Field latex from Hevea brasiliensis is coagulated to make sheet rubber. Which treatment achieves this, and why?

Adding a weak acid such as formic acid, which brings the pH to about 5 and cancels the protein film's charge

Adding ammonia, which raises the pH and neutralises the charge on the rubber particles so that they merge

Adding a weak acid such as formic acid, which brings the pH to about 5 and cancels the protein film's charge

Diluting with a large volume of water, which washes away the serum and crosslinks the cis-polyisoprene chains

Heating the latex above 200 °C, which melts the particles into a single mass without any chemical change

A tyre sidewall made of an unprotected diene rubber develops fine cracks perpendicular to the strain after months in open air, with no oil or heat involved. Which rubber is most likely to behave this way without an antiozonant?

SBR, whose butadiene–styrene backbone is rich in C=C bonds that ozone attacks

EPDM, whose backbone is saturated and carries the C=C only in a pendant group

Silicone rubber, whose backbone is an alternating chain of silicon and oxygen atoms

CSM, whose backbone is chlorinated polyethylene with a saturated carbon chain

A seal must work for years in contact with petrol and mineral oil. Which rubber is the sound choice among the following?

Nitrile rubber, because the polar nitrile groups make it swell far less in non-polar hydrocarbons

EPDM, because its saturated backbone makes it the most chemically resistant of all rubbers, oils included

Natural rubber, because its cis-1,4 chain crystallises on stretching and blocks the fuel

Butyl rubber, because its high isobutylene content makes it hydrocarbon resistant

Nitrile rubber, because the polar nitrile groups make it swell far less in non-polar hydrocarbons

Isobutylene–isoprene rubber contains only about 1–2 mol % isoprene. Which consequence is correct?

Few cure sites make sulfur cure slow, while ozone and heat resistance exceed those of NR

It cannot be crosslinked by sulfur at all, because its few double bonds are too unreactive, so only peroxide or resin cures are possible

It has high gas permeability because the isoprene units open up the chain packing and let gas molecules diffuse through

Few cure sites make sulfur cure slow, while ozone and heat resistance exceed those of NR

It strain-crystallises more strongly than NR, so it has the highest resilience of any rubber and the best tear strength

Why does an SBS block copolymer behave like a vulcanised rubber at room temperature yet flow when heated above about 100 °C?

The polystyrene end-blocks collect into glassy domains that tie the polybutadiene mid-blocks together, and those domains soften above their Tg

Sulfur crosslinks form in the polybutadiene on heating and break again on cooling, which is why the material is soft at room temperature but flows when hot

The polystyrene end-blocks collect into glassy domains that tie the polybutadiene mid-blocks together, and those domains soften above their Tg

The polybutadiene blocks crystallise at room temperature and melt at about 100 °C

Polystyrene forms the continuous rubbery matrix and polybutadiene the glassy filler

A PLA cup left on a garden surface shows almost no change after a year, but in a hot industrial compost pile it breaks down in weeks. Which explanation is correct?

PLA is glassy below its Tg of about 60 °C, so ester hydrolysis is very slow in the garden; the warm, moist compost pile is at or above that Tg

PLA has no hydrolysable bond, and compost destroys it by thermal oxidation of its all-carbon backbone, which the cool garden surface cannot do in a year

Garden soil contains no microbes at all, whereas every compost pile contains a large and varied population

PLA is melted by compost heat at 60 °C, and the melt is then broken down by microbes

Copolymerising 3-hydroxybutyrate with a few mole percent of 3-hydroxyvalerate, to give PHBV from PHB, has which effect?

The extra ethyl side groups disturb crystal packing, so Tm and crystallinity fall and the polymer becomes tougher and easier to melt-process

Tm and crystallinity rise, so the material becomes stiffer and more brittle than PHB, because valerate units pack tighter into the crystal

The ester links are removed, so the copolymer is no longer degraded by bacterial depolymerases

The extra ethyl side groups disturb crystal packing, so Tm and crystallinity fall and the polymer becomes tougher and easier to melt-process

The copolymer becomes a thermoset because valerate units carry a second reactive group

Which linkage creates the branch points of amylopectin in starch?

α-1,6 glycosidic links off the α-1,4 linked backbone chains

β-1,4 glycosidic links between anhydroglucose units, as in cellulose

α-1,6 glycosidic links off the α-1,4 linked backbone chains

Ester links between glucose and fatty acids

Disulfide bridges between neighbouring chains

Why can cellulose not be melt-processed like polyethylene, and how is it shaped instead?

