Development of Stereocomplex Crystal of Polylactide in High-Speed Melt Spinning and Subsequent Drawing and Annealing Processes

Abstract

High-speed melt spinning of racemate polylactide (r-PLA), which is a blend of equal amounts of poly(l-lactide) and poly(d-lactide) molecules, was performed up to the take-up velocity of 7.5 km/min. In the fiber structure analysis, particular attention was paid to the formation of stereocomplex crystals, because this crystal form has a melting temperature about 60° higher than the homocrystals. It was found that highly oriented and highly crystallized fibers containing the α-form and stereocomplex crystals were obtained when the take-up velocity exceeded about 4 km/min. The amount of stereocomplex crystal was higher under the spinning conditions of higher take-up velocity, lower throughput rate, and lower extrusion temperature. Under these conditions, higher tensile stress can be applied to the spinning line, and therefore, the orientation-induced crystallization is promoted. Annealing of the fibers obtained at high-take-up velocities, such as 6 km/min, which already have the crystalline structure with a certain amount of stereocomplex crystal, at a temperature between the melting temperatures of α-form and stereocomplex crystals, yielded the fiber structure mainly consisting of highly oriented stereocomplex crystal. The annealed fibers showed fairly high mechanical properties and good thermal stability.