Structure-property relationships

Chains that are long, straight and closely packed give a denser, stronger, more rigid polymer with a higher softening temperature, while shorter or more branched chains pack less closely.

Branched chains cannot pack closely together, so the polymer is less dense and more flexible, while straight chains pack closely, so the polymer is denser, stronger and more rigid.

Within a layer the covalent bonds are strong, giving a very high melting point, while between layers there are only weak forces, so the layers can slide over one another.

The boron trioxide alters the structure of the silicon dioxide network, raising the melting point, so borosilicate glass softens only at much higher temperatures.

Weak forces between layers allow them to slide, making graphite soft and slippery, even though the covalent bonds within each layer are strong.

The link between the strong forces holding the regular structure together and the high melting and boiling points of ionic compounds.