Hao-Wen Ko

Porous Polymer Nanostructures Fabricated by the Surface-Induced Phase Separation of Polymer Solutions in Anodic Aluminum Oxide Templates

We study the formation of porous polymer nanostructures fabricated by the surface-induced phase separation of polymer solutions in anodic aluminum oxide (AAO) templates. Poly(methyl methacrylate) (PMMA) and tetrahydrofuran (THF) are used to investigate the evolution process of the surface-induced phase separation. With the longer immersion time of the AAO template in the polymer solution, the size of the solvent-rich droplet is increased by the coarsening process, resulting in the formation of porous polymer nanostructures. The coarsening mechanism is further evaluated by changing the experimental parameters including the immersion time, the polymer concentration, the polymer molecular weight, and the solvent quality. Under conditions in which polymer solutions have higher viscosities, the coarsening process is slowed down and the formation of the porous nanostructures is prohibited. The prevention of the porous nanostructures can also be realized by adding water to the PMMA/THF solution before the immersion process.

Microwave-annealing-induced nanowetting: a rapid and facile method for fabrication of one-dimensional polymer nanomaterials

Template wetting methods have been broadly applied to fabricate diverse one-dimensional polymer nanomaterials. The currently used template wetting methods, however, have shortcomings and disadvantages such as long processing times, thermal degradation, and difficulties in controlling the lengths. In this work, we develop a novel microwave-annealing-induced nanowetting (MAIN) method to fabricate one-dimensional polymer nanomaterials using porous anodic aluminum oxide (AAO) templates. Upon microwave annealing, the polymer chains are infiltrated into the nanopores of the AAO templates, and the morphologies of the polymer nanomaterials can be controlled by the annealing conditions. The growth rates of the polymer nanomaterials using the MAIN method are faster than those using the traditional thermal annealing method. This work not only provides a time-saving method to fabricate one-dimensional polymer nanomaterials with controlled morphologies, but also offers a better understanding of the effect of microwave annealing on the wetting behaviors of polymer melts.

Morphology control of three-dimensional nanostructures in porous templates using lamella-forming block copolymers and solvent vapors

The microphase separation behavior of block copolymers confined in cylindrical nanopores has been extensively investigated. Recently, the solvent-annealing-induced nanowetting in templates (SAINT) method has been demonstrated to be a versatile approach for the infiltration of block copolymers into the nanopores of porous templates. The function of the annealing solvents, however, is still not well understood, especially in the morphology control of the fabricated block copolymer nanostructures. In this work, we elucidate the function of the annealing solvents in the SAINT method using a lamella-forming block copolymer, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and anodic aluminum oxide (AAO) templates. By changing the composition of the annealing solvents, different morphologies such as the concentric lamellar morphology, the winding cylinder morphology, and the irregular hybrid morphology are observed, mainly caused by the annealing-solvent-induced volume change. The morphology of the block copolymer nanostructures can be further confirmed using an HF solution to remove the PDMS domain selectively.

Selective Template Wetting Routes to Hierarchical Polymer Films: Polymer Nanotubes from Phase-Separated Films via Solvent Annealing

We demonstrate a novel wetting method to prepare hierarchical polymer films with polymer nanotubes on selective regions. This strategy is based on the selective wetting abilities of polymer chains, annealed in different solvent vapors, into the nanopores of porous templates. Phase-separated films of polystyrene (PS) and poly(methyl methacrylate) (PMMA), two commonly used polymers, are prepared as a model system. After anodic aluminum oxide (AAO) templates are placed on the films, the samples are annealed in vapors of acetic acid, in which the PMMA chains are swollen and wet the nanopores of the AAO templates selectively. As a result, hierarchical polymer films containing PMMA nanotubes can be obtained after the AAO templates are removed. The distribution of the PMMA nanotubes of the hierarchical polymer films can also be controlled by changing the compositions of the polymer blends. This work not only presents a novel method to fabricate hierarchical polymer films with polymer nanotubes on selective regions, but also gives a deeper understanding in the selective wetting ability of polymer chains in solvent vapors.

