Wim Thielemans

Cellulose nanowhisker aerogels

Aerogels were prepared through the self-assembly of cellulose nanowhiskers in a benign manner. Preparation of these aerogels only requires sonication in water to form a hydrogel, solvent exchange with ethanol and supercritical CO2 drying. Aerogels were prepared with varying cellulose nanowhisker content and characterised with X-ray diffraction, BET analysis and electron microscopy. Their density and porosity varied linearly with varying concentrations of cellulose nanowhiskers in the initial hydrogel and confirmed that gel shrinkage upon drying was limited to, on average, 6.5%. We achieved densities down to 78 mg cm−3 with high specific surface areas up to 605 m2 g−1. Mesopores displaying a bimodal size distributions with maxima centred around 4.3 and 15.5 nm accounted for 5–11% of total pore volume. Micropores accounted for less than 1% of total pore volume with the remaining fraction being macropores.

Dual fluorescent labelling of cellulose nanocrystals for pH sensing.

Cellulose nanocrystals were converted into ratiometric pH-sensing nanoparticles by dual fluorescent labelling employing a facile one-pot procedure. A simple and versatile three-step procedure was also demonstrated extending the number of fluorophores available for grafting. In this method an amine group was introduced via esterification followed by a thiol-ene click reaction.

Biodegradability of organic nanoparticles in the aqueous environment.

Synthetic nanoparticles have already been detected in the aquatic environment. Therefore, knowledge on their biodegradability is of utmost importance for risk assessment but such information is currently not available. Therefore, the biodegradability of fullerenes, single, double, multi-walled as well as COOH functionalized carbon nanotubes and cellulose and starch nanocrystals in aqueous environment has been investigated according to OECD standards. The biodegradability of starch and cellulose nanoparticles was also compared with the biodegradability of their macroscopic counterparts. Fullerenes and all carbon nanotubes did not biodegrade at all, while starch and cellulose nanoparticles biodegrade to similar levels as their macroscopic counterparts. However, neither comfortably met the criterion for ready biodegradability (60% after 28 days). The cellulose and starch nanoparticles were also found to degrade faster than their macroscopic counterparts due to their higher surface area. These findings are the first report of biodegradability of organic nanoparticles in the aquatic environment, an important accumulation environment for manmade compounds.

Imidazolium grafted cellulose nanocrystals for ion exchange applications

An imidazolium salt was grafted to cellulose nanocrystals (also called nanowhiskers) using copper(I) catalysed azide-alkyne cycloaddition and the bromide anion was successfully exchanged for bistriflimide and an anionic dye, providing the opportunity to synthesize a wide variety of ion exchange systems or catalysts using cellulose nanocrystals as a support medium.

Permselective nanostructured membranes based on cellulose nanowhiskers

Nanostructured thin films of cellulose nanowhiskers derived from cotton were formed using a simple drop-coating procedure. The hydrogen-bonded cellulose films were stable in aqueous solutions and their permselective properties were probed using voltammetric techniques. The nanowhisker extraction procedure produces cellulose nanowhiskers with negatively-charged sulfate surface groups that inhibit the transfer of negatively-charged species through the nanowhisker membrane, while the diffusion of neutral species is only slightly hindered. Using rotating-disk electrode measurements, the diffusion of various species within the film was studied and it was shown that the positively-charged species, Ru(NH3)63+, was adsorbed by the film, whereas the negatively-charged species, IrCl63−, was excluded by the film. The thermodynamics of adsorption of the positively-charged species by the cellulose nanoparticles were then studied using isotherm data. These observations open up new possibilities in electrochemical sensor development using renewable cellulosic materials as building blocks. Furthermore, charge-based permselective membranes can also be formed using free standing cellulose nanowhisker films, which offer the promise of renewable, selective membranes for separation technologies.

Synthesis of platinum nanoparticles using cellulosic reducing agents

Platinum nanoparticles were formed by reduction of H2PtCl6 using nanocrystalline cellulose from cotton as the reducing agent.

