The microscopic origin for this fibril structure is the presence of entanglements between long polymer chains. The image on the left shows a craze in one of our MD simulations. Each of the 1024 chains in the simulation cell is composed out of 256 beads. The individual chains have bundled up to form fibrils, which are connected irregularly by smaller cross-tie fibrils. Crazes are characterised by their inverse volume fraction or extension ratio.
Glassy polymers are very tough materials.
The reason for this great enhancement is the energy dissipation in the previously described crazing process. The craze, however, cannot widen forever, but will eventually fracture due to a crack that progagates through the crazed material. The width of the craze d that allows this crack propagation limits the fracture toughness. This process, however, takes place on a length scale too large to be modeled with Molecular Dynamics. Methods on longer scales like finite element calculations or continuum models must be employed.