Fabrication of Multicomponent Polymer Nanostructures Containing PMMA Shells and Encapsulated PS Nanospheres in the Nanopores of Anodic Aluminum Oxide Templates

Multi-component polymer nanomaterials have attracted great attention because of their applications in areas such as biomedicine, tissue engineering, and organic solar cells. The precise control over the morphologies of multi-component polymer nanomaterials, however, is still a great challenge. In this work, the fabrication of poly(methyl methacrylate)(PMMA)/poly-styrene (PS) nanostructures that contain PMMA shells and encapsulated PS nanospheres is studied. The nanostructures are prepared using a triple solution wetting method with anodic aluminum oxide (AAO) templates. The nanopores of the templates are wetted sequentially by PS solutions in dimethylformamide (DMF), PMMA solutions in acetic acid, and water. The compositions and morphologies of the nanostructures are controlled by the interactions between the polymers, solvents, and AAO walls. This work not only presents a feasible method to prepare multi-component polymer nanomaterials, but also leads to a better understanding of polymer-solvent interactions in confined geometries.

Hierarchical hybrid nanostructures: controlled assembly of polymer-encapsulated gold nanoparticles via a Rayleigh-instability-driven transformation under cylindrical confinement

We develop a novel route based on the solution wetting method using anodic aluminum oxide (AAO) templates to fabricate hierarchical hybrid nanostructures assembled from polystyrene-encapsulated gold nanoparticles (Au@PS NPs). Hybrid nanostructures including nanotubes and nanospheres can be reliably prepared, in which the spatial arrangement of the Au@PS NPs is determined by the pore diameters of the templates and the molecular weights of the thiol-ended polystyrene (PS-SH) ligands. In particular, the Rayleigh-instability-driven transformation plays a key role in the formation of the hybrid nanospheres.

Selective solvent-induced reconstruction in confined space: one-dimensional mesoporous block copolymer structures in cylindrical nanopores

We investigate the formation of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) nanostructures confined in cylindrical nanopores via a novel selective solvent-induced reconstruction process. The key factors of the morphologies of the nanostructures are determined, including the choice of solvents, the molecular weights of the polymers, and the pore diameters of the templates.

Rapid separation of gold nanorods in multilayer aqueous systems via centrifugation

In this work, we demonstrate multilayer aqueous systems as media to separate gold nanoparticles of different shapes and sizes. The multilayer aqueous systems are composed of CTAB/EG solutions with different viscosities. By centrifugation, gold nanoparticles with different shapes and sizes can be dispersed in different layers of the aqueous systems. Gold nanorods synthesized by different conditions, such as changing the amount of AgNO3 or different aging times, are used to examine the multilayer aqueous systems. Gold nanorods with higher aspect ratios can be obtained by increasing the concentration of AgNO3, as confirmed by UV-Vis spectra and TEM studies. The separated gold nanorods are also characterized by surface-enhanced Raman scattering (SERS), showing lower activity for gold nanorods with higher aspect ratios, due to the size-dependent absorption of the longitudinal plasmon band. The multilayer aqueous systems can effectively separate gold nanorods prepared by different conditions and can be applied to separate other types of nanoparticles.

Polymer Nanostructures Using Nanoporous Templates

The template method is one of the most commonly used techniques to fabricate polymer nanostructures. Polymers can be introduced into the nanopores of porous templates via various template wetting methods. In recent years, there have been significant progresses in the fabrication and characterization of polymer nanostructures using the template method. Here, we first introduce porous templates and the basic concept of the template wetting method. The anodic aluminum oxide (AAO) template, one of the most commonly used porous templates, will be discussed in more detail. We then cover some of the most important template wetting methods used for polymer materials, such as the melt wetting method, the microwave annealing wetting method, and the solution wetting method. Additionally, some of the newly developed wetting methods such as the nonsolvent-induced solution wetting method and the solvent annealing wetting method are described. By applying these methods, different types of polymers can be infiltrated into the nanopores, such as homopolymers, polymer blends, block copolymers, and polymer/inorganic hybrid materials. Some recent works on the fabrication of porous polymer nanostructures using templates are also summarized. Finally, recent studies on the morphology changes of the polymer nanostructures driven by the Rayleigh-instability transformation are reviewed.