Different deformation mechanisms of two modified-polystyrene bimodal systems

It is well known that the dominant toughening mechanism of rubber-modified polystyrene is multiple crazing. Some researchers have investigated polystyrene that can be modified by rubbers with dual particle sizes, leading to better mechanical properties. That is, the way to absorb energy during the deformation process is crazing and cavitation induced by rubber particles. Two types of polybutadiene-graft-polystyrene (PB-g-PS) rubber modifiers which have core-shell structures were synthesized via an emulsion graft polymerization using redox and oil-soluble initiators, respectively. To balance the yield strength, general-purpose polystyrene was blended with the PB-g-PS modifiers, as well as commercial high-impact polystyrene. Blends were defined as R-bimodal and O-bimodal corresponding to dispersed PB-g-PS particles formed using the redox and oil-soluble initiators, respectively. The impact strength of R-bimodal was improved significantly by altering the ratio of core to shell. However, little change of impact strength was observed for O-bimodal. Transmission electron microscopy images of fracture surfaces indicated that the deformation mechanism of R-bimodal is shear-yielding induced by multi-crazing. Moreover, PB-g-PS particles dispersed in O-bimodal can form a 'cluster' structure, leading to crazing to absorb energy. Scanning electron microscopy images also showed obvious distinctness between the R-bimodal and O-bimodal systems due to different deformation mechanisms. (C) 2010 Society of Chemical Industr