Dynamics of reversible melting revealed from frequency dependent heat capacity

Heat capacity of semi-crystalline polymers shows frequency dependence not only in the glass transition range. Also above glass transition and below melting temperature a frequency dependent heat capacity can be observed. The asymptotic value of heat capacity at high frequencies equals base-line heat capacity while the asymptotic value at low frequencies yield information about reversing melting. For polycarbonate (PC), poly(3-hydroxybutyrate) (PHB) and syndiotactic polypropylene (sPP) the asymptotic value at high frequencies can be measured by temperature-modulated DSC (TMDSC). For polycaprolactone (PCL) and sPP the frequency dependence of heat capacity can be studied in quasi-isothermal TMDSC experiments. The heat capacity spectra were obtained from single measurements applying multi-frequency perturbations (spikes in heating rate) like in StepScan™ DSC or rectangular temperature–time profiles. Actually, the dynamic range of commercial TMDSC apparatuses is limited and only a small part of the heat capacity spectrum can be measured by TMDSC. Nevertheless, comparison of measured base-line heat capacity with expected values from mixing rules for semi-crystalline polymers yield information about the formation (vitrification) and disappearance (devitrification) of the rigid amorphous fraction (RAF). For PC and PHB the RAF is established during isothermal crystallization while for sPP only a part of the RAF is vitrified during crystallization. Devitrification of the RAF seems to be related to the lowest endotherm.