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FUNCTIONALISATION OF
CELLULOSE-BASED MATERIALS
JOHAN ERLANDSSON
Doctoral Thesis
KTH Royal Institute of Technology
School of Engineering Sciences in Chemistry, Biotechnology and Health Department of Fibre and Polymer Technology
Stockholm, Sweden 2019
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Professor Lars Wågberg, Dr. Per A. Larsson
Copyright © Johan Erlandsson, 2019 All rights reserved
Paper I © Elsevier 2016
Paper II © Royal Society of Chemistry 2018 Paper III © American Chemical Society 2019 Paper IV © American Chemical Society 2018 Paper V Manuscript
TRITA-CBH-FOU-2019:44 ISBN 978-91-7873-295-1
Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknisk doktorsexamen fredagen den 11 oktober 2019, kl. 10 i sal F3, Lindstedtsvägen 26, KTH, Stockholm
Fakultetsopponent: Professor Sang-Young Lee, Ulsan National Institute of Science and Technology, Ulsan, South Korea
Avhandlingen försvaras på Engelska
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Till Erik och Gustav
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intensified and driven society and research towards new materials and processes that utilise renewable resources. Within the development of new materials, wood has been identified as a raw-material from which high performing materials can be derived. One such material is cellulose nanofibrils (CNFs) which are capable of replacing several currently used fossil-based materials. However, for CNFs to exhibit the required material properties they need to be chemically or physically modified. This means that the properties of the CNFs can be specifically adapted to fit the demand in particular areas, for example electrical energy storage. In these applications it is the mechanical properties; the large, easily functionalised surface and ability to be moulded into 3D shapes that make CNFs a highly interesting raw material.
This thesis explores the formation and functionalisation of CNF- and fibre- based materials and their novel use in applications such as energy storage.
The wet stability of the materials was achieved by crosslinking and ice templating the fibrils by a novel freezing procedure, which makes it possible to avoid the use of freeze-drying and subsequent crosslinking.
Using colloidal probe atomic force microscopy adhesion measurements, hemiacetals were shown to be formed between the aldehyde-containing fibrils when they are brought into molecular contact, for example during ice templating. Hemiacetal crosslinked aerogels have been shaped and functionalised to demonstrate their application as biomimetic structural composites, electrical circuits and electrical cells. In addition, crosslinked, light-weight 3D fibre networks were prepared with á similar chemistry by a self-assembly process of pulp fibres. These networks could be dried under ambient conditions and the materials formed were wet-stable due to the hemiacetal crosslinks formed in the fibre–fibre contacts, which provided the networks with excellent mechanical properties and shape recovery capacity in water.
Finally, using a newly developed polyampholyte and mixing it with CNFs, heterofunctional composite films and aerogels could be prepared. By activating crosslinkable groups in these composite materials, they were able to undergo further water based chemical functionalisation. In this
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Keywords: cellulose nanofibrils, porous materials, freeze-linking, functionalisation
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av fossila resurser har intensifierat sökandet efter alternativa lösningar.
Detta har i sin tur inriktat forskningen att undersöka och utveckla nya processer och material från förnyelsebara råvaror. I detta sökande har skogsråvaror presenterats som en kandidat som kan tillgodose behovet av förnyelsebara råmaterial för tillverkning av högpresterande material.
Cellulosananofibriller (CNF:er), frilagda från fibrer från ved, är ett av materialen som identifierats och dessa har ur flera synvinklar möjligheten att konkurrera med dagens fossilbaserade lösningar. För att lyckas ersätta de fossilbaserade materialen måste dock cellulosan modifieras, antingen fysiskt eller kemiskt, så att egenskaperna motsvarar kraven som ställs i applikationen, t.ex. elektrisk energilagring.
Arbetet i föreliggande avhandling undersöker olika metoder för bildning och funktionalisering av våtstabila CNF-och cellulosafiberbaserade nätverk ochderas användning i för dessa material nya applikationsområden t.ex. energilagringr. Resultaten visar att våtstabiliteten uppnås genom bildning av hemiacetalbindningar mellan fibrillerna genom att istemplera dem i en konventionell frysningsprocess.
Detta gör det möjligt att undvika processer så som frystorkning och efterföljande aktivering av tvärbindningsreaktioner.
Atomkraftsmikroskopistudier av adhesionen mellan fibriller i vått tillstånd visade att hemiacetalerna bildas mellan fibrillerna när dessa är i molekylär kontakt under kemiskt neutrala förhållanden och att den molekylära kontakten kan bildas av växande iskristaller under frysningsprocessen.
Aerogeler med olika skräddarsydda former användes därefter som substrat för ytfunktionlisering och strukturella biomimetiska kompositer. Bland annat demonstrerades elektriska kretsar och elektroder baserade på aerogeler. Porösa och hemiacetaltvärbunda fibernätverk med god formretention, våtstabilitet och goda mekaniska egenskaper tillverkades också med liknande kemi.
