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 ( X c ) within a definite range. Below the lower X c limit, only the low endotherm corresponding to the free amorphous phase was observed; above the upper X c 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 X c range increased with an increase in isothermal crystallization temperature from the melt. Long‐term physical aging at 52 °C, which did not affect X c , allowed the evolution of the free amorphous phase to the constrained amorphous phase in PLLA with X c within the definite range. The effects of physical aging at various temperatures on the enthalpy recovery endotherms were also investigated. Copyright © 2011 Society of Chemical Industry