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

Abstract

Understanding the interaction between boehmite and epoxy and the formation of their interphases with different mechanical and chemical structures is crucial to predict and optimize the properties of epoxy-boehmite nanocomposites. Probing the interfacial properties with atomic force microscopy (AFM)-based methods, especially particle-matrix long-range interactions, is challenging. This is due to size limitations of various analytical methods in resolving nanoparticles and their interphases, the overlap of interphases, and the effect of buried particles that prevent the accurate interphase property measurement. Here, we develop a layered model system in which the epoxy is cured in contact with a thin layer of hydrothermally synthesized boehmite. Different microscopy methods are employed to evaluate the interfacial properties. With intermodulation atomic force microscopy (ImAFM) and amplitude dependence force spectroscopy (ADFS), which contain information about stiffness, electrostatic, and van der Waals forces, a soft interphase was detected between the epoxy and boehmite. Surface potential maps obtained by scanning Kelvin probe microscopy (SKPM) revealed another interphase about one order of magnitude larger than the mechanical interphase. The AFM-infrared spectroscopy (AFM-IR) technique reveals that the soft interphase consists of unreacted curing agent. The long-range electrical interphase is attributed to the chemical alteration of the bulk epoxy and the formation of new absorption bands.

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

“…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. This heterogeneity in the bulk EP, with a broad stiffness distribution, is another pathway for energy dissipation, which makes the system more resistant to fracture.…”

Bulk chemical composition contrast from attractive forces in AFM force spectroscopy

“…The second model sample consists of an organic phase and an inorganic phase and is described in detail by Khorasani and coworkers [ 16 ]. A layer of boehmite is sandwiched between epoxy layers, as seen in Figure 5a –c which shows the cross section of the three layers.…”

“…This led to an interphase region in the right-side epoxy, which differs significantly in its properties and structure when compared to the neat bulk epoxy. This is discussed in detail in [ 16 ]. Analogous to the first model sample, the material phases can be clearly identified by the topography shown in Figure 5a .…”

“…For the second sample, boehmite was hydrothermally synthesized from elementary Al. The hydrothermal route is described elsewhere [ 16 , 37 ]. For the third sample, commercially available boehmite NP (HP14, Sasol) with an average primary size of 20 nm was acquired.…”

“…There is a number of AFM-based methods, such as tip-enhanced Raman spectroscopy (TERS) [ 21 ], AFM-based infrared spectroscopy (AFM-IR) [ 16 , 22 ], noncontact AFM (ncAFM ) [ 23 – 24 ], chemical AFM (cAFM) [ 25 – 26 ], and Kelvin probe force microscopy (KPM) [ 27 ] which were developed to identify local chemical or structural specificities in the samples. All the methods mentioned above are, to different degrees, advanced techniques which require additional equipment and expertise.…”