Pressure sensitive adhesives are unique in that they do not undergo any physical transformation or chemical reaction during their bonding processes. Instead, their properties are entirely controlled by their base polymer and the other components chosen to make up the final adhesive.
As with all adhesives, they must "wet out" their substrates, flowing to allow as much surface molecular contact as possible, to provide the maximum surface area for bonding. To enable this to happen, without the adhesive simply flowing off the substrate, formulators typically ensure that the pressure sensitive adhesive (PSA) is viscoelastic. This is normally taken to mean that the final mixture behaves to some extent like both a liquid and a solid. However, recent research suggests that even within PSA mixtures there is a clear distinction between viscoelastic and elastic solid behaviour - and this in itself is a function of the three dimensional architecture of the polymer.
When an adhesive wets and bonds, interaction with the substrate drives it to spread across the substrate surface.