Temperature-dependent Crystallization and Phase Transition of Poly(L-lactic acid)/CO2Complex Crystals

Semicrystalline polymers can crystallize in the unique crystalline polymorph and show different phase behaviors under the high-pressure CO(2)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 CO(2)at different pressures (1-13 MPa) and crystallization temperatures (T-c's, 10-110 degrees C). PLLA crystallized in the PLLA/CO(2)complex crystals under 7-13 MPa CO(2)atT(c)<= 50 degrees C but the commonacrystals under the high-pressure CO(2)atT(c)>= 70 degrees C. Solid-state nuclear magnetic resonance analysis indicated that the PLLA/CO(2)complex crystals possessed weaker interactions between the PLLA chains than the commonacrystals. The PLLA/CO(2)complex crystals were metastable and transformed into the thermally stableacrystalsviathe solid-to-solid route during heating or annealing at the temperature above 50 degrees C. The complex crystals of PLLA produced at lowT(c)was more ductile than theacrystals due to the lower crystallinity and the plasticizing effect of CO2.

Automated summary

Through treating PLLA with CO2 at different pressures and temperatures, this study found that PLLA can transform between complex crystalline states and conventional crystalline states, with the complex crystals having weaker interchain interactions. Additionally, the complex crystals transform into thermally stable crystals above 50 degrees Celsius, and they generally exhibit lower crystallinity and a plasticizing effect of CO2.