Céline Croutxé-Barghorn

New insights into photoinduced processes in hybrid sol–gel glasses containing modified titanium alkoxides

Hybrid organic–inorganic sol–gel compounds have been widely used for the design of new optical devices, since they combine the characteristics of both glasses and polymers and improve the properties of the final material. Hybrid precursors in which both phases are chemically grafted are of increasing interest: volume shrinkage is minimized and phase separation can be kept below the level of Rayleigh scattering, thus leading to highly transparent glasses. Polymerizable acrylate or methacrylate functions grafted onto modified silicone alkoxides can react ia a free-radical mechanism initiated either by heating or UV-light. Considerable attention has already been focused on the chemical mechanisms involved in the construction of the inorganic network. However, no extensive study of the organic part of the process has so far been conducted. This paper points out the characteristics of the photopolymerization process taking place in hybrid sol–gel materials. In particular, the influence of inorganic moieties on the photopolymerization kinetics was studied by UV and real time FTIR spectroscopy. Particular interest was focused on the incorporation of alkoxymetals that are usually added to improve the optical and physical properties of the final material. The role of the titanium component in the photopolymerization process is emphasized. The results provide insights into processes leading to simultaneous formation of interpenetrating organic–inorganic networks and are of crucial importance for the generation of optical devices.

Ordered Hybrids from Template-Free Organosilane Self-Assembly

Despite considerable achievements over the last two decades, nonporous organic-inorganic hybrid materials are mostly amorphous, especially in the absence of solvothermal processes. The organosilane self-assembly approach is one of the few opportunities for creating a regular assembly of organic and inorganic moieties. Additionally, well-established organosilicon chemistry enables the introduction of numerous organic functionalities. The synthesis of periodically ordered hybrids relies on mono-, bis-, or multisilylated organosilane building blocks self-assembling into hybrid mesostructures or superstructures, subsequently cross-linked by siloxane Si-O-Si condensation. The general synthesis procedure is template-free and one-step. However, three concurrent processes underlie the generation of self-organized hybrid networks: thermodynamics of amphiphilic aggregation, dynamic self-assembly, and kinetically controlled sol-gel chemistry. Hence, the set of experimental conditions and the precursor structure are of paramount importance in achieving long-range order. Since the first developments in the mid-1990s, the subject has seen considerable progress leading to many innovative advanced nanomaterials providing promising applications in membranes, pollutant remediation, catalysis, conductive coatings, and optoelectronics. This work reviews, comprehensively, the primary evolution of this expanding field of research.

Insights into photoinduced sol-gel polymerization: an in situ infrared spectroscopy study.

Photoacid-catalyzed sol-gel polymerization is now recognized as a powerful single-step synthetic approach to the synthesis of hybrid films, which can be distinguished from conventional sol-gel methods by higher reactivity and a solvent-free process. Despite its utility, the mechanism is not yet understood, in particular what chemical, physical, and photochemical parameters determine the precise sequence, kinetics, and advancement of this UV inorganic photopolymerization. Here, using mainly transmission real-time Fourier transformed infrared (RT-FTIR) spectroscopy, we characterize in situ the hydrolysis-condensation reactions of oligomeric silicon alkoxides and the formation of byproducts. Systematic review and assessment of numerous processing variables (relative humidity, film thickness, precursor structure, nature, and the concentration of photoacid generator) prove that the reaction kinetics are controlled by the two independent phenomena: the intrinsic chemical reaction rates and the water vapor permeation into the film.

Fabrication of microlenses by direct photo-induced crosslinking polymerization

Growing interest in the manufacture of microlenses results from their standard use as optical components with current applications in the telecommunication industry. A number of techniques were developed by using various materials and processes. One approach involved the ability of self-processing photopolymers to generate microlens arrays. Spatially controlled illumination of a photosensitive layer induced an inhomogeneous photopolymerization involving formation of 3-D polymer network, mass-transport process of reactive species and bending of the surface resulting from a gradient of surface free energy. The imaging process that generated as a relief in the photopolymer layer, exhibited a completely self-processing character without any chemical post-treatment. The lens arrays displayed diameters ranging from less than 100 μm to 1 mm and focal lengths from 100 μm to a few millimeters, depending on photonic, optical and physico-chemical parameters.

