An Arrhenius method to study the hydrolytic stability of polymer-metal adhesion

Polymer-metal joints are durable and strong while dry, yet become fragile and susceptible to subcritical crack growth when wet. The dry joint fails catastrophically in the adhesive layer under sufficiently large loads, but gradually at the polymer-metal interface under small loads. In this study, the fracture toughness of glassy, 343,000 MW, polystyrene-aluminum joints with various metal surface preparations is investigated using both a dry and a temperature controlled, water submerged, 90° peel test. While surface treatments of aluminum improve the physical interaction between polystyrene and aluminum, their chemical interaction is controlled using a styryl silane coupling agent added directly into the styrene monomer solution that polymerizes against the aluminum. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analyses of the fractured surfaces show that the locus of failure is mainly interfacial at low loads, but it gradually moves away from the interface at higher loads. This movement of the failure locus reflects a transition of the mechanism of interfacial de-bonding from the hydrolysis of the siloxane bonds to the cleavage of the main polymer chains. A rate-dependent bond failure model qualitatively describes the above process, in which the activation energy of bond dissociation is assumed to be a time-dependent parameter. Ellipsometric measurements on smooth silicon surfaces verify that the thickness of bound polymer (t_p) is controlled by the mole ratio of silane to polystyrene. For mole ratios less than one, t_p is less than the polymer radius of gyration. The fracture energy at high crack propagation velocity increases dramatically with mole ratio for acid-etched aluminum, but stays almost constant and high for anodized aluminum. At mole ratios yielding t_p greater than the average distance between entanglement points (d _e) for bulk polystyrene, the acid-etched joints produce a failure locus that is interfacial at small loads and low velocity, but moves away from the surface at large loads. Optical microscopy shows that bulk polymer fracture increases at high crack velocity as the mole ratio increases when t_p < d_e. Bulk polymer fracture also increases as the load and velocity increase for high mole ratios when t_p > d_e.