Open-cell microcellular foams with high surface area as well as well-defined porosities display a variety of attributes that are good candidates for a number of applications such as separation media, catalysts supports, liquid absorbents, gas storage membranes, and materials for scaffolds in tissue engineering.
The synthesis of microporous foams with open cell sizes through high internal phase emulsions has been studied before. High Internal Phase Emulsions are emulsions where the volume fraction of the dispersed internal phase is greater than the packing volume fraction of 0.74, and the internal water phase is dispersed within the external oil phase. Drops in this high internal phase emulsion are polyhedral shaped and their interface is ruptured by drop coalescence during polymerization. The cells are then linked up to form open-cell structures. The resulting microcellular foams are obtained by removing the aqueous phase after the process of polymerization.
The obtained microcellular foams display high porosity, low density, open-cell structure and smaller cell size when compared to conventional gas extrusion foams. However, while many research works have focused on the application of these foams, those focusing on the synthesis of electrically conductive microcellular foams are rare. In a recent paper published in Polymer, researchers led by professor Seong Jae Lee from The University of Suwon, Republic of Korea, implemented multi-walled carbon nanotubes as electrically conductive fillers, and synthesized conductive microcellular foams based on the poly(styrene-co-divinylbenzene) implementing the high internal phase emulsion polymerization approach.
In a bid to improve the degree of dispersion of the carbon nanotubes in the aqueous phase, the authors prepared dopamine solution by dissolving dopamine hydrochloride in Tris-hydrochloride buffer solution. Carbon nanotubes were then dispersed in the dopamine solution where the mixture was them magnetically stirred to induce self-polymerization.
Before preparing the high internal phase emulsion system, the authors prepared the organic as well as aqueous phase separately. Organic phase consisted of momomers and surfactant, whereas aqueous phase consisted of water, initiator and polydopamine-coated carbon nanotubes.
The authors added the aqueous phase dropwise while the organic phase was being stirred. When the resulting emulsions were stable, they formed the water-in-oil emulsions. The obtained emulsions were then polymerized to poly(high internal phase emulsion) foams in an oven. The emulsions were then purified with water and methanol.
The authors increased the amount the allowable carbon nanotubes without damaging the emulsion stability, and successfully prepared poly(high internal phase emulsion) foams with an electrical conductivity of approximately 10-2 Sm-1.
It was found that small amounts of polydopamine-coated carbon nanotubes did not induce significant difference in the insulating behavior of the polymer matrix. At a particular content, electrical conductivity was noted to increase rapidly, then levelled off gradually. The point of rapid electrical conductivity increase is the electrical percolation threshold to which flowing electrons are subjected owing to the carbon nanotube network. The percolation threshold was determined to be less than 1 wt% of carbon nanotubes.
From hydrodynamic theory and morphology, the authors found that the cell sizes of poly(high internal phase emulsion) foams were decreased, storage modulus and yield stress increased, and interfacial tension decreased with increasing content of the polydopamine-coated carbon nanotubes.
Haseung Kim, Kyung Hyun Ahn, Seong Jae Lee. Conductive poly(high internal phase emulsion) foams incorporated with polydopamine-coated carbon nanotubes. Polymer, volume 110 (2017), pages 187-195.Go To Polymer