Epoxies are a class of high-performance thermosetting polymers which have been
widely used in many industrial applications. However, unmodified epoxies are
susceptible to brittle fracture due to their highly crosslinked structure. As a result,
epoxies are normally toughened to ensure the feasibility of these materials for practical
applications. Recently, a new generation of toughening agents such as polysiloxane
based core-shell rubber (CSR) particles, amphiphilic block copolymers and silica
nanoparticles have been developed to toughen epoxies. These new toughening agents
will be studied in this thesis to pursue ultra-tough and stiff epoxies without
compromising other desirable properties.
Polysiloxane based CSR particles were able to toughen an anhydride cured
epoxy over a wide range of temperatures from -109 °C to 20 °C. At -109 °C, the
fracture energy of the epoxy could still be increased from 174 to 481 J/m2 with the
addition of 20 wt% of the CSR particles. The toughening mechanisms of these CSR
particle modified epoxies were identified as shear band yielding and plastic void growth.
A series of commercial poly(methyl methacrylate)-b-poly(butylacrylate)-b-poly(
methyl methacrylate) (MAM) triblock copolymers were studied as toughening
agents in two epoxy systems. The fracture toughness was generally increased by these
block copolymers, although their toughening performance was dependent on the
crosslink density of the epoxies and the morphologies of the modified epoxies. The
MAM modified epoxies were also studied as the matrix materials in fibre-reinforced
composites to investigate the transfer of toughness from the matrix materials to the
composites. Full (1 to 1) and nearly full toughness transfer was obtained for the
composites.
Hybrid toughening using a combination of the MAM block copolymer and silica
nanoparticles has also been investigated in the same epoxy systems. The addition of the
silica nanoparticles further increased the toughness of the MAM modified epoxies if
micron-sized MAM particles present.