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