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Characterization Techniques for PolyHIPE Polymers

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Added on  2019/10/18

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PolyHIPE polymers, excluding the first phase of silica PHP, were washed using iso-propanol and double distilled water to remove surfactants. The samples were then analyzed using Scanning Electron Microscopy (SEM), which provided information on topographical features, morphology, and compositional differences. Sample preparation involved drying in a vacuum oven at 60°C for 4 hours and mounting the sample on an aluminium stub with carbon cement or copper tape. To achieve better clarity, some samples were coated with a thin layer of gold using a sputter coater. The surface area and pore size distribution of PHPs were analyzed using Coulter SA 3100 analyser, which used Gas Sorption technique to obtain total surface area and pore size distributions. Adsorption and desorption isotherm data was obtained volumetrically using the static fully equilibrated method.

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PolyHIPE polymer (PHP)
PolyHIPE polymer is a highly porous material that can be easily prepared by
polymerisation of the monomeric continuous phase of a high internal phase emulsion
(HIPE). These polymeric foams were coined the generic name PolyHIPE by researchers
at Unilever Research Port Sunlight Laboratory, UK (Barby and Haq, 1982).
The process of preparing polyHIPE polymer is quite simple. Droplets of aqueous phase
are added to the mixture of oil phase, consisting of monomer, crosslinker and surfactant
while mixing. Mixing is needed to break up large droplets. Mixing is further continued
after addition of the internal phase to get a smaller pore volume. The emulsion is then
cured in the oven; the resulting porous material was then washed in the soxhlet, and dried.
Overview of High Internal Phase Emulsions (HIPE) and
PolyHIPE Polymer (PHP)
As defined by Lissant (1974), high internal-phase ratio emulsions are those with more
than 74% internal phase volume, Φ. The Φ value of 74% represents the maximum
volume that can be occupied by uniform non-deformable spheres when packed in the
most efficient manner. These days, also known as high internal phase emulsion (HIPE),
the value of Φ can be as high as 99%. At this high value of Φ, closely packed
monodispersed spheres is no longer physically possible in internal or dispersed phase.
Thus, at this high internal phase volume, the shape is deformed into non-spherical
polyhedral droplets which appeared to be monodispersed in size, as quoted by Cameron
and Sherrington (1996) on the work done by Lissant. The droplets have relatively large
contact area, are surrounded by continuous phase and stabilized by thin surfactant films.
The continuous phase, which generally constitutes less than 26% of the final volume of
HIPE, normally contains monomer, cross-linking agent, surfactant and oil-phase initiator.
Due to HIPE unique characteristics, HIPE have been used for many years in many
applications such as food preparation, cosmetics, oil recovery and many others. One of
the most important applications of HIPE is the ability to be used as template systems for
the synthesis of a range of polymeric materials.
The HIPE processing can be divided into two stages as discussed by Akay et al.
(2005). During the first stage of the processing, the dispersed (aqueous) phase is
continuously dosed into a mixing vessel containing the continuous phase (oil phase). Care
is taken in minimizing the jet mixing of the two phases since addition of aqueous phase
alone creates mixing. There is a reduction in the droplet size of the aqueous phase due to
the rotation of the impeller during dosing. In the second stage of processing, further
mixing is carried out upon completion of dosing in order to reduce aqueous phase droplet
size (i.e. size of pores after polymerization) and to obtain HIPE of narrow droplet size
distribution. No additional mixing (homogenization) stage is needed for the case of a very
low dosing rate.
The relative dosing rate having a dimension of deformation rate is used to characterize the
aqueous phase dosing rate.
In the case of very large relative dosing rate and small mixing rate, instead of HIPE
formation, dilute (low) internal oil-in-water (O\W) emulsion is formed. When HIPE is
stable, polymerization without phase separation will take place.

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The monomer-based HIPE can be polymerised to obtain micro-porous polyHIPE
polymers (PHP). Barby and Haq (1982) discovered that open-cell HIPE-based polymer
can be polymerised by using relatively simple low HLB (Hydrophile-Lipophile Balance)
surfactant and HIPEs composing of styrene-divinylbenzene (DVB) as shown
schematically in Figure 2.1.
The internal (aqueous) phase used in preparing the HIPE can be easily and rapidly
removed from the PHP to produce a highly porous material with very low density.
