Thermodynamics of Densely Grafted Strong Polyel ctrolyte Brushes in the Swollen State

摘要

The possibility of controlling the physical and chemical properties of surfaces by means of neutral and charged polymer brushes has attracted great attention in both, industry and academia in the past decades. In most applications, a solvent is present and if the solvent quality is high, the chains stretch in order to increase the polymersolvent interactions and thus reduce the enthalpy of the system. For strong polyelectrolytes, where the number of charges is fixed this effect is even larger because of the electrostatic repulsion between chain segments. This stretching causes a very high entropy loss in the system which can even provoke scission of carbon-carbon bonds as shown by Sheiko et al. in order to reduce the overall free energy of the system. The swelling properties of weak polyelectrolytes have been largely studied theoretical and experimentally45 but until now there is no evidence to our knowledge of bond breaking upon exposure of covalently-attached strong polyelectrolyte brushes to good solvents. In an attempt to study the thermodynamic stability of highly dense strong polyelectrolyte brushes on the swollen state, we synthesize Poly(4- methyl-vinylpyridine) (PMeVP) by the "grafting from" technique' and expose it to water -which is known to be a good solvent for these polyelectrolytes- for different time frames. Tirrel et al.' have also reported on strong polyelectrolyte brushes in contact with good solvents, but the surfaceattachment technique was different from our approach. Poly(N,N, dimethylacrylamide) (PDMAA) is used as a neutral water-soluble brush for comparison and to investigate the effect of solvent temperature on the stability of neutral brushes.