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

Abstract

The effect of compressed CO 2 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 CO 2 at 2 MPa and temperature g65 °C and at pressures of 4-16 MPa and temperatures as low as 0 °C. At 2 MPa, the R crystal form is formed predominantly in the crystallized samples. The results also indicated that the PLLA-CO 2 system exhibited the property of retrograde vitrification.

Calorimetric analysis of the multiple melting behavior of melt-crystallized poly(l-lactic acid) with a low optical purity

“…It was reported that both the lowered equilibrium melting point and decreased crystallization rate could lead to the reduction of critical temperature for generating the PLLA a 0 -and a-form crystals [60]. A previous study has evidenced that a crystals rather than a 0 ones were induced in the PLLA with an optical purity of 88 % by compressed CO 2 at temperatures as low as 65°C [50]. In contrast, the molecular weight was found to hardly affect the temperature range for polymorphism of PLLA [14].…”

“…It is well known that the optical purity (OP) of PLLA strongly affects its crystallization kinetics and melting behavior [50][51][52][53][54]. The crystallization rate, crystallinity, and melting point decrease with a reduction of optical purity [52,53], and PLLA loses crystallizability when the level of OP is smaller than 76 % [54].…”

How high pressure CO2 impacts PLA film properties

“…Based on those considerations, there is a clear interest in studying the interactions between CO 2 and PLA, and the modifications that such molecule could induce in the PLA material, which is suitable as a substitute of PET. For now, most of the related literature is focused on the effects of supercritical CO 2 (or conditions near to it) on the PLA properties, such as foaming properties [16], thermal behavior [17][18][19], hydrophobic drug encapsulation [20,21] or swelling [22,23]. Nevertheless, there is not enough available information related to the effects of CO 2 at conditions lower than its supercritical point, which are often the conditions encountered in food packaging and food processing.…”

Structural changes and crystallization kinetics of polylactide under CO2investigated using high-pressure Fourier transform infrared spectroscopy

“…The effects of compressed CO 2 on the crystallization behavior of polymers have been extensively studied . Because of the plasticization effects of CO 2 , the glass transition temperature and melting temperature of polymers decrease, resulting in the shifting of the crystallization temperature to lower values.…”