Credit: © 2010 ACS

Thin films with a honeycomb structure can be prepared using the so-called breath figure technique, whereby water — condensed on the surface of a polymer solution and arranged in a hexagonal array — acts as a template around which the film forms. Such films are particularly attractive to chemists and materials scientists because their thickness, surface area and pore dimensions are controllable. Their preparation, however, has typically been difficult to scale up and the films obtained have lacked stability. Now, Koh Hou Wong and co-workers at the University of New South Wales in Sydney have made films with improved mechanical properties, adapting the technique by using two silicone-based branched co-polymers and subsequently crosslinking them1.

Two highly branched co-polymers were first prepared by free radical polymerization — a process that is cheap, easily scalable and enables good control over the structures formed. The co-polymers were then assembled into a honeycomb-structured porous film through the breath figure technique. Alkoxysilane functional groups were subsequently crosslinked by the sol–gel process to give highly porous, stable and flexible films.

The researchers studied the thermal conductivity of the film by seeding a thermosensitive polymer inside its pores and observing its phase transition on changing the temperature. The thermo-sensitive polymers occupying the pores reflect a range of visible-light wavelengths and appear multicoloured, whereas the porous film appears transparent. On heating, the phase transition of the seeded polymer can therefore be directly observed. The honeycomb films exhibited good heat conduction and thermal stability and combined with their toughness, flexibility and ease of preparation, show promise for industrial applications.