Epoxy network homogeneity

As explained in Sec. 7.1, epoxy networks have been and are still the subject of controversy. This is mainly based on the particular interpretation of results obtained using microscopy techniques. On the contrary, results obtained with small-angle neutron scattering (SANS) proved that typical diepoxy-diamine networks were homogeneous (Wu and Bauer, 1985). 

Biobased epoxy nanocomposites can be reinforced with organo montmorillonite clay and carbon fibers obtained from poly(acryl-onitrile) (45). To get the organically modified clay into the glassy biobased epoxy networks, a sonication technique was used. In this way, clay nanoplatelets were obtained that were homogeneously dispersed and completely exfoliated in the matrix. 

It can be stated that networks based on a simple formulation (one monomer reacting with a comonomer), obtained from the step-polymerization process will exhibit a homogeneous structure. This is the case for epoxy-amine networks (the most studied) and polyurethane networks that have been used very often as ideal networks for structure-property correlations. 

The structure of precursors, the number of functional groups per precursor molecule, and the reaction path leading to the final network all play important roles in the final structure of the polymer network. Some thermosets can be considered homogeneous ideal networks relative to a reference state. It is usually the case when networks are prepared by step copolymerization of two monomers (epoxy-diamine or triol-diisocyanate reactions) at the stoichiometric ratio and at full conversion. 

Homogeneous ideal networks, also called closed networks, result from a single-step polymerization mechanism of a stoichiometric mixture of monomers, reacted to full conversion. Many amine-crosslinked epoxies of Tg < 200°C and polyurethanes obtained using a single isocyanate monomer and a single polyol belong to this family. 

A molecularly interlocked IPN of epoxy and polyimide was developed by Gaw et al. to form molecular composites of ODA-PMDA polyimide and DGEBA epoxy [73]. In this system the epoxy monomers were homogeneously mixed with a fully polymerized precursor to the polyimide, polyamic acid, that contained reactive groups that could react with the epoxy forming the three dimensional network. This system overcame many of the problems of previous systems by the use of a novel solvent system. 

The simple chemistry of curing, the homogeneous nature, and good properties of glasses based on the above-mentioned reactants, allow a better understanding of some important aspects of structure-properties relationships of these polymers as compared to more complicated epoxy systems. Many of these results seem to be generally valid and applicable to networks of different chemical nature. 

Here, we shall consider several macroscopic features of the plastic deformation of glassy epoxy-aromatic amine networks. Mostly, the tensile or compression deformation has an inhomogeneous character. Usually, diffuse shear zones (or coarse shear bands) are clearly seen at room temperature deformation. Shear zones start from the defects on the sample boundaries or voids (dust) in the bulk. At higher temperatures, the samples are homogeneously deformed with neck formation (DGER-DADPhS, P = 1) 34>. 

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

---