Hydrogen bonding of its –OH groups makes it decompose before melting, so it is shaped as a derivative or after regeneration

It has too low a molar mass to form a melt, so it is shaped only after crosslinking with sulfur like a rubber

It has no hydroxyl groups, so it cannot be dissolved and can only be cut mechanically

Hydrogen bonding of its –OH groups makes it decompose before melting, so it is shaped as a derivative or after regeneration

It melts at 60 °C but the melt is unstable, so it is shaped by compression below that temperature

Which of the following statements about synthetic rubbers are correct?

In polychloroprene the chlorine sits on the double bond, which gives flame, oil, weather and ozone resistance

EPDM is sulfur-curable because the diene third monomer leaves a pendant C=C while the backbone stays saturated

In polychloroprene the chlorine sits on the double bond, which gives flame, oil, weather and ozone resistance

EPDM is sulfur-curable because the diene third monomer leaves a pendant C=C while the backbone stays saturated

Polybutadiene has the highest Tg of the common diene rubbers, so it is the least resilient

SBR strain-crystallises like NR, so unfilled SBR is as strong as unfilled NR

Which of the following statements about silicone rubber and CSM are correct?

Silicone rubber has a Tg near −125 °C because its Si–O backbone is very flexible, so it stays elastic far below 0 °C

Unfilled silicone gum is weak, so fumed silica is added to reinforce it

CSM has a saturated carbon backbone, so it resists ozone and weathering well

Silicone rubber has a Tg near −125 °C because its Si–O backbone is very flexible, so it stays elastic far below 0 °C

CSM is made by copolymerising chloroprene with sulfur dioxide

Unfilled silicone gum is weak, so fumed silica is added to reinforce it

CSM has a saturated carbon backbone, so it resists ozone and weathering well

Which of the following statements about the biodegradation of polymers are correct?

Enzymes and water reach the amorphous regions first, so a more crystalline grade of the same polyester degrades more slowly

A thin film with a large surface-to-volume ratio degrades faster than a thick moulding of the same polymer

Enzymes and water reach the amorphous regions first, so a more crystalline grade of the same polyester degrades more slowly

A thin film with a large surface-to-volume ratio degrades faster than a thick moulding of the same polymer

PBAT cannot be biodegradable because it is made from fossil raw materials

Any polymer made from biomass is biodegradable, whatever its backbone and the medium

Rubbers and Bio-polymers: Natural and Synthetic Rubbers, Compostable and Natural Polymers

This chapter answers the rubber and bio-polymer part of Section XE5.3, “Synthesis, Manufacturing and Properties”, of XE5 Polymer Science and Engineering, one of the nine optional sections of the Engineering Sciences (XE) paper, any two of which a candidate chooses in the exam. The heading reads: “Natural and synthetic rubbers: recovery of NR hydrocarbon from latex; SBR; Nitrile; CR; CSM; EPDM; IIR; BR; Silicone; TPE; … Bio-compostable polymers such as PCL, PLA, PBAT, PHA/PHB, natural and biodegradable polymers such as cellulose, starch, alginate.” For each material the chapter gives the source or monomer, the route, the properties that follow from the structure, the uses and, for the bio-polymers, how they biodegrade. The numericals are chlorine and comonomer content by mass, yield with loss of water, degree of polymerisation and dry rubber content.

1. Natural rubber and recovery of the hydrocarbon from latex

Natural rubber (NR) is cis-1,4-polyisoprene, –CH₂–C(CH₃)=CH–CH₂–, made by the rubber tree Hevea brasiliensis. The tapped field latex is a colloid: about 30–35 % rubber hydrocarbon (the dry rubber content, DRC) as particles of about 0.1–1 µm, with water and small amounts of protein, lipid and sugar. Each particle is held apart by a negatively charged protein–lipid film. Recovery means breaking that stabilisation and collecting the rubber.

Coagulation: latex is diluted to about 15 % DRC and a weak acid, formic or acetic, is stirred in. The pH falls to about 5, near the isoelectric point of the protective protein; the charge is neutralised and the particles merge into a coagulum.
Sheet: the coagulum is rolled between smooth and then ribbed rolls into thin sheets, which are dried in a smokehouse at roughly 50–60 °C for some days. The result is ribbed smoked sheet (RSS); the smoke leaves preservative phenols.
Crepe: the coagulum is passed through grooved rolls under a water spray, which washes out serum and gives a thin crinkled sheet that is air-dried. Pale crepe starts with bleached latex and has the lightest colour; lower grades use scrap.
Concentrate: latex preserved with ammonia is centrifuged to about 60 % DRC for dipped goods (gloves, balloons) and foam. Crumb or block rubber (technically specified) is made by granulating, washing and drying the coagulum.

