Syntactic foams are lightweight, high specific strength materials used in the aerospace and naval
10 industries. Their utility is limited by their brittleness. The epoxy polymer matrix in an epoxy/hollow
11 glass microsphere (GMS) syntactic foam was modified using carboxyl-terminated butadiene-acrylonitrile
12 (CTBN) rubber with the aim to increase fracture toughness. The microstructure and fracture properties
13 were investigated, and compared to CTBN modified bulk epoxy polymers. The formation of complex
CTBN microstructures was responsible for the increase in fracture energy, from 193 J/m2
14 for the
unmodified syntactic foam, to 296 J/m2
15 at 12 wt% CTBN modification. However, this increase is much
smaller than for the CTBN modification of bulk epoxy polymers, where an increase from 101 J/m2
16 to
1112 J/m2
17 was measured for the same CTBN concentration. There is little toughness transfer from the
18 bulk epoxy polymers to the syntactic foams, attributable to small interstitial regions between the GMS,
19 restricting plastic zone size. A statistical approach to the analytical modelling of fracture energy in the
20 bulk epoxy polymers highlights the importance of considering the underlying distribution of rubber
21 particle and void sizes. The increase in fracture energy achieved in this work can increase the overall
22 usefulness of syntactic foams in structural applications.