Banded spherulites in poly(L-lactic acid): Effects of the crystallization temperature and molecular weight

The spherulitic morphology of pure poly(L-lactide) (PLLA) was investigated with polarized optical microscopy as a function of the crystallization temperature and molecular weight. After being melted at 210 degrees C for 3 min, samples were cooled quickly to designated temperatures for isothermal crystallization. It was shown for the first time that a clear banding-to-nonbanding morphological transition took place at a critical temperature for PLLA with a number-average molecular weight of 86,000. With the increasing molecular weight of the material, the spherulite growth rates decreased notably, and the band spacing decreased significantly. On the basis of the main-chain chirality in PLLA and the observation of a nonbanded spherulitic morphology in a certain temperature region, it was suggested that the crystallization temperature might have an effect on the relationship between the sense of lamellar twisting and the main-chain chiral structure in PLLA. (C) 2007 Wiley Periodicals, Inc.