NR has Tg near −70 °C, high resilience and low heat build-up. Its regular cis chain crystallises on stretching, which gives high tensile and tear strength and strong tack even without filler. The C=C bonds allow sulfur vulcanisation but also make it weak against ozone, heat ageing and hydrocarbon oils. Uses: truck and aircraft tyres, engine mounts, bearings, gloves.

⚠️ Same monomer, different polymer

Gutta-percha and balata are trans-1,4-polyisoprene. The trans chain packs into crystals at room temperature, so they are hard and leathery, not rubbery. Only cis-1,4 (NR, and synthetic polyisoprene, IR) is an elastomer.

2. Diene synthetic rubbers: SBR, BR, nitrile, CR and IIR

An elastomer needs a flexible chain (low Tg), no crystallinity at use temperature and crosslinks to stop flow. Synthetic rubbers differ in which monomer sets those properties. Three rules cover most of the table: polar groups (nitrile, chlorine) give oil resistance; unsaturation in the backbone gives sulfur-curable sites but lets ozone cut the chain; a very flexible or very stiff chain moves Tg and resilience.

Diene and related rubbers
Rubber	Monomers and route	Key properties	Uses
SBR	Butadiene + about 23 % styrene by mass; free-radical emulsion (cold, redox initiator, about 5 °C) or anionic solution	Does not strain-crystallise, so needs reinforcing filler; good abrasion and ageing resistance; Tg about −55 °C	Car tyre treads, belts, footwear
BR	Butadiene; Ziegler–Natta (Ni, Co, Nd) gives high cis-1,4	Lowest Tg (about −100 °C), best resilience and abrasion resistance, low heat build-up; poor tear and wet grip	Blended with NR or SBR in tyre treads and sidewalls; impact modifier for HIPS
Nitrile (NBR)	Butadiene + 18–50 % acrylonitrile; emulsion	Polar nitrile gives oil and fuel resistance, rising with acrylonitrile content, while low-temperature flexibility falls	Fuel hoses, oil seals, gaskets, rollers
CR (polychloroprene)	2-Chlorobuta-1,3-diene (chloroprene); emulsion, mostly trans-1,4	Chlorine on the double bond gives flame, oil, weather and ozone resistance; crystallises on stretching; cured with ZnO and MgO	Wetsuits, conveyor belts, cable jackets, adhesives
IIR (butyl)	Isobutylene + 1–2 mol % isoprene; cationic, AlCl₃ in methyl chloride at about −90 to −100 °C	Very low gas permeability (packed chain with methyl groups), good ozone and heat resistance; low resilience; few C=C so slow cure	Inner tubes, tubeless tyre inner liners, pharmaceutical stoppers

🧠 Reading the table

Ask two questions of any rubber. Is it polar (NBR, CR: swells less in oil)? Does it have backbone C=C (NR, SBR, BR, NBR: ozone-sensitive; CR is shielded by its chlorine and IIR has too few)? The same two questions place EPDM, CSM and silicone in the next section.

3. Saturated and specialty rubbers: EPDM, CSM, silicone and TPE

EPDM is a terpolymer of ethylene, propylene and a few percent of a non-conjugated diene (ethylidene norbornene, dicyclopentadiene or 1,4-hexadiene) made with soluble vanadium Ziegler–Natta or metallocene catalyst. Propylene units break up ethylene crystallinity, so the chain is amorphous and rubbery. The diene reacts through only one of its double bonds, leaving a pendant C=C for sulfur cure, so the backbone is saturated and ozone, weather and heat resistance are excellent. It is non-polar, so it swells in oil. Uses: roofing membranes, door and window seals, radiator hoses, cable insulation. Plain EPM, with no diene, must be cured with peroxide.

CSM (chlorosulfonated polyethylene) is made by treating polyethylene in solution with chlorine and sulfur dioxide: it carries about 25–43 % chlorine and about 1 % sulfur as –SO₂Cl groups. The chlorine breaks the PE crystallinity, giving rubber; the –SO₂Cl groups are the cure sites (metal oxides). The backbone is saturated, so ozone, weather, chemical and flame resistance are excellent. Uses: roofing sheet, hose covers, tank and pond liners.

Silicone rubber has a –Si(CH₃)₂–O– backbone, made by hydrolysing dimethyldichlorosilane to silanols that condense to linear polymer or to cyclic tetramer, which is then ring-opened by acid or base. The long, flexible Si–O bonds give Tg near −125 °C, so it stays rubbery from about −55 to 250 °C; it is inert, biocompatible and has high gas permeability. The unfilled gum is weak, so fumed silica is added. Cure is by peroxide, platinum-catalysed addition, or moisture (room-temperature vulcanising). Uses: gaskets, medical tubing, implants, sealants, bakeware.

