Abstract
Temperature-responsive photonic coatings are appealing for a variety of applications, including smart windows. However, the fabrication of such reflective polymer coatings remains a challenge. In this work, we report the development of a temperature-responsive, infrared-reflective coating consisting of a polymer-stabilized cholesteric liquid crystal siloxane, applied by a simple bar coating method. First, a side-chain liquid crystal oligosiloxane containing acrylate, chiral and mesogenic moieties was successfully synthesized via multiple steps, including preparing precursors, hydrosilylation, deprotection, and esterification reactions. Products of all the steps were fully characterized revealing a chain extension during the deprotection step. Subsequently, the photonic coating was fabricated by bar-coating the cholesteric liquid crystal oligomer on glass, using a mediator liquid crystalline molecule. After the UV-curing and removal of the mediator, a transparent IR reflective polymer-stabilized cholesteric liquid crystal coating was obtained. Notably, this fully cured, partially crosslinked transparent polymer coating retained temperature responsiveness due to the presence of non-reactive liquid-crystal oligosiloxanes. Upon increasing the temperature from room temperature, the polymer-stabilized cholesteric liquid crystal coating showed a continuous blue-shift of the reflection band from 1400 nm to 800 nm, and the shift was fully reversible.
Highlights
Stimuli-responsive photonic polymer coatings are able to change color in response to an external stimulus [1]
This change can be achieved by either using temperature-sensitive chiral dopants [6,7,8] or by utilizing the pre-transitional effect, which causes a strong redshift of the reflection band near the phase transition between the cholesteric liquid crystals (Ch-LCs) phase and the smectic (Sm) phase as a result of unwinding of the helix [9,10,11,12]
We designed a side-chain polysiloxane LC terpolymer with both cholesteric and smectic LC phases and that is crosslinkable (TP, Scheme 1). This polysiloxane-derived terpolymer contained three mesogenic side groups (P1, P2, and P3) prepared from three precursor molecules (M1, M2, and M3), which were attached to the polysiloxane backbone by hydrosilylation
Summary
Stimuli-responsive photonic polymer coatings are able to change color in response to an external stimulus [1]. With the full conversion of the acrylate groups under UV-initiated polymerization, crosslinked networks were obtained in the coating This partially crosslinked polymer coating still shows a ~600 nm reflection band shift within the IR region in response to a temperature change between 20–66 ◦C, owing to the Sm-Ch phase transition. Such temperature-responsive polymer-stabilized polysiloxane photonic coatings would be attractive for a variety of applications including foils for energy-saving windows [3,26,27,28,29,30,31]
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