Ultrathin films (12–125 nm)
of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blends
of different compositions have
been crystallized between 180 and 210 °C, i.e., above the melting
point of each polymer crystallized separately. The overall crystal
shape depends on the temperature, film thickness and ratio of the
two polyenantiomers in the blends. In nonequimolar blends, lamellae
show curvatures, and the sense of the curvature is determined by the
chirality of the polyenantiomer in excess, blend ratio, film thickness,
crystallization temperature and lamellar orientation (flat-on or edge-on).
The curvature of the stereocomplex lamellae is ascribed to the unequal
amount of PLLA and PDLA segments at the crystal growth front, creating
an unbalanced mechanical stress at the chain folding surfaces which
can be released by a curvature of the growth tip