Effect of the cast-solvent on the morphology of cast films formed with a mixture of stereoisomeric poly(lactic acids)

Semicrystalline films were prepared by casting chloroform (CHL), acetonitrile (ACN) or tetrahydrofuran (THF) solutions of a mixture (mix-poly(lactic acid) (PLA)) of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). All of the films involve stereocomplex crystals (sc-PLA) of PLLA and PDLA, but no crystals of PLLA or PDLA. The origin of the marked cast-solvent dependence of the morphology was studied by assuming that the structure of sc-PLA would be constructed by the balance of the interaction energy E-1 between the PLLA and PDLA chains in solution, and the interaction energy E-2 between PLLA or PDLA chains and solvent molecules. With the relation between E1 and E2, the scheme for the formation of sc-PLA was classified into two categories, Category-I and -II. In Category-I, the relation E-1>E-2 holds over the entire concentration range, and in Category-II, the relation E-1