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

Abstract

AbstractThe isothermal cold crystallization and melting behaviors of poly(L‐lactic acid)s (PLLAs, weight average molecular weight, Mw, 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 Mw, reached a maximum at Mw of ca. 21,000 and then decreased again. The crystallinity of PLLA can be controlled in the range 30–50% by crystallization temperature (Tc) and time to fulfill the requirement of subsequent solid state polycondensation. The melting behavior strongly depends on Tc. The samples crystallized at high Tc melted with a single peak but those crystallized at low Tc melted with double peaks. The higher melting point (TmH) kept almost constant and the lower melting point (TmL) increased clearly with Tc. But the TmL changed in jumps and a triple melting peak appeared at the vicinity of a characteristic crystallization temperature Tb, possibly because of a change of crystal structure. The equilibrium melting temperature of PLLA with Mw of 21,300 was extrapolated to be 222°C with nonlinear Hoffman‐Weeks method. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010