Self-assessing photoluminescent polyurethanes

Due to excimer formation upon self-assembly, the photoluminescence (PL) spectra of isolated and aggregated cyano-substituted oligo(p-phenylene vinylene) (cyano-OPV) molecules can be very different. With the objective to create self-assessing elastomers, which reversibly change their PL color as a function of applied strain, cyano-OPVs were used here as built-in deformation sensors in thermoplastic polyurethanes (TPUs). Binary blends of a commercial TPU and 0.05-0.4% w/w of various cyano-OPVs were prepared by melt processing. The phase behavior and emission characteristics of these materials were strongly influenced by the chromophore's chemical structure, the blend's composition, and the processing protocol. However, only a modest PL color change was observed upon deformation of these blends. Also investigated was the covalent incorporation of 1,4-bis(alpha-cyano-4-(12-hydroxydodecyloxy)styryl)-2,5-dimethoxybenzene into TPUs based on poly(tetramethylene glycol), butanediol, and 4,4'-methylene-bis(phenyl isocyanate). The TPU composition was systematically varied, and the composition was related to the material's PL response. This approach afforded TPUs which in their relaxed state display predominantly excimer emission and exhibit a significant PL color change upon deformation. This mechanochromic response was largely reversible and mirrored the stress-strain curves of the materials.