Size effect on the mechanical properties of polymer particles

This study reports a new phenomena observed for monodisperse polymer particles. Micron-sized polymer particles are widely used in food, chemical industries and biotechnology. Recently, there is a surging interest in extending the polymer particle technology for microsystem applications by producing nanostructured polymer particles. One typical example is the development of anisotropic conductive adhesives. In these applications, the mechanical properties of polymer particles are of crucial importance. It is well recognized that the strength of metals is grain size as well as thickness dependent. However, for amorphous polymer particles the size effect on mechanical properties remains largely unexplored. A nanoindentation based flat punch method has been developed to study the stress-strain behaviour of single micron-sized polystyrene-co-divinylbenzene slightly cross-linked particles in compression. Five groups of particles with identical chemical compositions but different diameters have been tested. The diameter of the particles varied from 2.6μm to 25.1μm. Results show that the particle compressive stress-strain behaviour is strongly size-dependent¬ — the smaller the particle diameter is, the stiffer the particle behaves. This finding is, however, in direct contradiction to the results of molecular dynamics simulations of nanoscale polymer particles where an inverse size effect has been recognized. A striking relation, similar to the Hall-Petch effect valid in micron range and the inverse Hall-Petch effect in nanoscale range for metals, has been proposed for the polymer particles.