Fabrication of Polymer Nanopeapods in the Nanopores of Anodic Aluminum Oxide Templates Using a Double-Solution Wetting Method

Polymerization within Nanoporous Anodized Alumina Oxide Templates (AAO): A Critical Survey

In the last few years, the polymerization of monomers within the nanocavities of porous materials has been thoroughly studied and developed, allowing for the synthesis of polymers with tailored morphologies, chemical architectures and functionalities. This is thus a subject of paramount scientific and technological relevance, which, however, has not previously been analyzed from a general perspective. The present overview reports the state of the art on polymerization reactions in spatial confinement within porous materials, focusing on the use of anodized aluminum oxide (AAO) templates. It includes the description of the AAO templates used as nanoreactors. The polymerization reactions are categorized based on the polymerization mechanism. Amongst others, this includes electrochemical polymerization, free radical polymerization, step polymerization and atom transfer radical polymerization (ATRP). For each polymerization mechanism, a further subdivision is made based on the nature of the monomer used. Other aspects of "in situ" polymerization reactions in restricted AAO geometries include: conversion monitoring, kinetic studies, modeling and polymer characterization. In addition to the description of the polymerization process itself, the use of polymer materials derived from polymerization in AAO templates in nanotechnology applications, is also highlighted. Finally, the review is concluded with a general discussion outlining the challenges that remain in the field.

Growth of Highly-Ordered Metal Nanoparticle Arrays in the Dimpled Pores of an Anodic Aluminum Oxide Template

A reliable, scalable, and inexpensive technology for the fabrication of ordered arrays of metal nanoparticles with large areal coverage on various substrates is presented. The nanoparticle arrays were formed on aluminum substrates using a two-step anodization process. By varying the anodization potential, the pore diameter, inter-pore spacing, and pore ordering in the anodic aluminum oxide (AAO) template were tuned. Following a chemical etch, the height of the pores in the AAO membrane were reduced to create a dimpled membrane surface. Periodic arrays of metal nanoparticles were subsequently created by evaporating metal on to the dimpled surface, allowing for individual nanoparticles to form within the dimples by a solid state de-wetting process induced by annealing. The ordered nanoparticle array could then be transferred to a substrate of choice using a polymer lift-off method. Following optimization of the experimental parameters, it was possible to obtain cm2 coverage of metal nanoparticles, like gold and indium, on silicon, quartz and sapphire substrates, with average sizes in the range of 50-90 nm. The de-wetting process was investigated for a specific geometry of the dimpled surface and the results explained for two different film thicknesses. Using a simple model, the experimental results were interpreted and supported by numerical estimations.

A decade of innovation and progress in understanding the morphology and structure of heterogeneous polymers in rigid confinement

When confined in nanoscale domains, polymers generally encounter changes in their structural, thermodynamics and dynamics properties compared to those in the bulk, due to the high amount of polymer/wall interfaces and limited amount of matter. The present review specifically deals with the confinement of heterogeneous polymers (i.e. polymer blends and block copolymers) in rigid nanoscale domains (i.e. bearing non-deformable solid walls) where the processes of phase separation and self-assembly can be deeply affected. This review focuses on the innovative contributions of the last decade (2010-2020), giving a summary of the new insights and understanding gained in this period. We conclude this review by giving our view on the most thriving directions for this topic.

In Situ Synthesis of Poly(butyl methacrylate) in Anodic Aluminum Oxide Nanoreactors by Radical Polymerization: A Comparative Kinetics Analysis by Differential Scanning Calorimetry and 1H-NMR

