Morphology and fracture of drawn crystalline polymers

Drawn crystalline polymers, fibers and films, exhibit a characteristic fibrous structure with the long and narrow microfibril as the basic element. The highly oriented microfibrils originating from the same stack of parallel lamellae of the starting material form the fibril which, as a rule, differs a little in drawing ratio from adjacent fibrils. As a consequence of this variation and the location of the ends of microfibrils on the outer boundary of the fibril, this is the area of reduced strength where the fracture phenomena start. The free ends of microfibrils are point vacancy defects of the microfibrillar superlattice which, under applied stress, open and form microcracks long before the sample fails. The radial crack propagation first breaks the microfibrils adjacent to the microcrack at the point vacancy and produces a great many radicals as a consequence of the rupture of tie molecules in at least one amorphous layer of each broken microfibril. Longitudinal crack propagation proceeds along the boundary between adjacent microfibrils and creates very few radicals because there are very few interfibrillar tie molecules which can be ruptured during this process. The coalescence of microcracks by radial and axial growth finally leads to a crack of critical size which by catastrophical growth leads to sample failure. If the longitudinal growth is sufficiently enhanced, the fracture surface is fibrous with individually broken fibrils which are separated from each other by longitudinal voids. Such an effect occurs in a torsional test, at the end of cold drawing, and very often in cyclic loading tests.