Are Cured Thermosets Inhomogeneous

However, similar structures were observed with etched surfaces of amorphous linear thermoplastics, such as polystyrene and poly(methyl methacrylate). Moreover, the small-angle X-ray scattering (SAXS) spectra of simple epoxy networks based on diepoxy and diamine monomers were 

However, this subject continued to be a controversial matter. Some authors used atomic force microscopy (AFM) in tapping mode to prove the existence of a two-phase structure in epoxy networks this structures comprises a hard microgel phase and a dispersed phase of soft partially reacted material (Vanlandingham et al, 1999). However, the interpretation of this kind of experimental result seems still to depend very much on what one wants to find. 

There have been attempts to relate the assumed nodular morphology with the physical properties of networks (Labana et al, 1971). This point is important, because if crosslinked polymers are considered as homogeneous three-dimensional structures, their ultimate properties can be related to the properties of such a continuum. On the other hand, if they are inhomogeneous, the supramolecular structure shown in Fig. 7.1 provides a more fruitful approach to interpreting of macroscopic properties. 

The presence of inhomogeneities in some polymer networks, particularly those formed by a free-radical chainwise polymerization, has long been recognized (Labana et al, 1971 Dusek, 1971) however, this does not mean that all thermosets must be inhomogeneous. 

It is not an easy task to define inhomogeneities in the structure of a polymer network. Every system will exhibit the presence of defects and fluctuations of composition in space when the scale of observation becomes smaller and smaller. A hierarchy of structures exists, from atomic dimensions to the macroscopic material. A scheme of different scale levels used to describe linear and crosslinked polymer structures is shown in Fig. 7.2. Inhomogeneities described in the literature for polymer networks are aseribed to permanent fluctuations of crosslink density and composition, with sizes varying from 10 nm up to 200 nm. This means that their size lies in the range of the macromolecular scale. 

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Dusek, K. Are cured thermoset resins inhomogeneous Angew. Makromol. Chemie 1996, 240, 1-15. 

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