Self-Assembly Of Polyoxometalate-Based Starlike Polymers Or Linear Polymers In Solvents Of Variable Quality

Nowadays more and more attention is being paid to star polymers because of their topologically branched nanostructures and practical applications in materials science and biological science, especially for the self-assembly of star polymers in different solvents. In order to observe self-assembled nanostructures of star polymers in the absence of staining, we need to introduce heavy metal particles, such as gold nanoparticles. In this article, we introduce polyoxometalate instead of heavy metal particle. The advantage of polyoxometalate is their multiple negative charges, which can be combined with cationic terminated polymers based on electrostatic interaction.Traditional self-assembly system mainly concentrated on small molecule surfactants and amphiphilic block copolymers. The driving forces behind these self-assembled nanostructures are essentially originated from a delicate competition of several interactions, including the ionic, hydrophilic or steric repulsion of the headgroups, hydrophobic attraction at the hydrocarbon-water interface, and entropic penalty for hydrophobic chain deformation. In a recent preliminary report, we described that POM-based supramolecular star polymers (PSPs) can self-assemble into unilamellar vesicles in the chloroform/methanol mixture solvents. In this article, we find that these totally hydrophobic polymers can self-assemble into free-standing sheets and oligolameller vesicles in the toluene/methanol mixture solvents. Moreover, close observation of TEM imaging reveals several intermediate nanostructures, which provide important mechanistic insights for this morphological evolution. The specific mechanism of the aggregate formation includes a delicate competition of tunable intra- and interstar steric repulsions and van der Waals attractions in the toluene/methanol mixture solvents with worsening quality.This article can be divided into two parts:In the first part, we use polyoxometalate-based supramolecular star polymers (PSPs) as a model of star polymers to study their intra-and interstar interactions and self-assembled behaviors in solvents of variable quality. They have an anionic polyoxometalate, which is closely encapsulated by a hydrophobic shell of cation terminated polystyrenes. These model stars can self-assemble to form unilamellar freestanding sheets in the toluene/methanol mixture solvent with a methanol volume ratio of 50%. When the methanol content increases to 75% or 80%, we find a coexistence of free-standing sheets with vesicles and several intermediates for the morphological evolution from sheet to vesicle. When further increasing the methanol content to 90%, pure vesicle phases form with unilamellar or oligolamellar features. All these nanostructures are packed by the totally hydrophobic starlike polymers, where the core-shell structures are retained. This is obviously different from the bilayered nanostructures formed by amphiphiles such as block copolymers and surfactants in selective solvents.In the second part, we give a research on the self-assembled nanostructures of linear polymers. Chemists have known that block copolymers can undergo a spontaneous self-assembly to generate spherical micelles, wormlike micelles, vesicles, and sheet-like aggregates. But in this part, we find that polystyrene can self-assemble into vesicles in the chloroform/methanol mixture solvents. The specific mechanism of the aggregate formation concerns a delicate competition of tunable intra-and inter-molecular steric repulsions and van der Waals attractions in the chloroform/methanol mixture solvents with worsening quality.