MORPHOLOGY AND MECHANICAL PROPERTIES OF CRYSTALLINE POLYMERS

Abstract

The composite structure consisting of extremely thin crystal lamellae or blocks imbedded in a pseudo-amorphous matrix is the main factor determining the mechanical properties of the crystalline polymer solid, much more important than the crystal perfection. In the spherulitic (slowly cooled) or microspherulitic (quenched) structure of the unoriented material the stacks of parallel lamellae interconnected by a few tie molecules are the basic structural element. The stacks themselves are not particularly strong. As a consequence of rejected impurity concentration on the boundaries between adjacent stacks and between spherulites which are bridged by very few tie molecules they can be easily sheared, rotated and by lamella destruction transformed into the fibrous structure. The highly oriented fibrous material has as the basic element the extremely long and narrow microfibril consisting of alternating crystal blocks and amorphous layers bridged by a great many rather taut tie molecules. Hence the microfibril acts in the fibrous structure as a very strong axial connection. The frictional forces on the extremely large surface transmit the load efficiently over long distances. The material connection in the axial direction by tie molecules is interrupted at the ends of microfibrils which act as point defects of the microfibrillar lattice. Upon loading they open as microcracks which by radial or axial coalescence, or both, grow to critical size leading to catastrophic crack propagation and sample failure.