1. business and industrial
2. chemicals industry
3. plastics and polymers

TAILORED CELLULOSIC MATERIALS BY
PHYSICAL ADSORPTION OF
POLYELECTROLYTES
Andrew Marais
Doctoral Thesis
KTH Royal Institute of Technology
School of Chemical Science and Engineering
Department of Fibre and Polymer Technology
Stockholm, 2015
ISBN 978-91-7595-500-1
TRITA-CHE Report 2015:14
ISSN 1654-1081
AKADEMISK AVHANDLING
Som med tillstånd av Kungliga Tekniska Högskolan i Stockholm
framlägges
till
offentlig granskning för
avläggande av
teknisk
doktorsexamen fredagen den 22:a maj 2015, kl. 10:00 i Kollegiesalen,
Brinellvägen 8, KTH, Stockholm. Avhandlingen försvaras på engelska.
Copyright © Andrew Marais, 2015
Stockholm 2015
-ii-
ABSTRACT
The growing interest in using bio-based resources has made forest-based
cellulose (used as fibre and nanofibril building elements) a good
candidate for the development of new materials. In order to be used in
commercial applications, cellulose must however be processed and/or
functionalized to provide the final material with specific properties. This
thesis presents (a) a way to improve the mechanical properties of
traditional cellulose-based materials (paper), (b) an investigation into the
structural and adhesive properties of the self-assembled hyaluronic acid
thin films used to tailor the mechanical properties of paper materials, and
(c) the preparation and functionalization of cellulose aerogels.
In the first part of this thesis, the adsorption of polyelectrolytes onto pulp
fibres (either as a monolayer or as multilayers) was studied as a way to
improve the mechanical properties of paper materials. It was found that
low amounts of adsorbed cationic amines were able to significantly
improve the tensile properties of sheets made from treated fibres. Tensile
testing of fibre crosses and microtomography revealed that this
improvement in mechanical properties was due to an increase in both the
interfibre joint strength and the interfibre contact area. By building up
polyelectrolyte multilayers of hyaluronic acid (HA) and polyallylamine
hydrochloride (PAH) onto the fibres, a threefold increase in both strain at
break and tensile strength was achieved.
In the second part, the structural and adhesive properties of (PAH/HA)
thin films were investigated. Such films showed adhesive features
stronger than those reported for bone structures containing mineralised
collagen.
Finally, wet-resilient porous cellulose aerogels were developed by freezedrying and crosslinking cellulose nanofibrillar gels. This material with
high porosity and a high specific surface area was then used as a
template to build three dimensional (3D) energy-storage devices using
the Layer-by-Layer approach. Thin films of conductive materials were
-iii-
deposited into the bulk of the material, and 3D-interdigitated
supercapacitors and batteries were built. The devices showed high
capacitance and operated under extreme conditions of compression and
bending, opening up numerous possibilities in the field of flexible
electronics.
-iv-
SAMMANFATTNING
Det växande behovet av att använda biobaserade resurser har dramatiskt
ökat intresset för att använda cellulosa ifrån ved för utvecklingen av nya
material. Cellulosa, i form av fibrer eller fibriller med cellulosa som
huvudbeståndsdel,
måste
funktionaliseras
att
för
naturligtvis
kunna
bearbetas
användas
i
nya
och/eller
kommersiella
tillämpningar. Denna avhandling omfattar (a) en metodik att förbättra
pappers mekaniska egenskaper mha av adsorberade polyelektrolyter, (b)
en undersökning av strukturen hos polymera multiskikt av PAH och
hyaluronsyra (HA), deras adhesiva egenskaper, och hur de kan användas
för att förbättra mekaniska egenskaper hos papper som tillverkats från
multiskiktsbehandlade fibrer, och (c) tillverkning och funktionalisering
av porösa cellulosa aerogeler.
I den första delen av avhandlingen studerades adsorption av
polyelektrolyter till massafibrer (i monolager eller multilager) och hur
denna
kan
förbättra
pappersmaterialets
mekaniska
egenskaper.
Dragprovning av fiberkors samt röntgen mikrotomografi visade att
förbättringen i de mekaniska egenskaperna kan förklaras både av en
ökning i fogstyrkan och av kontaktytan mellan fibrerna. Genom att
bygga upp multilager av hyaluronsyra och PAH på fibrerna ökade båda
töjningen och draghållfastheten med en faktor tre.
I den andra delen av avhandlingen undersöktes den övermolekylära
strukturen och adhesions egenskaperna hos multilager av PAH och HA.
Tunna filmer av (PAH/HA) uppvisar bättre adhesion egenskaper än vad
som rapporteras för mineraliserade kollagenstrukturer i ben.
Slutligen utvecklades metoder för att preparera en våtstabil, porös
cellulosa aerogel. Materialet karakteriserades av en hög porositet (99%)
och en hög specifik yta. Den våtstabila aerogelen användes sedan som
templat för att kräddarsy olika 3D energilagrings material med
multiskiktstekniken. Tunna filmer av ledande material byggdes upp i
aerogelen,
vilket
gjorde
det
möjligt
-v-
att
sedan
tillverka
3D
superkondensatorer
och batterier. De
tillverkade
komponenterna
uppvisade hög kapacitans och fungerar under extrem kompression och
böjning,
vilket
öppnar
nya
möjligheter
för
cellulosabaserade material inom flexibel elektronik.
-vi-
användning
av
LIST OF PUBLICATIONS
This thesis is based on the following papers, referred to in the thesis by roman
numerals, and appended in the final section of the thesis.
I.
The use of polymeric amines to enhance the mechanical properties of
lignocellulosic fibrous networks
A. Marais and L. Wågberg
Cellulose (2012), 19, 1437-1447
II.
New insights into the mechanisms behind the strengthening of
lignocellulosic fibrous networks with polyamines
A. Marais, M.S. Magnusson, T. Joffre, E.L.G. Wernersson and L. Wågberg
Cellulose (2014), 21, 3941-3950
III. Towards a super-strainable paper using the Layer-by-Layer technique
A. Marais, S. Utsel, E. Gustafsson and L. Wågberg
Carbohydrate Polymers (2014), 100, 218-224
IV. Robust and tailored wet adhesion in hyaluronic acid thin films
T. Pettersson, S. Pendergraph, S. Utsel, A. Marais, E. Gustafsson, and L. Wågberg
Biomacromolecules (2014), 15, 4420-4428
V.
Nanometer-thick hyaluronic acid self-assemblies with outstanding
adhesive properties
A. Marais, S. Pendergraph and L. Wågberg
Manuscript
VI. Nanocellulose aerogels functionalized by rapid Layer-by-Layer assembly
for high charge storage and beyond
M. Hamedi, E. Karabulut, A. Marais, A. Herland, G. Nyström, and L. Wågberg
Angewandte Chemie International Edition (2013), 52, 12038-12042
VII. Self-assembled three dimensional and compressible interdigitated thin
film supercapacitors and batteries
G. Nyström, A. Marais, E. Karabulut, L. Wågberg, Y. Cui and M. Hamedi
Nature Communications (2015), accepted for publication
-vii-
CONTRIBUTIONS TO THE PAPERS
The author’s contributions to the appended papers are as follows:
I.
All the experimental work and preparation of the manuscript
II.
Part of the experimental work and preparation of the manuscript
III.
All the experimental work and preparation of the manuscript
IV.
Part of the experimental work and part of the manuscript
V.
All the experimental work and preparation of the manuscript
VI.
Part of the experimental work, and part of the manuscript
VII.
Most of the experimental work and part of the manuscript
-viii-
RELATED MATERIAL
In addition to the appended papers, the work has resulted in the
following presentations:

The use of polymeric amines to enhance the mechanical
properties of fibrous networks
A. Marais, L. Wågberg
International Paper and Coating Chemistry Symposium (PCCS),
Stockholm, Sweden (2012)

Tailoring of the LbL structures for optimized adhesion
A. Marais, S. Pendergraph and L. Wågberg
American Chemical Society (ACS) meeting, New Orleans, USA
(2013)

The build-up of structured polyelectrolyte multilayers on pulp
fibres to prepare stretchable paper materials
A. Marais, S. Utsel, E. Gustafsson, L. Wågberg
International
Symposium
on
Wood,
Fibre
and
Pulping
Chemistry (ISWFPC), Vancouver, Canada (2013)

Layer-by-Layer films of polyallylamine and hyaluronic acid with
superadhesive properties
A. Marais, S. Pendergraph and L. Wågberg
American Chemical Society (ACS) meeting, Dallas, USA (2014)