Slutligen visades att det var möjligt att bilda CNF-baserade kompositmaterial i form av aerogeler och filmer genom att blanda en specialtillverkad polyamfolyt med CNF:er på nanoskala. Den heterofunktionella polyamfolyten kunde sedan användas för att kemiskt tvärbinda materialen vilka därmed kunde användas för vidare
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Nyckelord: cellulosananofibriller, porösa material, fryslänkning, funktionalisering
ix in energy storage devices
Johan Erlandsson, Verónica López Durán, Hjalmar Granberg, Mats Sandberg, Per A. Larsson, Lars Wågberg
Applied Materials Today (2016), 5, 246-254
Paper II – On the mechanism behind freezing-induced chemical crosslinking in ice-templated cellulose nanofibrils aerogels
Johan Erlandsson, Torbjörn Pettersson, Tobias Ingverud, Hjalmar Granberg, Per A. Larsson, Lars Wågberg
Journal of Materials Chemistry A (2018), 6, 29371-19380
Paper III – Cross-linked and shapeable porous 3D substrates from freeze-linked cellulose nanofibrils
Johan Erlandsson, Hugo Françon, Andrew Marais, Hjalmar Granberg, Lars Wågberg
Biomacromolecules (2019), 20, 728-737
Paper IV – Novel cellulose-based light weight, wet resilient materials with tuneable porosity, density and strength
Verónica López Durán, Johan Erlandsson, Lars Wågberg, Per A. Larsson ACS Sustainable Chemistry &. Engineering (2018), 6, 9951-9957
Paper V – The combination of a dendritic polyampholyte and cellulose nanofibrils – a new type of functional material Tobias Ingverud, Johan Erlandsson, Lars Wågberg, Michael Malkoch Manuscript
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Paper I Main author responsible for planning and performing the experimental work and data analyses except for the x-ray µ-tomography. Wrote first draft of manuscript and compiled comments and contributions from the other authors.
Paper II Main author responsible for planning and performing the experimental work and data analyses except for the AFM- study which was performed together with Torbjörn Pettersson. Wrote first draft of manuscript and compiled comments and contributions from the other authors.
Paper III Main author, performed the majority of the material preparation, experimental work and data analyses.
Prepared first draft of the manuscript and compiled contributions and comments from the other authors.
Paper IV Co-author, performed part of the material preparation and characterisation, all the mechanical evaluation and contributed to the manuscript preparation
Paper V Shared main authorship with Tobias Ingverud, performed part of the material preparation, characterisation and data analyses. Manuscript was prepared and contributions and comments from the other authors were compiled together with Tobias Ingverud.
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influence on structure and mechanical properties Kaldéus T., Nordenström M., Erlandsson J., Wågberg L., Malmström E., Manuscript in preparation,
Thickness, Temperature and Humidity Dependence of Conducting Carbon Nanotube Layer-by-Layer networks Johan Erlandsson, Deyu Tu, Hjalmar Granberg, Mats Sandberg, Robert Forchheimer and Lars Wågberg,
Manuscript in preparation
xii contributions:
Macro- and mesoporous spherical nanocellulose beads for use in energy storage devices
Johan Erlandsson, Verónica López Durán, Per A. Larsson, Hjalmar Granberg, Mats Sandberg, Lars Wågberg,
251st American Chemical Society Meeting, San Diego, California, USA (2016)
Nanocellulose aerogel beads: Structurable and printable energy storage
Johan Erlandsson, Hjalmar Granberg, Mats Sandberg, Lars Wågberg, 253rd American Chemical Society Meeting, San Francisco, California, USA (2017)
xiii CNF – Cellulose nanofibril
CNT – Carbon nanotube CPD – Critical Point Drying DLS – dynamic light scattering
d-VAS – dynamic volume filling arrested state PEI – Poly(ethyleneimine)
LbL – Layer-by-Layer
PEDOT:PSS - poly(3,4-ethylenedioxythiophene) polystyrene sulfonate PPy - polypyrrole
PVAm – poly(vinyl amine)
QCM-D – quartz crystal microbalance with dissipation SEM – Scanning electron microscopy
SPAR – stagnation point adsorption reflectometry TEMPO - 2,2,6,6-Tetramethylpiperidine 1-oxyl
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1.1 Context ... 1
2. BACKGROUND ... 2
2.1 Cellulose in wood ... 2
2.2. Wood fibres ... 3
2.3. Cellulose nanofibrils... 4
2.4.1 Nanopapers ... 5
2.4.2 Aerogels ... 6
2.4.3 Hydrogels ... 8
2.4.4 CNF-matrix composites... 9
2.4. Functionalisation of CNF and nanocellulose-based materials ... 10
2.5.1 Chemical modification and use of modified CNFs ... 10
2.5.2 Physical modification of cellulose-based materials .... 11
2.5.3 LbL in energy-storage applications ... 12
3. EXPERIMENTAL ... 14
3.1 Materials ... 14
3.1.1 Fibres and fibrils ... 14
3.1.2 Chemicals ... 14
3.2 Methods ... 15
3.2.1 Preparation of wet-stable CNF aerogels ... 15
3.2.2 Preparation of wet-stable fibre networks ... 15
3.2.3 Total charge determination ... 16
3.2.4 Aldehyde content determination ... 16
3.2.5 Mechanical evaluation of CNF-based materials ... 17
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3.2.8 Atomic Force Microscopy Imaging ...18 3.2.9 Colloidal Probe AFM ...18 3.2.10 Polyelectrolyte titration...19 3.2.11 Layer-by-Layer modification of cellulose aerogels ...19 3.2.12 Model surfaces ...20 3.2.13 Stagnation Point Adsorption Reflectometry ...20 3.2.14 Quartz-crystal microbalance with Dissipation ...21 3.2.15 Electrochemical evaluation of CNF-based electrode materials ...21 4. RESULTS AND DISCUSSION ...22
4.1 Preparation and properties of wet-stable CNF aerogels using ice-templating ...22
4.1.1 Aerogel beads (Paper I)...22 4.2 The establishment of the molecular mechanism behind freeze-linking (Paper II) ...25
4.2.1 The effect of aldehydes on the interactions between CNFs ...26 4.2.2 The origin of the interfibril chemical bond ...29 4.2.3 The effect of aldehyde content on aerogel formation .31 4.3 Shapeable CNF aerogels using freeze-linking (Paper III) .34
4.3.1 Principle of preparation and properties of aerogels ....34 4.3.2 Mechanical properties of freeze-linked aerogels ...37 4.3.3 Re-shaping and resilience of aerogels ...40 4.4 Wet-stable fibre networks (Paper IV) ...44
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4.5.1 Polyampholyte and CNF aqueous dispersions ... 47
4.5.2 Composite CNF:Helux films and aerogels ... 53
4.5.3 Irreversible crosslinking CNF-Helux d-VAS structures57 4.6 Functionalisation and applications of CNF-based materials ... 59
4.6.1. LbL functionalisation and use of aerogel beads in energy storage devices (Paper I) ... 59
4.6.2. Polymer and wax coatings towards functional and bio- mimicked materials (Paper IV) ... 61
4.6.3. Using the heterofunctionality of CNF-polyampholyte composite as basis for functionalisation through NHS- mediated grafting ... 63
5. SUMMARY AND CONCLUSIONS ... 65
6. OUTLOOK ... 67
7. ACKNOWLEDGEMENT ... 69
8. REFERENCES ... 71
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1. INTRODUCTION
1.1 Context
The awareness that the use of fossil-based resources has severe effects on the global environment emphasises the need for new energy and material solutions to be developed and implemented.
