DoITPoMS

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Questions

  1. What is the main reason for the predominance of trans states in crystalline polyethylene?

    a Allows better chain packing to produce a denser structure
    b Each trans state is lower in energy due to steric hindrance effects
    c The all-trans conformation is entropically favoured
    d Formation of chain-folded lamellae requires trans states

  2. Rank these structural features of a quenched high molecular weight semi-crystalline polymer in order of their size, smallest first (based on average values - there will be some overlap in ranges):

    1. Chain length
    2. Lamellar thickness
    3. Spherulite radius
    4. Root-mean-squared end-to-end distance of chains

    a B, D, C, A
    b A, C, B, D
    c D, C, A, B
    d C, B, D, A

  3. PHB (polyhydroxybuturyate) is a naturally occurring, biodegradable polymer produced by bacteria, which melts at around 200°C and forms large spherulites readily on cooling. What sort of heat treatment would result in a solid with mechanical properties most suitable for a making a fizzy drinks bottle?

    a Annealing at 100°C for one hour
    b Heating to just above 200°C until melted, followed by rapid quench to room temperature
    c Heating to just above 200°C until melted, followed by rapid quench to room temperature, and annealing at 100°C for one hour
    d Heating to just 200°C until melted, followed by a slow cooling to room temperature

  4. During a tensile test of a polyethylene dumbbell sample at room temperature using a strain rate of 10 mm/min, at what position relative to the sample is failure most likely to occur? Justify your answer in terms of the changes in polymer chain orientation in the sample during tensile draw.

    a Exactly at the mid-point between the jaws of the tensile tester
    b Exactly in the middle of the neck
    c Equally likely at either end of the dumbbell, where the neck meets the amorphous region
    d At one end of the dumbbell, furthest from the point at which the necking originated

  5. How are polymer chains able to form crystallites?

  6. Which distinctive shape is seen when observing a spherulite between crossed polars? Explain why this shape is seen.

  7. State two techniques that could be used to determine the degree of crystallinity of a polymer.

  8. Describe and explain the form of a stress-strain curve for a semi-crystalline polymer subjected to a uniaxial stress above its glass transition temperature.

  9. Describe how Considère's construction can explain the phenomenon of necking and stable cold drawing in polymers. What other types of behaviour may be observed for a polymer sample subjected to a uniaxial stress?. What other types of behaviour may be observed for a polymer sample subjected to a uniaxial stress?

  10. Compare and contrast features of the crystallisation in polymers with that in other types of materials, such as metals and ceramics.


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