Continuous-flow synthesis of polymer nanoparticles in a microreactor via miniemulsion photopolymerization

An efficient continuous synthesis of nanolatex was achieved in water using a single-lane photochemical microreactor combined with an energy-saving and safe UV fluorescent lamp. Acrylate and thiol-ene miniemulsions were polymerized in high yields at low irradiance (3 mW cm−2) upon controlling droplet size, temperature and residence time.

Bridged polysilsesquioxane films via photoinduced sol–gel chemistry

The synthesis and characterization of bridged polysilsesquioxane films was performed via a photoacid-catalyzed sol–gel method using a series of three precursors with different organic moiety structures.

Influence of zirconium propoxide on the radical induced photopolymerisation of hybrid sol–gel materials

This study emphasizes the role of zirconium propoxide on the free-radical photopolymerisation of hybrid organic–inorganic material based on 3-methacryloxypropyltrimethoxysilane (MAPTMS). By means of atomic force microscopy in pulsed force mode and real-time Fourier-transform infrared spectroscopy, it is shown that the addition of zirconium complex in the hybrid sol–gel material leads to a well-polymerised and hydrophilic surface under UV-illumination. By contrast, sols without zirconium complex exhibit soft and sticky surfaces. The study also demonstrates an unexpected role of oxygen in the photopolymerisation process. Indeed oxygen is well-known being a strong inhibitor of the radical polymerisation. However, in the present case, it interacts with Zr(OPr)4 and participates in a positive way to the photochemical process. During the irradiation, radicals react with O2 to form peroxyl radicals that are generally inert towards the photopolymerisation. By using laser flash photolysis, it was proved that these radicals combine with zirconium propoxide to form new initiating species that contribute to further free-radical photopolymerisation.

Depth characterization of photopolymerized films by confocal Raman microscopy using an immersion objective.

The depth characterization of photopolymer films by confocal Raman microscopy is often troublesome due to refraction effects. To minimize these effects, we used an oil immersion objective and a method was developed to avoid penetration of the oil without damaging the sample surface. Since the surface may be sticky if oxygen in the air inhibits the photopolymerization, a protective layer could not be put onto the film. Therefore, the method consists in using a thin polypropylene foil as substrate for the coating and placing the sample upside down under the objective. In this manner, the immersion oil could be deposited on top of the polypropylene. The advantage of this setup is that the oil, polypropylene substrate, and photopolymer film have close refractive indices. Basic calculations showed that the depth resolution is hardly affected in that configuration and double-bond conversion profiles could be plotted as a function of reliable nominal depth. The validity of the methodology was confirmed by experiments carried out with a dry metallurgical objective on the sample surface, face up, where refraction effects are still minor. In addition, infrared spectroscopy, which was used to follow the photopolymerization, corroborated the Raman conversion of the films over their thickness. The confocal Raman microscopy method can be applied to various photopolymerized systems to characterize their behavior towards oxygen inhibition and other heterogeneities in conversion arising from inner filter effects or interactions between additives for instance.

Self-Organized Poly(n-octadecylsilsesquioxane) Films via Sol–Gel Photopolymerization

We describe a novel solvent- and water-free sol-gel process for n-octadecyltriclorosilane (C(18)H(37)SiCl(3)) film catalyzed by photogenerated Brönsted acids. Driven by hydrophobic van der Waals interactions, a photoinduced self-assembly process occurs to afford a long-range ordered lamellar mesostructure, characterized by X-ray diffraction and transmission electron microscopy. Real-time Fourier transform IR spectroscopy was instrumental to probe the fast hydrolysis kinetics and assess the change of conformational behavior of the alkyl chains during UV irradiation. A unique combination of different solid-state NMR techniques ((29)Si, (13)C, (1)H) provided an insight into the supramolecular organization of this hybrid film.

Simultaneous sol–gel and anionic photopolymerization of 3-(glycidyloxypropyl)trimethoxysilanevia photobase catalysis

Type II polyether–silica hybrid films were successfully synthesized from 3-(glycidyloxypropyl)trimethoxysilane (GPTMS) through a novel photobase-catalyzed sol–gel and anionic polymerization using an α-amino acetophenone derivative as a tertiary amine photogenerator. We report here some characterization evidence establishing that the hybrid precursor undergoes two different polymerization reactions in a single step. FTIR analysis as well as solid-state NMR (13C, 29Si) shed light onto the tandem formation of the silica network and the polyether chains. The prominent role played by α-amino acetophenone in the dual catalysis was also highlighted by comparison with other O-acyloxime-based photobase generators.