Polymer

A small, reactive molecule that joins with others to form a polymer. In unsaturated polyester resins, for example, styrene monomer acts both as a diluent and as the cross-linking agent that reacts during cure.

A polymer that softens when heated and hardens again on cooling without chemical change, so it can be reshaped, welded or recycled. Thermoplastics range from commodity materials such as polypropylene and PVC to high-performance matrices such as PEEK used in advanced composites.

A polymer that cures irreversibly through crosslinking, forming a rigid three-dimensional network that cannot be re-melted or reshaped by heating. Epoxy, polyester, vinyl ester, phenolic and polyurethane resins are common thermosets used as composite matrices, adhesives and coatings.

The degree to which a polymer's molecular chains pack into ordered, crystalline regions. Higher crystallinity generally increases stiffness, chemical resistance and melting point but also raises moulding shrinkage, as in semi-crystalline thermoplastics such as polypropylene and PEEK.