Preparation of SiO2@EVA core–shell particles via post-emulsification method with polydisperse SiO2 particle sizes and the critical thickness-to-diameter ratio in PA6/SiO2@EVA system

Core-shell particles with rigid core and soft rubber shell have been proven to be effective in strengthening and toughening polymers simultaneously when the rubber shell thickness is controlled within a critical range. However, the critical value of the so-called thickness-to-diameter ratio τ (the ratio of the rubber shell thickness to the rigid core diameter) obtained in the previous work are all from polymer blends containing core–shell particles with monodispersed particle size. Does the critical τ value still exist in the core–shell toughening system with polydisperse particle sizes? Is it possible to produce core–shell particles with polydisperse particle sizes but the same τ value? In this paper, the post-emulsification method is first reported to prepare core–shell particles with polydisperse SiO2 particles as the rigid core and ethylene–vinyl acetate copolymer grafted maleic anhydride (EVA-g-MAH) as the rubber shell, in which the thickness of the rubber shell can be well controlled and tuned via varying the feeding weight ratio of SiO2 and EVA-g-MAH. The τ value is only determined by the weight ratio of SiO2 and rubber, and has nothing to do with the SiO2 particle size. A series of polyamide 6 (PA6)/SiO2@EVA core–shell toughening systems are prepared via melt blending process. It is proved the existence of the critical τ value (τcbd) above which brittle-ductile transition happens in the core–shell toughening system with polydisperse core–shell particle sizes. The value of τcbd is 0.069 in PA6/SiO2@EVA core–shell toughening system.