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Polymers are large molecules formed by repeating monomer units linked by covalent bonds into chains. Their mechanical and thermal properties depend strongly on molecular weight distribution, chain architecture (linear, branched, crosslinked), degree of crystallinity, and the nature of intermolecular interactions (van der Waals, hydrogen bonds). Thermoplastics soften upon heating (reversible), while thermosets form irreversibly crosslinked networks. The degree of crystallinity — quantified by the fraction of chain segments in ordered regions — controls stiffness, transparency, and chemical resistance.
Compare the properties of HDPE (high crystallinity), LDPE (low crystallinity due to branching), and polycarbonate (amorphous thermoset) to connect structural features to measured properties.
From organic chemistry you know that carbon forms four covalent bonds, and that chains of carbon atoms can grow arbitrarily long. From intermolecular forces, you understand that these chains interact with each other through van der Waals forces, dipole-dipole interactions, and hydrogen bonds. Polymers are what you get when these two concepts combine: a monomer (a small reactive molecule) is linked covalently thousands of times into a polymer chain, and the bulk material is a dense tangle of these chains interacting through intermolecular forces. The properties of the material emerge from the interplay between the covalent backbone and those intermolecular interactions.
Molecular weight (or more precisely, the molecular weight distribution) is the first key structural variable. Short chains slide past each other easily — the material flows like a liquid at low temperatures. As chains get longer, they become entangled, like a bowl of spaghetti, and entanglement dramatically increases melt viscosity and mechanical strength. The glass transition temperature Tg — the temperature below which chain segments can no longer rotate freely and the material behaves like a rigid glass — also increases with chain length. Above Tg, an amorphous polymer behaves like a rubbery solid; below it, like a hard brittle glass. Most engineering applications require knowing whether the service temperature is above or below Tg.
Chain architecture is the second key variable. A linear chain (no branches) can pack efficiently; branching disrupts packing and reduces crystallinity. Crosslinks — covalent bonds between chains — prevent chains from ever sliding past each other, producing a network that cannot flow when heated. This is the defining feature of thermosets: once crosslinked (cured), the material is set permanently. Epoxy, vulcanized rubber, and polyester resins are thermosets. Thermoplastics, by contrast, have no crosslinks — only intermolecular forces hold the chains together. Heat weakens those interactions, the material flows, and it can be remolded on cooling. Polyethylene, polypropylene, and polystyrene are thermoplastics. This distinction dictates the entire processing route (injection molding vs. reaction casting) and end-of-life options (recycling vs. not).
Crystallinity is the third key variable. A polymer chain can, in principle, fold back and forth in a regular pattern to form a crystalline lamella — but this requires a regular, symmetric chain that can pack efficiently. Chains with large side groups (polystyrene), irregular sterochemistry (atactic PP), or branching (LDPE) cannot crystallize readily and remain amorphous. Linear, regular chains (HDPE, nylon) achieve 50–80% crystallinity. In the crystalline regions, chains are ordered and densely packed, giving stiffness and opacity (crystallite interfaces scatter light). In the amorphous regions between crystallites, chains are disordered and more mobile, giving toughness and permeability. Engineering with polymers means designing around this balance: HDPE milk jugs are stiff and opaque (high crystallinity); LDPE squeeze bottles are flexible and translucent (low crystallinity); polycarbonate safety glasses are rigid and clear (amorphous, no crystallinity to scatter light).