Cellulose consolidation under high-pressure and high-temperature uniaxial compression

Materials based on cellulose cannot be obtained from thermoplastic processes. Our aim is to prepare all-cellulose materials by uniaxial high pressure thermocompression of cellulose. The effect of moisture content (0–8 w/w%) and temperature (175–250 °C) was characterized through the mechanical properties (bending and tensile), morphology (scanning electron microscopy, X-ray tomography) and microstructure (viscometric degree of polymerization, Raman spectroscopy, X-ray diffraction, solid-state NMR) of the specimens. The specimens were mechanically stronger in bending than in tension. They exhibited a more porous heart, a dense but very thin skin on the faces (orthogonal to the compression axis) and thick and extremely dense sides. During thermocompression severe friction between fibers caused a decrease in molecular weight while heating above the glass transition temperature was responsible for water migration towards the specimen heart. Most of the cohesion came from the small sides of the test samples (parallel to the compression axis) and seemed mainly related to the entanglement of amorphized cellulose at the interface between particles. Around 200 °C water accumulated and provoked delamination upon pressure release, but at higher temperatures water, in a subcritical state, may have been consumed during the hydrolysis of amorphous cellulose regions. The all-cellulose material with the best mechanical properties was obtained at 2% moisture and 250 °C. This work shows that thermocompression at high temperature with limited moisture may be viable to produce renewable, sustainable all-cellulose materials for application in biobased plastic substitutes including binderless boards

Williams, S. Kim R.; Caldwell, Karin D. (Eds.) : Field-flow fractionation in biopolymer analysis

Book’s topic This book covers the application of field-flow fractionation (FFF) to different biopolymers, for (bio)medical applications (medical research, pharmaceutical industry, diagnostic), but also food, membrane technology, etc. FFF is a relatively old method which has encountered new youth and strong development in recent years. The book focuses mainly on asymmetric flow field-flow fractionation (AF4), which dominates the literature and the market, but other methods are also described and assessed

Using apparent molecular weight from SEC in controlled /living polymerization and kinetics of polymerization

Apparent molecular weights from size exclusion chromatography, that is molecular weights relative to standards of a nature different to that of the polymer sample being studied, are frequently used. We use calculations corresponding to realistic cases to provide guidelines for situations when, and to what extent, apparent molecular weights (MWs) can be meaningful. In controlled polymerization, we show how, without due care, use of apparent MW, could lead to the incorrect conclusion that the reaction was not controlled, whereas the true MWs would be close to theoretical values. We show here that the quality of the eluent as a solvent for the standard and the polymer sample is a good indication of the accuracy and the significance of the apparent polydispersity index. Accurate Mark–Houwink–Sakurada parameters are of limited availability, but the data about solvent quality available in handbooks or available from static light scattering measurements. Apparent Mn is of no use in controlled polymerization if simple simulations as performed in this work do not validate their use. The determination of transfer constants by the Mayo plot can be performed using apparent Mn without introducing any significant error, contrary to apparent weight-average molecular weight Mw or apparent ln number distribution

Cinétique de la polymérisation radicalaire des acrylates par polymérisation par LASER pulsé et chromatographie d'exclusion stérique multi-détection (PLP-SEC) (analyse critique)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Size-exclusion chromatography (SEC) of branched polymers and polysaccharides

Branched polymers are among the most important polymers, ranging from polyolefins to polysaccharides. Branching plays a key role in the chain dynamics. It is thus very important for application properties such as mechanical and adhesive properties and digestibility. It also plays a key role in viscous properties, and thus in the mechanism of the separation of these polymers in size-exclusion chromatography (SEC). Critically reviewing the literature, particularly on SEC of polyolefins, polyacrylates and starch, we discuss common pitfalls but also highlight some unexplored possibilities to characterize branched polymers. The presence of a few long-chain branches has been shown to lead to a poor separation in SEC, as evidenced by multiple-detection SEC or multidimensional liquid chromatography. The local dispersity can be large in that case, and the accuracy of molecular weight determination achieved by current methods is poor, although hydrodynamic volume distributions offer alternatives. In contrast, highly branched polymers do not suffer from this extensive incomplete separation in terms of molecular weight

