Cristina E. Hoppe

Synthesis of silver nanoparticles coated with OH-functionalized organic groups: dispersion and covalent bonding in epoxy networks.

The introduction of reactive functionalities in the organic groups used to stabilize inorganic nanoparticles (NPs) enables multiple applications based on their covalent fixation to a variety of materials, substrates and interfaces. In this paper we report the synthesis of silver nanoparticles (NPs) with an average diameter of about 4 nm, coated with particular organic groups that allow their solubility in a variety of organic solvents and the covalent bonding through secondary hydroxyl groups present in their structure. Water-dispersible NPs stabilized with 11-mercaptoundecanoate anions were first synthesized. The esterification of carboxylate groups with phenyl glycidyl ether generated 2-hydroxyester functionalities and made the NPs dispersible in a variety of organic solvents. To illustrate the multiple possible applications of the synthesized NPs, their incorporation to an epoxy network is discussed. A solution of the silver NPs in diglycidyl ether of bisphenol A was polymerized in the presence of benzyldimethylamine as initiator. This led to an epoxy network containing a homogeneous dispersion of silver NPs as revealed by the constancy of the plasmon band location. Covalent bonding of the NPs to the epoxy matrix was produced by chain transfer reactions involving the hydroxyl groups. Nanocomposites were strongly colored and exhibited a dependence of the glass transition temperature on the concentration of NPs. Several applications are envisaged for these materials.

Hierarchical Assemblies of Gold Nanoparticles at the Surface of a Film Formed by a Bridged Silsesquioxane Containing Pendant Dodecyl Chains

Hierarchical aggregates of gold nanoparticles (NPs) on different length scales were in situ generated at the surface of a bridged silsesquioxane during the process of film formation by polycondensation and solvent evaporation. A precursor of a bridged silsesquioxane based on the reaction product of (glycidoxypropyl)trimethoxysilane (2 mol) with dodecylamine (1 mol) was hydrolytically condensed in a THF solution at room temperature in the presence of formic acid, water, and variable amounts of dodecanethiol-stabilized gold NPs (average diameter of 2 nm). The initial compatibility of the precursor with gold NPs was achieved by the presence of dodecyl chains in both components. Phase separation of gold NPs accompanied by partitioning to the air-polymer interface took place driven by the polycondensation reaction and solvent evaporation. A hierarchical organization of gold NPs in the structures generated at the air-polymer interface was observed. Small body-centered cubic (bcc) crystals of about 20 nm diameter were formed in the first step, in which the 2 nm gold NPs kept their individuality (high-resolution transmission electron microscopy, field emission scanning electron microscopy, and small-angle X-ray diffraction). In the second step, bcc crystals aggregated, forming compact micrometer-sized spherical particles. Under particular evaporation rates a third step of the self-assembly process was observed where micrometer-sized particles formed fractal structures. Increasing the initial concentration of gold NPs in the formulation led to more compact fractal structures in agreement with theoretical simulations. The surface percolation of NPs in fractal structures can be the basis of useful applications.

Functional nanocomposites based on the infusion or in situ generation of nanoparticles into amphiphilic epoxy gels

The production of nanocomposites with functional properties via the infusion of preformed nanoparticles (NPs) or their in situ generation inside an amphiphilic epoxy gel is reported. The gel was synthesized by the reaction of a diepoxy monomer based on diglycidyl ether of bisphenol A with an n-alkylamine, followed by annealing the resulting linear polymers above their glass transition temperatures to produce physical gelation through tail-to-tail association of pendant alkyl chains. Some of the advantages of these polymer gels are: (a) they have a low crosslink density and can therefore be significantly swollen by several organic solvents, (b) the presence of pendant alkyl chains provides a convenient chemical environment for the stabilization of NPs coated with alkyl chains, (c) the presence of secondary hydroxyls and tertiary amine groups in the polar backbone of polymer chains can be used to coordinate and reduce different precursors of NPs. Preformed NPs could be successfully infused into the gels keeping their optical properties (e.g., CdSe quantum dots) or magnetic behavior (e.g., γ-Fe2O3@oleic acid NPs) in the resulting nanocomposite. In situ generation of Au and Ag NPs (average size close to 10 nm) inside the amphiphilic gels was produced by infusing HAuCl4 or AgNO3 followed by reduction to the corresponding metals with secondary alcohols present in the polymer backbone, at 100 °C. Amphiphilic gels were employed as hosts for the in situ precipitation of gold(I)-dodecanethiolate leading to films exhibiting a red emission (638 nm) when excited with UV light (300 nm).

