Local Mechanical Properties of Heterogeneous Nanostructures Developed in a Cured Epoxy Network: Implications for Innovative Adhesion Technology

The physical performance of an epoxy-based material is largely influenced by the formation of heterogeneous nanostructures (HNSs) in the glassy three-dimensional epoxy network. Thus, understanding the connectivity and local mechanical properties of HNSs formed in cured epoxy networks is of critical importance in the development of advanced epoxy-based materials for innovative adhesion technology. Here, we use bimodal atomic force microscopy (AFM) to map the local mechanical responses of HNSs evolved in an epoxy-amine system during curing. Both modulus and dissipated energy quantities describing the elastic and adhesive responses, respectively, were simultaneously obtained at different harmonic vibrations in each bimodal AFM measurement. HNSs exhibiting different elastic and adhesive responses were clearly visualized for all epoxy networks at different levels of curing. The architecture and local mechanical responses of HNSs were visibly altered with increasing degree of conversion. Soft domains representing network regions with relatively lower elastic moduli and weaker adhesive interactions were dominant and continuously connected at initial stages of curing up to conversion degrees of ∼91%. However, hard domains became increasingly connected upon approaching the fully cured stage. In combination with previously reported results obtained using the particle tracking technique to study the network heterogeneity at lower conversion degrees of the same epoxy system, we were able to provide a full picture describing the generation and development of mechanical heterogeneities in the network during curing in which the length scale of the heterogeneities decreased from several hundred nanometers at the pregelation stage to ∼10 nm at the fully cured stage.