Intermolecular interactions on amine‐cured epoxy matrices with different crosslink densities. Influence on the hole and specific volumes and the mechanical behavior

Abstract

The architecture of an epoxy matrix was modified by curing the resin with mono‐/diamine mixtures having identical chemical structures. Both hole volume and specific volume variations were studied by positron annihilation lifetime spectroscopy and pressure‐volume‐temperature/density measurements, respectively. The average hole volume of the networks at room temperature slightly increased when the monoaminic chain extender content increased. The increment in the intermolecular interactions between functional groups of the networks chains, due to the less hindered nitrogen introduced by the monoamine, appears to be the responsible for the observed behavior. Besides, only small variations on the specific volume were observed on increasing the monoamine content, which points out that for a cured epoxy system, the chemical structure of the curing agent is mainly responsible for chain packing in the networks. On the other hand, intermolecular interactions between chains were considered as the key factor for fixing stiffness and strength. Thus, it was observed that the increase of the intermolecular interactions with the monoamine content produced a decrease in the sub‐Tg small‐range cooperative motions, which increased the low‐deformation mechanical properties at temperatures between β and α relaxations. This conclusion could be applied to previous investigations with epoxy matrices not fully crosslinked (nonstoichiometric or noncompletely cured formulations). Finally, it was found that fracture properties do not significantly depend either on the hole volume or on the intermolecular interactions. Fracture properties are more dependent on the crosslink density and the glass transition temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1240–1252, 2009

Network architecture and shape memory behavior of cold-worked epoxies

“…The molecular architecture of cured epoxies can be modified in different ways, for instance, by changing the chain stiffness of the network, its cross-link density, or its connectivity (Bignotti et al, 2011;Crawford, 1997, 1998;Yang et al, 2007). Several studies have shown that the network architecture greatly affects many properties of epoxy resins, in particular their mechanical properties (Blanco et al, 2009;Crawford, 1997, 1998;Nakka et al, 2011;Pandini et al, 2011;Sindt et al, 1996;Yang et al, 2007) and glass transition temperature (T g ) (Bignotti et al, 2011;Lesser and Crawford, 1998). With regard to the effect of the network architecture on the shape memory behavior of epoxies, the few articles published so far were mainly focused on the tailoring of the switching temperature (Liu et al, 2010;Xie and Rousseau, 2009).…”

Thermoplastic‐modified epoxy resins cured with different functionalities amine mixtures: Morphology, thermal behavior, and mechanical properties

“…The chain extender introduction in the aminic formulation increases intermolecular interactions thus diminishing the secondary b relaxation corresponding to short-range cooperative motions involving ACH 2 ACH(OH)ACH 2 AOA units in both temperature and strength. 41 The lower flexural strength and fracture toughness for the linear polymer obtained by reacting the epoxy resin with the monoamine is surely linked to the low molecular weight in the linear DGEBA/monoamine mixture due to the practical impossibility for achieving very high conversion on this system. These epoxy/amine mixtures were modified with PMMA.…”

“…Mechanical properties of neat matrices tested showing that modification of the network architecture by introducing an increasing amount of chain extender in the aminic hardener formulation is a reliable method to improve fracture toughness, as it was also shown elsewhere. 41 Mechanical properties of neat and modified systems are collected in Figures 1-3. First, the influence of chain extender is discussed.…”

“…Another method to achieve this modification is by reacting a bifunctional epoxy resin with a mixture of amines of different functionalities, one of which being a monoamine that plays the role of a chain extender. [17][18][19][30][31][32][33][34][35][36][37][39][40][41] In this way, full reacted networks are obtained without lateral chains and secondary reactions are avoided working at epoxy/amine stoichiometric ratio. Selecting amines with a similar chemical structure, the change in the system composition is minimum avoiding the variation in chain flexibility between crosslink points.…”

Stress Relaxation in Epoxy Thermosets via a Ferrocene-Based Amine Curing Agent

“…Previous publications have examined the effect of curing agent structure on the mechanical properties of epoxy thermosets, from which several general trends are apparent. − For example, an aliphatic amine-cross-linked epoxy was observed to transition from brittle fracture to yield at ca. 60–70 °C below its T g .…”