Switchable adhesion between oppositely charged polyelectrolytes.

Adhesion is a well-studied phenomenon, mainly for its industrial importance. We
consider a smart water-based adhesive that is switchable, i.e. the adhesion may be
turned on and off by an environmental trigger, in this case the pH.
The interaction investigated is between a weak polyacid hydrogel of poly(methacrylic
acid) (PMAA) and poly [2-( dimethyl amino )ethyl methacrylate] (PDMAEMA, a weak
polybase) chemically grafted to planar silicon substrates (brushes) by atom transfer
radical polymerisation.
The interaction between PDMAEMA and PMAA is of great interest because it
represents a situation where a surface adhesive (a polybase in contact with a polyacid)
can be turned on and off simply by changing the external environment. In particular
we observe that at pH less than 2, there is no significant interaction between the brush
and hydrogel, whereas above pH 3, there is strong adhesion comparable to epoxy
glue. The interaction between the brush and the gel is pressure sensitive so that the
adhesion energy is a function of the applied load.
To understand the mechanism involved in the pressure sensitive behaviour we
performed neutron reflectivity experiments of the brush in contact with the hydrogel
after known pressures were applied. Comparison of the conformations of brushes of
different thicknesses but with the same applied pressure shows that the interaction
between the brush and hydrogel takes place at the interface and is mainly due to
electrostatic interactions between the carboxylic group of the hydrogel and the amino
group into the brush. Viscoelastic dissipation in the hydrogel also contributes to the
total work of adhesion.