Wood, as an abundant, renewable and versatile raw material, is able to provide several material types, such as fibres, biopolymers and monomers, and these wood-derived materials have the potential to substitute conventional fossil-derived materials and is therefore of great interest in materials science. However, in many of the required applications, the extracted wood constituents have to be refined or modified in order to fit their properties to the requirements of the new application areas. This is currently a highly researched area, and the current work is in the centre of this research field.
This thesis focuses on the use of wood fibres and cellulose nanofibrils (CNFs) derived from cellulose-rich wood fibres for the development of materials suitable for environmentally friendly functionalisation and use in, for example, electrical energy storage and structural materials. More specifically the aims were:
• To develop CNF- and wood fibre-based materials able to be used as substrates for environmentally friendly functionalisation,
• To understand the molecular mechanisms behind the formation of the materials and how this knowledge can be used to tune the properties for optimal use,
• To demonstrate how the CNF-based materials can be functionalised and the properties tuned to serve in areas where there is a demand for materials with a low environmental impact.
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2. BACKGROUND
2.1 Cellulose in wood
Cellulose is a linear biopolymer, poly (1→4)-β-D-glucopyranose, the repeating unit being two anhydroglucose rings connected by an acetal bond between carbons 1 and 4 in the two rings. In wood, the degree of polymerisation of native cellulose is about 10 0001 and the cellulose chains are stacked and held together by van der Waals interactions and intra- and inter-chain hydrogen bonds2 into CNFs which have a square cross-section and are 3–5 nm in width and up to a micrometre in length.3 This corresponds to 4–6 cellulose chains being exposed on each side of the fibril.4 In the fibrils, the cellulose chains are stacked parallel to the fibril direction and it is suggested that they create regions of both crystalline cellulose (cellulose I), which are 150–300 nm long,5 and regions of less ordered cellulose in between the crystalline regions.6 The fibrils are in turn assembled into fibril aggregates that are embedded in hemicellulose and lignin in the layered structure of the wood fibre wall and the fibres are subsequently the building blocks of the wood itself. Figure 1 displays the hierarchical structure of wood.
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Figure 1. The hierarchical structure of wood from tree to fibre, nanofibrils and cellulose polymer chain.
2.2. Wood fibres
Through mechanical treatment and/or a chemical pulping process, such as the Kraft7 or sulfite8 process, the fibres in wood can be individually liberated from the wood composite. The chemical composition and the properties of the fibres depend on both the raw material and on the method used to liberate the fibres.9 Mechanically liberated fibres retain the chemical composition of the wood to a large extent whereas the composition of fibres produced by chemical pulping can differ significantly from that in the wood since the pulping process solubilises the hemicelluloses and lignin which can subsequently be washed away. The pulping is commonly combined with a bleaching stage which removes most of the residual lignin and results in lignin-free fibres with a general white appearance. The dimensions of the liberated fibres depend on the raw material but the fibres are approximately 20-40 µm in diameter and typically a few millimetres long.1 The liberated fibres can be
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used to produce a range of products such as printing grades and tissue paper but also packaging board grades by consolidating and drying the fibre mat in a paper machine.
2.3. Cellulose nanofibrils
Individual CNFs can be liberated from cellulosic wood fibres through a mechanical or chemical treatment, or a combination of the two. To overcome the adhesive forces holding the fibre structure together in the fibre wall, and achieve efficient liberation of the individual nanofibrils, chemical modification of the fibril surfaces inside the fibre wall is required.10, 11,12, 13 The chemical treatment introduces charged groups, such as carboxyl and phosphoryl groups, which create an electrostatic repulsion between the fibrils, facilitating their liberation from the fibre wall when the fibres are passed through a high pressure homogeniser10 which is the most common procedure employed to mechanically liberate the CNFs from the fibres. The CNFs are usually produced at a low solids contents, around 20 g/L, where the efficiency of liberation is higher, and they are obtained as a highly viscous gel.14 A 20 g/L CNF gel, a diluted (1g/L) fibril water dispersion and an AFM height image of individual CNFs are shown in Figure 2.
Figure 2. A 20 g/L CNF gel prepared from carboxymethylated fibres with a carboxylic acid content of 600 µmol/g (left) and a 1 g/L electrostatically stablilised dispersion of CNFs (middle). On the right an AFM-height image showing individual fibrils adsorbed onto a silicon oxide surface, scale bar represents 200 nm.
