Thermotropic Icy Road Sign with Light Scattering and Fluorescence Response

Prototypes of flexible, electricity-free, ice warning signs for roads and pavements have been developed. A thermotropic response in the form of an upper critical solution (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 conceptual road sign application is a multi-lamellar flexible strip with an active layer of a polystyrene-based solution. The solvent is a plasticizer, here either dioctylphthalate (DOP) or its alternative 1,2-cyclohexane dicarboxylic acid diisononyl ester (DINCH). A collection of styrene-based macromolecules were made by free radical (co)polymerization, varying molecular weight and monomer feed composition. UCST type phase diagrams for the polymer solutions were constructed from cloud point data measured by a bespoke photographic set-up, in which up to 30 samples were measured simultaneously using both light scattering, that is opacity, and fluorescence. For the latter, 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 contrast between ‘ON’ and ‘OFF’ states in the prototype ice warning signs was optimized by tuning the polymer concentration and the active layer thickness. Our prototype signs show full reversibility over many temperature cycles. We believe the concept can be of wider use in electricity-free signs and labels.