Self-organization of diblock and triblock copolymers of poly(L-lactide) and poly(oxyethylene) into nanostructured bands and their network system. Proposition of a doubly twisted chain conformation of poly(L-lactide)

Monodispersed poly(L-lactide)-block-poly(oxyethylene) (PLLA-PEG) and poly(L-lactide)block-poly(oxyethylene)-block-poly(L-lactide) (PLLA-PEG-PLLA) were synthesized, and their nanoparticles were successfully prepared in an aqueous medium. Both particles of the di- and triblock copolymers were nanoscale in size, consisting of hydrophobic PLLA. and hydrophilic PEG in the core and shell, respectively. The size of the PLLA-PEG-PLLA particles increased with increasing polymer concentration in the medium, while that of the PLLA-PEG particles was constant irrespective of the polymer concentration. When these particles were cast and heat-treated on a mica substrate, band structures having 1-2 nm thickness were formed on the surface for both copolymers as observed by atomic force microscopy. The band structures of PLLA-PEG showed parallel alignment with each other, while those of PLLA-PEG-PLLA formed a characteristic network system resembling the neuron system observed in animal tissues. Transmission electron micrographs of these band structures by using a gelatin-exfoliation method confirmed that the bands had a width of 10 nm or less in both cases. The nanobeam diffraction diagram of the band structures suggested that the main part of the band structures is composed of PLLA crystals whose c axis (molecular axis) is perpendicular to the substrate surface, while the a and b axes are perpendicular and parallel to the longitudinal direction of the bands, respectively. On the basis of these results, a plausible mechanism for the self-organization of block copolymers is proposed in addition to a model crystal structure of PLLA.