Eve Péré

Effect of the synthetic methodology on molecular architecture: from statistical to gradient copolymers

Styrene-butyl acrylate (S-BuA) copolymers were synthesized by nitroxide-mediated controlled radical polymerization using an alkoxyamine as an initiator. Using different synthetic methodologies, statistical copolymers were be obtained by batch nitroxide mediated polymerization while the gradient composition was a forced gradient by continuous addition of S during BuA polymerization (semi-batch process). These gradient copolymers have been studied by H NMR and size exclusion chromatography to characterize the gradient composition molecular structure. The evolution of the composition was correlated with the glass transition temperature () of the copolymers. The gradient copolymers exhibit one with a value in between the of polystyrene and poly(butyl acrylate), indicating that the materials did not present well defined microphase separation. Specific organization at the air-polymer interface of such copolymers has also been demonstrated by comparison between classical and attenuated total reflection (ATR) Fourier transform infra-red spectra. This bulk soft matter assembly was confirmed by AFM analysis, which showed a different morphology at the surface and in the bulk following removal of the top layer. Moreover, for the most well defined gradient composition, a specific nano-structuring was demonstrated by small angle neutron scattering. The preliminary rheological properties of these gradient copolymers were studied and are discussed in relation with their molecular structure.

Gradient or statistical copolymers by batch nitroxide mediated polymerization: effect of styrene/methyl acrylate feed

Monomer reactivity ratios in controlled radical copolymerization of styrene (S) and methyl acrylate (MA) monomers at 120 °C were determined. The Fineman–Ross method was used to calculate rs and rMA. Using this method, monomer reactivity ratio values of 0.89 and 0.22 were calculated for styrene and methyl acrylate, respectively. Because of the different reactivity ratios between the two monomers, and according to the molar fractions of two monomers, S/MA statistical or gradient copolymers were synthesized by nitroxide-mediated polymerization. Indeed, for different monomer ratios, the same statistical or gradient copolymers can be obtained by batch nitroxide mediated polymerization. These copolymers have been characterized by 1H nuclear magnetic resonance and size exclusion chromatography, and evolution of the composition has been correlated with the glass transition temperature measured by differential scanning calorimetry. Integrated intensities of the three observed peaks of (–OCH3) group in the 1H NMR spectra were used to determine the MA/MA/MA, MA/MA/S and S/MA/S triad sequences in the copolymers. Specific organization at the air/polymer interface of such copolymers has also been demonstrated by comparison between classical and attenuated total reflection (ATR) Fourier transform infra-red spectra.

Dispersion Improvement of Carbon Nanotubes in Epoxy Resin Using Amphiphilic Block Copolymers

Interface between Carbon NanoTubes (CNT) and epoxy matrix is admitted to play an important role in the dispersion quality and in the mechanical stress transfer. To improve the interfacial adhesion, we propose to chemically graft molecules onto CNT surface. To achieve this chemical modification, a controlled radical polymerization, named Nitroxide Mediated Polymerization NMP, is used to synthesize a diblock copolymer based on Acrylic Acid (PAA block) and Methyl MethAcrylate (PMMA block). In the present paper, this polymerization is performed “in situ”. The PAA block presents a good affinity with the CNT which enable grafting. The PMMA miscibility with epoxy is expected to give a good adhesion - between the CNT and the matrix - and to bring a better dispersion. In order to compare the chemical modification and the physical adsorption of the copolymers onto CNT dispersion, the same block copolymer was synthesized with and without CNT. The copolymer synthesis was controlled and characterized by different methods as NMR 1H (conversion and composition), SEC (molecular weight) and TGA (grafting density). We show that the better dispersion quality and better physical properties have been obtained with grafted CNT.

New Insights on Gas Hydroquinone Clathrates using in situ Raman Spectroscopy: Formation/Dissociation Mechanisms, Kinetics and Capture Selectivity

Hydroquinone (HQ) is known to form organic clathrates with different gaseous species over a wide range of pressures and temperatures. However, the enclathration reaction involving HQ is not fully understood. This work offers new elements of understanding HQ clathrate formation and dissociation mechanisms. The kinetics and selectivity of the enclathration reaction were also investigated. The focus was placed on HQ clathrates formed with CO2 and CH4 as guest molecules for potential use in practical applications for the separation of a CO2/CH4 gas mixture. The structural transition from the native form (α-HQ) to the clathrate form (β-HQ), as well as the reverse process, were tracked using in situ Raman spectroscopy. The clathrate formation was conducted at 323 K and 3.0 MPa, and the dissociation was conducted at 343 K and 1.0 kPa. The experiments with CH4 confirmed that a small amount of gas can fill the α-HQ before the phase transition from α- to β-HQ begins. The dissociation of the CO2-HQ clathrates highlighted the presence of a clathrate structure with no guest molecules. We can therefore conclude that HQ clathrate formation and dissociation are two-step reactions that pass through two distinct reaction intermediates: guest-loaded α-HQ and guest-free β-HQ. When an equimolar CO2/CH4 gas mixture is put in contact with either the α-HQ or the guest-free β-HQ, the CO2 is preferentially captured. Moreover, the guest-free β-HQ can retain the CO2 quicker and more selectively.

Double Beam FT-IR Reflection Spectroscopy on Monolayers

For in situ infrared studies of ultrathin films deposited on solid substrates or spread at an air/liquid interface, we have developed a new FT-IR double beam approach based on differential reflectivity by polarization modulation. Because only the s-component (perpendicular to the plane of incidence) of the electromagnetic field is incident on the studied surface, the spectral features are easy to interpret in terms of molecular orientation. Spectra of a cadmium arachidate Langmuir-Blodgett film deposited on a gold mirror and a DMPC Langmuir film spread at the air/water interface are reported, to illustrate the advantages of this method.

Real double-beam FTIR spectroscopy by polarization modulation

For transmission FT-IR spectroscopy of ultrathin films, we have developed a new double beam approach based on the technique of polarization modulation. Simultaneous recording of sample and reference interferograms makes detection insensitive to atmospheric (CO2, H2O) and substrate absorptions. In situ spectra of ultrathin films are obtained in a few minutes with high signal-to-noise ratio (S/N).