Thermochromic Liquid Crystal Oligomers for Solar Infrared Control in Windows
Henk Sentjens successfully defended his PhD thesis at the Department of Chemical Engineering and Chemistry on April 9th.
In modern society, it is of great important to find ways to more efficiently use energy. Therefore we are searching for areas both on large and individual levels to save energy. A significant area of energy consumption is the temperature control of living and working spaces. The use of air conditioning is rapidly increasing worldwide, which puts significant strain on the global energy demand. Keeping solar heat out rather than actively cooling indoor spaces could potentially save large amounts of energy. Sunlight consists of approximately 50% infrared light; this is light humans cannot see, but it does contribute to heating effects. Keeping this infrared light from passing through windows would therefore help keep indoor spaces significantly cooler without affecting visibility. This could be achieved by using layers that are transparent to visual light, but reflect the infrared potion of sunlight. Henk Sentjens performed research in the Stimuli-Responsive Materials and Devices group with promising results showing that liquid crystals are an interesting option for indoor temperature control.
Reflective materials based on such liquid crystals
One material capable of forming such a layer are cholesteric liquid crystals. Thanks to their helical shape, these are not only able to reflect specific wavelengths of light while remaining transparent to the remainder, they can also be temperature responsive, meaning they can reflect different wavelengths at different temperatures. Thanks to that property, they can be used to form a layer that can reflect increasing amounts of infrared light as the layer gets warmer. This allows them to keep heat out during summer, when it鈥檚 not desirable, but also lets them pass heat during winter, saving on heating costs. The research described in this thesis describes the development of reflective materials based on such liquid crystals.
Oligomers
To make the liquid crystals more easily coatable and give them a broader temperature response, they are processed into oligomers; molecule chains where a few liquid crystals are bonded together. The oligomers are carefully designed to ensure they have the right combination of reflective properties and thermal responsiveness. This is achieved by tuning the average length, and adjusting the liquid crystal composition. The final result are transparent, responsive coatings that can reflect either visible or infrared light as desired.
The oligomers, by design, have reactive groups at their ends to allow them to grow. However, to enable common liquid crystal processing techniques and prevent the loss of responsiveness, it is crucial these groups are removed. However, letting them react away with small 鈥渆ndcappers鈥� destabilizes the coatings and results in the loss of their reflective properties. Therefore, several different liquid crystals were designed and synthesized to remove the reactive groups while maintaining the reflection and temperature response. In doing so, we demonstrated a strong dependence of the isomeric purity of these endcapping molecules on the optical quality of the coatings.
Results
The researchers used the endcapped oligomers to create a proof-of-concept reflector. Liquid crystal reflectors fundamentally are only able to reflect 50% of sunlight; natural light consists of equal parts right- and left handed polarized light, and most cholesteric liquid crystals can only reflect one or the other. This can be overcome by using a halfwaveplate, a thin layer which is able to flip light from one handedness to the other. Combining two cholesteric layers with such a waveplate makes it possible to reflect 100% of the light. By carefully tuning the oligomers and the waveplate, it was possible to create a device with a solar modulation of 5.2%. These promising results show liquid crystals are an interesting option for indoor temperature control.