It has been nearly two decades since Klein's group discovered that polymer brushes are remarkably effective at reducing the friction between sliding surfaces <i>. Nine years later, Klein and coworkers showed that the friction could be reduced a further two orders of magnitude by using polyelectrolyte brushes [ii]. The main source of friction is understood to be the collisions between polymer chains from the two opposing brushes. Thus, the reduced friction suggests that the interpenetration and hence the number of collisions is less between polyelectrolyte brushes than between neutral brushes under equivalent loads (i.e., normal forces), but an explanation for this has been lacking.Here we examine the equilibrium behavior of two opposing polyelectrolyte brushes as a function of their separation, 2d, using self-consistent field theory (SCFT). We find that a significant amount of contraction occurs prior to physical contact, in contrast to neutral brushes which do not contact until they make physical contact. As a consequence, polyelectrolyte brushes are able to acquire a much larger normal force before they start interpenetrating, which explains their improved performance [iii]. There is also interest in whether or not the superb lubrication of polyelectrolyte brushes survives in the presence of salt, because of potential biological applications involving, for example, artificial joints [iv]. Therefore, we conclude by showing the consequences of adding salt to the solution. As expected, salt degrades the performance of the brushes, but not by too much provided that the salt concentration remains below that of the salted-brush regime [v]. Schematic diagram of two opposing polyelectrolyte brushes compressed together to a separation of 2d in a salt solution.Acknowledgements.  This work was supported by the EPSRC (grant no. EP/F068425/1).<i> Klein, J.; Kumacheva, E.; Mahalu, D.; Perahia, D.; Fetters, L.J. Nature 1994,  370, 634.[ii] Raviv, U.; Glasson, S.; Kamf, N.; Gohy, J.-F.; Jerome, R.; Klein, J. Nature 2003,  425, 163.[iii] Matsen, M.W. Eur. Phys. J. E 2011,  34, 45.[iv] Moro, T.; Takatori, Y.; Ishihara, K.; Konno, T.; Takigawa, Y.; Matsushita, T.; Chung, U.-I.; Nakanura, K.; Kawaguchi, H. Nat. Mater. 2004,  3, 829.[v] Matsen, M.W. Eur. Phys. J. E 2012,  35, 13.

Explanation for the low frictional force between polyelectrolyte brushes

244th National Meeting (2012)

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