Solvent-induced crystallization behaviors of PLLA ultrathin films investigated by RAIR spectroscopy and AFM measurements.

The crystallization of poly(L-lactide acid) (PLLA) ultrathin films induced by different solvents was investigated using reflection-absorption infrared (RAIR) spectroscopy and atomic force microscopy (AFM). Irregular PLLA dendrite lamellae grew in the flat-on orientation with dichloromethane solvent before being redissolved after longer induction times owing to the strong interaction between the PLLA segments and solvent molecules. Faster formation of PLLA spherulites was induced with acetone than with dichloromethane, and these remained unchanged with increasing induction time because of the polarity difference between the PLLA segments and acetone molecules. PLLA ultrathin films could not be induced to crystallize using chloroform because of the very strong interactions between the chloroform (CHCl3) molecules and PLLA amorphous chains, which caused the CHCl3 solvent molecules to rapidly permeate the PLLA random coils and dissolve the amorphous chains. These phenomena are attributed to solvent-specific competition between solvent-induced crystallization and dissolution effects in PLLA ultrathin films, which ultimately leads to the higher degree of crystallinity obtained with acetone than with dichloromethane.