Thermochromic Liquid Crystal Sheets

Thermochromic Liquid Crystals (TLCs) are a specialized class of thermotropic liquid crystals (TTLCs) known for their unique ability to change color with temperature variations. This optical behavior is directly tied to the molecular arrangement and structural transitions within the material, making them valuable in a variety of applications, including temperature sensing, medical diagnostics, and dynamic display technologies.

Molecular Structure and Classification

TLCs are a subset of TTLCs, which are complex organic compounds that exhibit an intermediate phase between conventional liquids and crystalline solids. This mesophase, or "liquid crystal phase," is temperature-dependent, providing a distinctive mix of fluidity and order. Not all TTLCs are thermochromic; the thermochromic property specifically depends on the mesophase's molecular arrangement.

TTLCs can be classified into three main categories based on their mesophase structure:

A. Nematic: Molecules exhibit high orientational order but low positional order. The molecules are aligned along a preferred direction without a systematic lattice structure. This phase is optically inactive and not thermochromic.

B. Smectic: Molecules possess both high orientational and positional order, forming a layered structure. Similar to the nematic phase, the smectic phase does not exhibit thermochromic properties.

C. Chiral-Nematic (Cholesteric): It is fundamental to the thermochromic behavior of TLCs. In this phase, the molecules are arranged in a helical pattern, where the "director" of each molecular layer is rotated at a specific angle relative to the adjacent layers, forming a continuous helix. This helical arrangement is responsible for the selective reflection of visible light, which varies with temperature changes.

Figure 1. Helical structure in chiral nematic or cholesteric liquid crystals^[1].

Working Principle

The change in the TLC structure leads to variations in the wavelength of light reflected, which results in a visible color shift. As temperature increases, the crystal structure becomes less ordered, altering the reflected color from red (lower temperature) to blue (higher temperature).

Applications of Thermochromic Liquid Crystals

Thermochromic Liquid Crystals find applications in several fields due to their unique, responsive optical properties:

• Temperature Sensors: TLCs are widely used in contact thermography, where color changes correspond to temperature variations on the surface, providing a non-invasive diagnostic tool.
• Smart Displays: The temperature-dependent color change of TLCs can be harnessed in innovative display technologies, enabling dynamic visual effects.
• Energy Efficiency: Incorporation into coatings and windows allows for temperature-dependent light modulation, improving energy efficiency in buildings.
• Medical Diagnostics: TLCs are used in devices that visualize thermal patterns in the human body, aiding in medical diagnostics by highlighting temperature anomalies.

Reference

1. Nadia Abdullah, et al. The basics and issues of Thermochromic Liquid Crystal Calibrations. Experimental Thermal and Fluid Science. 2010, 34(8): 1089-1121.