Color change in plastic, specifically referring to heat-sensitive plastics utilizing polydiacetylenes, occurs because heat disrupts the polymer's structure, which alters its light absorption properties.
Here's a more detailed breakdown:
-
The Key Material: Polydiacetylenes (PDAs): These are polymers with special side chains. The arrangement of these chains is crucial for their color-changing abilities.
-
Coplanarity and Conjugation: Under normal conditions, the side chains of the PDA are arranged in a relatively flat, or "coplanar," configuration. This coplanarity leads to a specific length of "conjugation" within the polymer's structure. Conjugation refers to alternating single and double bonds in the polymer backbone. This system of alternating bonds allows electrons to move freely along the chain.
-
Light Absorption and Color: The length of this conjugated system determines which wavelengths of light the polymer absorbs. Different wavelengths correspond to different colors. When a specific wavelength of light is absorbed, the plastic appears to be the complementary color (the color not absorbed).
-
The Role of Heat: When heat is applied, it introduces more motion into the PDA side chains. This increased motion disrupts the coplanarity, effectively shortening the length of the conjugated system.
-
Shift in Absorption: Because the conjugation length changes, the polymer now absorbs different wavelengths of light. This shift in absorption leads to a visible change in the color of the plastic. For example, a PDA might shift from absorbing green light to absorbing blue light, causing the plastic to change from blue to green.
In summary, color-changing plastic using PDAs works by altering the polymer's structure with heat, specifically disrupting the coplanarity of side chains. This change affects the length of conjugation, which in turn influences the wavelengths of light absorbed and reflected, resulting in a color change.
