High internal phase emulsion gels (HIPE-gels) from polymer dispersions reinforced with quadruple hydrogen bond functionality

Article abstract of DOI:10.1039/c2cc16670d

A convenient route to organogels templated by high internal phase emulsions has been developed. Key is the use of a waterborne polymer latex loaded with a multiple hydrogen bond (MHB) functionality that becomes disentangled and transfers across the oil-wa

Full text of DOI:10.1039/c2cc16670d

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High internal phase emulsion gels (HIPE-gels) from polymer dispersions
reinforced with quadruple hydrogen bond functionalityw
Yunhua Chen, Nicholas Ballard, Florence Gayet and Stefan A. F. Bon*
Received 27th October 2011, Accepted 28th November 2011
DOI: 10.1039/c2cc16670d
A convenient route to organogels templated by high internal
phase emulsions has been developed. Key is the use of a waterborne
polymer latex loaded with a multiple hydrogen bond (MHB)
functionality that becomes disentangled and transfers across the
oil–water interface forming a gel network in the oil phase via
hydrogen bond interactions.
to prevent disintegration via swelling once assembled at a
liquid–liquid interface in which the organic phase was a
solvent for the polymer, as this ultimately resulted in a loss
of Pickering stabilization.8 Zhang and Chen10 showed that
non-crosslinked polymer particles could be used in the pre-
paration of poly(HIPE)s if upon exposure to the organic phase
the swollen and disentangled individual polymer chains acted
as an amphiphilic polymeric stabilizer. An interesting develop-
ment was recently reported by Ngai and coworkers11 who
employed poly(methacrylic acid)-based microgel particles to
prepare a HIPE without liquid-like flow behavior, in other
words a colloidal gel based HIPE. Relatively large amounts of
microgel particles are required (up to 5 wt%) in order to
establish the fractal-like gel structure.
One speaks of a high internal phase emulsion (HIPE), for
example water droplets dispersed in oil, when the total volume
fraction of droplets exceeds the value of 74% by volume
(maximum packing fraction of monodisperse hard spheres).
HIPEs are widely used in gels and creams in cosmetic products,
petroleum gels for safety fuels, and the food and bio-related
industries.1 HIPEs are commonly stabilized by molecular
surfactants, which include amphiphilic block copolymers.3 In
recent years, HIPEs stabilized solely by particles that adhere to
the liquid–liquid interface, also known as Pickering HIPEs,
have attracted a surge of interest. A variety of particles serving
as stabilizer have been reported including silica,4 titanium
oxide5 and organic microgel particles.6 To obtain a stable
Pickering HIPE, the wettability of particles is very important,
which can also usually determine the type of emulsion to be
water-in-oil or vice versa. Generally, relatively hydrophobic
particles prefer to stabilize water-in-oil (w/o) emulsions
whilst hydrophilic particles tend to stabilize oil-in-water (o/w)
emulsions.7
Here we report
a facile strategy to produce strong
HIPE-gels, more specifically HIPE-organogels. The approach
involves the use of waterborne polymer colloids which are
loaded with multiple hydrogen bond (MHB) arrays, in our
case 2-ureido-4[1H] pyrimidinone (UPy) which can undergo
very strong and highly directional self-complementary quad-
ruple hydrogen bond interactions through dimerization, as
first shown by Sijbesma, Meijer et al.12 These were introduced
during the synthesis of the polymer latex by emulsion poly-
merization using a functional comonomer (see ESIw for
synthesis and Fig. 1). The latex particles are triggered to
adhere to the oil–water interface upon addition of salt, which
Solidification of the continuous phase of a HIPE in order to
prepare a porous monolith can be done through polymeriza-
tion, the product then is referred to as a poly(HIPE).2 We
pioneered the fabrication of particle-stabilized poly(HIPE)s
using polymer microgels, hereby employing sedimentation
as a tool to concentrate an originally medium internal phase
Pickering emulsion.8 Bismarck and co-workers9 reported on
the successful preparation of hybrid poly(HIPE)s stabilized by
titania and silica nanoparticles with internal phase volumes of
up to 90%. In our previous study we stated that it was essential
that the polymer colloids used as stabilizer were cross-linked
Department of Chemistry, University of Warwick, Coventry CV4 7AL,
UK. E-mail: s.bon@warwick.ac.uk; Web: www.bonlab.info
w Electronic supplementary information (ESI) available: Details of experi-
mental preparation and characterization of colloidal particles, Rhodamine
B piperazine methacrylamide, UPy monomer, HIPE gels and dye release
profiles of HIPE-organogels. See DOI: 10.1039/c2cc16670d
Fig. 1 Schematic of the method for preparation of HIPE-organogels.
