Percolation phenomenon in SAN toughened by core-shell polymer MBS

Abstract

The fracture toughness of polymeric materials is a decisive factor in material selection. Many effective toughening methods for polymeric materials have been developed in the last two decades. Rubber toughening is one of the methods successful in toughening brittle or notch sensitive polymers. Craze/shear yielding mechanisms have been thought the main source of toughness [1]. However, it has been found that these mixing mechanisms are only valid for the stress behavior of single particles, but it is difficult to describe the relationship between toughness and phase structure of blends containing impact modifiers. Some evidence has shown that the toughening effect depends on the phase structure of the impact modifier strongly [2]. Recently, there has been a preference for core-shell polymers to toughen the polymer matrix. It was proved that toughening by incorporation of core-shell polymer is much more effective than customary rubber. This is because the structure and size of core-shell particles are controllable in synthesis and virtually unchanged during processing [3]. Furthermore, the soft core cannot deform to form detrimental holes under external stress owing to the the rigid shell at the outer layer. In this paper, the core-shell polymer, poly(methacryco-butylene-styrene) (MBS) is adopted to toughen poly(styrene-co-acrylonitrile) (SAN) to investigate percolation phenomena occurring in SAN/MBS blends. The scanning electron microscope (SEM) photographs of the impact fracture surface of SAN and SAN/MBS blends (70/30 by weight) are shown in Fig. 1. The fracture surface of SAN is smooth, showing the characteristic of brittle failture. On the contrary, the fracture surface of SAN/MBS blends shows extensive matrix yielding, the characteristic of tough failure. Fig. 2 shows the relationship between the mechanical properties and the weight fraction of MBS particles in SAN/MBS blends. It reveals that the impact strength is practically unchanged when MBS content is low and 0.248 by weight (or 0.233 by volume) seems the critical value at which the impact strength remarkably increases and an obvious brittle-tough transition occurs. The tensile strength increases somewhat when MBS content is less than 10 wt%, and decreases promptly with increasing MBS content. When MBS content is up to 40 wt%, the tensile strength is 40 MPa, less than that of pure SAN matrix (65 MPa). It seems easier for us to explain the brittle-tough transition on the basis of the so-called ‘surface-to-surface interparticle distance’ model. Wu [2] described the brittle-tough transition of nylon containing rubber particles with the same size and various volume fraction (φr) using this model.