Another important characteristic of PHP is that it can be specifically tailor-made
according to its application. For example, PHP can be produced with specific
interconnect size, d, e.g. as d of 0<d/D<0.5, D is pore size. Moreover, a highly porous
interconnected monolithic material of PHP with a well-defined and uniform
microstructure of very low dry density can also be produced. The structure of PHP is
shown in Figure 2.2 (adapted from (Akay et al., 2005b)). The materials can be produced
over a wide range of pore size, D, (0.5 μm<D<5000 μm), based on the conditions of the
starting emulsions. PHP having pore size greater than 200 μm can be produced through
a coalescence polymerisation technique (Akay et al., 2005a; Akay et al., 2002).
Furthermore, the porosity of PHP surface can be controlled by varying the surface
chemistry of materials against which the HIPEs are polymerised. This allows the
production of asymmetric materials.
Due to PHPs unique structures and properties, PHPs have made ways in diverse fields of
intensified processes, especially in biology, where their applications include the
discovery of a number of of size-dependent phenomena in bioprocesses (Akay, 2006a;
Akay et al., 2004; Akay et al., 2002), tissue engineering (Akay, 2005; Akay et al., 2004;
Bokhari, 2003; Bokhari et al., 2003; Umez-Eronini, 2003; Byron, 2000) and other
intensified bioprocesses (Akay, 2006b; Akay, 2005).
Both hydrophilic and hydrophobic PHPs have been utilised in several other
applications such as intensification demulsification processes (Akay et al., 2005d;
Noor et al., 2005; Vickers, 2001), gas liquid separation (Akay et al., 2005b;
Calkan et al., 2005; Dogru and Akay, 2004), and metal ion removal in water
treatment (Katsoyiannis, 2002; Wakeman et al., 1998). PHPs have also been
applied in other intensified processes , for instance, foams and filtration
fabrications (Tai et al., 2001; Walsh, 1996; Bhumgara, 1995a; Bhumgara, 1995b),
metal plating (Akay et al., 2005b; Calkan et al., 2005; Brown et al., 1999;
Sotiropoulos et al., 1998), and organic chemistry processes (Moine et al., 2003).
As listed by (Noor, 2006) and discussed by (Akay et al., 2005b), for PHP to be utilized in
the applications mentioned above, the preparation and modification of PHP materials has
to meet the following criteria:
i) able to produce PHP with a required internal architecture or morphology, for instance,
specific pore/interconnect sizes and the presence of arterial channels;
ii) able to form monolithic structures;
iii) able to chemically/biologically functionalise or optimise the PHP for a specific
application;
iv) and ensure the sustainable production and modification of PHP.
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PolyHIPE Polymer Morphology
PHPs are being widely utilised in various applications based on each specific required
property of the materials, for instance, morphology, physical, mechanical, or thermal
properties. Therefore, control over PHP properties is essential to ensure viability of
application. Having several advantages of accessibility of the pores, controllability of
internal architecture, such as the pore and interconnect structures, versatility of
fabrication and chemical modification of the walls, PHP is a high potential material.
Another advantage of PHP is that it can also be fabricated from a very thin membrane to a
very large well-organised monolithic article.
The typical structure of PHP is an open cellular structure of spherical cavities. These
cavities are known as voids or pores having windows for interconnecting the pores. This
phenomenon is possible due to the trapped internal (aqueous) phase inside the continuous
phase during the polymerisation process. Generally, the stability level of the prepared
HIPE has a direct relation to the pore size of PHP. In a system with high emulsion
stability, a smaller droplet size will be produced due to the lower interfacial tension which
allows larger interfacial area. In a less stable emulsion system, emulsion droplets tend to
coalesce and lead to a larger cell once the polymer is formed. There are several factors
that govern the stability of HIPEs. Similar to other emulsions, HIPE stability is highly
dependent on the preparation parameters, which are shear stress (mixing speed) and
mixing time. In order to produce a more stable emulsion, high mixing speed is needed to
uniformly break the emulsions into small droplets. Similar effect was also observed by
(Walsh, 1996) when a mixing time was increased. The study showed that there was a
reduction in the size of water cavity and an increase in the number of windows leading to
production of more micro-size open structure material with the increase of mixing time.
