CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.7.7 Effect of Side-Chains on Properties of Polymeric Materials

Effect of Side-Chains on Properties of Polymeric Materials

Polyalkenes

  • Polyalkenes are formed through addition polymerisation of monomers containing C-C double bonds
  • The double bonds in monomers break open and add to each other to form long polymeric chains

Low density polyalkenes

  • Low density polymers are produced at high temperatures and pressure
  • As a result of the reaction conditions, low density polymers tend to have many side chains
    • Groups that branch off the main hydrocarbon backbone of the polymer chain
  • In Low density poly(ethene), the side chains prevent polymer chains from aligning in a neat arrangement and gives rise to weak van der waals forces between the chains
    • As a direct result of the weak van der waals interactions, low density polymers tend to have very low melting and leads to weak polymers
    • The inability for polymer chains to align due to the side chains leads to polymers with low crystallinity
  • Low density polymers are widely applied in areas where soft polymeric materials are needed
    • For example as plastic shopping bags and food coverings

High density polyalkenes

  • High density polymers are produced at lower temperatures and very low pressuresp
  • A metal catalyst is also present in the formation of these polymers
  • The resulting polymers have very few side chains
  • This leads to better alignment with other polymer chains and more opportunities for stronger van der waals forces
  • High density polymers have areas high in crystallinity within their polymeric structures
  • The lack of side chains lead to stronger more resilient polymers with high melting points

PTFE (Teflon)

  • PTFE is poly(tetrafluoroethene)
  • It’s monomer is similar to ethene except all 4 hydrogen atoms are replaced with fluorine atoms
  • When tetrafluoroethene is polymerised, a long carbon chain with fluorine side groups is formed
  • The fluorine atoms arrange themselves in a way to cover the carbon chain from being attacked
  • As all of the side groups are of the same size and chemical nature, the polymer chain arranges itself neatly and packs together well
  • PTFE has a very high melting point and it is resistant to chemical attack
    • This is not particularly due to strong van der waals forces
  • These properties lead to its application non-corrosive coating and non-stick appliances,

Kevlar

  • Kevlar is formed through condensation polymerisation of benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene
  • The Kevlar polymer contains amide links along the length of the polymer
  • When the chains align and arrange themselves, the amide groups promote hydrogen bonding between the functional group on their adjacent length of the chain
  • The oxygen atom of the C=O forms hydrogen bonds with the hydrogen atom of an N-H group

Hydrogen bonds form between Oxygen atoms of the C=O and hydrogen atom of the N-H groups

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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