Thermoplastic elastomers (TPE) are rubbery at use temperature yet melt-processable, because they replace chemical crosslinks by physical ones: hard glassy or crystalline domains embedded in a soft rubbery matrix. Examples: SBS (polystyrene–polybutadiene–polystyrene, made by sequential anionic polymerisation; the polystyrene domains, Tg about 100 °C, tie the rubber mid-blocks), its hydrogenated form SEBS (better ageing), thermoplastic polyurethane, copolyester and polyamide block copolymers, and TPV, polypropylene with finely dispersed, dynamically vulcanised EPDM. They soften above the hard-phase Tg or Tm, so scrap is reprocessable but the working temperature is lower than for a vulcanisate, and creep and compression set are worse.

4. Bio-compostable polyesters: PCL, PLA, PBAT and PHA/PHB

Their ester links can be hydrolysed and then used by microbes. Bio-based (made from biomass) and biodegradable are different things: PBAT is fossil-based yet compostable, bio-polyethylene is bio-based yet not biodegradable.

Compostable polyesters
Polymer	Source and route	Properties	Uses
PCL	ε-Caprolactone (petrochemical); ring-opening polymerisation, tin octoate with an alcohol initiator	Semicrystalline, Tm about 60 °C, Tg about −60 °C; soft, tough; degrades by lipases and fungi	Drug delivery, sutures, 3D-printing filament, blend partner
PLA	Lactic acid from fermenting sugar or starch → lactide (cyclic dimer) → ring-opening with tin octoate; direct condensation gives only low molar mass	Tg about 55–60 °C, Tm about 150–180 °C (PLLA); stiff, clear, brittle; softens near 60 °C	Cups, films, fibre, 3D printing, resorbable implants
PBAT	1,4-Butanediol + adipic acid + terephthalic acid (or DMT); melt polycondensation; fossil-based	Random aliphatic–aromatic copolyester: flexible, Tg about −30 °C, Tm about 110–120 °C; compostable	Compost bags, mulch film; toughens PLA and starch
PHA, PHB	Made by bacteria (e.g. Cupriavidus necator) as an energy store when sugars or oils are in excess and N or P is scarce; extracted from the cells	PHB (3-hydroxybutyrate): Tm about 175 °C, highly crystalline, stiff and brittle; PHBV with 3-hydroxyvalerate lowers Tm and crystallinity and toughens	Packaging, disposable cutlery, medical sutures; degrades even in soil and sea water

🎯 Why compost, not the soil heap?

PLA has Tg near 60 °C. An industrial compost pile runs at about 55–60 °C and is moist, so the chains are mobile enough for water to cut the ester links quickly. At 20 °C, below its Tg, PLA is glassy and hydrolysis is very slow, so a PLA cup in a garden persists for a long time. PCL (Tm 60 °C) and PHB, by contrast, are also attacked by soil microbes at ambient temperature.

5. Natural polymers: cellulose, starch and alginate

Cellulose is the most abundant polymer: β-1,4-linked glucose units (anhydroglucose, C₆H₁₀O₅, 162.14 g/mol) in a straight, flat chain, with degree of polymerisation up to about 10⁴ in cotton. Every unit has three –OH groups, so extensive hydrogen bonding makes it crystalline, strong and infusible and insoluble in water: it decomposes before it melts. It is processed through derivatives or special solvents: cellulose acetate (plastics, film, fibre), cellulose nitrate, and regenerated cellulose (viscose via cellulose xanthate, or lyocell from an amine-oxide solvent). Natural sources are wood pulp, cotton and jute.

Starch is the energy store of plants, built from α-glucose: amylose (linear, α-1,4, about 20–30 %) and amylopectin (α-1,4 chains with α-1,6 branch points, the rest). It is stored in semicrystalline granules. Heated with water and sheared, the granules gelatinise (swell, lose crystallinity) and, with a plasticiser such as glycerol, form thermoplastic starch (TPS). TPS is cheap and biodegradable but water-sensitive and it retrogrades (recrystallises, becomes brittle) with time, so it is blended with PBAT, PCL or PLA.

Alginate is extracted from brown seaweed: a linear copolymer of β-D-mannuronic (M) and α-L-guluronic (G) acid, with M-blocks, G-blocks and mixed blocks. Divalent cations such as Ca²⁺ bind between G-blocks of neighbouring chains (the “egg-box” model) and turn the sodium-alginate solution into a gel at room temperature. Uses: wound dressings, food thickener and gel, cell and drug encapsulation. The gel dissolves when the Ca²⁺ is removed, for example by a chelating agent.

