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,. 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 degrees C with nonlinear Hoffman-Weeks method. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 702-708, 2010