Pierre Terech

The network structure of a steroid/cyclohexane physical gel

Abstract Steroid/cyclohexane gels have been observed by the freeze-etching replication electron microscopy technique. Stereo images reveal clearly the three-dimensional gel network which has a mesh size of about 300 nm and consists of long filaments mostly 9.1 nm in diameter in agreement with previous SANS results. A 4.6-nm-diameter protofilament is observed. The 9.1-nm filaments appear to be made from two or more twisted protofilaments. There are two principal types of contact zone between the main filaments in which the filaments are either in parallel juxtaposition or fuse to form a filament of diameter less than the sum of the two incoming filaments.

Kinetics of aggregation in a steroid derivative/cyclohexane gelifying system

Abstract The kinetics of aggregation in a spin-labeled steroid/cyclohexane gelifying system is studied by electron spin resonance (ESR) and small-angle neutron scattering (SANS). The observed induction times are interpreted in terms of germination laws. The aggregation phenomenon is described by models used in heterogeneous and homogeneous chemical kinetics. The heterogeneous model supports a one-dimensional growth for the gel network. The homogeneous model is a two-step process where the first step is autocatalytic and the second one is first order. Arrhenius plots show that energies involved in both steps are about the same (6.5 kcal/mole). In addition, study of the transition gel → sol by infrared (IR) absorption spectroscopy supports first-order hydrogen-bonding mechanisms. A scheme of gelling domains growing in or out of phase is proposed to explain all observed kinetic curves.

Correlations of properties and structures at different length scales of hydro- and organo-gels based on N-alkyl-(R)-12-hydroxyoctadecylammonium chlorides.

The self-assembly and gelating ability of a set of N-alkyl-(R)-12-hydroxyoctadecylammonium chlorides (NCl-n, where n = 0-6, 18 is the length of the alkyl chain on nitrogen) are described. Several are found to be ambidextrous (gelating both water and a variety of organic liquids) and very efficient (needing less than ca. 0.5 wt % at room temperature). Structure-property correlations at different distance scales of the NCl-n in their hydro- and organo-gels and neat, solid states have been made using X-ray diffraction, neutron scattering, thermal, optical, cryo-SEM and rheological techniques. The self-assembled fibrillar networks consist of spherulitic objects with fibers whose diameters and degrees of twisting differ in the hydro- and organo-gels. Increasing n (and, thus, the molecular length) increases the width of the fibers in their hydrogels; an irregular, less pronounced trend between n and fiber width is observed in the corresponding toluene gels. Time-dependent, small angle neutron scattering data for the isothermal sol-to-gel transformation of sols of NCl-18/toluene to their gels, treated according to Avrami theory, indicate heterogeneous nucleation involving rodlike growth. Rheological studies of gels of NCl-3 in water and toluene confirm their viscoelastic nature and show that the hydrogel is mechanically stronger than the toluene gel. Models for the different molecular packing arrangements within the fibrillar gel networks of the hydro- and organogels have been inferred from X-ray diffraction. The variations in the fibrillar networks provide a comprehensive picture and detailed insights into why seemingly very similar NCl-n behave very differently during their self-assembly processes in water and organic liquids. It is shown that the NCl-n provide a versatile platform for interrogating fundamental questions regarding the links between molecular structure and one-dimensional self-aggregation, leading to gelation.

Cosolvent gel-like materials from partially hydrolyzed poly(vinyl acetate)s and borax.

A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydro-HVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. (11)B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions.

Soluble heterometallic coordination polymers based on a bis-terpyridine-functionalized dioxocyclam ligand.

Soluble homo- and heterometallic coordination polymers containing transition metal cations (Cu(2+), Fe(2+), Co(2+), and Ni(2+) ions) were prepared in a two-step procedure using a polytopic bis(terpyridine)dioxocyclam ligand 1H(2) (dioxocyclam = 1,4,8,11-tetraazacyclotetradecane-5,7-dione). These supramolecular systems incorporate two different metal complexes, the metal cations being located both between two terpyridine units and in the macrocyclic framework. The characterization of these soluble architectures was investigated by cyclic voltammetry, mass spectrometry, viscosimetry, and UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies. Our results clearly indicate the formation of well-organized heterometallic polymers in which two different metal ions alternate in the self-assembled structure. These investigations furthermore brought to light an original acid-controlled disassembling process of the homometallic copper(II) polymer into dinuclear complexes.