Superadhesive properties of LbL thin films of polyallylamine and
hyaluronic acid
A. Marais, S. Pendergraph and L. Wågberg
Layer-by-Layer Assemblies: Science and Technology Conference,
Hoboken, USA (2014)
-ix-
LIST OF ABBREVIATIONS
ADS2000P
Sodium poly[2-(3-thienyl)ethyloxy-4-butylsulfonate]
AFM
Atomic force microscopy
BTCA
1,2,3,4-Butanetetracarboxylic acid
CAT
Contact adhesion testing
CNF
Cellulose nanofibrils
CNT
Carbon nanotubes
CuHCF
Copper hexacyanoferrate
CV
Cyclic voltammetry
DPI
Dual polarization interferometry
HA
Hyaluronic acid
KPVS
Potassium polyvinyl sulphate
LbL
Layer-by-Layer
OTB
Ortho-toluidine blue
PAA
Polyacrylic acid
PAH
Polyallylamine hydrochloride
PDMS
Polydimethylsiloxane
PEDOT:PSS
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
PEI
Polyethyleneimine
PVAm
Polyvinylamine hydrochloride
QCM-d
Quartz crystal microbalance with dissipation
SEM
Scanning electron microscopy
SHP
Sodium hypophosphite
X-μCT
X-ray micro-computed tomography
-x-
TABLE OF CONTENTS
1.
2.
INTRODUCTION .................................................................................... 1
1.1.
Context ............................................................................................... 1
1.2.
Purpose of the study ........................................................................ 2
BACKGROUND ....................................................................................... 3
2.1.
From wood to fibres/nanofibres ..................................................... 3
2.2.
Materials from wood ....................................................................... 5
2.2.1.
Paper.......................................................................................... 6
2.2.2.
Aerogels .................................................................................... 8
2.3.
3.
2.3.1.
Polyelectrolyte adsorption ..................................................... 9
2.3.2.
Layer-by-Layer assembly ..................................................... 11
EXPERIMENTAL ................................................................................... 17
3.1.
Materials .......................................................................................... 17
3.1.1.
Fibres / Nanofibres ................................................................ 17
3.1.2.
Model surfaces ....................................................................... 17
3.1.3.
Chemicals................................................................................ 18
3.2.
4.
Surface modification of materials .................................................. 9
Methods ........................................................................................... 18
3.2.1.
Fibres and paper (Papers I-III) ............................................. 18
3.2.2.
Layer-by-Layer assemblies of (PAH/HA) (Papers IV-V) . 23
3.2.3.
Wet-resilient cellulose aerogels (Papers VI-VII) ................ 26
RESULTS & DISCUSSION .................................................................... 31
4.1.
Lignocellulosic
fibre
networks
with
tailored
properties
(Papers I-III) ................................................................................................ 31
4.1.1.
Adsorption of polyamines monolayers on fibres for the
preparation of strong paper materials ................................................. 31
-xi-
4.1.2.
Build-up of polyelectrolyte multilayers onto fibres for
strong and stretchable paper materials ............................................... 37
4.2.
Layer-by-Layer assemblies of (PAH/HA) (Papers III-V) .......... 39
4.2.1.
Build-up .................................................................................. 39
4.2.2.
Adhesive properties .............................................................. 42
4.3.
Nanocellulose aerogels as a platform for a variety of
applications (Papers VI-VII) ...................................................................... 48
4.3.1.
Preparation and characterization of the aerogels .............. 48
4.3.2.
LbL functionalization of the aerogels.................................. 49
4.3.3.
Preparation of three dimensional and compressible energy
storage devices ........................................................................................ 51
5.
CONCLUSIONS ..................................................................................... 57
6.
FUTURE WORK ..................................................................................... 59
7.
ACKNOWLEDGEMENTS .................................................................... 61
8.
REFERENCES ......................................................................................... 63
-xii-
INTRODUCTION
1. INTRODUCTION
1.1. Context
Both ecological and economic crises have recently dramatically increased
the need for a more sustainable society, and industries are facing new
demands aiming at reducing production costs as well as utilising
renewable resources to develop new functional materials.
In this respect, one renewable raw material of major interest is wood and
its main component: cellulose. Cellulose is the most abundant
biopolymer on Earth and its annual worldwide production is estimated1
to be of the order of 1010-1011 t. Since its discovery by Payen in the
nineteenth century2, cellulose has been widely investigated and used
industrially to produce materials such as paper.
Industries producing materials from forest resources are nowadays
facing new challenges and it is necessary to produce traditional paperbased materials in a more cost-effective way, for example by decreasing
the weight of packaging materials without influencing the mechanical
properties of the end-product. In order to maintain the properties of such
materials, the strength of its components must be enhanced, using
monolayers or multilayers of polyelectrolytes for instance, and it is
important to understand the structural mechanisms governing the
strength enhancement of paper materials at the microscopic level.
Moreover, paper can be envisioned as a substitute for oil-based plastics in
3D-shaped packaging, provided that its stretchability can be increased to
allow 3D forming. A further challenge is to develop a new class of
materials from wood that can be functionalized for use in a broad range
of applications.
-1-
INTRODUCTION
1.2. Purpose of the study
The overall aim of the work presented in this thesis has been to address
some of the challenges faced by the forest, paper and cellulose industries,
and to develop new functional materials from cellulose. More
specifically, the objectives have been:
(a) To prepare paper materials with enhanced dry strength by the
adsorption of polyamines, and to investigate the microscopic
mechanisms behind the strength enhancement.
(b) To prepare stretchable paper materials by building up
polyelectrolyte multilayers on the fibres by Layer-by-Layer
deposition
of
polyallylamine
hydrochloride
(PAH)
and
hyaluronic acid (HA).
(c) To investigate the structural and adhesive properties of Layerby-Layer assemblies containing PAH and HA.
(d) To prepare and utilize porous materials from cellulose
nanofibrils: characterization and functionalization via Layer-byLayer to prepare energy storage devices.
-2-
BACKGROUND
2. BACKGROUND
2.1. From wood to fibres/nanofibres
Wood consists of an assembly of fibres held together by lignin, and the
fibre wall is an ingenious nanocomposite where cellulose nanofibrils are
embedded in a matrix of hemicellulose and lignin. The main constituents
of the wood fibres are cellulose (about 50%), hemicelluloses (about 25%)
and lignin (about 25%). The middle lamella holding the fibres together
has a very high lignin content, but there is a distribution of lignin,
hemicellulose and cellulose across the fibre wall and the secondary wall
is dominated by cellulose and hemicellulose3. The wood fibres are
typically a few mm long and about 20-50 μm in diameter3. The cylindrical
fibre wall is composed of concentric layers, each consisting of fibril
aggregates that are oriented in a certain direction along the fibre axis,
usually called the fibril angle. The difference in fibril angle in the
different layers leads to the characteristic mechanical properties of the
fibres. The fibril aggregates are bundles of aggregated cellulose
nanofibrils, which are in turn composed of cellulose chains (consisting of
β-(1-4)-D-glucose repeating units) packed into cellulose crystals (a few
Å), and are typically a few nm in diameter and over 1 μm in length3,4. The
hierarchical structure of wood is depicted in Figure 1.
Wood fibres can be extracted from wood chips by a chemical treatment, a
mechanical treatment, or a combination of the two. The fibres used in the
work presented in this thesis were extracted by a chemical process called
the kraft process, in which the wood chips are reacted in an alkaline
medium (sodium hydroxide and sodium sulphide) at elevated
temperature (typically around 150 °C for several hours)5. Under such
cooking conditions, the lignin is dissolved to an extent that allows the
fibres to be extracted from the wood matrix. The wood chips are cooked
until a desired yield (the ratio of the mass of pulp to the initial mass of
wood) is reached. The remaining amount of lignin in the fibres is
evaluated as the kappa number6 of the pulp, and this value is usually
-3-
BACKGROUND
used to follow the cooking process and to characterize the pulp. After the
kraft process, the liberated fibres can be used to prepare various paper
materials and depending on the end application of the fibres, the pulp
can be bleached in various sequences. If the extraction process is taken
one step further, it is possible to extract nano-sized fibres, or cellulose
nanofibrils (CNF), from wood fibres.
HO
HO
HO
O
HO
O
O
HO
O
OH
HO
O
O
HO
OH
Figure 1. The hierarchical structure of wood: from the tree to the fibre,
nanofibril, and cellulose chain
The liberation of CNF was first introduced in the 1980s by Turbak et al.7,
but due to the high energy consumption and a lack of applications at that
time, the process did not reach commercial application in the 1980s.
-4-
BACKGROUND
Later, methods were developed to reduce the energy consumption by
enzymatic treatment, beating and/or charging of the fibres prior to the
mechanical treatment8–10. For example, the fibres can be chemically
treated (a carboxymethylation is performed to increase the charge) before
being mechanically treated in a high-pressure homogenizer. If shearing
forces are applied to the carboxymethylated fibres, the fibre wall
delaminates and it is possible to extract fibrils with dimensions in the
nanometer range. Such fibrils have remarkable properties, among which
their high charge (in the range of 500 μeq/g9), high aspect ratio and high
modulus11 can be specially mentioned. Due to their high charge and high
aspect ratio, CNF dispersions form gels at very low concentrations and
their colloidal behaviour in solution is naturally directly influenced by
the pH and salt concentration12. Significant efforts have been made to
make possible the production of CNF at the industrial scale13. Nowadays,
several plants around the world are producing CNF in industrial
quantities and the scaling-up of CNF production paves the way for
future applications in which the fibrils can be used to produce materials
for packaging, electronic and magnetic applications, to mention only a
few14–16.
2.2. Materials from wood
Although a large variety of materials can be produced from wood, the
focus in the work presented in this thesis is on fibrous networks (paper)
and porous nanofibrous networks (aerogels), as depicted in Figure 2.
-5-
BACKGROUND
Figure 2. Materials from wood: fibres used to prepare paper materials, and
nanofibrils (extracted from the wood fibres) used to prepare aerogels
2.2.1.
Paper
Paper is a fibrous network formed during a process in which a fibre
suspension is first dewatered, to form a wet web that is then pressed and
dried. As the water is removed, the fibres are brought into intimate
contact (due mainly to capillary forces) where van der Waals forces and
hydrogen bonds can form, which are crucial for the development of
paper strength17. The structure of a typical paper material is shown in
Figure 3.
The mechanical properties of paper materials are governed by three main
parameters: the individual fibre strength, the interfibre joint strength, and
the number of efficient interfibre joints per unit volume18–20. The endless
need for fibrous materials with higher performances has led to the
development of techniques aiming to increase the mechanical properties
of paper.
-6-
BACKGROUND
Figure 3. (Left) photograph of paper sheets and (right) SEM image of a paper
sheet, showing the microscopic structure of a lignocellulosic fibrous network
One of the most common methods, with low specificity, is to beat (or
refine) the fibres21. The shear forces in the presence of water during the
beating process produce hydrated/swollen fibres (which increases their
flexibility) and promote both internal and external fibrillation (which
increases the mechanical entanglements) as well as the creation of fines
(increasing the specific surface area and hence favouring the contact
between fibres), giving rise to a paper material with stronger mechanical
properties22. However, beating also increases the density of the network
