Polyelectrolyte brushes studied by surface forces measurement

Brush layers of polyelectrolytes, ionized chains of poly(glutamic acid) (PLGA) and poly(lysine) (PLL), prepared by the Langmuir-Blodgett method, were characterized using the surface forces measurements at various pHs, salt concentrations and chain densities. This paper reviews the major results: (1) the effective charge density of the brush layer calculated from the force profiles was much lesser than the density of the ionized groups of the polyelectrolyte brushes, indicating that nearly all the ionized groups were neutralized by the counterions in the brush layer; (2) the thickness of the brush layers agreed with the length of the extended polyelectrolytes and was practically independent of the salt concentrations studied (0.43-10mM). The thickness was proportional to the polymerization degree of polyelectrolytes; (3) the initial elastic compressibility modulus of the brush layer of PLGA or PLL increased with increasing ionization degree, while it decreased with increasing salt concentration because of a decrease in the osmotic pressure of the counterions; (4) stress profiles between the brush layers were scaled for polyelectrolytes of various polymerization degrees according to the contour length of the polyelectrolyte. Similar scaling was also found for stress profiles obtained at various salt concentrations (0.43-10mM) and pHs; (5) the "osmotic pressure of counterion" model reproduced well the steric components of the stress profiles, thus supporting that the steric repulsion was mainly due to the osmotic pressure of the counterions; and (6) a density-dependent jump in the properties of polyelectrolyte brushes such as transfer ratio, compressibility and surface potential has been found, indicating the existence of the density (interchain distance)-dependent transition of polyelectrolytes in solutions. We have proposed a counterion model to account for this transition.