Toughness enhancement of thermosetting polymers using a novel partially reacted substructure curing protocol: A combined molecular simulation and experimental study

Abstract

Curing epoxies with a mixture of low- and high-Tg diamines has been proposed as a way to increase thermoset toughness. We seek here to understand the origins of toughness enhancement in systems comprised of the diamines poly(oxypropylene)diamine (POPDA) and diethyltoluenediamine (DETDA) together with the epoxy resin diglycidyl ether of bisphenol A (DGEBA) via control of network isomerization. Two curing protocols at constant overall DGEBA/DETDA/POPDA 2:1 amine:epoxy stoichiometric composition are compared: (i) curing a liquid mixture of DGEBA, DETDA and POPDA, and (ii) partially curing DGEBA with POPDA (60% of amines reacted), then adding DETDA and more DGEBA to continue to a fully cured stoichiometric sample; the latter is referred to as the “partially reacted substructure” (PRS) method. PRS samples are 50% tougher than the compositionally-identical mixed samples yet have higher Tg's than the mixed samples. We show here that MD simulations of model systems provide a molecular-level rationale for this observation. First, MD yields reasonably accurate densities and Tg's. Lower Tg's in the mixed systems are correlated to larger thermal fluctuations in positions of monomer centers enabled by more uniform dispersion of the POPDA molecules. Furthermore, the onset of crosslink bond stretching under steady uniaxial tensile strain occurs at lower strains in the mixed samples, which correlates to their lower experimental ductility. This behavior is shown to arise from POPDA molecules in the PRS system more easily deforming from their unstrained conformations than they can in the mixed systems. These findings provide further guidance in the use of control over network isomerization at constant composition to enhance toughness of thermoset systems.