Hierarchical structures are commonly observed in nature and possess unique properties. The fabrication of hierarchical structures with well-controlled sizes in different length scales, however, is still a great challenge. To further understand the morphologies and properties of the hierarchical structures, here we present a novel strategy to prepare hierarchical polymer structures by combining the modified breath figure method and the template method. Poly(methyl methacrylate) (PMMA) honeycomb films with regular micropores are first prepared using the modified breath figure method by dipping PMMA films into mixtures of chloroform and methanol. The polymer chains on the honeycomb films are then annealed and wetted into the nanopores of anodic aluminum oxide templates via capillary forces, resulting in the formation of hierarchical polymer structures. The morphologies of the polymer structures, which can be controlled by the molecular weights of the polymers and the concentrations of the polymer solutions, are characterized by scanning electron microscopy. The surface wettabilities of the polymer structures are also examined by water contact angle measurements, and the hierarchical structures are observed to be more hydrophobic than the flat films and honeycomb films. This work not only provides a feasible approach to fabricate hierarchical polymer structures with controlled sizes but also gives a better understanding of the relationship between surface morphologies and properties.
Electrospun polymer core−shell fibers have gained much attention because of their promising applications in areas such as electronic devices, drug delivery, and tissue engineering. The morphology transformation of polymer core− shell fibers, however, has been rarely investigated. Here, we study the effect of thermal annealing on the morphology transformation of electrospun polystyrene (PS)/poly(methyl methacrylate) (PMMA) core−shell fibers on PMMA films. Two types of transformation processes are discovered. In the first type of the transformation process (type I), the PS cores transform to hemispherical particles after the annealing process; in the second type of the transformation process (type II), the PS cores transform to spherical particles after the annealing process. The measured sizes of the hemispherical and spherical PS domains fall into two classified regions, as predicted for the two different types of transformation processes. It is also observed that the growth rates of the undulated amplitude are similar for the two different types of transformation processes, but the type I fibers start to undulate at later annealing times than the type II fibers do. When the PS particles are selectively removed, the PMMA films with linearly arranged cavities are used for the tearing experiments, demonstrating a proof of concept on the potentials in studying the mechanical properties of cavity-containing films.
Designing anisotropic particles of various shapes draws great attention to scientists nowadays. We develop a facile and simple method to fabricate anisotropic polymer particles from spherical polymer particles. Poly(vinyl alcohol) (PVA) films spin-coated with polystyrene (PS) microspheres are confined on both sides using binder clips and are heated above the glass-transition temperatures of the polymers. During the thermal annealing process, the PS particles sink into the PVA films and transform to anisotropic particles. Depending on the distances to the bound regions, oblate spheroid PS particles or prolate spheroid particles with different aspect ratios can be obtained. The transformation of the particles is mainly driven by the stretching forces and the squeezing forces. The main advantage of this method is that anisotropic particles with different shapes can be fabricated simultaneously on a single film. We expect that this novel method can be helpful to various fields including colloids science, suspension rheology, and drug delivery.
Anisotropic polymer particles such as Janus particles have attracted significant attention in recent years because of their unique properties and unusual self-assembly behavior. Most anisotropic polymer particles synthesized so far, however, only have different chemical regions compartmentalized on the particles. It remains a great challenge to fabricate anisotropic polymer particles with different shapes within a single particle. A novel approach is developed to prepare anisotropic polymer particles that contain two hemispheres with different curvatures by annealing polystyrene microspheres on poly(vinyl alcohol) films. During the annealing process, the polymer microspheres gradually sink into the polymer films and transform to asymmetric polymer particles, driven by the surface and interfacial tensions of the polymers. Selective removal techniques are also used to confirm the morphologies of the asymmetric particles.
Over the past few decades, anisotropic polymer particles are of significant interest because of their unique properties which can be applied in various areas, such as drug delivery, biotechnology, and electronics. Most approaches to synthesize anisotropic polymer particles, however, are complicated, and the three-dimensional shapes of the anisotropic particles are usually difficult to be controlled. In this work, we develop a solvent on-film annealing (SOFA) method to fabricate anisotropic polymer particles by studying the morphological evolution of polystyrene (PS) microspheres on poly(methyl methacrylate) (PMMA) films annealed in toluene vapor. At different annealing stages, the isotropic PS microspheres gradually transform to anisotropic PS particles with different morphologies, such as UFO-, cymbal-, peanut-, and bowl-shaped particles. The morphology evolution is driven by the surface and interfacial tensions during the annealing processes and can be confirmed by a selective removal technique. Acetic acid, a selective solvent for PMMA, and cyclohexane, a selective solvent for PS, are also used as the annealing solvents to further investigate the effect of the annealing solvent.
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