Application Cases

Experiment Name: Optical Properties of Cholesteric Liquid Crystals, Their Reflection and Transmission Spectra, and Optical Response Under Different Conditions

Research Direction:MnSmart windows with adjustable visible light transmittance have garnered widespread attention from both academia and industry due to their ease of control, aesthetic appeal, and significant enhancement of comfort for building occupants. These smart windows have found applications in various fields, including architecture, transportation, and electronic or optical devices. Polymer-dispersed liquid crystals (PDLCs) have been widely used as smart windows. Since the liquid crystal molecules forming the droplets are randomly oriented, light passing through the matrix is strongly scattered by the droplets due to their refractive index mismatch, resulting in an opaque, milky appearance. When the LC molecules are aligned with the electric field in the powered state, the window becomes transparent. Compared to PDLCs, polymer-stabilized liquid crystals (PSLCs) tend to form a loose polymer network with nodular or fibrous conformations. It has been reported in the literature that PSLCs can be initialized in either a transparent or opaque state and can be rapidly switched with a lower external voltage. However, the relative light haze of 40%-50% in the scattering state of PSLCs severely limits their practical application. Cholesteric liquid crystals (CLCs) consist of elongated molecules that self-assemble into helical structures, not only capable of switching between transparent and scattering states but also reflecting different wavelengths. Polymer-stabilized cholesteric liquid crystals (PSCTs) can successfully replace nematic liquid crystals in PSLCs. PSCTs can be initialized in a transparent mode, where the cholesteric liquid crystal (ChLC) exhibits a planar texture (P) and reflects incident light due to the original Bragg reflection of ChLC.

When an appropriate voltage is applied, the LC molecules tend to transition to an isotropic texture (H state) under the influence of the electric field. However, the interaction between the polymer network and the LC molecules tends to maintain the P state. As a result, the final texture formed in PSCTs is a multidomain focal conic texture (FC state), which is opaque due to the refractive index mismatch between the polymer network and the LCs. After the external voltage is removed, PSCTs return to the initial P state under the combined effect of the LC molecule interactions and the balanced orientation of the polymer network, becoming transparent again and reflecting infrared light. Thanks to the stabilizing effect of the polymer network, the ChLC cell can stably switch between the transparent and opaque states. PSCTs can also be initialized in a normally opaque mode (opposite to the previously mentioned normally transparent mode), where the ChLC is in a scattering texture (FC) without an external electric field and can laterally switch to a transparent mode under the influence of an external electric field.

Experiment Objective:To gain a deep understanding of the optical characteristics of cholesteric liquid crystals and how this structure affects light propagation and modulation, thereby providing a theoretical basis for the dynamic light transmittance control of smart windows.

Testing Equipment:ATA-2021B High-Voltage Amplifier, Signal Generator, Oscilloscope, Precision Temperature-Controlled Stage, Polarizing Light Microscope

Experiment Process:The working principle of the smart window based on highly thermally stable polymer-stabilized cholesteric liquid crystals mainly relies on the optical modulation performance of cholesteric liquid crystals and the thermal stability of the polymer. When external conditions (such as temperature, electric field, etc.) change, the orientation of the cholesteric liquid crystal molecules changes, thereby altering the window's light transmittance and thermal radiation transmission. The highly thermally stable polymer ensures the overall structural stability and performance durability of the window during this process. YP-011PSCT was added to the ChLC. Subsequently, YP-011PSCT was injected into a 10μm cell and placed on a precision temperature-controlled stage to control the temperature. The electro-optical performance was evaluated using an automatic liquid crystal material testing system. A signal generator produced the excitation signal, which was then amplified by the high-voltage amplifier ATA-2021B and applied as an electric field to the sample. Meanwhile, the oscilloscope was used to observe the output voltage, thereby controlling the electric field. A polarizing light microscope was employed to observe the experimental phenomena.

Figure 1: Experimental Flowchart of the High-Thermal-Stability Polymer-Stabilized Cholesteric Liquid Crystal Smart Window

Experiment Results:

Figure 2(a) Relationship between transmittance and voltage; (b) Transmittance changes over time for 3%, 4%, and 5% RM257-doped YP-011PSCT cells.

The electro-optical performance of 3%, 4%, and 5% RM257-doped YP-011PSCT was evaluated using an automatic LCD tester, as shown in Figure 2. By increasing the doping concentration of RM257, the threshold voltage for driving the normally transparent YP-011PSCT to become opaque significantly increased, but the corresponding rise and decay times were significantly reduced. For the 3% RM257-doped YP-011PSCT, the threshold voltage, rise time, and decay time were 22.908V, 85.442ms, and 18.617ms, respectively. When the doping concentration of RM257 was increased to 5%, the threshold voltage rose to 31.172V, and the rise and decay times were shortened to 66.001ms and 5.336ms, respectively. The threshold voltage increased by 36.07%, the rise time decreased by 22.75%, and the decay time decreased by 71.34%. The more robust and dense the polymer matrix, the stronger the anchoring energy of the ChLC and the higher the threshold voltage. At the same time, a more robust and dense polymer matrix also increases spatial hindrance, which significantly shortens the rise and decay times and hinders the size switching of the ChLC by the external electric field.

Voltage Amplifier Recommendation: ATA-2021B

Figure: ATA-2021B High-Voltage Amplifier Specifications

The above case was compiled by Aigtek Xi’an Antai. Xi’an Antai Electronics is a high-tech enterprise specializing in the research, development, production, and sales of electronic measurement instruments such as power amplifiers, high-voltage amplifiers, power signal sources, preamplifiers for micro-signals, high-precision voltage sources, and high-precision current sources. It provides users with competitive testing solutions. Aigtek has become a widely recognized supplier of instruments and equipment in the industry, with a broad product line and considerable scale. Sample units are available for free trial.