Ralf Ruhmann

Thermotropic and Thermochromic Polymer Based Materials for Adaptive Solar Control

The aim of this review is to present the actual status of development in adaptive solar control by use of thermotropic and organic thermochromic materials. Such materials are suitable for application in smart windows. In detail polymer blends, hydrogels, resins, and thermoplastic films with a reversible temperature-dependent switching behavior are described. A comparative evaluation of the concepts for these energy efficient materials is given as well. Furthermore, the change of strategy from ordinary shadow systems to intrinsic solar energy reflection materials based on phase transition components and a first remark about their realization is reported. Own current results concerning extruded films and high thermally stable casting resins with thermotropic properties make a significant contribution to this field.

First example of a non-toxic thermochromic polymer material – based on a novel mechanism

Until now, all thermochromic materials have contained at least one toxic or carcinogenic component which is a major shortcoming for their application especially in medical or food technology products. This paper describes the development of the first non-toxic and non-carcinogenic thermochromic polymer material. The material consists of the biopolymer poly(lactic acid) (PLA), a natural dye of the anthocyanidine class, a gallate derivative and a fatty acid. The origin of the observed thermochromic effect is a conformational change of the polymer backbone which reversibly triggers the formation of polymer–dye complexes. Accordingly thermochromism represents an immanent property of the novel polymer material. The presented results establish a novel mechanism to realize the visualization of temperature changes in a way detectable by the human eye without any kind of auxiliary and provide a new approach for the development of thermochromic materials.

Thermotropic Materials for Adaptive Solar Control

Thermotropic materials offer an immense potential in adaptive solar control. They combine specific optical properties like absorbance and reflection, and high stability against solar radiation and heat with technology compatible processing capacities. Therefore, they represent perfect energy efficient materials. In detail, polymer blends, polymer-based hydrogels, casting resins, and thermoplastic films with a reversible temperature-dependent switching behavior have been investigated. Here a comparative evaluation of the different concepts with a view to their application in adaptive solar control is presented. Own current results exploit the well-known phase change materials and describe its use for adaptive solar control with extruded films or highly stable casting resins with thermotropic properties. Therewith, the status has changed from diffuse sunblind systems to intrinsic solar energy reflecting materials and a first smart window system based on phase change materials has now commercialized [1]. In summary: It is amazing that the solar energy itself is used as a promoter against solar heat.

Photochemically induced structure transfer from analogous azobenzenes and stilbenes onto a liquid crystalline phase

The photoinduced structure transfer from different 4-N,N-diethylamino-4′-alkoxyazobenzenes and analogous stilbenes onto a nematic liquid crystalline phase has been investigated. The structure transfer caused by the azobenzenes was found to be 7–8 times smaller than for the corresponding stilbenes. The photoinduced E→Z isomerization of the azobenzenes occurred in the liquid crystal more slowly than the thermal back reaction. In contrast, despite lower rate constants for the E→Z photoisomerization, the stilbenes under investigation brought about a greater structure transfer because no thermal back reaction was found at 25 °C.Investigations carried out with the azobenzenes in ethanol and cyclohexane showed the great influence of the polarity of the surrounding medium on the ratio between the rate constants for the E→Z photoisomerization and the thermal Z→E back reaction. This ratio kE→Z/kZ→E,therm increased at 25 °C from ca. 0.15–0.5 in the highly polar liquid crystal to ca. 13–19 in ethanol and nearly 500 in the apolar cyclohexane.This paper concludes that only the investigated stilbenes are suitable for photoaddressed displays using highly polar, thermotropic liquid crystals.