Reversing and Nonreversing Heat Capacity of Poly(Lactic acid) in the Glass Transition Region by Temperature-modulated Differential Scanning Calorimetry

摘要

A study of the glass transition region of an amorphous poly(lactic acid) (PLA) is performed withrntemperature-modulated differential scanning calorimetry (TMDSC) and standard differentialrnscanning calorimetry (DSC). The total, reversing, and non reversing heat capacities of PLA withrndifferent thermal histories were evaluated on heating, using TMDSC with an underlying heating raternand quasi-isothermal TMDSC. The resulting information on hysteresis, in form of enthalpyrnrelaxation, and the glass transition measured as the total heat capacity can be separated into thernreversing and non reversing heat-capacity components. The glass transition temperature, from thernreversing heat capacity and the enthalpy relaxation peaks from the non reversing component werernboth shifted to higher temperature for more slowly cooled (or aged) PLA. The activation energy andrnrelaxation time depends on the thermal pretreatment of the amorphous PLA. All quantitativernthermal analyses in the glass transition region use baselines derived from the extrapolated,rnvibrational heat capacity of the solid, based on low-temperature measurements, and the extrapolatedrnheat capacity of the liquid, based on measurements at high temperatures.