Draft:PolyHIPE Foams

• Comment: A good start, but I think you need to do some more work first:
  1. Read WP:MOS. One example that needs correction is the section titles - first letter caps only.
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PolyHIPE foams (polymerized high internal phase emulsion foams) are highly porous polymeric materials synthesized through emulsion templating using high internal phase emulsions (HIPEs). These materials have interconnected open-cell structures with high porosity (typically 74-99%) and are used in tissue engineering, separation processes, catalysis, energy storage systems, environmental remediation, and advanced electronics.

History and development

The foundational work on polyHIPE materials originated in 1982 when David Barby and Zoltan Haq at Unilever filed the patent (EP0060138A1) titled "Low density porous cross-linked polymeric materials and their preparation and use as carriers for included liquids." In 1982, the scientific literature first introduced the term "Polymerised High Internal Phase Emulsion (PolyHIPE)" to describe the porous materials that are created when High Internal Phase Emulsions become solidified.

The field was advanced during the 1990s and 2000s by Neil R. Cameron and David C. Sherrington at the University of Strathclyde, whose collaboration built upon earlier work during Sherrington's secondment to Unilever where he had been involved in polymeric high internal phase emulsions research. Cameron's PhD research with Sherrington from 1991-1995 established many principles of polyHIPE synthesis and characterization.

Early investigations of morphology formation using cryo-SEM techniques revealed that the transition from discrete emulsion droplets to interconnected cellular structures occurs around the gel point of the polymerizing system.

Synthesis and structure formation

High internal phase emulsions

PolyHIPE foams are synthesized from high internal phase emulsions, which are concentrated emulsion systems where the dispersed (internal) phase comprises more than 74% of the total volume—corresponding to the maximum theoretical packing fraction of monodisperse hard spheres. In practice, HIPEs can achieve internal phase contents of up to 99% due to droplet deformation and polydispersity effects.

Emulsion systems with internal phase volumes between 30-74% are classified as medium internal phase emulsions (MIPEs), while those with less than 30% internal phase are termed low internal phase emulsions (LIPEs).

Emulsion formation and stabilization

The most common polyHIPE synthesis involves water-in-oil emulsions where the continuous phase contains monomers, crosslinking agents, initiators, and surfactants, while the dispersed aqueous phase typically contains electrolytes such as calcium chloride. Sorbitan monooleate (Span 80) is the most widely used surfactant for W/O HIPE stabilization.

Polymerization and morphology development

Polymerization typically occurs through free radical mechanisms initiated by thermal decomposition of initiators such as azobisisobutyronitrile (AIBN) or potassium persulfate (KPS). The choice of initiator location (oil-soluble versus water-soluble) significantly affects the final foam morphology, with oil-soluble initiators typically producing open-cell structures with spherical pores, while water-soluble initiators can result in closed-cell polyhedral structures.

During polymerization, interconnecting holes or "windows" develop between adjacent emulsion droplets at the thinnest points of the continuous phase films, transforming the initially discrete droplet structure into a continuous, interconnected porous network. Removal of the internal phase yields voids in place of the internal phase droplets, resulting in the characteristic highly interconnected, open-cell, emulsion-templated porous structure.

Chemical compositions and material types

Styrenic polyHIPEs

The most extensively studied polyHIPE systems are based on styrene and divinylbenzene copolymers, which were among the first polyHIPE materials developed. These materials exhibit high mechanical strength but are inherently brittle due to their highly crosslinked structure and elevated glass transition temperatures. Chemical modification of styrenic polyHIPEs through electrophilic aromatic substitution reactions such as sulfonation, nitration, and bromination enables functionalization for specific applications.

Elastomeric polyHIPEs

Elastomeric polyHIPE materials have been developed by incorporating monomers with long hydrocarbon side chains, such as 2-ethylhexyl acrylate or 2-ethylhexyl methacrylate, which act as internal plasticizers to reduce glass transition temperatures and introduce flexibility. These materials exhibit improved compressibility and are instrumental in applications requiring flexible separation membranes, energy storage in soft robotics, and 3D-printed soft tissue engineering scaffolds.

Biodegradable polyHIPEs

Biodegradable polyHIPE materials have been synthesized for biomedical applications using various degradable monomer systems. Polycaprolactone (PCL)-based polyHIPEs have been prepared for tissue engineering applications, while fumarate-based systems offer controlled degradation rates suitable for bone tissue engineering.

Electrically conductive polyHIPEs

Electrically conductive polyHIPE materials have been developed for applications in electronics, energy storage, electromagnetic interference shielding, and bioelectronics. Electrical conductivity is introduced through various strategies including incorporation of conductive nanofillers, intrinsically conducting polymers, and carbonization processes.

Graphene-stabilized polyHIPEs

Graphene has emerged as a 2D surfactant for creating conductive polyHIPE materials. Pristine, unoxidized graphene can be spontaneously exfoliated at high-energy aqueous/organic interfaces, where it self-assembles into percolating networks and incorporates into the polyHIPE cell walls. These graphene-stabilized polyHIPEs exhibit properties including: Compressive strengths up to 7.0 MPa at densities of 0.22 g/cm³, electrical conductivities reaching 0.36 S/m, enhanced mechanical reinforcement even at low polymer content, and continuous conductive networks at low graphene loading.

