Temperature-dependent Crystallization and Phase Transition of Poly(L-lactic acid)/CO2 Complex Crystals

Abstract

Semicrystalline polymers can crystallize in the unique crystalline polymorph and show different phase behaviors under the high-pressure CO2 treatment. Understanding such unique crystallization and phase transition behavior is of fundamental importance for the CO2-assisited processing of semicrystalline polymers. Herein, we investigated the polymorphic crystalline structure, phase transition, and structure-property relationships of poly(L-lactic acid) (PLLA) treated by CO2 at different pressures (1–13 MPa) and crystallization temperatures (Tc’s, 10–110 °C). PLLA crystallized in the PLLA/CO2 complex crystals under 7–13 MPa CO2 at Tc ≤ 50 °C but the common a crystals under the high-pressure CO2 at Tc ≥ 70 °C. Solid-state nuclear magnetic resonance analysis indicated that the PLLA/CO2 complex crystals possessed weaker interactions between the PLLA chains than the common a crystals. The PLLA/CO2 complex crystals were metastable and transformed into the thermally stable a crystals via the solid-to-solid route during heating or annealing at the temperature above 50 °C. The complex crystals of PLLA produced at low Tc was more ductile than the a crystals due to the lower crystallinity and the plasticizing effect of CO2.