AN APPROACH TO YIELDING AND TOUGHNESS IN RUBBER-MODIFIED THERMOPLASTICS

An interpretation of yielding and fracture of rubber-toughened polymers is attempted, considering the fracture mechanics behavior of the matrices, with the rubber particles as stress-intensification sites. The fit of effective tensile yield stresses of composites vs. particle radii defines a stress-intensity factor K(Yc) for craze yielding much smaller than the classical fracture factor K(c), and a critical particle radius for yielding. Different K(Yc) values are found for polystyrene and poly(styrene-co-acrylonitrile)-based polymers. These factors are considered characteristic for craze initiation and propagation in the matrices, while K(c), in turn, would include also the craze-crack transformation contribution. K(Yc) appears independent of the rubbery-phase volume fraction and characteristics, but two different values are found and discussed for poly(styrene-co-acrylonitrile)-based materials in two different particle-size ranges. A similar treatment on notched specimens' yield stress indicates the presence of a maximum in different radius ranges for polystyrene and poly(styrene-co-acrylonitrile) matrices, with higher values than their breakdown stresses. This stress increment is in relation to the minimum particle size inducing and still stabilizing crazes and preventing crack formation. This maximum seems to control the reinforcing extent of the polymer matrix conditioning the Izod fracture initiation energy.