6. How the bio-polymers biodegrade

Biodegradation is not instant disappearance. It has stages. Fragmentation: water, heat or extracellular enzymes (lipases and esterases for polyesters, cellulases for cellulose, amylases for starch, PHA depolymerases for PHB) cut chains at the surface; a polymer too large to enter a cell must be cut first, so degradation is a surface erosion that lowers molar mass. Assimilation: microbes take up the short fragments. Mineralisation: they are converted to CO₂ and H₂O plus biomass in air, and to CO₂ and CH₄ without air.

Crystallinity: enzymes and water enter the amorphous regions first, so a more crystalline grade (PHB, PLLA, PCL) degrades more slowly than an amorphous or copolymer grade.
Chemistry: hydrolysable links (ester, glycosidic) are needed; an all-carbon backbone such as polyethylene or polypropylene is not attacked in practice. Hydrophilic polymers (starch, alginate) take up water and degrade faster than hydrophobic ones.
Environment: temperature, moisture, pH and microbial population. PLA needs hot compost; PCL, PHB and cellulose degrade at ambient temperature in soil.
Shape: a thin film with a large surface-to-volume ratio degrades much faster than a thick moulding.

⚠️ Judge the claim, not the label

“Biodegradable” without a medium and a time is not a property. A polymer can be biodegradable in industrial compost yet persist in sea water. “Bio-based” says nothing about end-of-life, and an oxo-additive that only fragments a polyolefin into microplastics does not make it biodegradable.

7. Worked numericals

Molar masses used (g/mol): H 1.008, C 12.011, N 14.007, O 15.999, Cl 35.45. Composition by mass from a repeat unit: polychloroprene repeat unit C₄H₅Cl = 4(12.011) + 5(1.008) + 35.45 = 88.53 g/mol, so chlorine is 35.45/88.53 = 40.0 % by mass. For CSM, the chlorine figure is set by the process, not by a fixed repeat unit.

Mole fraction to mass fraction (nitrile rubber). For a copolymer with 30 mol % acrylonitrile (C₃H₃N, 53.06 g/mol) and 70 mol % butadiene (C₄H₆, 54.09 g/mol): per 100 mol of units, acrylonitrile mass = 30 × 53.06 = 1591.8 g and butadiene mass = 70 × 54.09 = 3786.3 g, so the mass fraction of acrylonitrile = 1591.8/5378.1 = 29.6 %. Because the two masses are close, mole % and mass % nearly coincide here; do not expect that when the monomer masses differ, as with styrene (104.15) and butadiene.

Yield with loss of water. Lactic acid C₃H₆O₃ = 90.08 g/mol; each condensation step releases one H₂O, so a long chain has repeat unit C₃H₄O₂ = 72.06 g/mol. Mass of PLA from 100 kg of lactic acid at full conversion = 100 × 72.06/90.08 = 80.0 kg, and 20.0 kg of water leaves. The lactide route loses no water in the ring-opening step.

Degree of polymerisation and DRC. Cellulose with Mₙ = 486,000 g/mol has DP = 486,000/162.14 ≈ 3000 (end groups neglected). A latex of 35 % DRC gives 350 kg dry rubber per 1000 kg; if 5 % is lost in serum and washing, the sheet is 0.95 × 350 = 332.5 kg.

Key takeaways

NR is cis-1,4-polyisoprene recovered by acid coagulation of latex (pH about 5) into sheet (RSS) or crepe; it strain-crystallises, so it is strong without filler, but its C=C bonds make it ozone- and oil-sensitive; the trans isomer is hard gutta-percha.
SBR (emulsion or solution, no strain-crystallisation, needs filler), BR (lowest Tg, best resilience and wear), NBR (polar, oil-resistant), CR (chlorine: flame, oil, weather resistance) and IIR (tiny unsaturation, very low gas permeability) are the diene rubbers; polar groups give oil resistance, backbone C=C gives ozone sensitivity.
EPDM, CSM and silicone have saturated or inorganic backbones: excellent ozone, weather and heat resistance. EPDM keeps a pendant diene for sulfur cure; CSM is chlorosulfonated PE cured through –SO₂Cl; silicone (Tg about −125 °C) needs silica filler.
TPEs use physical crosslinks from hard domains (SBS: polystyrene domains in a polybutadiene matrix); they are reprocessable but limited by the hard-phase Tg or Tm.
PCL (ring-opening, Tm about 60 °C), PLA (lactide ring-opening, Tg about 60 °C, needs hot compost), PBAT (fossil-based but compostable, flexible) and PHA/PHB (bacterial, brittle PHB, tougher PHBV) are the compostable polyesters; bio-based does not mean biodegradable.
Cellulose (β-1,4, H-bonded, infusible), starch (amylose and amylopectin, TPS retrogrades) and alginate (M/G blocks, Ca²⁺ egg-box gel) biodegrade by hydrolytic enzymes; degradation is surface erosion, faster for amorphous, hydrophilic, thin samples.