In this work, we explore the ability to generate well-defined poly(butyl methacrylate) (PBMA) nanostructures by “in situ” polymerization of butyl methacrylate monomer (BMA). PBMA nanostructures of high and low aspect ratios have been successfully obtained through the free radical polymerization (FRP) of a BMA monomer in anodic aluminum oxide (AAO) nanoreactors of suitable size. A polymerization kinetics process has been followed by differential scanning calorimetry (DSC) and proton Nuclear Magnetic Resonance spectroscopy (¹H-NMR).The determination of the kinetics of polymerization through DSC is based on a quick and direct analysis of the exothermic polymerization process, whereas the analysis through ¹H-NMR also allows the unambiguous chemical analysis of the resulting polymer. When compared to bulk polymerization, both techniques demonstrate confinement effects. Moreover, DSC and ¹H-NMR analysis give the same kinetics results and show a gel-effect in all the cases. The number average molecular weight (Mn) of the PBMA obtained in AAO of 60–300 nm are between 30·10³–175·10³ g/mol. Even if the Mn value is lower with respect to that obtained in bulk polymerization, it is high enough to maintain the polymer properties. As determined by SEM morphological characterization, once extracted from the AAO nanoreactor, the polymer nanostructures show controlled homogeneous aspect/size all throughout the length of nanopillar over a surface area of few cm². The Young’s modulus of low aspect ratio PBMA nanopillars determined by AFM gives a value of 3.1 ± 1.1 MPa. In this work, a 100% of PBMA polymer nanostructures are obtained from a BMA monomer in AAO templates through a quick double process: 30 min of monomer immersion at room temperature and 90 min of polymerization reaction at 60 °C. While the same nanostructures are obtained by polymer infiltration of PBMA at 200 °C in about 6 h, polymerization conditions are much softer than those corresponding to the polymer infiltration process. Furthermore, the ¹H-NMR technique has been consolidated as a tool for studying the kinetics of the copolymerization reactions in confinement and the determination of monomer reactivity ratios.

Hierarchical self-assembly of a PS-b-P4VP/PS-b-PNIPAM mixture into multicompartment micelles and their response to two-dimensional confinement

Finely tuned synergistic effects among different blocks could realize intriguing hierarchical self-assembly of block copolymers and such hierarchical self-assembly could be manipulated by cylindrical confinement to tune the structures of assemblies.

Replication of Mesoscale Pore One-dimensional Nanostructures: Surface-induced Phase Separation of Polystyrene/Poly(vinyl alcohol) (PS/PVA) Blends

Mesoscale pore one-dimensional (1D) nanostructures, or vertically aligned porous nanostructures (VAPNs), have attracted attention with their excellent hydrophobic properties, ultra-high surface area, and high friction coefficient, compared to conventional vertically aligned nanostructures (VANs). In this study, we investigate the replication of VAPNs produced by the thermal nanoimprint process using anodic aluminum oxide (AAO2) templates (100 nm diameter). Polystyrene/poly(vinyl alcohol) (PS1/PVA) blends, prepared by the advanced melt-mixing process with an ultra-high shear rate, are used to investigate the formation of porosity at the nanometer scale. The results reveal that domain size and mass ratios of PVA precursors in the PS matrix play a dominant role in the interfacial interaction behavior between PS1-PVA-AAO2, on the obtained morphologies of the imprinted nanostructures. With a PVA nanodomain precursor (PS1/PVA 90/10 wt%), the integration of PVA nanodroplets on the AAO2 wall due to the hydrogen bonding that induces the phase separation between PS1-PVA results in the formation of VAPNs after removal of the PVA segment. However, in the case of PVA microdomain precursors (PS1/PVA 70/30 wt%), the structure transformation behavior of PS1 is induced by the Rayleigh instability between PVA encapsulated around the PS1 surfaces, resulting in the PS1 nanocolumns transforming into nanopeapods composed of nanorods and nanospheres.

Flexible Self-Cleaning Broadband Antireflective Film Inspired by the Transparent Cicada Wings

Cicada wings, covered with arranged nanostructures, were widely studied owing to their high transparency and low reflection. However, limited by technologies, their exquisite surface structures and multifunctional features were not inherited and applied by most artificial materials adequately. Here, the excellent optical properties of the cicada wing were investigated in detail experimentally and theoretically. Besides, a flexible self-cleaning broadband antireflective film inspired by the cicada wing has been successfully fabricated by a well-designed biological template method and sol-gel process. The cicada wing ( Megapomponia intermedia) was selected as the original template directly, and a SiO₂ negative replica was obtained by a sol-gel process. Then, chemical corrosion was used to remove the original template, retaining the pure negative replica. Subsequently, the polymethyl methacrylate (PMMA) positive replica could be rebuilt after another sol-gel process. Compared with a flat PMMA film, the average reflectivity of the structured PMMA film over the visible region was reduced from 10 to 2%. Besides, the bio-inspired film with a thickness of 0.18 mm exhibited satisfactory comprehensive performances with low reflectance (≤2%) in most of the visible region, as well as superhydrophobic property and perfect flexibility. Our results offered a quick and simple method to rebuild the nanostructured functional materials, promoting the practical applications of the bionic nanostructured materials. Meanwhile, the modified biomimetic fabrication method provides a solution for rebuilding exquisite biological materials and designing multifunctional surfaces. Moreover, the multifunctional antireflective film with wider universality will exhibit an enormous potential application value in optical communications, photoelectric devices, flexible display screens, and antidazzle glasses.

