with internal water volume
fraction of
φ
w = 0.8. Laponite clay was modifi ed using CTAB
Table 2. Summary of the properties of some of the fabricated polymeric materials.
System Stabilizer Internal phase
volume fraction
Average size of
microparticles [ μ m]
Average size of
pores [ μ m]
BET surface area
[m
2 g − 1 ]
Morphology
polyLIPDE-1 50/80% SiOH
silica
Φ
w/o (0.2) 20–50 – – multicore/
spherical
polyLIPDE-2 50/80% SiOH
silica
Φ
w/o (0.2) 25–65 – – multicore/
spherical
polyMIPDE-3 50% SiOH silica
Φ
w/o (0.5) 30–90 20–40 9.60 porous non-
spherical
microparticles
polyHIPE-4 50% SiOH silica
Φ
w (0.8) – 150–500 0.02 mostly closed
cells
polyHIPE-5 1.5 wt%
Laponite
Φ
w (0.8) – 200–1000 7.90 large closed cells,
brittle
polyHIPE-6 3 wt% Laponite
Φ
w (0.8) – 50–600 8.45 open-closed-
interconnected
cells
polyHIPE-7 silica, zinc
Φ
w (0.8) – 100–300 – mostly closed
cells
polyMIPTE-8 50/80% SiOH
silica
Φ
w/o/w (0.5) 200–800 3.81 porous
microparticles in
microcapsules
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show the porous structure and the monolith formed by
incorporating sporopollenin with the HIPE where many
of these microcapsules were seen embedded within the
polymer matrix of the resulted PolyHIPE foams, hence
adding more sites capable of further functionalization.
These porous monoliths can fi nd possible applications
in separation, enzyme immobilization, or even in gas
storage.
We have explored the possibility of adding a different
functionality to the polyHIPE via the incorporation of
zinc metal particles in the polymer cells or matrix. This
could provide a kind of conductive porous materials and/
or a possible use of zinc particles as templates for other
applications such as adsorption or catalysis. The w/o HIPE
was prepared in a similar way as in polyHIPE-4 shown in
Figure 5 a but with the inclusion of 2 wt% of zinc metal
particles to the oil phase before the mixing step. The
resulted HIPE was viscous and did not exhibit sedimen-
tation or coalescence for the time of the experiment.
Figure 7 a–d show the obtained zinc functionalized poly-
HIPE-7 with pore sizes ranging from 100–300 μ m, which
is comparable to that obtained for the same monolith
but without zinc particles (polyHIPE-4). As can be seen in
Figure 7 a–c, mostly closed cells were obtained with the
evidence of the existence of zinc metal particles as aggre-
gates inside many cells’ surfaces having sizes in the range
of 2–10 μ m. In addition, the gray color of the obtained
monolith ensuring the incorporation of zinc particles.
3.4. Monoliths From Pickering Triple w/o/w/o Emulsion
Templates
Having prepared polymeric materials ranging from porous
microparticles to solid foams, we explored the possibility
of including both structures in on single system based also
on Pickering emulsions. In this respect, we develop and
demonstrate for the fi rst time a method for fabrication of
novel porous hierarchical structures based on polymeriza-
tion of Pickering w/o/w/o triple emulsions as shown in
Figures 8 and 9 . In this method, the emulsifi cation of a Pick-
ering double emulsion of the type w/o/w in a second oil
phase was successfully achieved, simply by hand shaking.
This allowed us to prepare a kind of stable microcapsules
dispersed in another microcapsules, which we believe is
diffi cult to achieve with surfactant-stabilized emulsions.
The double w/o/w emulsion shown in Figure 3 d was reho-
mogenized by hand shaking for 10 s in styrene containing
2 wt% of 50% silica nanoparticles and 2 wt% AIBN. In the
resulted triple emulsion, the water volume fraction in the
primary w/o emulsion was
φ
w = 0.2, the primary emulsion
volume fraction in the double w/o/w emulsion was
φ
w/o =
0.5 and fi nally the volume fraction of the double emulsion
in the triple w/o/w/o emulsion was
φ
w/o/w = 0.5. An optical
micrograph of the resulted triple w/o/w/o emulsion is
Figure 6 . (a) Optical micrograph for w/o HIPE emulsion stabilized
by 3 wt% organo-modifi ed laponite nanoparticles and
φ
w = 0.8.
