Degrafting reaction of polymer brushes: the determining role of molecular weight and grafting density

Abstract

The chemical stability of polymer brushes is mandatory to ensure their applicability in a wide range of environments. However, it is well-known that, in presence of water, polymer chains detach from the substrate due to the hydrolytic cleavage of the anchoring bonds at the polymer-substrate interface. Furthermore, this degrafting reaction is promoted by the tension generated on the labile anchoring bonds by the swelling of the brush. It is now widely demonstrated that the brush tension increases, speeding up the degrafting reaction, as the number of chains per unit of area (grafting density) increases. On the contrary, the role of the molecular weight of the grafted polymer on the degrafting reaction is much less investigated. In this work, polymer brushes made up of polystyrenes with different molecular weights were obtained by the grafting to approach. Chain degrafting was then induced by a THF/water mixture and the progress of the degrafting reaction was followed by thickness measurements on the dry brushes. The reaction rate constant of the degrafting process was calculated as a function of both the grafting density and the molecular weight of the polymer, confirming that both parameters have a significant role. Furthermore, the degrafting reaction was also evaluated in brushes containing long and short chains in different percentages. A reciprocal influence was observed between the two components, with the overall degrafting process being precisely dictated by the majority one.