Multiple melting behaviour of poly(l-lactide-co-glycolide) investigated by DSC

The melting behaviour of poly(l-lactide-co-glycolide) (PLGA) isothermally crystallized from the glassy state was investigated by differential scanning calorimetry (DSC). Triple, double or single melting endotherms were observed in subsequent heating scans for the samples isothermally crystallized at different temperatures. These endothermic peaks were labelled as I, II, and III for low-, middle-, and high-temperature melting endotherms, respectively. The observations of the independence of endotherm III from the crystallization temperature, and the heating rate dependence of endotherm III, indicated that endotherm III was due to the remelting of recrystallized lamellar during a heating scan. The influence of crystallization time on the melting behaviour of PLGA showed that secondary crystallization appeared even at an early stage of crystallization, since endotherm II was still developing with time after the appearance of endotherm I. Further investigation showed that the peak temperature of endotherm I increased linearly with the logarithm of the crystallization time. It is suggested that endotherm II is attributed to the melting of the primary lamellae, while endotherm I is assigned to the melting of secondary lamellae.