Sylvain Robert

Grafting of Polycaprolactone on Oxidized Nanocelluloses by Click Chemistry

The main objective of this work is the grafting of polycaprolactone diol (PCL) on the surface of oxidized nanocelluloses (ONC) in order to enhance the compatibility between the hydrophilic cellulose nanofibres and the hydrophobic polymer matrix. This grafting was successfully realized with a new strategy known as click chemistry. In this context, the oxidized nanocelluloses bearing alkyl groups (ONC-PR) were prepared by reacting amino groups of propargylamine (PR) with carboxyl groups of ONC. In parallel, PCL was converted into azido-polycaprolactone (PCL-N3) in two steps: (i) tosylation of polycaprolactone (PCL-OTs) and (ii) conversion of PCL-OTs into PCL-N3 by nucleophilic displacement using sodium azide. Finally, ONC-PR was reacted with PCL-N3 in heterogeneous conditions through click chemistry in order to prepare polycaprolactone grafted oxidized nanocellulose (ONC-g-PCL), which could be suitable for improving the interfacial adhesion in the composite materials. The grafted samples were characterized by transmission electron microscopy and by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Carbon-13 nuclear magnetic resonance spectroscopy (13C-NMR) spectroscopic techniques.

Interaction effects between cellulose and water in nanocrystalline and amorphous regions: a novel approach using molecular modeling

The hydrophilic/hydrophobic nature of cellulose is based on its structural anisotropy. Cellulose chains are arranged in a parallel manner and are organized in sheets stabilized by interchain OH-O hydrogen bonds, whereas the stacking of sheets is stabilized by both van derWaals (vdW) dispersion forces and weak CH-O hydrogen bonds. Cellulose has a strong affinity to itself and materials containing hydroxyls, especiallywater. Based on the preponderance of hydroxyl functional groups, cellulose polymer is very reactive with water. Water molecular smallness promotes the reaction with the cellulose chains and immediately formed hydrogen bonds. Besides that, vdW dispersion forces play an important role between these two reactive entities. They stabilize the cellulose structure according to the considerable cohesive energy in the cellulose network. Hydrogen bonding, electrostatic interactions, and vdW dispersion forces play an important role in determining the cellulose crystal structure during the cellulose-water interactions. As a result of these interactions, the volume of cellulose undergoes a meaningful change expressed not only by an exponential growth in amorphous regions, but also by an expansion in nanocrystalline regions. In addition, the volume change is associated with the swelling material expressed as a weight gain of the cellulose polymer. Molecular modeling using Accelrys Materials Studio allowed us to open a new horizon and is helpful for understanding cellulose-water interactions.

Self-assembly and intermolecular forces when cellulose and water interact using molecular modeling

Cellulose chains are linear and aggregation occurs via both intra- and intermolecular hydrogen bonds. Cellulose has a strong affinity to itself and toward materials containing hydroxyls groups. Based on the preponderance of hydroxyl functional groups, cellulose is very reactive with water. At room temperature, cellulose chains will have at least a monomolecular layer of water associated to it. Theformation of hydrogen bonds at the cellulose/water interface is shown to depend essentially on the adsorption site, for example, the equatorial hydroxyls or OH moieties pointing outward from the cellulose chains. The vdWforces also contribute significantly to the adsorption energy. They are a considerable cohesive energy into the cellulose network. At the surface of the cellulose chains, many intermolecular hydrogen bonds of the cellulose chains are lost. However, they are compensated by hydrogen bonds with water molecules. Electronic clouds can be distorted and create electrostatic dipoles. The large antibonding electron cloud that exists around the glucosidic bonds produces an induced polarization at the approach of water molecules. The electron cloud can be distorted and create an electrostatic dipole. It applies to the total displacement of the atoms within the material. Orbitals play a special role in reaction mechanism. Hydrophilic/hydrophobic nature of cellulose is based on its structural anisotropy. Cellulosewater interactions are exothermic reactions. These interactions may occur spontaneously and result in higher randomness of the system. They are denoted by a negative heat flow (heat is lost to the surroundings). Energy does not need to be inputted in order for cellulose-water interactions to occur.

Study of the Effect of Grafting Method on Surface Polarity of Tempo-Oxidized Nanocellulose Using Polycaprolactone as the Modifying Compound: Esterification versus Click-Chemistry

Esterification and click-chemistry were evaluated as surface modification treatments for TEMPO-oxidized nanocelluloses (TONC) using Polycaprolactone-diol (PCL) as modifying compound in order to improve the dispersion of nanofibers in organic media. These two grafting strategies were analyzed and compared. The first consists of grafting directly the PCL onto TONC, and was carried out by esterification between hydroxyl groups of PCL and carboxyl groups of TONC. The second strategy known as click-chemistry is based on the 1,3-dipolar cycloaddition reaction between azides and alkyne terminated moieties to form the triazole ring between PCL and TONC. The grafted samples were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Thermogravimetry analysis (TGA). Further, the effects of the two treatments on the surface hydrophobization of TONC were investigated by contact angle measurements. The results show that both methods confirm the success of such a modification and the click reaction was significantly more effective than esterification.

Yellowing mechanism and kinetics of thick handsheets of softwood thermomechanical pulp

Abstract Two major UV absorption peaks were observed when thermomechanical papers (black spruce, balsam fir) were exposed to a UV light in the range 300–400 nm for up to 65 hours. One of the peaks around 425 nm was the result of the formation of three different chromophores. The other one at 360 nm was a composite band resulted from the disappearance of one chromophore, and the appearance of a different chromophore, which seemed to be the intermediate molecule of two steps photoinduced reaction. The kinetic observed always follows a first order reaction rate. We believe that the two chromophores observed at 360 nm were related to the formation of the chromophores appearing at 425 nm. This resulted in a system of four chromophores which occurred in yellowing involving three reaction pathways. One of the reaction involved a chromophore which was affected by peroxide and borohydride bleaching. The elimination of this chromophore by either oxidation or reduction minimized its effect on color reversion. The ot...