Chem. Commun., 2012, 48, 1117–1119 1117
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compresses their electrostatic double layer. Upon contact with
the organic phase (which is by necessity a good solvent for the
polymer) the particles can swell and the individual polymer
chains can disentangle and diffuse further into the organic
phase. The pendant UPy groups in the polymer chains create a
physically crosslinked and self-supporting gel through self-
complementary quadruple hydrogen bond interactions.
The polymer colloids functionalized with UPy moieties were
prepared by soap-free emulsion polymerization, a well docu-
mented method for the synthesis of monodisperse polymeric
nanoparticles. A 50 : 50 molar ratio (44 : 56 wt% ratio) of
methyl methacrylate (MMA) and butyl acrylate (BA) was used
as a monomer mixture with approximately 1.2 mol% (based on
total monomer) of UPy functionalized methacrylate (ESIw).
Particles were fluorescently labelled using small quantities of
Rhodamine B piperazine methacrylamide. This fluorescent
monomer was made from the nucleophilic substitution reaction
between Rhodamine B piperazine and methacryloyl chloride
in 80% yield (Fig. 2). Full experimental details and characteri-
sation are provided in the ESI.w
A reference latex which had carboxylic acid functionality
was prepared using methacrylic acid (MAA, 4.6 mol%) as a
functional monomer, instead of the UPy monomer. Mono-
disperse particle size distributions were obtained for both the
UPy and the reference carboxylic acid functionalized latex
particles (dispersity of 0.025 and 0.012, respectively) with
z-average diameters of 223 nm and 235 nm, respectively, as
measured by dynamic light scattering (ESIw, Fig. S1 and S2).
Our starting point for fabricating the HIPE-organogels is a
0.1 wt% waterborne dispersion of our polymer colloids (0.8 mL).
To this we add 50 mg (1.05 mol dmꢀ3) of solid NaCl after
which 0.2 mL of toluene was added. This amount of NaCl was
required in order to compress the double layer sufficiently
and trigger particle adhesion to the toluene–water interface
(the dependency of the z potential of PMMA–PBA–PUPy
particles on sodium chloride concentrations at pH = 7 is
shown in Fig. S3, ESIw). At this salt concentration coagulation
of the latex particles is relatively fast (the half-life time for fast
coagulation is B2 seconds according to the equation developed
by Smoluchowski) so the subsequent emulsification step
should be conducted directly after mixing. Next the mixture
was shaken vigorously by hand for just 5 s. Alternatively the
HIPE organogel can be easily made by placing on a vortex
mixer (Fisherbrand, Whirlimixer) at 2800 rpm for 10 seconds
(see ESIw for video). High internal phase emulsion self-standing
organogels were formed immediately under these relatively
low-shear conditions. Stable HIPE-organogels were made
using particle weight fractions of 5% down to an impressive
0.1% with internal phase volumes of up to 85 vol%. An example
of one of the HIPE-organogels in which the toluene contin-
uous phase is tagged with hostasol dye is shown in Fig. 3a.
Fig. 3 (a) Photograph of an inverted vial containing a typical HIPE-
organogel with aqueous dispersed phase volume of 80% (toluene
containing a hostasol fluorescent dye as continuous phase) stabilized
using UPy functionalized soft latex particles (0.1 wt%), (b) confocal
image of the same fluorescently tagged HIPE-gel, scale bar: 50 mm.
Confocal laser scanning microscopy (CLSM) was used to
investigate the porous microstructure of our HIPE gels.
Fig. 3b confirms that water-in-oil HIPE gels were formed,
with toluene (hostasol dye labelled) as the continuous organogel
phase with droplets of water entrapped. The size of the water
droplets varied from several to tens of micrometres. CLSM
analysis of our rhodamine tagged polymer colloids indicates
that the polymer chains and thus the supramolecular hydrogen
bond reinforced gel network distributed themselves through-
out the continuous toluene phase, proving that indeed success-
ful phase transfer and disentanglement of the polymer chains
have taken place. Note however that there is a logical enrich-
ment at the w/o interface (Fig. 4a). Exposure of the HIPE gel
to the open atmosphere for 30 min led to preferential evapora-
tion of the toluene phase, hereby deforming the structure into
a polyhedral morphology, more commonly observed in foams
(Fig. 4b). The continuous cellular network is preserved and no
fusion of neighbouring cells that contained water was observed.
Prolonged drying periods throughout which the majority of
both liquid phases evaporated led to relaxation of the poly-
hedral structure back into a more spherical porous morpho-
logy. Eventually upon drying some macroscopic buckling of
the HIPE-gel monoliths was observed. The structure remained
self-supporting.
