Expansion and contraction of Materials

[derekmohammed]

Hi,

I was wondering why some plastics seem to shrink when heat is applied? Does the plastic expand when it is colder? Is it some how due to the molecular bonds in the material rather then the ionic bonds in something like a metal that expands when heated?
Thanks
Derek Mohammed

 

[Gokul43201]

Yes, if a plastic shrinks upon heating, it will usually expand when cooled within that same range. This phenomenon is known as negative thermal expansion (NTE), and is seen in other materials besides plastics/polymers. Many ceramic oxides (particularly molybdates and tungstates), phosphates, cyanides, and even graphite exhibit NTE.

The cause of the phenomenon is different for different materials, but is almost always related to the geometry of the molecules (crystal structure or polymer arrangement). In oxides and cyanides, the contraction upon heating is often attributed to the rotational energy of "bridging" atoms/groups.

   |           |
 - M --- O --- M -
   |           |
   |           |
 - M           M -    COLD, Bridging 'O' atoms are nearly stationary
   |           |
   |           |
 - M --- O --- M -
   |           |


  |     |
- M     M -
  | \ / |
  |  O  |
- M     M -    HOT, Bridging 'O' atoms are rotating rapidly about the M-M axis 
  |  O  |
  | / \ |
- M     M -
  |     |

The above illustration shows a simplified mechanism for one such type of negative expansion. What's happening here is that increasing the temperature makes the bridging oxygen atoms to want to oscillate more. The only way they can do this is by pulling their neighboring metal atoms closer to each other. This causes a contraction along the M-O-M direction.

Most NTE materials exhibit anisotropic expansion, as in the illustration above. What this means is that the extent of (negative) expansion is dependent on the direction. However, in a polycrystalline material, you have little grains of crystal each pointing along a different direction. Due to this randomness, the overall expansion of the material ends up being isotropic. However, if you have a single-crystal, you will see uniaxial NTE in most of these materials.

 

[brewnog]

However, if you're thinking about things like shrink wrapping processes, and heat-shrink tube, these plastics do not expand back to their original size when cooled. Off the top of my head, these plastics are only partially polymerised, becoming fully polymerised when heat is applied (and shrinking), and this is not reversible.

 

[Gokul43201]

However, if you're thinking about things like shrink wrapping processes, and heat-shrink tube, these plastics do not expand back to their original size when cooled. Off the top of my head, these plastics are only partially polymerised, becoming fully polymerised when heat is applied (and shrinking), and this is not reversible.

Oh, yes, I completely forgot about this class of plastics. Heat-shrink tubing would hardly be useful if it expanded on allowing to cool.

 

[derekmohammed]

Actually this is what I was talking about in my organal post (my fault should have stated that) But what is happening at the atomic level that causes the molecules not to expand again?

 

[Gokul43201]

But what is happening at the atomic level that causes the molecules not to expand again?

The short (not implying that it's incomlpete) answer is : bond formation.

If that makes sense to you, great ! Else, I (or someone else) could elaborate when I (they) find a little more time.

 

[derekmohammed]

Is it that the high electromegativity of the central atom holds it in place but it needs energy to "push" the terminal atoms "in".

(This is a dumbed down version if even the corect version. (only taken physical chem))

 

[Gokul43201]

Actually, the specific reason can vary from one material to another.

But the general idea is that the unpolymerized material is in a local energy valley. Heating it, allows it to jump across the energy barrier to the polymerized state. In the polymerized state, there is more bonding between the atoms, resulting in their wanting to be closer to each other.

(see attached picture)