A new series of monoepoxide terminated controlled epoxy networks (CENs) and a corresponding soluble fraction polymer (SFP) were prepared to further investigate the effects of chain termination on epoxy thermoset structure-property relationships. CENs having an initial molecular weight between crosslinks (M c,i ) of $3000 g/mol using phenylglycidyl ether (PGE) as the chain terminator have thermal and mechanical properties consistent with previously studied monophenol terminated CENs. Glass transition temperature (T g ) decreases monotonically with PGE concentration (e), whereas fracture toughness decreases sharply at a critical PGE concentration (e c ). A PGE terminated SFP was prepared corresponding to the soluble fraction expected for the CEN composition at e c . The SFP behaves as a weak antiplasticizer in these epoxy thermosets; T g is reduced and follows the inverse rule of mixtures, and fracture toughness is slightly reduced. By difference it is inferred that most of the deterioration of epoxy thermoset properties resulting from incorporation of chain terminators above e c is a result of the presence of nonelastically active pendant chains and by the increase in M c .

Five families of new controlled epoxy thermosets (CENs) using three monophenol chain terminators were prepared to study systematic changes in the structure and amount of the monophenol and the initial molecular weight between crosslinks (M c,i ) on the properties of epoxy thermosets. Glass transition temperature (T g ) decreases with monophenol mole fraction (v) in proportion to both the concentration and flexibility of the chain terminator. Distinct serial relations for T g depression were observed for the three M c,i families. Dynamic mechanical analysis (DMA) shows significant perturbations of the relaxation behavior with added terminator as evidenced by decrease in peak tan d and in post T g damping. The rubbery coefficients of thermal expansion (CTE) increases with monophenol concentration only at v > 0.05 and shows distinct curvature versus temperature, but is largely invariant with monophenol flexibility. The thermal stability of terminated CENs decreases only slightly with v and little difference was found with monophenol structure. Most surprisingly, fracture toughness decreases markedly and discontinuously with v depending on M c,i . The values of the critical monophenol concentration at which fracture toughness markedly decreases (v c ) are inversely proportional to M c,i but are independent of monophenol flexibility. No correlation of v c with any of the calculated network structure parameters was apparent.