Dual glassy relaxations in physically aged semi‐crystalline poly( L ‐lactic acid)

Semi‐crystalline poly(L‐lactic acid) (PLLA) was physically aged below the glass transition temperature for various times to investigate its amorphous phase behavior. During a differential scanning calorimetry heating scan, dual enthalpy recovery endotherms were found to appear in the glass transition region of PLLA, aged at 52 °C, of a particular degree of crystallinity (Xc) within a definite range. Below the lower Xc limit, only the low endotherm corresponding to the free amorphous phase was observed; above the upper Xc limit, only the high endotherm corresponding to the constrained amorphous phase was observed. Dual tan δ peaks in dynamic mechanical analysis confirmed the coexistence of the dual amorphous phases. Both lower and upper limits of the Xc range increased with an increase in isothermal crystallization temperature from the melt. Long‐term physical aging at 52 °C, which did not affect Xc, allowed the evolution of the free amorphous phase to the constrained amorphous phase in PLLA with Xc within the definite range. The effects of physical aging at various temperatures on the enthalpy recovery endotherms were also investigated.