and slows down the dewatering at the wet-end and in the press sections
of the paper machine, and thus decreases the productivity of the
papermaking process. In addition, beating is an energy-consuming
process. A further drawback of beating is the shrinkage of the fibres
during drying which leads to a shrinkage of the entire web during
drying, which lowers both the productivity and the mechanical
properties of the paper.
Another possible way to modify the mechanical properties of a cellulosic
network is to use chemical additives to modify the interfacial interactions
between the fibres. Cationic starch23 and polyacrylamide resins24 have
been widely used for decades as dry-strength additives, and more
recently, other polyelectrolytes have been investigated as an easy and
inexpensive way to improve the mechanical properties of paper25–28. The
focus in the present study is on two polyamines: polyallylamine
-7-
BACKGROUND
hydrochloride (PAH) and polyvinylamine (PVAm), used as dry-strength
and, to some extent, wet-strength additives.
Apart from conventional macroscopic tensile tests, the mechanical
properties of a fibrous network can be investigated at the microscopic
scale by measuring the tensile strength of the interfibre joints. This has
been challenging in the past because of the difficulty in handling
individual fibre crosses, and also because of the heterogeneity of the
fibres, giving rise to measurements with large variations. In the present
study, a direct method was used, where individual fibre crosses are
formed and mechanically tested. This was first shown in the 1960s29, and
was followed by a range of similar studies30,31. Nowadays, new
techniques are available to test single fibre crosses in various modes of
loading following a protocol which reduces the probability of sample
failure during preparation32–34.
2.2.2.
Aerogels
A significant amount of research has recently been undertaken in order
to develop new types of materials from CNF, including porous materials
such as foams or aerogels35–37. An aerogel can be defined as a material
initially in a gel state in which the liquid has been replaced by a gas. The
main characteristics of an aerogel are: very low density, high porosity,
high specific surface area and insulation properties. A cellulose aerogel
can be prepared by freeze-drying a CNF gel. The freeze-drying method
consists in freezing the sample in liquid nitrogen (i.e. at -196 °C) followed
by sublimation of the frozen water. The sublimation prevents the
structure from collapsing, and a porous network can be obtained36. The
macroscopic and microscopic structures of a CNF aerogel are shown in
Figure 4. The porosity of a cellulose aerogel can reach 99% and the
density is typically 0.02 g/cm3. Despite many interesting properties, the
use of cellulose aerogels is somewhat limited because they are not stable
in water and decompose.
-8-
BACKGROUND
Figure 4. (Left) Photograph and (right) SEM image of a cellulose aerogel
One way of solving this problem is to chemically crosslink the CNF
network. Zhang et al.
38
suggested a route to crosslink the aerogels using
polyamide-epichlorhydrin (PAE). PAE resins have been extensively used
in the paper industry as wet strength agents for hygienic papers, filter
papers and wallpaper17. However, the toxicity of PAE is a strong
hindrance to wider use, and it is therefore important to develop other
crosslinking routes for wet-resistant cellulose aerogels where the
negative charges of the aerogel are preserved or increased.
2.3. Surface modification of materials
Although wood-based materials display many interesting features, it is
often necessary to modify them in order to give them specific
functionalities. There are two ways of modifying the surface of a
material: a chemical way (by covalently grafting species onto the surface
for example) and a physical way, relying on non-covalent, physical
adsorption. The focus in the present work is on the physical adsorption of
charged species such as polyelectrolytes.
2.3.1.
Polyelectrolyte adsorption
A polyelectrolyte is by definition a polymer containing one (or more)
potentially charged groups in its repeating unit. Polyelectrolytes are
ubiquitous and play a major role in our lives: from our body (e.g. DNA)
-9-
BACKGROUND
to the food we eat everyday (e.g. carboxymethyl cellulose, used as a
thickener). Polyelectrolytes may be either strong or weak. The former are
not affected by the pH whereas the charge density of the latter is directly
linked to the pH. Due to the presence of charges, polyelectrolytes are
soluble in water.
The adsorption of polyelectrolytes onto oppositely charged surfaces has
been extensively investigated over the past decades and the driving force
for this phenomenon is the gain in entropy following the release of
counter-ions upon adsorption39–42. The following parameters are crucial to
an understanding of the behaviour of the polyelectrolyte/substrate
system:
-
The pH, which controls the degree of dissociation and thereby
the charge density of the polymer (in the case of weak
polyelectrolytes) and the substrate;
-
The surface charge of the substrate, since there is a 1:1 ratio
between the amount of adsorbed charge and the surface charge
of the substrate (also termed pure electropsorption)43;
-
The salt concentration and salt type, since an increased salt
concentration decreases the entropy gain upon adsorption, which
in turn decreases the adsorption, but also reduces the
intramolecular repulsion, making the polymer coil-up and
increasing the adsorbed amount in the case of a porous substrate;
-
The porosity of the surface (or the molecular mass of the
polymer), which determines whether the polyelectrolyte is
adsorbed exclusively onto the surface or whether it penetrates
into the substrate;
-
The solvent: in a so-called good solvent or poor solvent, the
polymer chains have respectively a strong or weak affinity for
the solvent and a tendency to adopt an extended or coiled
conformation. The quality of the solvent may also influence the
-10-
BACKGROUND
adsorption if there is a considerable non-ionic contribution to the
adsorption process.
Since pulp fibres are negatively charged due to the carboxylic groups on
the hemicelluloses and oxidized lignin4, it is possible to adsorb cationic
polymers onto their surfaces. Polyelectrolytes are a key feature of the
papermaking process, since they are used to tailor the properties of paper
from the wet state (e.g. retention, flocculation) to the dry final material
(e.g. wet and dry strength)44–47, and they are nowadays used in the
production of almost all paper grades.
Polyelectrolytes can be deposited onto charged substrates as monolayers,
as multilayers or as complexes to physically modify the surface
properties of the material onto which they are adsorbed48,49.
2.3.2.
Layer-by-Layer assembly
Principle
The Layer-by-Layer (LbL) technique involves building up a film by
depositing alternating cationic and anionic polyelectrolytes and/or
particles onto a charged substrate, as shown in Figure 5. The principle of
LbL assembly was first introduced for colloids in the 1960s50 and was
then developed and extended to a broad range of polymers and particles
by Decher in the 1990s49,51,52. This work was a breakthrough in physical
surface modification and the interest in this technique has continuously
increased since then.
-
-
-
- - - Substrate
-
-
P+
Rinse
+
+ + + +
+ + + + +
+
+
+
- - - - - - - - Substrate
PRinse
-
- - - - - + + + + +
+ + +
+
+
+
- - - - - - Substrate
-
-
+
+ +
-
-
Figure 5. Simplified scheme of the Layer-by-Layer deposition technique, starting
from a negatively charged substrate and depositing consecutively cationic (P+)
and anionic polymers (P-), forming the first bilayer
-11-
BACKGROUND
LbL films can be assembled either by dipping the substrate in a solution
or dispersion, or by spraying the solution onto the surface. The spraying
technique is much faster than the dipping technique because of the
shorter time necessary to deposit each layer and also because the rinsing
step can be avoided. However, the thickness of the LbLs assembled by
dipping is greater than that of films obtained by spraying53,54. The fibres
and model surfaces investigated in the present work were LbL-coated
using the dipping approach, but a new filtration method was introduced
for the LbL deposition on aerogels.
The assembly of LbL films is a very versatile technique that can be used
to tailor the properties of a surface, and specific functionalities can be
achieved with only a few layers, i.e. with nanometre-thick films. The
protocol for the buildup of LbL structures is very simple and
environment-friendly, since water is most commonly used as solvent.
Moreover, the LbL technique allows a large variety of combinations since
the only requirement is to use charged species. It is even possible to use
uncharged species, provided that other driving forces, of enthalpic or
entropic origin, can ensure interactions with the second LbL component,
as is the case for LbL films of polyethylene oxide (PEO) and polyacrylic
acid (PAA)55.
For all these reasons, the LbL technique is nowadays used in a broad
range of applications including gas barrier56,57, biomimicking of natural
structures58, superhydrophobic surfaces59, electronics60, drug delivery61–64,
and other biomedical applications65–69.
Linear vs. super-linear growth
As
with
monolayer
polyelectrolyte
systems,
the
build-up
of
polyelectrolyte multilayers is extremely dependent on the pH and on the
salt concentration. Together with the intrinsic properties of the polymers
(such as molecular weight and charge), these parameters control the
growth of the multilayer structure, which can be either linear or superlinear70–73. Although the mechanisms governing the transition from linear
-12-
BACKGROUND
to super-linear growth are still debated in the literature, two models can
be mentioned: the first relying on the interdiffusion of one species
through the LbL film71, and the second being based on the so-called
Island model74, where the adsorbed polymer forms islands on the
substrate, increasing the surface area available for further deposition. The
thickness of the LbL films can thereby be controlled and tailored by
adjusting the number of layers deposited, the pH of the solutions, the salt
concentration, and also the nature of the species deposited (polymers or
particles).
LbL on wood fibres
The LbL technique has already been used for wood fibres as a way to
functionalize75,76 or to increase the strength of fibrous materials, typically
using PAH and PAA77, or cationic and anionic starch78. The deposition of
polyelectrolyte multilayers onto the surface of lignocellulosic fibres has
been shown to have a strong impact on the interfibre joints, which is the
molecular/microscopic reason why differences can be seen on a
macroscopic scale79.
Adhesive properties of LbL thin films
Different approaches can be used to determine the adhesive properties of
solid materials, where different length scales are involved. In the present
work, two approaches were used to characterize the adhesive properties
of nanometer-thick LbL films: a colloidal probe atomic force microscopy
(AFM) technique, and a contact adhesion testing (CAT) technique.
AFM is a microscopy technique that can be used to image the topography
of a substrate with a nanometre resolution. It is based on a cantilever
scanning the substrate and detecting the force of interaction between the
tip of the cantilever and the substrate. This technique can also be used to
measure adhesion between films/particles80. A common procedure
consists of attaching a colloidal probe (typically a silica particle with a
diameter in the micrometre range). The probe can be LbL-coated and
-13-
BACKGROUND
allowed to approach a similarly coated surface. The force profile is
recorded throughout the measurement. The force necessary to separate
the two surfaces brought into contact is called the pull-off force, and is
one parameter describing the adhesive properties of a material. The AFM
colloidal probe technique allows the measurement of pull-off forces for
films with thicknesses down to the nanometre level and is therefore
adapted to LbL studies. The number of layers, the outermost layer, the
molecular mass of the two compounds and the contact time have been
shown to be crucial parameters for the adhesive properties of LbL films81.
Colloidal probe AFM have been used to investigate LbL systems such as
(PAH/PAA)81 and (PAH/PSS)82 to mention a few.
For CAT measurements, model surfaces such as polydimethylsiloxane
(PDMS)83, glass and silicon wafers can be used as substrates for LbL
deposition and evaluation of the adhesive properties of LbL films. The
JKR technique84, based on the adhesion between a sphere/hemisphere
and a flat surface, can be used to provide information about the
properties of LbL films, as shown by Nolte et al.85, who investigated
adhesive interactions of (PAH/PAA) films. However, it was shown that
the roughness and stiffness of LbL films in the dry state hinders the
establishment of intimate contact between the coated substrates, and
subsequently the adhesion between the LbL thin films.
LbL films of hyaluronic acid
One of the polymers often used in LbL systems for biomedical