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On an individual level, the semi-crystalline CNFs exhibit impressive mechanical properties. The crystalline segments have a theoretical elastic modulus of 138 GPa15 and the fibrils, being composed of both less ordered and crystalline regions, a modulus of 84 GPa16 and a strength of up to 3 GPa.17 Completely liberated fibrils also have a large specific surface area reaching several hundreds of square meters per gram,18 and the charged groups introduced onto the surface of the fibrils allow electrostatically stabilised low-viscosity water dispersions (Figure 2) of individual CNFs to be prepared.19 Due to the large aspect ratio of the CNFs, the dispersions have a low overlap concentration, 0.4-0.7 g/L,20 and the well-dispersed state of the CNFs can be disturbed by altering the ionic strength through either addition of salt or changing the pH.19 These changes reduce the repulsion between the CNFs in the dispersion and if the repulsion is low enough the CNFs can come into physical contact. If the CNF concentration is below the overlap concentration, aggregates are formed19 and if the solids content is above the overlap concentration, a low-solids-content colloidal glass or gel is formed.21, 22
2.4.1 Nanopapers
CNFs, although they are nano-sized, still resemble wood pulp fibres in the sense that they have a huge aspect ratio and can still be consolidated to form sheets. These nanopapers can be formed either by vacuum-assisted filtration or by solvent casting where the CNFs are randomly packed together in a flat gel-sheet, as a result of the removal of water. The gel-sheet can subsequently be dried by heating or by further evaporation to form the nanopapers.
Nanopapers produced from well liberated CNFs are dense structures and have densities close to that of crystalline cellulose23 (1500 kg/m3) and have a high visible light transmittance because the nano-sized CNFs are too small to scatter visible light.24 The dense CNF films also have a low oxygen permeability at low relative humidities.25, 26 and display elastic moduli of 7-14 GPa, tensile
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strengths in the range of 250 MPa and a strain at break of approximately 6%.27, 28
2.4.2 Aerogels
CNFs are also able to form the complete opposite of densely packed nanopapers in the form of light-weight materials such as aerogels.
Images of a typical CNF aerogel and its cross-section are shown in Figure 3. In the CNF-based aerogels, the CNFs are dispersed in 3D- space. The exact organisation of the CNFs either as interconnected 2D-sheets (Figure 3), as a fibrillary structure of aggregated CNFs or, most ideally, as individual CNFs, depends on the procedure used to produce the aerogel. CNF-based aerogels are usually produced by rapid freezing of the CNF dispersions, and the frozen CNFs are subsequently lyophilised. During the freezing step, the CNFs and any other components are excluded from the growing ice-crystals and are thus templated by the final ice-crystals.29 As the freezing temperature affects the ice crystal nucleation and growth, the temperature thus also affects the final structure of CNFs in the aerogel. The lower the temperature, the smaller will the pores in the aerogel be.30 Also, by using a temperature gradient, anisotropic aerogels with unidirectional and aligned pores can be prepared.31 While temperature affects the pore size and direction, the concentration of the CNFs determines how the CNFs are dispersed in the ice and hence also the aerogel morphology. At lower concentrations, closer to the overlap concentration of CNFs, the structure formed is of long aggregates extending throughout space, but at higher concentrations, the CNFs are squeezed into interconnected sheets extending in the structure.32 To obtain aerogels without greatly altering the distribution of CNFs in the gel state due e.g. to ice-crystal formation, a commonly utilised technique is critical point drying (CPD) where the solvent is removed by replacing it with supercritical CO2 which is then transformed into gas which can leave without perturbing the CNF structure.
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Figure 3. A CNF aerogel cube (left) and its porous interior (right).
CNF aerogels are generally not resilient in the dry state and the shapes of aerogels are often limited to the shapes of the moulds containing the gel during freezing. However, due to the shear thinning behaviour and high zero shear viscosity of the CNF gel, the shaping can be addressed by 3D-printing the gel and subsequently freeze-drying the printed gel shape, forming complex shapes that are difficult to mould.33
Water is a known plasticiser for cellulose and dry CNF-based aerogels are thus inherently sensitive to water and the structure easily disintegrates when soaked in water.34, 35 In order to obtain aerogels that are wet-stable and retain their structure when wet, which can be further utilised in water-based applications, several methods for crosslinking the CNF aerogels have been developed.
Many of these strategies include the addition of crosslinking agents to the CNF-gels prior to freezing where the crosslinking reaction can be activated after drying by e.g. heating.36, 37 Another technique to crosslink CNF aerogels involves a post treatment of the final dried aerogel with fluorine gas, without any prior addition of crosslinking chemicals.38 It has also been reported that crosslinking of the CNFs in the wet state prior to freezing can be achieved by the addition of diamines which can react with aldehydes remaining from e.g.
TEMPO-mediated oxidation,39 or by cyclic freeze-thawing of the CNFs40. Crosslinked nanocellulose aerogels have also been prepared
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from CNCs functionalised with two different reactive groups which can react and form chemical bonds.41
2.4.3 Hydrogels
Due to the low overlap concentration of CNFs, highly entangled, non-flowing, networks are easily formed by inducing gelation by increasing the CNF concentration or by adjustment of the ionic strength or pH.22 Gelation of CNFs in these ways create physically locked CNF networks, volume-filling arrested states (VAS), and these are in certain cases (a colloidal glass) reversible i.e. flowing CNF dispersions can be re-obtained from the locked state by dilution which causes the physically locked CNFs to separate and move freely.21 Such reversible physical locking CNFs in a hydrogel can also be induced by adding polymers and complexing agents.42 During mechanical deformation, the individual components move, and bonds can subsequently be reformed when the system is at rest.
By using multivalent ions to induce the gelation of CNF networks, the network will become physically crosslinked, where the multivalent ion acts as the crosslinking agent due to its interaction with charged groups on two different fibrils.43, 44 It is also possible to use multivalent metal ions to physically crosslink CNF/polyelectrolyte blends to produce highly stable hydrogels with tuneable properties depending, on the metal ion used to crosslink the structure.45
By performing chemical crosslinking of the water-swollen CNF network, permanent, non-dispersible hydrogels can be produced.
