Macromolecular Organization of Poly(L-lactide)-block-Polyoxyethylene into Bio-Inspired Nano-Architectures

Abstract

Block copolymers create various types of nano-structures, e. g., spheres, rods, cubes, and lamellae. This review discloses the dynamic macromolecular organization of block copolymers comprising poly(L-lactide) (PLLA) and poly(oxyethylene) (PEG) that allows to simulate elaborate biological systems. The block copolymers, AB- (PLLA-PEG) and ABA-type (PLLA-PEG-PLLA), are synthesized by ordinary lactide polymerization to have a controlled block length. They are dispersed into an aqueous medium to prepare nano-scale particles, consisting of hydrophobic PLLA and hydrophilic PEG in the core and shell, respectively. Then, the particles are placed on a flat substrate by the casting method. The particles are detected as discoids by AFM, having shrunk with loss of water. Heat-treatment of these particles at 60°C (above Tg of PLLA) gives rise to a collapse into small fragments, which then aggregate into bands with nano-size width and thickness. The PLLA-PEG bands align parallel to each other, while the PLLA-PEG-PLLA bands form a characteristic network resembling the neuron system created in animal tissue. As analyzed by TEM diffraction, each is composed of α-crystal of PLLA whose c-axis (molecular axis) is perpendicular to the substrate surface. Based on this fact, a doubly twisted chain structure of PLLA is proposed in addition to a plausible mechanism for the self-organization of the block copolymers. Derivatives of the PLLA-PEG block copolymers can form far more interesting nano-architectures. An equimolar mixture of enantiomeric copolymers, PLLA-PEG-PLLA and PDLA-PEG-PDLA, forms a hydrogel that is thermo-responsive. The terminal-modified poly(L-lactide)-block-polyoxyethylene monocinnamate (PLLA-PEG-C) forms a highly stabilized nanofiber by the photo-reaction of the cinnamates placed in the outer layer of the nanobands.