4e. Covalent bonding in polymers and 1, 2 and 3 'dimension' concepts in macromolecules

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes

Currently being re-edited today!


(c) doc bCovalent bonding in polymers and 1, 2 and 3 'dimension' concepts in macromolecules

'1D, 2D and 3D dimension concepts'

A crude order of increasing material strength and increasing thermal stability

  • The bonding in polymers or plastics is no different in principle to the examples described above, but there is quite a range of properties and the difference between simple covalent and giant covalent molecules can get a bit 'blurred'.

    • Bonds between atoms in molecules, e.g. C–C in polymer molecule chains are called intramolecular bonds and very strong.

    • The much weaker electrical attractions between individual molecules are called intermolecular forces.

  • A sort of 'one dimensional' situation in terms of strong covalent bonds

  • In thermosoftening plastics like poly(ethene) the bonding is like ethane except there are lots of carbon atoms linked together to form long chains.

    • They are moderately strong materials but tend to soften on heating and are not usually very soluble in solvents.

    • The structure is basically a linear 1 dimensional strong bonding networks.

    • The polymer molecules are held together by weak intermolecular forces and NOT by the strong chemical bonds holding the carbon chain 'backbone' together.

    • The long polymer molecules mean the intermolecular forces are appreciable but the material is flexible and softens on heating.

  • A two dimensional array of strong covalent bonds

  • Graphite structure is a layered 2 dimensional strong bond network made of 2D layers of joined hexagonal rings of carbon atoms with weak inter–molecular forces between the layers. (more details on graphite)

    • Kevlar is a very strong polymer, which like graphite, consists of 2D arrays of strongly bonded atoms - it is used in bullet proof protective clothing.

    • The layers are held together by intermolecular forces - as in graphite - but the covalent bonding in the layers is very strong.

    • When hit by a bullet, the Kevlar layers deform and absorb the kinetic energy without ('ideally') being penetrated.

  • Three dimensional networks of strong covalent bonds

  • Thermosetting plastic structures like melamine have a 3 dimensional cross–linked giant covalent structure network similar to diamond or silica in principle, but rather more complex and chaotic!

    • Because of the strong 3D covalent bond network they do not dissolve in any solvents and do not soften and melt on heating and are much stronger than thermoplastics.


Some representations of 'dimension'

BUT they all amount to 2D diagrams, so you have to think spatially beyond the simple diagrams

Below are 2D representations of the molecular structure of the polymer materials

(c) doc b

the formation of poly(ethene) doc b oil notes

The repeating unit is shown in brackets e.g. for poly(ethene) it is CH2-CH2

To indicate the repeating nature of the molecule, bond dashes go through the brackets giving the 'shorthand' or 'abbreviated' representation of the structure of poly(ethene)   -(-CH2-CH2-)n-   where n is the very large number, and can represent hundreds or even thousands of repeating units in one polymer molecule, in this case poly(ethene).

The repeating unit shown in brackets for poly(chloroethene) is -(CH2-CHCl)-

a section of poly(chloroethene), PVC

 

Other representations of a poly(ethene) molecule, typically a long chain molecule formed from lots of repeating units joined together by strong carbon-carbon bonds.

The intermolecular forces between polymer molecules are bigger than those between small molecules like water and great enough to ensure plastic polymers like poly(ethene) and PVC are solid at room temperature.

The greater the intermolecular forces the greater the energy needed to overcome them and melt a material.

The higher the temperature, the greater the kinetic energy of particles to overcome the molecular attractive forces.

These intermolecular forces are much weaker than ionic or covalent bonds so the melting points of polymers are still much less than those of giant covalent structures like diamond or silica or ionic compounds like sodium chloride.

Also, giant covalent structures like diamond and silica have very strong 3D covalent bond networks making them much more thermally stable.


What next?

Recommend next: Part 5 Metallic Bonding – structure and properties of metals

 

Sub-index for: Part 4 Giant covalent structures and other big molecules

 

Index for ALL chemical bonding and structure notes

 

Perhaps of interest?

Materials science pages

Nanoscience – Nanotechnology – Nanochemistry (index of pages)

Smart Materials Science (alphabetical index at top of page)

Polymers

Addition polymers - plastics - properties and uses

Comparing thermoplastics, fibres, thermosets

Natural polymer molecules - starch and DNA

 

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