Photothermal triggering of self-healing processes applied to the reparation of bio-based polymer networks

Green laser irradiation successfully activated self-healing processes in epoxy-acid networks modified with low amounts of gold nanoparticles (NPs). A bio-based polymer matrix, obtained by crosslinking epoxidized soybean oil (ESO) with an aqueous citric acid (CA) solution, was self-healed through molecular rearrangements produced by transesterification reactions of β-hydroxyester groups generated in the polymerization reaction. The temperature increase required for the triggering of these thermally activated reactions was attained by green light irradiation of the damaged area. Compression force needed to assure a good contact between crack faces was achieved by volume dilatation generated by the same temperature rise. Gold NPs dispersed in the polymer efficiently generated heat in the presence of electromagnetic radiation under plasmon resonance, acting as nanometric heating sources and allowing remote activation of the self-healing in the crosslinked polymer.

Poly(dodecyl methacrylate) as Solvent of Paraffins for Phase Change Materials and Thermally Reversible Light Scattering Films

Paraffins are typical organic phase change materials (PCM) used for latent heat storage. For practical applications they must be encapsulated to prevent leakage or agglomeration during fusion. In this study it is shown that eicosane (C20H42 = C20) in the melted state could be dissolved in the hydrophobic domains of poly(dodecyl methacrylate) (PDMA) up to concentrations of 30 wt %, avoiding the need of encapsulation. For a 30 wt % solution, the heat of phase change was close to 69 J/g, a reasonable value for its use as a PCM. The fully converted solution remained transparent at 80 °C with no evidence of phase separation but became opaque by cooling as a consequence of paraffin crystallization. Heating above the melting temperature regenerated a transparent material. A high contrast ratio and abrupt transition between opaque and transparent states was observed for the 30 wt % blends, with a transparent state at 35 °C and an opaque state at 23 °C. This behavior was completely reproducible during consecutive heating/cooling cycles, indicating the possible use of this material as a thermally reversible light scattering (TRLS) film.

Epoxy-Based Organogels for Thermally Reversible Light Scattering Films and Form-Stable Phase Change Materials

Alkyl chains of β-hydroxyesters synthesized by the capping of terminal epoxy groups of diglycidylether of bisphenol A (DGEBA) with palmitic (C16), stearic (C18), or behenic (C22) fatty acids self-assemble forming a crystalline phase. Above a particular concentration solutions of these esters in a variety of solvents led to supramolecular (physical) gels below the crystallization temperature of alkyl chains. A form-stable phase change material (FS-PCM) was obtained by blending the ester derived from behenic acid with eicosane. A blend containing 20 wt % ester was stable as a gel up to 53 °C and exhibited a heat storage capacity of 161 J/g, absorbed during the melting of eicosane at 37 °C. Thermally reversible light scattering (TRLS) films were obtained by visible-light photopolymerization of poly(ethylene glycol) dimethacrylate-ester blends (50 wt %) in the gel state at room temperature. The reaction was very fast and not inhibited by oxygen. TRLS films consisted of a cross-linked methacrylic network interpenetrated by the supramolecular network formed by the esters. Above the melting temperature of crystallites formed by alkyl chains, the film was transparent due to the matching between refractive indices of the methacrylic network and the amorphous ester. Below the crystallization temperature, the film was opaque because of light dispersion produced by the organic crystallites uniformly dispersed in the material. Of high significance for application was the fact that the contrast ratio did not depend on heating and cooling rates.

