Enthalpy of melting of alpha'- and alpha-crystals of poly (L-lactic acid)

Literature values of the enthalpy of melting (Dhm°) of 100 % crystalline poly (L-lactic acid) (PLLA) vary in a wide range (82-203 J g-1). In most cases, such values were determined without taking into account the presence of alpha'- or alpha-crystal modifications, or the influence of D-lactic acid units in the analyzed poly (lactic acid). The bulk enthalpy of melting was determined for the stable orthorhombic a form in only few studies, and thermodynamic properties of the metastable alpha'-modification are not available yet. In the present work, the temperature dependence of Dhm° for both the alpha'- and alpha-crystal polymorphs of PLLA are determined, by combining differential scanning calorimetry and X-ray diffraction. It is proven that the enthalpies of melting of alpha'- and alpha-crystals of 100% crystalline PLLA are different. At the respective melting temperatures of 150 °C and 180 °C, the Dhm° values of the alpha'- and alpha-forms are 107 and 143 J g-1. At identical temperature, the enthalpy of melting of alpha'-crystals is approximately 25 J g-1 lower than that of the alpha-form. This observation is rationalized taking into account the presence of conformational defects in the disordered alpha'-modification.