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Why can linear and branched polymers melt when heated?

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Diagram illustrating a homopolymer arrangement using polyvinyl chloride as an example, showing a chain of identical repeating vinyl chloride units with the chemical structure highlighted. — polymer Industrial polymers are synthesized from simple compounds joined together to form long chains. For example, polyvinyl chloride is an industrial homopolymer synthesized from repeating units of vinyl chloride.

Linear and branched polymers are capable of melting because their molecular structures consist of individual, separate chains that are not linked together by covalent bonds. In their solid state, these molecules are held together by relatively weak intermolecular forces, such as van der Waals forces.12

When these materials are heated, the molecules gain enough energy to overcome these intermolecular attractions. Because the chains are not cross-linked into a rigid, permanent network, they are free to move and flow past one another, causing the polymer to soften and eventually become molten. This ability to be repeatedly melted and reshaped characterizes them as thermoplastic polymers.213

In contrast, network polymers are held together by strong, permanent covalent bonds that form a complex, interconnected structure. This structure prevents the molecules from flowing, meaning they cannot melt upon heating and will instead undergo chemical decomposition if temperatures are raised too high.23