María L. Gómez

A mechanistic and laser flash photolysis investigation of acrylamide polymerization photoinitiated by the three component system safranine-T/triethanolamine/diphenyliodonium chloride

Abstract The photopolymerization of acrylamide (AA) initiated by the synthetic dye safranine-T in the presence of triethanolamine (TEOHA) as co-initiator has been investigated in aqueous solution. It was found that the addition of diphenyliodonium chloride (DPIC) to the system has a marked accelerating effect on the polymerization rate. In the absence of DPIC an inhibiting effect of TEOHA is observed at high concentrations of the amine. This effect is suppressed by the presence of the onium salt. At the same time the photobleaching of the dye caused by the amine is suppressed by the presence of AA or DPIC. Laser flash photolysis experiments show that the presence of the onium salt increases the yield of triplet state of the dye. Also the yield of radicals in the quenching of the dye triplet by TEOHA increases by the presence of DPIC. The semi-reduced form of the dye decays faster in the presence of DPIC, suggesting a possible way of generating extra active radicals at the same time that the dye is regenerated. It is concluded that are several factors that contribute to the accelerating effect of DPIC on the polymerization rate.

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).

Photophysical Behavior of Modified Xanthenic Dyes Embedded into Silsesquioxane Hybrid Films: Application in Photooxidation of Organic Molecules

Polymeric materials based on a bridged silsesquioxane with pendant dodecyl chains were synthesized and modified with different xanthenic dyes with the aim of developing a material with potential application in photooxidation of organic compounds. The employed dyes constitute a family of novel xanthenic chromophores with outstanding properties as singlet oxygen photosensitizers. The hybrid matrix was chosen for its enhanced properties such as flexibility and chemical resistance. The employed dyes were easily incorporated into the hybrid polymer obtaining homogeneous, transparent, and low-refractive-index materials. The polymeric films were characterized using UV-Vis absorption, fluorescence, and laser flash photolysis techniques. The ability of these materials to produce singlet oxygen was tested following the photooxidation of 9,10-dimethylanthracene which is a well-known chemical trap for singlet oxygen. High photooxidation efficiencies were observed for these materials, which present the advantage of being easily removed/collected from the solution where photooxidation takes place. While photobleaching of the incorporated dyes is commonly observed in the solution, it takes place very slowly when dyes are embedded in the hybrid matrix. These properties bode well for the potential use of these materials in novel wastewater purification strategies.