Thermomechanical and large deformation behaviors of antiplasticized epoxy resins: Effect of material formulation and network architecture

Abstract

A series of amine-hardened epoxies were prepared by systematically varying the stoichiometric ratio and the relative amounts of diepoxide and monoepoxide resin to chemically control the material composition and macromolecular architecture (chain segments flexibility; crosslink density; amounts of dangling groups). Positron Annihilation Lifetime Spectroscopy was used to investigate the free volume associated to the various epoxy formulations, while dynamic-mechanical analysis was employed to investigate their network density and their primary and secondary mechanical relaxations. The mechanical behavior at small and large strain was studied by means of tensile and compression tests. The results pointed out that deviations from the ideal stoichiometric composition and the addition of monoepoxide resins lead to significant room temperature stiffening, together with a reduction of the crosslink density and glass transition temperature. This behavior, phenomenologically associated to antiplasticization, was interpreted according to the specific macromolecular architecture and ascribed to chain mobility hindrance, as revealed by secondary transitions, whereas no significant contribution from the free volume could be evidenced. Furthermore, it was shown that depending on the strain scale and on the corresponding deformational mechanisms, the mechanical response may be differently influenced either by the presence of dangling groups or by the network density

New Eco-Friendly Synthesized Thermosets from Isoeugenol-Based Epoxy Resins

“…Due to this higher crosslinking density, molecules are no more able to fill free volumes, increasing the distance between neighbouring chains and reduces intermolecular interactions. This effect has been observed in non-stoichiometric epoxy/amine system, where unreacted species fill the free volume, enhancing chain interactions and increasing stiffness [28][29][30]. Furthermore, the stiffness reduction with the increasing crosslinking density is moderate in the glassy state (12% at the maximum) and the values superior to 2.2 GPa are still suitable with the development of high-grade composite materials.…”

The Influence of the Furan and Maleimide Stoichiometry on the Thermoreversible Diels–Alder Network Polymerization

“…Varying the stoichiometric ratio of the reactive functional groups in a crosslinking polymerization results in changes in the thermophysical and thermomechanical properties of the formed network, as reported in literature for irreversible network polymerizations, such as epoxy-amine reactions [ 19 , 20 , 21 ] and polyester formation [ 22 ]. Moreover, changing the (ratio between the) concentrations of the functional groups results in important changes in the kinetics of the polymerization reactions.…”

Convenient Preparation of Graphene Oxide from Expandable Graphite and Its Characterization by Positron Annihilation Lifetime Spectroscopy

“…Positron annihilation lifetime spectroscopy (PALS) is a non-destructive experimental technique able to supply information on defects in metals [23] and semiconductors [24] as well as on voids in non-metallic materials, especially polymers [25][26][27][28][29][30]. For this last class of materials, the probe is positronium (Ps), an unstable electron-positron bound state which may be formed when a positron introduced in the sample and rapidly thermalized encounters an electron of the medium.…”