The Development of Smart Window Stabilized by Polymer Walls and Liquid Crystal Physical Gels

Liquid crystal (LC) smart windows are attractive technology because they can protect privacy as well as control indoor environments by switchable electro-optical properties. However, conventional LC smart windows require a high driving voltage and nematic LC (NLC) in LC smart windows can leak when severe mechanical stimuli are applied. Therefore, we newly developed a dual stabilization (DS) process to fabricate mechanically stabilized LC smart windows which operate at a lower voltage than conventional LC smart windows.<br/>The DS cell was fabricated with two components. The first one is composed of NLC (5CB) and macrogelator with photoisomerizable azobenzene moiety (BTAG) for the formation of liquid crystal physical gel (LCPG). The other component is composed of photopolymerizable low molecular weight monomers (LMWM) for the construction of partition walls. The DS process is performed by irradiating UV light under a photomask. Upon irradiating UV light on the DS cell, LMWMs in the UV light-exposed region are polymerized. Because of the concentration gradient, LMWMs move from the UV light-protected region to the UV light-exposed region resulting in the construction of partition walls. At the same time, BTAG molecules in the UV light-exposed region are photoisomerized from <i>trans</i>-isomers to <i>cis</i>-isomers resulting that the 3D network structures which are formed by intermolecular hydrogen bonding between BTAG are collapsed. The <i>cis</i>-isomers move to the UV light-protected region by the concentration gradient of the macrogelator, resulting in the formation of physical 3D networks.<br/>The electro-optical properties of the DS cell were investigated for smart window application. The DS cell is opaque due to the mismatch of the refractive index between the randomly dispersed NLC domains. Under the electric field, the NLC molecules are homeotropically aligned because the 5CB has a positive dielectric anisotropy. Thus, the mismatch of the refractive index is disappeared, and the DS cell turns transparent. When the electric field is removed, the NLC molecules are randomly dispersed in the LCPG domain again, and the DS cell returns to the opaque state. According to the voltage-dependent transmittance curves, the driving voltage of the DS cell is lower than the conventional smart windows.<br/>To demonstrate this phenomenon, the DS cell was observed by polarized optical microscopy (POM) equipped with a quarter-wave plate. Unlike the randomly dispersed NLC molecules, the NLC molecules near the partition wall are homogeneously aligned. Hence, the NLCs in the LCPG domain can be easily reoriented at low voltage.<br/>To confirm the mechanical stability, we fabricated a flexible DS smart window with the partition wall in the ITO-coated polyethylene terephthalate (PET) films. Because the partition wall improves the adhesion between the flexible films as well as serves as spacers, the flexible DS smart window is mechanically stable even if external stimuli such as pressing, cutting, and bending are applied. In conclusion, the dual stabilized LC smart window by the partition wall not only operates under a low voltage but also has excellent mechanical stability. This work was supported by the BK21 FOUR, Basic Research Laboratory Program (2020R1A4A1018259), and Mid-Career Researcher Program (2021R1A2C2009423).