Effects of Contact Time and Polarity Level on Adhesion

We have measured the rate at which adhesion develops between two surfaces that interact by hydrogen bonding. A polydimethylsiloxane elastomer lens with a slightly oxidized surface was pushed against a polystyrene based copolymer substrate that contained acid groups. The interaction was measured on both forming and breaking the contact using the JKR technique. The toughness of the joint, G_c, increased considerably with increasing acid content in the substrate whilst the apparent work of adhesion, W, measured whilst making the contact, decreased with increasing acid content. This decrease in W implies that the acid groups caused repulsion between the surfaces when they were not in contact, but the increase in G_c shows that they formed bonds after contact was made. The rate of increase of G_c with time was found to depend on the acid content in the substrate but, for moderate acid levels, G_c was found to saturate at values that varied approximately linearly with acid content. For 8%acid and 47%acid substrates the rate of adhesion development over a period of 24 hours could be fitted by a model assuming (i) the toughness increased linearly with areal density of bonds between the substrate and the elastomer and (ii) the rate of reaction between the substrate and the elastomer varied as the cube of the density of unreacted acid groups. This cube law may possibly be explained by the kinetics of motion of the elastomer chains on the substrate.