What is a polymer?
Plastics are tangled spaghetti made of molecules — that one picture explains melting, softening, stretching, and creep.
Molecular spaghetti
A polymer is a molecule built by repeating one small unit thousands of times into a chain — 'poly-mer' literally means 'many parts'. Polyethylene, the plastic of shopping bags, is just —CH₂— repeated tens of thousands of times. A lump of plastic is billions of these chains tangled together like a bowl of cooked spaghetti.
Nearly everything odd about plastics follows from that picture. Chains slide past each other, so plastics are softer and stretchier than metals. Sliding takes time, so plastics slowly keep stretching under a constant load (creep). Warm them and the chains wriggle more, so plastics soften long before they burn.
The glass transition: hard candy vs gummy bear
Every amorphous (non-crystalline) polymer has a special temperature called the glass transition, Tg. Below Tg the chains are frozen — the material is glassy: hard, stiff, sometimes brittle. Above Tg the chains can wriggle — it turns rubbery and soft. Same molecule, entirely different personality.
That is why a PVC window frame (Tg ≈ 82 °C) is rigid on a summer day but a car dashboard can slump in desert heat, and why rubber (Tg far below 0 °C) is always in its soft state. The Tg column in the materials database is one of the most important numbers on a polymer's card: it tells you the temperature range where the material keeps its stiffness.
Thermoplastics and thermosets
Thermoplastics (PP, PET, nylon, PEEK) are spaghetti with no permanent connections: heat them and they melt, so they can be molded, welded, and recycled. Some of them crystallize partially as they cool — those also have a melting point Tm above which the crystals let go.
Thermosets (epoxy, polyester, phenolic) are chemically crosslinked during curing — the strands get welded into one giant molecule. They cannot melt again (they char instead), but the network makes them dimensionally stable and creep-resistant, which is why epoxy is the standard glue holding carbon-fiber laminates together. The cure kinetics simulation on this site models exactly that welding process happening over time.
Below Tg: chains are frozen in place — the material is glassy, stiff, and can be brittle.
1. Below its glass transition temperature Tg, an amorphous polymer is…
2. Which family can be melted down and recycled by remelting?
3. Plastics slowly keep stretching under a constant load (creep) because…