Recycling of Polymers: A Review

Plastics are inexpensive, easy to mold, and lightweight. These and many other advantages make them very promising candidates for commercial applications. In many areas, they have substantially suppressed traditional materials. However, the problem of recycling still is a major challenge. There are both technological and economic issues that restrain the progress in this field. Herein, a state-of-art overview of recycling is provided together with an outlook for the future by using popular polymers such as polyolefins, poly(vinyl chloride), polyurethane, and poly(ethylene terephthalate) as examples. Different types of recycling, primary, secondary, tertiary, quaternary, and biological recycling, are discussed together with related issues, such as compatibilization and cross-linking. There are various projects in the European Union on research and application of these recycling approaches; selected examples are provided in this article. Their progress is mirrored by granted patents, most of which have a very limited scope and narrowly cover certain technologies. Global introduction of waste utilization techniques to the polymer market is currently not fully developed, but has an enormous potential.

Synthesis of carbon-supported Pt nanoparticle electrocatalysts using nanocrystalline cellulose as reducing agent

Pt nanoparticles have been synthesized at relatively low temperatures in aqueous solution from hexachloroplatinic acid using cellulose nanocrystals (CNXLs) from cotton as reducing agents. The Pt nanoparticles were characterised using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. X-ray diffraction and X-ray photoelectron spectroscopy showed that the particles have a metallic Pt core and an oxidised surface layer. TEM analysis showed that the nanoparticles have an average diameter of approximately 2 nm, which is independent of the reactant concentrations. By performing the reduction reaction in the presence of a carbon-black support (Vulcan XC-72R), and removing the cellulosic material by heating in air, it was possible to produce carbon black supported Pt nanoparticles. Electrochemical analysis revealed that this Pt/C was highly active towards electrocatalysis of the oxygen reduction reaction, suggesting that this method may be very useful for fabricating Pt/C electrocatalysts.

Starch Nanocrystal Stabilized Pickering Emulsion Polymerization for Nanocomposites with Improved Performance

Latex/starch nanocrystal (SNC) nanocomposite dispersions were successfully synthesized via a one-step surfactant-free Pickering emulsion polymerization route using SNC as the sole stabilizer. The effect of the SNC content, initiator type and comonomer on the particle size, colloidal stability, and film properties were investigated. Both HCl and H2SO4-hydrolysed starch nanocrystals, each bearing different surface charges, were used as Pickering emulsion stabilizing nanoparticles. SNCs from HCl hydrolysis were found to provide a better stabilization effect, giving rise to a polymer dispersion with a lower average particle size. The mechanistic aspects of the Pickering emulsion polymerization were also discussed. Nanocomposites formed by film-casting the polymer Pickering emulsions showed better mechanical properties and optical transparency than those obtained by blending the polymer emulsion with a nanocrystal dispersion, showing the one-pot route to nanocomposite precursors to be doubly advantageous. Therefore, this in situ polymerization technique not only facilitates the use of SNC nanoparticles, it also provides a valuable nanocomposite with enhanced mechanical properties and high transparency level.

Citric acid as a benign alternative to metal catalysts for the production of cellulose-grafted-polycaprolactone copolymers

Monocrystalline cellulose nanocrystals (nanowhiskers), prepared by acid hydrolysis of cotton wool followed by Soxhlet extraction in ethanol to remove adsorbed impurities, were modified with poly(e-caprolactone) using a “grafting from” approach, with citric acid, a benign naturally available organic acid, as the catalyst. The influence of catalyst concentration, monomer concentration, reaction time, and reaction temperature was studied to determine the optimal conditions for the ROP, and to enable us to control the grafted polymer content. The resulting materials were analysed by FTIR, elemental analysis, XPS and contact angle. In addition, homopolymer byproducts were analysed by GPC. Modified nanoparticles with a PCL content (PCL shell around the crystalline cellulose core) of up to 58 wt% were obtained, about 4 times higher than earlier reported grafted nanoparticles using the common tin(II) ethylhexanoate catalyst. Nanoparticles with a controlled PCL content could be prepared and maximum PCL content was obtained for [e-CL] : [CA] : [OH]surf = 660 : 10 : 1 at 150 °C for 2 hours under inert atmosphere. Increasing the reaction time did not result in a statistically significant increase in the PCL content. As it is virtually impossible to remove all catalyst after polymerisation reactions, the use of a benign, naturally available catalyst in the production of materials aimed at increasing global sustainability is an important step forward.