In-situ polymerization of styrene in AAO nanocavities

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.

Electrochemistry under confinement

Although the term 'confinement' regularly appears in electrochemical literature, elevated by continuous progression in the research of nanomaterials and nanostructures, up until today the various aspects of confinement considered in electrochemistry are rather scattered individual contributions outside the established disciplines in this field. Thanks to a number of highly original publications and the growing appreciation of confinement as an overarching link between different exciting new research strategies, 'electrochemistry under confinement' is the process of forming a research discipline of its own. To aid the development a coherent terminology and joint basic concepts, as crucial factors for this transformation, this review provides an overview on the different effects on electrochemical processes known to date that can be caused by confinement. It also suggests where boundaries to other effects, such as nano-effects could be drawn. To conceptualize the vast amount of research activities revolving around the main concepts of confinement, we define six types of confinement and select two of them to discuss the state of the art and anticipated future developments in more detail. The first type concerns nanochannel environments and their applications for electrodeposition and for electrochemical sensing. The second type covers the rather newly emerging field of colloidal single entity confinement in electrochemistry. In these contexts, we will for instance address the influence of confinement on the mass transport and electric field distributions and will link the associated changes in local species concentration or in the local driving force to altered reaction kinetics and product selectivity. Highlighting pioneering works and exciting recent developments, this educational review does not only aim at surveying and categorizing the state-of-the-art, but seeks to specifically point out future perspectives in the field of confinement-controlled electrochemistry.

Impact of dimensionality and confinement on reaction dynamics and thermodynamics within 1D and 2D nanostructures

Confinement has been shown to contribute to the dynamics of small molecules within nanoscale hydrophobic or hydrophilic cavities. Enclosure within a confined space can also influence energy transfer pathways, such as the enhancement of fluorescence over thermal relaxation. In this paper, the effect of confinement on the thermodynamic properties and reaction kinetics of small hydrophobic molecules confined in a soft polymeric template is detailed. A quasi-elastic neutron scattering experiment identified a substantial decrease in translational diffusion of pyrrole after solubilization within a hydrophobic cavity. This decrease in mobility is due to pyrrole's closer packing and increased density under confinement vs the bulk liquid. The decreased mobility and increased density explain the spontaneous polymerization reaction of pyrrole observed within the cavity. The precise characterization of the polymerization kinetics under confinement found that the reaction is independent of pyrrole concentration, consistent with the close packing density. Kinetic data also show that confinement dimensionality finds a thermodynamic expression in the transition state entropy. The dynamics and kinetics experiments reported here offer rare empirical insight into the important influence that cavity geometry places on the reactions they host.

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.

Ion Dynamics of Monomeric Ionic Liquids Polymerized In Situ within Silica Nanopores

Polymerized ionic liquids are a promising class of versatile solid-state electrolytes for applications ranging from electrochemical energy storage to flexible smart materials that remain limited by their relatively low ionic conductivities compared to conventional electrolytes. Here, we show that the in situ polymerization of the vinyl cationic monomer, 1-ethyl-3-vinylimidazolium with the bis(trifluoromethanesulfonyl)imide counteranion, under nanoconfinement within 7.5 ± 1.0 nm diameter nanopores results in a nearly 1000-fold enhancement in the ionic conductivity compared to the material polymerized in bulk. Using insights from broadband dielectric and Raman spectroscopic techniques, we attribute these results to the role of confinement on molecular conformations, ion coordination, and subsequently the ionic conductivity in the polymerized ionic liquid. These results contribute to the understanding of the dynamics of nanoconfined molecules and show that in situ polymerization under nanoscale geometric confinement is a promising path toward enhancing ion conductivity in polymer electrolytes.

