Structure and properties — Polymers

Two samples of polyethylene can be a shopping bag and a drainpipe. Same monomer, same repeat unit, same formula — and the difference is entirely in how the chains are arranged relative to each other. This section is about reading structure and predicting the material.

Crystalline and amorphous regions

A polymer is not a crystal and not a liquid. Stretches of chain that lie neatly against their neighbors form crystalline regions; stretches that are tangled form amorphous ones. Almost every real polymer has both, and the fraction that is crystalline is what most of its properties track.

More crystalline → denser, stronger, stiffer, more opaque, higher melting.
More amorphous → softer, more flexible, more transparent, no sharp melting point.

The opacity is a useful tell. Crystalline regions scatter light because their refractive index differs from the amorphous regions around them, so a highly crystalline polymer is cloudy and a fully amorphous one can be glass-clear. That is why PMMA and polycarbonate are used for windows and high-density polyethylene never is.

What lets chains pack

Feature	Effect on packing	Result
Linear chains	Pack well	HDPE: rigid, dense
Branched chains	Pack less well	LDPE: flexible, less dense
Isotactic or syndiotactic	Pack well	Strong, crystalline
Atactic	Cannot pack	Amorphous, soft
Hydrogen bonding between chains	Holds them together	Nylon, Kevlar: very strong
Bulky side groups	Raise T_g, slow crystallization	Polystyrene: brittle, glassy

Be careful with the last row. Ordinary polystyrene is amorphous because radical polymerization makes it atactic, not because the phenyl group is bulky; syndiotactic polystyrene carries the same bulky group and is crystalline, melting near 270 °C. What bulk does is raise T_g and slow crystallization down, which is enough to keep a randomly placed chain from ever finding order.

This is the same argument as the fatty acids in the biomolecules chapter. Straight chains stack and melt high; kinked or branched chains do not. One idea, applied to a triglyceride in one chapter and to polyethylene in another.

Two transition temperatures, not one

A small molecule has a melting point. A polymer has up to two transitions, and they describe different regions:

T_g, the glass transition temperature. Below it the amorphous regions are frozen and the material is hard and brittle; above it they can move. Whether the sample then feels rubbery depends on what else is holding it: with little crystallinity and no cross-links it is rubbery, but HDPE, PET and nylon are all far above their T_g at room temperature and stay rigid, because their crystallites act as physical cross-links up to T_m. Essentially every polymer has a T_g, because essentially every sample has some amorphous material in it.
T_m, the melting temperature. Where the crystalline regions melt. Only polymers with crystalline regions have one.

T_g is why the same material behaves differently with temperature in ordinary life. Natural rubber has a T_g well below room temperature, so it is elastic; cool it in liquid nitrogen and it shatters like glass, because you have taken it below T_g.

Thermoplastic against thermoset

Two transitions, describing two different parts of the same sample. The glass transition is where the tangled amorphous regions stop being frozen; the melting temperature is where the packed crystalline regions come apart. A fully amorphous polymer simply has no Tₘ.How soft it gets above Tₑ depends on how much crystallinity is left holding the sample: with little of it you get rubber, while HDPE, PET and nylon are all far above their Tₑ at room temperature and stay rigid, because the crystallites act as physical cross-links up to Tₘ. Natural rubber is the clean case — cool it in liquid nitrogen, take it below Tₑ, and the same material shatters like glass because its chains can no longer move.

	Thermoplastic	Thermoset
Structure	Separate chains	One covalent network
Held together by	Intermolecular forces	Cross-links
On heating	Softens, can be remolded	Decomposes
Recyclable by melting	Yes	No
Examples	PE, PET, nylon	Epoxy, Bakelite, vulcanized rubber

The difference is covalent, and it comes back to counting reactive sites. Two sites per monomer gives chains, which can slide past each other when heated. Three or more gives a network, and a network cannot melt because melting would mean breaking covalent bonds.

Cross-linking as a dial

Vulcanization is the clearest case. Natural rubber is a polymer of isoprene with a double bond in every repeat unit; heat it with sulfur and short S–S bridges form between chains. The number of bridges is a dial:

None — chains slide past each other permanently. Sticky, and it flows.
A few percent — chains can stretch apart and are pulled back. This is elastic, and it is what a tire is.
Heavily cross-linked — nothing moves. Hard and brittle, like ebonite.

Elasticity is an entropy effect, which is worth stating plainly because it is not obvious. Stretching a rubber pulls its coiled chains straight, and a straight chain has far fewer available conformations than a coiled one — so it has lower entropy. Releasing it lets entropy pull the chains back into a coil. The cross-links are what stop them simply sliding apart instead.

What carries forward

Read the structure and predict the material: linear and regular means crystalline, strong and opaque; branched or atactic means amorphous and flexible; hydrogen bonds between chains mean strong; cross-links mean it cannot be melted. And two numbers rather than one — T_g for the amorphous regions, T_m for the crystalline ones.