Pores for thought: Just shine a light

Illuminating polymer pores 

Nuclear magnetic resonance spectroscopy and mass spectrometry, among other techniques have been used in work on polymer pores. While, irradiation with light is a well-established approach to the initiation of polymerization as well as cross-linking (or curing of polymers) during plastics production, researchers in the USA have now demonstrated that light can be used to retroactively increase the size of the pores within a polymer network.

Writing in the Wiley journal Angewandte Chemie, the team suggests that this new approach allows for the production of polymer gels with tailored mechanical properties.

Huaxing Zhou and Jeremiah Johnson of the Massachusetts Institute of Technology have incorporated photoreactive groups - organic side-chain derivatives of trithiocarbonate units - into their polymers to endow the with the ability to respond to light irradiation. When UV light shines on the materials, the bonds between the sulfur atoms and the carbons of the side chain are photolysed. This generates highly reactive free radicals that then rapidly form new bonds. If this process occurs in the presence of N-isopropylacrylamide monomers, the monomers will polymerise so that the side groups grow into long polymeric chains. As long as the UV shines bond breaking and new bond formation can propagate around the sulfur atoms only stopping once the UV is switched off.

Chemistry that gels

The team then treat their resulting polymer with tris-tetrazine, which causes polymer cross-links to form leading to the formation of a polymer gel with tiny pores embedded within it. They point out, however, that even in this cross-linked state, the polymer chains are capable of continuing to grow outward from the trithiocarbonate groups if more monomer is added and the UV lamp switched on again. The team explains that the longer the irradiation lasts, the longer the chains can grow, which increases the size of the pores in the network. Bigger pores mean a less "rigid" gel, which can then swell still further. Intriguingly, the photoprocesses can occur even under incident sunlight.

Light control

As with many light-activated chemical processes it should be possible to "pattern" the photoreactions by applying a mask to particular areas and so control which parts of the initial material are affected by the UV. The process could thus be controlled spatially, using a mask, as well as temporally by switching on and off the UV light source at appropriate times. If the polymerization is carried out stepwise with different monomers, it should also be possible to make a chemical pattern or a chemical gradient, allowing for the controlled tuning of the size and composition of the pores within a gel, the team points out.

One might imagine a material that would emulate phototropism in plants, growing towards a light source. Such a polymer might, if connected to appropriate light-harvesting system be used to "grow" an optimal structure for a solar-energy converting device that would attain maximal conversion for a given location and pattern of incident sunlight if not placed entirely in the open air, for instance. Similarly, the polymers could be grown to create space filling filters for soaking up toxic agents for a structure or as cell scaffolds for studying cell growth or engineering new materials that self-heal.