Determination of the equilibrium enthalpy of melting of two-phase semi-crystalline polymers by fast scanning calorimetry

Abstract

The equilibrium enthalpy of melting ΔHm0 [J·g−1] is an extrapolated thermodynamic quantity attributed to crystallizable macromolecules and widely used to characterize polymers in their semi-crystalline state, for it allows estimating the degree of crystallinity by direct comparison with the enthalpy of melting obtained from differential scanning calorimetry. ΔHm0 is typically obtained by cross-comparing the results obtained by at least two techniques. This work proposes a simplified experimental protocol to determine ΔHm0 by the use of Fast Scanning Calorimetry (FSC). This approach applies to any crystallizable polymer for which a specific microstructure can be obtained (i.e. a two-phase semi-crystalline microstructure with a negligible amount of rigid amorphous fraction) and that can also be quenched to its fully amorphous state. Such a two-phase microstructure can be obtained on nanoscale samples through an annealing process performed in situ on the FSC sensor at crystallization temperatures as close as possible to the melting temperature. The enthalpy of melting is then evaluated from the two-phase model for different crystallization times (i.e. different crystallinities) and the ΔHm0 is obtained by extrapolating the data to the 100% crystalline state. This procedure was applied on samples whose ΔHm0 values are already available in the literature, but also on more recent biobased polyesters whose thermal properties are still under investigations.