Toughness of Particle Modified Semi-Crystalline Polymers: General Principles and Specific Examples

Many structural polymers exhibit brittle behavior either generally such as the thermoset epoxies, or notch brittleness under impact loading, such as the semi-crystalline polyamides (nylon). In many instances this brittleness has been aleviated more or less dramatically by the incorporation of rubbery particulate components which in the course of improvement of toughness cavitate. There exist many ad-hoc explanations for the effectiveness of these practices which are often in violation of basic principles. Here we discuss some general principles for improvement of toughness that rely on the reduction of plastic resistance of the cavitated matrix. As a special case, the dramatic improvement of notch brittle polymides by the incorporation of a well grafted rubbery component is presented, in which the mechanism of toughening relies on the percolation through the structure of a crystalline component of low plastic resistance. This is achieved when the half thickness of the inter-particle matrix ligament become smaller than the characteristic thickness of the layer of crystallization with preferred orientation.