PolyHipe Polymer: Doc
Added on - 17 Oct 2019
PolyHIPE polymer (PHP)PolyHIPE polymer is a highly porous material that can be easily prepared bypolymerisation of the monomeric continuous phase of a high internal phase emulsion(HIPE). These polymeric foams were coined the generic name PolyHIPE by researchersat Unilever Research Port Sunlight Laboratory, UK (Barby and Haq, 1982).The process of preparing polyHIPE polymer is quite simple. Droplets of aqueous phaseare added to the mixture of oil phase, consisting of monomer, crosslinker and surfactantwhile mixing. Mixing is needed to break up large droplets. Mixing is further continuedafter addition of the internal phase to get a smaller pore volume. The emulsion is thencured in the oven; the resulting porous material was then washed in the soxhlet, and dried.Overview of High Internal Phase Emulsions (HIPE) andPolyHIPE Polymer (PHP)As defined by Lissant (1974), high internal-phase ratio emulsions are those with morethan 74% internal phase volume,Φ. TheΦvalue of 74% represents the maximumvolume that can be occupied by uniform non-deformable spheres when packed in themost efficient manner. These days, also known as high internal phase emulsion (HIPE),the value ofΦcan be as high as 99%. At this high value ofΦ, closely packedmonodispersed spheres is no longer physically possible in internal or dispersed phase.Thus, at this high internal phase volume, the shape is deformed into non-sphericalpolyhedral droplets which appeared to be monodispersed in size, as quoted by Cameronand Sherrington (1996) on the work done by Lissant. The droplets have relatively largecontact 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 ofHIPE, normally contains monomer, cross-linking agent, surfactant and oil-phase initiator.Due to HIPE unique characteristics, HIPE have been used for many years in manyapplications such as food preparation, cosmetics, oil recovery and many others. One ofthe most important applications of HIPE is the ability to be used as template systems forthe 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 iscontinuously dosed into a mixing vessel containing the continuous phase (oil phase). Careis taken in minimizing the jet mixing of the two phases since addition of aqueous phasealone creates mixing. There is a reduction in the droplet size of the aqueous phase due tothe rotation of the impeller during dosing. In the second stage of processing, furthermixing is carried out upon completion of dosing in order to reduce aqueous phase dropletsize (i.e. size of pores after polymerization) and to obtain HIPE of narrow droplet sizedistribution. No additional mixing (homogenization) stage is needed for the case of a verylow dosing rate.The relative dosing rate having a dimension of deformation rate is used to characterize theaqueous phase dosing rate.In the case of very large relative dosing rate and small mixing rate, instead of HIPEformation, dilute (low) internal oil-in-water (O\W) emulsion is formed. When HIPE isstable, polymerization without phase separation will take place.
The monomer-based HIPE can be polymerised to obtain micro-porous polyHIPEpolymers (PHP). Barby and Haq (1982) discovered that open-cell HIPE-based polymercan be polymerised by using relatively simple low HLB (Hydrophile-Lipophile Balance)surfactant and HIPEs composing of styrene-divinylbenzene (DVB) as shownschematically in Figure 2.1.The internal (aqueous) phase used in preparing the HIPE can be easily and rapidlyremoved 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-madeaccording to its application. For example, PHP can be produced with specificinterconnect size, d, e.g. as d of 0<d/D<0.5, D is pore size. Moreover, a highly porousinterconnected monolithic material of PHP with a well-defined and uniformmicrostructure of very low dry density can also be produced. The structure of PHP isshown in Figure 2.2 (adapted from (Akay et al., 2005b)). The materials can be producedover a wide range of pore size, D, (0.5 μm<D<5000 μm), based on the conditions of thestarting emulsions. PHP having pore size greater than 200 μm can be produced througha coalescence polymerisation technique (Akay et al., 2005a; Akay et al., 2002).Furthermore, the porosity of PHP surface can be controlled by varying the surfacechemistry of materials against which the HIPEs are polymerised. This allows theproduction of asymmetric materials.Due to PHPs unique structures and properties, PHPs have made ways in diverse fields ofintensified processes, especially in biology, where their applications include thediscovery 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 otherintensified bioprocesses (Akay, 2006b; Akay, 2005).Both hydrophilic and hydrophobic PHPs have been utilised in several otherapplications 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 watertreatment (Katsoyiannis, 2002; Wakeman et al., 1998). PHPs have also beenapplied in other intensified processes , for instance, foams and filtrationfabrications (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 inthe applications mentioned above, the preparation and modification of PHP materials hasto 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 specificapplication;iv) and ensure the sustainable production and modification of PHP.
PolyHIPE Polymer MorphologyPHPs are being widely utilised in various applications based on each specific requiredproperty of the materials, for instance, morphology, physical, mechanical, or thermalproperties. Therefore, control over PHP properties is essential to ensure viability ofapplication. Having several advantages of accessibility of the pores, controllability ofinternal architecture, such as the pore and interconnect structures, versatility offabrication 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 avery large well-organised monolithic article.The typical structure of PHP is an open cellular structure of spherical cavities. Thesecavities are known as voids or pores having windows for interconnecting thepores. Thisphenomenon is possible due to the trapped internal (aqueous) phase inside the continuousphase during the polymerisation process. Generally, the stability level of the preparedHIPE has a direct relation to the pore size of PHP. In a system with high emulsionstability, a smaller droplet size will be produced due to the lower interfacial tension whichallows larger interfacial area. In a less stable emulsion system, emulsion droplets tend tocoalesce and lead to a larger cell once the polymer is formed. There are several factorsthat govern the stability of HIPEs. Similar to other emulsions, HIPE stability is highlydependent on the preparation parameters, which are shear stress (mixing speed) andmixing time. In order to produce a more stable emulsion, high mixing speed is needed touniformly 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 areduction in the size of water cavity and an increase in the number of windows leading toproduction 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 poresize. 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 wasobserved that the emulsion with DVB alone can easily and more uniformly get blendedcompared to the emulsion with styrene alone. Thus, increasing the ratio of styrene/DVBin a HIPE led to the increase in emulsion stability, leading to the decrease in pore sizediameter from 15 to 5μm. In addition, it was also observed that even a small increase inthe 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 tocrumbled or weak PHP. Furthermore, the influence of electrolyte concentration in theaqueous phase was also studied. The study showed that in the test with 5% DVB in theoil phase and azobisisobutyronitrile (AIBN) as an initiator, a 10-fold reduction in cell sizewas observed when the salt concentration in the aqueous phase was increased from 0to 10g/100ml.A study by Akay et al. (2005b) has shown that the temperature also plays a role in thepore size of PHP. The study showed that the pore size can be controlled by elevating theemulsification temperature. This information is useful whenever a large pore size isneeded. 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 thecellular condition of the material was first studied by Williams and Wrobleski (1988). Theresult showed that the surfactant is more important in determining the cellular structure