Prof. Mark Geoghagen "Polyelectrolyte brushes: Friction, adhesion, and lubrication"

Polyelectrolytes are known for their responsive nature; they change shape with pH or salt, and sometimes with temperature. In aqueous solution they might change from hydrophilic to hydrophobic, but when chemically tethered by one end (brushes) to a substrate, control of their behavior becomes subtler. One aspect of this, is that counterions can be trapped within the brush (osmotic behavior) or above it (double-layer), depending on parameters such as the grafting density of brushes on the surface. A rich phase diagram arises with chemically grafted brushes, and much work has been applied to understanding their behavior. Hydrophilic brushes are well solvated, and are thus lubricious, whereas brushes in their hydrophobic state will exhibit frictional behavior. Also, the charged nature of brushes, coupled with their capacity to hydrogen bond, is important in their adhesive behavior.

In this presentation, the frictional and adhesive nature of polyelectrolyte brushes will be discussed. Frictional behavior will be discussed in the context of the pH and salt-dependent interaction of polyelectrolytes and polyzwitterions with a single-asperity point, i.e. the tip of an atomic force microscope, which may be plain (i.e. silicon nitride), gold-coated, or coated with a hydrophobic or hydrophilic self-assembled monolayer. The adhesion between a polyelectrolyte brush and an oppositely charged hydrogel is also discussed. The pH-dependent nature of the adhesion gives rise to switchable behavior.

 Friction force microscopy (FFM) studies on the frictional properties of poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA) polybase brushes were made as a function of pH. Here, we observe a transition between different contact mechanics; we observe that the contact mechanics is a strong function of pH, with regions whereby both single asperity contact mechanics behavior (both Johnson-Kendall-Roberts (JKR) and Derjaguin-Müller-Toporov (DMT) models) and multiple contact mechanics (Amontons’ law) observed, depending on the pH [1]. Generally, Amontons’ law fitted the data best in the extreme pH regions (1, 2, and 12). Between these values at high and low pH, DMT behavior was appropriate, whereas the observation of JKR behavior depended on the surface in contact with the brush, but generally at relatively neutral pH values.

Brushes of the polyzwitterion, poly[2-(methacryloyloxy)ethylphosphorylcholine] (PMPC) was also studied. PMPC is known to exhibit co-nonsolvency in water and ethanol mixtures, and this is highlighted by an increase in the friction coefficient due to loss of hydration of the brush chains and hence substantially reduced lubrication [2]. It was also found that the friction coefficient of PMPC brushes decreases by the addition of salt in surrounding medium. It is believed that cations or anions could associate with the phosphorylcholine group to different degrees, hinder the inter- and intramolecular interactions, which increase the lubricity of PMPC brushes. The salt-dependent behavior of the PMPC brushes is in contrast to neutron reflectometry results, which hint at an independence of brush conformation with increased added salt [3].

The interaction between (polybase) PDMAEMA brushes and a polyacid presents an intriguing pH-dependent adhesion problem. Oppositely charged polyelectrolytes present an intuitively appealing means of creating a switchable adhesive because pH-induced conformational transitions mean that when one polymer is extended due to osmotic pressure in its counter-ions, another is more collapsed. Changing pH can therefore changes the nature of the interactions between the two polymers and therefore provides a means of controlling adhesion. Early experiments show that the adhesion between a grafted layer of polybase and a polyacid hydrogel adhere very strongly in aqueous environment, with a thermodynamic work of adhesion of > 0.4 N/m, but detach at pH ~ 1 [4,5]. Experiments are described to reveal the dominant factors in this adhesion. Mechanical testing was performed by comparing the adhesion between polyelectrolytes and neutral polymers in order to determine whether hydrogen bonding or electrostatic attraction dominated the adhesion process. These experiments suggest that electrostatic interactions dominate. We also note that using more rigid (more crosslinked) hydrogels, there is a decrease in adhesion, indicating that real-world adhesion (total work of adhesion) has a strong dependence on viscoelastic dissipation within the hydrogel.

[1] M. Raftari, Z. Zhang, S. R. Carter, G. J. Leggett, and M. Geoghegan Soft Matter, 10, 2759-66 (2014).
[2] Z. Zhang, A. J. Morse, S. P. Armes, A. L. Lewis, M. Geoghegan, and G. J. Leggett, Langmuir, 27, 2514-21 (2011).
[3] M. Kobayashi, K. Mitamura, M. Terada, N. L. Yamada, and A. Takahara, J. Phys.: Conf. Ser., 272, 012019 (2011).
[4] R. La Spina, M. R. Tomlinson, L. Ruiz-Pérez, A. Chiche, S. Langridge, and M. Geoghegan Angew. Chem. Int. Ed., 46, 6460-3 (2007).
[5] R. La Spina, A. Chiche, M. R. Tomlinson, and M. Geoghegan, Eur. Coat. J., 22-8 (2011).