Microfibrillar Structure, Radical Formation, and Fracture of Highly Drawn Crystalline Polymers

Summary

The highly drawn crystalline polymer with the almost perfectly oriented fibrous structure differs from the unoriented more or less spherulitic material not only in the orientation of crystal lattice and crystal lamellae but also in the basic structural elements: stacks of parallel lamellae in the latter and well-aligned microfibrils in the former case. This difference explains very well not only the anisotropy of mechanical properties but also the superior elastic modulus and strength of fibrous material. The extremely long and thin macrofibril consists of folded chain crystal blocks connected axially with a great many tie molecules. At the ends of microfibrils the axial molecular connection in the fiber is interrupted. Under tensile load such point vacancies yield microcracks — — about 10¹⁵ per cm³ — — thus concentrating the stress on adjacent microfibrils. Depending on the ratio of microfibril strength and autoadhesive forces between adjacent microfibrils the coalescence and growth of microcracks occurs by axial (longitudinal) cracks along the boundary between the microfibrils or by radial (transverse) cracks through adjacent microfibrils. In the former case, very few and in the latter case a great many tie molecules are ruptured producing radicals detectable by ESR. Polyethylene and polypropylene with weak Van der Waals forces between adjacent microfibrils yield few and nylon 6 and 66 with stronger hydrogen bridges a great many radicals in excellent suppport of the microfibrillar model of fibrous structure.