Programming Thermochromic Liquid Crystal Hetero-Oligomers for Near-Infrared Reflectors
Unequal Incorporation of Similar Reactive Mesogens in Thiol-ene Oligomers
Henk Sentjens (Eindhoven University of Technology)
A.J.J. Kragt (ClimAd Technology B.V., Eindhoven University of Technology, Architectural Technology)
Johan Lub (Eindhoven University of Technology)
Mart D.T. Claessen (Eindhoven University of Technology)
Vera E. Buurman (Eindhoven University of Technology)
Joris Schreppers (Eindhoven University of Technology)
Henk A. Gongriep (Eindhoven University of Technology)
Albertus P.H.J. Schenning (Eindhoven University of Technology)
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Abstract
Cholesteric liquid crystal oligomers are widely researched for their interesting thermochromic properties. However, structure-property relationships to program the thermochromic properties of these oligomers have been rarely reported. In this work, we use the versatile thiol-ene click reaction to synthesize a series of hetero-oligomers and study the impact of different compositions on the thermochromic behavior of the resulting material. Characterization of the oligomers shows significantly different rates of reaction for the monomers despite their very similar structures, which leads to oligomer compositions that do not match the original reaction feed. The oligomers are then used to produce thin near-infrared reflecting coatings. The best-performing thermochromic reflector has a room-temperature reflection band that shifts a total of 510 nanometers upon heating to 120 °C. The shift is repeatable for up to 10 times with no appreciable degradation. The room temperature reflection of the coatings is shown to be tunable not only by adjusting the chiral dopant concentration but also by the ratio of the monomers. Finally, we show that the oligomers can be chemically modified by making their reactive end groups undergo a reaction with monothiol compounds. These modifications allow for further fine-tuning of liquid crystal oligomers for heat-regulating window films, for example.
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