Warwick researchers develop new method for generating stable foam structures

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Chemists and engineers at Warwick University have found that exposing particular mixtures of polymer particles and other materials to sudden freeze-drying can create a high-tech armoured foam they say could be used for a number of purposes, including a new range of low-power room temperature gas sensors.

Freeze-drying has been used to create structured foams before, however generating stable polymer foam structures normally requires complicated production techniques. The most straightforward of these methods is the so-called foaming or expanding process, which involves introducing small discontinuities (for example by dispersing a compressed gas) into a soft polymer, and then reinforcing the cellular structure created, upon polymerisation or cooling.

The new approach of fabricating polymer foams by “ice-templating” differs from previous strategies in that it uses a special range of colloids, (mixtures of small particles dispersed in water) with crucial differences in their hardness and size, as key building blocks. In particular, the technique uses a blend of larger soft polymer latexes, with diameters in range of 200–500nm, in conjunction with a range of much smaller hard nanoparticles such as silica, with diameters in range of 25–35nm.

When such a mixture is exposed to freeze-drying the difference in diameters induces a concentration enrichment of the smaller harder particles in the mix near the wall of each growing ice crystal. This creates a cell-structured foam in just one step, in which each cell in effect has an armored layer of the smaller, harder nanoparticles.

The Warwick researchers also found that by changing parameters, such as the nanoparticle/polymer latex ratios and concentrations, as well as the nanoparticle type, it was possible to fine-tune the pore structure, and the overall porosity, of the polymer foams. The team has also used various types of inorganic nanoparticles to create this instant freeze-dry foam armouring including, silica, Laponite clay, aluminium oxide, as well as small polystyrene latex particles.