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

Abstract

SynopsisTo 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 ( T N ) in the region of TN >> iW. The long spacing, determined by SAXD, of the twisted yarn increases with increase 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 a t the initial stage, i.e., a t 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.

Thermomechanical effects in the torsional actuation of twisted nylon 6 fiber

“…Twisted fiber samples annealed at 200°C appear to be immune from these irreversible processes, suggesting that the higher annealing temperature allows a stable molecular structure to form even when the fiber was held at constant length during heat treatment. Indeed, Tsujimoto et al have shown that annealing of twisted nylon 6 fibers at 170°C at constant length stabilises the molecular structure, possibly by increased molecular packing in amorphous regions 19 . torsional stroke for all cases).…”

“…Twist strain is accommodated by distortion of the amorphous regions and be re-orientation and partial destruction of the lamellae. It was further observed that annealing of the twisted fibers at 170°C for 15 minutes when held at constant length did not alter the degree of crystallinity, but stabilised the structure possibly due to better packing within the amorphous regions19 .…”

“…Both materials show similar DSC thermograms with a slight increase in the melting temperature of 1-2°C and a small decrease in the degree of crystallinity by 1-2% when compared with the as-received fiber. The similar phenomenon was observed from the recrystallization peaks (on cooling) of different samples while recrystallization enthalpies decreased for twisted fibers compared to the as-received one.Tsujimoto et al and co-workers have previously investigated the effect of twist and annealing on the microstructure of nylon 6 monofilaments19 . They observed that twisting slightly decreases the degree of crystallinity by partial destruction of lamellae.…”

Morphology modulation of artificial muscles by thermodynamic-twist coupling

“…Briefly, the spiral molecular chains were modulated by the thermodynamic relaxation in the twisted, coiled fibres (with coil length L 0 ). Twist insertion into a polymer fibre generates internal stress, and thermal annealing (at T ann ) produces a self-supporting artificial muscle, where heating-induced partial thermodynamic relaxation of molecular chains can inhibit fibre twist release [ 25 , 26 ]. This self-supporting artificial muscle can actuate under heating ( T act ), while for a sufficiently high T act , further relaxation of the molecular chains occurs, transforming the muscle into another muscle with a different return coil length ( L’ ) (Fig.…”