Scanning electron microscopy analysis confirms the porous
morphology of the HIPE organogel (Fig. S4, ESIw). No
individual particles can be identified in HIPE gels after drying
at room temperature, which confirms the theory that the
particles (at least partially) film forms upon exposure to
toluene. Increasing the concentration of the particle dispersion
from 0.1 wt% up to 5 wt% showed no marked change in the
average pore size and pore size distribution, presumably due to
the low shear force employed limiting the droplet size (Fig. S5,
ESIw).
The quadruple hydrogen bond network formed by UPy
arrays reinforces the physical properties of the HIPE gel
scaffolds and allows weight fractions down to 0.1% of colloidal
particles in the aqueous phase to support the solid-like struc-
tures of the HIPE gel. To test the strength of the HIPE gel
network reinforced by the UPy arrays, a HIPE organogel was
made with enclosed water droplets containing 1 M NaCl and
Rhodamine B as a dye. Submersion of this monolith into pure
water created a high osmotic pressure difference between the
dye labelled internal water phase and the outer water bulk phase.
Fig. 2 Synthesis of Rhodamine B piperazine methacrylamide from
Rhodamine B piperazine and methacryloyl chloride.
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In conclusion, HIPE gels were prepared by simple hand-
shaking of a water–toluene mixture, the former containing
small quantities (down to 0.1 wt%) of UPy functionalized
(1.2 mol%) PMMA-co-PBA latex particles. Up to 85 vol% of
water droplets could be dispersed into a continuous organic
gel phase composed of toluene and unravelled polymer chains,
originating from the polymer latex particles. The gel was
physically reinforced through quadruple hydrogen bond inter-
action of the UPy moieties. The HIPE-gels were able to
withstand a large osmotic pressure gradient, meaning that
individual cells (water droplets) were kept intact and thus
compartmentalized. Upon removal of all volatiles self-supporting
cellular monoliths were obtained. We believe that our approach
to the fabrication of the HIPE gels opens routes for potential
applications in medical injectable gels, pressure sensors, and
other interesting multiphase soft matter materials.
Fig. 4 Confocal images of (a) the HIPE gel and (b) the HIPE gel after
30 min of preparation, excited by laser with a wavelength of 480 nm
for the rhodamine labeled UPy functionalized particles, both scale
bars: 50 mm.
The authors would like to thank the China Scholarship
Council (Y.C.) and Unilever (N.B.) for partial funding. Part of
the equipment used in this research was obtained through
Birmingham Science City: Innovative Uses for Advanced
Materials in the Modern World (West Midlands Centre for
Advanced Materials Project 2), with support from Advantage
West Midlands (AWM) and part funded by the European
Regional Development Fund (ERDF).
Fig. 5 Storage modulus G0 (closed symbol) and loss modulus G00
(open symbol) of HIPE-organogels (0.1 wt%) as a function of
frequency in the oscillatory frequency sweep.
Notes and references
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No rupture of the closed cells of the organogel was observed,
as this would lead to a rapid release of the dye into the
continuous phase. Dye release was slow and occurred through
diffusion, as confirmed by UV-Vis measurements of the con-
tinuous bulk water phase (Fig. S6, ESIw). This opens an
opportunity to employ our organogels as potential controlled
release vehicles, pressure sensors, and compartmentalized
chemical reaction microenvironments.
We investigated the rheological properties of the HIPE-
organogels. Fig. 5 shows the dependence of the storage and
loss moduli of our HIPE-organogels (0.1 wt% of particles
used) on the frequency in the oscillatory frequency sweep.
Throughout the entire measured frequency range, the storage
modulus (G0) is considerably higher than the loss modulus
(G00), which indicates that stable gels were formed. This is also
consistent with the observation of Fig. 3a that the gels cannot
flow even after inverting the vial.
To shine a light on the role and the reinforcement of the
UPy groups we compared the rheological properties of HIPEs
formed by PMMA–PBA–PMAA latex particles which have
carboxylic acid providing double hydrogen bonding inter-
actions. Upon loading 0.1 wt% of these particles, no HIPE-
organogel can be prepared as the system can undergo flow
easily. Only upon increasing the particle concentration by a
factor of 10 up to 1 wt%, and also noting that the molar
amount of carboxylic acid groups is fivefold with respect to
UPy, can a HIPE-organogel be formed exhibiting comparable
rheological performance (Fig. S7, ESIw).
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J. H. Hirschberg, R. F. Lange, J. K. Lowe and E. W. Meijer,
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