The network structure of epoxy systems and its relationship to toughness and toughenability
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Beginning in about 1980, epoxy system networks were developed via an Edisonian approach that formed tough high Tg polymers (e.g., CET, crosslinkable epoxy thermoplastic resins). The driving force for the innovation was the need for inherently tough resins that could provide a base for formulation of aerospace prepregs and adhesives. Classical approaches to toughening epoxies either involved addition of toughening agents (rubber or thermoplastic) or reducing network crosslink density. Both approaches had significant drawbacks in their effects on polymeric properties and/or processability. This inherently tough CET chemistry was modified and extended in about 1990, to make it more suitable for liquid molding applications (e.g. resin transfer molding, pultrusion, etc.). Even though hundreds of CET formulations were investigated, no systematic investigation of the structure property relationships for this resin type was undertaken until recently. Model CET epoxy networks, with variations in crosslink density and monomer rigidity, were prepared to study how the network structure affects modulus, Tg, and toughness (toughenability). Diglycidyl ethers of bisphenol-A, tetrabromobisphenol A, and tetramethylbisphenol-A, along with the corresponding chain extenders, were chosen to study how monomer backbone rigidity and crosslink density affect physical and mechanical properties of epoxy polymers. The present study indicates that, as expected, the backbone rigidity of the epoxy network, not the crosslink density alone, will strongly influence the modulus and Tg, of epoxy resins. Upon rubber toughening, it is found that the rigidity of the epoxy backbone and/or the nature of crosslinking agent utilized are critical to the toughenability of the polymer formed. That is, the well-known correlation between toughenability and the average molecular weight between crosslinks (Mg). which usually corresponds to the ductility of the epoxy resin, does not necessarily hold true when the nature of epoxy backbone molecular mobility is altered. The potential significance of the present findings for a better design of toughened thermosets for structural applications is discussed.