Thermochromic polyethylene films doped with perylene chromophores: experimental evidence and methods for characterization of their phase behaviour

We report on a thermochromic system suitable for sensing temperature changes in the 30–70 °C regime based on linear low density polyethylene (LLDPE) films doped with N,N′-bis-(1′-phenylethyl)-perylene-3,4,9,10-tetracarboxydiimide (PE-Pery), a fluorescent aggregachromic dye. At low PE-Pery concentration (0.01–0.02 wt%), the dye monomers were well dispersed in the polymer matrix showing their maximum fluorescence intensity at 525 nm. As the dye content was increased, monomers emission quenched whereas dye aggregates prevailed above 0.05 wt% as well as their red fluorescence band at 620–680 nm. Upon heating from 30 to 70 °C, all films displayed a thermochromic response, more evident for the less concentrated samples (<0.05 wt%) in which the emission of the dye as a monomer continuously increased with increasing temperature. This phenomenon promoted effective color changes from a dull red-violet at 30 °C to a bright yellow-green at 70 °C. Combined DSC and variable-temperature Solid State NMR (SSNMR) measurements addressed the thermochromic behavior to the increased amount of the available amorphous phase and to the increased mobility of both the interphase and amorphous components with temperature, which favored PE-Pery dispersion and diffusion, thus recovering their fluorescence. Overall, the present results support the use of PE-Pery-enriched LLDPE films as a chromogenic material suitable for the detection of temperature changes close to the physiological regime.