Evidence for a three-zone interphase with complex elastic–plastic behaviour: nanoindentation study of an epoxy/thermoplastic composite

Abstract

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 tip–sample 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.

Observations on interphase characterisation in polymer composites by nano-scale indentation using AFM and FEA

“…This suggests that indentations on the edge of the glass fibre-reinforcement may lead to moduli values that are between that of the fibre and the matrix giving the impression of an interphase. This interpretation is consistent with the suggestions of Gao et al [1] and Munz et al [2] that any transition region within two to three times the indentation diameter is associated with restriction of the indentation. The average experimental contact diameters were 87.9 nm and 31.7 nm for indentations in the bulk phenolic and glass fibre, which are in good agreement with the FEA modelled indentations of 87.6 nm and 34.4 nm for indentations at 678 nm (phenolic) and -678 nm (glass).…”

“…A limitation of these techniques is that the plastic and/or elastic deformation produced during the indentation may be restricted by the proximity of the reinforcement [1][2]. This leads to transition measurements that relate to the properties of the constituent materials, and obscure the effects due to any interphase which might be attributable to a number of factors, including the presence of a coupling agent.…”

Nanoindentation study of interphases in epoxy/amine thermosetting systems modified with thermoplastics

“…Quantitative knowledge of interphase properties is a problem that remains to be solved. Although the importance of the interphase properties seems to be generally accepted, there are only a limited number of investigations to provide quantitative information on mechanical interphases, including parameters such as the interphase width, total modulus change through the interphase, or radial stiffness profiles [1,[30][31][32][33][34][35][36]. Currently, due to the availability of advanced techniques of micro-and nanoindentation, mapping of local stiffness has become possible with a high spatial resolution.…”

Effective properties and size effects in filled polymers

“…They assumed that the interphase is a homogeneous region of finite thickness with properties differing from the bulk properties. Nevertheless, the material properties in the interphase region are distributed inhomogeneously on the microscale and this distribution strongly influences the overall behaviour of the filled polymer [13,17,20,25,26]. On the RVE scale an existing interphase can be incorporated phenomenologically [39,10,17].…”

“…Different experimental methods on different scales have been used to examine the interphase's properties but a general explanation for the formation of the interphase is still missing, especially due to the diversity of the observed effects [5,6,30,13,21]. With respect to the bulk material changes have been observed in the entanglement density, in the chemical composition, in the glass transition temperature, etc.…”