Folded chain concept of fiber structure

Abstract

The fringed fibril concept of fiber structure is suported by the high longitudinal and smaller lateral tensile strength and elastic modulus, but one experiences difficulty in explaining the longitudinal periodicity and the accommodation of amorphous chains between subsequent crystals. The situation is exactly opposite with the extreme folded chain concept. A compromise between the two concepts seems to fit all experimental evidence and is supported by electron microscopy and X-ray investigation of plastic deformation of single crystals and bulk samples of PE. The partial chain folding at the transversal surface of the crystals provides enough space for the amorphous conformation of tie molecules that connect the subsequent crystal lamellae and produce the good longitudinal mechanical properties of the fiber. The lamellae are roughly perpendicular to the fiber axis and consist of slightly mismatched folded chain blocks with skewed fold-containing surfaces. The lateral dimensions of blocks are between 100 A and 300 A, and of lamellae a few thousand A. The blocks are arranged in long microfibrils. The lamellae are stacked in fibrils extending over many tens of microns in the fiber direction. The boundaries between fibrils are a consequence of different deformation rate and draw ratio between adjacent fibrils. They are regions of weakened strength and may even result in longitudinal voids.