STRESSED POLYMERS CHANGE COLOR
Materials incorporate fluorescent dyes as internal strain sensors
Light-emitting polymer blends that respond to stresses and strains by changing their hue have potential uses ranging from early internal failure sensors to antitampering films for packaging, according to the polymer scientists who developed the materials at Case Western Reserve University, Cleveland.
Associate professor Christoph Weder and graduate student Brent R. Crenshaw prepared blends of conventional polymers such as linear low-density polyethylene and a series of highly photoluminescent dyes [Chem. Mater., 15, 4717 (2003)].
"The dyes serve as integral sensors," Weder says. They allow a material's mechanical deformation to be traced through a change in its fluorescence--for example, from red to green. Under UV light, the effect is readily seen by the naked eye.
The phenomenon relies on phase separation of small aggregates of the cyano-oligo(p-phenylene vinylene) dye molecules in the polymer matrix. The phase behavior of the blend is controlled by varying the dye's chemical structure, the blend composition, and the processing conditions.
Dissolved "isolated" dye molecules exhibit "monomer emission." When the molecules are assembled as aggregates, in a polymer matrix for example, they display "excimer emission," which has a different fluorescence color.
"Our work indicates that the color change requires numerous very small aggregates of the dye molecules embedded in the polymer matrix," Weder explains. "Only with this architecture can mechanical deformation of the matrix efficiently break up the dye aggregates and lead to dispersion and dissolution of the dye molecules.
"We are currently working on applying the concepts to other polymer systems--for example, elastomers, in which the effect can be reversible," Weder adds. "We are also extending the approach to other stimuli, such as changes in temperature."
||HUE AND CRY Dye molecules, such as the one shown below, change their emission color when the polymer matrix is deformed.
PHOTO BY B. R. CRENSHAW