applications is hyaluronic acid (HA), a linear biopolymer present in the
human body (eyes, synovial liquid etc.). It is commonly used in medical
applications such as biomedicine, surgery and drug delivery, and it is a
polysaccharide of major interest due to some of its properties
(hydrophilicity, rheological behaviour, lubrication ability and so on, to
mention only a few)86–88. The presence of a negatively charged group in
its repeating unit makes it possible to use hyaluronic acid in LbL films.
Often associated to poly-L-lysine (PLL), HA has been used in LbL films
-14-
BACKGROUND
for drug delivery and cell adhesion, for example61–64. Biological structures
can also be mimicked using HA, as shown for example by investigations
where a skin tissue replica was designed by LbL89. Other examples of
tissue engineering using LbL have also been reported90.
Although well studied in the above-mentioned applications, the use of
HA in biomedical applications could be extended. In this thesis, we
investigate HA as a strong bio-adhesive by studying the adhesive
properties of LbL films where HA is associated with PAH.
LbL and energy storage
Among the several approaches that can be used to store electrical energy,
the
most
common
ones
are
batteries
and
electrochemical
supercapacitors91,92. The main difference between these two types of
device is that batteries store energy as chemical reactants, whereas
supercapacitors
store
energy
as
charge.
Both
batteries
and
supercapacitors are composed of two electrodes (anode and cathode)
separated by an electrolyte, which ensures the ion transport between the
two electrodes and prevents direct contact between them. The
functioning of a battery is based on redox reactions (faradaic processes)
at the interface between the electrodes and the electrolyte. On the other
hand, supercapacitors store charge in an electric double layer at the
interface between the electrode and the electrolyte. These operating
differences give rise to devices with different properties. The processes
involved in the charge storage in supercapacitors are much faster than
the faradaic processes in batteries. One consequence is that the
charge/discharge timescale is seconds for supercapacitors, and can be up
to days for batteries. This translates into lower energy densities but
higher power densities for supercapacitors than for batteries. Upon
operation, the stability of the electrode materials in a battery is affected
(as the molecular structure can be changed with the redox reactions), and
this makes the lifetime of a battery (in terms of number of
charge/discharge cycles) shorter than that of a supercapacitor.
-15-
BACKGROUND
Among the conductive materials used in emerging storage energy
technologies, carbon nanotubes (CNT) have shown promising features as
electrode material93,94. CNT are nanometer-sized cylindrical structures
made of graphite sheets, characterized by a high aspect ratio, and
displaying high mechanical and electrical conductivity properties95,96.
One way of incorporating charges in the CNT structures is to
functionalize them with carboxylic acid groups. The structure of a singlewall carbon nanotube functionalized with carboxylic acid groups is
schematically shown in Figure 6. The presence of charged group makes
CNT suitable for LbL deposition63, and LbL films containing CNT have
already been assembled for strong composites60 or energy storage
applications97,98.
O
C
OH
Figure 6. The structure of a carboxymethylated CNT, used as active material in
the 3D energy storage devices
Traditionally, energy storage devices are two-dimensional and although
several studies have been carried out to build 3D energy storage
devices99, these are limited due to the design complexity of such devices,
and the difficulty in depositing material in a porous template. The ability
to assemble three-dimensional devices with interdigitated electrodes
using a simple and fast procedure such as LbL would pave the way for a
range of potential applications.
-16-
EXPERIMENTAL
3. EXPERIMENTAL
The main procedures and chemicals used in the different studies are
described in the following section. For more information, the reader is
referred to the relevant section in the corresponding paper.
3.1. Materials
3.1.1.
Fibres / Nanofibres
The fibres used in Paper I were from specially prepared, laboratorycooked kraft pulps (SCA Research AB, Sundsvall, Sweden). They were
never-dried, unbleached and unbeaten softwood fibres (pure spruce)
cooked to three different kappa numbers: 34, 75 and 107 (denoted K34,
K75 and K107 respectively). The K75 fibres were used in Paper II. In
Paper III, a totally chlorine-free bleached kraft pulp from softwood was
used (SCA Forest Products, Östrand Mill, Sundsvall, Sweden). The pulp
was received as dry sheets which were disintegrated, washed and
converted to the sodium form prior to use, following a procedure
described earlier23.
The CNF used in Papers III, VI and VII were provided as a 2 wt% gel by
Innventia AB, Stockholm, Sweden. The material was produced and
characterized following procedures described by Wågberg et al.9
3.1.2.
Model surfaces
The silicon wafers (p-type, MEMC Electronics Materials, Inc., Novara,
Italy) used in Papers III, IV and V as substrates for the LbL deposition
were first rinsed with Milli-Q water, ethanol and Milli-Q water, and then
blown dry with N2. They were then plasma-treated (Model PDC 002,
Harrick Scientific Corporation, NY, USA) for 2 min at 30 W.
-17-
EXPERIMENTAL
3.1.3.
Chemicals
PAH (with molecular masses 15 kDa and 56 kDa), PAA (240 kDa), HA
(1.6 MDa), 1,2,3,4-Butanetetracarboxylic acid (BTCA) and sodium
hypophosphite (SHP) were provided by Sigma Aldrich and used without
further treatment. Polyvinylamine (PVAm) polymers with molecular
masses 45 kDa and 340 kDa and a rate of hydrolysis greater than 90%
were provided by BASF (Ludwigshafen, Germany) under the commercial
names Lupamin 5095 and Lupamin 9095 respectively. These commercial
products were dialysed against Milli-Q water and freeze-dried prior to
use. Branched PEI (60 kDa) was provided as a 50 wt% aqueous solution
by Acros Organics (U.S.) and was used as received. The Sylgard 184 and
curing agent used to prepare the PDMS probes were purchased from
Dow Corning (U.S.). Single Wall carbon nanotubes (CNT) functionalized
with carboxyl groups were purchased from Carbon Solutions. CNT
dispersions were prepared by ultrasonication and centrifugation to
remove the largest nanoparticle aggregates.
3.2. Methods
3.2.1.
Fibres and paper (Papers I-III)
Polyelectrolyte titration
The polyelectrolyte titration method was used to determine the surface
charge of the fibres and the charge density of the cationic polymers, and
to determine the adsorption isotherms of polyelectrolytes on the fibres100.
In this method, a solution is titrated with potassium polyvinyl sulphate
(KPVS) in the presence of a cationic indicator, ortho-toluidine blue (OTB).
Since KPVS is negatively charged, it neutralizes the cationic charges
present in the solution. When all the cationic charges have been
neutralized, KPVS accumulates and forms a coloured complex with OTB.
By
recording
the
absorbance
of
-18-
the
solution
throughout
the
EXPERIMENTAL
measurement, it is possible to detect the equivalent point and thereby
determine the amount of charges initially present in the solution101,102.
Polyelectrolyte adsorption
A polyamine monolayer was adsorbed by pouring the desired amount of
polymer into a 5 g/L fibre suspension under stirring with a background
salt concentration of 5·10-4 M of NaHCO3. The pH was close to 8 and the
adsorption time was 10 min.
The LbL build-up on the pulp fibres was carried out on a larger scale
since a sheet was made after each deposited layer. Adsorption isotherms
were first determined in order to evaluate the amount of polyelectrolyte
to be added as the first layer. To predict the amount of polymer to be
added in the next layer, dual polarization interferometry was used103. The
cationic polymer (i.e. the first layer) was added to a 3.6 g/L fibre
suspension under stirring. The adsorption time was 10 min and, for the
systems with added salt, the salt concentration was set to 10 mM of NaCl.
After the adsorption, 1L of the suspension was taken out and used to
prepare a sheet.
P-
S0
V0
S2i+1
[
(V0-2i-1)
S2i+2
(V0-2i-2)
[
P+
n
Figure 7. Schematic of the protocol to build LbL films on fibres, for i ϵ ⟦0;n-1⟧,
where V0 is the initial volume of fibre suspension, Sk is the k-layer treated sheet,
P+/P- is the cationic/anionic polymer, and n is the number of bilayers
The remaining suspension was dewatered in a Büchner funnel in order to
remove the excess polymer. Water was thereafter added to form a 3.6 g/L
-19-
EXPERIMENTAL
suspension to which the next polymer was added. This sequence was
repeated until the desired number of layers (i.e. 10 in this study) was
achieved, as depicted in Figure 7.
Handsheet preparation and evaluation
A Rapid Köthen instrument (PTI, Vorchdorf, Austria) was used to
prepare the handsheets. Each sheet was dried under restrained
conditions at 93 °C under a reduced pressure of 95 kPa for 15 min. All the
sheets were then conditioned at 23 °C and 50% RH before further testing.
Uniaxial tensile testing was conducted according to the SCAN-P 67:93
Standard using a horizontal tensile tester (PTI, Vorchdorf, Austria). The
tensile index, as well as the stress and strain at break, were used to
evaluate the mechanical properties of the paper.
Nitrogen analysis was used to determine the nitrogen content of paper
samples, and thus to assess the amount of polymer adsorbed. This
technique consists in burning the sample at 1050 °C in an oxygen-poor
medium. All the nitrogen-containing groups are then converted to
nitrogen oxides, which in turn react with ozone to form excited nitrogen
oxide molecules. As these excited molecules decay, light is emitted. A
photomultiplier detects the emitted light and the instrument returns a
number of counts, corresponding to the amount of emitted light and thus
to the amount of nitrogen. For each polymer, a calibration curve was
established in order to link the number of counts to the amount of
polymer. The tests were performed with the aid of an ANTEK MultiTek
(by PAC, Houston, Texas, USA).
Interfibre joint strength evaluation
The fibre crosses for interfibre joint strength measurements were
prepared from untreated and PAH-saturated K75 fibres, according to the
following procedure: a few fibres were suspended in droplets of
deionized water on a Teflon-coated steel surface. A similar steel plate
was then placed on top of the former and a weight was added to reach a
-20-
EXPERIMENTAL
nominal drying pressure of 2.6 kPa. The system was heated for 2 hours at
108°C. After drying and conditioning at 23°C and 50% RH, overlapped
fibres were selected and tested. The individual fibre crosses were
thereafter fixed on specially designed sample holders using liquid
adhesives as shown in Figure 8.
Crossed fibre
Loaded fibre
0.5 mm
Glue
X
Y
Figure 8. Photo micrograph showing the fibre cross mounted on the sample
holder prior to tensile testing
The fibre crosses were subjected to either a shearing or a peeling mode of
loading (only shearing is discussed in the present thesis). The mechanical
testing was performed with the aid of a commercial tensile testing
machine with an in-house constructed grip-system. The conventional
shearing type of loading, commonly considered to be the interfibre joint
strength, was performed by fixing both ends of one fibre while one end
of the loaded fibre was subjected to a prescribed displacement at a rate of
2 μm/s in the direction of the fibre axis.
The recorded force at rupture (Pmax) was normalised with respect to the
macroscopic overlapping area (Aoverlap) of the crossing fibres, measured in
a high magnification transmission microscope, in order to take into
account differences in fibre dimensions, as wider fibres are expected to
carry more load since a larger area is available for bonding. The interfibre
joint strength is defined as:
-21-
EXPERIMENTAL
 overlap 
Pmax
Aoverlap
Since the strength distribution of interfibre joints prepared and tested
using this method is skewed34, the median is the best measure of the
central tendency of the distribution. The median can be used to compare
the populations of modified and unmodified specimens. Moreover, since
the direct testing of interfibre joints is difficult, time-consuming and
subject to large variations, relatively small sample sizes are usually
obtained. A univariate bootstrap analysis was therefore performed using
10000 resamplings of the data to provide an evaluation of the stability of
the measurements. The median was then determined as the arithmetic
mean of the resampled median calculations.
X-ray micro-computed tomography (X-μCT) and image analysis
In order to acquire information about the network structure, X-μCT scans
were performed using a Bruker Skyscan 1172 at constant room
temperature (23°C) and a relative humidity of 30% (±2%). 1200
projections were acquired and the resulting images had 4000 x 2664
pixels, 0.8 μm in size. 3D images with a voxel size of 0.8 μm were
reconstructed from the images using NRecon 1.6.9 (Bruker, Belgium). A
resolution of about 1 μm is too poor to resolve molecular interactions on
