Adhesion energy between polymer networks and solid surfaces modified by polymer attachment

Abstract

The adhesion energy, or strain energy release rate, G, between polydimethylsiloxane (PDMS) elastomers and flat silica surfaces modified by irreversible adsorption and end grafting of PDMS chains, has been characterised through peel tests and ‘JKR’ experiments. The 90° peel tests have been conducted down to very low velocity (5 nm s^–1). In the ‘JKR’ experiment, a small spherical cap of elastomer is pushed against the surface, and the area of contact is monitored as a function of the applied load, allowing the determination of both G and the elastic modulus of the elastomer, E. Systematic measurements have been made, varying the molecular architecture of both the elastomer and the grafted layer (chain length and surface chain density). Our central result is that G, measured at the lowest possible accessible rates, passes through a maximum when the surface density of grafted chains increases. This can be rationalised in the framework of recent models of polymer–polymer junctions, provided that the ability of the grafted chains to penetrate into the network is accounted for. When the velocity of the advancing fracture is increased, the measured adhesion energy increases strongly and non-linearly, contrary to the predictions of molecular models based on the suction mechanism.