Enthalpy of melting of α′- and α-crystals of poly(L-lactic acid)

Literature values of the enthalpy of melting (Delta h(m)degrees) 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 alpha 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 Delta h(m)degrees 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 degrees C and 180 degrees C, the Delta h(m)degrees 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 251 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. (C) 2015 Elsevier Ltd. All rights reserved.