Overview

Chromogenic polymers change their visible optical properties in response to an external stimulus [1]. In terms of the specific stimulus they are subclassified as thermochromic (temperature), photochromic (light), electrochromic (electric field), piezochromic (pressure), ionochromic (ion concentration) or biochromic (biochemical reaction). The chromogenic phenomena enable the integration of sensor and actuator functionality or any kind of information respectively into a material itself.

This update is focused on thermochromic phenomena in polymers. Thermochromism can appear in all different classes of polymers: thermoplastics, duroplastics, gels, inks, paints or any type of coatings. The polymer itself, an embedded thermochromic additive or a supermolecular system built by the interaction of the polymer with an incorporated non-thermochromic additive can cause the thermochromic effect. Also, from the physical point of view, the origin of the thermochromic effect can be multifarious. It can come from changes of light reflection, absorption and/or scattering properties with temperature [2].

The content of this review is structured according to the thermochromic material type. Section 2 gives an overview of polymers exhibiting thermochromic properties themselves. Thermochromic polymer systems obtained by doping the polymer matrix with thermochromic additives are reported in Section 3 and Section 4 reviews polymer systems in which the interaction between the polymer matrix and an incorporated additive is the origin of thermochromism. Finally, in Section 5, polymer systems changing their transparency with temperature by reversible switching between a transparent and a light-scattering state are reviewed. In the literature, this subclass of thermochromic materials is termed thermotropic. At the end...