Multiple light scattering and cavitation in two phase tough polymers

In glassy polymers toughened by inclusion of nanometric rubber particles, t he high impact strength is due to cavitation of the rubber particles follow ed by the appearance of microshear bands in the glassy matrix. These materi als are mostly opalescent or even opaque, which renders difficult any optic al investigation of the damage process. Simple light scattering techniques were employed in earlier work to study the onset of damage in transparent t oughened polymers. As demonstrated in one previous paper, multiple Light sc attering can be employed to further investigate opaque materials and hence highly damaged polymers. Coherent light backscattering in strongly opaque m aterials arises from the fact that an incident light beam, if not absorbed, is scattered successively by several scatterers before emerging again at t he front surface of the body. The so-called coherent backscattering cone ma y be analyzed in terms of the size, shape, and density of the scatterers. I n the present work, this technique was applied to a semicrystalline polymer and to rubber toughened PMMA containing core-shell (hard core) particles, an initially transparent material which becomes progressively opaque in the course of mechanical damage under stress. During the damage process, both the number of cavitated particles and their individual void fraction may in crease, and a cavitated particle acts as a light scatterer of cross-section proportional to its void content. The weakness of such scattering techniqu es resides in the fact that the light scattering pattern is determined by t he product of the density of the scatterers and their scattering cross-sect ion. Consequently, the number of damaged particles cannot be separated from the particle void content. This study describes a new method based on the superposition of small elastic unloadings on the main tensile strain. Durin g these unloadings, the number of damaged particles remains constant but th eir optical cross-section changes, thus leading to a supplementary equation describing the scattering properties of the body. Hence, the number of cav itated particles and their individual void fraction may be calculated separ ately from the experimental data. Since the use of coherent light backscatt ering to investigate damage mechanisms in polymers is relatively new, the p aper also recalls the basic principles of multiple light scattering. (C) 19 99 John Wiley & Sons, Inc.