Reactive groups on the CNF surface, such as aldehydes, have been used together with an added crosslinking agent to produce CNF hydrogels with tuneable mechanical properties. 46 Such materials are tentatively useful in e.g. tissue engineering. If a crosslinking agent with an inherent responsiveness to an external stimulus, e.g.
poly(N-isopropylacrylamide) particles, is used the crosslinked CNF structure produced can adopt the responsiveness of the crosslinking agent.39
9 2.4.4 CNF-matrix composites
The inclusion of materials such as polymers and/or inorganic particles in a matrix of CNFs to create a composite is associated with the task of mixing the two components on the nanoscale. This is usually done by adding the second component to a low solids content aqueous dispersion of CNFs in order to facilitate the mixing.
If necessary, the solids content of the resulting mix can subsequently be increased by careful evaporation of water47. This mixture is subsequently used to prepare the composite material, using the same techniques as for making CNF-only materials such as films and aerogels. However, this method generally relies on the prerequisite that the second component is soluble or dispersible in water, which limits the choice of materials that can be used. It also requires that the CNFs and the second component can be mixed without any type of aggregation, which prevents nanoscale mixing, which is the case when mixing anionic CNFs with e.g. a cationic polyelectrolyte or an polymer which interacts attractively with the CNF. It is thus difficult to achieve cationic polyelectrolyte/interacting polymer-CNF composites whereas composites of CNFs and anionic polyelectrolytes are achievable,45 as are composites of CNF and anionic nanoclays, both low-charged sepiolite48 and highly anionically charged montmorillonite.28 An uncharged water-soluble polymer and CNFs can also be effectively mixed and prepared into a composite aerogel49 as well as ternary aerogel composites of CNFs, clay and water-soluble, uncharged polymer.47
CNF composites are not however limited to water-dispersible or water-soluble materials, since CNFs can be used to stabilise water- based dispersions of nanocarbons such as graphene and carbon nanotubes (CNTs).50 Utilising this, CNF/CNT composite films, aerogels and filaments have been prepared.51 Nanocarbons are not the only conducting material that displays an interaction with anionic CNFs. PEDOT:PSS, a conducting polymer complex that forms particles in water,52 interacts with CNFs, and composite films
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of CNF and PEDOT:PSS have been made that exhibited remarkably high combined ionic and electronic conductivity.53
2.4. Functionalisation of CNF and nanocellulose- based materials
2.5.1 Chemical modification and use of modified CNFs
The fibril surface inherently contains hydroxyl groups which are available for further functionalization. A common water-based chemical modification used on fibrils is periodate oxidation, which selectively cleaves the C2–-C3 bond of the anhydroglucose ring and oxidises the primary alcohols to two aldehyde groups, one on each of the two carbons.54 These aldehydes are much more reactive than the hydroxyl groups and can be used to modify the properties of CNF-based materials and to introduce new functionalities to the fibrils. A direct use of the aldehydes is to crosslink the oxidised CNF- based materials by formation of inter- or intrafibril hemiacetal bonds between an aldehyde and an hydroxyl.55, 56 This crosslinking of the CNFs has been used, for example, to decrease the moisture sensitivity of CNF-films.23 A subsequent reduction of the aldehydes to dialcohol cellulose creates a presumed core-shell structure of the CNFs with the dialcohol cellulose units present on the surface of the fibrils. Dialcohol core-shell fibrils forms highly ductile CNF materials that can be used in e.g. 3D-forming.57 The aldehydes also readily react with primary amines by reductive amination, and nanocellulose grafted with various amine-containing groups have been produced.58-60 Grafting from the hydroxyls via other routes is also viable and vinyl and allyl groups have been grafted directly onto the surface of CNFs.61
In addition, the CNF surface often contains the charged groups introduced to facilitate their liberation from the fibre wall, and these groups are also readily available for further chemical modification in water. They have for example been used to improve colloidal stability through the grafting of poly(ethylene glycol) from the
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CNFs62 and adhesion properties through the grafting of dopamine units which can complex multivalent ions.63 Dopamine-modified fibrils have also been used as strength additives in paper.64
It is also possible to modify the CNFs in the form of already prepared macroscopic materials such as aerogels, and their properties can be significantly changed. The surface energy of the aerogels can be lowered, by for example silanisation and the material then switches from absorbing water to absorbing hydrocarbons.65, 66
2.5.2 Physical modification of cellulose-based materials Physical modification of cellulosic materials, such as pulp fibres, using the Layer-by-Layer (LbL) technique67, schematically shown in Figure 4, has been extensively used to change their properties. By adsorption of polyelectrolytes and/or charged nanoparticles onto the fibre surface, it has been possible to prepare materials with increased strainability,68 flame retardancy69 and antibacterial properties70, and this has significantly extended the range of application of cellulosic fibre-based materials.
Another great advantage of the LbL-technique, in addition to its great range of applications, is its tuneability. Properties of the layers such as growth behaviour and thickness of the adsorbed layers, are for example, easily tuned by adjusting the adsorption conditions such as solvent quality, pH, ionic strength and ion type.71 The large range of conditions made possible by the LbL-technique also allows non-charged systems, based on non-ionic interactions, to be assembled into multilayers.72, 73
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Figure 4. Schematic representation of the LbL-process where a polycation (Poly+) and polyanion (Poly-) are sequentially adsorbed onto an anionic substrate to build up a multi-layered structure.