(b) Digital image of the emulsion shown in “a” after prepara-
tion showing its stability. (c) Immediate phase separation in an
attempt to prepare w/o emulsion stabilized by 3 wt% of the
unmodifi ed Laponite particles.
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Figure 7 . (a–c) SEM images for polyHIPE-7 prepared from Pickering HIPE stabilized by 3 wt% of 50% SiOH silica nanoparticles and 2 wt% zinc
particles,
φ
w = 0.8. (d) A digital image of the resulted monolith.
Figure 8 . Emulsions and monoliths formed from polymerization of pre-made water-in-styrene-in-water-in-styrene triple Pickering emulsion.
(a,b) Optical and digital images for a triple w/o/w/o emulsion before polymerization, showing its original structure. (c) A monolith formed after
polymerization. (d) A fl uorescent image for the monolith showing the large cells entrapping a number of macroporous microparticles.
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fl uorescent image of one large cell entrapping a number
of macroporous particles, indicative of the formation of a
complex structure.
Figure 9 a–f shows detailed microstructures of the
resulted porous polyMIPTE-8 using SEM. In this unique
structure, we realized three distinct interfaces, which
may rationalize many of the phenomena observed and
the key parameters for excellent stabilization of solid-
stabilized emulsions studied here. From the fi rst look
at the structure shown in Figure 9 a–e, one can realize
the evidence of encapsulation of porous microparticles
(30–90 μ m) inside the large (200–800 μ m) water globules
shown in Figure 8 a. The size range of the outer water glob-
ules was 150–600 μ m and the emulsion exhibited excel-
lent stability to coalescence for several weeks. However,
the emulsion sedimented within 30 min after prepara-
tion, releasing a layer of free oil on top as can be seen in
Figure 8 b. This spontaneous sedimentation enhanced the
packing of the sedimented emulsion; hence, increased
the effective internal volume fraction of the double emul-
sion. The resulted monolith, polymerized medium internal
phase triple emulsion (polyMIPTE-8), was very hard to cut,
without the help of extra tools, indicative of high mechan-
ical strength of the product (Figure 8 c). Figure 8 d shows a
Figure 9 . (a–f) SEM images of the hierarchical porous polyMIPTE-8 structure formed from polymerization of pre-made water-in-styrene-in-
water-in-styrene (w/o/w/o) triple Pickering emulsion stabilized by 3 wt% of 50% SiOH silica nanoparticles in the inner oil phase, and 2 wt%
of the same silica type in the outer oil phase. Oil globules was stabilized by 2 wt% of 80% SiOH silica based in second water phase. The
fi rst water phase contains Rhodamin B and 1 wt% Fe
3 O 4 . The fi rst oil phase contains 1 wt% DVB and 2 wt% AIBN, both based on monomer
mass. The outer second oil phase contains only 2 wt% AIBN in addition to the silica. The volume fraction of the phases was
φ
w = 0.2,
φ
w/o =
0.5, and
φ
w/o/w = 0.5.
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outermost oil phase during the fi nal homogenization
step. It is worth mentioning that the parent w/o/w/o
emulsion underwent sedimentation without coalescence
leaving around 1/3 of the original oil (outermost phase)
in top of the emulsion (Figure 8 b). Therefore, the effective
volume fraction of w/o/w in the polymerized triple emul-
sion was
φ
w/o/w = 0.6, that is, MIPTE was actually formed
by spontaneous sedimentation. Our results shown above
suggested that we can control the fi nal structure of the
resulted materials by polymerize the fi rst/second water
(oil) phases in the four phases triple emulsions (either
w/o/w/o or o/w/o/w) to fabricate a wide range of dif-
ferent structures such as microcapsules in microcapsules,
microcapsules in large hydrogels, porous materials, and
others. Such study is currently underway.
