The bulk enthalpy of melting of α’-crystals of poly (l-lactic acid) determined by fast scanning chip calorimetry

Abstract

Related to the wide scatter of information in the literature, recently the bulk enthalpy of melting of α-crystals of poly (l-lactic acid) (PLLA) has been re-determined by correlating the crystallinity-dependent heat capacity at a temperature slightly higher than the glass transition temperature and the corresponding enthalpy of melting [Macromol. Rap. Comm. 2022, 43, 2200148], using fast scanning chip calorimetry (FSC). For α-crystals, a value of 104 J/g is suggested, valid at 190 °C. Here, the initial study is expanded such to obtain the bulk enthalpy of melting of PLLA α’-crystals, containing conformational defects and forming at lower crystallization temperatures than the α-phase. Samples containing different amount of α’-crystals were generated by varying the time of isothermal crystallization at 90 °C. Extrapolation of the obtained relationship between the heat capacity at the crystallization temperature, that is, the crystallinity, and the enthalpy of melting on subsequent fast heating, suppressing the transition into α-crystals and simplifying therefore the analysis of the melting process of the formed α’-crystals, toward fully crystallized PLLA yields a bulk enthalpy of melting of 72 J/g at the thermal stability limit of α’-crystals of 150 °C. The used experimental approach to obtain the bulk enthalpy of melting of polymer crystals relies on the presence of a two-phase structure/absence of a glassy rigid amorphous fraction at the temperature of analysis of the crystallinity via the heat capacity (90 °C). Annealing experiments slightly below the analysis temperature support this assumption by absence of enthalpy-recovery peaks on subsequent heating.