Interfacial Chemical Bonds Research Articles

The Role of Chemical Bonding in the Adhesion of Elastomers

Abstract A review is given of several studies of the effect of interfacial bonding upon the mechanical strength of an adhesive joint. In the first, polybutadiene layers were crosslinked by a free radical process while in contact with silane-treated glass. A direct proportionality was found between the minimum peel strength of the joint, at high temperatures and low rates of peeling, and the vinyl content of the silane treatment liquid. Covalent bonding between the diene polymer and vinyl groups on the treated glass was inferred. When radioactively tagged silanes were employed, extensive combination with the glass substrates was demonstrated. Again, the greater the amount of vinyl silane found on the treated glass surface, the greater the mechanical strength of adhesion between the treated glass and a polybutadiene overlayer. In another series of experiments, two partially crosslinked sheets of polybutadiene were pressed together before the crosslinking was taken to completion. The additional crosslinking was determined from measurements of the elastic properties and of the degree of equilibrium swelling by a compatible liquid. Again, the mechanical strength of adhesion between the two sheets under threshold conditions was found to be directly proportional to the inferred degree of interfacial interlinking. Thus, at least at high temperatures and low rates of peel, there is substantial evidence for a direct correlation between the mechanical strength of a joint and the degree of interfacial chemical bonding. Moreover, the relationships established in these studies allow other bonding systems to be diagnosed as chemical or physical in nature. For example, a dramatic increase in the strength of adhesion between two crosslinked polybutadiene layers was observed if they were exposed to air or oxygen for periods of an hour or two before being pressed into contact. It is inferred that interfacial chemical bonds are formed as a consequence of rapid surface oxidation reactions.

’’Hot’’ atom interactions with polymer surfaces

Sputter deposition of metal films is being used more widely in the industrial sector for the coating of many materials. In an effort to identify interface chemical bonds, if any, the interactions of sputter‐deposited (’’hot’’) Ni atoms with polyvinyl alcohol (PVA), polyvinyl methyl ether (PVME), polymethyl methacrylate (PMMA), and polyvinyl acetate (PVAC) surfaces have been investigated using x‐ray photoemission spectroscopy (XPS). Nickel atoms and ions resulting from the bombardment of clean Ni foil by 1000‐eV argon ions were impinged normally on clean polymer surfaces. At Ni coverages of about one third monolayer or more on both PVA and PVME, a new peak in the oxygen 1s core level spectrum was observed which had a binding energy 1.7 eV smaller than the original bulk polymer oxygen. The resulting Ni complex can be contrasted to that formed when thermally evaporated Ni atoms interact with the same surface. In the latter, a Ni–O–C chelate complex was formed having a new oxygen 1s binding energy 1.3 eV smaller than the bulk. This difference implies that the hot or sputter‐deposited Ni atoms are more strongly bound to the oxygen than they are in the chelate complexes produced by thermally evaporated Ni. The interactions of the hot Ni atoms with the more complex PMMA and PVAC are more difficult to interpret. On these polymer surfaces, XPS cannot distinguish between metal complexes formed by evaporation and those complexes formed by sputtering.

Soft X-Ray Photoemission Measurement of Schottky Barrier Formation at The Pd-si Interface

The binding energy of the bulk Si 2p levels observed with soft X-ray photoemission can be used to monitor the band bending in the silicon space charge region when a metal is deposited onto the silicon surface. Changes in the 2p binding energy with metal coverage can then be used to determine the change in the Schottky barrier height as the metal-silicon contact is formed. By tuning the photon energy and therefore the photoemitted electron escape depth, chemical shifts (atomic environment effects) at the interface can be separated from the bulk band bending effects. When combined with annealing to produce in-depth atomic intermixing, the result may reveal information on the distribution of metal atoms at the interface and its effect on the barrier height.Measurements of the Schottky barrier height change as a function of palladium deposition were made on (2 × 1) p-type and (7 × 7) n-type Si(111) surfaces by monitoring Si 2p core level shifts in a bulk sensitive mode. The barrier height change reached 1/e of its final value at a palladium coverage of 2.9 Å. Several experiments have made it possible to relate the measured Si 2p core level monitor of band bending to absolute Schottky barrier heights in a fully consistent fashion. Therefore, these results provide a means to measure the barrier height in the initial stages of Schottky barrier formation (i.e. at low metal coverage) and to compare these observations with the chemical behavior of the interface at low coverage and with electrical measurements on bulk contacts. Since the Pd/Si Schottky barrier height is established at the bulk value within a coverage of 3–5 Å (even before the overlayer is metallic), the role of interface chemical bonds in determining the barrier height is paramount.This work was supported in part by the U.S. Office of Naval Research.

