Thermal Control of Nanostructure and Molecular Network Development in Epoxy-Amine Thermosets

Abstract

Epoxy-amine resins find wide application as the matrix material of high performance polymer composites because of their favorable mechanical properties, thermal properties and solvent stability. These properties result from the complicated, highly cross-linked molecular network that is characteristic of epoxy-amine thermoset polymers. The connectivity of the molecular network has a strong influence on the physical performance of the finished part. Nonhomogeneity in the network structure can degrade these favorable properties through the introduction of low-energy pathways for solvent penetration or fracture propagation. This work examines the influence of cure temperature on the network-building cross-linking reaction and the subsequent effect on the homogeneity of the cross-linked molecular network. Specific attention is paid to nanoscale variation in the distribution of cross-link density. Thermal, rheological, and spectroscopic techniques are used to monitor key chemical and structural changes during network growth. Atomic force microscopy is used to understand nanoscale fracture behavior in terms of the low energy pathways that result from a nonhomogeneous distribution of cross-link density. The influence of processing-induced changes in molecular connectivity is discussed in terms of observed nanoscale morphology and fracture properties of the cured material.

The degradation mechanism of an epoxy-phenolic can coating

“…It is, on the other hand, well established that nanoscale fluctuations in cross-linking density do occur within thermoset epoxy resins. [33][34][35] Furthermore, we have previously shown that short term water uptake into model epoxy-phenolic resins is indeed heterogeneous at the nanoscale, and corresponds to slight variations in the degree of cross-linking density, in keeping with the Nguyen model. [16] Since water has been shown to diffuse through the intact coatings rapidly, it is feasible that the distribution of free-volume voids provided by these fluctuations in cross-link density provide favourable diffusive pathways through the coating.…”

Insights into Epoxy Network Nanostructural Heterogeneity Using AFM-IR

“…28 More recently, examination of fracture surfaces has become possible under ambient conditions (i.e., circumventing any sample preparation effects) using the atomic force microscope (AFM), and numerous reports have emerged of nodular morphologies detected in tapping mode height or phase images. 26,29,30,31 However, Duchet et al countered that such features may be attributed to imaging artefacts generated by a blunt SPM probe tip scanning over rough surfaces, and a similar argument has recently been made for peakforce tapping mode modulus maps showing an apparently heterogeneous nanostructure for epoxy amine fracture surfaces. 32,33 These differences in interpretation are unsurprising, given that the evidence for heterogeneous network formation has largely been limited to topographical measurements of fracture interfaces.…”

“…26 Examination of resins cured in the presence of 0.5 % catalytic accelerator (under otherwise identical conditions) revealed that a comparable nodular morphology was established after a shorter curing period (20 min), demonstrating that a heterogeneous network developed more rapidly, Figure 3. For samples cured using a still higher content of tetrabutylphosphonium bromide catalyst (1.0 %), fracture surfaces were found to display heterogeneous nodular morphologies regardless of cure time (> 10 min), Figure 4.…”

“…19,20,21,22,23,24 Heterogeneous cross-linking on this scale is expected to provide low energy pathways through the resin, yielding a structural basis for the relatively low fracture toughness of epoxy materials. 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.…”

Interlaminar fracture toughness, adhesion and mechanical properties of MWCNT–glass fiber fabric composites: Effect of MWCNT aspect ratios

“…From Fig. b, two peaks are clearly observed on the tan δ curves, which is probably due to the presence of amine‐cured epoxy networks with two different crosslinking densities . As shown in Fig.…”