Practice questions (16)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Field latex from Hevea brasiliensis is coagulated to make sheet rubber. Which treatment achieves this, and why?
1. Adding ammonia, which raises the pH and neutralises the charge on the rubber particles so that they merge
2. Adding a weak acid such as formic acid, which brings the pH to about 5 and cancels the protein film's charge
3. Diluting with a large volume of water, which washes away the serum and crosslinks the cis-polyisoprene chains
4. Heating the latex above 200 °C, which melts the particles into a single mass without any chemical change
Show answer
Answer: B — Adding a weak acid such as formic acid, which brings the pH to about 5 and cancels the protein film's charge
The rubber particles are kept apart by a negatively charged protein–lipid film. A weak acid brings the pH close to the isoelectric point of the proteins, so the repulsion vanishes and the particles coalesce. Ammonia does the opposite: it keeps the latex stable and is used as the preservative for concentrate. Dilution does not crosslink the chains (that needs vulcanisation), and heating to 200 °C would degrade the rubber.
A tyre sidewall made of an unprotected diene rubber develops fine cracks perpendicular to the strain after months in open air, with no oil or heat involved. Which rubber is most likely to behave this way without an antiozonant?
1. SBR, whose butadiene–styrene backbone is rich in C=C bonds that ozone attacks
2. EPDM, whose backbone is saturated and carries the C=C only in a pendant group
3. Silicone rubber, whose backbone is an alternating chain of silicon and oxygen atoms
4. CSM, whose backbone is chlorinated polyethylene with a saturated carbon chain
Show answer
Answer: A — SBR, whose butadiene–styrene backbone is rich in C=C bonds that ozone attacks
Ozone cracking needs backbone C=C bonds, which ozone cleaves while the rubber is under strain. SBR has many. EPDM, CSM and silicone have saturated or inorganic backbones, so they resist ozone; this is the main reason EPDM replaced diene rubbers in outdoor seals.
A seal must work for years in contact with petrol and mineral oil. Which rubber is the sound choice among the following?
1. EPDM, because its saturated backbone makes it the most chemically resistant of all rubbers, oils included
2. Natural rubber, because its cis-1,4 chain crystallises on stretching and blocks the fuel
3. Butyl rubber, because its high isobutylene content makes it hydrocarbon resistant
4. Nitrile rubber, because the polar nitrile groups make it swell far less in non-polar hydrocarbons
Show answer
Answer: D — Nitrile rubber, because the polar nitrile groups make it swell far less in non-polar hydrocarbons
Oil resistance follows polarity: like dissolves like, so non-polar rubbers (NR, EPDM, butyl) swell in hydrocarbons, while polar NBR does not. EPDM is excellent against ozone, steam and polar fluids, but it swells in petrol and oil, which is the tempting wrong answer; its saturation helps ozone resistance, not oil resistance.
Isobutylene–isoprene rubber contains only about 1–2 mol % isoprene. Which consequence is correct?
1. It cannot be crosslinked by sulfur at all, because its few double bonds are too unreactive, so only peroxide or resin cures are possible
2. It has high gas permeability because the isoprene units open up the chain packing and let gas molecules diffuse through
3. Few cure sites make sulfur cure slow, while ozone and heat resistance exceed those of NR
4. It strain-crystallises more strongly than NR, so it has the highest resilience of any rubber and the best tear strength
Show answer
Answer: C — Few cure sites make sulfur cure slow, while ozone and heat resistance exceed those of NR
The few isoprene units supply the only C=C for sulfur vulcanisation, so cure is slow and the rubber is unreactive to ozone and oxygen. Sulfur cure is possible (butyl inner tubes are sulfur-cured), so the claim that it cannot be sulfur-cured is false. Butyl is known for very low gas permeability and low resilience, which rules out the other two.
Why does an SBS block copolymer behave like a vulcanised rubber at room temperature yet flow when heated above about 100 °C?
1. Sulfur crosslinks form in the polybutadiene on heating and break again on cooling, which is why the material is soft at room temperature but flows when hot
2. The polystyrene end-blocks collect into glassy domains that tie the polybutadiene mid-blocks together, and those domains soften above their Tg
3. The polybutadiene blocks crystallise at room temperature and melt at about 100 °C
4. Polystyrene forms the continuous rubbery matrix and polybutadiene the glassy filler
Show answer
Answer: B — The polystyrene end-blocks collect into glassy domains that tie the polybutadiene mid-blocks together, and those domains soften above their Tg
