Fracture mechanism of drawn oriented crystalline polymers

Abstract

During the fracture of highly oriented drawn films and fibers of crystalline polymers, one observes a great many radicals produced by chain rupture. The description of such material by the logarithm of the time to break under applied load as a linear function of stress with the activation energy of the covalent bond seems to designate single bond rupture as the most time-consuming step in creep failure. According to the fibrous morphology of the structure, one ruptures the tie molecules which connect crystals in the direction of the fiber axis and transmit the major part of the load through the amorphous layer sandwiched between subsequent crystals. The maximum number of ruptured chains, however, is between one hundredth (nylon 6 and 66) and one thousandth (polyethylene) of the total number of tie molecules in the sample. That means that chain rupture does not occur in all amorphous layers with tie molecules but only in isolated defects of the structure where the inhomogeneity of the stress and strain field produces a sufficient stress concentration on the tie molecules for rupture. In the microfibrillar model of fibrous structure the ends of individual microfibrils are point vacancies of the microfibrillar lattice where the absence of tie molecules yields a larger strain and eventual microcrack formation. As a consequence, the adjacent microfibrils are so much more strained than the rest of the sample that they may fail by rupture of all tie molecules in one single amorphous layer between two subsequent crystal blocks. The microcracks initiated at point vacancies grow in the radial direction by breaking micro-fibrils or in the axial direction along the boundary between adjacent micro-fibrils. In the former case, favored by strong autoadhesion of microfibrils and a small fraction of tie molecules, a great many radicals are formed (about 5 × 10¹⁷ cm⁻³ in nylon 6 and 66). In the latter case, favored by a high fraction ***of