STEREOCOMPLEX FORMATION BETWEEN ENANTIOMERIC POLY(LACTIC ACID)S .3. CALORIMETRIC STUDIES ON BLEND FILMS CAST FROM DILUTE-SOLUTION

Blend films were obtained by casting mixed solutions of poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA). Crystallization in these films was studied by differential scanning calorimetry (DSC). When the casting solution was dilute (1 g/dL), racemic crystallites of PDLA and PLLA were preferentially produced, as long as either PDLA or PLLA had a low molecular weight, e.g., 5 x 10(3). In contrast, crystallites of PDLA or PLLA homopolymers were predominantly formed when the molecular weight of both polymers was high, e.g., 1 x 10(5). Racemic crystallization, however, occurred more predominantly than homopolymer crystallization even for the pair of PDLA and PLLA with the high molecular weight when the solution was aged for a long time at 10 g/dL prior to casting. On the basis of these DSC findings, a mechanism is proposed for the crystallization of enantiomeric poly(lactic acid)s when they are cast from dilute solution.