We set out to develop a prototype for “icy road” warning signs which was able to operate autonomously without the use of electricity, and which could be easily placed onto existing road features, such as street boundary pillars and road safety barriers.

The number of road accidents in the UK under frosty or icy conditions runs in the thousands. Our concept would aid to reduce these numbers, without the introduction of a digital, and thus electric, infrastructure.

The results from our studies are now published open access in the Journal of Materials Chemistry C from the Royal Society of Chemistry. The conceptual road sign application is a multi-lamellar flexible strip.

A temperature triggered response in the form of an upper critical solution temperature (UCST) type phase separation targeted near the freezing point of water manifests itself through light scattering as a clear-to-opaque transition. It is simultaneously amplified by an enhanced photoluminescence effect. The essence is summed up in the supporting video.

The active layer in the strip is a polystyrene-based solution. When the temperature drops to near freezing the polymer molecules undergo a phase transition, from a coil happily dissolved, into a globule that crashes out of solution. This crashing out (phase separation) triggers the transparent active layer to become cloudy white, and thus opaque. The temperature at which this occurs is referred to as the cloud point. It is the result of the scattering of light caused by the different refractive indices of the two phases.

The clear-to-opaque transition of the active layer on the black strip allows us to introduce colour, either by adding a dye dissolved in the plasticizer liquid, or by printing a coloured translucent image on the top layer of the strip. This effect is shown in the image below where Pac-Man’s ghost only appear under freezing conditions.

We wanted our road sign to be more visible in the dark. For this the concept of restricted motion enhanced photoluminescence, often referred to as aggregation-induced emission (AIE), was used. Polystyrene labelled with tetraphenylethylene (TPE) was used for this. The fluorescent tag emits light vividly under icy conditions, as the combination of phase separation, increased viscosity of the system, and lower temperatures restricts its motion and thus enhances its light emission (see video).

Prof. dr. ir. Stefan Bon says: “We worked on this project for a number of years, with a team of talented people which included undergraduate, master, and PhD students and research fellows. The project was co-led by Robert Young and Joshua Booth, who are shared first author on the paper. We are delighted with how it turned out, and are pleased that all is now published open access in the Journal of Materials Chemistry C. We hope that people are enthusiastic about the concept.”

The paper can be accessed from here:

https://doi.org/10.1039/D1TC01189H