[en] Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic−polyelectrolytic diblock copolymer, poly(methyl methacrylate)-block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA-b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10−4 w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (±20−30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 ± 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.
Research Center/Unit :
Center for Education and Research on Macromolecules (CERM)
Disciplines :
Materials science & engineering Chemistry
Author, co-author :
Raviv, Uri; Weizmann Institute of Science, RehoVot, Israel > Department of Materials and Interfaces
Giasson, Suzanne; Laval UniVersity, Québec, Canada > Department of Chemical Engineering and CERSIM
Kampf, Nir; Weizmann Institute of Science, RehoVot, Israel > Department of Materials and Interfaces
Gohy, Jean-François; Université de Liège - ULiège > Department of Chemistry > Center for Education and Research on Macromolecules (CERM)
Jérôme, Robert ; Université de Liège - ULiège > Department of Chemistry > Center for Education and Research on Macromolecules (CERM)
Klein, Jacob; Weizmann Institute of Science (RehoVot, Israe)l and Oxford UniVersity (UK) > Department of Materials and Interfaces (Israel) > Theoretical Chemistry Laboratory (UK)
Language :
English
Title :
Normal and frictional forces between surfaces bearing polyelectrolyte brushes
Publication date :
19 August 2008
Journal title :
Langmuir
ISSN :
0743-7463
eISSN :
1520-5827
Publisher :
American Chemical Society, Washington, United States - District of Columbia
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The polyelectrolyte concentration is (3.5 ± 0.3) × 10 -6, and each chain has 80 ions; thus, the total ion concentration is on the order of 10-4. The polyelectrolyte localizes the ions in its vicinity and at this concentration the chains are about 100 nm apart, whereas their size is only a few nanometers and their neutralization length is less then 1 nm (see the Discussion). Thus, counterions associated with different chains do not overlap. In addition, the chains essentially do not adsorb onto the mica surfaces and are not expected to remain in the gap. We therefore conclude that the contribution of the nonadsorbing polyelectrolytes to the screening length in this case is very small. Recently, a quantitative treatment of this case was reported by Tadmor et al. (Macromolecules 2002, 35, 2380). Within the scatter, our data is in agreement with this treatment.
We note that by assuming this additivity we may double count at D < 2L the contribution of the small fraction of counterions that are outside the brush. However, this should have a negligible effect.
The magnitudes of A and B (Figure 5 caption) are, at ca. 10-2, significantly smaller than expected from a scaling treatment as we have here. The reason may be that using N2 in the expression for the elastic constant, k, in our treatment implies ideal freely jointed chains. In reality, the number of equivalent freely jointed segments must be significantly smaller then the number of monomers N 2, and this could correspondingly lead to larger and more reasonable (order unity) A and B values.
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