Chemistry and the Consumer

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GEK2500Living with Chemistry
Chemistry and the Consumer (actually
Chemistry and the Economy, toosince polymers are
a major component of industrial
manufacturing) Polymers
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Look around us and identify some common plastic
materials...
Shoes, watch straps, clothes, seats, pens, bags,
handphones, spectacles, etc., etc.
Plastic refers to the end use of polymers
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• Plastics - desirable properties

Mouldable
Flexible
Water-proof
Light weight
Cheap
Transparent/semi-transparent
Colourful
Unbreakable
Non-sticking
Elastic/stretchable
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Looking at the composition of matter ..

• All matter is made up with smaller units of a
  component
• Water molecule (H2O)

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The composition of matter

• Table salt sodium chloride (NaCl) crystals

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What is the composition of a glass of salt water ?
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What is the compositionof a polymer?

• Polymer is a Macromolecule
• Large molecule consisting of repeating smaller
  units (called monomer).
• e.g.
• (polymany merosunit)

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Polymers

• Natural or synthetic
• Synthetic polymers - plastics, textiles and
  rubber
• have totally changed modern life - we live in a
  plastics world (most plastics come from
  hydrocarbons)
• Hermann Staudinger (German, 1881-1965) Father of
  Polymers, awarded Nobel Prize in 1953

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Natural polymers

• e.g. DNA, natural rubber, diamonds, starch,
  cellulose
• Natural rubber
• rubber latex produced by 500 species of plants
• very sticky, difficult to handle
• no practical use for 400 years until Goodyear
  created vulcanization process in 1839

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Synthetic polymers

• e.g. polyethylene (PE)
• Repeating unit is ethylene -CH2CH2-
• Shorthand notation
• -CH2CH2n-
• n the number of repeat units

http//chemincontext.eppg.com/chapter9/cic_interfa
ce9.swf
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Synthetic polymer - PE

• A sample of PE contains many individual polymer
  chains of different lengths
• like a bowl of noodles
• Talk about average molecule weight or average
  number of repeat units
• Typical PE molecule
• molecular weight 106 , n 35700

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Different average molecular weights (or average
n) for different applications

• e.g. wax polish 2000 atomic mass units
• the repeat unit -CH2CH2- is 28 amu
• n 2000 ? 28 71 units
• e.g. toys 134400 amu
• n 134400 ? 28 4800 units

C12 H1 O16 N14
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Consider this!

• Leisure/recreational activities and plastics
• Impact of plastics
• Think of the plastics used in the making of a
  personal computer

http//pslc.ws/macrog/maindir.htm
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Experiment with this !

• Making Casein
• Boil low-fat milk with vinegar
• After curd forms, boil for 10 min
• Separate the solid polymer and wash it repeatedly
  until wash water is clear
• This polymer is elastic
• Real world applications coatings and adhesives
  also used in buttons (but not anymore)

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Classification of synthetic polymers

• Classification according to
• Physical structure
• Thermal behaviour
• Use
• Method of preparation

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Physical structure

• the arrangement or morphology of the polymer
  chains
• 3 types linear, branched and crosslinked
• Linear resembles cooked spaghetti

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Physical structure

• Branched - similar but with branches coming out
  of main chains
• Crosslinked - chains are interconnected into a
  network

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Physical structure

• affects various properties
• e.g. linear, branched polymers generally soluble
  in solvents and can flow under pressure and
  heat
• crosslinked polymers generally insoluble and do
  not flow.

Why ?
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Thermal behaviour

• Most important property the behaviour of
  plastics when heated
• characterized by Tg - glass transition
  temperature
• temperature at which polymer becomes soft and
  flexible and not rigid
• 2 types thermoplastic and thermosetting polymer

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Most matter melts or sublimes when heated, but
polymers can be softened when heated
Why ?
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Thermoplastic

• branched or linear molecules
• softens each time it is heated above Tg
• thus, can be remoulded repeatedly
• 80 of industrial plastics are thermoplastic
• polyethylene (PE)
• polyvinyl chloride (PVC)
• polyethylene terephthalate (PET)
• nylons

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Thermosetting polymer

• crosslinked molecules
• crosslinks form when heated and then set (harden)
• do not soften, cannot be remoulded with heat, but
  reshaped using mechanical means
• Thermosetting plastics include
• phenolic resins (phenolformaldehyde)
• urea-formaldehyde
• melamine-formaldehyde

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Uses

• 3 terms refer to the uses of polymers
• Plastics
• Fibres (textiles)
• Elastomers (rubber)
• (Classifications that non-chemists (generally
  consumers) use)

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Plastics

• Plastic means capable of being moulded
• either thermosetting or thermoplastic polymers
• moulded during manufacturing process
• commonly amorphous, i.e. no fixed atomic lattice
  structure

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Crystallinity of polymers

• Amorphous, slightly crystalline or highly
  crystalline

Arranged neatly, tightly
In a mess, loosely arranged
Visualise Uncooked spaghetti totally boiled
spaghetti partially boiled spaghetti
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Fibres or textiles

• Thermoplastics with individual linear polymer
  chains held together
• more crystalline
• easily drawn into thin filaments
• More synthetic fibres are produced than natural
  fibres (plant- or cellulose-based)
• e.g. Nylon, polyesters

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Elastomers

• stretches easily and returns readily to its
  original shape
• natural rubber is still the single most produced
  elastomer
• synthetic rubbers, e.g. styrene-butadiene rubber
  (SBR), butadiene rubber (BR)
• the monomers usually contain more than one CC
  double bond

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Vulcanization

• Crosslinking increases strength and elasticity

heat
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• Charles Goodyear
• inventor of vulcanized rubber accidentally
• later copied by Thomas Hancock (UK)
• Hancocks friend coined the term vulcanization
  after Vulcan, the Roman God of Fire
• Goodyear died penniless in fact 200,000 in
  debt. Yet..

"Life," he wrote, "should not be estimated
exclusively by the standard of dollars and cents.
I am not disposed to complain that I have planted
and others have gathered the fruits. A man has
cause for regret only when he sows and no one
reaps."
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Method of preparation

• 2 types of preparation methods
• addition polymers
• condensation polymers
• (rearrangement polymers)?
• differences
• condensation produces a byproduct, e.g. water
• condensation polymers often have non-C atoms in
  backbone chains

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Addition polymers

• formed by adding many monomer units together
  through chemical bonds
• e.g. polyethylene
• n CH2CH2 ? -CH2- CH2-n
• ethylene monomer polyethylene
• Other addition polymers polypropylene,
  poly(methyl methacrylate), polystyrene,
  poly(vinyl chloride)

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Condensation polymers

• two molecules combine with the formation and loss
  of another smaller molecule
• e.g. poly(ethylene terephthalate) (PET)

Terephthalic acid
Ethylene glycol
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Desirable properties
details
Everyday products
PE
definition
Natural
Synthetic
Plastics
Vulcanisation
Physical
Classification
Mol wt
Linear
Use
Preparation
Thermal behaviour.
Branched
Addition
Thermoset
Condensation
Plastics
Crosslinked
Thermoplastic
Fibres
Elastomers