Abraham Chemtob

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.

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.

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.

Photoinduced synthesis and ordering of lamellar n-alkylsiloxane films†

Self-assembled organosilica films exhibiting various lamellar structures were prepared without solvent and watervia the fast photoacid-catalyzed sol–gel process of n-alkyltrimethoxysilanes (CnTMS, n = 8, 10, 12, 16 and 18). This facile photochemical route towards nanostructured hybrid films relies on superacid photocatalyst generated by the UV photolysis of a soluble lipophilic iodonium salt: (C12H25)2Φ2I+ SbF6−. A main emphasis has been on discussing the effect of the alkyl chain length on the level of ordering of the layered mesostructure (short- or long-range order), the alkyl chain packing arrangement (bilayered or interdigitated) and its conformational order (gauche and trans). X-Ray diffraction and electron microscopy (TEM and SEM) proved the high periodicity achieved with the long chain alkylsilanes (n = 16 and 18). Real-time Fourier transform infrared spectroscopy (RT-FTIR) was also instrumental in giving a unique insight into the sol–gel reaction kinetics and the progressive conformational ordering of the alkyl chains during the photoinduced self-assembly. Further, the combination of different solid-state NMR techniques (29Si and 13C) shed light on the siloxy layer microstructure and the conformational structure of the alkyl chain respectively.

Macroporous organosilica films via a template-free photoinduced sol–gel process

In the present work, we have designed a simple and original procedure for the spontaneous formation of macroporous organosilica films by photopolymerization-induced phase separation, avoiding the intervention of a template. The simple mixture of 3-(glycidyloxypropyl)trimethoxysilane (CH3O)3Si(CH2)3OCH2-CHCH2O (GPTMS), and poly(diethoxysiloxane) (C2H5O)2SiO)n (PDEOS, n ≈ 5), without solvent and water followed by UV illumination in the presence of a diaryl iodonium salt (photoacid generator) afforded macroporous hybrid films. This method relies on a single-step photoinduced sol–gel process with photogenerated Bronsted acids, resulting from the iodonium salt photolysis, which catalyse the hydrolysis–condensation reactions of both precursors. The formation of macropores was found to be strongly dependent on the concentration of PDEOS. It has been indeed proven that an increase in the fraction of PDEOS yielded macropores of increased diameters from 100 nm to 50 μm. In this study, our efforts have been focused on two essential aspects. On the one hand, the characterization of the macroporous films with regard to morphology, organic–inorganic microstructure and optical properties was commented upon thoroughly; on the other hand a general mechanism leading to the development of macropores was proposed and discussed.

Activation of the sol–gel process by visible light-emitting diodes (LEDs) for the synthesis of inorganic films

Photoinduced sol–gel polymerization is an efficient one-step and solvent-free process to synthesize inorganic or hybrid films. Highly condensed films are achieved using photoacid generators (PAGs) producing in situ Bronsted superacids without thermal densification. However, most commercial PAGs exhibit short light absorption below 300 nm and display limited overlapping with the emission spectra of conventional UV sources, or with the newly emerging visible LED light sources. Therefore, the development of PAGs with extended absorption in UVA (λ > 380 nm) is of paramount importance. In this study, a simple one-step visible LED induced sol–gel process of poly(dimethoxysiloxane) (PDMOS) was conducted by virtue of different photosensitized acid generating systems. The sol–gel process was followed using real-time Fourier transform infrared (RT-FTIR) spectroscopy. The hydrolysis rate and condensation of the siloxane network were directly correlated with the concentration of the photogenerated acid. Different types of photosensitizers, onium cations and counter anions were investigated. Isopropylthioxanthone combined with iodonium salts bearing high charge delocalized anions led to the fastest and more efficient release of protonic acid, yielding tack-free and condensed transparent silica films.

Block Copolymer Self-Assembly in Mesostructured Silica Films Revealed by Real-Time FTIR and Solid-State NMR

Over the past ten years, understanding the self-assembly process within mesostructured silica films has been a major concern. Our characterization approach relies on two powerful and complementary techniques: in situ time-resolved FTIR spectroscopy and ex situ solid-state NMR. As model systems, three silica/surfactant films displaying various degrees of mesostructuration were synthesized using an amphiphilic block copolymer (PEO-b-PPO-b-PEO) via a UV light induced self-assembly process. The key idea is that the hydration state of the hydrophobic PPO chain is expected to be different depending upon whether the sample is amorphous (blend) or mesostructured (segregated). With real-time FTIR experiments, we show that the methyl deformation mode can act as a signature for the PPO microenvironment so as to trace the progressive copolymer self-association throughout the irradiation time. In (1)H solid-state NMR, the dependence of the (1)H chemical shift on the PPO hydration state has been exploited to evidence the extent of mesostructuration.