Isothermal cold crystallization and melting behaviors of poly(L‐lactic acid)s prepared by melt polycondensation

The isothermal cold crystallization and melting behaviors of poly(L ‐lactic acid)s (PLLAs, weight average molecular weight, M_w, 6000–80,000) prepared via melt polycondensation were studied with differential scanning calorimeter in this work. It is found that the crystallization rate increased with decreasing M_w, reached a maximum at M_w of ca. 21,000 and then decreased again. The crystallinity of PLLA can be controlled in the range 30–50% by crystallization temperature (T_c) and time to fulfill the requirement of subsequent solid state polycondensation. The melting behavior strongly depends on T_c. The samples crystallized at high T_c melted with a single peak but those crystallized at low T_c melted with double peaks. The higher melting point (T_mH) kept almost constant and the lower melting point (T_mL) increased clearly with T_c. But the T_mL changed in jumps and a triple melting peak appeared at the vicinity of a characteristic crystallization temperature T_b, possibly because of a change of crystal structure. The equilibrium melting temperature of PLLA with M_w of 21,300 was extrapolated to be 222°C with nonlinear Hoffman‐Weeks method. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010