Classes, Structure and Properties of Polymers; Vulcanisation of Rubber – Polymers and Polymerisation (Senior 6 Chemistry) | Ganzaa

Polymers and Polymerisation

Lessons

Senior 6 · Chemistry

Polymers and Polymerisation

4 lessons

Lesson 3 of 4

Classes, Structure and Properties of Polymers; Vulcanisation of Rubber

25 min

Three polymers on one bicycle

Look closely at a bicycle taxi in Musanze or Huye. The tyre stretches under load and springs back thousands of times a day. The seat cover is soft PVC, but the same PVC in a drainpipe is hard and stiff. The reflector is clear and brittle. All four are polymers, yet they behave completely differently. The difference lies in how the chains are built and how they are held to each other: branching, the forces between chains, the order in which they pack, and whether they are tied together by cross-links.

Definition

Thermoplastic: a polymer that softens when heated and hardens again when cooled, as often as you like, because its chains are held only by intermolecular forces. Example: poly(propene).

Thermosetting polymer (thermoset): a polymer that is hardened ("set") by forming many covalent cross-links, and does not soften on reheating; strong heating chars it. Example: Bakelite.

Elastomer: a polymer that can be stretched to several times its length and returns to its shape when released. Example: vulcanised rubber.

Cross-link: a covalent bond, or short bridge of atoms, that joins one polymer chain to another. Example: the –S–S– bridge in vulcanised rubber.

Copolymer: a polymer made from two or more different monomers; a homopolymer is made from one. Example: styrene–butadiene rubber (SBR), used in car tyres, is a copolymer of phenylethene and buta-1,3-diene.

Ways of classifying polymers

Table 3.1: Classes of polymers

Basis	Classes	Examples
origin	natural; semi-synthetic (a natural polymer chemically modified); synthetic	starch, cellulose, proteins, silk, natural rubber; cellulose ethanoate, vulcanised rubber; PE, PVC, nylon
polymerisation	addition; condensation	PE, PP, PVC; PET, nylon, proteins
monomers	homopolymer; copolymer	PE, PVC; SBR, nylon-6,6 (two monomers), proteins
chain shape	linear; branched; cross-linked	HDPE; LDPE; vulcanised rubber, Bakelite
effect of heat	thermoplastic; thermoset	PE, PP, PVC, PET, nylon; Bakelite, melamine–methanal, epoxy glues
use (physical behaviour)	plastics; fibres; elastomers	PP chairs; nylon, polyester, acrylic; rubber, SBR

Figure 3.1 Chain structures. In (a) and (b) the chains are separate molecules held by intermolecular forces, so on heating they slide past each other and the plastic melts. In (c) a few cross-links (orange) let the coiled chains stretch but pull them back. In (d) so many cross-links join the chains that the whole object is close to one giant molecule, which cannot melt.

How structure controls properties

Chain length. Longer chains have larger total London forces and tangle more, so the polymer is stronger and softens at a higher temperature.
Branching. Branches keep chains apart. LDPE (density about 0.92 g cm⁻³) softens at about 110 °C and is flexible; unbranched HDPE (about 0.95 g cm⁻³) melts at about 130 °C and is stiffer and stronger.
Intermolecular forces. Polar C–Cl bonds give permanent dipole–dipole forces in PVC, so it is more rigid than PE. Hydrogen bonds between amide groups make nylon fibres strong (Lesson 2).
Crystallinity. Regular chains pack into ordered, crystalline regions, which make a polymer stronger, denser and cloudy. Irregular chains give amorphous, often transparent, polymers such as polystyrene and Perspex. In isotactic poly(propene), made with Ziegler–Natta catalysts, all the $\mathrm{CH_3}$ groups lie on the same side of the chain; the chains pack well, and the polymer is strong enough for chairs and crates. Atactic PP, with the $\mathrm{CH_3}$ groups placed at random, is soft and sticky.
Plasticisers. Small molecules mixed into PVC push the chains apart and weaken the forces between them. Unplasticised PVC is a rigid pipe; plasticised PVC is a flexible hose or cable coating.
Cross-linking. A few cross-links give an elastomer; many give a hard, rigid thermoset that does not melt.
Glass transition temperature, $T_{g}$ . Below $T_{g}$ an amorphous polymer is hard and glassy, because its chain segments cannot move; above it, it is rubbery. Natural rubber has $T_g \approx -70\,^\circ\mathrm{C}$ , so it is flexible in any weather; polystyrene has $T_g \approx 100\,^\circ\mathrm{C}$ , so a polystyrene cup is rigid and brittle at room temperature.

