Sonia Molina-Boisseau

Gas-Phase Surface Esterification of Cellulose Microfibrils and Whiskers

A new and highly efficient synthetic method has been developed for the surface esterification of model cellulosic substrates of high crystallinity and accessibility, namely, freeze-dried tunicin whiskers and bacterial cellulose microfibrils dried by the critical point method. The reaction, which is based on the gas-phase action of palmitoyl chloride, was monitored by solid-state CP-MAS (13)C NMR. It was found that the grafting density not only depended on the experimental conditions, but also on the nature and conditioning of the cellulose samples. The structural and morphological modifications of the substrates at various degrees of grafting were revealed by scanning electron microscopy and X-ray diffraction analysis. These characterizations indicated that the esterification proceeded from the surface of the substrate to their crystalline core. Hence, for moderate degree of substitution, the surface was fully grafted whereas the cellulose core remained unmodified and the original fibrous morphology maintained. An almost total esterification could be achieved under certain conditions, leading to highly substituted cellulose esters, presenting characteristic X-ray diffraction patterns.

The molecular structure of waxy maize starch nanocrystals

The insoluble residues obtained by submitting amylopectin-rich native starch granules from waxy maize to a mild acid hydrolysis consist of polydisperse platelet nanocrystals that have retained the allomorphic type of the parent granules. The present investigation is a detailed characterization of their molecular composition. Two major groups of dextrins were found in the nanocrystals and were isolated. Each group was then structurally characterized using beta-amylase and debranching enzymes (isoamylase and pullulanase) in combination with anion-exchange chromatography. The chain lengths of the dextrins in both groups corresponded with the thickness of the crystalline lamellae in the starch granules. Only approximately 62 mol% of the group of smaller dextrins with an average degree of polymerization (DP) 12.2 was linear, whereas the rest consisted of branched dextrins. The group of larger dextrins (DP 31.7) apparently only consisted of branched dextrins, several of which were multiply branched molecules. It was shown that many of the branch linkages were resistant to the action of the debranching enzymes. The distribution of branched molecules in the two populations of dextrins suggested that the nanocrystals possessed a regular and principally homogeneous molecular structure.

Reinforcing mechanisms of starch nanocrystals in a nonvulcanized natural rubber matrix.

A phenomenological modeling approach was developed to try to understand the reinforcing mechanism of starch nanocrystals in a nonvulcanized natural rubber matrix. Natural rubber was not cross-linked to maintain the biodegradability of the biosourced materials. Nonlinear dynamic mechanical experiments highlighted the significant reinforcing effect of starch nanocrystals and the presence of the Mullins and Payne effects. Two models were used to predict the Payne effect considering that either filler-filler (Kraus model) or matrix-filler (Maier and Göritz model) interactions are preponderant. The use of the Maier and Goritz model demonstrated that phenomena of adsorption and desorption of NR chains on the filler surface governed nonlinear viscoelastic properties, even if the formation of a percolating network for filler contents >6.7 vol % was evidenced by the Kraus model.

Fine grinding of polymers in a vibrated bead mill

Abstract Three polymers, having various glass temperatures, have been ground in a shaker bead mill. One is a friable polymer (poly(vinyl acetate), PVA) and two are thermoplastic polymers (polystyrene (PS) and polyethylene (PE)). These materials have been ground for various operating conditions (bead and polymer loads, vibration frequency, temperature). It was found that, in the case of the friable polymer, the size reduction occurs mainly according to a cleavage mechanism. Nevertheless, in specific operating conditions, attrition may happen. In the case of the thermoplastic polymers, different steps appear during the size reduction: abrasion, creation of cracks and fragmentation. Then, the fragmentation mechanism depends on the polymer nature. It is shown that it is necessary to work with a temperature below the glass temperature of polymer. Generally, the rate of size reduction increases with a decrease of the bead load and of the powder load. In the case of PVA, the evolution of the size distributions has been predicted by two statistical laws (the normal law at the beginning of the experiments and a combination of the normal and the log-normal laws at the end), whereas for PS, only log-normal laws have permitted to predict the evolution of the size distributions.

