Morphology and Properties of Crystalline Polymers with Fiber Structure

The morphological properties as derived from wide- and small-angle x-ray scattering, IR dichroism, electron spin resonance, and electron microscopy, and the anisotropy of mechanical properties of the fiber structure can be well explained by the microfibrillar concept developed on the basis of a detailed study of plastic deformation and fracture of polymer single crystals. The microfibrils bridging the cracks of a fractured crystal contain in axial direction a regular alternation of crystal blocks and amorphous layers containing chain folds, chain ends and a large ratio (between 5 and 30%) of tie mole cules providing the high longitudinal elastic modulus and strength. With lateral dimensions between 100 and 300 Å and a length of about 10 μ, they represent the strong structural element of the fiber. Lateral fit of crystal blocks of adjacent microfibrils produces the lamellae oriented more or less perpendicular to the fiber axis. They are a secondary structural element contributing to the lateral elastic modulus and strength. But the relatively easy axial slip of microfibrils, not substantially hampered by lamellae, yields the high shear compliance which is inexplicable by any model based on lamellae as primary element of fiber structure. The fracture of fibers in the tensile test most probably initiates at point defects of the microfibrillar or fibrillar superlattice, i.e., at the ends of microfibrils. The density of such defects as derived from the probable dimensions of microfibrils agrees very well with the observed number of ruptured chains.