Natural rubber and its vulcanisation

Natural rubber comes from latex, the milky liquid that flows from cuts in the bark of the rubber tree, Hevea brasiliensis. Adding a little methanoic acid to latex makes the rubber particles clump together (coagulate), and the solid is pressed into sheets.

Rubber is poly(2-methylbuta-1,3-diene), a polymer of isoprene. Each monomer adds at carbons 1 and 4, and the remaining double bond moves to the middle of the unit:

$n\,\mathrm{CH_2{=}C(CH_3){-}CH{=}CH_2} \longrightarrow \mathrm{\bigl[{-}CH_2{-}C(CH_3){=}CH{-}CH_2{-}\bigr]_{\mathit{n}}}$

In natural rubber every double bond has the cis arrangement (Figure 3.2a), which puts a kink in the chain, so the chains coil and cannot pack closely. Raw rubber has three faults: it turns soft and sticky when warm, hard and brittle when cold, and when stretched its chains slide past each other, so it does not fully spring back.

In 1839 Charles Goodyear found that heating rubber with sulfur cures all three. In vulcanisation, rubber is heated to about 140 to 160 °C with a few per cent of sulfur (and, in industry, zinc oxide and accelerators). Sulfur atoms form bridges, one to several atoms long, between carbon atoms next to the double bonds of neighbouring chains (Figure 3.2b). The chains can still uncoil when the rubber is stretched, but the cross-links stop them sliding, and when the force is removed the chains return to their coiled shape. Vulcanised rubber stays elastic from well below 0 °C to about 100 °C, is stronger, is not sticky and resists solvents better. With about 30 % sulfur, so many cross-links form that the product, ebonite, is a hard, rigid solid.

Figure 3.2 (a) The cis repeat unit of natural rubber. (b) In vulcanised rubber, sulfur bridges (orange) tie neighbouring chains together at a few places, which turns a sticky, soft material into an elastomer.

Worked example

Problem. A sample of vulcanised rubber contains 2.0 % sulfur by mass; the rest is poly(2-methylbuta-1,3-diene). Assuming all the sulfur forms –S–S– cross-links, estimate how many isoprene units there are for each cross-link. ( $M_{r}$ of $\mathrm{C_5H_8}$ = 68.11; S = 32.06)

Solution. Take 100 g of rubber: 98.0 g polymer and 2.0 g sulfur.

$n(\mathrm{C_5H_8}\ \text{units}) = \frac{98.0}{68.11} = 1.439\,\mathrm{mol} \qquad n(\mathrm{S}) = \frac{2.0}{32.06} = 0.0624\,\mathrm{mol}$

Each cross-link uses 2 S atoms: $n(\text{cross-links}) = \frac{0.0624}{2} = 0.0312\,\mathrm{mol}$ .

$\frac{1.439}{0.0312} = 46$

Answer: about one cross-link for every 46 isoprene units (2 significant figures, as for the 2.0 %). The chains are free to uncoil between cross-links, which is why the rubber stretches.

Worked example

Problem. A school replaces a broken poly(propene) chair and a cracked melamine–methanal plate. A recycler says the chair can be melted and remoulded but the plate cannot. Explain, using structure and bonding.

Solution. Poly(propene) is a thermoplastic: its chains are separate molecules held only by London forces. On heating these weak forces are overcome, the chains slide past each other and the plastic melts; on cooling the forces re-form in the new shape. Melamine–methanal is a thermoset: its chains are joined by many covalent cross-links into a three-dimensional network. Heating cannot separate the chains without breaking covalent bonds, so the plate does not melt; at high temperature it chars (decomposes) instead.

