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Reaction enthalpies of metal-polyimide interface formation observed by calorimetry Murdey, Richard


A calorimeter has been developed to study metal-polymer interfaces in ultrahigh vacuum (UHV) conditions. Metal atoms are pulsewise deposited on a polymer substrate, gradually forming a metallic overlayer, while the reaction heat is measured in-situ with a pyroelectric sensor. Differential enthalpies of interface formation are derived from the calorimetry data and reported as a function of metal coverage. The ability of the instrument to resolve the underlying chemical reactions and growth morphology that characterize these interfacial systems is confirmed by the widely different enthalpy curves observed for each of the three metals studied in the present work - calcium, chromium and copper - deposited on PMDA-ODA polyimide substrates and the correspondingly distinct conclusions about the interfacial reactivity that were made in each case: Strong binding was discovered at the interface between calcium and polyimide. A calcium-polyimide complex with a binding energy of 600 ± 20 kJ/mol is suggested from the high initial intensity and exponential decay of the observed enthalpies with increasing metal coverage. The surface density of calcium binding sites was estimated at 4.1 nm- 2 . A more complicated reactivity was observed for the chromium-polyimide interface. Reaction enthalpies of approximately 60 ± 20 kJ/mol were observed at low coverages, six times lower than the enthalpy of formation of bulk chromium metal. This indicates the creation of a thermodynamically unstable interface. Chromium deposition apparently induces an endothermic disruption of the polyimide surface, which both lowers the net reaction enthalpy and suppresses the formation of metal-metal bonds. The copper results, meanwhile, are consistent with the formation of spherical metal clusters. The calorimeter records the change in surface energy from which the physical properties of the metal clusters, such as the cluster size, can be determined as a function of coverage.

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