4e. Covalent bonding in all sorts of 'polymers' and a sort of 1, 2 and 3 'dimension' approach to the structure of macromolecules - big molecules!

This page is not written to suit any exam board specification - its just a page of my own thoughts!

This is my own personal approach to try to correlate in my head the different sort of structures you find when dealing with large molecules that have some kind of larger or polymeric structure.

The phrases '1, 2 or 3 dimension' refer to the main chain of strongly covalent bonded atoms - usually carbon).

However, other atoms or groups of atoms may be attached to the main 1D, 2D or 3D bond networks.

e.g. poly(propene) is essentially linear chains of carbon atoms with methyl groups attached to alternate carbon atoms.

On this page I'm not interested in the use of these materials, and only a brief mention of their properties, BUT lots about their relative spatial structure!

A sort of 'one dimensional' view of the strong bonds in polymer chains

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 holding a chain of atoms together.

In thermosoftening plastics like poly(ethene) and PVC, the bonding is like ethane, propane etc. 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 (though there can be some branching with a side-chain group, but very little cross-linking).

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.

The above diagram shows the formation of poly(ethene).

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 - these give a sort of 'one dimensional' of this kind of polymer.

So this is a sort of essentially one dimensional network of bonds in terms of the polymer chain, BUT, the strength of the intermolecular forces should not be underestimated.

For more on these kinds of thermosoftening polymers:

Addition polymers - plastics e.g. poly(ethene), poly(propene) and PVC etc.

A sort of 'two dimensional' view of strong covalent bond networks in large molecules

graphite  AND    graphene

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

Graphene is essentially a single layer of graphite and also consist of a giant covalent network of hexagonal rings of carbon atoms.

Kevlar is a very strong polymer, which like graphite, consists of 2D arrays of strongly bonded molecules, BUT is it a true 2D arrangement?

A layer of Kevlar

The polymer chains are NOT cross-linked or joined together like the atoms in a layer of graphite or a sheet of graphene. BUT the chains are bound together by strong intermolecular bonds (a special type called a hydrogen bond).

The layers are also held together by intermolecular forces - as in graphite

  Kevlar sheets

Kevlar is used in bullet proof protective clothing.

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

(More on Kevlar)

A sort of 'three dimensional' view of the strong network of bonds in giant covalent structures

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

silicon or diamond  AND     Silicon dioxide SiO₂

Diamond (allotrope of carbon, C_n) is a 3D giant covalent structure (n is a VERY large number).

Elemental silicon, Si_n, is a 3D giant covalent structure.

Silicon dioxide (SiO₂)_n, is a 3D giant covalent structure.

 (more details on diamond)  and (more details on silicon and silicon dioxide)

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.

How do fullerenes and nanotubes fit into a 1D to 3D view of large molecules?

and when is a carbon based molecule considered a 'nanoparticle'?

One of the simplest 'buckyballs'  C60 (diameter 0.71 nm)	A longer buckminsterfullerene which is 'rugby ball' or 'sausage' shaped, C72 etc. (length ~1.4 nm)
A short section of a carbon nanotube e.g. 6 nm diameter x 100 nm in total length, the ends would be like those of the 'sausage' fullerene above right.

The two main guiding principles of 'nanoparticles' is

(i) Their size is usually between 1 and 100 nm (the diameter of a single carbon atom is ~ 0.140 nm).

(ii) Nanoparticles of material behave in a physical or chemical manner that is significantly different from the bulk material.

Some representations of 'dimension'

BUT they all amount to 2D diagrams in the plane of the screen, so you have to think spatially beyond the simple diagrams!

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

In terms of the covalent bonding networks of carbon atoms of the polymer chain

we can think of the structures simplistically and dimensionally as 1D, 1D, 3D, 2D, 3D