Chemistry of charging an insulator material

Earlier this year, I had learnt in Physics class that rubbing two different materials against each other can charge the materials (i.e. cause each material to either be positively-charged or negatively-charged). I had learnt that the charging process is an electron transfer phenomenon. The charging of conductors by rubbing (i.e. friction) is easily understood because conductors allow charge carriers to flow through them easily. Such as in the case of metals; adding more electrons to the sea of delocalised electrons is not difficult. 


However, what happens during the charging of an insulator material, such as a polymer like poly(ethene), what happens to electrons which are transferred to these materials? Do they go into particular orbitals? 


One thing I am sure of is that the electrons are surely localised to the region of the insulator they are transferred to. Thus, they are definitely not free moving. 


