Harri Setälä

A simple process for lignin nanoparticle preparation

A lack of renewable resources and their inefficient use is a major challenge facing the society. Lignin is a natural biopolymer obtained mainly as a by-product from the pulp- and paper-making industries, and is primarily burned to produce energy. However, interest for using lignin in more advanced applications has increased rapidly. In particular, lignin based nanoparticles could find potential use in functional surface coatings, nanoglue, drug delivery, and microfluidic devices. In this work, a straightforward method to produce lignin nanoparticles from waste lignin obtained from kraft pulping is introduced. Spherical lignin nanoparticles were obtained by dissolving softwood kraft lignin in tetrahydrofuran (THF) and subsequently introducing water into the system through dialysis. No chemical modification of lignin was needed. Water acts as a non-solvent reducing lignins degrees of freedom causing the segregation of hydrophobic regions to compartments within the forming nanoparticles. The final size of the nanoparticles depended on the pre-dialysis concentration of dissolved lignin. The stability of the nanoparticle dispersion as a function of time, salt concentration and pH was studied. In pure water and at room temperature the lignin nanoparticle dispersion was stable for over two months, but a very low pH or high salt concentration induced aggregation. It was further demonstrated that the surface charge of the particles could be reversed and stable cationic lignin nanoparticles were produced by adsorption of poly(diallyldimethylammonium chloride) (PDADMAC).

Immobilization–Stabilization of Proteins on Nanofibrillated Cellulose Derivatives and Their Bioactive Film Formation

In a number of different applications for enzymes and specific binding proteins a key technology is the immobilization of these proteins to different types of supports. In this work we describe a concept for protein immobilization that is based on nanofibrillated cellulose (NFC). NFC is a form of cellulose where fibers have been disintegrated into fibrils that are only a few nanometers in diameter and have a very large aspect ratio. Proteins were conjugated through three different strategies using amine, epoxy, and carboxylic acid functionalized NFC. The conjugation chemistries were chosen according to the reactive groups on the NFC derivatives; epoxy amination, heterobifunctional modification of amino groups, and EDC/s-NHS activation of carboxylic acid groups. The conjugation reactions were performed in solution and immobilization was performed by spin coating the protein-NCF conjugates. The structure of NFC was shown to be advantageous for both protein performance and stability. The use of NFC allows all covalent chemistry to be performed in solution, while the immobilization is achieved by a simple spin coating or spreading of the protein-NFC conjugates on a support. This allows more scalable methods and better control of conditions compared to the traditional methods that depend on surface reactions.

Experimental evidence on medium driven cellulose surface adaptation demonstrated using nanofibrillated cellulose

This paper combines theoretical considerations with experimental evidence to explain the behavior of cellulose when exposed to different media. The observations are explained based on the amphiphilic character of the cellulose molecule and fundamental physicochemical phenomena. Nanofibrillated cellulose was chosen to demonstrate the phenomena since due to its high surface area the effects at issue are pronounced. X-Ray photoelectron spectroscopy and contact angle measurements were used to demonstrate the chemical and energetical changes taking place on the cellulose surface, and atomic force microscopy was used to follow nanoscale structural changes. Due to its hydrophilicity cellulose is well dispersed in water. However, when exposed to non-polar media like air or organic solvents cellulose undergoes partly irreversible reorganization like aggregation or surface passivation in order to find the energetically most favorable state. We show that when NFC is dried directly from water it aggregates strongly and accumulates a very high amount of non-cellulosic material on the surface. Very similar effects also occur when using non-polar media like toluene. Hence, both the reactivity and nanoscale structure are lost. In contrast, NFC retains its reactivity and nano-scaled structure in amphiphilic media like dimethyl acetamide as is confirmed with a simple silylation reaction. We conclude that the interfacial phenomenon is general for cellulosic material but has the most practical impact on applications of nanoscaled cellulose or ultrathin cellulose films.

Modified nanofibrillated cellulose-polyvinyl alcohol films with improved mechanical performance

In this study chemically surface-modified nanofibrillated cellulose (NFC) was used at low levels (0.5 to 3 wt%) as reinforcement in a polyvinyl alcohol (PVA) matrix. The modified NFC–PVA films prepared by a solution casting technique showed improved mechanical performance and good optical properties. NFC was allylated and further epoxidised with hydrogen peroxide. The addition of 1 wt% epoxy-NFC enhanced the modulus and strength of the pure PVA film, 474% and 224%, respectively. This composite film exhibited visible light transmittance of 83%. The results also showed that 1 wt% epoxy-NFC loading was beneficial to improve the crystallinity of PVA. SEM characterization confirmed better dispersion of modified NFC within the PVA matrix compared to unmodified NFC. The result showed the favourable effect of chemically modified NFC on the mechanical properties of PVA compared to unmodified NFC as reinforcement.

