Polymers
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Polymer
◼ A polymer is a molecule, made from joining
together many small molecules called monomers.
The word "polymer" can be broken down into
"poly" (meaning "many" in Greek) and "mer"
(meaning "unit").
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Characteristics of Polymers
◼ Very resistant to chemicals.
◼ Can be both thermal and electrical insulators.
◼ Very light in weight and very strong.
◼ Have a limitless range of characteristics and colors.
◼ Can be processed in various ways.
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Polymerization
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Polymerization
◼ Polymerization, any process in which relatively small
molecules, called monomers, combine chemically to
produce a very large chainlike or network molecule,
called a polymer.
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Addition Polymerisation
◼ Addition polymerisation involves addition reactions in which a large
number of small molecules (monomers) join together to form very
large molecules (polymers).
◼ In addition polymerisation the monomers have at least one double
bond between carbon atoms. Alkenes are particularly useful
monomers as they contain double bonds and can be made to undergo
addition reactions amongst themselves.
◼ Addition reactions involving alkenes are reactions in which the
carbon-carbon double bond is converted to a single bond and atoms
or group of atoms are added to each of the two carbon atoms.
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Examples
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Condensation Polymerisation
◼ Condensation polymerisation involves a reaction between
monomers with two different functional groups. When these
types of monomers react they join together, usually losing a
small molecule such as water, and so the reactions are called
condensation reactions.
◼ Most natural polymers are condensation polymers, and many
synthetic polymers are made by condensation polymerisation as
substitutes to natural fibres such as wool and silk.
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Example
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◼ Q.1: What are 3 differences of addition and condensation
polymerization.
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Explain the characteristics of thermoplastic and
thermoset by using the following diagrams
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Thermoplastic
◼ They have weak forces between the separate polymer chains.
◼ They soften easily and can be remoulded into new shapes.
◼ They are sometimes called thermosoftening plastics.
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Thermosets
◼ The polymer chains are fixed together by strong bonds which
are called cross-links.
◼ They do not soften.
◼ If heated strongly enough, they eventually break down and
char.
◼ They are hard and rigid.
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Thermoset or thermoplastic?
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Uses of Plastic
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Changing properties of Plastics
◼ Some plastics are made of polymers with straight chains, which
pack together neatly.
◼ Other polymers have large group of molecules sticking out.
◼ Regular packing of polymer chains makes the polymer denser
than those containing side chains.
◼ Low density polymers- polymers with side branches.
◼ High density polymers- polymers with straight chains.
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Low to High Density Polymers
◼ Low density polymers can be converted to high density
polymers using different catalysts and by changing pressure.
◼ For example- low density poly(ethene) can be converted to
high density poly(ethene) by using a special catalyst and low
pressure in the reaction
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Mixing additives with Plastic
◼ In addition to changing monomers and reaction conditions, other
things can be added to modify the properties of plastics.
◼ Examples of additives include:
◼ Plasticisers- these make a plastic easier to mould and more flexible.
◼ Preservatives- these stop the plastic from breaking down in harsh
conditions or when it is being heated.
◼ Antioxidants stop the plastics from reacting with oxygen.
◼ UV stabilizers protect the plastic from sunlight.
◼ Flame Retardants- these make it more difficult to ignite a plastic and
more difficult for flames to spread.
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Vulcanisation
◼ Some substances are also added to make plastics impact
resistant or harder wearing. For example, if sulfur is added to
rubber, it becomes much more durable. This is because the
sulfur forms cross-links between the rubber polymer chains. By
bonding neighboring chains together, the rubber is now suitable
for making tyres. The rubber is said to be vulcanised.
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Degradation of Plastic
Ways to degrade plastic faster have been discovered.
◼ By light: polymer chains can have atoms that absorb light and
this energy will be used to split the molecules.
◼ Bacteria: modified bacteria has been made which can produce
plastic granules which is biodegradable but is expensive.
◼ Water: By changing the reacting mixture, plastics that are
soluble in water can be made.
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Questions Please
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