Question Bank: Polymers and Composites Flashcards
What is meant by a polymer repeat unit?
The smallest section of a polymer chain that can be used to construct the chain by translation.
State the three aspects of a polymer chain that can be altered to change a polymer’s properties.
Chain length
Side groups
Cross-linking
State six effects of changing a polymer from amorphous to semi-crystalline.
Increased resistance to chemical deterioration Improved strength Improved stiffness Reduced ductility Loss of transparency Increased shrinkage upon cooling
What are the 3 main categories of polymers as defined by the degree of cross-linking? Specify the degree of cross-linking for each category.
Thermoplastics - no cross-linking
Thermosets - Extensive cross-linking
Elastomers - light cross-linking
What is meant by the glass-transition temperature?
The temperature at which a polymer transitions from a hard, brittle material to a ductile, rubbery material.
What aspect of a polymer reaches a critical value at the glass-transition temperature and allows extensive chain movement?
The amount of free volume around the polymer chains.
Why would adding a large side group to a polymer cause an increase in the glass transition temperature?
A big side group makes it harder for the chain to rotate so more free volume (i.e. a higher thermal energy input) is needed.
Describe stress-strain curves for a polymer above and below its glass transition temperature.
Below - a straight line, akin to a brittle material’s curve
Above - standard stress-strain curve; elastic region followed by a plastic region
What is the modulus of a polymer as a function of the fraction of covalent bonds it has?
Exponential, modulus on y-axis, fraction on x-axis
What aspect of viscoelasticity distinguishes it from deformation in most materials?
There is a temporal component to deformation.
How does the modulus of a composite vary as a function of fibre fraction when fibres are aligned with the load?
Modulus is directly proportional to fibre fraction
How does the modulus of a composite vary with the volume fraction of fibres when fibres are aligned perpendicular to the applied load?
Quadratically, lower than for aligned
State the main advantage and disadvantage to having short, randomly aligned fibres in a composite.
Advantage: reduced anisotropy
Disadvantage: reduced peak stiffness
State the main advantage and disadvantage to having long, aligned fibres in a composite.
Advantage: Very high peak stiffness
DIsadvantage: Highly anisotropic
