Recent developments in x‐ray diffraction studies of high polymers

Examples of preferred ‘tilted crystal’ orientations in fibres during the early stages of drawing and also during contraction on heating are described. The crystallographic direction of tilt in drawing is, in the examples studied so far, not the same as the direction of tilt in relaxation. Tilted‐crystal X ‐ray diffraction patterns give very valuable aid in the determination of the crystal structures of high polymers. The ability of high polymers to crystallize depends on geometrical regularity of molecular structure; chemical or stereochemical irregularity prevents crystallization unless the alternative groups of atoms are similar in size and shape. The determination of the proportions of crystalline and amorphous material in polymer specimens by the X ‐ray diffraction method is discussed. The experimental requirements and the underlying theoretical assumptions are pointed out. A reversible transition of a novel type has been detected in polytetrafluoroethylene: the onset of partial disorder in the crystals at room temperature, 300° c. below the melting point.