Direct formation of banded spherulites in poly(l-lactide) from the glassy state: Unexpected synergistic role of chain structure and compressed CO2

Abstract

Abstract Ring-banded spherulites have never been formed directly from the glassy state of poly( l -lactide) (PLLA), particularly in case of its copolymers containing some stereodefective D-lactic units, which kinetically hinder the crystallization. By utilizing compressed CO 2 , banded spherulites have been directly obtained by treating the glassy PLLA containing 4–6% d -isomer. Specifically, ring-banded spherulites with regular rings, consisting of periodically parallel-aligned lamellar crystals twist in the left-handed sense, were exclusively formed in CO 2 of high pressures (8−16 MPa) and low temperatures below the normal glass transition temperature ( T g ). Owing to the relatively high d -isomer content, PLLA possessed the adequate resistance to crystal nucleation during melt-quenching, which is inherently favorable for obtaining a fully glassy sample with negligible crystal nuclei and plays a significant role in promoting the formation of banded spherulites. The formation of banded spherulites in PLLA from the glassy state is plausibly attributed to a synergistic effect of the specific molecular structure and the suitable CO 2 atmosphere (i.e., low temperatures and high pressures). The unbalanced stress at opposite fold-surfaces responsible for the lamellar twisting was intensified by the d -isomer and released by the compressed CO 2 , which eventually resulted in cooperatively twisted lamellae for constituting the ring-banded spherulites.