Pascal Griesmar

Sol–gel–xerogel evolution investigated by electroactive probes in silica and transition-metal oxide based gels

Electrochemical techniques, including chronoamperometry, have been used to explore the evolution of composition and structure of silica and transition-metal gels prepared by sol–gel processes. Electroactive probes have been either freely dissolved into the gels or attached to the polymeric and oligomeric species. Their diffusion kinetics have been analysed in relation to parameters such as the gelation catalyst, the gel aging and drying. The results confirm predictions made by other techniques and shed new light on the behaviour of such systems.

One-shot synthesis of a poly(N-isopropylacrylamide)/silica hybrid gel.

Class I hybrid poly(N-isopropylacrylamide)/silica hydrogels, PNIPAM/SiO2, were prepared by a new one shot synthesis. In this approach, the free-radical polymerization of vinyl groups of N-isopropylacrylamide (NIPAM) and the hydrolysis-condensation of alkoxy groups of tetramethoxysilane (TMOS) are performed concomitantly using sodium persulfate and 3-(dimethylamino)-propionitrile, a well-known couple to initiate the organic polymerization. The cross-linker is N,N-methylenebisacrylamide. The kinetic study of mechanical properties from the sol-to-gel state for different ratios of TMOS/NIPAM was investigated by rheological ultrasonic measurements. The thermoresponse of hybrid materials was investigated by differential scanning calorimetry and the measurements showed that hybrid gels present a lower critical solution temperature, which is similar with one of single organic hydrogel.

Sol-gel chemistry for nonlinear optics

Nonlinear optical materials have been synthesized via the sol-gel route. A chemical control of the reactivity of molecular precursors toward hydrolysis and condensation allows the deposition of transparent thin or thick films. Potassium titanyl phosphate (KTP) can be better synthesized from phosphoester precursors [PO(OH)3-x(OR)x] rather than phosphoric acid [PO(OH)3] or phosphate esters [PO(OR)3]. These precursors lead to crystalline transparent KTP thin films around 600 degree(s)C. Organic molecules for nonlinear optics can be embedded into an oxide sol-gel matrix. Second harmonic generation has been obtained after poling with thick films in which organic chromophores are chemically bonded to the oxide backbone via the use of functionalized silicon alkoxide precursors.

Influence of Ni2+ on urease activity produced by biofilms of Arthrobacter oxydans 1388

New TiO2-based hybrid materials composed of an organic polymer, cellulose acetate butyrate and copolymer of acrylonitrile acrylamide (AN + AA) were prepared. The effectiveness of immobilization of microbial strain Arthrobacter oxydans 1388 on the newly synthesized hybrid membranes was investigated by biochemical methods. The obtained results revealed that the matrix more suitable for biofilm formation was composed of organic polymers without a metal component in the membrane composition. The influence of Ni2+ on urease activity produced by biofilms was investigated. The experimental results demonstrated that 2 mg L−1 concentration of Ni2+ in the nutrient medium is more appropriate for biofilm proliferation.

Characterization of New Titanium Oxide Polymer Hybrid Membranes for Biofilm Formation

Abstract Presently, there is great need to create new matrices for diverse applications particularly in the fields of biotechnology and food industry. Here, we report on the synthesis of new matrices based on a mixture of polymer cellulose acetate butyrate/copolymer polyacrylonitrile acrylamide/TiO2. The characterization of the new matrices was performed using IR spectroscopy, QCM technology, and SEM. The tests revealed that when we use our polymers as carriers, there is a limit for the concentration of titanium of 5%. Further increasing of Ti concentration leads to precipitation processes. The QCM analyses show that a low concentration of Ti(OBu)4 does not influence the viscosity of the matrices obtained, but their elasticity changes significantly. The membranes obtained were successfully applied for biofilm formation of Yeast strain Saccharomyces cerevisiae. GRAPHICAL ABSTRACT

Gelation monitoring by quartz microbalance in pulse mode

Classical viscoelastic measurement setup with a quartz crystal microbalance uses a steady state input signal in order to measure the complex equivalent electrical parameters. Using a network analyzer, this method enables the measurement of the equivalent impedance around the first resonance peak of the quartz within a tiny frequency range (typically 10 kHz around a 6 MHz resonance frequency). However, due to the network analyzer acquisition time, such a setup cannot make two successive acquisitions in less than 15 s for one resonance peak. We excite the quartz by a short pulse and record its time impulse response. This kind of excitation allows us to record higher resonance peaks (up to the 11th order) and to reduce drastically the acquisition time, enabling up to 100 acquisitions per second.

