Micromechanics of deformation in particle-toughened polyamides

It is now well appreciated that a number of semicrystalline polymers can be effectively toughened by the addition of a well-dispersed secondary phase, when the average interparticle matrix ligament thickness, A, of the blend is reduced below a critical length parameter, nc, This critical parameter is a specific material characteristic of the base polymer and can be achieved by various combinations of filler particle volume fraction and particle size. Recently, the significant improvements in toughness achieved when Lambda less than or equal to A(c) were attributed to a morphological transition taking place when interface-induced crystallization of characteristic thickness Lambda(c)/2 successfully percolates through the primary phase. These transcrystallized layers are highly anisotropic in their mechanical response and, as a result, change the preferred modes of plastic deformation in the material, enabling the large plastic strains, which provide the high toughness. This study aims to elucidate the micromechanics and micromechanisms responsible for the high toughness exhibited by these morphologically altered heterogeneous systems via a series of micromechanical models. The case of polyamide-6 modified with cavitating spherical elastomeric particles treated as voids is modeled. It is found that the mechanical response and local modes of plastic deformation of these systems depend strongly on the morphology of the primary phase, the volume fraction of filler particles and the level of applied stress triaxiality. In particular, when Lambda < Lambda(c) and highly textured material percolates through the matrix, the material is found to deform by multiple shear banding along crystallographic planes of low shear resistance in the matrix ligaments diagonally bridging particles. This mode of plastic deformation is found to be robust to increases in triaxiality and, indeed, the textured material acts to resist void growth while promoting shear along the preferentially oriented crystallographic planes. (C) 2000 Elsevier Science Ltd. All rights reserved.