From Block Copolymer Nanotubes to Nanospheres: Nonsolvent-Induced Morphology Transformation Using Porous Templates

Block copolymer nanostructures have attracted great attention because of the wide range of applications such as sensors and drug delivery. The fabrication of block copolymer nanostructures with controlled morphologies and sizes, however, is still challenging. Here, we study the fabrication of nanotubes and nanospheres of polystyrene- block-polybutadiene (PS- b-PBD) using anodic aluminum oxide (AAO) templates. When PS- b-PBD solutions in N-methyl-2-pyrrolidone are introduced into the nanopores of the AAO templates applying the traditional solution wetting method, PS- b-PBD nanotubes can be obtained. When PS- b-PBD solutions in the nanopores are in contact with a nonsolvent, acetic acid, PS- b-PBD nanospheres are formed. Two possible mechanisms are proposed to discuss the formation of the nonsolvent-driven morphology transformation, including the Rayleigh-instability-type transformation mechanism and the nucleation and growth mechanism. The effect of the polymer concentrations on the internal morphologies of the PS- b-PBD nanostructures is discussed; at higher concentrations, PS- b-PBD nanocapsules can also be prepared. Furthermore, core-shell PS- b-PBD/polymethylmethacrylate nanospheres can be fabricated using this strategy with polymer blend solutions. This work not only demonstrates a simple strategy to control the morphologies of block copolymer nanostructures but also deepens the understanding of the interactions between polymer solutions and solvents.

Gecko-like Branched Polymeric Nanostructures from Nanoporous Templates

Here, we report a simple method to produce hierarchically shaped polymeric one-dimensional nanostructures. More specifically, dual-sized polymer nanowires are fabricated employing multibranched anodic aluminum oxide templates. By fine selection of the anodization conditions, we achieve branched nanopores having a first segment of 400 nm in diameter from which seven further 55 nm in diameter pores arise. Wetting of such nanopores with polymer melts-for example, poly(ε-caprolactone) and polystyrene-allows for the nanomolding of their respective inverse nanostructures, that is, dual-sized multibranched polymer nanowires that, when supported on a flat surface, strongly resemble the spatulae of geckos' toes. The structural features of the dual-sized polymer nanostructures, namely, crystalline phase, crystallinity, texture, and so on, are furthermore characterized and interpreted within the context of polymer phase transitions in confined media. Our work presents a readily applicable approach to produce soft nanomaterials of high morphological complexity, thereby with promising implications in the nanotechnology area, for example, in biomimetic solid adhesion.

Nanoscale crystallization and thermal behaviour of 1,2,4,5-tetrabromobenzene

Nanocrystals of the thermosalient compound 1,2,4,5-tetrabromobenzene are indefinitely stable in the metastable phase from cryogenic temperatures to 80 °C and sublime upon further heating.

Recent progress in the self-assembly of block copolymers confined in emulsion droplets

When the self-assembly of block copolymers (BCPs) occurs within a deformable emulsion droplet, BCPs can aggregate into a variety of nanoscaled particles with unique nanostructures and properties since the confinement effect can effectively break the symmetry of a structure.

Determining the Effect of Centrifugal Force on the Desired Growth and Properties of PCPDTBT as p-Type Nanowires

In this study, low-bandgap polymer poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b;3,4-b')dithiophen]-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl} (PCPDTBT) nanostructures have been synthesized via a hard nanoporous alumina template of centrifugal process. Centrifuge has been used to infiltrate the PCPDTBT solution into the nanoporous alumina by varying the rotational speeds. The rotational speed of centrifuge is directly proportional to the infiltration force that penetrates into the nanochannels of the template. By varying the rotational speed of centrifuge, different types of PCPDTBT nanostructures are procured. Infiltration force created during the centrifugal process has been found a dominant factor in tuning the morphological, optical, and structural properties of PCPDTBT nanostructures. The field emission scanning electron microscopy (FESEM) images proved the formation of nanotubes and nanowires. The energy-dispersive X-ray spectroscope (EDX) analysis showed that the nanostructures were composed of PCPDTBT with complete dissolution of the template.