The morphology and conductivity of these materials can be controlled by varying graphite flake size, concentration, and the use of additives such as tannic acid to modulate graphene hydrophilicity.

Carbon nanotube composite polyHIPEs

Carbon nanotubes (CNTs), both single-walled and multi-walled varieties, have been incorporated into polyHIPE structures to create lightweight, conductive foams. These materials demonstrate percolation thresholds as low as 0.35 vol% CNT content and enhanced electromagnetic interference (EMI) shielding effectiveness exceeding 30 dB

Intrinsically conducting polymer polyHIPEs

Intrinsically conducting polymers (ICPs) such as polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT) have been integrated into polyHIPE systems to create materials with unique electroactive properties.

CarboHIPEs: carbonized polyHIPE materials

Carbonization of polyHIPE materials through controlled pyrolysis produces CarboHIPEs—porous carbon foams with electrical conductivity and enhanced surface area. These materials serve as supports for hybrid electrodes, and are useful in energy storage and conversion applications.

Applications

Tissue engineering and biomedical applications

PolyHIPE materials have found extensive use as scaffolds for tissue engineering due to their high porosity, interconnected pore structure, and ability to support cell growth and proliferation. Studies have demonstrated that cells grown on 3D polyHIPE scaffolds exhibit enhanced drug tolerance and more physiologically relevant behavior compared to traditional 2D culture systems.

Separation and chromatography

The high surface area and interconnected pore structure of polyHIPE materials make them suitable materials for separation applications. Functionalized polyHIPEs have been employed as chromatographic supports and ion-exchange materials.

Catalysis and chemical synthesis

PolyHIPE materials serve as supports for heterogeneous catalysts due to their high surface area, ease of functionalization, and enhanced mass transfer properties enabled by convective flow through large interconnected pores.

Energy storage and conversion

Recent developments have explored polyHIPE materials for energy-related applications including battery separators, supercapacitor electrodes, and phase change material encapsulation for thermal energy storage. Conductive polyHIPEs are used in supercapacitor applications as lightweight current collectors with enhanced electrolyte accessibility.

Supercapacitor applications

PolyHIPEs can be used as templates to create electrodes for supercapacitors, which require materials with a high surface area. In this process, the polyHIPE foam is first synthesized and then converted into a porous carbon structure through pyrolysis (a high-temperature heating process in the absence of oxygen). The resulting carbon foam retains the interconnected, porous architecture of the original polyHIPE.

These carbonized polyHIPEs have a hierarchical pore structure, combining large macropores for easy electrolyte access with smaller meso- and micropores that create a large surface area for ion adsorption. For example, a nitrogen-doped porous carbon derived from a polyHIPE template was shown to have a specific capacitance of 209 F/g. Another study demonstrated that by optimizing the templating process, a hierarchically porous carbon with a specific surface area of 2289 m²/g and a specific capacitance of 306 F/g could be achieved.

Thermal energy storage

PolyHIPEs can be used to encapsulate phase-change materials (PCMs) to solve the common problem of leakage when the PCM is in its liquid state. The molten PCM is infiltrated into the pores of the polyHIPE, where it becomes trapped by capillary forces. This creates a "form-stable" composite, preventing the PCM from leaking even when it is fully melted. This technique has been used to encapsulate various PCMs, including n-hexadecane, octadecane, and polyethylene glycol (PEG), within different polyHIPE frameworks.

Additionally, the polyHIPE framework itself can be enhanced. For instance, incorporating thermally conductive additives like graphene into the polymer structure improves the rate at which the composite can absorb and release heat, further improving its performance as a thermal management system.

Battery separators

PolyHIPEs are effective as battery separators because their high porosity can be filled with a large volume of liquid electrolyte, resulting in very high ionic conductivity. One method involves polymerizing the HIPE in situ with the electrolyte already present as the internal phase. This creates a ready-to-use separator membrane in a single step, with an ionic conductivity close to that of the pure electrolyte.

Further advancements have focused on creating stretchable polyHIPE separators for use in soft and wearable electronics. By selecting elastomeric monomers for the polymer framework, researchers have developed separators that remain functional even when stretched to over 50% strain. These flexible separators have been used to build intrinsically stretchable zinc-carbon and other types of soft batteries for applications like wearable sensors and on-skin devices.

See also

Emulsion – The type of liquid-liquid mixture used as the template for PolyHIPEs.

Polymeric foam – The broader class of materials to which PolyHIPEs belong.

Porous medium – The general scientific classification for materials with pores.

Pickering emulsion – An alternative method for stabilizing emulsions using solid particles instead of surfactants.

Aerogel – Another class of highly porous, low-density solid material, often compared to PolyHIPEs.

Tissue engineering – A major application area for PolyHIPEs, particularly as scaffolds for cell growth.

Separator (electricity) – The general component in batteries that PolyHIPEs can be engineered to function as.