Polymers and Plastics Questions and Answers

Answer: C

Correct answer: 80 g

If 60 per cent is crystalline, 40 per cent is amorphous:

No polymer is fully crystalline; the chains are too long to fold perfectly, so crystalline lamellae are always interspersed with amorphous regions. The crystalline fraction governs stiffness, density and opacity, and the amorphous fraction governs the glass transition and the impact behaviour.

Answer: D

Correct answer: a linear chain

A bifunctional monomer can react at two points, so it links to one neighbour on each side and the growth is one-dimensional - a linear chain. Three or more reactive sites allow branching in a third direction and lead to a cross-linked network, which is how thermosets are made. Functionality is therefore what determines whether a polymerisation gives a thermoplastic or a thermoset.

Answer: A

Correct answer: lowers the molecular mass by capping chains

A monofunctional monomer reacts once and can grow no further, so it terminates whatever chain it joins. Adding a small proportion of it therefore caps chains before they reach full length and reduces the average molecular mass. It is used deliberately for exactly that purpose - a chain stopper, controlling the molecular mass and hence the melt viscosity of the product.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

Three condensation polymers with three pairs of monomers. Bakelite is phenol with formaldehyde, and because phenol is trifunctional the product is a network thermoset. Nylon-6,6 is adipic acid with hexamethylenediamine, giving amide linkages. PET is terephthalic acid with ethylene glycol, giving ester linkages. All three eliminate a small molecule at each step, which is what makes them condensation rather than addition polymers.

Answer: C

Correct answer: 1-Q, 2-R, 3-P

Each rubber is chosen for one dominant property. Nitrile rubber is polar because of its acrylonitrile content, so hydrocarbon oils do not swell it - hence oil seals. Silicone rubber has a flexible and thermally stable siloxane backbone, so it stays elastic from below \( -60 \) to above 200 degrees. SBR wears slowly against a road surface, so it goes into tyre treads.

Answer: D

Correct answer: 3, 4, 2, 1

The three stages of a chain polymerisation. Initiation has two parts: the initiator decomposes into radicals, and a radical then attacks the first monomer’s double bond. Propagation follows, with monomer adding repeatedly to the growing radical - thousands of additions in a fraction of a second. Termination ends it, here by two radicals combining. Note that a very long chain is formed almost instantly, so high molecular mass polymer is present from the start.

Answer: A

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: a thermoset’s covalent network cannot be melted, and \( M_w \) always exceeds \( M_n \) because it weights the longer chains. Statement 3 reverses the effect of a plasticiser - it inserts small molecules between the chains, separating them and letting them move more freely, which lowers the glass transition. That is exactly why plasticised PVC is flexible and unplasticised PVC is rigid.

Answer: B

Correct answer: polystyrene

Polystyrene is a linear thermoplastic: its chains are separate, so it softens on heating and can be remoulded. The other three are all thermosets, cured into covalent networks - epoxy by a hardener, and Bakelite and urea-formaldehyde by condensation with formaldehyde, both of which involve a trifunctional component and so build in three dimensions.

Answer: C

Correct answer: adding a plasticiser

A plasticiser separates the chains and lets them move past one another, lowering the glass transition and making the material softer and more flexible - the opposite of stiffening. The other three all raise stiffness: crystallinity packs the chains into ordered regions, cross-links tie them together covalently, and a glass fibre filler carries load in a much stiffer phase.

Answer: D

Correct answer: loss of plasticiser and evolution of hydrogen chloride

Both processes act together in aged PVC. The plasticiser is a small molecule and migrates out or evaporates over time, so the material stiffens and embrittles; and heat and ultraviolet light strip hydrogen chloride from the chain, leaving conjugated sequences that absorb visible light and discolour it. The two together produce exactly the brittle, yellowed article described.