The effects of monophenol chain terminators on the structure–property relationships of controlled epoxy networks

Abstract

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.

The effects of monoepoxide chain termination and their derived soluble fractions on the structure‐property relationships of controlled epoxy networks

“…The equivalent fraction of monofunctional component relative to the total equivalents of all of the monomers is defined as ε. This generally applicable term is used for these new PGE terminated CENs and is recalculated for the previous monophenol terminated CENs5 to compare the structure‐property relationships.…”

“…These well‐known recursive calculations utilize the amount, molecular weight, and functionality of the respective monomers, namely the epoxy resin, BA, THPE, and PGE, and the degree of functional group conversion (here taken as 100%). As before, M c,i for this series is calculated assuming no excess epoxy in the formulation,5 and its value (∼3000 g/mol) is nearly half‐way between that of the higher two values (∼1500 and ∼5500 g/mol) used in the previous study. Over the range of ε values studied, the network fraction decreases to < 50 wt %, the M w of the soluble fraction nearly doubles, and M c increases by a factor of > 4.…”

“…Varying amounts of the synthesized soluble fraction polymer were then added to the CEN formulation which upon curing gave thermosets having gel and soluble fractions but minimal pendant fraction. As in the previous study5 the thermomechanical properties of these new monoepoxide derived CENs were studied in detail.…”

“…Glass transition temperature ( T g ) decreases linearly in proportion to both the concentration and flexibility of the added monophenol 5. Distinct serial relations for T g depression were observed for the three families of M c,i materials.…”

“…Also, the effects of monofunctional diluents on the sub‐ T g relaxations in the dynamic mechanical analysis (DMA) of some amine‐cured epoxy thermosets have been studied 3, 4. The effects of chain termination on the thermal and mechanical properties of epoxy thermosets were studied in detail recently using D.E.R.™ 332 epoxy resin, bisphenol A (BA) as the chain extender, tris(4‐hydroxyphenyl)ethane (THPE) as the crosslinker, and monophenols as the monofunctional component 5. In that work five families of new controlled epoxy networks (CENs) using three different 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 thermal and mechanical properties of epoxy thermosets.…”