Freeze-drying for foam fabrication

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

Freeze-drying has been used to create structured foams before, the first such experiments being with rubber in the 1940s with the ice crystals formed throughout this process acting as templates to form the porous foam structure. However, when trying to create particularly strong, stable polymer foam structures, engineers and chemists today tend to rely on more complicated processes. The most straightforward of these methods is the so-called foaming or expanding process, which consists of introducing small discontinuities (for example, by dispersing a compressed gas) into a soft polymer and then taking a further step to reinforce the cellular structure created upon polymerisation or cooling.

The University of Warwick team’s new approach to fabricate polymer foams by ‘ice-templating’ differs from previous strategies in that they use 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, they employ a blend of larger ‘soft’ polymer latexes (with diameters in the range of 200-500 nm) in conjunction with a range of much smaller ‘hard’ nanoparticles such as silica (with diameters in the range of 25-35 nm).

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 cellular structured foam in just one step in which each cell is effectively given an armoured 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 were also able to employ 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.

The armoured polymer foams have a range of applications but one of the most interesting could be a new range of room-temperature, low-power gas sensors. The team increased the complexity of their mixture of colloids by the addition of a third colloidal component, carbon black particles with approximate diameters of 120 nm, which allowed them to produce a conductive foam - 14% of the weight of which was carbon black particles.

Lead researcher Dr Stefan Bon from the University of Warwick’s Department of Chemistry said: “This new process allows us to create interesting foam-based nanocomposite materials which show promising results as gas sensors that can operate at room temperature and differ from traditional metal-oxide-based sensors. We know that existing chemical sensors formed from composites of carbon black particles and insulating polymers have been previously shown to form room-temperature (thus low-power) chemical sensors for detecting a range of volatile organic compounds. Now in one step, we can place the same material in a high-tech polymer foam to create a new range of gas-sensor devices. We believe these materials could become a new generation of sensing porous films.”