Effects of rubber particle cavitation on the yielding of high impact polystyrene
A pre-damage method was developed to determine whether rubber particle cavitation is responsible for craze initiation, or vice versa. Tensile tests were carried out on pre-strained high impact polystyrene (HIPS) specimens which had been annealed above 100 °C to heal any crazes formed in the PS matrix during pre-straining. Moderate prestraining followed by annealing was found to reduce the yield stress of the HIPS, but not the post-yield flow stress. These observations are related to cavitation for the rubber particles, which results in a fibrillar structure within the rubber membranes of a typical "Salami' particle. The reduction in yield stress provides evidence for primary chain scission in the rubber phase during yielding: on subsequent loading of the annealed tensile bar, the rubber particle exhibits a reduced resistance to cavitation because less energy is required to form a void. It was found in the follow-up pre-straining tests on HIPS blends and at different testing temperatures that the critical pre-strain is a function of rubber content and temperature. Comparable effects are seen in creep tests. Small levels of pre-straining have little effect if the specimen is not annealed before reloading. The TEM work also confirms rubber particle cavitation as a rate-determining step in the deformation of HIPS and supports the view that cavitation precedes crazing. A modified cavitation model is proposed to account for the deformation of a cavitated particle under tension. The predictions of this model agree with the experiments that the resistance of the particle is weakened by cavitation and crazes could initiate at the lower tension. The technique of pre-straining and annealing specimens, before submitting them to conventional tensile tests, provides valuable insight into the mechanisms and kinetics of toughening.