There are some other less apparent parameters that can have influence on PHP pore
size. Williams et al.(1990) discovered that the ratio of styrene/DVB (divinyl benzene)
used in preparation of HIPE play an important role in the formation of PHP. It was
observed that the emulsion with DVB alone can easily and more uniformly get blended
compared to the emulsion with styrene alone. Thus, increasing the ratio of styrene/DVB
in a HIPE led to the increase in emulsion stability, leading to the decrease in pore size
diameter from 15 to 5μm. In addition, it was also observed that even a small increase in
the amount of surfactant used would result in reducing the pore size even though 50 %
and more of surfactant concentration (w/w relative to the monomer content) led to
crumbled or weak PHP. Furthermore, the influence of electrolyte concentration in the
aqueous phase was also studied. The study showed that in the test with 5% DVB in the
oil phase and azobisisobutyronitrile (AIBN) as an initiator, a 10-fold reduction in cell size
was observed when the salt concentration in the aqueous phase was increased from 0
to 10g/100ml.
A study by Akay et al. (2005b) has shown that the temperature also plays a role in the
pore size of PHP. The study showed that the pore size can be controlled by elevating the
emulsification temperature. This information is useful whenever a large pore size is
needed. Findings from the research are as shown Figure 2.3 and Figure 2.4.
A closed–cellular cell structure can also be produced. The factors that determine the
cellular condition of the material was first studied by Williams and Wrobleski (1988). The
result showed that the surfactant is more important in determining the cellular structure
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of PHP although internal phase volume has some effects. It was discovered that low
concentration of surfactant, (i.e. <5% in term of w/w) relative to the monomer phase
resulted in closed-structure materials whereas high concentration (i.e. >7%) of the same
surfactant resulted in opened-cellular materials. This phenomenon occurred due to the
decrease in the thickness of monomer film separating the adjacent droplets when the
surfactant concentration was increased. During polymerisation process, windows
between adjacent droplets appeared at a specific critical film thickness. On the other
hand, when the monomer was less dense than the polymer (with low concentration of
surfactant), these windows shrank to produce a closed-cellular structure.
PolyHIPE Polymer Properties
PHP has highly permeable pores and interconnected walls; however, the surface area is
still low, with a typical range of 3-10 m2/g. This is explainable due to the relatively large
pore size of 10s of microns. This low surface is a drawback for application of PHP in
chromatographic support (Krajnc et al., 2005) which requires high surface area of
hundreds m2/g . This leads to further study by Hainey et al. (1991) to significantly
enhance the surface area of PHP. They discovered that the surface area can dramatically
be increased by substituting one of the monomers with organic porogenic (porogen –
pore-forming component) solvent and by adding big amount of cross linker (DVB) in the
continuous phase. This results in producing PHP with large surface area of 350 m2/g.
However, despite having a high surface area, the mechanical properties of the materials
were seriously affected. The monolith structure easily collapsed when the material was
subjected to low to moderate stress and to a flow through liquid.
Barbetta and Cameron (2004a; 2004b) then carried out a further study in finding a better
porogenic solvent without sacrificing the mechanical properties of the material. They
discovered that by substituting the solvent from toluene (T) to chlorobenzene (CB) and to
2-chloroethylbenzene (CEB), BET (Brunauer-Emmet-Teller) surface area was increased
from 350 to 550 m2/g. For PHP with CEB, not only is the surface area increased
significantly, the morphology also changed resulting in the formation of larger size of
windows. Unfortunately, the mechanical properties of the produced PHP were not
improved.
Cameron (2005) carried out a study using a 1:1 volume ratio of CEB:CB. The
material produced retained the original morphology of original PHP with the same
value of surface area, 550 m2/g. However, this type of PHP is not as robust as
the PHP produced using styrene/DVB. Recently there have been studies done by
Haibach et al.(2006) and Menner et al (2006) on synthesis low-density polymer
foams with superior mechanical properties. The continuous phase of the
emulsions was increased up to 40%. Haibach et al. discovered that the Young’s
modulus of silica reinforced foams increased by 280% and the crush strength by
218% when compared to non-reinforced foams. However, the surface area was
not significantly improved. Menner et al. used polyethylene glycol dimethacrylate
(PEGDMA) as a main crosslinker. The produced foams did not exhibit any
brittleness or chalkiness. They discovered that Young’s Modulus and crush
strength of silica reinforced foams increased by up to 360% and by up to 300%,
respectively, when compared to non-reinforced foams. There was no report on
surface area of the materials.
