Giant Vesicles of Amphiphilic Diblock Copolymer with Branched Side Chains

Abstract

Biomolecules of lipids and proteins optimize the hydrophobicity to precisely form their three-dimensional structures. The branching of hydrocarbon chains is appropriate for fine-tuning the hydrophobicity of the molecules because it provides a slight reduction in hydrophobicity. This paper describes the morphological changes of vesicles formed by an amphiphilic diblock copolymer supporting branched side chains in the hydrophobic block to elucidate the steric effect of the side chains on the morphology. The vesicles consisting of poly(methacrylic acid)-<i>block</i>-poly(isopropyl methacrylate-<i>random</i>-methacrylic acid), PMAA-<i>b</i>-P(iPMA-<i>r</i>-MAA), were obtained by the polymerization-induced self-assembly using the photo-nitroxide mediated controlled/living radical polymerization (photo-NMP) in an aqueous methanol solution (methanol/water=3/1 <i>v</i>/<i>v</i>). PMAA-<i>b</i>-P(iPMA-<i>r</i>-MAA) with a very high ratio of the iPMA units produced spherical vesicles. As the iPMA ratio decreased, the vesicles shrank, expanded again, and changed into sheets. The chain length of the hydrophobic block also dominated the morphology. For the high iPMA ratio constant, the copolymers with a short block produced two domains of micron-sized spherical vesicles and unspecific nano-sized particles fused. An increase in the block length changed the nanoparticles into nanospheres, accompanied by an increase in the number of micron-sized spherical vesicles. By further extending the block length, most of the nanospheres grew into micron-sized spherical vesicles. On the other hand, for the low iPMA ratio constant, an increase in the length of the hydrophobic chain changed the unspecific nanoparticles sheets into aggregates with an inverted hexagonal phase reticularly perforated. The PMAA-<i>b</i>-P(<i>n</i>-propyl methacrylate-<i>r</i>-MAA) copolymer produced flexible sheets with a smooth surface without any pores, while PMAA-<i>b</i>-P(methyl methacrylate-<i>r</i>-MAA) provided rods of laminated sheets. It was found that the formation of the inverted hexagonal phase was due to the steric repulsion of the bulky isopropyl groups in the hydrophobic blocks. These findings indicate that the morphology of the vesicles is manipulated not only by the hydrophobicity of the copolymer, but also by the bulkiness of the branched side chain in the hydrophobic block.