Extraction and controlled functionalization of lignin for materials applications

Lignin is a renewable aromatic polymer that due to its abundance and unique chemical structure is a promising candidate to replace aromatic materials that are currently sourced from fossil oil. The same structure of lignin poses however drawbacks for its valorization. The high temperature and strong pH that are generally used to isolate lignin, make this polymer prone to reactions of condensation during its extraction. This translates into isolated lignins where most of their native functional groups are eliminated in favor of the formation of interunit carbon-carbon linkages. The lack of control on lignin's chemical functionalities ultimately hinders the use of this polymer for the development of new materials. Many research groups have been trying to tackle these challenges by developing new processes that could allow for the efficient isolation of lignin without compromising on its functional groups. Amongst these, the Aldehyde-Assisted Fractionation (AAF), which relies on the formation of acetal groups between lignin's ß-O-4 units and an added aldehyde, was proven to be efficient in isolating high yields of lignin, that could be then quantitatively depolymerized into small aromatic monomers. Inspired by this research, the aim of my work is to take advantage of the AAF to quantitatively isolate lignin from lignocellulose, at the same time introducing on the biopolymer scaffold new functional groups that could allow for more effective development of lignin-based materials. My work is based on a Multifunctional-AAF, in which the multifunctional aldehydes can react with the lignin via the formation of acetals, at the same time leaving on the biopolymer additional functionalities. In the first part of my thesis, I show how the extraction and simultaneous functionalization of lignin can be achieved by using terephthalic aldehyde (TALD). In addition to successfully functionalizing the lignin, I demonstrate how the degree of functionalization can be precisely controlled and monitored via 1H NMR, yielding a material with high reactivity towards phenolation. In the second part, I show how the isolation and controlled functionalization of lignin can be translated to the use of other aldehydes, namely glyoxylic acid (GA), to functionalize lignin with carboxylates, quantified via 31P NMR. The isolated lignin offers in this case unique avenues for the development of more sustainable surfactants. In the last chapter of the thesis, I show that the Multifunctional-AAF can be expanded to isolate and functionalize lignin with multiple functionalities by using mixtures of aldehydes (TALD and GA), demonstrating again that each functional group can be easily quantified. These lignins, extracted with different ratios of the two functional groups, showed dissimilar properties in terms of solubility and reactivity towards gelatin. We used these lignins to develop lignin-gelatin based hydrogels, showing how the bi-functionalization was pivotal for the formation of hydrogels with improved rheological, mechanical, self-healing and adhesive properties. In summary, this thesis shows the development of a new approach for lignin extraction and simultaneous valorization. The versatility of this process enables more efficient functionalization, allowing the obtainment of lignins which are "tailor-made" for their end use. The control over lignin's chemical structure and functionalization will pave the way for the use of lignin in high-end application.

Production and characterization of stabilized lignin

Masoud Talebi Amiri

Lignocellulosic biomass is a sustainable source of renewable carbon, and it is the most abundant form of terrestrial biomass. The three main constituents of lignocellulosic biomass are cellulose, hemicellulose, and lignin. The monomers from these biopolymers can be valuable feedstocks for a future sustainable chemical industry, and they include glucose from cellulose, predominantly xylose from hemicellulose, and aromatic molecules from lignin. Even though both glucose and xylose are already established feedstocks for further upgrading in biorefineries, the upgrading of lignin to higher value-added chemicals has not achieved the same success, despite the tremendous need for renewable aromatic molecules. Developing methods for the high-yield depolymerization of lignin can dramatically increase the efficiency and profitability of biorefineries. The main limiting factor in producing high-quality lignin is the inter-unit carbon-carbon linkages in the chemical structure of lignin, and also the formation of these bonds during the extraction process, known as repolymerization or condensation. In the first part of my thesis, I study the possibility of avoiding the condensation of lignin by preventing the formation of C-C linkages. This is achieved by adding formaldehyde, as a protection group, to the pretreatment of lignocellulosic biomass. The results show a near theoretical yield of monomers from the stabilized lignin after depolymerization by hydrogenolysis. This method was then further expanded by screening other protecting groups in order to develop a comprehensive picture of the chemistry involved in this process, and to optimize the process notably to be able to yield a narrower product distribution. Even though this method provides high-quality lignin, there is a need for developing a fractionation process for large-scale production of isolated lignin. This can facilitate the implementation of the process in biorefineries and also further studies on developing new upgrading pathways. The second part of this thesis presents a protocol that fractionates lignocellulosic biomass into pure parts of three main constituents in large laboratory-scale with high extraction yields. The results show extraction yields higher than 95% for each biopolymer. The highly digestible cellulose fraction and high-quality lignin can be depolymerized into their constituent monomers with conventional methods. However, the hemicellulose fraction is obtained as functionalized sugars due to its reaction with aldehyde during pretreatment. The third part of my work presents a non-destructive method for the prediction of the potential yield of monomers from the depolymerization of isolated lignin. A novel 2D heteronuclear single quantum coherence nuclear magnetic resonance (2D-HSQC NMR) spectroscopy technique can be used for simultaneous identification and quantification of chemical groups in the structure of lignin. Standard 2D-HSQC NMR spectroscopy does not provide accurate quantification for polymers and oligomers such as lignin due to errors that are principally caused by differences in relaxation times for different parts of the oligomer chain. However, these errors can be avoided by extrapolation to time-zero 13C HSQC (HSQC0) for series of spectrums, acquired with different repetition times. The prediction results show agreement within a few percentage points with experimental results determined by gas chromatography (GC) after hydrogenolysis.

