High internal phase emulsions

Cameron, N.R. and Sherrington, D. C. High Internal Phase Emulsions (HIPEs)-Structure, Properties and Use in Polymer Preparation. Vol. 126, pp. 163-214. 

Perhaps the most important and striking features of high internal phase emulsions are their rheological properties. Their viscosities are high, relative to the bulk liquid phases, and they are characterised by a yield stress, which is the shear stress required to induce flow. At stress values below the yield stress, HIPEs behave as viscoelastic solids above the yield stress, they are shear-thinning liquids, i.e. the viscosity varies inversely with shear rate. In other words, HIPEs (and high gas-fraction foams) behave as non-Newtonian fluids. 

A number of factors greatly influence the stability of high internal phase emulsions, including the nature of the surfactant, its concentration, the nature of the continuous phase, the temperature and the presence of salts in the aqueous... 

There exists, in the literature on high internal phase emulsions, a small number of publications on possible applications of HIPEs, involving a diverse range of topics. The production of petroleum gels as safety fuels is one such example [124,125] this was mentioned in the section on non-aqueous HIPEs. The main advantage over conventional fuels is the prevention of spillage, which reduces the risk of fire in an accident. Also, studies on the flash-point of emulsified fuels [127] showed a considerable increase, compared to the liquid state, for commercial multicomponent fuels. In addition, there may be an enhancement of the efficiency of combustion of the fuel on emulsification, as it is known that a small amount of water in fuel can improve its performance [19]. 

If a high internal phase emulsion is prepared in which the continuous phase contains one or more monomeric species, and polymerisation is initiated, a novel type of highly porous material is produced. Polymers of this type are referred to as PolyHIPE, using the nomenclature devised by Unilever scientists [128],... 

The idea of the preparation of porous polymers from high internal phase emulsions had been reported prior to the publication of the PolyHIPE patent [128]. About twenty years previously, Bartl and von Bonin [148,149] described the polymerisation of water-insoluble vinyl monomers, such as styrene and methyl methacrylate, in w/o HIPEs, stabilised by styrene-ethyleneoxide graft copolymers. In this way, HIPEs of approximately 85% internal phase volume could be prepared. On polymerisation, solid, closed-cell monolithic polymers were obtained. Similarly, Riess and coworkers [150] had described the preparation of closed-cell porous polystyrene from HIPEs of water in styrene, stabilised by poly(styrene-ethyleneoxide) block copolymer surfactants, with internal phase volumes of up to 80%. 

PolyHIPE materials have also been prepared by polycondensation in high internal phase emulsions [153]. Thus, a resorcinol-formaldehyde (RF) porous copolymer was synthesised from an o/w HIPE of cyclohexane in an aqueous solution of resorcinol, formaldehyde and surfactant. Addition of an acid catalyst to the emulsion, followed by heating, resulted in copolymerisation. Other systems prepared included urea-formaldehyde, phenol-formaldehyde, melamine-formaldehyde and a polysiloxane-based elastomeric species. 

The dispersed phase of high internal phase emulsions may also be used to prepare polymeric materials in this case, conversion of monomer dispersed droplets to polymer results in latexes or particulates. 

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