Thermodynamic vs Kinetic Control of Brush Composition in Grafting to Reactions of Disperse Polymer Systems in Melt

Grafting to reactions are one of the most investigated approaches to producing polymer brushes due to their unique capability to control both the brush thickness and grafting density. Despite their widespread use, some aspects of the grafting to reactions remain not fully understood. Among these, particularly relevant is the preferential grafting of the shortest chains in dispersed polymer systems. This phenomenon is investigated in the present paper by grafting on silicon oxide substrates polystyrene blends containing equimolar amounts of two telechelic polystyrenes with different molecular weights to model disperse polymer systems. Furthermore, for each blend, different end groups, namely, hydroxy or phosphate end groups, are considered. For hydroxy-terminated polystyrene blends, a preferential grafting of the shortest component is observed and derives from the lower entropy loss for short chains when the reactive end group approaches the surface, but the brush composition, in terms of the ratio between short and long chains, is independent of grafting time and temperature. In contrast, at a short time, the grafting of phosphate-terminated polystyrene blends takes place with an increase of short chains definitely higher than the one observed for the hydroxy-terminated blends. In turn, after a sufficiently long time, the brush composition converges to the same final composition of the hydroxy-terminated polystyrenes. This result indicates that at short times the control of the brush composition is dictated by a fast absorption of the short chains on the surface substrate due to the strong polarity of the phosphate group. This adsorption takes place before the grafting to process and results in an observed excess of grafted short chains. In turn, as grafting proceeds, the brush composition evolves toward the thermodynamic composition, which is therefore equal to the one of the hydroxy-terminated polymer blends.