Banded spherulites in PS-PLLA chiral block copolymers

In this study, thin-film samples of semicrystalline block copolymer (BCP), polystyrene-b-poly(L-lactide) (PS-PLLA), were prepared by solution-casting from dichloromethane solution (1 wt % of PS-PLLA). As observed by polarized light microscopy (PLM), banded spherulites can be found in the PS-PLLA thin-film samples crystallized from melt. By contrast, no banded spherulites can be obtained in crystallized PLLA homopolymer under similar crystallization conditions. However, the formation of banded spherulites for PLLA homopolymer can be achieved by blending compatible poly(epsilon-caprolactone) (PCL) homopolymer. Also, blends of PS-PLLA and poly(D-lactide) (PDLA) were prepared; no banded spherulites can be obtained in crystallized blends having equimolar PLLA and PDLA (namely, racemic blends). As a result, we suggest that the effect of chirality on polylactide crystallization is necessary for the induction of cooperative lamellar twisting along the radial growth direction to form banded spherulites. Moreover, the chiral effect might be enhanced by the introduction of diluents, such as compatible PCL homopolymer that promotes the formation of banded spherulites; this is referred to the effect of diluents. In contrast to the PLLA/PCL blends, a significant decrease in the band spacing of PS-PLLA banded spherulites (namely, the increase in twisting power) can be found. We speculate that the incompatible PS block chemically jointed with the PLLA block may intensify the effect of diluents so as to increase the imbalanced stress at crystalline lamellar fold surfaces for lamellar twisting. The morphological evolution of the banded spherulites was further investigated by using transmission electron microscopy and field-emission scanning electron microscopy. A hypothetic model was thus proposed to reveal the mechanism for the formation of curved crystalline lamellae in crystallized polylactide BCP.