| Research Abstract |
Interactions between opposed brash layers of polyelectrolytes, ionized poly(L-glutamic acid) (PLGA) or poly(L-lysine) (PLL), in water were directly investigated using the surface forces measurement. The brush layers were prepared by the Langmuir-Blodgett deposition of amphiphiles bearing PLGA (degree of polymerization, n = 21,44 and 48) or PLL (n = 41 and 52) as hydrophilic groups. Surface force profiles, consisting of a long-range electrical double layer repulsion and a short-range steric repulsion, were measured and analyzed with varying pH, salt concentration, and polyelectrolyte chain length and chain density.
The surface potential obtained from the double layer repulsion indicated that nearly all of the ionized groups in the brush layers were neutralized by couterions. The steric repulsion of the brush layer increased with increasing the ionization degree of polyelectrolytes by varying pH, while it decreased with increasing salt concentration (0.43 〜 10 mM) at a fixed pH likely due
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to a decrease in the osmotic pressure of the counterions.
The distance at which the steric repulsion appeared (D_0) corresponded to twice the length of an extended polyelectrolyte molecule. This distance D_0 remained practically the same under the pH and salt conditions studied for the brush of an identical polymerization degree. Twice the length of polyelectrolyte chains, L_0 was defined as D_0 - 6 (nm) where 6 nm was twice the length of the long alkyl chains in polyelectrolyte amphiphiles, and found to be proportional to the polymerization degree of polyelectrolyte n. The stress profiles, obtained by differentiating the force profiles, were scaled according to the distance to provide identical profiles for different polymer chain lengths. An equation describing the stress profiles was derived based on a model that attributed the steric force to the osmotic pressure of the counterions.
The density-dependent transition in polyelectrolyte brushes was observed at a chain density 〜 0.2 chain/nm^2, which was interpreted by the change in the counterion binding. Less
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