a joint level, but it can provide information about the changes in the sheet
structure on a submicrometre level when chemical additives are added.
A segmentation of the fibres was performed using the method described
by Wernersson et al.104, in which the user manually follows the centre line
of a fibre introducing control points using different cutting planes, as
illustrated in Figure 9. By following the demarcated fibre and analysing
its neighbourhood, it is possible to estimate whether the fibre is free or
bonded, as well as the cross-sectional area and circumference of the
fibres. It is thus possible to segment the marked fibre and count the
number of contacts per unit length. At the intersection of two segmented
fibres, the contact area of the interfibre joint can also be evaluated.
-22-
EXPERIMENTAL
Figure 9. Illustration of the different planes used to mark the centre line of a
fibre: (a) top view, (b) section used to indicate the depth of the centre line, (c)
section perpendicular to the centre line of the fibre, used to manually ensure that
the spline stays at the centre of the marked fibre
3.2.2.
Layer-by-Layer
assemblies
of
(PAH/HA)
(Papers IV-V)
LbL deposition on model surfaces
The LbL deposition of the (PAH/HA) system onto model surfaces (silicon
wafers or PDMS spheres) was achieved by dipping the surfaces
consecutively in PAH and HA solutions (of concentration 0.1 g/L), with a
rinsing step in Milli-Q water after each polymer deposition. The dipping
times were 10 min in the polyelectrolyte solutions and 5 min in the
rinsing bath. The pH of the PAH and HA solutions was 4.9 and 5.8
respectively.
-23-
EXPERIMENTAL
Thickness and growth of (PAH/HA) assemblies
Dual polarization interferometry (DPI) is an optical method used to
study the build-up of a polymer film on a substrate103,105. A laser light
beam is passed through waveguides composed of a reference and the
substrate to be modified. Depending on the amount of material deposited
onto the substrate, the output from the two waveguides generates
interference fringes that can be traced back to the thickness of the
adsorbed film103. A Dual Polarization Interferometer Analight Bio200
from Farfield Sensors Ltd (Manchester, UK) was used to study the
(PAH/HA) assemblies with a continuous flow of 100 μL/min. All the
samples had a concentration of 0.1 g/L during the adsorption step.
Nitrogen-doped silica chips were used as deposition substrates.
Quartz-crystal microbalance with dissipation (QCM-d) is a technique
used to record the real time adsorption of a polymeric compound to a
surface in terms of mass and viscoelastic properties. A crystal is subjected
to an oscillation and the change in frequency due to the adsorption of a
polymer can be recorded and related to the mass of adsorbed material,
while
the
energy
dissipation
provides
information
about
the
viscoelasticity of the adsorbed layers . A QCM E4 (Q-Sense AB, Västra
106
Frölunda, Sweden) was used to study the multilayer build-up with a
continuous flow of 100 μL/min. The substrates were silicon oxide crystals
with an active surface of sputtered silica prepared in the same way as the
silicon oxide wafers described earlier. AT-cut quartz crystals with a 5
MHz resonance frequency were used. The change in frequency is
equivalent to a change in adsorbed mass according to the Sauerbrey
model107. This model assumes rigidly attached layers, and the adsorbed
amount determined contains both polymer and water coupled to the
adsorbed layer.
Imaging and adhesion properties by Atomic Force Microscopy
An AFM MultiMode IIIa (Veeco Instruments Inc. Santa Barbara, CA) was
used both for imaging and for adhesion measurements. For tapping
-24-
EXPERIMENTAL
mode imaging in air, an EV scanner was employed using standard noncontact mode silicon cantilevers with a spring constant in the range of 32
to 70 N/m (TAP150, Bruker, Camarillo, CA). The force measurements
were performed by capturing normal force curves in water. A silica
particle (Thermo scientific, CA) with a diameter of approximately 10 μm
was attached, with the aid of a manual micromanipulator and an
Olympus reflection microscope, to the end of the tipless cantilever, using
a small amount of a two-component epoxy adhesive (Strong epoxy rapid,
Casco). For each probe, the exact diameter of the probe was determined.
Adhesion properties of (PAH/HA) using contact adhesion testing
(CAT)
PDMS spheres with diameters of about 3 mm were prepared by injecting
droplets of PDMS into a bath of water heated to 65 °C. The spheres were
then cured by standing in this water bath for 2 hours. In order to remove
the non-cured PDMS fractions, the spheres were thereafter washed in
heptane. For both the adsorption steps and the adhesion measurements,
the spheres were attached to a piece of glass slide using PDMS, as shown
in Figure 10. The PDMS spheres were LbL-coated by dipping in alternate
PAH and HA solutions, with a rinsing step between each step.
Figure 10. Photograph of a probe consisting of a PDMS sphere attached to a
glass slide
A specially designed CAT instrument was used to characterize the
adhesive properties of the LbL thin films. The instrument consists of a
stage that can be moved up and down with a micrometer-controlled
-25-
EXPERIMENTAL
translation stage with a horizontal aluminium holder on which the probe
(PDMS sphere) is attached to a transparent glass slide. The flat surface
(silicon wafer) is attached to a sample holder standing on a precision
balance and the two surfaces are brought into contact until a certain
trigger load is reached. The stage is then moved upwards to pull the
surfaces apart and the evolution of the load as a function of time (and
distance) is recorded. The instrument is connected to an optical
microscope as well as a camera, allowing images of the contact zone
between the two surfaces to be recorded throughout the measurement.
The never-dried LbL coated surfaces were brought into contact in the wet
state, dried overnight in contact, and thereafter pulled apart at a velocity
of 100 μm/min, as depicted in the schematic shown in Figure 11.
Contact
Dry
Pull-off
Figure 11. Schematic of the adhesion measurements, where the LbL-coated
PDMS sphere was brought into contact with the LbL-coated silicon wafer in the
wet state. The surfaces were then dried in contact before being pulled-apart
3.2.3.
Wet-resilient cellulose aerogels (Papers VI-VII)
Preparation
1,2,3,4-butanetetracarboxylic acid (BTCA) and sodium hypophosphite
(SHP) were added to a CNF gel (2 wt%) in ratios of 1:1 and 1:2
respectively. The mixture was then stirred for 20 min using an Ultra
Turrax T25 (IKA, Germany) at 10 000 rpm.
The gel was thereafter frozen in aluminium cups using liquid nitrogen,
and freeze-dried. The gels were then cross-linked in the dry state by
heating at 170°C for 4 min. The aerogels were thoroughly rinsed with
-26-
EXPERIMENTAL
Milli-Q water prior to use in order to remove any unreacted crosslinker
or catalyst. The procedure is depicted in Figure 12.
O
HO
O
O
HO
O
HO
HO
O
O
O
O
O
HO
O
O
O
HO
HO
O
OH
HO
O
OH
O
HO
O
O
HO
OH
O
O
O
O
HO
O
OH
OH
Figure 12. Schematic of the preparation of the crosslinked cellulose aerogel by
freeze-drying and heating the gel
LbL functionalization of the aerogels
The aerogels were functionalized by LbL deposition. Since aerogels are
porous materials, the commonly-used dipping approach could not
provide a homogeneous coating through the bulk aerogel in a reasonable
time. A new method was therefore introduced, where the liquid polymer
solution (or particle suspension) was forced through the aerogel by
pouring the solution on top of the aerogel and applying suction to the
aerogel, as schematically outlined in Figure 13. The concentration of the
solutions/dispersions used was 1 g/L and no pH adjustment was made.
Figure 13. Schematic of the filtration method used to build LbL assemblies
within the aerogels by forcing the liquid through the porous structure
-27-
EXPERIMENTAL
Characterization of the aerogels
The total charge of the aerogels was determined using conductometric
titration. An automatic titrator was used and HCl was added to
protonate the carboxylic groups of the aerogel.
Scanning electron microscopy (SEM) was used to image the structure of
the aerogels, using a Hitachi S-4800 field emission scanning electron
microscope.
Compression tests were performed on wet samples using an Instron
5566. The samples were tested at a speed of 50%/min in compression and
30% in extension, and cycles were performed between 10% and 80%.
Preparation of energy storage devices
The vacuum-assisted LbL-method was used to build layers of active
materials inside the aerogel structure to assemble 3D interdigitated
supercapacitors. An aerogel sample was first cut to the desired shape,
and a (PEI/CNT)5 multilayer was then formed in the aerogel to create the
first electrode. After drying, the first electrode was contacted using silver
paint and a copper wire, and the contact region was waxed using paraffin
wax to prevent any wetting of this region. The separator was then built
using an LbL system composed of (PEI/PAA)30. Finally, the second
electrode was assembled and contacted in a similar way as the first
electrode, as outlined in Figure 14. Hybrid batteries were also formed by
incorporating copper hexacyanoferrate nanoparticles (CuHCF) as a
cathode material.
-28-
EXPERIMENTAL
Aerogel substrate
Substrate
Electrode1
LbL Electrode 1
Masking + contact
LbL Separator
LbL Electrode 2
Masking + contact
Separator
Electrode 2
Figure 14. Schematic of the step-by-step procedure to build 3D interdigitated
energy storage devices in the aerogels at the (top) macro-, (middle) micro- and
(bottom) nano-level
Electrochemical characterization of the energy storage devices
The LbL-assembled 3D supercapaci tors and batteries were characterized
electrochemically using a PGSTAT302N potentiostat (Eco Chemie, The
Netherlands). The supercapacitors were tested in a 1 M Na2SO4
electrolyte, and cyclic voltammetry was performed using scan rates of 5–
50 mV/s. Constant current charging and discharging was carried out
using currents between 25 and 200 μA, corresponding to current
densities between 0.07–0.5 A/g based on the total carbon mass of the
device. The voltage window was 0–0.8 V.
The copper hexacyanoferrate cathode and hybrid 3D batteries were
tested using cyclic voltammetry at a scan rate of 1 mV/s in a (1 M KNO3;
0.01 M HNO3) electrolyte.
-29-
EXPERIMENTAL
For measurements under compression, a setup using a digital caliper was
used, as shown in Figure 15, and CVs were recorded at each compression
step using a scan rate of 10 mV/s.
Figure 15. Photograph of the setup used to characterize the electrochemical
behaviour of the devices under compression
-30-
RESULTS & DISCUSSION
4. RESULTS & DISCUSSION
4.1. Lignocellulosic
fibre
networks
with
tailored
properties (Papers I-III)
4.1.1.
Adsorption of polyamines monolayers on fibres
for the preparation of strong paper materials
Adsorption isotherms
In order to estimate the amount of polyelectrolyte that could be adsorbed
onto the fibres, adsorption isotherms were determined for different
combinations of fibre/polyelectrolyte using polyelectroyte titration of the
filtrates obtained from dewatered fibres suspensions with different
amounts of added polyelectrolyte. Fibres from pulps with different
kappa numbers were used and the charge characterization of these is
displayed in Table 1. As expected, the pulp yield had a strong influence
on both the surface and total charge of the fibres, since charged species
such as oxidized lignin and charged hemicelluloses are removed during
the cooking process. A maximum in the surface charge was observed due
to a competition between the accessibility of charges within the fibres
(which increases during cooking) and the total charge of the fibres (which
decreases during cooking)108.
The adsorption isotherms for the K75 pulp are shown in Figure 16. The
typical behaviour (an increase followed by a levelling-off of the adsorbed
amount, corresponding to the saturation adsorbed amount) can be
detected, and lower molecular mass polymers gave rise to higher
adsorbed amounts, which shows that, in addition to adsorption at the
surface, there can also be penetration of polymer into the fibre wall to a
certain extent. In the case of the K75 pulp, the pore radius (11 nm) is in
the same range as the radius of gyration of low molecular mass PAH (10
nm)109, making it possible for polymer chains to penetrate the fibre wall.
-31-
RESULTS & DISCUSSION
Table 1. Surface charge, total charge, lignin content and average pore size of the
fibres from the K34, K75 and K107 pulps
Pulp
Yield
Surface
Total
Lignin
Pore radiusd
chargea
chargeb
contentc
(%)
(μeq/g)
(μeq/g)
(w%)
K34
49.7
5.6
74
3.3
17
K75
56.4
9.3
137
10.0
11
K107
60.4
8.5
177
15.1
10
(nm)
Determined by polyelectrolyte titration
Determined by conductometric titration
cDetermined by chemical analysis
dFrom Andreasson et al. (2003)108
a
b
Figure 16. Adsorption isotherms for PAH and PVAm with different molecular
masses, for pulp K75, measured in 5·10-4 M NaHCO3
-32-
RESULTS & DISCUSSION
Mechanical properties of the handsheets
Hansheets were prepared from fibres treated with different amounts of
polyelectrolyte, and the tensile indices of the sheets prepared from pulp
