PolyHIPE

A high internal phase liquid-liquid emulsion (HIPE) is one where the internal or dispersed phase droplets occupy >74% of the total volume of the emulsion. At this point the droplets contact each other and beyond this volume % the droplets are forced into distorted polyhedra. If for example styrene and divinylbenzene are employed as the continuous phase and water droplets dispersed in this oil phase using a suitable surfactant to form a HIPE, the comonomers can be polymerized to form a poly(styrene-divinylbenzene) polyHIPE. Typically the water droplets are... 

Fig. 1.12 Scanning electron micrograph of poly(styrene-divinylbenzene) polyHIPE (x2000, bar=10 pm).

Another important consideration was the choice of resin. The use of Hmb was developed using resins composed of polydimethylacrylamide polymerized within the pores of a solid, macroporous support, either Pepsyn or polyhipe, both are exemplified below. The use of Hmb involves the switching of solvents from DMF to dichloromethane the resins mentioned have excellent swelling properties in both solvents. However, some commercial polystyrene supports have given poor results because of the need to switch solvents, which can cause problems due to resin shrinkage. 1 ... 

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 range of monomers which can be employed is largely dictated by the physical chemistry of the emulsion system. For instance, monomers must be sufficiently hydrophobic to allow the formation of stable w/o HIPEs. In addition, most systems which have been studied have used polymerisation methods which require either an initiation step, or addition of a catalyst. This is due to the fact that the first step in the preparation of the polymer is the preparation of HIPE this can only proceed satisfactorily in the absence of any significant degree of polymerisation. Thus, it can be seen that radical addition polymerisation is suitable for the synthesis of PolyHIPE polymers, whereas condensation polymerisation can be more problematical. Also, the latter reactions often generate water as the by-product, hence the aqueous component of the HIPE is inhibiting to the polycondensation. 

By far the most studied PolyHIPE system is the styrene/divinylbenzene (DVB) material. This was the main subject of Barby and Haq s patent to Unilever in 1982 [128], HIPEs of an aqueous phase in a mixture of styrene, DVB and nonionic surfactant were prepared. Both water-soluble (e.g. potassium persulphate) and oil-soluble (2,2 -azo-bis-isobutyronitrile, AIBN) initiators were employed, and polymerisation was carried out by heating the emulsion in a sealed plastic container, typically for 24 hours at 50°C. This yielded a solid, crosslinked, monolithic polymer material, with the aqueous dispersed phase retained inside the porous microstructure. On exhaustive extraction of the material in a Soxhlet with a lower alcohol, followed by drying in vacuo, a low-density polystyrene foam was produced, with a permanent, macroporous, open-cellular structure of very high porosity (Fig. 11). 

Fig. 12. Scanning electron micrograph (SEM) of poly(styrene/DVB) PolyHIPE...

The concentration of the surfactant in the monomer phase was found to be critical to the formation of a stable polymer foam [129,130]. At least 4% surfactant, relative to the total oil phase, was required for PolyHIPE formation, whereas formulations containing above 80% resulted in the formation of an unconnected or closed-cell material. Surfactant levels between 20 and 50% were deemed to be optimum at all internal phase volumes. Additionally, Litt et aL [131] demonstrated that block copolymer surfactants can be used to prepare water-in-styrene HIPEs. From these, highly porous uncrosslinked polystyrene PolyHIPE materials were synthesised. 

Williams [129] discovered that the surfactant level had a profound effect on the cellular structure of PolyHIPE materials. Below about 5% surfactant, the... 

PolyHIPE materials possess many peculiar properties as a result of their unique cellular structure. Referring specifically to open-cell polymers (which have received the most attention in the literature), they are characterised by a very low dry bulk density, typically less than 0.1 gem-3, due to their highly porous, interconnected structure. 

Another important property of open-cell PolyHIPE materials is their ability to absorb large quantities of solvent, by capillary action [128]. Simply immersing a piece of the material in the liquid causes absorption, with displacement of the air from inside the matrix. This occurs until all voids are filled. The nature of the liquid will affect the volume which can be taken up [133]. Methanol, which is a non-swelling solvent for crosslinked polystyrene, is absorbed to a lesser... 

Table 1. Properties of poly(styrene/DVB) polyHIPE prepared with a porogenic solvent in the continuous phase...

PolyHIPE polymer HIPE composition Surface area (m2g-1)a... 

Similarly, it is possible to pump liquids through blocks of PolyHIPE, taking advantage of the fully interconnected microstructure. The rather large cell size of the material means that back pressures are relatively low. This is potentially of great importance in liquid chromatography, and will be discussed later. 

PolyHIPE has found a successful application in the field of solid phase peptide synthesis (SPPS), where the highly porous microstructure acts as a support material for a polyamide gel [134]. The polystyrene matrix is functionalised to give vinyl groups on its internal surfaces, and is then impregnated with a DMF solution of N, JV -dimethylacrylamide, acryloylsarcosine methyl ester, crosslinker and initiator. Polymerisation grafts the soft gel onto the rigid support, giving a novel composite material (Fig. 16). 

Reaction of flavin with chloromethylated PolyHIPE (Method 1). 

Fig. 17. Structure of flavin and preparation of flavin-containing PolyHIPE materials...

PolyHIPE, in granular form, has recently been employed as a support for bicatalyst systems [136]. Styrene/DVB porous polymers were prepared with free vinyl groups (acryloyl, allyl and vinylbenzyl) on the surfaces of the cavities. Impregnation with solutions of quaternary onium monomers, with subsequent polymerisation, resulted in grafting onto the pendant double bonds, to give a surface-quaternised material (Fig. 19). 

Fig. 19. Preparation of quaternary onium-containing PolyHIPE polymers...

Fig. 21. Heck-type vinylation reaction with PolyHIPE-supported Pd catalyst...

The ability of PolyHIPE materials to absorb liquids has been exploited in experiments on their potential use as carrier materials for the safe transport of hazardous or flammable liquids [128]. A poly(styrene/DVB) sample was able to absorb twenty times its own weight of liquid paraffin, simply by immersing the material in the liquid. However, the problem in this application is the subsequent removal of the liquid from the polymer. This can only be achieved by vacuum distillation, which is very difficult with high boiling liquids. 

Other applications suggested for PolyHIPE materials include their use as inert matrices for the immobilisation of cells and enzymes [144]. Ruckenstein... 

Another more esoteric use for polystyrene PolyHIPE materials was suggested by Williams et al., and involves the intact capture of microparticles of cosmic dust [147],... 

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