Direct Interfacial Modification of Nanocellulose Films for Thermoresponsive Membrane Templates

This letter proposes a strategy to construct tunable films combining the physical characteristics of cellulose nanofibrils and smart polymers for membrane applications. A functional membrane template was obtained by first fabricating a water stable film from cellulose nanofibrils and subsequently surface grafting it with a thermoresponsive polymer, poly(N-isopropylacrylamide). The behavior of the membrane template was dependent on temperature. The increment in slope of relative water permeance around the lower critical solution temperature of poly(N-isopropylacrylamide) increased from 18 to 100% upon polymer attachment. Although the membrane template essentially consisted of wood-based materials, the benefits of smart synthetic polymers were achieved.

Correlation between cellulose thin film supramolecular structures and interactions with water

Water interactions of ultra-thin films of wood-derived polysaccharides were investigated by using surface sensitive methods, Quartz Crystal Microbalance with Dissipation (QCM-D) and Atomic Force Microscopy (AFM). These approaches allow systematic molecular level detection and reveal information on the inherent behaviour of biobased materials with nanosensitivity. The influence of structural features of cellulose films i.e. crystallinity, surface roughness and porosity on water interactions was clarified. Cellulose films were prepared using spin-coating and Langmuir-Schaefer deposition to obtain thin films of equal thickness, identical cellulose origin, simultaneously with different supramolecular structures. The uptake/release of water molecules and swelling were characterized using QCM-D, and the structural features of the films were evaluated by AFM. More crystalline cellulose film possessed nanoporosity and as a consequence higher accessible surface area (more binding sites for water) and thus, it was capable of binding more water molecules in humid air and when immersed in water when compared to amorphous cellulose film. Due to the ordered structure, more crystalline cellulose film remained rigid and elastic although the water binding ability was more pronounced compared to amorphous film. The lower amount of bound water induced softening of the amorphous cellulose film and the elastic layer became viscoelastic at high humidity. Finally, cellulose thin films were modified by adsorbing a layer of 1-butyloxy-2-hydroxypropyl xylan, and the effect on moisture uptake was investigated. It was found that the supramolecular structure of the cellulose substrate has an effect not only on the adsorbed amount of xylan derivative but also on the water interactions of the material.

Understanding the mechanisms of oxygen diffusion through surface functionalized nanocellulose films

A concept for direct surface modification on self-standing films of cellulose nanofibrils (CNF) is demonstrated using an aminosilane group in cellulose compatible solvent (dimethyl acetamide, DMA). The chemically modified structure efficiently prevents the oxygen molecules from interacting with the nanocellulose film in the presence of water molecules. Oxygen permeability values lower than 1mLmmm-2day-1atm-1 were achieved at extremely high levels of relative humidity (RH95%). The aminosilane reaction is compared to conventional hydrophobization reaction using hexamethyldisilazane. The differences with respect to interactions between cellulosic nanofibrils, water and oxygen molecules taking place with aminated and silylated CNF films correlated with the degree of surface substitution, surface hydrophilicity and permeability of the formed layer. The self-condensation reactions taking place on the film surface during aminosilane-mediated bonding were decisive for low oxygen permeability. Experimental evidence on the importance of interfacial processes that hinder the water-cellulose interactions while keeping films low affinity towards oxygen is demonstrated.

The effect of oxyalkylation and application of polymer dispersions on the thermoformability and extensibility of paper

Wood fiber-based packaging materials, as renewable materials, have growing market potential due to their sustainability. A new breakthrough in cellulose-based packaging requires some improvement in the mechanical properties of paper. Bleached softwood kraft pulp was mechanically treated, in two stages, using high- and low-consistency refining, sequentially. Chemical treatment of pulp using the oxyalkylation method was applied to modify a portion of fiber material, especially the fiber surface, and its compatibility with polymer dispersions including one carbohydrate polymer. The results showed that the compatibility of the cellulosic fibers with some polymers could be improved with oxyalkylation. By adjusting mechanical and chemical treatments, and the thermoforming conditions, the formability of paper was improved, but simultaneously the strength and stiffness decreased. The results suggest that the formability of the paper is not a direct function of the extensibility of the applied polymer, but also depends on the fiber network structure and surface energy.

Modular modification of xylan with UV-initiated thiol-ene reaction

Birch xylan was functionalized with various thiols through UV initiated radical thiol-ene reaction under mild conditions. Xylan was allylated through etherification with allyl glycidyl ether under alkaline conditions. The allylated xylan was then reacted with thiols containing varying functional groups: trimethylbenzyl mercaptan, dodecanethiol, thioglycolic acid, L-cysteine and cysteamine hydrochloride. The reactions were conducted under homogeneous conditions at room temperature, either in water (hydrophilic thiols) or in DMF (hydrophobic thiols). The effect of reaction parameters to the functionalization efficiency was studied, including, for example, thiol excess, thiol character, initiator amount and reaction mixture concentration. The reactions were fast and 100% conversion of allyl groups was reached in most cases, sometimes already within 10 min. Water as solvent resulted generally in faster reactions when compared to DMF, and it was possible to conduct the aqueous reaction even without added UV initiator. It was also possible to incorporate two functionalities simultaneously during one reaction into the xylan structure.