Enthalpy relaxation of unconstrained and constrained amorphous phase for low isotacticity polypropylene

The influence of crystallization on the amorphous phase of low isotacticity polypropylene (LT-PP) was studied by analyzing the enthalpy relaxation behavior using fast scanning calorimetry (FSC) and temperature modulated FSC. The high temperature shift of the enthalpy recovery peak and the glass transition temperature during crystallization was interpreted as a transformation of the unconstrained amorphous (UCA) to constrained amorphous (CA) phase. Enthalpy relaxation approach had the same result as the conventional heat capacity analysis of mobile amorphous (MA) phase as proposed by Wunderlich. Our study revealed that successive glass transition of rigid amorphous (RA) phase in a three-phase model and the gradual increase in the heat capacity of the MA phase in a two-phase model are experimentally indistinguishable above the crystallization temperature.