The glassy polystyrene domains (Tg about 100 °C) act as physical crosslinks between rubbery polybutadiene chains. Above the polystyrene Tg the domains soften and the material flows, and on cooling they re-form, so scrap can be reprocessed. There is no sulfur and polybutadiene (Tg about −100 °C) does not crystallise appreciably; the glassy phase is the minor, dispersed one.
A PLA cup left on a garden surface shows almost no change after a year, but in a hot industrial compost pile it breaks down in weeks. Which explanation is correct?
1. PLA is glassy below its Tg of about 60 °C, so ester hydrolysis is very slow in the garden; the warm, moist compost pile is at or above that Tg
2. PLA has no hydrolysable bond, and compost destroys it by thermal oxidation of its all-carbon backbone, which the cool garden surface cannot do in a year
3. Garden soil contains no microbes at all, whereas every compost pile contains a large and varied population
4. PLA is melted by compost heat at 60 °C, and the melt is then broken down by microbes
Show answer
Answer: A — PLA is glassy below its Tg of about 60 °C, so ester hydrolysis is very slow in the garden; the warm, moist compost pile is at or above that Tg
Hydrolytic chain cutting needs mobile chains and water. Below its Tg PLA is glassy and the ester links are only slowly attacked, so a cup persists outdoors; at 55–60 °C in moist compost the chains are mobile, hydrolysis is fast, and microbes then consume the fragments. PLA is a polyester with hydrolysable links, soil has abundant microbes, and PLLA melts at about 150–180 °C, not 60 °C.
Copolymerising 3-hydroxybutyrate with a few mole percent of 3-hydroxyvalerate, to give PHBV from PHB, has which effect?
1. Tm and crystallinity rise, so the material becomes stiffer and more brittle than PHB, because valerate units pack tighter into the crystal
2. The ester links are removed, so the copolymer is no longer degraded by bacterial depolymerases
3. The extra ethyl side groups disturb crystal packing, so Tm and crystallinity fall and the polymer becomes tougher and easier to melt-process
4. The copolymer becomes a thermoset because valerate units carry a second reactive group
Show answer
Answer: C — The extra ethyl side groups disturb crystal packing, so Tm and crystallinity fall and the polymer becomes tougher and easier to melt-process
The valerate unit carries an ethyl side group instead of the methyl of butyrate. This irregularity interrupts packing, lowering crystallinity and Tm, which reduces brittleness. The ester links remain, so biodegradation continues, and nothing makes the chain crosslink. Higher crystallinity would be the PHB homopolymer, not the copolymer.
Which linkage creates the branch points of amylopectin in starch?
1. β-1,4 glycosidic links between anhydroglucose units, as in cellulose
2. α-1,6 glycosidic links off the α-1,4 linked backbone chains
3. Ester links between glucose and fatty acids
4. Disulfide bridges between neighbouring chains
Show answer
Answer: B — α-1,6 glycosidic links off the α-1,4 linked backbone chains
Both amylose and the main chains of amylopectin are α-1,4 linked; amylopectin has α-1,6 links every 20 or so units that start the branches. The β-1,4 linkage is cellulose, and starch has neither fatty-acid esters nor disulfide bridges.
Why can cellulose not be melt-processed like polyethylene, and how is it shaped instead?
1. It has too low a molar mass to form a melt, so it is shaped only after crosslinking with sulfur like a rubber
2. It has no hydroxyl groups, so it cannot be dissolved and can only be cut mechanically
3. Hydrogen bonding of its –OH groups makes it decompose before melting, so it is shaped as a derivative or after regeneration
4. It melts at 60 °C but the melt is unstable, so it is shaped by compression below that temperature
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Answer: C — Hydrogen bonding of its –OH groups makes it decompose before melting, so it is shaped as a derivative or after regeneration
Each anhydroglucose unit has three –OH groups that hydrogen-bond strongly in a rigid, crystalline chain, so the decomposition temperature is below any melting point. Cellulose acetate is thermoplastic and soluble, and viscose or lyocell regenerate the cellulose from solution. Cellulose has very high DP and many hydroxyl groups, and it has no 60 °C melting point.
Which of the following statements about synthetic rubbers are correct?
1. In polychloroprene the chlorine sits on the double bond, which gives flame, oil, weather and ozone resistance
2. EPDM is sulfur-curable because the diene third monomer leaves a pendant C=C while the backbone stays saturated
3. Polybutadiene has the highest Tg of the common diene rubbers, so it is the least resilient
