A thin shopping bag and a stiff drainpipe can both be made of polyethylene. They share the same monomer, the same repeat unit and the same formula. What differs is how the chains are arranged relative to each other. This page shows how to read that arrangement from a structure and predict how the material behaves.
Crystalline and amorphous regions
Follow one chain through a piece of polyethylene. In some stretches it lies straight, side by side with its neighbors, packed as neatly as the particles in a crystal. These ordered stretches make up the crystalline regions. In other stretches the same chain loops and tangles with the chains around it, with no order at all. These tangled stretches make up the amorphous regions. Amorphous means “without a regular shape.”
Most polymers whose chains can pack have both kinds of region. Some, such as PMMA and ordinary polystyrene, are amorphous throughout. The share of a sample that is crystalline is its crystallinity, and most of its properties follow that number:
- More crystalline → denser, stronger, stiffer, more opaque, higher melting.
- More amorphous → softer, more flexible, more transparent, no sharp melting point.
Opacity gives crystallinity away. A material’s refractive index is how strongly it bends light, and a crystalline region bends light by a slightly different amount than the amorphous material around it. Light therefore scatters at every boundary between the two, so a highly crystalline polymer looks cloudy. A fully amorphous polymer has no such boundaries and can be glass-clear. That is why PMMA and polycarbonate are used for windows and high-density polyethylene never is. A clear plastic is usually an amorphous one.
What lets chains pack
Start with two fatty acids from Lipids. Stearic acid has a straight chain and melts at 69 °C. Oleic acid has the same eighteen carbons with one cis kink, and it melts at 13 °C. Straight chains lie flat against each other, so the London dispersion forces between them add up along their whole length. A kink holds them apart.
Polymer chains follow the same rule. A chain that is straight and regular can lie against its neighbors and crystallize, while a branch or an irregular pattern of side groups keeps the chains apart. Addition polymers gave two structural variables, and both work this way. The first is branching. The second is tacticity, the pattern of side groups along the chain: all on one side (isotactic), alternating (syndiotactic) or random (atactic). Hydrogen bonds between chains, which you met in nylon and Kevlar in Condensation polymers, go further: they tie neighboring chains together.
| 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 |
| Hydrogen bonding between chains | Holds them together | Nylon, Kevlar: very strong |
An atactic polymer is amorphous, but that does not settle whether it is soft. Atactic polypropylene is a sticky goo, while ordinary polystyrene, also atactic, is a rigid glass. The difference is the subject of the next section.
Two transition temperatures, not one
Natural rubber is soft and stretchy at room temperature. Cool a rubber ball in liquid nitrogen, at −196 °C, and drop it: it shatters like glass. No bond in it has changed. Its chains have simply lost the ability to move.
A small molecule has one melting point. A polymer has up to two transitions, and each belongs to a different kind of region:
- Tg, the glass transition temperature, belongs to the amorphous regions. Below it, their tangled chains are frozen in place and the material is hard and brittle, like glass. Above it, the chains can wriggle and slide. Essentially every polymer has a Tg, because essentially every sample contains some amorphous material.
- Tm, the melting temperature, belongs to the crystalline regions: it is where they melt. Only a polymer with crystalline regions has one.
Above Tg, whether a sample feels rubbery depends on what else holds it together. At room temperature HDPE is about 145 °C above its Tg, yet it stays rigid. Its crystalline regions hold the mobile chains in place until they melt at Tm. A cross-link is anything that ties one chain to another. Crystalline regions tie chains by packing rather than by a bond, so they are called physical cross-links.
| Polymer | Tg | Tm | At room temperature |
|---|---|---|---|
| Polyethylene (HDPE) | −120 °C | 135 °C | far above Tg, but crystalline → rigid |
| Natural rubber | −70 °C | none in practice | above Tg, amorphous, cross-linked → elastic |
| PVC, unplasticized | 80 °C | none in practice | below Tg → glassy, rigid |
| Polystyrene (atactic) | 100 °C | none | below Tg → glassy, brittle |
| PET | 70 °C | 260 °C | below Tg → rigid; crystallizes on heating past it |
| Nylon 6,6 | 50 °C | 265 °C | near Tg, crystalline → tough |
Read the table with two questions in hand. Is room temperature above or below Tg? And is there any crystallinity? Below Tg the polymer is a glass: rigid, and brittle unless something else toughens it. Above Tg with neither crystalline regions nor cross-links, it is a gum or a thick liquid that flows. Above Tg with either of them, it is a rubber or a tough plastic.
The two temperatures are not independent of each other. As a rough rule, Tg is between a half and two-thirds of Tm when both are measured in kelvins. PET (343 K and 533 K, a ratio of 0.64) and nylon 6,6 (323 K and 538 K, 0.60) fit the rule; polyethylene (0.38) falls below it.
The case. Polystyrene carries a bulky phenyl group, a benzene ring, on every other carbon of its chain. Ordinary polystyrene is glass-clear, rigid and brittle, and it has no melting point. Is the bulk of the phenyl group the cause?
Step 1 — check the pattern. Ordinary polystyrene is made by radical polymerization, which gives an atactic chain: each phenyl group points one way or the other at random. Random chains cannot pack, so the polymer is amorphous, and that is why it is clear.
Step 2 — compare a regular chain. Syndiotactic polystyrene carries the same phenyl groups, alternating from side to side. It is made with a metal catalyst related to the Ziegler–Natta catalysts. Its chains pack, and it is crystalline, melting near 270 °C. So the bulk alone does not prevent packing; the random pattern does.
