Crystallization-Induced Orientation for Microstructures of Poly(l-lactide)-b-poly(ε-caprolactone) Diblock Copolymers

The formation of large-sized, well-oriented microdomains of poly(l-lactide)-b-poly(ε-caprolactone) (PLLA−PCL) was successfully achieved by using different crystalline substrates including benzoic acid (BA) and hexamethylbenzene (HMB), whereas spherulitic crystalline texture was obtained on amorphous substrates. Crystallization-induced orientation of PLLA−PCL microstructures is attributed to directional crystallization of eutectic type between crystalline PLLA and substrate, regardless of microphase separation. Surprisingly, a flat-on crystalline morphology (i.e., with the chain axis normal to the substrate) was obtained, as evidenced by single-crystal-like [001] zonal electron diffraction (ED). By contrast, an edge-on crystalline morphology (i.e., with the chain axis parallel to the substrate) was observed for PLLA homopolymer under the same condition. The crystallographic organization of PLLA−PCL microdomains as observed by transmission electron microscopy, ED, and scanning probe microscopy suggests that the growth direction of PLLA crystals is along the a-axis direction, which suggests that the growth direction of PLLA crystals is along the a-axis direction that corresponds well with the preferential growth direction (i.e., b-axis direction) of BA or HMB crystals. As a result, the phase-separated lamellar morphology consists of an alternation of a flat-on layer of PLLA lamellae and of an amorphous layer of PLLA and PCL components.