4. SBR strain-crystallises like NR, so unfilled SBR is as strong as unfilled NR
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Answer: A — In polychloroprene the chlorine sits on the double bond, which gives flame, oil, weather and ozone resistance; B — EPDM is sulfur-curable because the diene third monomer leaves a pendant C=C while the backbone stays saturated
The statements on polychloroprene and on EPDM are standard. Polybutadiene has the lowest Tg (about −100 °C) and the highest resilience of the group, and SBR, being irregular, does not strain-crystallise, so unfilled SBR is weak and needs reinforcing filler.
Which of the following statements about silicone rubber and CSM are correct?
1. Silicone rubber has a Tg near −125 °C because its Si–O backbone is very flexible, so it stays elastic far below 0 °C
2. CSM is made by copolymerising chloroprene with sulfur dioxide
3. Unfilled silicone gum is weak, so fumed silica is added to reinforce it
4. CSM has a saturated carbon backbone, so it resists ozone and weathering well
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Answer: A — Silicone rubber has a Tg near −125 °C because its Si–O backbone is very flexible, so it stays elastic far below 0 °C; C — Unfilled silicone gum is weak, so fumed silica is added to reinforce it; D — CSM has a saturated carbon backbone, so it resists ozone and weathering well
Silicone is flexible to very low temperature and needs silica for strength, and CSM, with a saturated backbone, resists ozone. CSM is made by chlorosulfonating polyethylene with Cl₂ and SO₂ in solution, not by copolymerising chloroprene.
Which of the following statements about the biodegradation of polymers are correct?
1. Enzymes and water reach the amorphous regions first, so a more crystalline grade of the same polyester degrades more slowly
2. A thin film with a large surface-to-volume ratio degrades faster than a thick moulding of the same polymer
3. PBAT cannot be biodegradable because it is made from fossil raw materials
4. Any polymer made from biomass is biodegradable, whatever its backbone and the medium
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Answer: A — Enzymes and water reach the amorphous regions first, so a more crystalline grade of the same polyester degrades more slowly; B — A thin film with a large surface-to-volume ratio degrades faster than a thick moulding of the same polymer
Biodegradation is surface erosion that starts in amorphous regions, so crystallinity slows it and a large surface area speeds it. Biodegradability depends on the chemical links and the medium, not on the raw-material origin: PBAT is fossil-based yet compostable, and bio-polyethylene is bio-based yet not biodegradable.
Polychloroprene has the repeat unit C₄H₅Cl. Take C = 12.011, H = 1.008 and Cl = 35.45 g/mol. What is the chlorine content of the polymer in mass percent? Give the answer to the nearest whole number.
Numerical answer — type the value.
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Answer: 40
Repeat-unit mass = 4(12.011) + 5(1.008) + 35.45 = 48.044 + 5.040 + 35.45 = 88.534 g/mol. Cl fraction = 35.45/88.534 = 0.4004, i.e. 40 mass percent.
Poly(lactic acid) is made by direct melt polycondensation of lactic acid, C₃H₆O₃ (M = 90.08 g/mol), with one water molecule released per ester link formed. For a long chain the repeat unit is C₃H₄O₂ (M = 72.06 g/mol). How many kilograms of PLA are obtained from 100 kg of lactic acid at complete conversion? Give the answer to the nearest whole number.
Numerical answer — type the value.
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Answer: 80
Mass of polymer per mole of lactic acid consumed = 72.06 g (90.08 − 18.02). PLA = 100 × 72.06/90.08 = 80.0 kg; the other 20.0 kg is water.
A nitrile rubber is a random copolymer of 30 mol % acrylonitrile (C₃H₃N, M = 53.06 g/mol) and 70 mol % butadiene (C₄H₆, M = 54.09 g/mol). What is the acrylonitrile content in mass percent? Give the answer to one decimal place.
Numerical answer — type the value.
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Answer: 29.6
Take 100 mol of units: acrylonitrile = 30 × 53.06 = 1591.8 g, butadiene = 70 × 54.09 = 3786.3 g. Mass % acrylonitrile = 1591.8/(1591.8 + 3786.3) × 100 = 1591.8/5378.1 × 100 = 29.6.
A PHBV copolymer contains 80 mol % 3-hydroxybutyrate units (C₄H₆O₂, M = 86.09 g/mol) and 20 mol % 3-hydroxyvalerate units (C₅H₈O₂, M = 100.12 g/mol). What is the mass percent of valerate units in the polymer? Give the answer to one decimal place.
Numerical answer — type the value.
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Answer: 22.5
Per 100 mol of units: butyrate = 80 × 86.09 = 6887.2 g, valerate = 20 × 100.12 = 2002.4 g, total 8889.6 g. Valerate mass % = 2002.4/8889.6 × 100 = 22.5. The mass percent exceeds the mole percent because the valerate unit is heavier.