A polymer intended for injection moulding is found to have a polydispersity index of 20. The likely consequence is
Answer: A
The abundant short chains act as internal lubricant and ease the flow, but they contribute little strength and act as flaws. Narrowing the distribution improves the properties at the cost of a higher moulding pressure.
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Assertion: a thermoplastic can be recycled by remelting but a thermoset cannot. Reason: the thermoset's chains are joined by covalent cross-links.
Answer: B
Heat can overcome the intermolecular forces holding thermoplastic chains together but not the covalent bonds of a network, which break only by destroying the material. The cross-linking is exactly why remelting is impossible.
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Assertion: increasing the sulphur content always improves a rubber. Reason: sulphur introduces cross-links between the chains.
Answer: C
Correct answer: the assertion is false although the reason offered is a true statement in itself
The reason is correct - sulphur does cross-link the chains - but the assertion overstates the consequence. More cross-links mean less chain mobility, so beyond a few per cent the rubber becomes progressively harder and less elastic, and at thirty per cent it is ebonite, a rigid material with no rubbery behaviour at all. The sulphur content is optimised for the application, not maximised.
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Comparing high-density with low-density polyethylene, the high-density grade is
Answer: D
Correct answer: linear, more crystalline and stiffer
High-density polyethylene is essentially unbranched, so the chains pack closely into crystalline regions - hence the higher density, stiffness, melting point and chemical resistance. Low-density polyethylene carries frequent short and long branches that prevent close packing, leaving it less crystalline, softer and more transparent. Both are polymers of ethylene, differing only in chain architecture, which is a good illustration of how much structure matters.
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None of the following is an addition polymer except
Answer: A
Correct answer: polystyrene
Polystyrene is made by addition polymerisation: the styrene double bond opens and the monomers join with nothing eliminated, so the repeat unit has the same composition as the monomer. The other three are condensation polymers, each eliminating a small molecule - water in nylon-6,6 and PET, water in Bakelite - so their repeat units are lighter than the sum of their monomers.
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