Poly(diethylene glycol methylether methacrylate) Brush-Functionalized Anodic Alumina Nanopores: Curvature-Dependent Polymerization Kinetics and Nanopore Filling

We report on the synthesis and characterization of poly(diethylene glycol methylether methacrylate) (PDEGMA) brushes by surface-initiated atom transfer radical polymerization inside ordered cylindrical nanopores of anodic aluminum oxide with different pore radii between 20 and 185 nm. In particular, the dependence of polymerization kinetics and the degree of pore filling on the interfacial curvature were analyzed. On the basis of field emission scanning electron microscopy data and thermal gravimetric analysis (TGA), it was concluded that the polymerization rate was faster at the pore orifice compared to the pore interior and also as compared to the analogous reaction carried out on flat aluminum oxide substrates. The apparent steady-state polymerization rate near the orifice increased with decreasing pore size. Likewise, the overall apparent polymerization rate estimated from TGA data indicated stronger confinement for pores with increased curvature as well as increased mass transport limitations due to the blockage of the pore orifice. Only for pores with a diameter to length ratio of ∼1, PDEGMA brushes were concluded to grow uniformly with constant thickness. However, because of mass transport limitations in longer pores, incomplete pore filling was observed, which leads presumably to a PDEGMA gradient brush. This study contributes to a better understanding of polymer brush-functionalized nanopores and the impact of confinement, in which the control of polymer brush thickness together with grafting density along the nanopores is key for applications of PDEGMA brushes confined inside nanopores.

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.

The application of spatially restricted geometries as a unique route to produce well-defined poly(vinyl pyrrolidones) via free radical polymerisation

We report, for the first time, the metal-free green synthesis of linear poly(vinyl pyrrolidone) (PVP) homopolymers of molecular weight higher than 100 kg mol-1 and narrow dispersities via thermal and photo-induced free radical polymerisation carried out within alumina nanoporous membranes acting as "nanoreactors".

Efficient metal-free strategies for polymerization of a sterically hindered ionic monomer through the application of hard confinement and high pressure

In this paper, we have studied the effect of both hard confinement (nanoporous membranes treated as nanoreactors) and high pressure (compression of system) on the progress of free-radical (FRP) and reversible addition-fragmentation chain transfer (RAFT) polymerizations of selected hardly polymerizable, sterically hindered imidazolium-based ionic monomer 1-octyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide ([OVIM][NTf₂]). These two innovative approaches, affecting (in a different way) the free volume of the polymerizing system, allows the reduction of the number of toxic substrates/catalysts, satisfying the requirement of green chemistry. It was found that at both conditions (high compression and confinement) the polymerizability of monomer, as well as the control over the reaction and the properties of the produced polyelectrolytes, have increased significantly. However, it should be added that there were noticeable differences between FRP carried out under confinement and at high pressures. Interestingly, by appropriate variation in thermodynamic conditions, it was possible to synthesize polymers of moderate molecular weight (M_n ∼ 58 kg mol⁻¹) and relatively low dispersity (Đ ∼ 1.7); while for the reaction performed within AAO pores of varying diameter (d = 35 nm and d = 150 nm), macromolecules of higher M_n but slightly broader dispersity indices (Đ ∼ 2.2–2.7) were recovered. On the other hand, RAFT polymerization carried out under confinement and at elevated pressures yielded polymers with well-defined properties. Noteworthy is also the fact that nanopolymerization leads to polymers of comparable M_n to those obtained at high-pressure studies but at significantly shorter reaction time (t ∼ 2 hours). We believe that the presented data clearly demonstrated that both examined approaches (the compression and application of alumina templates, treated as nanoreactors) could be successfully used as additional driving forces to polymerize sterically hindered monomers and produce well-defined polymers in relatively short times. At the same time, it should be mentioned that both proposed polymerization methods enabled us to omit the addition of metal-based initiators/catalysts, which seem to be a crucial step towards further development of the alternative green synthesis of polyelectrolytes in the future.

The effect of hard confinement and high pressure on the progress of free-radical and reversible addition-fragmentation chain transfer polymerizations of sterically hindered 1-octyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide ([OVIM][NTf2]) has been investigated.

