High Strength and High Thermal Stability of Polylactide Fibers with Stereocomplex Crystal Obtained by High-Speed Melt Spinning and Subsequent Drawing and Annealing Processes

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To clarify the effect of molecular orientation on crystal structure change of racemate polylactide, structure formation of fibers obtained by high-speed melt spinning was investigated during heating and cooling processes. Changes in crystal structure of racemate polylactide fibers and films were analyzed by in-situ WAXD, DSC, and birefringence measurements during heating and cooling processes. The co-existence of α-form and stereocomplex crystals was observed above the take-up velocity of 4 km/min, and reflection intensity of stereocomplex crystal increased with an increase in the take-up velocity. During heating process, α-form crystals of as-spun fibers started to increase at around 80℃ which was independent of molecular orientation of as-spun fibers. The reflection intensity of stereocomplex crystal of as-spun fibers appeared when α-form crystal started to decrease. Existence of the α-form crystals in these fibers was confirmed even at melting temperature of α-form crystal of 170℃. The development of α-form crystal was also observed in the fibers drawn at 70℃. Birefringence of drawn fibers showed the highest value at 2 km/min, and decreased with increasing take-up velocity. On the other hand, birefringence of drawn decreased by annealing at 190℃ and became close to the birefringence of fibers obtained by directly annealing the high-speed spun fibers at the same temperature.