Questions
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What is the main reason for the predominance of trans states in crystalline polyethylene?
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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):
- Chain length
- Lamellar thickness
- Spherulite radius
- Root-mean-squared end-to-end distance of chains
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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?
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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.
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How are polymer chains able to form crystallites?
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Which distinctive shape is seen when observing a spherulite between crossed polars? Explain why this shape is seen.
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State two techniques that could be used to determine the degree of crystallinity of a polymer.
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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.
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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?
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Compare and contrast features of the crystallisation in polymers with that in other types of materials, such as metals and ceramics.