Half way through the last century, the first reports detailing the complex internal nanostructure of epoxy resins were published. Most of these pioneering works detailed the observation of nodular internal structures using scanning electron microscopy. In later publications, atomic force microscopy studies emerged showing that these internal morphologies were made up of highly cross-linked nodules, embedded in a more lightly cross-linked matrix. Since then, researchers have proposed that the nodules form during pre-gelation, as a result of cluster formation followed by predominately intra-nodule cross-linking reactions. However, the presence of nodular nanostructures within epoxy resins has historically been disputed. Recent technological advances have invigorated research in this area since advanced high-resolution techniques have been developed and successfully used to prove the presence and formation of nodular structures. Unfortunately, little is understood about how to control their formation and thereby potentially tailor resin properties.
Recently, University of Manchester researchers, Dr. Suzanne Morsch, Zoi Kefallinou, Yanwen Liu, and Professor Stuart Lyon from the Corrosion and Protection Centre in collaboration with Dr. Simon Gibbon from AkzoNobel examined a lightly crosslinked epoxy-phenolic system; namely bisphenol-A and diglycidyl ether of bisphenol-A. To be precise, their main focus was to investigate the effects of catalytic content and stoichiometry on the development of an internal nanostructure. In addition, they hoped to assess the significance of moisture sorption since it has been linked to service failure of network polymers through cracking, plasticization and swelling. Their work is currently published in the research journal, Polymer.
The research method used by the scientists commenced with preparation and separation of samples that were to be used during the subsequent steps. Next, the researchers obtained bulk infrared spectra using a Fourier transform infrared spectrometer. Atomic force microscopy images of cured samples coated onto pre-scored steel and fractured under liquid nitrogen immediately before analysis were then taken. Lastly, gravimetric water uptake, electrochemical impedance spectroscopy and nano-thermal analysis were also performed.
The authors observed that the heterogeneous stoichiometric resins actually retarded water uptake and displayed enhanced corrosion resistance when compared to less polar homogeneously structured resins. This was seen to be as a result of the higher overall cross-linking density within stoichiometric specimens. Additionally, they noted that the nanostructure was not the controlling factor for the resin’s attributes.
In summary, University of Manchester scientists presented experimental evidence on the development of an internal nodular morphology even in very lightly cross-linked epoxy-phenolic networks. Generally, this proved that for the specimens used, the nanoscale structure represented an intrinsic feature of network systems. Moreover, chemically similar stoichiometric resins with a heterogeneous nanostructure displayed improved resistance to corrosion breakdown and lower water uptake than the homogeneous resins. Altogether, further investigations controlling for the overall cross-linking density are required to fully ascertain the effects of internal topology on transport properties.
Suzanne Morsch, Zoi Kefallinou, Yanwen Liu, Stuart B. Lyon, Simon R. Gibbon. Controlling the nanostructure of epoxy resins: Reaction selectivity and stoichiometry. Polymer, volume 143 (2018) page 10-18.Go To Polymer