Benoit Loppinet

Characterization of nanoparticles in diluted clear solutions for Silicalite-1 zeolite synthesis using liquid 29Si NMR, SAXS and DLS

Clear solutions for colloidal Silicalite-1 synthesis were prepared by reacting tetraethylorthosilicate in aqueous tetrapropylammonium hydroxide solution. A dilution series with water resulting in clear solutions with a TEOS ratio TPAOH ratio H2O molar ratio of 25 : 9 : 152 up to 25 : 9 : 15,000 was analysed using liquid 29Si nuclear magnetic resonance (NMR), synchrotron small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Particle sizes were derived independently from DLS and from the combination of SAXS and NMR. NMR allowed quantitative characterization of silicon distributed over nanoparticles and dissolved oligomeric silicate polyanions. In all samples studied, the majority of silicon (78-90%) was incorporated in the nanoparticle fraction. In concentrated suspensions, silicate oligomers were mostly double-ring species (D3R, D4R, D5R, D6R). Dilution with water caused their depolymerisation. Contrarily, the internal condensation and size of nanoparticles increased with increasing dilution. SAXS revealed a decrease of effective nanoparticle surface charge upon dilution, reducing the effective particle interactions. With DLS, the reduction of nanoparticle interactions could be confirmed monitoring the collective diffusion mode. The observed evolution of nanoparticle characteristics provides insight in the acceleration of the Silicalite-1 crystallization upon dilution, in view of different crystallization models proposed in the literature.

Polyelectrolyte effect in perfluorosulfonated ionomer solutions

Ubbelohde viscosity measurements were performed on dilute solutions of perfluorosulfonated ionomers (PFSI) in various solvents exhibiting quite different dielectric constants. With aliphatic alcohols as solvents an increase of the reduced viscosity with dilution is evident. This effect is analogous to the so-called ‘polyelectrolyte effect’ observed for highly charged ionic polymers in polar solvents. The polyelectrolyte effect increases with the dielectric constant for alcohols, is similar in water and in methanol solutions, and disappears for high polarity solvents. This evolution is discussed in terms of an increase of the electrostatic persistence length. The viscosimetric behaviour of PFSI solutions is shown to fit well with the Liberti-Stivala equation.

Interrelation Between Phase State and Gas Permeation in Polysulfone/Polyimide Blend Membranes

The phase state of polysulfone/polyimide (PSF/PI) blends has been studied by differential scanning calorimetry, rheology, and X-ray scattering. The blends rich in PSF form miscible blends when prepared by solution casting from a common solvent. In these PSF-rich blends, the single dynamic process in rheology shifts and broadens, with composition reflecting the change in local friction and the enhancement of concentra- tion fluctuations, respectively. Heating to temperatures above the glass transition temperature results in phase separation into PSF- and PI-rich domains. An apparent phase diagram has been constructed, and helium permeability has been measured in different regimes corresponding to miscible, partially miscible, and completely phase- separated states. We find that one component (PI) controls the permeability values and activation energies for helium permeation in the blends. Gas permeation is found to be very sensitive to local concentration fluctuations and thus can be used as a probe of the phase state in polymer blends.

Dynamics of swollen gel layers anchored to solid surfaces

We employ a dynamic micro light scattering technique to probe the thermal concentration fluctuations in surface-attached poly-N-isopropylacrylamide (PNIPAAm) gel layers swollen in ethanol as a good solvent. At the equilibrium swelling state, the relaxation function exhibits two decays in the time range between microseconds and seconds and the characteristic rates display a pure diffusive behavior. The fast cooperative diffusion increases with crosslinking density as a result of the decrease in the dynamic network mesh size. This increase is significantly stronger than the concentration dependence of the cooperative diffusion in uncrosslinked linear PNIPPAm solutions. Uniaxial swelling due to the surface attachment and structural inhomogeneities intrinsic to photo-crosslinked gels alter the dynamics of the surface anchored networks compared to the solutions. In contrast to the frozen inhomogeneities in conventional gels, the slow diffusion in the present anchored layers was found to be ergodic. It might relate to structural inhomogeneities but its nature is not clarified yet.

Colloidal structure of ionomer solutions in polar solvents

Abstract The structure of ionomer solutions in polar solvents which are not able to dissolve the neutral polymer analogue was investigated using small-angle X-ray and neutron scattering. The colloidal nature of these solutions is demonstrated for two different families of ionomers. The perfluorinated ionomers form rod-like particles. The value of the radius varies from 15 to 25 A depending on the nature of the ionomer and on the solvent. The polyethylene co-methacrylic acid partially neutralized ionomers form ellipsoidal particles. The semi-major axis and the semi-minor axis are, respectively, 125 A and 350 A. In both cases, no solvent penetrates the particles.

