Pitch Gradation by Ion-Dragging Effect in Polymer-Stabilized Cholesteric Liquid Crystal Reflector Device.

Xiaowen Hu,Weijie Zeng,Kai Wang,Xinmin Zhang,Lingling Shui,Xiao-Fang Jiang,Xinshuai Jiang,Mingliang Jin,Guofu Zhou,Xiaoling Liao

doi:10.3390/polym12010096

Xiaowen Hu, Weijie Zeng + Show 8 more

Open Access

https://doi.org/10.3390/polym12010096

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Abstract

An IR reflector based on polymer-stabilized cholesteric liquid crystal (PSCLC) can selectively tune IR light reflection for smart window application. Broadening the reflection bandwidth to block more IR heat radiation requires the expansion of the pitch distribution in the PSCLC. Traditional attempts using ex situ direct current (DC) bias upon an already polymerized PSCLC reflector usually require a sustaining potential difference holding the pitch gradient of the reflector. Removing the DC bias will lead to a reflect bandwidth comeback. Here, we have developed an in situ DC curing strategy to realize an irreversible reflect bandwidth broadening. Briefly, a DC bias was used to drive the redistribution of impurity cations, which can be captured by the ester group of oligomers, during the photopolymerization. During the slow polymerization process, such trapped cations will drag the oligomers towards the cathode and compress the pitch length near the cathode before the oligomers form the long polymer chain. Consequently, a frozen pitch gradient by such an in-situ-electric-field-assisted dynamic ion-dragging effect leads to the formation of a pitch gradient along the electrical field direction. After removing the DC bias, the as-cured polymer is observed to have frozen such a gradient pitch feature without recoverable change. As a result, the PSCLC reflector exhibits steady bandwidth broadening of 480 nm in the IR region, which provides the potential for saving energy as a smart window.

Highlights

Today, global overheating has triggered various threats to our planet and modern civilization [1].Since 1970, Earth’s heat content has risen at a rate of 6 × 1021 joules a year [2], the equivalent of the energy output of about 190,000 nuclear power plants
Applying an external electric field on a polymer-stabilized cholesteric liquid crystal (PSCLC) reflector cell can modify the orientation of the Cholesteric liquid crystal (CLC) molecules and drive the translational motion of the polymer network, inducing a pitch change in the cell
We have reported in this work on the preparation of a PSCLC with a negative

Summary

Introduction

Global overheating has triggered various threats to our planet and modern civilization [1]. Since the ∆n of colorless organic materials is usually less than 0.3, the band width of a single-pitch CLC in the IR region is limited to a few tens of nanometers [10], making it insufficient for heat reflection In this regard, broadening the reflection bandwidth requires a non-uniform pitch distribution or a pitch gradient in the CLC gel films. Applying an external electric field on a polymer-stabilized cholesteric liquid crystal (PSCLC) reflector cell can modify the orientation of the CLC molecules and drive the translational motion of the polymer network, inducing a pitch change in the cell Such a pitch change is highly reversible and the broadened pitch will recover as soon as the holding bias is removed, as the polymer network has already been cured before the direct current (DC) electric field is applied. The PSCLC reflector which is based on a polymer-stabilized CLC displays a steady bandwidth broadening of 480 nm in the IR region

Materials

Device Preparation

Characterization

Results

Conclusions