Formation of Stereocomplex Crystals in Fiber Processing of Racemate Polylactide
Takeshi Kikutani, Midori Takasaki, Motohiko Kuroda, Hiroshi Ito
Tokyo Institute of Technology
Japan
Keywords: racemate polylactide, melt spinning, stereocomplex crystal
Much attention has been devoted to polylactides (PLA) recently because the polymer can be produced from sustainable natural resources such as corn. PLA is also recognized as an
environment-friendly material because of its biodegradability. One of the drawbacks of this polymer is its low melting temperature. Therefore attempts have been devoted for the improvement of the dimensional stability of the PLA products at high temperature, which
includes the preparation of PLA/clay nano-composites.
PLA has asymmetric carbons in its main chain. Pure poly(L-lactide) (PLLA) or copolymers of L-lactide with a small amount of D-lactide are mainly used in the market. In general, PLLA crystallizes into two main crystalline forms, alpha-form and beta-form, both of which have the melting temperature of around 170oC. On the other hand, it was reported that racemate PLA, in which the same amount of PLLA and PDLA chains are mixed, forms stereocomplex crystals. The stereocomplex crystals have the melting temperature of 230oC, which is about 60 degrees higher than that of alpha- or beta-form crystals.
In this study, high-speed melt spinning and subsequent drawing and annealing of racemate polylactide (r-PLA) were carried out to obtain PLA fibers with good thermal stability. It was found that alpha-form crystals and stereocomplex crystals coexist in the as-spun fibers
obtained at the take-up velocities higher than 4 km/min. Amount of the stereocomplex crystals increased with an increase in the spinning speed and with a decrease in the extrusion temperature.
With the intention of preparing fibers only composed of highly oriented stereocomplex crystals, as-spun fibers of the r-PLA obtained at 1 and 6 km/in were subjected to drawing and annealing processes. The drawing and annealing of as-spun fibers obtained at 1 km/min, which are amorphous fibers, yielded the fibers structure composed of stereocomplex crystals. The crystalline orientation of these fibers, however, were low probably because of the occurrence of orientation relaxation while melting of the alpha-form crystals and re-crystallization into the stereocomplex crystals proceeded at the annealing temperature higher than 170oC. On the other hand, annealing of the 6 km/min fibers, which originally have a certain amount of stereocomplex crystals, yielded fiber structure with highly oriented stereocomplex crystals. These fibers showed high mechanical properties and good thermal stability.