Mechanism of particle formation in silver/epoxy nanocomposites obtained through a visible light-assisted in situ synthesis

A detailed understanding of the processes taking place during the in situ synthesis of metal/polymer nanocomposites is crucial to manipulate the shape and size of nanoparticles (NPs) with a high level of control. In this paper, we report an in-depth time-resolved analysis of the particle formation process in silver/epoxy nanocomposites obtained through a visible-light-assisted in situ synthesis. The selected epoxy monomer was based on diglycidyl ether of bisphenol A, which undergoes relatively slow cationic ring-opening polymerization. This feature allowed us to access a full description of the formation process of silver NPs before this was arrested by the curing of the epoxy matrix. In situ time-resolved small-angle X-ray scattering investigation was carried out to follow the evolution of the number and size of the silver NPs as a function of irradiation time, whereas rheological experiments combined with near-infrared and ultraviolet-visible spectroscopies were performed to interpret how changes in the rheological properties of the matrix affect the nucleation and growth of particles. The analysis of the obtained results allowed us to propose consistent mechanisms for the formation of metal/polymer nanocomposites obtained by light-assisted one-pot synthesis. Finally, the effect of a thermal postcuring treatment of the epoxy matrix on the particle size in the nanocomposite was investigated.

A modifier that enables the easy dispersion of alkyl-coated nanoparticles in an epoxy network

Nanoparticles (NPs) coated with alkyl chains cannot be dissolved in diglycidylether of bisphenol A (DGEBA), which is a typical monomer used in the synthesis of epoxy networks. We show that adding small amounts of the linear amphiphilic polymer obtained by reaction of DGEBA with dodecylamine, produced a stable dispersion of dodecanethiol-coated gold NPs in DGEBA. The anionic homopolymerization of this blend initiated by a tertiary amine led to a nanocomposite with a uniform dispersion of gold NPs. The selected crosslinking chemistry allowed covalent bonding of the modifier to the matrix, avoiding phase separation and enabling easy tuning of the thermal properties of the matrix.

Tailoring the self-assembly of linear alkyl chains for the design of advanced materials with technological applications

The self-assembly of n-alkyl chains at the bulk or at the interface of different types of materials and substrates has been extensively studied in the past. The packing of alkyl chains is driven by Van der Waals interactions and can generate crystalline or disordered domains, at the bulk of the material, or self-assembled monolayers at an interface. This natural property of alkyl chains has been employed in recent years to develop a new generation of materials for technological applications. These studies are dispersed in a variety of journals. The purpose of this article was to discuss some selected examples where these advanced properties arise from a process involving the self-assembly of alkyl chains. We included a description of electronic devices and new-generation catalysts with properties derived from a controlled two-dimensional (2D) or three-dimensional (3D) self-assembly of alkyl chains at an interface. Then, we showed that controlling the crystallization of alkyl chains at the bulk can be used to generate a variety of advanced materials such as superhydrophobic coatings, shape memory hydrogels, hot-melt adhesives, thermally reversible light scattering (TRLS) films for intelligent windows and form-stable phase change materials (FS-PCMs) for the storage of thermal energy. Finally, we discussed two examples where advanced properties derive from the formation of disordered domains by physical association of alkyl chains. This was the case of photoluminescent nanocomposites and materials used for reversible optical storage.

Unidirectional freezing as a tool for tailoring air permeability in macroporous poly(ethylene glycol)-based cross-linked networks

Unidirectional freezing followed by photopolymerization at subzero temperatures was used to obtain highly air-permeable monoliths with ordered porous structures. Scaffolds were obtained from aqueous solutions of a poly(ethylene glycol)dimethacrylate (PEGDMA) oligomer, a photosensitizer and a reducing agent. Solutions were vertically frozen in liquid nitrogen at a controlled rate to induce the oriented growth of ice crystals and then cryo-photopolymerized under blue-light irradiation. Ice crystals were finally removed under vacuum producing macroporous hydrophilic networks with aligned pores. Porosities ranged between 80 and 95%, depending on the initial concentration of PEGDMA. The influence of processing variables on the final properties of the materials was addressed, concerning particularly the effect of porosity and freezing directionality on air permeability. Compared to porous PEGDMA-based monoliths with non-aligned macropores, gas permeability was two to three times higher for oriented scaffolds at the same porosity level, a fact explained by the easier transport of gas molecules through the aligned structures. However, the role of pore orientation on gas permeability was shown to be less marked as porosity increased. The results demonstrate that the use of unidirectional freezing strongly increases the permeability of monolithic samples up to values usually required, for instance, in tissue engineering applications (higher than 2D). These findings provide new perspectives on pore design principles toward future scaffolding of polymeric cross-linked matrices.