Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

1. Materials expand when heated because:

   	a	The mean interatomic distance increases as the thermal energy of the atoms increases
   	b	The heat takes up extra volume forcing the material to expand
   	c	The rubber melts
   	d	There are more chemical bonds being formed

2. When rubber is put under uniaxial tension it:

   	a	Contracts
   	b	Extends
   	c	Explodes
   	d	Rises

3. Rubber differs from all other materials in that:

   	a	The density of cross-links is higher
   	b	It can be extended by more than 100%
   	c	Its stiffness increases with increasing temperature
   	d	It demonstrates visco-elastic behaviour

4. Approximately how many monomers are there in between cross-links in rubber?

   	a	5-10
   	b	50-100
   	c	500-1000
   	d	5000-10000

5. What is meant by a change of conformation:

   	a	Stretching of C-C bonds
   	b	Any change of torsion angle between polymer segments
   	c	Chains sliding over each other easily
   	d	Breaking of C-C bonds and therefore failure of the material

6. If a load is suspended from a rubber strip at room temperature, and the temperature is then reduced by 20°C, the load will be:

   	a	Pulled upwards
   	b	Lowered
   	c	Released
   	d	Unaffected

7. If a rubber strip is laid on a bench, unstretched at room temperature, and the temperature is then reduced by 20°C the strip will:

   	a	Contract
   	b	Expand
   	c	Stay The Same
   	d	Decompose

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

8. A bungee jumper, Bill, whose mass is 82.5 kg prepares to do a bungee jump. The bridge from which he will jump is 130 m high. The jumper understands why rubber behaves the way it does under tension because he has read the DoITPoMS TLP "Stiffness of Rubber". He asks the instructor what the dimensions of the bungee cord are and is told it is 60 m long when not stretched and has an initial diameter of 10 cm. Immediately after his successful jump he hangs stationary 20 m from the ground waiting to be pulled up again. While dangling there he estimates the cross-link density of the rubber. What should his answer be?

9. Explain why the bungee jumping cord should be made from butyl rubber (especially if it is to be used in a sunny country). You should explain what might happen if butyl rubber is not used and why this occurs.

10. Explain how the stiffness of a rubber membrane changes when it is put under biaxial tension, paying special attention to the three separate regimes of behaviour and explaining how each one occurs. Why do rubber bands snap when over-stretched whereas Silly Putty flows and necks until it separates into two pieces?

Open-ended questions

The following questions are not provided with answers, but intended to provide food for thought and points for further discussion with other students and teachers.

11. Can rubber be used as a structural material? Give examples of cases where it would be suitable.

12. You should now understand the way rubber behaves in uniaxial tension and the effect of temperature on its behaviour. Using what you have learnt consider how a large block of rubber would deform under uniaxial compression. You should use what you know about the way polymer chains behave in your answer.

13. Look back over the balloon experiment, where the deformation of rubber under biaxial tension was demonstrated. What do you think would happen if the balloon had been heated in the same way the rubber strip was heated in the first experiment? Would the balloon have contracted? What would have been the effect of heating the air inside the balloon?