To test our hypotheses, we prepared a triple w/o/w/o
emulsion in similar way as described above but lowering
both the volume fraction of the primary w/o emulsion
in the second water phase to
φ
w/o = 0.2 and that of the
double emulsion (
φ
w/o/w = 0.3). Figure 10 a shows a fl u-
orescent image of the obtained stable w/o/w/o triple
emulsion. The outer water globules showed diameters in
the range of 100–400 μ m and encapsulated many spher-
ical double w/o/w emulsion globules. It is obvious from
this fl uorescent image that the encapsulated double
emulsion is spherical and showed remarkable structure
differences form that shown in Figure 3 d, where non-
spherical structure was evident. Therefore, selecting the
desired volume fraction of the primary w/o emulsion in
the double emulsion is crucial for the structure of the
resulted emulsions as well as for the porous materials.
The parent triple emulsion was monitored for several
weeks and showed excellent stability against coales-
cence. Figure 10 b shows digital images of the obtained
monolith after polymerization and it fl oated in pure eth-
anol indicative of low density of the monolith and the
formation of closed cells.
The microstructure of the triple emulsion after poly-
merization was studied by SEM and is shown in Figure
11 a and b. Large closed cells were observed (100–500 μ m)
with few spherical microcapsules seen entrapped inside.
Smaller pores were also seen in some of the large cells.
It is obvious that different morphologies, size range, and
number of encapsulated microparticles were achieved
by changing the
φ
w/o of the primary w/o simple emul-
sion. It can be noticed from Figure 11 b that the number
of microparticles that remained inside the large cells are
smaller than that of the triple emulsion prepared with
higher
φ
w/o/w (Figure 9 b), which we expected as depicted
in Figure 1 b and from the previous studies on Pickering
systems.
[ 25 ] These new triple emulsion systems could be
used in potential applications either as nonpolymerizable
emulsions in their liquid state or as polymerized solid
triple emulsions.
(cells), which themselves were dispersed in the outer-
most oil phase. We can see also few open and mainly
closed cells at different depths in the outermost poly mer
matrix. Both open and closed cells have a number of
small pores having sizes of 10–40 μ m (Figure 9 b and e).
The closed cells are similar to colloidosomes where the
shell is composed of densely packed silica nanoparti-
cles. In addition, this polymerized (frozen) structure is
a replica of the parent w/o/w/o emulsion shown in the
optical image in Figure 8 a indicative of excellent coa-
lescence stability of the parent emulsion even during
polymerization at 75 ° C for 24 h. The fi rst interface rep-
resents the contact between the fi rst water phase and
the fi rst oil phase, containing the hydrophobic silica, in
the multi-hollow porous microparticles that formed from
polymerization of a double w/o/w emulsion just before
the phase inversion (Figure 3 e–i). This interface can be
represented as the closed pores formed in the polymer-
ized double w/o/w emulsion. We hypothesized that there
were no more smaller water drops remained inside that
double emulsion as all the smaller drops has merged into
larger ones to form these closed large pores. The second
interface is the contact between the fi rst oil phase and
the second water phase where the double emulsion is
encapsulated in large water globules. We can see clearly
how the excess of hydrophilic 80% SiOH silica nanopar-
ticles, based on the second water phase, form aggregates
inside the large water cells that surround the porous
microparticles as shown in Figure 9 c and d. This “frozen”
structure gave us a good indication about the steric stabi-
lization of these microparticles through the formation of
3D network of the stabilizing silica. The third interfaces is
formed between the large water globules and the outer-
most oil continuous phase, again containing hydrophobic
silica, where one can see the rough interface (shell)
having average thickness of 1–1.5 μ m, estimated from
SEM. This interface separates hydrophilic and hydro-
phobic water and oil phases, respectively. Therefore, both
types of silica nanoparticles might have been contrib-
uted in forming such a thick shell. For Pickering simple
o/w emulsions studied by low-temperature fi eld-emis-
sion electron microscopy (LTFE–SEM), it was concluded
that the thickness of the interface was around 0.5 μ m
for oil drops stabilized by similar hydrophilic silica used
in our study.
