Insertion-Crystallization-Induced Low-Temperature Annealing Peaks in Melt-Crystallized Poly(l-Lactic Acid)

Abstract

Annealing of poly(l-lactic acid) (PLLA) at temperatures lower than the temperature of primary melt-crystallization leads to appearance of distinct low-temperature annealing peaks in subsequently recorded calorimeter heating scans. Reason is continuation of crystallization by growth of crystals of lower stability as formed during primary crystallization. The assignment of low-temperature annealing peaks to crystallization is based on a robust analysis of the decrease of the heat capacity of the semicrystalline structure slightly above the glass transition temperature and its correlation with the corresponding enthalpy of crystallization. Comparison of the form of annealing peaks with classical enthalpy-recovery peaks obtained on devitrification of the glassy amorphous phase of semicrystalline PLLA supports the interpretation regarding the origin of annealing peaks.