Polymer changes colour in the heat of the moment

Scientists in China, the UK and the Netherlands have engineered a polydiacetylene polymer that reversibly changes colour within 1 second of being heated or cooled.

Thermochromic polymers have a wide range of potential uses, from biological sensors to smart windows. However, the irregular structure and weak molecular interactions in established thermochromic polymers results in long response times, slow reversibility and a narrow working temperature range.

Now, a team led by Zhengzhong Shao of Fudan University in China report that introducing peptide side chains into the polymer gives fibres that are strong and exhibit a remarkably rapid colour change even at temperatures up to 200 °C. The critical temperature of the transition can be tuned by varying the length of alkyl chains in the polymer.

Not only do the peptides stabilise the diacetylene but they also enable the material to self-assemble into continuous fibres. Heat disrupts the coplanarity of the polymer by introducing more motion into the polydiacetylene side chains and the resulting change in conjugation length changes the polymer’s colour by affecting which wavelengths are absorbed. The presence of the peptide localises the disruption and also acts as a stabilising agent, re-establishing the conjugation and the original colour as soon as the heat is removed so the process can be repeated whenever heat is applied as long as the peptide is not disrupted.

‘The colorimetric response maintains a stable value even after scores of thermal cycles,’ says Shao. ‘There is hardly any loss of sensitivity or colour change with repeated use, as long as the organisation from the peptide remains.’ If the temperature exceeds 200 °C, the peptide is irreversibly damaged and the material no longer presents the thermochromic properties.

The team are now looking into adjusting the temperature range at which the colour change takes place. Polymer chemist Dmitriy Paraschuk of Moscow State University in Russia agrees that this is a priority. For the system to be a suitable biosensor, ‘the thermochromic change needs to be closer to that of the human body.’ However, he adds that the team ‘have managed to combine strong thermochromism with excellent processibility’.