Evolution of elastic and thermal properties during TMOS-gel formation determined by ringing bottle acoustic resonance spectroscopy, impulsive stimulated scattering, photopyroelectric spectroscopy and the hot ball method

The evolution of the elastic and thermal properties of a tetramethylorthosilicate (TMOS)-based gel that exhibits an extraordinary ringing effect when enclosed in a bottle is investigated during the sol–gel transition. The results demonstrate the feasibility of three proposed experimental methods for monitoring of gels during their formation. The shear stiffening evolution during gelation is monitored by ringing bottle, resonant acoustic spectroscopy and by an ultrasonic technique using piezo electric excitation and detection. The evolution of the longitudinal modulus and the thermal diffusivity of the gel during stiffening are simultaneously determined by a combined photoacoustic and photothermal method based on heterodyne diffraction detection of impulsive stimulated scattering by, respectively, a propagating acoustic wave grating and a decaying thermal expansion grating that were both thermo elastically generated using a pulsed laser. Also, the feasibility of an inverse photopyroelectric method and a hot ball technique to monitor the thermal transport efficiency and thermal impedance of a forming gel by tracking the thermal conductivity, the thermal diffusivity, and the thermal effusivity is demonstrated. The network polymerization and stiffening during the sol–gel transition in TMOS-gel corresponds with substantial changes in the shear acoustic velocity and in all thermal properties, while the longitudinal acoustic velocity is only weakly affected.

TITAN BASED HYBRID ORGANIC-INORGANIC GELS COMPRISING CARBOHYDRATE MOIETY

Abstract A series of titanium based gels comprising carbohydrate moieties was synthesized. The sugars used include a specific hydrazide group as complex forming agent. Additionally, this group forms stable hydrates which allow further slow water release. As a result, the compounds used in this study form transparent and stable gels with titanium alkoxide without addition of external water. Interpretation of the IR-spectra reveals that some compounds form monodentate ligands and other bidentate ligands. Further Tian-Calvet calorimetric measurements confirmed our IR-based conclusions. This strategy of synthesis gives a new opportunity to obtain hybrid materials by addition of natural chelating ligands to slow down the hydrolysis/condensation reactions that occur during the sol-gel process. Therefore, carbohydrate moieties have been incorporated in the gels obtained and different type of Ti coordination was proposed based on the IR spectroscopy studies. SEM investigations show the influence of the ligand on the morphology of the xerogel. GRAPHICAL ABSTRACT

Monitoring of different complex materials with TSM resonators array

To perform a spatial monitoring of complex fluids, a new sensors made of an array of Thickness Shear Mode Resonator (TSMR) is used. TSMR gives access to the spatial mechanical parameters such as the complex shear modulus. Using a network analyzer, the variation of the impedance of the TSMR around its resonance frequency can be measured. From this impedance measurement, previous works have shown that the complex shear modulus can accurately be extracted using an appropriate electrical model. To perform a spatial monitoring in inhomogeneous materials, a new sensor made of a TSMR array is proposed. This paper presents a new associated model of the TSMR array. Experimental study using water-glycerol mixtures confirm its validity. The efficiency of the rheological measurement using this type of sensor is also proved for gels formation monitoring.

Kinetic reaction monitoring of acidified milk gels with a quartz resonator: effect of temperature and GDL quantities

The behavior of weak gels during their formation particularly attracts the attention of yoghurt factories. Our study is related to an acidified pre-heated milk gel formation fairly often used to produce soft cheeses and yoghurts. These formations require a tight control of the kinetic reaction parameters. To achieve the measurement, a thickness shear mode quartz resonator technique has been used in the MHz frequency range. This method is singularly effective to ensure a complete and reliable follow up of the viscoelastic parameters (the storage G- and the loss -G-shear moduli) of gel materials. Using a suitable model, this technique gives access to a new accurate characteristic time t/sub vs/ (named viscoelastic time). This time corresponds to the instant from which the material is no longer a Newtonian liquid. Using the Arrhenius and modified Flory-Stockmayer relationships, the activation energy is extracted from a study of the gelation process at different temperatures. The experimental results of the G and G moduli are discussed in this paper, on one hand versus temperature, and on the other versus glucono-delta-lactone (GDL) added to milk.