What occurs during melting of a polymer?

• A. Decomposition into monomers.
• B. Increase in brittleness without flow.
• C. Transformation of a solid material having an ordered structure into a viscous liquid in which the structure is highly random.
• D. Transformation to a crystalline solid.

Answer: (C)

Melting in polymers is the loss of long-range order as temperature reaches the melting point, allowing the chains to become mobile. In a semicrystalline polymer, the crystalline regions (lamellae) break their orderly packing and melt, so the material becomes a viscous liquid where the overall structure is highly random. The chains can slide past one another, enabling flow, unlike in a solid where they are locked in place. This is different from chemical decomposition, which would break the molecules apart, and from forming a crystalline solid, which would require cooling or crystallization. Some polymers are fully amorphous and don’t have a sharp melting point; they soften via a glass transition instead.