K75 are shown in Figure 17 as a function of the adsorbed amount of
polymer. The density of the networks was not affected by the adsorption
of polymer, so that a direct comparison can be made between the
different configurations. Each fibre/polyelectrolyte combination gave rise
to an increase in the tensile index, with a tensile index increasing from
about 80 Nm/g (unmodified fibres) to 120 Nm/g (at saturation
adsorption, i.e. about 2 mg/g) in the case of PAH 56 kDa.
Figure 17. (Left) Tensile index as a function of the adsorbed amount of
polyelectrolyte (determined by nitrogen analysis), and (right) stress-strain
curves of handsheets adsorbed with different polymers at saturation levels, both
for pulp K75
The stress-strain curves of the samples, also displayed in Figure 17 for
paper samples made from the K75 pulp, show that each polymer, when
adsorbed at saturation level, gave rise to a significant improvement in the
mechanical properties, although the improvement was slightly greater
for the combination K75/PAH 56 kDa. Handsheets prepared from the
K34 and K107 pulps showed lower relative improvements in the
mechanical properties when polyelectrolytes were adsorbed. This is
probably because fibres from the K75 pulp are the most swollen fibres110,
and consequently more flexible, increasing the probability for a fibre to
-33-
RESULTS & DISCUSSION
achieve intimate contact with another fibre to form a strong interfibre
joint, which is even stronger after adsorption of strength additives such
as PAH or PVAm.
Interfibre joint strength
To obtain a better understanding of the microscopic mechanisms
involved in the dry strengthening of paper materials following the
adsorption of polyamines, the strength of interfibre joints was evaluated
for the K75 fibres, untreated and treated with PAH 56 kDa at saturation
adsorption, which was the combination for which the largest relative
increase (47%) in tensile index was observed. The results are presented in
Table 2, and show that an increase of 18% in the interfibre joint strength
was achieved when PAH was adsorbed.
Table 2. Median and standard deviation of the interfibre joint strength in
shearing mode
Median
Standard deviation
(MPa)
(MPa)
Untreated
1.94
0.60
40
PAH 56 kDa
2.28
0.45
35
Fibres
Number of specimens
Another important feature of the interfibre joint strength is the
distribution of the joint strength values. As can be seen in Figure 18,
treatment with PAH shifted the interfibre joint strength distribution
towards stronger joints, which means that the number of weak joints
decreased and/or the number of strong joints increased after PAH
adsorption onto the fibres. The variation in interfibre joint strength is
generally high, but the strength variation found here was comparable to
that reported in previous studies21,30, and a clear trend for the joint
strength to increase as a result of the adsorption of PAH was evident.
However, the difference in joint strength accounts for only 18% of the
-34-
RESULTS & DISCUSSION
increase in median strength and cannot solely explain the 47% increase in
strength at the macroscopic level.
0.35
Unmodified
Modified
Distribution unmodified
Distribution modified
0.3
Density
0.25
0.2
0.15
0.1
0.05
0
0
5
σ
overlap
/MPa
10
15
Figure 18. Interfibre joint strength distribution and the fitted Weibull
distribution for the unmodified and PAH-modified specimens respectively tested
in shear
Interfibre joints, contact area and number
X-μCT was used to provide information on the microstructure of the
fibrous network, and to investigate the influence of the adsorption of
PAH on the number and area of the interfibre joints.
3D images of the fibrous network and of one isolated interfibre joint are
shown in Figure 19. Using image analysis, the contact area of the
interfibre joints as well as the number of interfibre joints in the fibrous
network were estimated, and the results are presented in Table 3. The
contact area and the number of joints increased by 14% and 26%
respectively when PAH was added. The relatively large standard
deviations show the large variety of interfibre joints formed during the
consolidation process.
-35-
RESULTS & DISCUSSION
Figure 19. 3D visualisations of (left) the untreated paper (sample width 0.8
mm) and (right) one interfibre joint from the same sample, with a contact area of
557 µm2. The fibre on top is semi-transparent in order to make the optically
joined area visible
Table 3. Mean contact area of interfibre joints and number of interfibre joints
per mm fibre for untreated and PAH-treated samples, evaluated from X-µCT
Mean contact area
(μm2)
Standard deviation
(μm2)
Number of
joints / mm
fibre
Untreated
214
95
14.7
PAH 56 kDa
245
110
18.5
The strength enhancement of a fibrous network upon adsorption of a
polyamine monolayer can therefore be explained at the microscopic level
by an increase in the interfibre joint strength together with an increase in
both the number of joints and the contact area. This is the first time, to the
knowledge of the author, that these different factors have been evaluated
separately and combined to explain the mechanism behind the dry
strengthening of paper materials using chemical additives. The results
demonstrate how important it is to separate the factors that are
influenced by the additives in order to further improve their
performance. In future investigations, it would be of interest to use X-ray
-36-
RESULTS & DISCUSSION
tomography with a higher resolution in order the better to evaluate the
molecular interactions across fibre surfaces.
4.1.2.
Build-up of polyelectrolyte multilayers onto
fibres for strong and stretchable paper materials
Polyelectrolyte multilayer films of (PAH/HA) were assembled on the
surface of bleached pulp fibres under two different salt conditions: no
salt, and 10 mM NaCl. Handsheets were thereafter prepared from the
unmodified and modified fibres. The mechanical properties (tensile index
and strain at break) of these handsheets are presented in Figure 20. A
major difference can be seen between systems without added salt, and
systems in which the LbLs were assembled in 10 mM NaCl. Neither
tensile index nor strain at break showed any significant change when the
fibres were treated with (PAH/HA) in the absence of salt, regardless of
the number of deposited layers. On the other hand, the mechanical
properties increased after each added layer when the multilayers were
deposited in a 10 mM NaCl environment, and both tensile index and
strain at break increased by a factor of 3 after 5 bilayers of (PAH/HA) had
been deposited.
Figure 20. (Left) Tensile index and (right) strain at break of handsheets
prepared from fibres coated with layers of (PAH/HA), assembled with 10 mM
NaCl and without salt
-37-
RESULTS & DISCUSSION
In order to explain how the multilayers influence the mechanical
properties of the paper, the molecular organisation in the interfibre joints
must be considered. In order to form a joint with properties providing
both strength and stretchability to the paper, two features are crucial: (a)
the polymers must be anchored at the surface of the fibre, and (b) the LbL
film must have ductile properties, i.e. the film must have a yield
stress/strain lower than that of the fibre wall, and the different layers in
the formed film must interpenetrate to create a film with a thickness of
10-20 nm. Figure 21 depicts these aspects. A solid anchoring of the first
polymeric layer to the fibre surface can be achieved both by charge
interactions and by partial penetration of the polymer chains into the
fibre wall. PAH with low molecular mass (15 kDa) was used as the
anchoring layer. The radius of gyration of the polymer chains in a 10 mM
NaCl medium was calculated to be 7 nm109, and the pore radius of a
bleached kraft pulp fibre is of the order of 10 nm108. It is thus possible for
these polymer chains to penetrate the outer layers of the fibre wall,
ensuring a good anchoring of the layer from which the build-up of the
structure is initiated.
Fibre
+
LbL film
+
-
+
-
Fibre
-
+
+
-
-
+
-
+
-
Fibre
Figure 21. Schematic of an interfibre joint between two LbL-treated fibres with
an enlargement of the fibre/LbL film interface, showing the anchoring of the
polymer to the fibre surface, and the intermixing within the LbL film
In order the better to understand the behaviour of the (PAH/HA) LbL
films leading to the significant improvements at the paper level
presented above, a more fundamental study of the build-up and adhesive
properties of (PAH/HA) films was carried out.
-38-
RESULTS & DISCUSSION
4.2. Layer-by-Layer
assemblies
of
(PAH/HA)
(Papers III-V)
4.2.1.
Build-up
LbL films of (PAH/HA) were assembled onto silicon wafers to study the
build-up of the multilayers. The influence of the number of layers
deposited and of the addition of 10 mM NaCl were investigated.
QCM-d was used to provide information on the film build-up and the
results are shown in Figure 22, in terms of the frequency shift (which
translates into the amount of material adsorbed at the solid/liquid
interface) and the dissipation (which translates into the visco-elastic
behaviour of the film). Without added salt, the LbL system showed a
very low build-up with increasing number of layers, indicated by a more
or less constant frequency shift regardless of the number of layers, and
stable low dissipation values, indicating a rigid film with low viscous
losses.
Figure 22. QCM-d data for (PAH/HA) assemblies with and without added salt:
(left) frequency shift and (right) dissipation as a function of the layer number
In the presence of 10 mM NaCl during the assembly of the layers, the
behaviour was totally different. The frequency shift decreased in a superlinear manner and an odd-even effect was observed, with a greater
frequency shift and thus more adsorbed material when HA was in the
-39-
RESULTS & DISCUSSION
outermost layer. The dissipation data showed an increased viscoelasticity of the film when the number of adsorbed layer increased, and
the odd-even effect was also observed.
The thickness of the (PAH/HA) films was estimated by DPI, and the
results are shown in Figure 23. As observed in QCM-d, the addition of
salt changed the build-up behaviour from a linear to a super-linear
growth, and an odd/even effect was again observed in the system with
added salt, where films with HA in the outermost layer were thicker than
those with PAH in the outermost layer.
Figure 23. Thickness of (PAH/HA) films as a function of the number of layers
deposited for systems assembled with and without added salt, obtained by DPI
measurements
AFM images after drying of the structures formed upon LbL deposition
are displayed in Figure 24. The images show a major difference in the
build-up of the assemblies depending on whether or not salt was added.
In the system without added salt, small structures were formed and their
number increased with increasing number of layers deposited. On the
other hand, with added salt, nano-sized structures were formed, and
these structures increased in size and number as the number of deposited
layers increased, until they coalesced to form even larger structures,
-40-
RESULTS & DISCUSSION
giving rise to a rough film composed of structures reaching typically 30
nm in height. As was observed in DPI and QCM-d, an odd-even effect
was observed, where structures with PAH in the outermost layer
appeared to be flatter while structures with HA in the outermost layer
showed a surface structure with greater height differences, and thus a
greater roughness.
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
10 mM NaCl
No added salt
1
Figure 24. AFM images of (PAH/HA) assemblies deposited on silicon wafers
(top) without added salt and (bottom) with 10 mM NaCl. The numbers indicate
the number of layers deposited. Images are 2x2 µm2, and the z-range is 30 nm
Such structural changes can be explained by different salt conditions
between the assembly (10 mM NaCl) and washing steps (pure water)111.
Furthermore, Kovacevic et al.112 suggested that multilayers can be in a
-41-
RESULTS & DISCUSSION
glassy state at low ionic strength and that an increase in ionic strength
can lead to a liquid state where the molecules have more freedom to
diffuse in the layer to reach an equilibrium conformation. The increased
mobility of the polymer chains in the presence of salt promotes the
interdiffusion within the LbL films, and it has been suggested that this is
an explanation for the super-linear growth of some LbL systems71. The
increase in film roughness shown in the AFM images gives rise to a
larger surface area available for further deposition of polymer, and this,
also known as the Island model74, can explain the super-linear growth
observed in the QCM-d and DPI studies.
4.2.2.
Adhesive properties
Two different approaches were used to investigate the adhesive
properties of (PAH/HA) films: AFM colloidal probe, providing
information about the adhesive behaviour at the nano-level, and CAT, at
the micro-level.
AFM colloidal probe
AFM force measurements were performed on LbL films of (PAH/HA)
assembled in-situ in the AFM, simultaneously on the silicon wafer and on
the colloidal probe. The force-displacement curves are shown in
Figure 25 for systems tested under conditions where the following
parameters were varied:

Number of layers deposited

Outermost layer

Dwell time

Presence of added salt
-42-
RESULTS & DISCUSSION
No added salt
Added salt
a
b
c
d
e
f
Figure 25. Force-displacement curves during separation of the colloidal probe
and flat surfaces. Graphs (a) and (b) show the influence of contact time for 5
layers in the systems, i.e. with PAH in the outermost layer, (a) without and (b)
with salt. Graphs (c) and (d) show the influence of contact time for 6 layers in
the systems, i.e. with HA in the outermost layer, (c) without and (d) with 10
mM NaCl. Graphs (e) and (f) show the influence of layer number at 10 seconds
contact time in the systems (e) without and (f) with 10 mM NaCl
-43-
RESULTS & DISCUSSION
As was observed in previously shown AFM, QCM-d and DPI results, the
systems assembled with and without added salt behaved totally
differently: an increase by one order of magnitude in the pull-off force
was observed when 10 mM NaCl was added for 10 layers and a contact
time of 10 s. The addition of salt promotes chain mobility, and the chain
mobility promotes the interactions between the films in contact to an
extent that is dependent on the contact time. Increasing the contact time
between the two surfaces increased the pull-off forces, regardless of
whether or not salt was added. This trend is expected since it is possible
for the polymer chains to establish a greater contact with the other
surface when the contact time is increased. When the number of layers
deposited was increased, the pull-off force also increased. The pull-off
forces were similar for the systems with PAH and the systems with HA
in the outermost layer, but the separation distances were greater when
HA was in the outermost layer, and separation forces were still detected
at separation distances of about 8 μm for the system with added salt and
a contact time of 10 s, showing that differences in the molecular weight of
the polyelectrolyte in the outermost layer allow the molecular
interactions to act over different distances, due to the unique properties
of the supramolecular structures containing long HA chains and short
PAH chains.
The work of adhesion was obtained by calculating the area under the
force-displacement curves, and the results are shown in Figure 26 as
functions of contact time and number of layers. The work of adhesion
values followed a similar trend to the pull-off forces, i.e. the work of
adhesion increased when salt was added, when the contact time was
increased, and when the number of layers deposited was increased. In
the films with up to six layers, the work of adhesion appeared to correlate
with the pull-off force of the interface. However, when subsequent layers
were added, the increase in work of adhesion was related to the ability of
the film to undergo a large displacement before total release, which
suggests that there is a transition between thin films where the work of
adhesion is governed mainly by the pull-off force and thicker films where
-44-
RESULTS & DISCUSSION
the critical force approaches a limit where the microscopic displacement
of the interfaces controls the work of adhesion. The work of adhesion for
9 and 10 layers formed in the presence of 10 mM NaCl was more than 20
times greater than the maximum work of adhesion reported earlier for
sacrificial bonds in bone structures113. Furthermore, adhesive chain
pulling interactions were recorded over a distance twice as long (> 8 μm)
as the greatest distance measured for the chains to rupture with collagen
and bone114. This shows the potential of the supramolecular structures
formed in the (PAH/HA) LbL system.
Figure 26. Work of adhesion as a function of (left) the contact time and (right)
the number of layers, for (PAH/HA) systems with and without added salt
The results suggest a mechanism involving two structural levels: an
intimate contact is achieved by bringing the nanoscopic rough (PAH/HA)
surfaces towards each other; molecular interdiffusion then leads to the
formation of an interpenetrating network. The supramolecular structure
of the (PAH/HA) assemblies, in combination with polymer chain
disentanglement, leads to the high separation forces and large separation
distances observed upon separation of the two surfaces. The suggested
mechanism is illustrated in Figure 27.
-45-
RESULTS & DISCUSSION
Figure 27. Schematic description of the approach and separation of two
(PAH/HA)-coated surfaces
CAT measurements
Adhesion measurements using a CAT equipment were conducted to
provide information on the adhesive properties of the (PAH/HA) system
formed at 10 mM NaCl on a micro-scale. In order to promote the contact
between the LbL-coated surfaces and thus optimize the adhesion, the
treated surfaces were brought in contact in the wet state, and dried in
contact before they were pulled apart. The force-displacement curves of
are shown in Figure 28. A dramatic increase in the pull-off force was
obtained for (PAH/HA) films from 1 to 3 bilayers, corresponding to a
greatest film thickness of 12 nm. The separation distances also
significantly increased. Images of the surfaces after the pull-off
experiments are shown in Figure 28, and these images reveal a transition
from a purely adhesive failure regime to a mixed adhesive and cohesive
failure regime, indicated by the presence of small fragments of PDMS
remaining on the silicon wafer for systems coated with 2 bilayers and
more. This means that the adhesion was strong enough to cause cohesive
failure of the surface of the PDMS probe.
-46-
RESULTS & DISCUSSION
Reference
(PAH/HA)1
(PAH/HA)
(PAH/HA)
2
3
Figure 28. (Left) Force-displacement curves for the reference system and the
system with 1, 2 and 3 bilayers of (PAH/HA) formed at 10 mM NaCl, and
(right) images of the corresponding surfaces after the pull-off experiments (the
scale bars are 250 µm)
The area under the force-displacement curves was calculated, and the
work of adhesion values were obtained for the different systems, and the
results are plotted in Figure 29. As envisioned in the force-displacement
curves, the work of adhesion increased by approximately one order of
magnitude for each bilayer deposited.
Figure 29. Work of adhesion as a function of the number of bilayers, formed in
the presence of 10 mM NaCl, for model surfaces coated with (PAH/HA) and
(PAH/PAA)
-47-
RESULTS & DISCUSSION
To put these values into context, similar experiments were performed
using a system commonly used in LbL studies, and known for its
adhesive properties: (PAH/PAA). The results are plotted on the same
graph and the increase in adhesion with an LbL coating of (PAH/PAA) is
much lower than that of the (PAH/HA) system. This shows the unique
adhesion features of HA films, and supports the trends seen in the AFM
force measurements.
4.3. Nanocellulose aerogels as a platform for a variety
of applications (Papers VI-VII)
4.3.1.
Preparation and characterization of the aerogels
Cellulose aerogels were prepared by freeze-drying a CNF gel. BTCA was
used to crosslink the material and render it solvent-stable. The BTCA
crosslinking leads to the incorporation of one carboxyl group per ester
linkage to the cellulose hydroxyls, resulting in an increase in the charge
(up to 2300 μeq/g for a 1:1 weight ratio of CNF and BTCA), which is
beneficial in a LbL perspective. The material has a porosity close to 99%
and the porous structure of the aerogel is shown in Figure 30.
100 µm
200 µm
Figure 30. (Left) X-ray microtomography and (right) SEM images of the aerogel
showing the porous structure of the material
-48-
RESULTS & DISCUSSION
The aerogels were tested in compression in the wet state by compressing
an aerogel in a step-by-step manner, with a relaxation between each
compressive step. The results, displayed in Figure 31, show that the
material undergoes shape-recovery up to very high compressive strains.
Figure 31. Compressive stress-strain cycling of a wet aerogel up to 80% strain
4.3.2.
LbL functionalization of the aerogels
LbL was used to coat the aerogels with active materials and provide them
with specific functionalities. The layering was achieved by a new way of
forming LbLs, by forcing a polyelectrolyte suspension through the
aerogel, ensuring complete coverage of the bulk of the material, achieved
in only a few seconds, which is impossible using common dipping
methods where diffusion processes in this type of low density material
take a long time. To show that the method allows full coverage of the
material in the bulk, confocal microscopy was used to analyse a cross
section of an aerogel that was LbL-coated with 5 bilayers of a fluorescent
polymer,
Poly[2-(3-thienyl)ethyloxy-4-butylsulfonate],
also
termed
ADS2000P. The confocal image, shown in Figure 32, reveals the structure
of the aerogel with full coverage of the structure by the fluorescent
polymer.
-49-
RESULTS & DISCUSSION
100 µm
Figure 32. Confocal image reconstructed from slice images of the cross section of
an aerogel LbL coated with 5 bilayers of ADS2000P
The versatility of LbL makes it possible to use the aerogel as a platform
for a variety of applications where active polymers and/or nanoparticles
can be incorporated. To show the variety of possibilities offered by this
template, LbL was used to incorporate a biopolymer (HA), a conductive
polymer,
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS), and conductive nanoparticles, carbon nanotubes (CNT), as
depicted in Figure 33 and Figure 34.
O
O
OH
OH
C
OH
O
O
HO
HO
O
O
OH
NH
O
n
Figure 33. CNF aerogels, LbL-functionalized with (from left to right) HA,
PEDOT:PSS and CNT
-50-
RESULTS & DISCUSSION
Among the possible applications of these LbL-functionalized CNF
aerogels, the preparation of energy-storage devices is particularly
interesting, and the next section focuses on supercapacitors and batteries
prepared in this manner.
Figure 34. Illustration of the porous aerogel coated with CNT
4.3.3.
Preparation
of
three
dimensional
and
compressible energy storage devices
LbL assembly of the electrodes and separator
Supercapacitors were built inside the aerogel using the cellulose aerogel
as a template by sequentially adsorbing (PEI/CNT)5 to assemble the first
electrode, followed by (PEI/PAA)30 to assemble the separator and finally
(PEI/CNT)5 again to form the second electrode. The electrodes were
contacted immediately after they had been assembled using copper wires
and silver paint. The contacts were protected from water by masking the
contact region with paraffin wax. A photograph of an actual device is
shown in Figure 35.
-51-
RESULTS & DISCUSSION
Figure 35. Photograph of a full device
Electron microscopy was used to show the sequential build-up of the full
device (Electrode 1 - Separator - Electrode 2). The SEM images shown in
Figure 36 show the build-up of CNT networks separated by a polymeric
film on both sides of a cellulose sheet constituting the aerogel. The
thickness of the CNT electrodes can be estimated to be 150 ±30 nm and
that of the separator 1.70 ±0.46 μm.
Figure 36. SEM images showing the sequential build-up of a full device at
different magnifications: (left) Electrode 1, (middle) Electrode 1 – Separator, and
(right) full device: Electrode 1 – Separator – Electrode 2. The scale bars are (top)
50 µm and (bottom) 2 µm
-52-
RESULTS & DISCUSSION
Electrochemical characterization
Cyclic voltammetry (CV) and galvanostatic cycling of the supercapacitors
were carried out in 1 M Na2SO4 electrolyte, and the plots are shown in
Figure 37. The CV graph shows square-shaped curves, characteristic of
carbon supercapacitors and associated with the charging of a double
layer at the interface between a carbon electrode and the electrolyte. The
capacitive
behaviour
of
the
device
is
also
reflected
in
the
charge/discharge curves with a linear dependence of the potential with
time. From these data, the specific capacitance of the devices was
estimated to be 25 F/g, using a current corresponding to a 1 min
discharge rate, and the devices showed stable cycling behaviour after 400
charge/discharge cycles. Although lower than some recently reported
values, the capacitance is still comparable to that of other traditional
carbon-based supercapacitors, showing that the concept of building 3D
interdigitated energy-storage devices inside porous soft materials, using
Current (A g-1)
0.2
0.1
0
5 mV s-1
10 mV s-1
25 mV s-1
50 mV s-1
-0.1
-0.2
0
0.2
0.4
0.6
Cell voltage (V)
30
20
10
25 μA
50 μA
0.8
0.6
0.4
0.2
0
0
200
400
600
800
Time (s)
0
0.8
Cell voltage (V)
0.3
Specific capacactiance (F g-1)
LbL, is possible.
0
100
200
Cycle number
300
400
Figure 37. (Left) Cycling voltammograms at different scan rates and (right)
galvanostatic cycling of the 3D supercapacitors
To take full advantage of the flexibility of the porous aerogels used as a
template, the devices were tested under compression. CVs were recorded
for a device compressed to different levels, up to 75% of its initial
thickness. The results shown in Figure 38 indicate that the device can
operate under extreme conditions of compression without any significant
-53-
RESULTS & DISCUSSION
change in its performance, and that the device recovers its initial shape
0.05
0
20
40
60
80
Compression (%)
0
0%
20%
40%
60%
75%
-0.05
-0.1
0
25 μA
25 μA (after compression)
30
0.2
0.4
Cell voltage (V)
0.6
0.8
20
Cell voltage (V)
Cuurent (A g-1)
0.1
c
1
0.8
0.6
0.4
0.2
0
Specific capacitance (F g-1)
b
0.15
Normalized capcity
after the pressure is released.
10
0
0
0.8
0.6
0.4
0.2
0
10600
100
10800
Time (s)
11000
200
300
Cycle number
400
Figure 38. Characterization of a full device under compression: (a) stepwise
compression and release of the device (scale bar 0.5 cm), (b) cyclic
voltammograms recorded under different compressions, and (c) relative losses in
capacity after compressing the device
When the device is compressed, the voids in the porous structure are
compressed, and even if two LbL-coated cellulose walls come into
contact, it is the external parts of the coating that come into contact, i.e.
the same electrode, and this is the reason why no short circuits were
observed even under extreme compression. Being able to operate a
device under such conditions paves the way for many potential
applications in flexible electronics.
Extension to batteries
Having demonstrated the concept of building 3D supercapacitors in
aerogels, and taking advantage of the versatility of the LbL technique,
hybrid batteries were built by incorporating copper hexacyanoferrate
(CuHCF) nanoparticles in the cathode. CuHCF is an open framework
material with the crystal structure of Prussian blue, and it can be used as
-54-
RESULTS & DISCUSSION
cathode for ultra-long-lifetime, large-scale cheap batteries115. To first
validate the concept, a single electrode was built, associating the
negatively charged CuHCF with PEI, and incorporating CNT to provide
conductivity,
so
that
the
electrode
was
composed
of
(PEI/CuHCF)3(PEI/CNT)3(PEI/CuHCF)3. The CV of the single electrode is
shown in Figure 39c (inset). The two characteristic peaks corresponding
to the faradaic processes that the cathode undergoes are clearly seen,
showing that the CuHCF particles were successfully incorporated into
the material. The single electrode was also shown to be stable over days
of charge/discharge cycling, showing that the LbL coated electrodes are
stable and that the incorporated CuHCF particles do not detach or
dissolve in the electrolyte.
A full hybrid battery was then built using this cathode and CNT as the
active material in the anode. SEM images of the device as well as
electrochemical characterisation are displayed in Figure 39. Small redox
peaks can be seen in the CV, although these were not as pronounced as
for the single electrode. One reason for this could be an asymmetry of the
amount of active materials in the anode and cathode, giving an
unmatched capacity of the electrodes. The loss in capacity upon cycling
shows that the device design needs to be optimized. However, the
general trends indicate that the battery operates as such and that the
incorporation of CuHCF into the cathode was successful, opening up a
range of possibilities where the properties of the device can be tailored by
incorporating specific particles.
-55-
RESULTS & DISCUSSION
d
10
1
0
Normalized capacity (C/C0)
0.8
-10
0.7
0.9
1.1
0.6
Potential vs. SHE (V)
8
0.4
4
0.2
0
Anode CNT
Anode CNT
-4
0.4
0.5
0.6 0.7 0.8
Cell voltage (V)
10
5
voltage (V)
Cuurent (μA)
16
12
Cuurent (μA)
c
0.6
10 μA
20 μA
10200
0
10800
Time (s)
0
0.9
1
0.8
50
100
150
Cycle number
200
Figure 39. SEM images of (a) the cross section of the hybrid battery and (b)
zoom in the CuHCF cathode, and (c) cyclic voltammograms of the 3D battery
with 5 and 10 bilayers of (PEI/CNT) in the anode, with the single electrode inset,
recorded with a scan rate of 1 mV/s. (d) Galvanostatic charging and discharging
of the CNT10 device and zoom in on one discharge cycle using 10 µA and 20 µA
charging current and device cutoff voltage 0.5 V and 1 V
-56-
CONCLUSIONS
5. CONCLUSIONS
The aim of the present thesis was to use physical adsorption of
polyelectrolytes and nanoparticles to enhance the properties of existing
cellulosic materials as well as to develop and functionalize a new class of
nanocellulosic material.
In this respect, the following results were achieved:

The strength of unbleached paper materials was enhanced by
about 50% by adsorbing monolayers of polyamines onto the
surface of the fibres in an amount of about 2 mg/g. The
mechanisms governing this strength enhancement on the
microscale were shown to be related to an increase in the
interfibre joint strength together with an increase in the interfibre
contact area and in the number of interfibre contacts.

The strength and strain at break of bleached paper materials
were increased by a factor of three by building up multilayers of
(PAH/HA) onto the surface of the fibres.

The fundamental study of (PAH/HA) LbL films revealed a
transition from linear to super-linear growth upon addition of
salt, as well as very high adhesive features on the nano- and
micro-scales. The adhesion was shown to be stronger than the
adhesion in bone structures.

Solvent-stable porous nanocellulosic soft aerogels were prepared.
The material had a high specific area, porosity, and charge. A
new method was introduced to functionalize this class of
material using LbL in a fast and robust way by forcing polymeric
or nanoparticle suspensions through the aerogel.
-57-
CONCLUSIONS

The CNF aerogels were used as a template to build 3D
interdigitated energy storage devices by LbL deposition inside
the aerogel. The devices operate under extreme conditions of
compression and their shapes can be controlled. Recently
developed
high-performance
cathode
nanoparticles
were
incorporated to build a hybrid battery and demonstrate the
versatility of the method, paving the way for the development of
a new class of materials where high capacitance and flexibility
can be achieved.
-58-
FUTURE WORK
6. FUTURE WORK
As a continuation of the work presented in this thesis, a number of
specific tracks can be suggested to provide an even more complete
picture
of
cellulosic
materials
functionalized
by
polyelectrolyte
adsorption.

Polyamines have been shown to be efficient dry-strength
additives for paper materials, and the microscopic mechanisms
of the dry-strengthening of lignocellulosic networks have been
identified. At the molecular level, however, the mechanisms are
still unclear and further investigations are necessary to identify
the molecular interactions between polyamines and the polymers
constituting the pulp fibres.

LbL assemblies of (PAH/HA) were characterized and its
adhesive properties were studied on different length scales. An
interesting future development of HA-containing LbL films
would be to use them as an adhesive for biomedical applications
such as tissue engineering. It would also be desirable to identify
polyelectrolytes that could provide properties similar to those of
HA, but with a greater availability and a lower cost.

CNF aerogels were developed and a simple, cheap and fast
method to functionalize them has been introduced. This
constitutes a totally new materials platform, where the aerogel
can be used as a template for many potential applications. In
addition to the energy-storage devices presented in this work,
other initial activities in our research team have shown that antibacterial materials, as well as fire-retardant insulation materials
can be prepared and it will probably also be possible to prepare
materials for controlled drug release, to mention a few new
routes, using a LbL approach similar to that demonstrated in the
present thesis. Another future step regarding the development of
-59-
FUTURE WORK
functional porous cellulose-based materials is to find efficient
and cost-effective ways to produce CNF aerogels and provide
them with specific functionalities on a larger scale.

The concept of assembling 3D supercapacitors and hybrid
batteries in a porous template via LbL was demonstrated. Taking
advantage of the versatility of the LbL method, other particles
could be incorporated to build high-performance batteries.
Scaling up the process would also be a next step of great interest.
-60-
ACKNOWLEDGEMENTS
7. ACKNOWLEDGEMENTS
First and foremost, I would like to thank my supervisor Lars Wågberg for
giving me the opportunity to work in his group, and for his endless
optimism, enthusiasm and support regarding both the results and the life
in the group.
BiMaC Innovation is acknowledged for financial support.
I would also like to thank all my co-authors for their assistance and
cooperation in the different steps of the projects. My office-mates,
Caroline, Simon, Rebecca, Louise and Maryam are acknowledged for
valuable scientific discussions, for ensuring a very nice atmosphere at
work, and last but not least for helping me to improve my Swedish!
I wish to thank the administrative and technical staff at the Department
of Fibre and Polymer Technology, Mia, Inga and Mona. Thank you for
answering questions and for making administrative procedures so fast
and simple.
Special thanks to Mahiar for helping me bring my research and my thesis
to a level that I would not otherwise have been able to reach!
All the colleagues at FPT (particularly Andreas, Christopher, Dongfang,
Emil, Erdem, Gustav, Johan, Louise, Mahiar, Maryam, Oruc, Petri,
Raquel, Rebecca, Rosana, Veronica, Yujia) are also acknowledged for all
the good times spent together, from the laboratory to the floorball court,
not to mention the after-works and conference trips!
Cora Thibeaut is acknowledged for her kind assistance in designing
Figure 1.
I would also like to address special thanks to my French colleagues and
friends Myriam and Thomas for sharing their Swedish experience as PhD
students. Thank you for your support and for all the discussions on
-61-
ACKNOWLEDGEMENTS
politics, sports, and science! And thank you Myriam for reviewing the
thesis and being my critic!!!
Anthony, Paul, Pia, Rey and Zhenya (aka the NT team) are also
acknowledged for all the fun times and great dinners.
Juan Pablo Parra Martinez (…to make it short!) is thanked for numerous
training and laughing sessions, not to mention the stress-releasing sugarand fat-based food occasions and other beer collaborations!
Pampi: thank you so much for all the good times spent together, at the
gym or around a glass of wine!
Franz L., Ludwig v. B., Frédéric C., Sergei R., Giacomo P. (non-exhaustive
liszt!) are acknowledged for providing inspiration, strength and
motivation throughout these years.
Finally, I would like to thank my closest friends from France, Benoît,
Florian, Frédéric, Julien, Karine, Margot, Michaël, Nathalie, Noémie, for
your support, for all the nice times spent together, and for being so close
after so many years and despite the distance!
Parvenu au terme de cette thèse, il me reste à remercier ceux qui m’ont
permis d’en arriver là! Maman, Papa, Jordan, Ophélie, Valentine, merci
pour tout!
-62-
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