Step 3 — find what the bulk does do. A large side group makes it harder for the backbone to twist, so the chains need more heat before they can move. That raises Tg to about 100 °C. At room temperature, then, ordinary polystyrene is an amorphous polymer below its Tg, with no crystalline regions to toughen it: a rigid, brittle glass.
Cross-linking as a dial
Raw natural rubber is sticky, and on a warm day it slowly flows. Heat it with sulfur and it becomes the springy material of a tire. That change is vulcanization. It works by adding cross-links made of covalent bonds: short bridges of one or more sulfur atoms, written –Sx–, each tying one chain to a neighbor. The figures show two-sulfur bridges.
Natural rubber is cis-polyisoprene. It has the structure that 1,4-addition of isoprene gives, so every repeat unit keeps one C=C (see Addition polymers). Ordinary sulfur is made of S8 molecules, rings of eight atoms. During the process these rings open, and the sulfur bonds at allylic carbons, the carbons right next to a C=C. Every CH2 and CH3 in the chain sits next to a C=C, so every one of them is allylic. Their hydrogens come off far more easily than those of an ordinary alkane, because the radical left behind is spread over the neighboring double bond, as Conjugated systems explains.
So the C=C in every repeat unit is not wasted. It is the handle vulcanization needs, and it is why a saturated chain such as polyethylene cannot be vulcanized with sulfur.
The number of bridges works like a dial, and the amount of sulfur sets it:
- No bridges — the chains slide past each other for good. The rubber is sticky, and it flows.
- A few bridges (a few percent sulfur) — the chains can be stretched out, and they spring back when released. The rubber is elastic; this is a tire or a rubber band.
- Many bridges — nothing can move. The result is hard and brittle, like ebonite (hard rubber).
Why stretched rubber springs back
Stretch a rubber band and it pulls back. A metal spring pulls back too, but for a different reason: stretching a spring strains its bonds, and the strained bonds pull back. In rubber the pull comes from entropy, which you met in Substrate & solvent effects. Entropy measures how many ways the molecules can arrange themselves.
A coiled chain can take an enormous number of shapes. Pull it nearly straight and very few shapes are left, so stretching lowers the entropy. Let go, and the chains return to the far more probable coiled state. The cross-links keep them from simply sliding past one another instead, which is why elastic rubber needs them.
The observations. Stretch a rubber band quickly and touch it to your lip: it feels warm. Hang a weight from a rubber band, enough to stretch it by well over a tenth of its length, and warm the band with a hair dryer: it shortens and lifts the weight, where almost any other material would expand.
The warmth. Stretching rubber barely changes the energy of its chains, because no bond is strained. The work you do has to go somewhere, so it comes out as heat.
The shortening. The pull toward the coil is an entropy effect, and entropy enters the free energy as TΔS. The higher the temperature, the larger that term, so the pull grows as the band warms, and a stretched band contracts.
Thermoplastic against thermoset
What a polymer is sorted plastics into thermoplastics and thermosets by counting reactive sites: separate chains soften and can be remolded, while a network cannot melt. Heat a polyethylene milk bottle and it melts and can be molded again. Heat a cured epoxy glue or a melamine plate and it may soften above its own Tg, but it never melts or flows. Heated far enough, it chars.
| Thermoplastic | Thermoset | |
|---|---|---|
| Structure | Separate chains | One covalent network |
| Held together by | Intermolecular forces | Cross-links |
| On heating | Melts, can be remolded | May soften above Tg, never melts; decomposes |
| Recyclable by melting | Yes | No |
| Examples | PE, PET, nylon | Epoxy, melamine, vulcanized rubber |
Cross-linking an existing polymer makes a network too. Vulcanized rubber is soft at room temperature, because it is above its Tg, yet it is a thermoset: its sulfur bridges join the chains into one network, which is why a worn tire cannot be melted down and remolded.
Plasticizers: lowering Tg without changing the polymer
Rigid PVC makes drainpipes and window frames. The same polymer, made soft, becomes cable insulation, flooring and shower curtains. The chains are identical in both. The soft kind also contains a plasticizer: a small molecule mixed in among the chains. It does not evaporate easily; a phthalate diester is typical.
The plasticizer molecules sit between the chains, hold them farther apart and weaken the forces between them. The chains can then start moving at a lower temperature. So a plasticizer lowers Tg, often by a hundred degrees, and the effect is entirely physical: no bond in the polymer is made or broken.
Fibers, plastics and elastomers
A nylon thread, a polystyrene cup and a rubber band belong to three different classes of material. Crystallinity, Tg and cross-linking together decide the class, and the class predicts most of the material’s properties.
- A fiber is highly crystalline and has strong forces between its chains. During manufacture it is drawn, that is, stretched, which lines its chains up along the fiber: nylon, PET, Kevlar.
- A plastic is partly crystalline, or else amorphous and below its Tg. It is stiff, but it can still be shaped when hot: polyethylene, polystyrene, PVC.
- An elastomer is amorphous, sits above its Tg, and is lightly cross-linked so that it springs back: vulcanized rubber.
What carries forward
Read the structure and predict the material. Linear, regular chains crystallize, so the material is strong and opaque. Branched chains crystallize less, and atactic chains stay amorphous. Hydrogen bonds between chains make a polymer strong, and cross-links mean it cannot be melted. Keep two temperatures in mind rather than one: Tg for the amorphous regions and Tm for the crystalline ones.