Polymers and Plastics Questions and Answers

Answer: A

Correct answer: oil resistant seals and gaskets

The acrylonitrile units make the chain polar, so hydrocarbon oils and fuels do not swell it - the property that decides almost every seal, gasket and hose application in contact with oil. The more acrylonitrile, the better the oil resistance and the worse the low temperature flexibility, so the grade is chosen by balancing the two.

Answer: B

Correct answer: retention of flexibility over a wide temperature range

The silicon-oxygen backbone is far more flexible than a carbon chain and much more thermally stable, so silicone rubber stays elastic from about \( -60 \) to over 200 degrees, where organic rubbers become brittle at one end and degrade at the other. It is also physiologically inert. Against that it is expensive, permeable to gases, and poor in abrasion and tear resistance.

Answer: C

Correct answer: conjugated, allowing charge to delocalise

An alternating single and double bond sequence puts the p orbitals in a continuous overlapping array along the chain, so a charge introduced by doping can move along it rather than being localised on one atom. Doping supplies that charge by partial oxidation or reduction. A saturated backbone has no such pathway, which is why ordinary polyethylene is one of the best insulators known.

Answer: D

Correct answer: hydrolysable ester linkages

Polylactic acid is a polyester, and the ester linkage in its backbone can be split by water - slowly in the environment, and faster with the help of microbial enzymes - so the chain breaks into fragments small enough to be metabolised. A carbon-carbon backbone offers no such point of attack, which is precisely why polyethylene and polypropylene persist for centuries.

Answer: A

Correct answer: it is collected in large quantities as a single clean stream

PET’s recyclability is largely a matter of logistics: drinks bottles are consumed and discarded in enormous numbers, they are almost all PET, and deposit and kerbside schemes collect them separated from other plastics. That clean single-polymer stream is what makes mechanical recycling economic. The polymer does degrade somewhat on each cycle, and it certainly requires sorting; it is not biodegradable at all.

Answer: B

Correct answer: 1000

The degree of polymerisation is the chain mass divided by the repeat unit mass:

so the chain contains a thousand ethylene residues. Multiplying instead of dividing gives 784,000, which would be a molecule of impossible size.

Answer: C

Correct answer: 15 000

The number average weights every chain equally:

which with equal numbers of the two species is simply their arithmetic mean. The weight average of the same sample is higher, at about 16,667, because it counts the heavier chains more.

Answer: D

Summing the products of number and mass squared and dividing by the total mass gives 5 \( \times \) 10\( ^{10} \) over 3 \( \times \) 10\( ^6 \), that is 16 667. It exceeds the number average because the heavier chains count for more.

Answer: A

Correct answer: 1.11

The weight average is

so

A narrow distribution, as one would expect from only two species. Commercial polymers, with a continuous spread of chain lengths, run from about 1.5 upward.

Answer: B

Correct answer: 100

Dividing as before,

so a hundred repeat units, each comprising one adipic acid and one hexamethylenediamine residue. Note that a nylon repeat unit is much heavier than an ethylene one, so a similar molecular mass corresponds to a far shorter chain.