Jill Kunzelman

Shape memory polymers with built-in threshold temperature sensors

The design, fabrication and characterization of a new shape memory polymer (SMP) with built-in temperature sensing capabilities are reported. The material was prepared by incorporating a fluorescent, chromogenic oligo(p-phenylene vinylene) dye into a cross-linked poly(cyclooctene) (PCO) matrix via guest diffusion. The dye concentration was chosen to allow for self-assembly of the dye upon drying, resulting in the formation of excimers. Exposure of these phase-separated blends to temperatures above the melting point (Tm) of the PCO leads to dissolution of the dye molecules, and therefore causes a pronounced change of their absorption and fluorescence color. The optical changes are reversible; i.e., the aggregate absorption and emission are restored upon cooling below Tm. The color is dictated by the phase behavior and is independent of the mechanical state of the SMP. Thus the effect allows one to monitor reaching of the set/release temperature of the polymer.

Self-assembly of chromogenic dyes—a new mechanism for humidity sensors

The design, fabrication and characterization of a novel type of humidity sensing polymer are presented. The new material was prepared by incorporating a chromogenic, cyano-substituted oligo(p-phenylene vinylene) dye into a polyamide by quenching the blend from the melt. Exposure of these kinetically trapped, molecularly mixed blends to water plasticizes the amorphous phase and leads to aggregation of the dye molecules, concomitant with pronounced and irreversible changes of the materials absorption and fluorescence color.

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 (Tg) between 131 and 149 °C. These dye/polymer blends were melt-processed and quenched from homogeneous melts to below their Tg to kinetically trap the dyes in dispersed states, which exhibit monomer fluorescence. Upon annealing the materials above Tg, 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 polymerTg.