The Origin of Annealing Peaks in Semicrystalline Polymers: Enthalpy Recovery or Melting?

The present work provides unambiguous evidence that annealing peaks in differential scanning calorimetry heating experiments of semicrystalline polymers are caused by crystallization and not enthalpy relaxation of a rigid amorphous fraction (RAF). This conclusion is derived by analysis of the heat capacity of poly(butylene succinate) slightly above the glass-transition temperature, which decreases during stepwise isothermal annealing at temperatures lower than the crystallization temperature, thus suggesting a further solidification of the system but not relaxation of the RAF toward a liquid-like behavior. Rather, quantitative correlation of the enthalpy of crystallization/annealing with the heat capacity reveals that in the late stage of the initial crystallization process and during additional low-temperature annealing, the RAF increases due to secondary crystallization.