Preparation of polyHIPE

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Polyhipe polymer (PHP) is a highly porous polymeric material made through a high
internal phase emulsion (HIPE) polymerisation route. The materials used in preparation
of polyHIPE were styrene, divinyl benzene as the monomer, sorbitan monooleate (SPAN
80) as the surfactant, and potassium persulfate as the initiator.. All the materials used
were reagent grade chemicals without any further purification. The ratio of the reactants
used was varied according to the desired characteristics of the polyHIPE.
PolyHIPE polymers were prepared using the HIPE polymerisation route as described in
the literature (Akay, 2005; Akay et al., 2005c; Akay, 2004; Akay et al., 2004; Akay and
Vickers, 2003; Akay et al., 2002; Akay, 1995; Akay et al., 1995). The techniques used in
this study ensure the preparation of microporous polymers with well controlled internal
architecture, pore and interconnect sizes and their distributions (Akay, 2005). The
schematic diagram of the experimental set up used in preparing the polymers is given in
Figure 3.1. The pictures of the set-up, the impeller and the mixing vessel are presented in
Figure 3.2.
In summary, the oil phase and aqueous phase prepared and used in the experiments were
listed in Table 3.1. The standard dosing time and mixing time used in this study are 10
minutes and 50 minutes, respectively. In some cases, the dosing and mixing times were
varied.
Mixing was carried out by using two sets of impellers with each set having two flat
paddles of 9 cm diameter. The two sets are stacked at right-angle to each other and the
impeller bottom is placed at the closest possible distance to the bottom of the mixing
vessel. The impeller was operated at constant rotational speed of 300 rpm. The pore and
interconnect sizes of the micro-porous polymer are controlled through the temperature of
the emulsification, mixing speed and total mixing time. Details of these parameters in
obtaining specific pore size and structure are available in the literature (Akay, 2005; Akay
et al., 2002).
The impeller was simultaneously started as the aqueous phase was dosed into the
vessel. Upon completion of dosing all the aqueous phase into the vessel, the mixture
may be further stirred for some period of times. The produced emulsion was then
transferred into 50 mL polypropylene containers having internal diameter of 2.6 cm. The
polypropylene containers filled with emulsion were then placed in a pre-heated, 60 °C
oven where polymerization took place for about 8 hours. After polymerization, the
solidified PHP blocks were removed out of the polypropylene containers and cut into 0.4
cm discs. The discs were then dried by leaving them on the paper towel overnight in a
fume cupboard, and can be stored for subsequent modifications and applications.
Washing of PolyHIPE polymers
All the PolyHIPE discs, except the first phase of silica PHP, were washed in a soxhlet set
up as shown in Figure 3.3 to remove the surfactants. The washing was first done using
iso-propanol for 9 hours, and then followed by 12 hours washing in double distilled water
to get rid of any remaining residues in the pores and interconnects.
Scanning Electron Microscopy (SEM)
The Scanning Electron Microscope (SEM) provides information relating to topographical
features, morphology, phase distribution, compositional differences, crystal structure,
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crystal orientation, and the presence and location of electrical defects. The SEM is a
microscope that uses electrons rather than light to form an image. There are many
advantages to using the SEM instead of a light microscope. The SEM has a large depth of
field, which allows a large amount of the sample to be in focus at one time. The SEM also
produces images of high resolution, which means that closely spaced features can be
examined at a high magnification. Preparation of the samples is relatively easy since most
SEMs only require the sample to be conductive. The combination of higher
magnification, larger depth of focus, greater resolution, and ease of sample observation
makes the SEM one of the most heavily used instruments in research areas today.
The SEM equipment used in this work was an environmental SEM model Hitachi S2400
Scanning Electron Microscope fitted with an Oxford Instrument Isis 200 Ultra-Thin
Window X-ray detector. The picture of the machine is presented in Figure 3.5. Since the
SEM operation uses vacuum conditions and electrons to form an image, special
preparation of the sample is compulsory. All water, solvents or other materials that may
vaporise while in a vacuum must be removed prior to analysing the sample. Therefore,
the sample was dried in a vacuum oven at 60°C for 4 hours before cutting and mounting it
on a stub.
