Polylactide Phase Transition in Acetone. A High‐Sensitivity Differential Scanning Calorimetry Study

ABSTRACT Polylactides (PLA) are insoluble in acetone under normal conditions. Upon heating, the PLA solubility in acetone increases sharply up to complete dissolution. The PLA transition from the solid semi‐crystalline state to the soluble one is first studied by the high‐sensitivity differential scanning calorimetry. The PLA dissolution thermograms are obtained. Thermodynamic characteristics of the dissolution, the transition temperature, and enthalpy ( T t = 51.0 ± 0.3 °C and 22 ± 2 J g −1 ), are shown to be independent of the polymer content in the system (1–7 mg mL −1 ). The dissolution thermograms are transformed to the temperature dependences of the relative PLA solubility (0 ≤ α( T ) ≤ 1). These dependences are well approximated by the model of phase equilibrium between the solid polymer and its ideal solution (correlation coefficient of 0.999). The parameters of the model, the apparent transition enthalpy and the apparent transition temperature, do not depend on the PLA concentration: Δ m h * = 420 ± 50 kJ mol −1 , and 324.9 ± 0.2 K. As a result of the consistent thermodynamic analysis of the calorimetric data, the PLA crystallization degree (20%) and the average molecular weight of the crystalline domains of PLA macromolecules (4000 Da) are estimated.