Infrared imaging of ring-banded spherulites formed in poly(L-lactide)-poly(ethylene glycol) diblock copolymer:...

Poly(L-lactide)-poly(ethylene glycol) (PLLA-PEG) diblock copolymer film was observed to exhibit ring-banded spherulite morphology when crystallized from melt. Polarized optical microscopy (POM) and atomic force microscopy (AFM) revealed that the ring-banded structure is composed of alternating concave and convex bands, which correspond to flat-on lamellae showing layered terrace-like morphology and edge-on lamellae showing fibrillar texture, respectively. The composition distribution and chain orientation within the ring-banded spherulite were further analyzed by Fourier transform infrared (FT-IR) imaging and polarized FT-IR. The data indicated that the concave bands are PLLA-rich domains, where PLLA chains are perpendicular to both the substrate and the radial direction of the spherulite, whereas the convex bands are rich in PEG, where the PLLA chains orient parallel to the substrate and the tangential direction of the spherulite. The orientation pattern of the PLLA segments in the concave and convex bands is attributed to the periodic twisting of PLLA lamellae along the radial growth direction in the formation process of the spherulite.