PolyHIPE Polymers: A Comprehensive Analysis of Properties and Uses

This report provides a detailed overview of PolyHIPE (High Internal Phase Emulsion) polymers, a class of materials synthesized from emulsions with a high internal phase volume. It begins by defining HIPEs and their unique characteristics, emphasizing the use of HIPEs as templates for polymeric materials. The report then delves into the two-stage HIPE processing method, including the critical control of mixing and dosing rates. It discusses the formation of micro-porous polyHIPE polymers (PHPs) and their morphology, highlighting the open-cell structure, pore size control, and the influence of factors like surfactant concentration and temperature. The report explores the properties of PHPs, including their permeability and the ability to tailor the surface area. Finally, it covers the diverse applications of PHPs in fields such as biology, intensified processes, gas separation, and metal ion removal, and concludes by summarizing the key criteria for PHP preparation and modification to meet specific application needs. The report emphasizes the importance of controlling PHP properties for application viability.

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.
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.
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.
2.1.1 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 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.
2.1.2 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.