Polymers and polymerisation

A polymer is a very large molecule (a macromolecule) built by chemically joining together many smaller repeating units, called monomers, through a process called polymerisation. A single polymer chain can contain from hundreds to millions of monomer units linked end to end by covalent bonds, and this sheer chain length is what gives polymers their characteristic combination of properties — flexibility, toughness, and the ability to be processed into films, fibres, and moulded shapes — that are distinct from both small-molecule solids and simple liquids.

How a bulk polymer material behaves under heat depends critically on whether individual chains are cross-linked to one another. In a thermoplastic, individual polymer chains are not covalently bonded to neighbouring chains; they are held together only by comparatively weak intermolecular forces (van der Waals forces, sometimes hydrogen bonding) between chains that are otherwise free to slide past one another once given enough thermal energy. Heating a thermoplastic above a characteristic softening temperature weakens these intermolecular forces enough for chains to flow past each other, letting the material be melted and reshaped; cooling restores the intermolecular forces and the material solidifies again in its new shape, without any covalent bonds having been broken or formed — this cycle can in principle be repeated many times, which is exactly why thermoplastics such as polyethylene and polystyrene are recyclable by remelting.

A thermoset behaves fundamentally differently because its polymerisation process, called curing, forms permanent covalent cross-links directly between neighbouring chains, in addition to the covalent bonds within each chain. Once cured, a thermoset is not really a collection of separate chains anymore but is closer to a single, enormous three-dimensional covalent network spanning the entire piece of material. Because flowing would require breaking covalent bonds throughout this network rather than merely overcoming weak intermolecular forces, a cured thermoset cannot be melted and reshaped — heating it further tends to break bonds destructively, causing charring or decomposition rather than flow, which is why thermosetting plastics such as epoxy resins and vulcanised rubber are valued for high-temperature dimensional stability and structural rigidity, at the cost of not being remeltable or reprocessable in the way thermoplastics are.