ELECTRICAL and EMISSION PROPERTIES OF CHLOROPHYLL a IN PLANAR ASYMMETRICAL MEMBRANES

Abstract— We have designed and constructed an experimental set‐up allowing the simultaneous determination of the electrical and fluorescence properties of pigments incorporated in asymmetrical lipid bilayers formed from monolayers. The system is composed of two Langmuir troughs machined in an annealed Teflon block. Each trough is mounted with a movable barrier for the compression of the monolayers as well as with a float type Langmuir torsion balance to measure the surface properties of the monolayers prior to membrane formation. The trough is also equipped with optical windows which permit the localization of a laser beam on the reconstituted membrane incorporating the chromophore. The reconstituted bilayers are further characterized by measurement of their specific resistance and capacitance properties. A detailed description of the experimental method proposed is given, stressing the main advantages of our technique. Finally preliminary results on asymmetrical phosphatidylethanolamine/phosphatidylserine bilayers incorporating chlorophyll‐a molecules are presented and discussed.

Interactions in mixed monolayers between distearoyl-l-phosphatidylethanolamine, rod outer segment phosphatidylethanolamine and all-trans retinal. Effect of pH☆

The interactions in mixed monolayers between distearoyl-L-phosphatidylethanolamine, natural phosphatidylethanolamine purified from bovine rod outer segments and all-trans retinal have been studied at the nitrogen/water interface at 21.0 +/- 0.5 degrees C. Seven mixtures of each phospholipid with all-trans retinal, covering the whole range of molar fractions, were studied. The monolayers were spread on a 1 X 10(-3) M phosphate buffer subphase at three different pH values, 5.5, 7.1 and 8.2. The results for the two series of mixtures are strikingly different. The surface phase rule shows that all-trans retinal is miscible with the natural phospholipid at the interface. Small, negative deviations with respect to the additivity rule are observed in this case. The excess free energies of mixing were also calculated as a function of concentration for this system at four different surface pressures, 5, 7, 10 and 13 mN X m-1. They are negative for the four surface pressures considered and symmetrical with respect to the mole fraction. On the other hand, when distearoyl-L-phosphatidylethanolamine is mixed with all-trans retinal, the components are no longer miscible at the interface. This marked difference in behaviour between the two lipids reflects the importance of hydrophobic interactions in the mixed monolayers of phospholipids with retinals. Furthermore, for the two series of mixtures, the surface pressure isotherms do not show any significant shift when the subphase pH is changed from 5.5 to 8.2. This behaviour raises questions about the formation of a Schiff base between phosphatidylethanolamine and retinal at the interface. It is suggested that, owing to the nature of the disk membranes, such an effect would also be observed in vivo. The possible implications of this are discussed, particularly with respect to questions pertaining to the stability of the retinal chromophore.

Quantum chemical calculations of pristine and modified crystalline cellulose surfaces: benchmarking interactions and adsorption of water and electrolyte

Abstract In this study, we investigate the hydration of three different functional groups present on cellulose nanocrystal (CNC) surfaces: hydroxyls, carboxylates and sulphates by means of quantum chemical calculation. The performance of several density functional theory (DFT) functionals in reproducing, against higher level MP2 benchmark calculations, relevant non-covalent CNC interactions is also assessed. The effect of a sodium ion on the hydration of the surface functional groups was also investigated. Major restructuring of the hydrogen-bonding network within cellulose was found in the presence of a sodium ion. The calculated binding energy of water with a surface group ion pair was also greater, which indicates a greater hydrophilicity of CNC surfaces in the presence of adsorbed sodium. Cellulose hydrophilic surfaces (1 1 0) and (1 −1 0) were also calculated using DFT methods. The results indicate that the surfaces possess different electrostatic potential maps. Hydrogen bond restructuring is found on the chemically modified surfaces. The adsorption energy of water and electrolyte is also found to be different on each surface.

Formamidine sulfinic acid used as a bleaching chemical on softwood TMP

In this work, we showed that formamidine sulfinic acid, FAS, can be efficiently used as a bleaching chemical for softwood TMP. The bleaching reaction was very rapid. The brightness achieved is dependent on the temperature and the chemical charge applied. The highest brightness was obtained at pH 10, and the lowest yellowness at pH 12. FAS treated pulps are much more stable toward light-induced yellowing. In all cases, FAS bleaching at pH 12 results in much slower and less intense aging than all other pulps, either untreated or bleached with FAS at pH 8 or 10.

Use of a Nitrogen‐Centered Peroxide Activator to Increase the Chromophore Removal Potential of Peroxide‐Lignin Model Compound Study

Abstract The kinetics of reaction of a lignin model compound, acetovanillone (4′‐hydroxy‐3′‐methoxyacetophenone), in tetraacetylethylenediamine (TAED)/H2O2 and peroxide‐alone systems were investigated. The oxidation of acetovanillone followed first‐order kinetics in both systems. In the TAED‐peroxide system, the first‐order rate constant, k, was 0.34 min−1 with a half‐life, t 1/2, of 2 min, compared to k of 0.0035 min−1 and t 1/2 of 220 min in the peroxide‐alone system. Thus, TAED/H2O2 oxidized acetovanillone about one hundred times faster than peroxide alone. The results of the study explain why, when using TAED/H2O2 to bleach TMP softwood pulp, the initial pH should be about 11 for increased production of peroxyacetic acid, and the final pH should be about 7 to minimize decomposition and increase bleaching efficiency and reactivity.