For the sample to withstand the vacuum inside the column, care must be taken in
preparing the sample. In order to protect the inner structure of the sample, sample was
carefully broken/ cut into a small piece prior to mounting it to the sample holder, an
aluminium stub. The sample was glued onto an aluminium stub with carbon cement or
copper tape. Carbon cement was only used for the samples that are not adhesive enough
on the copper tape. For the samples glued with carbon cement; they were left overnight
for the carbon to dry off, so that it can withstand the vacuum. The extra caution in
mounting procedure is very important to ensure a good quality result. Since the samples
were analysed in an environmental SEM, non-conductive samples could be examined
without being coated with conductive material. However, for achieving better clarity,
samples were coated with a very thin layer of gold using a gold sputter coater. Figure 3.6
shows how the sample was mounted on the aluminium stub.
Surface Area and Pore Size Analysis
In this study, the instrument used to measure the surface area and pore size distribution of
PHPs is Coulter SA 3100 analyser, manufactured by the Beckman-Coulter company. The
picture of the machine is shown in Figure 3.7. This instrument uses Gas Sorption
technique to obtain total surface area and pore size distributions of 0.4 to 200 nm
diameter. Nitrogen was used as the adsorbates. The technique can be described as the
physical characterisation of material structures whereby gas molecules of known size are
condensed (adsorbed) on surfaces of the sample at a constant temperature. The quantity of
the gas adsorbed and the resultant sample pressure are recorded and used in constructing
isotherm. The data from the isotherm are then used in subsequent calculation models. In
this study, BET (Brunauer, Emmet and Teller) calculation model is used for specific
surface area and BJH (Barret, Joyner and Halenda) calculation model is used for pore size
distribution.
During the adsorption process, when the adsorbate molecules are attached to the
surface of the materials, the molecules are then retained by physisorption or
chemisorption process. The machine assumes that all adsorption detected is due
to physically adsorbed gas. Hence, all calculation models are based on
physisorption process and not on chemisorption process.
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Adsorption and desorption isotherm
The SA 3100 measures both, adsorption and desorption isotherm branches. The surface
area is determined based on the adsorption branch whereas the pore size distribution is
determined based on either adsorption or desorption branch and on both branches. The
adsorption isotherm is a set of incremental data based on quantity of adsorbate gas
condensed on the surface of the materials at a given pressure and at a constant
temperature. The gas volume is measured at STP (Standard Temperature and Pressure)
conditions and reported in cc/g. Desorption branch represents the reverse adsorption
process and reported as a set of decremental data. Adsorptive pressures vary with
temperature; hence, isotherm data is unique at a given analysis temperature.
The isotherm is represented as volume adsorbed (cc/g) against the relative pressure. The
relative pressure is calculated as the sample pressure divided by the saturation vapour
pressure. The sample pressure is the residual pressure in the sample chamber that formed
from the leftover molecules that are not adsorbed during the gas adsorption on to the
material surface. In other words, not all adsorbate gas molecules get adsorbed. The range
of measurable values of relative pressure with SA3100 is 0 to 0.995. The boiling pressure
of the liquid gas may be taken as the saturation vapour pressure. However, due to the
contamination of the liquid nitrogen by the condensation of atmospheric gases, it is
important that the saturation vapour pressure is measured throughout the duration of
sample analysis.
The adsorption process is measured volumetrically using the static fully equilibrated
method. Adapting this method, discreet data points are taken and each point is
equilibrated to pre-defined limits. High resolution results are obtainable using a large
number of data points. Volume of the sample tube unoccupied by the sample is measured
by using Helium gas and is termed freespace. The pressure of each data point is measured
and subsequently used to calculate the volume of adsorbate gas retained by the sample.
The volume of each dose of gas is constant and has been pre-calibrated at the factory. The
isotherm data for adsorbed gas (y-axis) is determined by subtracting the freespace of the
sample tube from the total volume of gas dosed to the sample. The isotherm data for
relative pressure (x-axis) is obtained by dividing sample pressure by saturation vapour
pressure.
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