Multifunctional nanoparticles with controllable dimensions and tripled orthogonal reactivity

Multifunctional nanoparticles featuring three distinct and orthogonal functionalities for performing catalyst-free click reactions of azide-alkyne and maleimide-thiol and atom transfer radical polymerization (ATRP) are fabricated using a simple chemical vapor deposition copolymerization approach with the flexibility to control the particle size and geometry.

Controlled self-assemblies of polystyrene-block-polydimethylsiloxane micelles in cylindrical confinement through a micelle solution wetting method and Rayleigh-instability-driven transformation

Block copolymer micelles have been extensively discussed for many decades because of their applications, such as lithography and drug delivery. However, controlling the morphologies of nanostructure assembly using block copolymer micelles as building elements remains a great challenge. In this work, we developed a novel route to induce micelle assembly in confined geometries. Polystyrene-block-polydimethylsiloxane (PS-b-PDMS) micelle solutions were used to prepare micelle nanostructures, and the critical parameters affecting the morphologies were determined. Micelle nanorods, micelle nanospheres, and multi-component nanopeapods were prepared by wetting anodic aluminum oxide (AAO) templates with micelle solutions. Rayleigh-instability-driven transformation was discovered to play an important role in controlling the morphologies of the micelle nanostructures. This study not only proposes a versatile approach to preparing block copolymer micelle nanostructures, but it also provides deeper insight into the controlling factors of block copolymer micelle morphologies in cylindrical confinement.

Microfluidic and Nanofluidic Resistive Pulse Sensing: A Review

The resistive pulse sensing (RPS) method based on the Coulter principle is a powerful method for particle counting and sizing in electrolyte solutions. With the advancement of micro- and nano-fabrication technologies, microfluidic and nanofluidic resistive pulse sensing technologies and devices have been developed. Due to the unique advantages of microfluidics and nanofluidics, RPS sensors are enabled with more functions with greatly improved sensitivity and throughput and thus have wide applications in fields of biomedical research, clinical diagnosis, and so on. Firstly, this paper reviews some basic theories of particle sizing and counting. Emphasis is then given to the latest development of microfuidic and nanofluidic RPS technologies within the last 6 years, ranging from some new phenomena, methods of improving the sensitivity and throughput, and their applications, to some popular nanopore or nanochannel fabrication techniques. The future research directions and challenges on microfluidic and nanofluidic RPS are also outlined.

New Double-Infiltration Methodology to Prepare PCL-PS Core-Shell Nanocylinders Inside Anodic Aluminum Oxide Templates

Melt nanomolding of core-shell nanocylinders of different sizes, employing anodic aluminum oxide (AAO) templates, is reported here for the first time. The core-shell nanostructures are achieved by a new melt double-infiltration technique. During the first infiltration step, polystyrene (PS) nanotubes are produced by an adequate choice of AAO nanopore diameter size. In the second step, PCL is infiltrated inside the PS nanotubes, as its melting point (and infiltration temperature) is lower than the glass transition temperature of PS. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) measurements verified the complete double-infiltration of the polymers. Differential scanning calorimetry (DSC) experiments show that the infiltrated PCL undergoes a confined fractionated crystallization with two crystallization steps located at temperatures that depend on which surface is in contact with the PCL nanocylinders (i.e., alumina or PS). The melt double-infiltration methodology represents a novel approach to study the effect of the surrounding surface on polymer crystallization under confinement.

Fabrication of Chemically Tunable, Hierarchically Branched Polymeric Nanostructures by Multi-branched Anodic Aluminum Oxide Templates

In this paper, a template-assisted replication method is demonstrated for the fabrication of hierarchically branched polymeric nanostructures composed of post-modifiable poly(pentafluorophenyl acrylate). Anodic aluminum oxide templates with various shapes of hierarchically branched pores are fabricated by an asymmetric two-step anodization process. The hierarchical polymeric nanostructures are obtained by infiltration of pentafluorophenyl acrylate with a cross-linker and photoinitiator, followed by polymerization and selective removal of the template. Furthermore, the nanostructures containing reactive pentafluorophenyl ester are modified with spiropyran amine via post-polymerization modification to fabricate ultraviolet-responsive nanostructures. This method can be readily extended to other amines and offers a generalized strategy for controlling functionality and wettability of surfaces.

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

A novel method to fabricate hierarchical hybrid nanostructures assembled from polystyrene-encapsulated gold nanoparticles is developed.

