Intriguing morphology transformation due to the macromolecular rearrangement of poly(L-lactide)-block-poly(oxyethylene):: from core-shell nanoparticles to band structures via fragments of unimolecular size

We report on anomalous structural organization from core-shell nanoparticles of a hydrophobic/hydrophilic diblock copolymer consisting of semi-crystalline poly(L-lactide) (PLLA) and poly(oxyethylene) (PEG). The copolymer was synthesized and suspended in an aqueous medium to prepare its core-shell particles. The resultant nanoparticles were spread on a germanium substrate to trace the particle aggregation by atomic force microscopy (AFM) and FT-IR spectroscopy. On the substrate surface, the core-shell particles were found to change their shape into disks, with the PLLA blocks being slightly crystallized. When heated to 60 degreesC, these disk-like aggregations burst into small fragments and then turned to band structures. On the other hand, any ordered structure was not. observed when a solution of PLLA-PEG was cast on the surface. In the freeze-dried sample of the suspension, it was found that a lamellar microphase-separated structure was created with crystallization of the PLLA blocks by annealing. The lamella thickness analyzed by the small- and wide-angle X-ray scatterings of this sample was reasonably correlated with the width of the band structures formed on the germanium surface. It is therefore concluded that the formation of the regular band structures can be guided by both the phase separation and crystallization behaviors of the semicrystalline block chains of PLLA-PEG. (C) 2000 Elsevier Science Ltd. All rights reserved.