The first direct observation of a chemically heterogeneous nanostructure within an epoxy resin is reported. Epoxy resins comprise the matrix component of many high performance composites, coatings and adhesives, yet the molecular network structure that underpins the performance of these industrially essential materials is not well understood. Internal nodular morphologies have repeatedly been reported for epoxy resins analysed using SEM or AFM, yet the origin of these features remains a contentious subject, and epoxies are still commonly assumed to be chemically homogeneous. Uniquely, in this contribution we use the recently developed AFM-IR technique to eliminate previous differences in interpretation, and establish that nodule features correspond to heterogeneous network connectivity within an epoxy phenolic formulation.

As linseed oil has a longstanding and continuing history of use as a binder in artistic paints, developing an understanding of its degradation mechanism is critical to conservation efforts. At present, little can be done to detect the early stages of oil paint deterioration due to the complex chemical composition of degrading paints. In this work, we use advanced infrared analysis techniques to investigate the UV-induced deterioration of model linseed oil paints in detail. Subdiffraction limit infrared analysis (AFM-IR) is applied to identify and map accelerated degradation in the presence of two different grades of titanium white pigment particles (rutile or anatase TiO). Differentiation between the degradation of these two formulations demonstrates the sensitivity of this approach. The identification of characteristic peaks and transient species residing at the paint surface allows infrared absorbance peaks related to degradation deeper in the film to be extricated from conventional ATR-FTIR spectra, potentially opening up a new approach to degradation monitoring.

The long-term failure of seemingly intact corrosion resistant organic coatings is thought to occur via the development of ionic transport channels, which spontaneously evolve from hydrophilic regions on immersion, i.e., as a result of localized water uptake. To this end, we investigate water uptake characteristics for industrial epoxy-phenolic can coatings after immersion in deionized water and drying. Moisture sorption and the changing nature of polymer-water interactions are assessed using FTIR for dry and pre-soaked films. More water is found to be absorbed by the pre-soaked coatings on exposure to a humid environment, with a greater degree of hydrogen-bonding between the polymer and water. Furthermore, morphological changes are then correlated to localized water uptake using the AFM-IR technique. Nanoscale softened regions develop on soaking, and these are found to absorb a greater proportion of water from a humid environment.

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a b s t r a c tThermoset coatings commonly rely on high cross-linking density to provide enhanced barrier properties. Hence it is surprising that for the industrial epoxy-phenolic network investigated, equilibrium moisture uptake is found to increase with respect to cure time, i.e., with greater cross-linking. Molecular interactions between absorbed water and the resin are characterised using infrared spectroscopy, and water uptake is correlated to network polymer features such as polarity and free volume.

The internal topology of epoxy resins is, for the first time, shown not to be the determining factor for small molecule transport. Whilst epoxy resins comprise the matrix component of many high performance composites, coatings and adhesives, the nanostructure and transport properties of these materials are not well understood. Here, peakforce AFM imaging, in-situ FTIR cure analysis and nanochemical AFM-IR imaging are used to establish the effects of reaction selectivity and stoichiometry on the nanostructure of epoxy-phenolic resins based on bisphenol-A and diglycidyl ether of bisphenol-A. In the presence of excess epoxy, resins transition from exhibiting homogeneous internal nanostructures to the familiar nodular morphology characteristic of epoxies. This occurs as a result of lower reaction selectivity in the presence of increasing catalyst concentrations.Surprisingly however, chemically similar stoichiometric resins with a heterogeneous nanostructure display improved resistance to corrosion breakdown (ion transport) and lower water uptake than the homogeneous resins.

Paint remains a widely employed approach to corrosion control due its relatively low cost and proven efficacy. Nonetheless, the processes governing long-term deterioration of intact organic coatings (in the absence of defects) are not fully understood. In this contribution, we investigate the degradation mechanism of a corrosion resistant epoxyphenolic can coating. In-situ time-resolved ATR FTIR is applied to monitor both the chemical integrity of the coating and water uptake as a function of immersion time in water or electrolyte. Ion transport is assessed across free standing films, and morphological changes accompanying immersion are examined using ex-situ advanced scanning probe microscopy techniques. Coatings are found to deform as a result of water sorption during immersion in electrolyte or water, yielding regions of heterogeneous hydrophilicity, yet no change in functional group chemistry is found to occur.  Non-Fickian water sorption is associated with deformation of the coating. HIGHLIGHTS AFM-IR is used to show that hydrophilic pores form during immersion.3

Water sorption in epoxy networks is associated with deleterious physical effects such as swelling, hydrolysis, lowering of the Tg, cracking and crazing.Nonetheless, water uptake in epoxy coatings is poorly understood in relation to macromolecular structure. In this contribution, we study the effect of cure time (closely related to cross-linking density and free volume) on water uptake for a model epoxy-phenolic coating. Localised water uptake is then mapped with nanoscale lateral resolution using AFM-IR, and correlated to cross-linking density.