A-level Applied Science/Choosing and Using Materials/Microscopic structure
We need to consider crystalline, polymeric and amorphous materials;
- Crystalline materials
- may be ionic, metallic or giant covalent.
- The atoms are packed in a regular pattern. This gives great strength, but in the case of ionic and covalent crystals, and molecules held by dipole-dipole or hydrogen bonds, the crystal structure fractures if placed under strain.
- Polymeric materials
- are either long covalent molecules, held together by intermolecular bonds; or a giant molecule. The latter are the thermoset polymers which behave like crystalline materials.
- Thermoplastic polymers become ductile when heated and can be moulded into shape. Cold-drawing the polymer (imposing plastic strain while the material is at room temperature) will line-up the polymers chains. Cold-drawn polymer is very strong in the direction it was strained, because its strength comes from the covalent bonds of it molecules, not the intermolecular bonds.
- Amorphous materials
- include glass and polymers. Amorphous polymers are discussed above, but note that if the intermolecular bonds are strong and directional (e.g. Perspex, polystyrene) then the polymers becomes brittle. Glass is also brittle, and for the same reason – straining the material breaks the bonds that hold it together. In the case of glass these are covalent bonds.
The structure of composite materials.
Composite materials combine the properties of two or more simple materials. For example, glass fibre combines a tough flexible resin with strong fibres of glass.
The interaction between the two materials is also important. Cracks may be stopped from opening because they form in one material but cannot pass through the other one.