2.5.3 LbL in energy-storage applications
One area where the LbL-technique has recently gained interest is in applications for electrical energy storage, where several of the advantages of the nano-engineering with the LbL-technique offers can be utilised. One of the advantages is that the nanometre-size of the conducting particles, such as graphene and CNTs, can be retained, i.e. aggregation is to a large extent avoided, and the electrode therefore possesses a large specific surface area and high porosity, which for example are important parameters for electrochemical double layer charge storage in supercapacitors.74, 75 Another advantage of LbL-assembled electrodes is the molecular contact that is achieved between current-carrying materials and e.g.
redox active materials which makes possible the efficient charge transfer which is a necessity for efficient electrical energy storage devices.76 In addition, the use of nano-sized materials in LbL reduces the diffusion lengths of the electrolytes77 by increasing the surface to bulk ratio and this provides access to more of the electroactive material.78 Furthermore, the ability of LbL-films to swell in water contributes to the surface accessible to the electrolyte which is important for pseudocapacitors where fast reversible redox
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reactions take place on the surfaces of redox active species such as polyaniline.77
By utilising the charged and accessible surface area of CNF-based electrodes, the LbL-technique has been used with CNTs and poly(ethylene imine) (PEI)36 to prepare compressible supercapacitor. In addition, 3D-interdigitated energy storage devices, i.e. a full device with two electrodes and a separator, has been formed by covering the entire internal surface area of a CNF aerogel.79
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3. EXPERIMENTAL
This section provides an overview of the materials, experimental methods and techniques used in the present work. For more detailed information, the reader is referred to the corresponding sections in the individual papers appended to this thesis.
3.1 Materials
3.1.1 Fibres and fibrils
The CNFs used in Papers I, II, III and V were supplied by RISE Bioeconomy (formerly Innventia AB; Stockholm, Sweden) as a 20 g/L fibril physical hydrogel. Prior to fibril liberation through fluid mechanical homogenisation, the softwood fibres (Domsjö dissolving Plus, Domsjö Aditya Birla AB, Domsjö) had been carboxymethylated according to a previously described procedure10 to a charge density of 600 µeq/g. In Paper IV, the fibres used were from a bleached solfwood kraft pulp supplied by SCA Forest Products (Östrand Mill, Timrå, Sweden). For the cellulose model surfaces in Paper V, dissolving pulp from Domsjö Aditya Birla AB was used.
3.1.2 Chemicals
Sodium metaperiodate (99%) and branched PEI (60 kDa) were purchased from Arcos Organics (USA). Hydroxylamine hydrochloride (99%), ethylene glycol, sodium hypochlorite (10-15 % available chlorine), 2-propanol (99.9%), hydrogen peroxide (30% in water), sodium carbonate (≥99.5%), pyrrole (99%), iron(III)chloride, carnauba wax (yellow), N-methylmorfolin-N- oxide and dimethyl sulfoxide were purchased from Sigma Aldrich.
Poly (vinyl amine) (PVAm) (340 kDa) commercial grade (Xelorex RS 1300) was obtained from BASF (Ludwigshafen, Germany).
Sodium poly(2-(3-tienyl)ethoxy-4-butyl-sulfonate) was purchased from American Dye Source Inc. (Montreal, QC, Canada). CNTs were purchased from Carbon Solutions (Riverside, CA, USA). Solid alkyl ketene dimer wax was supplied by EKA Chemicals (Bohus, Sweden).
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Helux was produced by DSM (Heerlen, the Netherlands) and supplied by Polymer Factory AB (Stockholm, Sweden). Ultrapure carbon dioxide was purchased from Strandmöllen AB (Ljungby, Sweden). Milli-Q water was used in all experiments unless otherwise stated.
3.2 Methods
3.2.1 Preparation of wet-stable CNF aerogels
The general procedure for the preparation of CNF aerogels involved the mixing of sodium periodate with the CNF hydrogel using a high shear dispersing equipment (Ultra Turrax, IKA® Werke GmbH, Staufen, Germany) and the mixture was subsequently shaped by moulding in pre-shaped moulds (Paper I, II and III) or extrusion (Paper III) into desired shapes. The oxidation was allowed to proceed for 1h in the absence of light before the samples were placed in a regular freezer (-18°C) for at least 2 h. Larger samples, requiring more time to completely freeze, were left in the freezer after complete solidification for at least 2h. The samples were then thawed at room temperature and subsequently solvent exchanged in several steps to acetone which was then allowed to evaporate at ambient conditions. The aerogels were stored dry until further use.
For the preparation of Helux-CNF composite aerogels (Paper V), the Helux and CNFs were mixed at pH 10 in a range of different CNF:Helux weight ratios. The composite mixture was subsequently frozen at -18°C for 2 h before the samples were lyophilised for three days. The crosslinking of the composite aerogels was thermally activated by treating the samples at 150 °C for 30 min.
3.2.2 Preparation of wet-stable fibre networks
Fibre networks (Paper IV) were prepared by an initial periodate oxidation of the fibres (20 g/L consistency, with 6.3 vol% 2- propanol added) under constant stirring for 30 min at 50 °C in the absence of light. Following the periodate oxidation, the fibres were washed and re-suspended (20 g/L) and subjected to an acidic
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chlorite oxidation for 20 h, under constant stirring. After being washed the fibres were re-suspended at 10 g/L and sheared using the Ultra Turrax® for 0–30 min (10 000 rpm) and subsequently divided into smaller portions in glass beakers after which sodium periodate was added to the beakers, at a ratio of 2.7 g NaIO4 / g of fibre. In the absence of light, the oxidation proceeded for 16 h at room temperature before the material was solvent exchanged to acetone in several steps and then dried under ambient conditions.
3.2.3 Total charge determination
The total amount of acid groups in the cellulosic materials was determined by conductometric titration following a previously reported method.80 Prior to the measurements, the material was washed with excessive amounts of water, and any acidic groups were converted into the proton form by equilibration in acidic water (pH 2) for 30 min. Following the equilibration, the material was again washed until the conductivity of the washing water was
<5 µS/cm. Prior to the conductometric titration, the aerogels were disintegrated to sub-millimetre pieces using an Ultra Turrax. The total charge density of the final aerogel beads was determined by titrating the material with sodium hydroxide using a Titrino 702 SM (Metrohm AG, Herisau, Switzerland).