[ 34 ] This also supports the results obtained
from the ellipsometric study of silica nanoparticles at a
planar oil–water interface where it was concluded that a
multilayer of silica particles is evident.
[ 35 ] Interestingly,
the third interface was generated by low energy homog-
enization, namely hand shaking for 10 s. We noticed the
existence of smaller pores having diameters between
10–40 μ m located among the large cells (Figure 9 f). These
pores might have been formed from the excess of small
water drops that dispersed and silica-stabilized in the
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that tailor-made block copolymers are by far the most
effi cient stabilizers compared with low-molecular-weight
surfactants.
[ 38 ] Polymerizable nonaqueous emulsions are
of interest as nanoreactor systems for the polymerization
of water-sensitive monomers or catalysts. Novel Pickering
simple and multiple emulsions containing ionic liquids
with aqueous or nonaqueous systems have been introduced
in previous report.
[ 39 ] Moreover, ionic liquids were used
as internal phases in nonaqueous open pores polyHIPEs
using surfactants as stabilizers.
[ 40 ] Unlike all the above
studies, which utilize either surfactants or block copoly-
mers as stabilizers, we have successfully prepared novel
polymerizable simple (o/o) and double (o/o/o) nonaqueous
emulsions stabilized solely by solid nanoparticles, which
upon polymerization resulted in polyHIPNAE (polymerized
high-internal-phase nonaqueous emulsion) or PolyMIP-
NADE (polymerized medium-internal-phase nonaqueous
double emulsion), respectively. Figure 12 a and b shows an
example of polyHIPNAE formed from polymerization of a
3.5. Monoliths from Non-Aqueous Pickering
Emulsion Templates
Water-based systems, containing only oil and water
phases, are by far the most studied in the literature as well
for emulsion, mini-emulsion and dispersion polymeriza-
tion processes. In comparison with aqueous emulsions,
oil-in-oil (o/o) emulsions also called nonaqueous emul-
sions have had relatively scarce attention in literature.
[ 38 ]
For this type of emulsions, almost exclusively steric sta-
bilization is adopted. For such emulsions, it was shown
Figure 10 . (a) A fl uorescent micrograph of a freshly prepared
triple w/o/w/o water-in-styrene-in-water-in-styrene Pickering
emulsion showing its original structure before polymerization.
The emulsion was stabilized by by 3 wt% of 50% SiOH silica nano-
particles in the inner oil phase and 2 wt% of the same silica in the
outer oil phase. Oil globules was stabilized by 2 wt% of 80% SiOH
silica based in second water phase. The fi rst water phase contains
Rhodamin B and 1 wt% Fe
3 O 4 . The fi rst oil phase contains 1 wt%
DVB and 2 wt% AIBN, both based on a monomer mass. The outer
second oil phase contains only 2 wt% AIBN. The volume fraction
of the phases were,
φ
w = 0.2,
φ
w/o = 0.2, and
φ
w/o/w = 0.3. (b) Dig-
ital images for the resulted monolith after polymerization of the
triple emulsion showing its low density when put on ethanol.
Figure 11 . (a,b) SEM images of the obtained Pickering polyMIPTE
after polymerization of the triple w/o/w/o emulsion represented
in Figure 10 a.
Macromol. Chem. Phys. 2012, 213, 1815−1832
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aqueous w/o/w emulsion shown in Figure 3 d. As can be
clearly seen in Figure 12 c and d, nearly spherical micro-
particles having size range of 25–100 μ m, which covered
with dense silica aggregates were formed as a result of
polymerization of the fa/s/fa double emulsion. Very few
microparticles with small pores with sizes of 10–25 μ m
were observed as shown in Figure 12 d. This morphology
is somewhat similar to that of microparticles obtained
after polymerization of w/o/w emulsion as shown in
Figure 3 c. These novel nonaqueous simple and double
Pickering emulsions can serve as versatile templates
for potential applications in synthesis of new types of
hybrid organic–inorganic polymer nano- or micropar-
ticles and macroporous polyHIPNAE-MIPNADE non-
aqueous foams.
