Amorphous and Polymeric Materials

Amorphous materials have no internal structure. The atoms are arranged almost randomly.

 Imagine a liquid where the particles are moving about freely, not particularly bound to any others. Imagine the liquid is frozen instantly so that the particles remain randomly arranged. This is a good way of thinking of amorphous solids, in fact there are some very viscous liquids which are hard to define as being solid or liquid e.g. treacle.

Some amorphous solids, e.g. some types of glass, "creep". They flow very slowly like a liquid.

Because amorphous solids have no internal structure crystal slip does not take place so there is almost no plastic deformation before fracture occurs. This is why materials like glass and ceramics tend to be brittle.

These materials are also prone to surface cracking. These cracks can propagate very quickly resulting in fracture. If we can prevent this cracking then amorphous materials can be surprisingly strong as in the case of fiberglass and bullet proof glass.

 

Polymeric materials, such as plastics and rubber, are made of long polymer chains.
In rubber these chains are tangled together like spaghetti on a plate. When stressed the chains unwind and elongate. The bonds between the chains are very weak so initially the rubber stretches easily. The bonds between the atoms themselves are very strong so once untangled the chains become very strong and the rubber much stiffer.

We can make rubber stronger and tougher by joining the chains together with sulphur bonds, a process known as vulcanisation.

 
On the graph you can see how the stiffness changes. Notice also how the deformed rubber returns to its original length but not all of the strain energy is recovered. This is due to friction between molecules as they rub together.

Someone thought this graph looked like a uterus so this effect is known as hysteresis.