Characterisation of the physicochemical properties of polymers ground in a vibrated bead mill

Abstract The paper presents a study on the evolution of polymer properties during fine grinding in a vibrated bead mill. Two polymers having different initial properties were tested: poly(vinyl acetate) (PVA) and polystyrene (PS). They were ground with various operating conditions and the evolution of the size, the morphology, the molecular weight, the crystalline structure and the glass transition temperature was characterised. It was shown that the fragmentation rate and the fragment morphology depend on the material nature and on the operating parameters. The energy supplied by the mill is first essentially used for particle fragmentation. Then, when a limit size is reached, chains are cut at their extremities. It is thus possible to grind a polymer without damage if the experimental time is well controlled. The crystalline structure of poly(vinyl acetate) becomes amorphous during the first grinding minutes, and the amorphisation time does not depend on the operating conditions. Finally, no variation of the glass transition temperature was observed in this study.

Production of Small Composite Particles by Co-Grinding in a Media Mill: Characterization of the Granulometric and the Mechanical Properties

The granulometric and mechanical properties of a poly(vinyl acetate) (the matrix) and calcium carbonate (the filler) composite system were investigated. This particulate system was prepared either by blending unground or ground particles of the two materials, or by co-grinding the two products in a laboratory dry agitated bead mill. The follow-up of the evolution of the particle size, illustrated by scanning electron microscope (SEM) photos, has permitted the identification of the mechanism by which the composite is formed by co-grinding. This composite is constituted by filler particles of less than 1 mm homogeneously dispersed in the poly(vinyl acetate) matrix. The characterization of the mechanical properties of blends has shown that the storage tensile modulus is enhanced when adding more than 20 weight percent filler. As for the co-ground composite, this filler produces a great reinforced effect, especially at a temperature higher than the glass transition temperature of the synthetic matrix.

Competitive adsorption of phenol and heavy metal ions onto titanium dioxide (Dugussa P25)

Abstract Single-solute adsorption of phenol (chosen as an organic pollutant) and single-solute adsorption of four heavy metal ions (Cu(II), Zn(II), Cd(II) and Fe(II) chosen as minerals pollutant) onto titanium dioxide (Dugussa P25) nanoparticles in aqueous suspension are studied. Temperature effect is studied for copper and for phenol. Furthermore, competitive adsorption between phenol and each metal cation is also studied. Phenol is dosed via HPLC while heavy metals are quantified by Atomic Adsorption Spectrophotometry. The single-solute adsorption results were well fitted by Freundlich, Langmuir, Temkin and Kiselev isotherm models. This study showed that adsorption of the different pollutants onto TiO2 is favourable and adsorption energy was calculated as well. Moreover it is concluded that phenol adsorption and heavy metal cation adsorption do not proceed with the same mechanism. In the case of competitive adsorption in the 4 studied bisolute systems (phenol in presence of each heavy metal ion), the ad...

Fragmentation mechanism of poly(vinyl acetate) particles during size reduction in a vibrated bead mill

The fragmentation mechanism taking place during dry grinding of poly(vinyl acetate) particles in a shaker bead mill has been analysed using various shape factors permitting a quantitative description of the particles morphology. The knowledge of the size evolution was not sufficient to understand the phenomena occurring in the mill. Several experiments were carried out with different operating conditions and it was shown that the predominant phenomenon governing the fragmentation depends mainly on the intensity of the stress applied to the particles. When this is high, the entire particles are broken while only small fragments are removed when the stress is low. This study permits the definition of the sub-populations of particles created after fragmentation, which can be used for modelling the grinding kinetics.

Size reduction of polystyrene in a shaker bead mill—kinetic aspects

This paper reports an experimental study of polystyrene grinding in a shaker bead mill. The objective is to characterize the influence of the operating conditions on the evolution of the particle size. The effects of temperature, vibration frequency, and bead and polymer loads have been examined. Grinding kinetics have been predicted by combinations of four log-normal laws. The number of sub-populations used for the prediction has been defined after SEM observations. The kinetic parameters (mean size, standard deviation and proportionality coefficient) of the different laws have been determined and their evolution during grinding has been analysed.

The mixing of a polymeric powder and the grinding medium in a shaker bead mill

Abstract In this work, we have studied the influence of the operating conditions on the mixing of a polymeric powder and the glass beads in a shaker bead mill. The parameters studied are the position of polymer, polymer load, bead load, diameters of beads and polymer, frequency and amplitude of vibration. In the system, three phenomena govern the mixing of the constituents and the homogeneity of the system: downward percolation, upward percolation and diffusion. The rate of each phenomenon depends on the operating conditions. We have noted that the acceleration of vibration is the most important parameter.