From dynamic mechanical properties to plastic strain behavior of epoxy networks. Effect of the network architecture

Abstract

AbstractClosed epoxy networks with various architectures, i.e., crosslink density and chain flexibility between crosslinks, were considered in this study. The crosslink density can be varied by preparing epoxy‐amine networks from a mixture of primary amines having different functionalities using mixtures of poly(oxypropylene)amines or a monofunctional amine having the same chemical structure acting as a chain extender. The chain flexibility is modified by considering aliphatic epoxy prepolymers instead of aromatic. The influence of the architecture of the epoxy networks on the low‐ and high‐stress mechanical properties is discussed. The dynamic mechanical spectroscopy conducted in wide ranges of frequency and temperature gives insight into the molecular level analyzing the data in both the glass transition region (α‐relaxation) and sub‐Tg relaxation zones. A model described by Perez and based on the existence of nanofluctuations of density or quasi‐point defects and the concept of hierarchically correlated molecular motions are also used to describe the low‐stress mechanical behaviour. The same theory is extended by taking into account the large‐stress effects and used to analyze the stress‐strain curves recorded in compression at various temperatures and strain rates. This modeling can describe variation of the flow stress with temperature and gives a unique description of the yielding of all the epoxy networks. From this model, the main physical parameter determining the flow stress is the cohesion of the solid state resulting from intermolecular interactions; the loss of cohesion, as temperature increases, corresponds to the α‐process which depends on the network architecture. Finally, the strain and stress at break (sr and ϵr) recorded in tension at various temperatures and strain rates for epoxy networks having various crosslink densities are correlated with the low‐stress behavior from the shift factors, aT, determined in the glass transition region.