3.2.4 Aldehyde content determination
The oxidized material was dispersed in water and the pH was adjusted to 4. The sample mixture was then mixed with a solution containing a stoichiometric excess of hydroxylamine hydrochloride, also adjusted to pH 4, and the sample was allowed to react with the hydroxylamine for 2 h. During this reaction, hydroxylamine reacts stoichiometrically with any aldehydes to form oximes, while releasing a corresponding amount of protons. The amount of aldehydes was then determined by titration of the reaction mixture back to pH 4 with sodium hydroxide.81
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3.2.5 Mechanical evaluation of CNF-based materials
The mechanical uniaxial compressive properties of the fibre networks and CNF-based aerogels, both dry and wet, were evaluated using an Instron 5566 Universal testing machine (Norwood, MA, USA). The samples were strained up to 80% compressive strain at which the strain was relieved. The shape-recovery capacity was determined as the percentage of regained strain at the point where a load of 0 N was recorded during unloading. The aerogel beads (Paper I) were tested in compression between two flat surfaces using a linear stage controlled by a high precision electric motor and the load was measured by a SAG-204 balance (Mettler Toledo, Columbus, OH, USA). The properties of CNF-Helux composite films (Paper V) were evaluated in tension using an Instron 5944 Universal testing machine (Norwood, MA, USA). The elastic moduli were calculated as the slopes of the initial linear part of the stress- strain curves and the yield stress as the y-coordinate where the two linear fits from the initial linear part and the following plateau region of the stress-strain curve intersected each other.
3.2.6 Fourier Transform Infrared Spectroscopy
A Perkin-Elmer Spectrum 2000 FTIR system (Waltham, MA, USA) equipped with a single reflection attenuated total reflectance system (GoldenGate) from Graseby Specac Ltd. (Orpington, Kent, England) was used to analyse the materials. Each spectrum was recorded with 4 cm−1 resolution between 600–4000 cm−1 as an average of 16 scans.
3.2.7 Scanning Electron Microscopy
The morphology of CNF-based aerogels and fibre networks (Papers I, II, III, IV and V) were studied using an S-4800 field emission scanning electron microscope (SEM) (Hitachi, Tokyo, Japan). The samples were carefully cut into thin pieces and attached to a conducting adhesive tape. Prior to imaging, the samples were sputtered in a Cressington 208 HR sputter coater (Cressington Scientific Instruments, Watford, UK) with Pt/Pd alloy to limit specimen charging.
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3.2.8 Atomic Force Microscopy Imaging
Atomic force microscopy (AFM) imaging was used to examine the morphology of both chemically functionalised (Paper II) and physically functionalised CNFs (Paper V). The imaging was performed with the CNFs adsorbed either on a freshly cleaved mica surface or on a QCM crystal coated with anchoring layers of a cationic polyelectrolyte. The cationic polyelectrolyte and CNFs were adsorbed sequentially by submersion, first in the polyelectrolyte solution, followed by washing, and then in a CNF dispersion (0.1 g/L). The surface was dried in a stream of N2 gas. The adsorbed CNFs were imaged in air using a Bruker Multimode 8 (Bruker, Santa Barbara, CA) run in the ScanAsyst mode with a cantilever having a nominal tip radius of 2 nm (SCANASYST-AIR, Bruker, Camarillo, CA, USA).
3.2.9 Colloidal Probe AFM
Colloidal probe AFM82 measurements were performed for a symmetric (CNF-CNF) system (Paper III) to study the interaction forces between CNFs in liquid. The interactions were monitored as a function of time in contact and of the chemical environment such as salt concentration (NaCl) and pH (6.5–12). The chemical environment in the test cell was changed by flushing with new liquid while the surfaces were out of contact and the system was then allowed to equilibrate before the measurements were continued.
The measurements were performed with a Veeco Instruments Multimode IIIa equipped with a Picoforce extension (Veeco Instruments Inc.), using a calibrated83-85 CLFC-NOCAL tipless cantilever (Bruker, Camarillo, CA) with an attached SiO2 particle (d = 5 µm, Thermo Scientific, USA). The SiO2 particle was glued with Epicote 1009 (Shell co) to the tipless beam (LxW, 400x29µm2).
The deflection sensitivity used to convert the raw data to force as a function of separation was determined in measurements between the clean probe and the unmodified substrate.86
A simultaneous in-situ modification of both probe and substrate was carried out by sequentially adsorbing an anchoring layer of PEI
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and a layer of CNFs with a washing after each adsorption. The measurements were then made by bringing the probe and surface into contact. Upon contact, defined as a measured force of 35 nN, the two surfaces were kept in contact for 0, 10 or 60 s, after which the probe was retracted from the surface. The results were evaluated using the AFM Force IT v2.6 (ForceIT, Järna, Sweden) software.
3.2.10 Polyelectrolyte titration
Polyelectrolyte titration87 was performed using a Titrino 716 titration unit (Metrohm AG, Herisau,Switzerland) to determine the amount of cationic polyelectrolyte adsorbed onto the cellulose aerogels (Paper I). In brief, the amount of non-adsorbed PEI was determined by titration with potassium polyvinylsulfonate (KPVS) in the presence of orthotoluidine blue as indicator. The equivalence point was detected optically, OTB is blue when an excess cationic charge is present and purple when an excess anionic charge is present, and the amount of adsorbed PEI could be back-calculated from the original concentration and the concentration after adsorption. The charge density of Helux (Paper V) was determined with a particle charge titration unit (Particle Metrix Stabino, Meerbusch, Germany) using poly(diallyldimethylammonium chloride) for the titration of Helux in its anionic form and KPVS in its cationic form. The equivalence point was determined when the streaming potential reached 0 mV.