A Patterned Butyl Methacrylate-co-2-Hydroxyethyl Acrylate Copolymer with Softening Surface and Swelling Capacity

The tunable swelling and mechanical properties of nanostructures polymers are crucial parameters for the creation of adaptive devices to be used in diverse fields, such as drug delivery, nanomedicine, and tissue engineering. We present the use of anodic aluminum oxide templates as a nanoreactor to copolymerize butyl methacrylate and 2-hydroxyethyl acrylate under radical conditions. The copolymer obtained under confinement showed significant differences with respect to the same copolymer obtained in bulk conditions. Molecular weights, molecular weight dispersities, Young's modulus, and wetting behaviors were significantly modified. The combination of selected monomers allowed us to obtain nanopillar structures with an interesting softening surface and extraordinary swelling capacity that could be of special interest to surface science and specifically, cell culture.

Thermo-responsive PNIPAm nanopillars displaying amplified responsiveness through the incorporation of nanoparticles

The possibility of combining more than one stimulus-responsive property into a single material holds interesting potential for the creation of adaptive devices to be used in diverse fields such as drug delivery, nanomedicine and tissue engineering. This paper describes a novel material based on thermo-responsive PNIPAm nanopillars with amplified surface properties through the incorporation of Fe₃O₄ nanoparticles. The incorporation of magnetic nanoparticles into the nanopillars, prepared via surface-initiated atom-transfer radical polymerization in anodized aluminum oxide templates, sharply increased their stiffness and hydrophobicity when increasing the temperature above the volume phase transition temperature. Furthermore, their magnetic response turned out to be proportional to the amount of the incorporated nanoparticles. The possibility of sharply increasing the stiffness with a temperature variation close to the human body temperature paves the way to the application of these substrates as "smart" scaffolds for cell culture. Additionally, the presence of superparamagnetic nanoparticles in the nanopillars offers the possibility of using these nanostructured systems for magnetic hyperthermia.

Studies on the hard confinement effect on the RAFT polymerization of a monomeric ionic liquid. Unexpected triggering of RAFT polymerization at 30 °C

For the first time, the RAFT polymerization of a monomeric ionic liquid under hard confinement was successfully carried out.

Nanostructured fumarate copolymer-chitosan crosslinked scaffold: An in vitro osteochondrogenesis regeneration study

In the tissue engineering field, the design of the scaffold inspired on the natural occurring tissue is of vital importance. Ideally, the scaffold surface must promote cell growth and differentiation, while promote angiogenesis in the in vivo implant of the scaffold. On the other hand, the material selection must be biocompatible and the degradation times should meet tissue reparation times. In the present work, we developed a nanofibrous scaffold based on chitosan crosslinked with diisopropylfumarate-vinyl acetate copolymer using anodized aluminum oxide (AAO) templates. We have previously demonstrated its biocompatibility properties with low cytotoxicity and proper degradation times. Now, we extended our studies to demonstrate that it can be successfully nanostructured using the AAO templates methodology, obtaining a nanorod-like scaffold with a diameter comparable to those of collagen fibers of the bone matrix (170 and 300 nm). The nanorods obtained presented a very homogeneous pattern in diameter and length, and supports cell attachment and growth. We also found that both osteoblastic and chondroblastic matrix production were promoted on bone marrow progenitor cells and primary condrocytes growing on the scaffolds, respectively. In addition, the nanostructured scaffold presented no cytotoxicity as it was evaluated using a model of macrophages on culture. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 570-579, 2018.

Molecular weight dependence of the intrinsic size effect on Tg in AAO template-supported polymer nanorods: A DSC study

Many studies have established a major effect of nanoscale confinement on the glass transition temperature (T_g) of polystyrene (PS), most commonly in thin films with one or two free surfaces. Here, we characterize smaller yet significant intrinsic size effects (in the absence of free surfaces or significant attractive polymer-substrate interactions) on the T_g and fragility of PS. Melt infiltration of various molecular weights (MWs) of PS into anodic aluminum oxide (AAO) templates is used to create nanorods supported on AAO with rod diameter (d) ranging from 24 to 210 nm. The T_g (both as T_g,onset and fictive temperature) and fragility values are characterized by differential scanning calorimetry. No intrinsic size effect is observed for 30 kg/mol PS in template-supported nanorods with d = 24 nm. However, effects on T_g are present for PS nanorods with M_n and M_w ≥ ∼175 kg/mol, with effects increasing in magnitude with increasing MW. For example, in 24-nm-diameter template-supported nanorods, T_{g, rod} - T_{g, bulk} = -2.0 to -2.5 °C for PS with M_n = 175 kg/mol and M_w = 182 kg/mol, and T_{g, rod} - T_{g, bulk} = ∼-8 °C for PS with M_n = 929 kg/mol and M_w = 1420 kg/mol. In general, reductions in T_g occur when d ≤ ∼2R_g, where R_g is the bulk polymer radius of gyration. Thus, intrinsic size effects are significant when the rod diameter is smaller than the diameter (2R_g) associated with the spherical volume pervaded by coils in bulk. We hypothesize that the T_g reduction occurs when chain segment packing frustration is sufficiently perturbed by confinement in the nanorods. This explanation is supported by observed reductions in fragility with the increasing extent of confinement. We also explain why these small intrinsic size effects do not contradict reports that the T_g-confinement effect in supported PS films with one free surface exhibits little or no MW dependence.

