At room temperature, the fracture surfaces of the S-CSR particle-modified polymers also showed crack forking and feather markings. However, these fracture surfaces are rougher than those of the unmodified epoxy, and scanning electron micrographs of the deformation zone for the 10 wt% and 20 wt% S-CSR-modified epoxy polymers are shown in Fig. 13. The fracture surfaces are covered with cavitated S-CSR particles, which can be identified as the circular features in Fig. 13. The cavitation process causes the originally solid rubber particles to deform into a rubbery shell surrounding a void. The mean diameter of these cavities was measured to be 0.296 μm. This is significantly larger than the mean diameter of the S-CSR particles measured from the AFM images, which was 0.18 μm. This observation clearly reveals that plastic void growth of the epoxy polymer has followed cavitation of the S-CSR particles. This is one of the main toughening mechanisms for such thermoset polymers toughened by the presence of well-dispersed rubber particles. Essentially, the cavitation of the particle creates voids which relieve the triaxial stress-state ahead of the crack tip and so enable plastic void growth to occur far more readily in the epoxy polymer. Cavitation, as opposed to particle debonding, will occur when the rubber particle is strongly bonded to the surrounding polymer. Indeed, based on the FEG-SEM observations, the core to shell adhesion must also be relatively high for the S-CSR particles, as no debonding is observed. For the low-temperature results, the fracture surfaces of the particle-modified polymers are very similar to the samples tested at room temperature, see Figs. 14 and 15. Indeed, all of the S-CSR particles cavitated, even at −109 °C, although the size of the cavities is reduced at low temperatures, which indicates a lesser extent of plastic void growth in the epoxy polymer.
S-CSR particle-modified polymers
room temperature
S-CSR-modified epoxy polymers
10 wt% and 20 wt%
%
cavities
mean diameter
0.296 μm
μm
S-CSR particles
mean diameter
0.18 μm
μm
samples
room temperature
S-CSR particles
cavitated
−109 °C
°C