Structural Transformation of Diblock Copolymer/Homopolymer Assemblies by Tuning Cylindrical Confinement and Interfacial Interactions

In this study, we report the controllable structural transformation of block copolymer/homopolymer binary blends in cylindrical nanopores. Polystyrene-b-poly(4-vinylpyridine)/homopolystyrene (SVP/hPS) nanorods (NRs) can be fabricated by pouring the polymers into an anodic aluminum oxide (AAO) channel and isolated by selective removal of the AAO membrane. In this two-dimensional (2D) confinement, SVP self-assembles into NRs with concentric lamellar structure, and the internal structure can be tailored with the addition of hPS. We show that the weight fraction and molecular weight of hPS and the diameter of the channels can significantly affect the internal structure of the NRs. Moreover, mesoporous materials with tunable pore shape, size, and packing style can be prepared by selective solvent swelling of the structured NRs. In addition, these NRs can transform into spherical structures through solvent-absorption annealing, triggering the conversion from 2D to 3D confinement. More importantly, the transformation dynamics can be tuned by varying the preference property of surfactant to the polymers. It is proven that the shape and internal structure of the polymer particles are dominated by the interfacial interactions governed by the surfactants.

On-Film Annealing: A Simple Method to Fabricate Heterogeneous Polymer Surfaces, Porous Films, and Hemispheres

Polymer microspheres have been widely investigated because of their applications in areas such as drug delivery, latex diagnostics, and affinity bioseparators. The effect of annealing on polymer microspheres, however, has been rarely studied. In this work, we demonstrate the morphology transformation of polystyrene (PS) microspheres annealed thermally on poly(methyl methacrylate) (PMMA) films. During the annealing process, the PS microspheres gradually sink into the PMMA films and transform into PS hemispheres, driven by the reduction of the surface and interfacial energies. The effect of the film thicknesses on the morphology transformation is also studied. In addition, porous PMMA films or PS hemispheres can be obtained by removing the PS or the PMMA domains of the polymer composites using cyclohexane or acetic acid, respectively.

Effect of the polymer concentration on the Rayleigh-instability-type transformation in polymer thin films coated in the nanopores of anodic aluminum oxide templates

We study the Rayleigh-instability-type transformation of polystyrene (PS) thin films coated in the nanopores of anodic aluminum oxide (AAO) templates. The PS thin films are fabricated using a solution-wetting method, in which the wall thicknesses are controlled by the concentrations of the polymer solutions and the diameters of the nanopores. By thermal annealing, the surfaces of the thin films undulate and the morphologies transform from nanotubes to Rayleigh-instability-induced nanostructures (short nanorods) and long nanorods. To understand the mechanism of the morphology transformation further, we construct the morphology diagrams by annealing the PS thin films at different temperatures and times. We observe that the morphology diagrams of the PS thin films prepared by different concentrations are similar, indicating that the transformation kinetics are not affected by the film thicknesses. The values of the undulation wavelengths, however, are controlled by the film thicknesses and the diameters of the nanopores.

Templated self-assembly of block copolymers and morphology transformation driven by the Rayleigh instability

In the current study, we investigate the self-assembly of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) confined in the nanopores of the anodic aluminum oxide (AAO) template and the subsequent morphology transformation induced by the Rayleigh instability. PS-b-P4VP nanotubes and nanorods with various internal nanostructures are fabricated by wetting the AAO template with PS-b-P4VP/chloroform solution, and then followed by solvent evaporation. After the removal of AAO template by potassium hydroxide solution, several different solvents (chloroform, toluene, and N,N-dimethylformamide) with different qualities are used to swell and anneal those nanotubes and nanorods suspended in aqueous media. Morphology transformation from nanostructured PS-b-P4VP nanotubes or nanorods to ordered nanospheres is observed by annealing upon chloroform and toluene while the morphology remains unchanged upon N,N-dimethylformamide annealing, indicating that solvent quality is a key factor in tuning the morphology and internal structures. Kinetics study and theoretical analysis for the morphology transition from two-dimensional (2D) block copolymer (BCP) nanotubes and nanorods to three-dimensional (3D) BCP nanospheres are further performed. From the morphological evolution and the quantitative calculation, it is confirmed that this transition is induced by the Rayleigh instability. This study provides a simple but promising method, that is, solvent annealing method, for the fabrication of BCP nanospheres with ordered internal nanostructures, which may have great application in drug delivery and other nanotechnology.

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.