EPFL2019

Approche expérimentale du comportement mécanique des polymères en sollicitation rapide

In the more recent applications of technical polymers, they are frequently submitted to high strain rates. Being viscoelastic by nature, their mechanical properties are rate dependent. It is therefore important to investigate their properties over a wide range of strain rates. The aims of this study are twofold: i) to propose an experimental technique applicable to a large domain of strain rates, included those covering impact tests, and ii) to apply this technique to the study of the rate dependence of the mechanical properties of some technical polymers. In the first section, the deformation mechanisms of polymers, and the way in which they are influenced by strain rate, are reviewed. Some basics of fracture mechanics are also briefly presented. A detailed study of our original approach to the high rates tests then follows. This technique is based on the reduction of dynamic effects which often affect the measurements at high loading rates. Rather than impact tests, our technique is a quasi-static tests at high rates. This results from the use of a damper as coupling element between the servohydraulic piston and the specimen, together with the specific design of load and displacement detectors. The transient acceleration, responsible for the dynamic effects is decreased, but the high loading rates are maintained. The second section is devoted to the investigation of the rate effect on the tensile and rupture properties of polyoxymethylene (POM) and impact modified polymethylmethacrylate (PMMA). It is shown that high molecular weight POM is less sensitive to the increase of the loading rates than for lower molecular weights. The physical structure, which depends on the processing conditions, plays an important role as well. Results show greater embrittlement by increasing the loading rate than by lowering the temperature. Finally, a molecular model of fracture is proposed. PMMA modified by different fractions and morphologies of the rubbery phase are submitted to tensile and fracture tests at room temperature. Several stages in the micromechanisms of deformation are identified by transmission electron microscopy. The progressive disappearance of some of these stages leads to ductile to brittle transitions in some of the materials studied. The velocity at which these transitions occur depends on the fraction and on the morphology of the modifying phase. It is also shown that a spherical morphology of the rubbery phase is not necessarily required to improve the toughness, but that this latter depends on the interactions between the phases.

EPFL1996

Fluid-structure thermal coupling and ablation effects in atmospheric entry

Ojas Joshi

This thesis deals with the modelling of fundamental aspects of the mission engineering, the physics, and the constraints of atmospheric entry of space vehicles. The aim of this work is to formulate a coupled approach for the modelling and the simulation of thermal fluid-structure interaction in atmospheric entries, taking into account the thermochemical and physical response of the material composing the solid structure. Numerous missions that aim to explore planets, comets or regions outside our galaxy are planned for the next few years. During the phase of entry into the atmosphere of a planet or a moon, space vehicles encounter extreme heating conditions that can damage the structure and the on-board equipment. A Thermal Protection System (TPS) is hence installed to insulate the vehicle’s internal parts from high temperatures and heat fluxes. Ablative materials, which are composed of high-temperature resistant fibre porous structure filled with an organic matrix, are often used to manufacture heat shields of high speed entry vehicles. The hot gas around the spacecraft transfers energy fluxes to the vehicle through the fluid-structure interface. The solid then reacts, with part of the heat being conducted in the deeper layers of the material, and part being absorbed by physical and chemical transformations of the material itself. The organic phase of the ablative material evaporates – this process is called pyrolysis – and these gases are blown into the outer flow, creating thus a protective layer. Moreover, the surface of the TPS may react with the atmospheric gases, consuming in this way a fraction of the material. A model for these two phenomena and the thermal fluid-structure interaction is proposed in this thesis. Numerical simulations are one of the most widely used tools to design the TPS, and a detailed mathematical model allows to reduce margins in the sizing. Atmosphere gases and the spacecraft structure form two distinct systems that interact through the external surface of the TPS, and through which energy and mass exchanges occur. Many computational tools exist to simulate the physics of each of the two domains. A higher level of accuracy is however achieved if the problem is approached in a coupled fashion, where each system takes into account the thermophysical response of the other. An innovative coupling algorithm, which includes the direct evaluation of the surface chemistry, is presented in this thesis. Using this coupling procedure, numerical simulations of the TPS of a European Space Agency project are performed and key parameters for the sizing of the heat shield are evaluated and discussed.

EPFL2013