Crystallization of Star-Shaped Poly(L-lactide)s with Arm Chains Aligned in the Same Direction in Two-Dimensional Crystals in a Langmuir Monolayer

Polylactide (PLA) crystallizes to form extendedchain crystals in a Langmuir monolayer because crystallization is accelerated on the water surface. This is a unique situation where chain packing can be analyzed by simply measuring the lamellar thickness. Herein, star-shaped poly(L-lactide)s (PLLAs) with 2-12 arms were synthesized through the polymerization of L-lactide with various polyols as initiators, and their crystallization behavior in a monolayer was studied via atomic force microscopy. The PLLAs comprising 2-4 arms crystallized with all arms aligned in the same direction and being folded at the central polyol unit. Meanwhile, the PLLAs comprising 6 and 12 arms crystallized with both halves of the arms extended from the center to the opposite directions, most likely due to the steric hindrance of the crowded arms. Considering that the PLLAs crystallized from a once-formed condensed amorphous state during compression, they have a strong tendency to crystallize with the arms aligned in the same direction. The crystallization rate of star-shaped PLAs is known to reduce compared with that of a linear PLA even if the number of arms is as few as 2. This should be closely related to the unique crystallization behavior of the star-shaped PLLAs with the arms aligned in the same direction.

Automated summary

Polylactide (PLA) forms extended-chain crystals in a Langmuir monolayer due to the acceleration of crystallization on the water surface. Star-shaped poly(L-lactide)s (PLLAs) with 2-12 arms were synthesized and their crystallization behavior in a monolayer was studied. PLLAs with 2-4 arms showed alignment in the same direction and folding at the central polyol unit, while PLLAs with 6 and 12 arms showed extension in opposite directions, likely due to steric hindrance. Star-shaped PLLAs have a reduced crystallization rate compared to linear PLA, possibly because of their unique crystallization behavior with aligned arms.