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9.5 Option - Polymers: 1. Natural polymers have been cultivated by many cultures
| Syllabus reference (2002 version) | ||
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| 1. A range of natural polymers has been cultivated by many cultures and used for many purposes including clothing and ropes | Students learn to:
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Students:
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Extract from Senior Science Stage 6
Syllabus (Amended October 2002) © Board of Studies, NSW.
[Edit: 12 May 11]
Prior learning: Science Stages 4–5
syllabus: 4.7.6b, 5.7.3b.
Background information: Polymers
are large molecules. Some are natural and have been used for many thousands
of years. Others have only been fabricated recently. They have many uses due
to their varied properties. Natural polymers include material from plants (cotton),
animals (wool) and insects (silk).
perform
a first-hand investigation
to model
a polymer molecule with its component monomers
- Decide what polymer you are going to model, after investigating the structure of different simple polymers and the monomers that make up the polymer.
- Choose appropriate material to model the polymers and monomers. To do this find out what material the science department has or discuss with your teacher what you could bring from home. Make sure you have material to form bonds to link the model atoms together when forming the different molecules.
- Write a key to state what each object represents.
- Perform the investigation by building the model of one monomer, then the model of the polymer that the monomers combine to form. When building your polymer you may decide to use a simplified form of the monomer, rather than have the detail that you had in the model of the monomer.
- Check with other groups to see how they built their models. Discuss the advantages and disadvantages of the different forms.
- Click here for a possible answer.
describe a polymer as a chemical substance that is a large molecule that
consists of identical repeating chemical units
- The word “polymer” is derived from the Greek word. ‘poly”, meaning “many”.
- Polymers are very large molecules consisting of thousands of simple repeating units called monomers. The monomers are chemically bonded to form the polymer.
- The type of monomer will affect the properties of the polymer.
relate the
term ‘polymerisation’ to the process of joining monomers to form
polymers
- Polymerisation is the chemical reaction that produces a polymer from its monomers.
- The monomers are held together by covalent bonds, that is, by sharing electrons.
A web site that may be useful is
Scientific Principles 
Materials Science and Technology Teachers Workshop,
Department of Materials Science and Engineering, University of Illinois Urbana-Champaign, USA
identify some natural examples of polymers including
- hair and fur
- wools
- silk
- cotton
- A fibre is a long thread-like structure. Animals, insects and plants can make natural polymers in the form of fibres. Animal fibres include hair, fur and wools.
- Animal fibres are complex proteins (mainly keratin), the monomers being amino acids.
- Silk is made by some insects and spiders. The silk industry uses silk made by mulberry silk moth. (also known as the “silkworm”).
- Cotton is a vegetable fibre. Cotton is mainly composed of the polymer cellulose – the monomer being glucose (a sugar).
compare various natural polymers for
- thermal properties,
- strength,
- elasticity,
- moisture absorbency,
- effects of acids/alkalis,
- resistance to biological attack,
- affinity for dyestuffs,
- shrinkage
- flammability
- Natural polymers have many useful properties.
- We need to know the properties of a specific polymer to decide how best to use it, eg in the manufacture of fibres and fabrics.
| Property |
What
it means |
| thermal
properties |
Does
it insulate against heat or cold? |
| strength |
How
strong is it? Will it tear in daily use. |
| elasticity |
How
well does it stretch and then return to its original shape? |
| moisture
absorbency |
How
well does it soak up moisture? |
| effects
of acids/alkalis |
What
happens when it is in contact with corrosive chemicals? |
| resistance
to biological attack |
Is
it eaten by insects or rotted by fungus and moulds? |
| affinity
for dyestuffs |
Can
it be easily dyed? Does the dye easily wash out or fade away? |
| shrinkage |
Does
the fabric shrink (become smaller) when washed? |
| flammability |
Does
it burn when heated? |
| Property | Material rating | ||
| wool | silk | cotton | |
| thermal
properties |
excellent |
good |
good |
| strength |
fair |
excellent |
good |
| elasticity |
good |
good |
fair |
| moisture
absorbency |
good |
good |
good |
| effects
of acids/alkalis |
good/acids
fair/alkalis |
poor |
fair |
| resistance
to biological attack |
fair |
good |
fair |
| affinity
for dyestuffs |
good |
good |
excellent |
| shrinkage |
average |
good
if prewashed |
good |
| flammability |
low |
low |
high |
describe
a range of properties of natural polymers and relate these to their uses in
society
- Natural fibres have varied properties and we choose specific fibres to make fabrics for certain applications based on these characteristics.
