polymers

Polymers are the plastics, rubbers, and fibers of everyday life: polyethylene bags, PET bottles, nylon, rubber tires, styrofoam. The name means 'many parts': each molecule is a giant chain built by linking thousands of small identical units (mers) end to end, like a long paperclip chain or a string of beads. Most are built on a carbon backbone and come from oil.

Within a single chain the atoms are held by strong covalent bonds, but neighboring chains are held to each other only by weak secondary forces (van der Waals forces). That mismatch explains polymer behavior: the chains slide and uncoil easily, so polymers are soft, flexible, and low-melting; they are light (density around 0.9 to 1.4 g/cm^3, close to water) and cheap and easy to mold. Thermoplastics soften and re-melt on heating (the chains just slide); thermosets are cross-linked into one giant network and char rather than melt; elastomers (rubbers) are lightly cross-linked so they stretch far and snap back.

Polymers win on light weight, corrosion resistance, electrical insulation, color, and cheap mass-shaping (injection molding). Their limits: low stiffness and strength, they creep (slowly sag under a steady load), they soften with modest heat, and they degrade in sunlight. Recycling is easy for thermoplastics but hard for thermosets, which cannot be re-melted.