High-Contrast Colorless-to-Colored Thermochromic Materials

Area of Science:

• Materials Science
• Chemistry
• Physical Chemistry

Background:

• Thermochromic materials change color with temperature.
• Controlling thermochromic properties is crucial for applications like sensors and security inks.
• Existing thermochromic systems often lack high contrast or tunable transition temperatures.

Purpose of the Study:

• To develop novel high-contrast, colorless-to-colored thermochromic materials.
• To establish a design rule for controlling the coloration state and transition temperature.
• To demonstrate the versatility of the system for various applications.

Main Methods:

• Synthesis of three-component mixtures including catechol/pyrogallol derivatives (color developers - CD), fatty acid phase change materials (PCM), and leuco dyes.
• Density functional theory (DFT) calculations to study hydrogen-bonding interactions.
• Thermal and spectroscopic measurements to analyze phase behavior and coloration.
• Preparation of structured mixtures like solid lipid particles and polymer composites.

Main Results:

• Achieved high-contrast colorless-to-colored thermochromic materials.
• Demonstrated that coloration in solid states is controllable by the relative alkyl chain lengths of CD (C_CD) and PCM (C_PCM).
• Colorless solids obtained when C_CD > C_PCM (ΔC<0) due to phase segregation; colored solids obtained when ΔC≥0 due to CD solubilization in PCM.
• System demonstrated tunability across different leuco dye families and PCMs, with preserved transition temperatures in structured forms.

Conclusions:

• A novel design strategy for tunable thermochromic materials based on CD-PCM interactions was established.
• The relative alkyl chain length is a key parameter for controlling solid-state color.
• The developed materials are suitable for applications in polymer composites, screen-printed patterns, sensors, and invisible security inks.