3.2.11 Layer-by-Layer modification of cellulose aerogels The LbL-technique was used to modify porous aerogel beads (Paper I) by sequential soaking of the beads in a cationic PEI solution (pH 10) and an anionic CNT dispersion or SPTBS solution (pH 4) for 5 min, with an intermediate washing step, creating up to 5 bilayers of (PEI/CNT) or (PEI/SPTBS). Aerogel beads with 1, 3 and 5 bilayers were prepared. After the final washing step, the modified beads were solvent exchanged to acetone and dried ambiently until further use.
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Model surfaces of cellulose88 with a charge density of 600 µeq/g for Stagnation Point Adsorption Reflectometry (SPAR) and quartz- crystal microbalance with dissipation (QCM-D)-measurements (Paper V) were prepared on both oxidised silicon wafers (p-type boron doped, MEMC Electronics, Novara, Italy) and QCM-D crystals having a 50 nm thick SiO2 layer (QSX303, Qsense AB, Västra Frölunda, Sweden) The silicon wafers were pre-treated at 1000°C for 1–3 h to create a 30–80 nm thick SiO2-layer. The oxidised wafers and the crystals were subsequently plasma treated at 30W for 3min in a plasma oven (PDC 002, Harrick Scientific Corp., NY, USA) and activated in 10 wt% NaOH before an anchoring layer of PVAm was adsorbed onto the surface, after which a thin film of cellulose was deposited by spin coating a heated NMMO/DMSO cellulose solution on the surfaces. The cellulose was precipitated on the surface and the excess solvent removed by washing the surfaces with water. The surfaces were dried in a stream of nitrogen gas and stored dry before use.
3.2.13 Stagnation Point Adsorption Reflectometry
The Helux-CNF interaction (Paper V) was evaluated using SPAR,89 where the adsorption of multilayers of Helux and CNFs onto cellulose model surfaces and oxidised silicon surfaces was studied.
The adsorption studies were performed with the aid of a SPAR instrument from the Laboratory of Physical Chemistry and Colloid Science, Wageningen University (Wageningen, The Netherlands).
Helux and CNFs were adsorbed sequentially with an intermediate rinsing step. The adsorption studies were performed at two pH- values: pH 10 and 6.5. The flow of the liquid through the cell was approximately 1 mL/min. The laser light reflected from the surface is divided into its perpendicular and parallel components and the intensity (Si and Sp) of each is measured continuously. The intensity ratio S is the primary output. A baseline intensity (S0) is initially recorded and the shift ΔS due to adsorption is measured. For thin films, the shift is proportional to the adsorbed amount. From the
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thickness of the anchoring SiO2-layer, the signal ratio and the refractive index increment of the adsorbed polymer the solid adsorbed amount in mg/m2 can be calculated.
3.2.14 Quartz-crystal microbalance with Dissipation
A QCM-D (E4, Q-sense AB, Göteborg, Sweden) was used to study and complement the SPAR-measurements (Paper V). The adsorbed mass of Helux and CNFs, including associated water, detected as a decrease in resonance frequency (Δf), and dissipative behaviour of the adsorbed material on the cellulose model surfaces and clean SiO2 surfaces at different pH values was monitored. The adsorption was performed by allowing Helux (0.15 mL/min) and CNFs (0.05 g/L) to flow consecutively over the surface and allowing them to adsorb onto the surface while the change in oscillatory frequency and dissipation were monitored. Between each layer of Helux and CNF, a thorough, rinsing step with water of the same pH was included.
3.2.15 Electrochemical evaluation of CNF-based electrode materials
The beads functionalised with the CNT/PEI system were electrochemically characterised using an EG&G 237A potentiostat/galvanostat (EG&G, Gaithersburg, Maryland, USA).
Cyclic voltammetry was performed with a three-electrode setup, with individual beads as working electrodes, using 0.1 M sodium sulphate solution as the electrolyte and scan rates ranging from 10–
100 mV/s. An Ag/AgCl and platinum wire were used as the reference and counter electrodes respectively. Constant current charging and discharging was carried out using a current of 80 µA, corresponding to a current density of 1 mA/mg with respect to the total mass of the bead-electrode. The specific capacitance was calculated from the discharge part of the curve as 𝐶 = 𝐼∙𝑡
∆𝑉∙𝑚, where 𝐼 is the discharge current, 𝑡 is the discharge time, ∆𝑉 is the voltage window and 𝑚 is the total mass of the bead-electrode.
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4. RESULTS AND DISCUSSION
4.1 Preparation and properties of wet-stable CNF aerogels using ice-templating
4.1.1 Aerogel beads (Paper I)
Millimetre-sized aerogel beads were obtained by periodate oxidation and shaping of periodate-containing CNF-gel into spheres according to the freeze-linking procedure schematically shown in Figure 5.
Figure 5. Schematic illustration of the steps involved in the preparation of freeze-linked aerogel beads. (A) shaping the CNF-periodate mixture into droplets, (B) oxidation and self-assembly, (C) freezing (-18 °C), (D) thawing at RT, (E) solvent exchange, (F) ambient drying.
The beads obtained their shape through the formation of CNF- periodate droplets which subsequently retained their more or less spherical shape by resting on a super hydrophobic surface90 where the surface tension of the droplet governs the spherical shape. The oxidation reaction subsequently proceeded inside the resting droplets and the periodate oxidised the C2 and C3 hydroxyls on the fibril surface to aldehydes. The oxidation also resulted in a volumetric shrinkage of the CNF-gel, so that after the oxidation the
