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

Abstract

Reproducible and uncharacteristic tensile stressstrain behavior of cured glassy epoxy-amine networks produces distinctive fracture surfaces. Test specimens exhibiting plastic flow result in mirror-like fracture surfaces, whereas samples that fail during yield or strain softening regions possess nominal mirror-mist-hackle topography. Atomic force microscopy and scanning electron microscopy reveal branched nodule morphologies in the 50-nm size scale that may be responsible for the unusual tensile properties. Current hypothesis is that plastic flow of the glassy thermoset occurs through the existence and deformation of these nodular nanostructures. The thermal cure profile of the epoxy-amine thermoset affects the size and formation of the nodular nanostructure. Eliminating vitrification during thermoset polymerization forms a more continuous phase, reduction in size of the nodules, and eliminates the capacity of the material to yield in plastic flow. This maximizes nanostructure connectivity of the glassy epoxyamine thermoset and reduces strain to failure significantly.

Mesoscopic Heterogeneity in the Curing Process of an Epoxy–Amine System

“…In general, epoxy resins are supposed to be fractured and then characterized. Scanning electron microscopy and atomic force microscopy have been commonly used for the structural analysis of thermosetting resins. − It has been reported that the nodular structure with particle sizes ranging from tens to hundreds of nanometers existed on an etched fracture surface of thermosetting resins. − A plausible explanation for the nodular structure is that it is composed of relatively higher cross-linking regions surrounded by an interstitial phase of lower cross-linking density. In this sense, the structure can be regarded as heterogeneous. − Another interpretation is that the nodular structure itself is not evidence for the presence of heterogeneity because the etched fracture surface of amorphous polymers may provide similar nodular structures. − The discrepancy between the two interpretations seems to arise from the limited number of available techniques for nondestructive characterization of the network.…”