Polymers and Plastics Questions and Answers

Answer: A

Correct answer: light cross-linking above its glass transition

An elastomer needs both conditions. Being above its glass transition, the chains are free to move and can be uncoiled by an applied stress; being lightly cross-linked, they cannot slide permanently past one another, so the network pulls them back when the stress is removed. Heavy cross-linking gives a rigid thermoset, and no cross-linking at all gives a material that flows and does not recover.

Answer: B

Correct answer: formaldehyde

Bakelite is the phenol-formaldehyde resin, made by condensing the two with elimination of water. Phenol is trifunctional towards formaldehyde, so the reaction builds a three-dimensional network and the product is a thermoset - hard, infusible, a good electrical insulator, and the first fully synthetic plastic, introduced in 1907.

Answer: C

Correct answer: hexamethylenediamine

Nylon-6,6 condenses adipic acid, a six-carbon diacid, with hexamethylenediamine, a six-carbon diamine - hence the two sixes in the name. Each condensation eliminates a molecule of water and forms an amide linkage. The regular chain and the hydrogen bonding between the amide groups give the crystallinity and the strength that make it a fibre.

Answer: D

Correct answer: terephthalic acid

PET is the polyester of ethylene glycol and terephthalic acid, made by esterification with elimination of water. The rigid aromatic ring in the acid raises the melting point and the stiffness well above what an aliphatic diacid would give, which is what makes PET suitable for fibres, films and bottles. Adipic acid with the same glycol would give a soft low-melting polyester.

Answer: A

Correct answer: strength of the carbon-fluorine bond

The carbon-fluorine bond is among the strongest in organic chemistry, so PTFE resists almost every chemical and withstands temperatures at which other plastics degrade. The fluorine atoms also sheathe the carbon backbone completely and interact very weakly with anything else, which gives the exceptionally low surface energy responsible for the non-stick behaviour and the low friction.

Answer: B

Correct answer: eliminate hydrogen chloride

Heating PVC strips hydrogen chloride from the chain, leaving a conjugated sequence that absorbs visible light and discolours the material, and the acid released catalyses further elimination - so the degradation accelerates once it begins. Stabilisers based on metal soaps or organotins scavenge the acid and interrupt that autocatalysis, and without them PVC could not be processed at all.

Answer: C

Correct answer: amorphous with a bulky side group

The phenyl ring is large and, in commercial atactic polystyrene, randomly placed, so the chains cannot pack into crystals. The material is therefore wholly amorphous, which makes it transparent, and its glass transition of about 100 degrees leaves it well below \( T_g \) at room temperature, which makes it hard and brittle. Toughening requires a rubber phase, as in high impact polystyrene.

Answer: D

An amine or anhydride reacts with the terminal rings and knits the molecules into a network, so the proportions must be right and the mixture has a working life. Its adhesion and chemical resistance make it the standard structural adhesive.

Answer: A

Correct answer: two or more kinds of repeat unit

A copolymer is made from more than one monomer, and the arrangement gives further variety: random, alternating, block or graft. It is the principal means of tailoring properties - SBR combines the toughness of styrene with the elasticity of butadiene, and ABS the rigidity of styrene-acrylonitrile with the impact resistance of a rubber phase.

Answer: B

The incompatible blocks segregate into separate domains, which is what makes a thermoplastic elastomer behave as a rubber yet melt for processing. A graft copolymer instead hangs one polymer as side chains from the other.