Polyelectrolyte Brush Layers Studied by Surface Forces Measurement:  Dependence on pH and Salt Concentrations and Scaling

Abstract

Interactions between opposed brush layers of polyelectrolytes ionized poly(L-glutamic acid) (PLGA) or poly(L-lysine) (PLL) in water were directly investigated using 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. The density of polyelectrolyte chains was around 0.4 chain/nm 2 . 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. The surface potential obtained from the double layer repulsion indicated that nearly all of the ionized groups in the brush layers were neutralized by counterions. In the case of PLGA, the steric repulsion of the brush layer increased with increasing pH from 9 to 10, reflecting the increase in the ionization degree of the carboxylic groups, and then decreased above pH 10 due to salt effect. Similar behavior was also observed for PLL: the steric repulsion increased when the pH was changed from 4 to 3 and decreased at pH 2. In the cases of both PLGA and PLL, the steric repulsion decreased with increasing salt concentration (0.43-10 mM) at a fixed pH, which was likely due to a decrease in the osmotic pressure of the counterions. The distance at which the steric repulsion appeared (D0) corresponded to twice the length of an extended polyelectrolyte molecule. This distance D0 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 L0 was defined as D0 -6 (nm), where 6 nm was twice the length of the long alkyl chains in polyelectrolyte amphiphiles, and was 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 equation reproduced well the stress profiles of PLGA and PLL brushes, where the osmotic coefficient was a variable parameter. The osmotic coefficient was estimated to be 0.004 for both PLGA and PLL and much less than the value determined in solution (∼0.2), indicating that the counterions bound more strongly to the polyelectrolytes in the brush layer than to isolated polyelectrolytes in solution. The advantage of characterizing polyelectrolytes by surface forces measurement was discussed.

Structure and Surface Properties of High‐density Polyelectrolyte Brushes at the Interface of Aqueous Solution

“…Similar results have been reported by Matsuoka [23] and Kurihara. [24] Negligible change was also observed between the swelling structure of poly(MPC) facing a pure D 2 O and a NaCl deuterium oxide solution, as shown in Figure 1 (c) and (d). In this case, another mechanism should be explained.…”

Characterization of swollen structure of high-density polyelectrolyte brushes in salt solution by neutron reflectivity

“…Similar results have been reported by Matsuoka [9,10], Kurihara [11], and Tran. [12] Huck et al reported the swelling behavior of the poly(METAC) brush in solution observed by AFM. [13] They suggested that thickness of brush decreased at much lower external salt concentrations than the concentration of chemically charged monomers of the chains.…”

Creation of dense polymer brush layers by the controlled deposition of an amphiphilic responsive comb polymer

“…In fact, the size of features in the SFM images matches exactly with the calculated area/molecule deposited with the LB trough further supporting an average molecular weight of the comb molecules as being around 130 kDa. Some research into creating dense layers via amphiphilic copolymer LB assembly has been performed in the past [26][27][28][29][30][31][32][33][34][35], but we have shown that multi-molecular micellation (creating structures > 100 nm) has been largely prevented through controlling polymer chain architecture. Such films could potentially be used as ultrathin size-selective nanofiltration films for sensor applications, templates for nanolithography, or an environmentally acceptable way to tune the physical and chemical properties of a flat surface.…”