PolyHIPEs: High-Porosity Polymers through Emulsion Templating

Porous polymers are essential for catalysis, chromatography, separation, absorption, ion exchange, insulation, tissue engineering, drug delivery, and energy damping applications, to name but a few. PolyHIPEs (PHs) are novel, high-porosity polymer monoliths with unique porous structures that are typically templated within water-in-oil (w/o) high internal phase emulsions (HIPEs) and synthesized using free radical polymerization. These w/o HIPEs typically consist of an external (continuous) phase (monomers, crosslinking comonomers, and emulsifier) and from 74 % to over 90 % internal (dispersed) phase (water and a stabilizing salt). The internal phase can be removed through holes within the thin polymer film that surrounds the discrete internal phase droplets, leaving a highly interconnected, high-porosity, void structure (average diameters range from 10 to 100 µm). Similarly, Hydrogel PHs (HG-PHs) are novel, high-porosity hydrogel monoliths with unique porous structures that are typically templated within oil-in-water (o/w) HIPEs and synthesized using free radical polymerization. These o/w HIPEs typically consist of an external (continuous) phase (water, monomers, crosslinking comonomers, and emulsifier) and from an internal (dispersed) phase (organic liquid). The advantages of PHs and HG-PHs include their low densities (as low as 0.03 g/cm3), their ability to absorb large amounts of liquid (typically 9 g/g), and their flow-through capabilities.

The phase compositions in a typical w/o HIPE and in a typical o/w HIPE picture
The phase compositions in a typical w/o HIPE and in a typical o/w HIPE.
A schematic illustration of a typical PH synthesis within a w/o HIPE Picture
A schematic illustration of a typical PH synthesis within a w/o HIPE.

 

A schematic illustration of a typical PH synthesis within a w/o HIPE Picture
A schematic illustration of a typical PH synthesis within a w/o HIPE.

 

Typical, highly interconnected, PH structure and a typical HG-PH structure. (90 % internal phase, styrene/divinylbenzene = 9/1) picture
Typical, highly interconnected, PH structure and a typical HG-PH structure.
(90 % internal phase, styrene/divinylbenzene = 9/1).

PolyHIPE Topics

Shape memory PHs
Poly(urethane urea) PHs for tissue engineering
Hydrophobic-hydrophilic bicontinuous PHs for controlled release
Encapsulation of individual aqueous solution droplets
Encapsulation of individual molten salt droplets
Hydrogel PHs (HG-PHs) from oil-in-water HIPEs
Hydrophobic-hydrophilic bicontinuous PHs for toughening

We have developed organic-inorganic hybrid and nanocomposite polyHIPE. Silsesquioxane-based nanocomposites were synthesized using in-situ formation of silsesquioxane (SSQ) networks and through pre-formed SSQ. SSQ can have the molecular formula Rx-(SiO1.5)z-Ry (where x + y = z). The effect of pre-formed SSQ reactivity was also investigated. These nanocomposites exhibited superior mechanical properties and thermal stability. We have definitively demonstrated the relative importance of cross-linking, inorganic reinforcement, and covalent bonding to such nanocomposites. In addition, we have shows that highly porous inorganic monoliths could be produced through the pyrolysis of these porous polymer nanocomposites.

Simultaneous formation of organic and SSQ interconnected networks in organic-inorganic hybrid polyHIPE Picture
Simultaneous formation of organic and SSQ interconnected networks in organic-inorganic hybrid polyHIPE.
Nanocomposite polyHIPE with pre-formed SSQ containing no reactive groups, one reactive group, and multiple reactive groups picture
Nanocomposite polyHIPE with pre-formed SSQ containing no reactive groups, one reactive group, and multiple reactive groups.

Other novel polyHIPE systems under development include hydrogel and bicontinuous polyHIPE for drug delivery, biodegradable polyHIPE for tissue engineering, conducting polyHIPE for sensors, crystallizable polyHIPE for shape memory, and interpenetrating polymer network polyHIPE. A recent discovery was the critical effect that the locus of initiation (organic phase or interface) has on the molecular structures, porous structure, and properties of polyHIPE.

Nanoporous inorganic monolith, density 0.06 g/cm3 (based on styrene/divinylbenzene/vinyltrimethoxysilane/toluene = 30/10/30/30). There was a 79% mass loss and 90 % shrinkage during polyHIPE pyrolysis picture
Nanoporous inorganic monolith, density 0.06 g/cm3 (based on styrene/divinylbenzene/vinyltrimethoxysilane/toluene = 30/10/30/30). There was a 79% mass loss and 90 % shrinkage during polyHIPE pyrolysis.
Nanoporous nanocomposite polyHIPE, density 0.06 g/cm3 (styrene/divinylbenzene/vinyltrimethoxysilane/toluene = 30/10/30/30) picture
Nanoporous nanocomposite polyHIPE, density 0.06 g/cm3 (styrene/divinylbenzene/vinyltrimethoxysilane/toluene = 30/10/30/30).
Nanocomposite polyHIPE, density 0.11 g/cm3 (styrene/divinylbenzene/methacryloxypropyltrimethoxysilane = 30/10/60) picture
Nanocomposite polyHIPE, density 0.11 g/cm3 (styrene/divinylbenzene/methacryloxypropyltrimethoxysilane = 30/10/60).

Phase composition and porous structure of a bicontinuous polyHIPE. The hydrophobic scaffold is filled with a hydrogel picture

Phase composition and porous structure of a bicontinuous polyHIPE. The hydrophobic scaffold is filled with a hydrogel.

Phase composition, molecular structure, and porous structure in a biodegradable polyHIPE picture

Phase composition, molecular structure, and porous structure in a biodegradable polyHIPE.