Structure and Rheology of Cationic Molecular Hydrogels of Quinuclidine Grafted Bile Salts. Influence of the Ionic Strength and Counter-Ion type

Quinuclidine grafted cationic bile salts are forming salted hydrogels. An extensive investigation of the effect of the electrolyte and counterions on the gelation has been envisaged. The special interest of the quinuclidine grafted bile salt is due to its broader experimental range of gelation to study the effect of electrolyte. Rheological features of the hydrogels are typical of enthalpic networks exhibiting a scaling law of the elastic shear modulus with the concentration (scaling exponent 2.2) modeling cellular solids in which the bending modulus is the dominant parameter. The addition of monovalent salt (NaCl) favors the formation of gels in a first range (0.00117 g cm(-3) (0.02 M) < T(NaCl) < 0.04675 g cm(-3) (0.8 M)). At larger salt concentrations, the gels become more heterogeneous with nodal zones in the micron scale. Small-angle neutron scattering experiments have been used to characterize the rigid fibers ( approximately 68 A) and the nodal zones. Stress sweep and creep-recovery measurements are used to relate the lack of linear viscoelastic domain to a mechanism of disentanglement of the fibers from their associations into fagots. The electrostatic interactions can be screened by addition of salt to induce a progressive evolution toward flocculation. SEM, UV absorbance, and SAXS study of the Bragg peak at large Q-values complete the investigation.

Thermoreversible gelation of organic liquids by arylcyclohexanol derivatives Synthesis and characterisation of the gels

A variety of organic liquids can be immobilised using certain 4-tert-butyl-1-arylcyclohexanol derivatives (BACOl). Only the isomers with axial aryl groups are active as gelling agents. The BACOl–hydrocarbon systems have been characterised with respect to their rheological properties under shear. The gels are viscoelastic solids with high elastic shear moduli (G′≈70350 Pa at C≈2.2 wt.% in dodecane) and high yield stresses (σ≈620 Pa). A 3D network with high cohesive energy and long lifetimes of the related structures is in thermal equilibrium with the surrounding solution. Diffraction experiments have characterised the crystallinity of the gel networks made up of assemblies of bimolecular BACOl associations (d≈14.3 A). Xerogels exhibit a mesomorphic organisation with a 76 A repeating unit. Phase diagrams have been determined as a function of the solvent and gelator types and the related thermodynamic parameters were deduced. IR spectroscopy has demonstrated that H-bonding is responsible for the aggregation of the BACOl molecules.

Characterization of strain recovery and “self-healing” in a self-assembled metallo-gel

We report a self-assembled metallo suprapolymer gel exhibiting remarkable self-healing features. The Ni2BTC metallo suprapolymer gels result from the complexation of Ni(2+) metal ions by a tritopic ligand (bis-terpyridine cyclam) in dimethylformamide (DMF) and an annealing step at 50 °C for 24 hours. The self-healing properties are characterized by visual inspection, rheological and impedance spectroscopy measurements: the results are compared with those of a fatty acid-based molecular organogel chosen as a reference system. The creep-recovery analysis uses the Burgers model for low strains and characterizes a recovery capability of up to 72% of the deformation in Ni2BTC gels while it is only 32% for the fatty acid organogel. At very large strains, the impedance spectroscopy confirms the slow repairing process consistently with the visual observations. Rheological measurements demonstrate the restructuring of the fractured networks. The fatigue of the self-healed gel networks undergoing long sequences of strain-relaxation steps is characterized.

Self-assembled composite nano-materials exploiting a thermo reversible n-acene fibrillar scaffold and organic-capped ZnO nanoparticles

An organic–inorganic composite material is obtained by self-assembly of 2,3-didecyloxy-anthracene (DDOA), an organogelator of butanol, and organic-capped ZnO nanoparticles (NPs). The ligand 3, 2,3-di(6-oxy-n-hexanoic acid)-anthracene, designed to cap ZnO and interact with the DDOA nanofibers by structural similarity, improves the dispersion of the NPs into the organogel. The composite material displays mechanical properties similar to those of the pristine DDOA organogel, but gelates at a lower critical concentration and emits significantly less, even in the presence of very small amounts of ZnO NPs. The ligand 3 could also act as a relay to promote the photo-induced quenching process.

Acid-base-driven interconversion between a mononuclear complex and supramolecular coordination polymers in a terpyridine-functionalized dioxocyclam ligand.

A polytopic cyclam-bis-terpyridine ligand has been used to accomplish an acid-base-triggered formation of either a mononuclear neutral complex or metallopolymers with Cu(2+) ions. A controlled interconversion between these two forms was achieved through the reversible displacement of a Cu(2+) cation from the macrocycle to the terpyridine units.