Mechanical properties and inhomogeneous nanostructures of dicyandiamide‐cured epoxy resins

Abstract

Dicyandiamide (DICY)-cured epoxy resins are important materials for structural adhesives and matrix resins for fiber reinforced prepregs. The objective of this study was to examine the mechanical and physical properties as well as the gel structures of the cured resins and discuss the relationships among them. Diglycidyl ether of bisphenol-A (DGEBA) oligomers were chosen as the common chemical structure of the epoxy resins. Four kinds of resin mixtures were formulated using the seven types of DGEBA oligomers having different molecular weight distributions. Three resin formulations having bimodal-type molecular weight distributions were designed to have almost identical rubbery plateau values of the storage modulus in dynamic mechanical analyses after curing, means that they had almost equivalent average crosslink density and basic chemical structure. However, the toughness, ductility, and environmental (heat and solvent) resistance of these three formulations were different. Atomic force microscopy revealed the existence of inhomogeneous nanoscale gel structures in these cured resins. The morphological differences in the gel structures in terms of their size, the connectivity, and the relative magnitude of the heterogeneity would cause the difference in several properties of the DICY-cured epoxy resins.

Insights into Epoxy Network Nanostructural Heterogeneity Using AFM-IR

“…25,26 Indeed it has recently been shown that the bulk toughness, solvent resistance, T g , compressive strain to failure and ductility of formation improve when narrow molecular weight distributions are maintained during the cure of epoxydicyanodiamide resins. 27 Nonetheless, the presence and origin of such internal nanostructure has remained a point of active debate in the literature. For example, early evidence for highly cross-linked 'nodule' features was predominately obtained using electron microscopy, where internal structures were exposed by fracturing.…”

Crack‐healing behavior of epoxy–amine thermosets

“…[27][28][29][30] The fracture surfaces generally appear to have a nodular topography. From AFM measurements, [27][28][29][30] the average height of each nodule (the root mean square distance) appears to vary largely between 5 and 200 nm and may be considered a characteristic length that interacts on either side of a fractured surface. When fractured surfaces come in contact during healing, the peaks and valleys will most likely not match perfectly at the nanoscopic scale, and this will limit contact and bonding.…”

Chemorheology investigation of a glassy epoxy thermoset on tensile plastic flow and fracture morphology

“…Sahagun and Morgan showed via atomic force microscopy (AFM) that epoxy‐amine thermosets exhibit detectable heterogeneous microstructures on the 50‐nm scale . Earlier studies supported these observations via electron microscopy of various epoxy‐amine polymers and other thermosets; however, dissenting authors argued that the electron beam was responsible for these heterogeneous surfaces through surface etching . Sahagun observed that in the absence of etched surfaces via AFM, heterogeneous nanostructures were present within crosslinked epoxy‐amine polymers.…”