The role of chemical bonding in the adhesion of elastomers

A review is given of several studies of the effect of interfacial bonding upon the mechanical strength of an adhesive joint. In the first, polybutadiene layers were crosslinked by a free radical process whilst in contact with silane-treated glass. A direct proportionality was found between the minimum peel strength of the joint, at high temperatures and low rates of peeling, and the vinyl content of the silane treatment liquid. Covalent bonding between the diene polymer and vinyl groups on the treated glass was inferred. When radioactively tagged silanes were employed, extensive combination with the glass substrates was demonstrated. Again, the greater the amount of vinyl silane found on the treated glass surface, the greater the mechanical strength of adhesion between the treated glass and a polybutadiene overlayer. In another series of experiments two partially-crosslinked sheets of polybutadiene were pressed together before the crosslinking was taken to completion. The additional crosslinking was determined from measurements of the elastic properties and of the degree of equilibrium swelling by a compatible liquid. Again, the mechanical strength of adhesion between the two sheets under threshold conditions was found to be directly proportional to the inferred degree of interfacial interlinking. Thus, at least at high temperatures and low rates of peel, there is substantial evidence for a direct correlation between the mechanical strength of a joint and the degree of interfacial chemical bonding. Moreover, the relationships established in these studies allow other bonding systems to be diagnosed as chemical or physical in nature. For example, a dramatic increase in the strength of adhesion between two crosslinked polybutadiene layers was observed if they were exposed to air or oxygen for periods of an hour or two before being pressed into contact. It is inferred that interfacial chemical bonds are formed as a consequence of rapid surface oxidation reactions.

Study of the electronic structure of model (110) surfaces and interfaces of semi-infinite III-V compound semiconductors: The GaSb-InAs system

This paper presents the results of a study of the electronic structure of model (110) surfaces and interfaces of semi-infinite III-V compound semiconductors, with specific application to the GaSb-InAs system. The description of the individual solids is sought within the empirical tight-binding method and combined with the Green's-function technique to account for their semi-infinite nature. Surface states are found to exist for ideal (110) surfaces, consistent with previous results obtained via the use of the finite-slab method. Results presented for the interface between two semi-infinite III-V semiconductors are the first to be reported and indicate the possible importance of the nature of the band-edge discontinuity of the heterojunction. It is shown that the expected unusual band lineup of the GaSb-InAs system gives rise to a finite density of states at the interfacial atomic planes, throughout the region of the bulk fundamental band gaps. Consequently, no interfacial barrier exists in this heterojunction. It is noted that this behavior of the GaSb-InAs heterojunction occurs in a regime of thickness of the individual materials which, in practice, is not accessible via the usual finite-slab method. Results of the influence of the interface geometry on the interface electronic structure are reported. For the interface characterized by interfacial chemical bonds of bulk bond strengths, we find no interface states in the fundamental-band-gap regions. This is found to be the case for both usual and unusual band lineup. Variations in the interface bond strengths, simulating the influence of the interface geometry, show that interface states first appear for unusually large departures from bulk bond strengths. It is thus suggested that interface states in lattice-matched III-V semiconductor heterojunctions are unlikely to arise solely due to variations in intrinsic chemical bonding effects. Results for interface states associated with dangling-bond are presented. It is found that the energy position of such states is relatively insensitive to those changes in surrounding geometry which do not appreciably alter the surface bond strengths of the host materials. These results suggest that the most likely origin of interface states for the class of systems investigated here is the presence of defects involving relaxation of the surrounding interfacial atoms.

Electron energy loss spectroscopy of the Si(111)—simple-metal interface

New data are presented on the formation of Schottky barriers on Si(111) 7 \ifmmode\times\else\texttimes\fi{} 7 with evaporated Al, Ga, or In metal. Electron-energy-loss spectra (ELS) have been taken as a function of metal overlayer coverage. The removal of clean surface state transitions has been observed at submonolayer coverage. The behavior of interface collective excitations of bulk-silicon-like transitions and of transitions from the metal core levels confirm the covalent character of the interface chemical bonds.

Surface spectroscopy of Schottky-barrier formation on Si(111) 7 × 7: Photoemission studies of filled surface states and band bending

The formation of Schottky barriers between Si(111) 7 \ifmmode\times\else\texttimes\fi{} 7 and group-III metals has been studied by ultraviolet photoemission spectroscopy. Modifications of the band bending and of the work function occur for low values of metal coverage (one to four monolayers). The "intrinsic," clean surface states are simultaneously replaced with "extrinsic" metal-related interface states. A two-step empirical model is proposed for the formation of the barrier. The first step is saturation of interface bonds, and the second step, critical in determining the junction properties, corresponds to the formation of a thin region with properties intermediate between that of a metal and of a semiconductor. Our experiments emphasize the need for a detailed theoretical treatment of the interface chemical bonds and underline the inadequacy of "macroscopic" models for metal-semiconductor junctions.

Effect of interfacial bonding on the strength of adhesion

AbstractThe strength of adhesion has been determined experimentally for an elastomer layer coupled to a rigid substrate by interfacial chemical bonds of varying surface density. Sites for interfacial bonding were obtained by treating glass plates with mixtures of vinyl‐and ethylsilanes in varying proportions. A layer of polybutadiene was then applied and cross‐linked in situ by a free‐radical process. Formation of interfacial bonds to vinyl groups (when present) on the glass substrate is inferred from the proportional increase in strength of adhesion under near‐equilibrium conditions, i.e., at low rates of detachment and at high temperatures. A 35‐fold increase in strength was found for vinylsilane, relative to ethylsilane, in rough agreement with the relative magnitudes of the strengths of covalent and dispersion bonds. However, the absolute magnitudes were much greater than predicted, by a factor of about 25 in both cases. This is attributed to the polymeric character of the elastomer: many molecular bonds must be stressed in order to detach or rupture one.4 In agreement with this hypothesis, the strength of adhesion decreased with increasing crosslinking. Anomalously high adhesion was found with clean glass. It presumably reflects a specific bonding mechanism of unknown type.