|University Federico II
of Naples, Italy
Polymers are very long chain molecules. They are generally solid at room temperature, but most of them become liquid by heating.
A thermoplastic polymer is a polymer that, when heated, becomes liquid, so it can be easily formed in any desired shape (hence the terms plastics, meaning deformable, and thermoplastic, by heat).
Such a polymer, when cooled, will keep its shape forever.
The polymer chain in the liquid is a random coil and can easily change its shape. Most polymers become liquid at moderate temperatures. Metals can be hot worked too, but they require much higher temperatures. This makes polymer processing easy and cheap compared to metals, but it also makes plastics nonresistant to heat, because they start deforming and then become liquid.
Polymers often pass from solid to liquid state by melting. Melting is a passage from an ordered crystal state, where atoms are arranged in a regular array, to a liquid state, where they are disordered. In this case the passage resembles melting metals or ice. Such polymers are called semicrystalline (semi stands for half, because order is not perfect in polymers). Polyethylene and nylon are examples of semicrystalline polymers.
Other polymers, on the contrary, cannot crystallize; when cooled from the liquid, they become solid, but remain disordered and similar to liquid in structure. They are glass-like: common glass is a disordered solid that becomes liquid by heating (though at much higher temperatures). Such polymers (polystyrene is an example) are called amorphous or glass polymers.
The ability of a polymer to crystallize is strongly influenced by its stereochemistry. Giulio Natta (1903 - 1979) showed that polymers must be stereoregular, i.e. the steric arrangement, not only the chemical structure, must repeat regularly along the chain, in order to crystallize. If the polymer is not stereoregular, it may still form amorphous plastics, but it will never crystallize. Natta's discovery started polypropylene industrial production: PP, one of the major plastics in the market, is a isotactic (stereoregular) polymer.
The passage from solid to liquid and vice versa implies no chemical transformation. This means that a thermoplastic polymer, at least in principle, can be melted again and recycled to form new items. In practice this is only partially true, because thermal processes inevitably provoke some chemical degradation, worsening the quality of the material.
Thermoplastics production is millions of tons per year. They are the most widely consumed plastics, including polyethylene (PE), polypropylene (PP), polystyrene (PS), Nylon.