Study of mechanical damage in rubber‐toughened poly(methyl methacrylate) by single and multiple scattering of light

Abstract

A s glassy polymers are usually somewhat brittle, several blending techniques are used to toughen these materials and to increase their impact strength. The microstructures of toughened polymers are complicated, as are their mechanical damage mechanisms. Moreover, these materials are mostly opalescent or even opaque, which renders difficult any optical investigation of the damage process. The scale of the damage generally ranges from nanometers to several micrometers, thus requiring a large panel of experimental techniques to investigate its microstructural evolution. When illuminated with a light beam, any non-perfectly transparent body will scatter light with a scattering pattern depending on the size, shape, and location of the microscopic light scatterers it contains. If the body is highly opaque, an incident light beam, if not absorbed, is scattered successively by several scatterers before emerging again at the front surface of the body. It has recently been shown that there is a coherent interference enhancement of the randomly multiple scattered light in a small angle range around the exact reverse direction of the incident light. This so-called coherent backscattering cone can be analyzed in terms of the size, shape and density of the scatterers. In this work, this technique is applied to a rubber-toughened PMMA containing core-shell (hard core) particles, a n initially transparent material that becomes progressively opaque during mechanical damage under stress. The damage mechanism within the rubber particles is compared with the size and shape of the light scatterers inferred from coherent backscattering and light transmission measurements.

Multiple light scattering and cavitation in two phase tough polymers

“…The cavities, as shown by transmission electron microscopy, may comprise a unique small cavity in the center of a pure rubber particle, or a distribution of several smaller holes in the rubber wall of a composite particle. As discussed in our earlier work, 24 the light scatterers in these materials may be defined as follows.…”

“…There are two boundary cases where and v f may be determined from L if (i) the void content remains constant, and (ii) the number of damaged particles remains constant. These two situations were discussed in detail in our previous work 24 and the theoretical results were compared with experimental values in the two extreme cases of damage basically due to an increase in the void content of the particles, or to an increase in the number of damaged particles. It was shown that at low strain rates in RT-PMMA, the void content of the damaged particles remains constant during the elongation process.…”

“…Hence, multiple light scattering and coherent backscattering must be employed in such cases. 20 -23 Our previous paper 24 reported a first attempt to use these techniques to investigate the damage mechanisms in polymers toughened with core-shell rubber particles. The main weakness of this type of optical experiment is that the measured optical mean free path L represents the scattering cross-section multiplied by the number of scatterers .…”

“…Hence, although the basic principles of the scattering experiment and damage process were described in our previous paper, 24 they are recalled here for the sake of clarity.…”

“…The small unloading procedure was already briefly employed in our earlier investigation 24 to estimate the loss of mechanical properties during the damage process. The purpose of the present work was to show that use of this new experimental procedure in loading and light scattering experiments allows separate calculation of the optical mean free path L, the scattering cross-section (the void content of a ruined particle), and the scatterer density (the number of ruined particles).…”