Toughened epoxy polymers via rearrangement of network topology

Abstract

The highly crosslinked molecular architecture of thermosets makes this class of material strong but brittle. It is advantageous to enhance the ductility without sacrificing strength, glass transition temperature (T_g), and modulus. The hypothesis was tested that without altering the chemical structure, the network topology of a dense thermoset can be engineered to dissipate more energy before rupturing covalent bonds, producing a tougher material without sacrificing ultimate tensile strength, density, or glass transition temperature. A processing technique termed “Reactive Encapsulation of Solvent/Drying” (RESD) was used in which epoxy curing was conducted in the presence of varying amounts of inert small-molecule solvent, followed by a drying/annealing process in which the solvent was removed. Density measurements and freeze-fracture surface analysis revealed that the resulting RESD materials are not porous in their relaxed state after annealing. Comparing the dynamic mechanical response of the modified (RESD) to the unmodified (conventional) structures revealed no significant differences in glassy modulus and T_g. However, quasi-static mechanical testing showed that upon stretching, the modified structures have a volumetric energy capacity of up to 2.5 times that of the unmodified ones. SEM micrographs of fracture surfaces of RESD specimens indicated nano-sized cavities on the surface of the modified thermosets, which were not present before breaking. Therefore, the presence of distinct topological features in the modified network is likely the origin of the large improvement in ductility. Topology-based toughening is potentially an important step toward developing better high performance network polymers and composites.