Time-temperature indicators for high temperature applications

New thermochromic materials are reported, which are useful for application as time-temperature indicators (TTIs) in temperature regimes that are complementary to those accessible with existing technologies. The design approach involves incorporating small amounts (0.3-2.9 wt.-%) of the excimer-forming, photoluminescent chromophores 4,4′-bis(2-benzoxazolyl) stilbene or a cyano-substituted oligo(p-phenylene vinylene) derivative into ethylene/norbornene copolymers with glass transition temperatures (T _g) between 131 and 149 °C. These dye/polymer blends were melt-processed and quenched from homogeneous melts to below their T_g to kinetically trap the dyes in dispersed states, which exhibit monomer fluorescence. Upon annealing the materials above T_g, self-assembly of the dye molecules into aggregates that allow for excimer formation occurs, concomitant with permanent and pronounced fluorescence color changes. This approach yielded TTI materials that demonstrate a predictable, Arrhenius-type behavior and which are useful in a temperature regime from ∼130 to 200 °C. The kinetics of the color change were tunable on time scales between seconds and days by changing dye concentration, dye structure, or host polymer T_g.