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Stiffness variation in the interphase of amine-cured epoxy adjacent to copper microstructures
(2007)
In an effort to expand the understanding of the mechanical properties of the polymeric interphase on a metal surface, a composite consisting of epoxy and copper was prepared and analyzed. Scanning force microscopy-based force modulation microscopy (SFM-FMM) was employed along with dynamic mechanical analysis (DMA) and energy dispersive X-ray analysis (EDX). Diglycidyl ether of bisphenol A (DGEBA)-based epoxy resins were applied with amine curing agents. The samples were made taking advantage of electron beam lithography (EBL) in order to produce sharp edges of copper structures and a flat surface suitable for the SFM-FMM analysis, which was able to depict the stiffness within the interphase. It is considered significant information because the mechanical characteristic within the narrow interphase was revealed. Comparing with DMA and EDX, the stiffness information of SFM-FMM demonstrated a matching correlation and agreement in terms of preferential adsorption of the curing agent in the vicinity of the interface. The stiffness profiles of the two epoxy systems turned out to be different, and it shows the material dependence of the interphase characteristics.
A detailed study of the mechanical interphase (IP) between an amine-cured epoxy and the amorphous thermoplastic polyvinylpyrrolidone (PVP) was performed. The amine curing agent was diaminodiphenylsulphone (DDS). With 170 °C, the curing temperature was close to the glass transition temperature of PVP. Using a depth-sensing indentation setup equipped with a Berkovich indenter, force penetration curves were measured at different positions on the cross-section of an epoxy/PVP/epoxy sandwich specimen. Profiles of the sample Young's modulus, Es, the hardness, H, and the plasticity index, ψ, were analysed as a function of the distance from the epoxy/PVP interface. Each of these quantities showed significant variations. Furthermore, the profile of the ratio between the reduced modulus of the tipsample contact, Er, and the hardness is shown to reflect the profile of the plasticity index, that is the relative amount of plastic indentation work. A relationship is presented that allows the calculation of the plasticity index from the ratios Er/H and hf/hc, where hf and hc denote the final depth and the contact depth, respectively. Although the Young's modulus shows strong variations, the hardness appears to be the most sensitive parameter for IP property variations. Three different zones can be identified from the hardness profile, with respective widths of ~21.3, 52.5 and 160.9 µm, which add up to a total IP width of ~234.7 µm. Zone Z1 is located next to the PVP layer. The outer edge of zone Z2 is located at a position similar to the one of the amine depletion zone. The latter was detected by means of energy-dispersive analysis of x-rays and its width is ~71 µm. Inspection of the residual imprints shows a larger plastic deformation for zone Z1, in agreement with the high values of the plasticity index and of the creep deformations. From these grounds, the different IP zones are discussed in terms of the diffusion processes taking place during the epoxy curing procedure. Zone Z1 is associated with bi-directional interdiffusion across the initial interface. Zone Z3 can be attributed to long-ranging effects reaching far beyond the amine depletion zone.