- Natural polymers vary in their properties.
- No one polymer is suitable for all uses.
- Wool is a good insulator, doesn’t absorb much moisture, has elasticity and can be dyed easily.
- Clothing made out of wool keeps the wearer warm and relatively dry.
- Silk is a very attractive, strong but expensive fibre.
- Silk is used for clothing when price is less important than appearance. Ties and scarves are examples.
- Cotton is a good insulator, is easily dyed and absorbs moisture well.
- Cotton clothing absorbs perspiration well. Towels are usually made from cotton.
- The disadvantages of fibres have to be borne in mind.
- Wool is attractive to some insects as a food supply.
- Cotton shrinks when washed. It is often pre-shrunk before use.
- Even relatively weak acids and alkalis affect silk.
plan,
choose appropriate equipment or resources
for, and perform a first-hand investigation
to compare natural polymers across a range of properties selected from
- thermal properties,
- strength,
- elasticity,
- moisture absorbency,
- effects of acids/alkalis,
- resistance to biological attack,
- affinity for dyestuffs,
- shrinkage,
- flammability
- To plan this investigation you will need to decide which properties you are going to test. Consider which ones you will be able to obtain the equipment to test. Your school should have physics material to test strength and elasticity as well as chemicals to test effects of acids and alkalis. When doing this test make sure you use dilute acids as clothing wouldn’t normally be exposed to concentrated acids. You could use weak acids like acetic acid and citric acid. If you decide to test flammability make sure that you have adequate safety precautions.
- Once you have decided what you are going to test, choose appropriate equipment in consultation with your teacher.
- Perform the investigation using a table to record your results. If necessary repeat the investigation. If one test doesn’t have clear cut results and they are difficult to determine, you may have to rethink some of your procedure.
- One example of an investigation is listed below.
| Background |
Thermal properties
- Materials that are good insulators will be chosen for clothing designed to keep people warm.
- Good insulators will keep a hot substance warmer for longer than a poor insulator.
- Set up a number of identical containers with a lid and thermometer.
- Leave one container uninsulated.
Wrap other containers with equal quantities of material being tested.
- Add equal amounts of hot water at the same temperature to each container.
- Measure the temperature in each container at regular intervals.
- Graph results. Time is the independent variable (X-axis) and temperature is the dependent variable (Y-axis).
- Use graphs to compare insulating properties of various materials.
Strength and elasticity of natural fibres
Strength of a fibre
- In some cases, especially thread, twine and rope, the strength of a fibre is important.
- Tie a measured length of thread to a fixed point.
- Attach a mass hanger to the end of the thread.
- Add masses to mass hanger until thread snaps.
- Note mass added.
- Repeat the experiment using threads of other fibres. (These must be of the same diameter for a fair test.)
- This experiment could also be repeated using wet fibres.
Elasticity of a fibre
- Elasticity is the property by which a fibre tends to go back to its original length after being stretched.
- Garments that are made of elastic fibres tend to keep their shape and be more comfortable to wear.
- Tie a length of thread to a fixed point
- Attach a mass hanger to the end of the thread and measure length of thread.
- Add a mass to mass hanger and measure new length
- Remove mass. Measure thread and check that length has returned to original size.
- Add two masses to mass hanger and measure new length
- Remove masses. Measure thread and check that length has returned to original size.
- Continue process, adding extra masses until thread no longer returns to original length.
- Repeat the experiment using threads of other fibres. (These must be of the same length and diameter for a fair test.)
- The thread which will return to its original length
with the greatest mass added has the greatest elasticity.
- Plot a graph of extension (increase in length when mass is added) against mass added
- Mass is the independent variable and is plotted on the X-axis.
- Extension is the dependent variable and is plotted on the Y-axis.
Moisture absorbency of materials
- Sometimes absorbency of moisture is a desirable feature of a fabric, eg towels.
- At other times it could be unwanted eg clothing meant to keep out rain.
- Suspend a piece of cloth with end dipped in water.
- Remove cloth after approximately 15 minutes.
- Measure distance water has moved up cloth.
- Repeat experiment with similar sized pieces of different cloths
- The more absorbent the cloth, the further the water will rise in a set time.
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