Optical spatial solitons and modulation instabilities in transparent entangled polymer solutions

Fully transparent nondilute polydiene solutions exhibit optical nonlinearities when irradiated by a low-power cw laser in the visible. The formation of optical spatial solitons is imaged through phase contrast microscopy. Both (2+1)D and (1+1)D modulational instabilities are evidenced as 2D and 1D arrays of linear filaments formed by beam defocusing or by using a cylindrical lens. The origin of the nonlinearity remains elusive.

Chiral cavity ring down polarimetry: Chirality and magnetometry measurements using signal reversals

We present the theory and experimental details for chiral-cavity-ring-down polarimetry and magnetometry, based on ring cavities supporting counterpropagating laser beams. The optical-rotation symmetry is broken by the presence of both chiral and Faraday birefringence, giving rise to signal reversals which allow rapid background subtractions. We present the measurement of the specific rotation at 800 nm of vapors of α-pinene, 2-butanol, and α-phellandrene, the measurement of optical rotation of sucrose solutions in a flow cell, the measurement of the Verdet constant of fused silica, and measurements and theoretical treatment of evanescent-wave optical rotation at a prism surface. Therefore, these signal-enhancing and signal-reversing methods open the way for ultrasensitive polarimetry measurements in gases, liquids and solids, and at surfaces.

Cavity ring-down ellipsometry

We demonstrate the enhancement of ellipsometric measurements by multiple reflections of a polarized light pulse on a highly reflective target surface, using an optical cavity. The principle is demonstrated by measuring the adsorbed amount of a molecular vapor (fenchone) onto the ring-cavity mirrors. A phase shift sensitivity of about 10(-2) degrees in a single laser pulse is achieved in 1 micros. Further improvements are discussed that should allow sensitivities of at least 10(-4) degrees , surpassing current commercial ellipsometers, but also surpassing their time resolution by several orders of magnitude, allowing the uses of sensitive ellipsometry to be expanded to include the study of fast surface phenomena with submicrosecond resolution.

Semifluorinated Alkanes at the Air-Water Interface: Tailoring Structure and Rheology at the Molecular Scale.

Semifluorinated alkanes form monolayers with interesting properties at the air-water interface due to their pronounced amphi-solvophobic nature and the stiffness of the fluorocarbons. In the present work, using a combination of structural and dynamic probes, we investigated how small molecular changes can be used to control the properties of such an interface, in particular its organization, rheology, and reversibility during compression-expansion cycles. Starting from a reference system perfluor(dodecyl)dodecane, we first retained the linear structure but changed the linkage groups between the alkyl chains and the fluorocarbons, by introducing either a phenyl group or two oxygens. Next, the molecular structure was changed from linear to branched, with four side chains (two fluorocarbons and two hydrocarbons) connected to extended aromatic cores. Neutron reflectivity at the air-water interface and scanning force microscopy on deposited films show how the changes in the molecular structure affect molecular arrangement relative to the interface. Rheological and compression-expansion measurements demonstrate the significant consequences of these changes in molecular structure and interactions on the interfacial properties. Remarkably, even with these simple molecules, a wide range of surface rheological behaviors can be engineered, from viscous over viscoelastic to brittle solids, for very similar values of the surface pressure.

Photoswitching the mechanical properties in Langmuir layers of semifluorinated alkyl-azobenzenes at the air–water interface

Semifluorinated alkyl-azobenzene derivatives (SFAB) can form stable Langmuir layers at the air-water interface. These systems combine the amphiphobic character of semifluorinated alkyl units as structure-directing motifs with photochromic behavior based on the well-known reversible cis-trans isomerization upon irradiation with UV and visible light. Herein, we report our investigations of the structural and dynamic tunability of these SFAB layers at the air-water interface in response to an external light stimulus. The monolayer structures and properties of [4-(heptadecafluorooctyl)phenyl](4-octylphenyl)diazene (F8-azo-H8) and bis(4-octylphenyl)diazene (H8-azo-H8) were studied by neutron reflectivity, surface pressure-area isotherms with compression-expansion cycles, and interfacial rheology. We find that UV irradiation reversibly influences the packing behavior of the azobenzene molecules and interpret this as a transition from organized layer structures with the main axis of the molecule vertically oriented in the trans form to random packing of the cis isomer. Interestingly, this trans-cis isomerization leads to an increase in surface pressure, which is accompanied by a decrease in viscoelastic moduli. These results suggest ways of tailoring the properties of responsive fluid interfaces.