Supramolecular ordered photochromic cholesteric polymers as smart labels for thermal monitoring applications

Abstract New reversible temperature-sensitive polymeric materials with a broad window of trigger temperatures were developed for application as thermal monitoring smart labels for perishable commodities. In particular new liquid crystalline copolymers functionalized with azoaromatic chromophores and chiral cholesteric moieties were synthesized by Controlled Radical Polymerization (CRP) using cheap and easily available reagents and mild reaction conditions. These materials exhibit a chiral nematic phase (N*) with an intense Bragg reflection which gives an iridescent and bright appearance to their thick films. By inducing the azoaromatic trans-cis isomerization through UV irradiation, it is possible to break the native chiral supramolecular conformational order thus leading to an amorphous transparent phase which is characterized by different UV–Vis and CD spectra and which is stable whilst the temperature is kept below the T g of the polymer. Hence, tags can be reused even though the required process is so specific that they cannot be easily tampered. Thus, owing to the high control of the polymer molecular weight and structure allowed by the ATRP technique, it is possible to finely tune the trigger temperature and the photoswitchable properties thus allowing the fabrication of tailor made labels suitable for a huge variety of products with potential applications as sensors and in photonic and display fields.