Chromogenic Polymers

In recent years functional polymers changing their visible optical properties in response to an external stimulus have met with growing interest. According to the external stimulus which affects the optical properties, these so-called chromogenic polymers are classified as thermochromic (stimulus: temperature), photochromic (stimulus: light), electrochromic (stimulus: electric field), piezochromic (stimulus: pressure), ionochromic (stimulus: ion concentration) and biochromic (stimulus: biochemical reaction). Due to their advanced properties the demand on chromogenic polymers for future application will become enormous.

Thermochromism is either caused by structural changes of functional dyes or by changing the repeat distance of periodic structures. The R&D activities of the Fraunhofer IAP in this field are focused on functional dye systems based on pH-indicator or leuco dyes. Leuco dye systems usually switch from a colored to a colorless state on heating, which we denote as conventional switching. For the opposite switching direction from colorless to color on heating we use the term inverse switching.

Thermotropic effects are based on a transition from a clear transparent into a light scattering state. The light scattering state consists of a domain structure, whereby the refractive index or indices of the domains differ from the refractive index of the surrounding matrix material. The Fraunhofer IAP has long-term experiences in the development of thermotropic materials.