INSIGHTS INTO THE ORIGIN OF THE AGGREGACHROMIC BEHAVIOUR OF PERYLENE DYES IN THERMOCHROMIC LLDPE FILMS

Films of semicrystalline LLDPE (Low Density PolyEthylene) doped with N,N"-bis-(1"- phenylethyl)-perylene-3,4,9,10-tetracarboxydiimide (PE-Pery), a fluorescent aggregachromic dye, showed a thermochromic response upon heating from 30 to 70 °C [1]. By increasing temperature the fluorencence of the dye as a monomer (emission bands at 525 and 565 nm) increased with respect to that of dye aggregates (emission band at 620-680 nm), with a corresponding change of color from a dull red-violet at 30 °C to a bright yellow-green at 70 °C. This behaviour makes PE-Pery-enriched LLDPE films suitable to detect temperature changes close to the physiological regime with a wide range of possible applications as plastic sensing devices [2]. In this work spectrofluorimetric, calorimetric, and solid-state NMR (SSNMR) techniques were combined to investigate the processes occurring upon heating in PE- Pery/LLDPE films, possibly driving PE-Pery aggregachromic behaviour. Understanding the origin of the thermochromic response is indeed fundamental to the reasearch for materials with similar and improved sensing properties. Variable-temperature SSNMR experiments gave important information on the phase properties of the LLDPE domains in the presence of PE-Pery and on the trasformations occurring upon heating. Interestingly, both 13C CP/MAS spectra and on resonance 1H FID"s showed evidences of the presence of a LLDPE interphase, intermediate between the crystalline and the amorphous phases [3] and showing a peculiar behaviour upon heating, which seems to play a crucial role in determining the thermocromic response. Finally, combining all the results obtained, we can propose a model explaining the aggregachromic behaviour of PE-Pery in LLDPE films. References [1] M. Carlotti, G. Gullo, A. Battisti, F. Martini, S. Borsacchi, M. Geppi, G. Ruggeri and A. Pucci Polym. Chem. 6, 4003-4012 (2015) [2] I. S. Park, H. J. Park and J.-M. Kim ACS Appl. Mater. Interfaces 5, 8805-8812 (2013) [3] P. J. Flory J. Am. Chem. Soc. 84, 2857-2867 (1962)