Thermally-induced softening of PNIPAm-based nanopillar arrays

The surface properties of soft nanostructured hydrogels are crucial in the design of responsive materials that can be used as platforms to create adaptive devices. The lower critical solution temperature (LCST) of thermo-responsive hydrogels such as poly(N-isopropylacrylamide) (PNIPAm) can be modified by introducing a hydrophilic monomer to create a wide range of thermo-responsive micro-/nano-structures in a large temperature range. Using surface initiation atom-transfer radical polymerization in synthesized anodized aluminum oxide templates, we designed, fabricated, and characterized thermo-responsive nanopillars based on PNIPAm hydrogels with tunable mechanical properties by incorporating acrylamide monomers (AAm). In addition to their LCST, the incorporation of a hydrophilic entity in the nanopillars based on PNIPAm has abruptly changed the topological and mechanical properties of our system. To gain an insight into the mechanical properties of the nanostructure, its hydrophilic/hydrophobic behavior and topological characteristics, atomic force microscopy, molecular dynamics simulations and water contact angle studies were combined. When changing the nanopillar composition, a significant and opposite variation was observed in their mechanical properties. As temperature increased above the LCST, the stiffness of PNIPAm nanopillars, as expected, did so too, in contrast to the stiffness of PNIPAm-AAm nanopillars that decreased significantly. The molecular dynamics simulations proposed a local molecular rearrangement in our nanosystems at the LCST. The local aggregation of NIPAm segments near the center of the nanopillars displaced the hydrophilic AAm units towards the surface of the structure leading to contact with the aqueous environment. This behavior was confirmed via contact angle measurements below and above the LCST.

Polymerization of Monomeric Ionic Liquid Confined within Uniaxial Alumina Pores as a New Way of Obtaining Materials with Enhanced Conductivity

Broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) have been employed to probe dynamics and charge transport of 1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide ([bvim][NTf₂]) confined in native uniaxial AAO pores as well as to study kinetics of radical polymerization of the examined compound as a function of the degree of confinement. Subsequently, the electronic conductivity of the produced polymers was investigated. As observed, polymerization carried out at T = 363 K proceeds faster under confinement with some saturation effect observed for the sample in pores of smaller diameter. Obtained results were discussed in the context of the very recent reports showing that the free volume of the confined material is higher with respect to the bulk one. It was also noted that conductivity of poly[bvim][NTf₂] is significantly higher with respect to the macromolecules obtained upon bulk polymerization. Moreover, charge transport of the confined macromolecules is even higher when compared to the bulk monomeric ionic liquid at some thermodynamic conditions. Additionally, the molecular weight, M_w, of the confined-synthesized polymers is significantly higher with respect to the bulk-synthesized material. Interestingly, both parameters, (i) the enhancement of σ_dc and (ii) the increase in M_w, can be tuned and controlled by the application of the appropriate confinement. Consequently, those results are quite promising in the context of development of the fabrication of polymerized ionic liquids (PILs) nanomaterials with unique properties and morphologies, which can be further easily applied in the field of nanotechnology.

Polymerization kinetics of a fluorinated monomer under confinement in AAO nanocavities

The study of the polymerization kinetics of a fluorinated acrylic monomer under confinement into AAO nanocavities.