4. Conclusion
We have fabricated various materials based on aqueous
and nonaqueous stable simple, double, and triple Pickering
emulsions systems stabilized solely by DCDMS-modifi ed
amorphous silica nanoparticles of well-controlled surface
hydrophobicities or organo-modifi ed laponite clay nano-
particles. The different types of emulsions and polymer
pre-made nonaqueous formamide-in-styrene (fa/s) emul-
sion with an internal phase volume fraction of
φ
f = 0.8.
The emulsion was stabilized by 3 wt% of 50% SiOH silica
nanoparticles, based in styrene. Mostly closed cells with
a size range of 50–500 μ m were formed. We noticed that
most cells contain silica aggregates inside their internal
interfaces as shown in Figure 12 a and b. These signifi -
cant silica aggregates did not appear in the corresponding
aqueous polyHIPE-4 system shown in Figure 5 a, indicative
of the occurrence of most of the silica particles outside the
polymer matrix.
In line with the results obtained above, magnetic and
fl uorescent polyMIPNADE was also prepared as shown
in Figure 12 c and d. The double nonaqueous formamide-
in-styrene-in-formamide (fa/s/fa) emulsion was stabi-
lized by 2 wt% of 50% SiOH silica particles in the styrene
phase,
φ
f = 0.2 while the styrene globules were stabilized
by 2 wt% of the same type of silica particles in outer for-
mamide phase,
φ
f/s = 0.4. We selected this value of volume
fraction of the primary fa/s emulsion in the formed
fa/s/fa double emulsion since it was catastrophically
phase inverted to simple fa/s type when we prepared it
with
φ
f /s = 0.5. Therefore, the nonaqueous fa/s/fa double
emulsion was catastrophically phase inverted at a lower
value compared to that obtained in the corresponding
Figure 12 . (a,b) SEM images of Pickering polyHIPNAE formed from polymerization of a pre-made nonaqueous (o/o) formamide-in-styrene
emulsion,
φ
f = 0.8, stabilized by 3 wt% of 50% SiOH silica nanoparticles dispersed initially in styrene. (c,d) SEM images of polyMIPNADE
microparticles formed from polymerization of a pre-made formamide-in-styrene-in-formamide double emulsion stabilized by 2 wt% of
50% SiOH silica particles in the styrene phase,
φ
f = 0.2. The inner formamide phase contains Rhodamin B and 1 wt% Fe
3 O 4 . The outer forma-
mide phase contains 2 wt% of the same type of silica particles,
φ
f/s = 0.4.
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materials described here open up several intriguing ave-
nues of research. We have prepared dual function mag-
netic and fl uorescent hybrid microparticles in different
morphologies, namely multi-core and multi-hollow porous
microparticles, via Pickering suspension polymerization of
double w/o/w emulsions. We have shown that micropar-
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used as microreactors or fi ne templates for synthesizing
advanced materials and therefore adding a new dimension
to Pickering multiple emulsions. We presented an example
of suspension polymerization of novel Pickering simple
nonaqueous o/o and double o/o/o emulsions, which
resulted in the formation of polyHIPNAE and polyMEPNAE,
respectively. Although we present a proof of concept, this
approach could be applied using different types of oil or
ionic liquid phases.
Supporting Information
Supporting Information is available from the Wiley Online
Library or from the author.
Acknowledgements : This project was supported by King Saud
University, deanship of scientifi c research, college of science,
research center. The author wishes to thank Wacker Chemie
(Germany) for kindly providing the silica nanoparticles.
Received: April 3, 2012 ; Revised: June 4, 2012; Published online:
July 27, 2012; DOI: 10.1002/macp.201200172
Keywords: hierarchical structures; macroporous polymers;
nanoparticles; nonaqueous emulsions; Pickering emulsions
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A. K. F. Dyab
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