Change in fine structure of nylon 6 gut yarn in twisting, annealing, and untwisting processes. I. Observation by WAXD, SAXD, and EM

To obtain information on the change in fine structure of nylon 6 taking place during practical false‐twisting processes, the manner of change in the three elemental processes, i.e., twisting, annealing, and untwisting, was studied. For simplicity, nylon 6 gut yarn was used instead of multifilament yarn. Wide‐ and small‐angle x‐ray diffraction (WAXD and SAXD) together with electron microscopy (EM) were used here. The degree of molecular orientation in the crystalline region of the twisted yarn gradually decreases with increase of the twist number ( TN ) in the region of TN ≫ 100. The long spacing, determined by SAXD, of the twisted yarn increases with increases in TN . The increase in long spacing cannot be interpreted only by macroscopic strain or elongation of the yarn in the twisting process. This difference seems to arise from the contribution of the decrease of lateral size of lamellae to the average long spacing; therefore the increase in long spacing should be attributed to the elongation of the amorphous region, deduced from the crystallinity measured by WAXD and the long spacing diffraction. The angle between the streak line on the surface of twisted yarn observed by EM and the fiber axis agrees well with the twist angle of the yarn. The crystal lamellae are stacked normal to the streak line at the initial stage, i.e., at a low value of TN , but they begin to deviate from the normal direction with increase in TN , accompanied by their partial destruction. Based on SAXD and density measurements, the internal strain of yarn annealed after twisting is fully relaxed. When the yarn is untwisted after twisting and annealing, the crystal orientation recovers gradually to that of the untreated yarn. The chain axis within the lamellae in the center region of the yarn becomes nearly parallel to the fiber axis, but the chain axis in the outer region does not.