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

Literature values of the enthalpy of melting (Δ h m °) 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 α′- or α-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 α form in only few studies, and thermodynamic properties of the metastable α′-modification are not available yet. In the present work, the temperature dependence of Δ h m ° for both the α′- and α-crystal polymorphs of PLLA are determined, by combining differential scanning calorimetry and X-ray diffraction. It is proven that the enthalpies of melting of α′- and α-crystals of 100% crystalline PLLA are different. At the respective melting temperatures of 150 °C and 180 °C, the Δ h m ° values of the α′- and α-forms are 107 and 143 J g −1 . At identical temperature, the enthalpy of melting of α′-crystals is approximately 25 J g −1 lower than that of the α-form. This observation is rationalized taking into account the presence of conformational defects in the disordered α′-modification. [ABSTRACT FROM AUTHOR]