Insights into Nano-Scale Physical and Mechanical Properties of Epoxy/Boehmite Nanocomposite Using Different AFM Modes

Abstract

Understanding the interaction between nanoparticles and the matrix and the properties of interphase is crucial to predict the macroscopic properties of a nanocomposite system. Here, we investigate the interaction between boehmite nanoparticles (BNPs) and epoxy using different atomic force microscopy (AFM) approaches. We demonstrate benefits of using multifrequency intermodulation AFM (ImAFM) to obtain information about conservative, dissipative and van der Waals tip-surface forces and probing local properties of nanoparticles, matrix and the interphase. We utilize scanning kelvin probe microscopy (SKPM) to probe surface potential as a tool to visualize material contrast with a physical parameter, which is independent from the mechanics of the surface. Combining the information from ImAFM stiffness and SKPM surface potential results in a precise characterization of interfacial region, demonstrating that the interphase is softer than epoxy and boehmite nanoparticles. Further, we investigated the effect of boehmite nanoparticles on the bulk properties of epoxy matrix. ImAFM stiffness maps revealed the significant stiffening effect of boehmite nanoparticles on anhydride-cured epoxy matrix. The energy dissipation of epoxy matrix locally measured by ImAFM shows a considerable increase compared to that of neat epoxy. These measurements suggest a substantial alteration of epoxy structure induced by the presence of boehmite.

Short- and Long-Range Mechanical and Chemical Interphases Caused by Interaction of Boehmite (γ-AlOOH) with Anhydride-Cured Epoxy Resins

“…There is no significant stiffness contrast between the bulk epoxy and bulk HydBo. This is in agreement with previous findings, where the stiffness values of cured epoxy and boehmite in nanocomposites did not show a remarkable difference [16]. The soft area at the interfacial region, which is approximately 100–200 nm thick, coincides with the location of the HydBo’s outer shell (nanoplatelets, described in Figure 2).…”

“…It is worth noting that the curing agent plays an important role in the properties of the nanocomposite since each curing agent can interact differently with boehmite nanoparticles. The different interaction between BNPs and epoxy components results in property variations, not only at the interface between polymer and filler but also the bulk properties of the cured polymer can vary significantly [16,17].…”

“…Recently, we showed that the analysis of ImAFM force curves enabled us to visualize the formation of the mechanical interphase as well as probing the van der Waals and dissipative forces, which indicated the short- and long-range alteration of bulk epoxy in the presence of BNPs [16]. However, a precise probing of mechanical interphases and obtaining enough data within the scale of such small nanoparticles (less than 20 nm) are still challenging, even with AFM approaches with a lateral resolution of a few tens of nanometers.…”

Nanomechanical study of polycarbonate/boehmite nanoparticles/epoxy ternary composite and their interphases

“…However, identifying BNP solely based on topographic features is a challenging task. It has been demonstrated by Ghasem Zadeh Khorasani et al 12 on similar composites that during SKPM measurements, BNP appear with a higher surface potential than epoxy. Here, we also apply SKPM to locate the BNP (brighter spots in Figure 7(b)) in the epoxy matrix.…”

“…Most remarkably, the increase of shear strength, shear modulus, and compressive strength, as well as an improvement of the fracture toughness of EP by the addition of boehmite were reported 10,11 . The role of BNP as a reinforcement agent was attributed to property alteration of the bulk EP matrix in their presence, and to the formation of a soft interphase, preventing crack propagation by increased energy dissipation 12 . It was demonstrated 13 for an anhydride‐cured EP‐BNP composite system, that the hardener molecules were preferentially absorbed by the BNP, resulting in the local alteration of the resin‐hardener ratio in the bulk.…”