New experimental procedure to determine the recombination rate constants between Nitroxides and Macroradicals

A new experimental procedure for the determination of the recombination rate constant, kc, between a propagating macroradical and a nitroxide is proposed. It is based on a single pulse−pulsed lamp polymerization where the main chain growth breaking event between two consecutive pulses is the recombination of the macroradicals with the nitroxide. The recovered polymer is analyzed by size exclusion chromatography, and the parameters of the molar mass distribution are used to determine kc, in a similar way as that traditionally applied in the determination of the chain transfer rate constants, i.e., the Mayo method using the number- or weight-average degrees of polymerization and the full chain length distribution method. We named the technique RNR−PLP−SEC for radical nitroxide recombination−pulsed lamp polymerization−size exclusion chromatography. The particular polystyryl macroradical−SG1 nitroxide system was tested to validate it. To apply a consistency check, the experimental parameters have been varied according to the recommendations made by the IUPAC for the measurements of the propagation rate constants via PLP. The recombination rate constant kc was measured over a temperature range where no cleavage of the formed alkoxyamine might occur. At 40 °C kc = 2.6 × 105 L mol-1 s-1, and the value increased from 1.1 × 105 L mol-1 s-1 at 15 °C to 4.0 × 105 L mol-1 s-1 at 82 °C. The extrapolation at 120 °C led to 5.3 × 105 L mol-1 s-1, in good agreement with the values already reported in the literature

Polymerization kinetics : monitoring monomer conversion using an internal standard and the Key Role of sample t0

The use of an internal standard is a conventional and convenient way to monitor the conversion of one or several monomers during a controlled radical polymerization. However, the validity of this technique relies on an accurate determination of the initial monomer-tointernal standard ratio, A0, because all subsequent calculations of the conversion are based on this value. In most kinetic studies using an internal standard for the determination of the conversion, an incorrect determination of the value of A0 will result in absurd conclusions and will thus be detected and corrected. However, an incorrect determination of A0 can be significantly misleading in the case of controlled polymerization. This article shows that deviation of the kinetic plots from their true shape because of an error in A0 will not look absurd in the case of controlled polymerization. On the contrary, the plots will fit with a possible deviation of the polymerization from a "quasi-living" to a less controlled process. As a consequence, an error on A0 may not be detected and can result in a misinterpretation of the behavior of the system

In-Situ Bulk Polymerization of Dilute Particle/MMA Dispersions

Composites of poly(methyl methacrylate) and various nanoscale inorganic particles (zinc oxide, titanium dioxide, zirconium dioxide, silicon dioxide, and aluminum nitride) were prepared by in-situ bulk polymerization using 2,2‘-azobis(isobutyronitrile) as initiator. The particles of ZnO, TiO2, and ZrO2 were surface-modified by alkylphosphonic acids to render them dispersible in the monomer. The effect of these nanoparticles on the free radical polymerization was investigated. Regardless of chemical nature and size, the particles suppress the autoacceleration which would otherwise occur in the bulk free-radical polymerization of methyl methacrylate (MMA). A degenerative chain transfer is proposed to take place between surface-adsorbed water on the particles and propagating chain radicals. This reaction competes with normal termination. Formation of vinylidene chains ends originating from disproportionation is suppressed. In consequence, thermal stability of PMMA produced in the presence of particles is improved. Aggregation of individual particles upon polymerization has been observed and presumably is due to interparticle depletion attraction, even though the particles are individually dispersed in the monomer. Formation of particle clusters is suppressed when a difunctional monomer (e.g., ethylene glycol dimethacrylate) is used as comonomer. The cross-linked medium slows down the diffusion of the particles and therefore interferes with particle aggregation via a depletion mechanism