Thermal Behavior of Poly(l-lactide) Having Low l-Isomer Content of 94% after Compressed CO2 Treatment

Abstract

The effect of compressed CO2 treatment on the thermal behavior of poly(l-lactide) (PLLA) with low l-isomer content of 94% was studied by using differential scanning calorimetry (DSC) and temperature modulated DSC. It was shown that the treated samples displayed rich thermal transition signals during DSC heating, which is differ from the PLLA having high l-isomer content, including enthalpy relaxation, endothermic annealing, melting−recrystallization process, and cold crystallization. The results suggested that the crystalline phase obtained has less perfection and low crystallinity regardless of the treatment conditions because of the low crystallizability of the PLLA. This PLLA was induced to crystallize after treatment under CO2 at 2 MPa and temperature ≥65 °C and at pressures of 4−16 MPa and temperatures as low as 0 °C. At 2 MPa, the α crystal form is formed predominantly in the crystallized samples. The results also indicated that the PLLA−CO2 system exhibited the property of retrograde vitrification.