Microstructure and origin of cross-tie fibrils in crazes

Crazes were grown in thin films of polystyrene (PS) at various temperatures and the resulting craze fibril microstructures were examined using low-angle electron diffraction (LAED). A quasi-regular array of cross-tie fibrils pull the main fibrils away from the tensile axis by an angle ± /2°. As a result, the LAED patterns from crazes grown at temperatures T<50°C exhibited split diffraction lobes centred about the equatorial axis of the LAED pattern. It was found that decreased with increasing crazing temperature and that the split lobes could no longer be resolved at the highest temperatures. Diffuse meridional diffraction spots due to scattering from the quasi-regular array of cross-tie fibrils were seen in the LAED patterns from crazes grown at low temperatures. The spacing of the cross-tie fibrils, R, determined from these patterns, was found to increase with the crazing temperature. A new model of craze widening was proposed that accounts for the formation of cross-tie fibrils by allowing some of the entangled polymer strands which bridge two fibrils in the active zone to survive fibrillation. Cross-tie fibrils are created when several such strands pile up locally, and the craze/bulk interface bypasses the pile-up.