The fine structure of polytetrafluoroethylene

Abstract

Electron micrographs of highly crystalline PTFE crystallized by slow cooling from temperatures not far above the melting point show well‐marked bands, often with striations perpendicular to the bands; optical evidence shows that the chain molecules are parallel to the striations. The structure is in marked contrast to the spherulitic structure of most polymers; it appears that in PTFE the molecules are straight and parallel for much greater distances than in other polymers, and it is suggested that the width of the bands is a measure of molecular length. The unusual structure is attributed to the unusual stiffness of the fluorocarbon chain; the molecules in the liquid are straighter and less tangled than in other polymer melts. Heating to 500°C., followed by slow cooling, gives a modified structure approaching the spherulitic type found in other polymers; it is suggested that this is due to increased molecular tangling in the melt at higher temperatures. One type of polymer shows bands of remarkably uniform thickness, suggesting unexpected uniformity of molecular length. PTFE wax made by thermal or radiation degradation of the high polymer shows well‐defined spiral growth steps due to dislocations; the step heights suggest an unexpected uniformity of molecular length.