Warning

Common mistakes. (1) "Thermoplastics are weak": nylon and HDPE are thermoplastics, and both are strong. The word describes behaviour on heating. (2) Saying that vulcanisation adds hydrogen bonds: the cross-links are covalent sulfur bridges. (3) Thinking that vulcanised rubber can be remelted: its cross-links make it behave like a thermoset, which is exactly why old tyres are hard to recycle (Lesson 4). (4) Forgetting that rubber keeps one C=C per repeat unit after polymerisation.

Tip

Link each property to one structural cause: stretchy = few cross-links; rigid and heat-proof = many cross-links; melts and remoulds = no cross-links; dense and stiff = unbranched; flexible = branched or plasticised.

MSJ Chem: Thermoplastic and thermosetting polymers. Watch how the presence or absence of cross-links decides whether a polymer softens on heating or stays rigid, with examples of each class.

✦

Practice

Classify each as natural or synthetic, and as thermoplastic or thermoset where this applies: starch, PVC, Bakelite, silk, nylon-6,6.
Distinguish a homopolymer from a copolymer, giving one example of each.
Write the repeat unit of natural rubber and name its monomer.
Explain why LDPE has a lower density and melting temperature than HDPE.
A PVC garden hose is flexible but a PVC water pipe is rigid. Explain.
Explain, in terms of structure, why vulcanised rubber returns to its shape after stretching, but raw rubber does not.
A tyre rubber contains 3.0 % sulfur by mass, all in –S–S– cross-links. Estimate the number of isoprene units per cross-link. ( $\mathrm{C_5H_8}$ = 68.11; S = 32.06)
Old tyres pile up behind garages. Evaluate why they are hard to recycle, and suggest one way they can still be reused.

Show answersHide answers

Starch: natural. PVC: synthetic, thermoplastic. Bakelite: synthetic, thermoset. Silk: natural (a protein). Nylon-6,6: synthetic, thermoplastic.
A homopolymer is made from one monomer (poly(ethene) from ethene); a copolymer from two or more (SBR from phenylethene and buta-1,3-diene).
$\mathrm{\bigl[{-}CH_2{-}C(CH_3){=}CH{-}CH_2{-}\bigr]_{\mathit{n}}}$ (cis); monomer 2-methylbuta-1,3-diene (isoprene).
LDPE chains are branched, so they cannot pack closely: there is more empty space (lower density) and fewer, weaker London forces between chains, so less energy is needed to separate them (lower melting temperature).
The hose contains a plasticiser, whose small molecules sit between the PVC chains, push them apart and weaken the dipole–dipole forces, so the chains move past each other easily. The pipe is unplasticised, so the chains are held firmly.
In vulcanised rubber, covalent sulfur bridges join the chains. Stretching uncoils the chains, but the bridges stop them sliding past each other, so they return to their coiled shape. Raw rubber has no cross-links, so stretched chains slide into new positions and stay there.
Per 100 g: $\frac{97.0}{68.11} = 1.424\,\mathrm{mol}$ units; $\frac{3.0}{32.06} = 0.0936\,\mathrm{mol}$ S, so 0.0468 mol cross-links; $\frac{1.424}{0.0468} = 30$ units per cross-link.
Tyre rubber is cross-linked, so it cannot be melted and remoulded like a thermoplastic; it also contains steel, fabric and carbon black. Burning it in the open gives toxic smoke. It can be retreaded, cut into crumb rubber for sports surfaces or road surfacing, used to make mats and sandals, or heated without air (pyrolysis) to recover oil and carbon.

Sources and credits

Image: A taxi-rider.jpg by Mutiganda, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Image: Bakelite Telephone by William Warby, via Wikimedia Commons, licensed under CC BY 2.0.
Image: Rubber tapping (48058).jpg by Manukrishnan80, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: A.5 Thermoplastic and thermosetting polymers (SL), MSJ Chem (Mike Sugiyama Jones), embedded from YouTube.
The text, diagrams, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Three polymers on one bicycle

2Ways of classifying polymers

3How structure controls properties

4Natural rubber and its vulcanisation

Three polymers on one bicycle

Ways of classifying polymers

How structure controls properties

Natural rubber and its vulcanisation