TRITA-PMT Report 2003:3
ISSN 1104-7003
ISRN KTH/PMT/R—03/3--SE
Model studies of cellulose fibers and films
and their relation to paper strength
Susanna Fält
Licentiate thesis
Stockholm 2003
TRITA-PMT Report 2003:3
ISSN 1104-7003
ISRN KTH/PMT/R—03/3—SE
@ Susanna Fält 2003
Model studies of cellulose fibers and films
and their relation to paper strength
Susanna Fält
Licentiate thesis
Model studies of cellulose fibers and films
and their relation to paper strength
Susanna Fält
MidSweden University, FSCN, SE-851 70 Sundsvall
KTH, Dept. Fibre and Polymer techn., Fibre Technology Div., SE-100 44 Stockholm
SCA Packaging Research, Box 716, SE-851 21 Sundsvall
ABSTRACT
The objectives of this work were (i) to develop a new method for the preparation of thin
cellulose model films, (ii) to use these model films for swelling measurements and (iii) to
relate the swelling of fibers and films to the dry strength of paper.
In the new film preparation method, NMMO (N-methylmorpholine-N-oxide) was used to
dissolve cellulose and DMSO (dimethyl sulfoxide) was added to control the viscosity of the
cellulose solution. A dilute solution of the cellulose was spin-coated onto a silicon oxide
wafer and the cellulose film thus prepared was then precipitated in deionised water. A
saturated layer of glyoxalated-polyacrylamide was used to anchor the film onto the silicon
oxide wafer. This procedure gave films with thicknesses in the range of 20-270 nm. The films
were cleaned in deionised water and were found by ESCA analysis and contact angle
measurements (θ< 20°) to be free from solvents. Solid state NMR measurements on fibers
spun from NMMO also indicated that the model film consisted of about 50% crystalline
material and that the crystalline structure was of the cellulose II type. Determination of the
molecular weight distribution of the cellulose surface material showed that the NMMO
treatment caused only a minor breakdown of the cellulose chains and that low molecular mass
oligomers of glucose were not created.
It was further shown that atomic force microscopy (AFM) measurements could be used to
determine the thickness of the cellulose films, in both the dry and wet states. The thickness
was determined as the height difference between the top surface and the underlying silica
wafer measured at a position where an incision had been made in the cellulose film.
The cellulose solutions were also directly spin-coated onto the crystal used in the Quartz
crystal microbalance (QCM-D), pre-treated with the same type of anchoring polymer. With
this application, these model surfaces were shown to be suitable for swelling measurements
with the QCM-D. The extent of swelling and the swelling kinetics in the presence of
electrolytes, such as NaCl, CaCl2 and Na2SO4, and at different pH were measured in this way.
The films were found to be very stable during these measurements and the results were
comparable to the swelling results obtained for the corresponding pulps. The swelling of both
fibers and films followed the general behavior of polyelectrolyte gels in the presence of
electrolytes and was in accordance with the Donnan equilibrium theory. The films have been
shown to differ from fibers with regard to the absence of a covalent interior network. This
influences the evaluation of the deswelling effects measured on the model films.
The swelling effect seen with different electrolytes has also been considered in relation to the
tensile strength of paper prepared from a kraftliner-pulp. In this study, it was found that there
was no direct relationship between the swelling of the fibers, measured as WRV, and the
strength of the paper in the presence of different electrolytes at pH 5.
KEYWORDS: absorption, carboxymethyl cellulose, cellulose, cellulose fibers, dissolving pulps, donnan
equilibrium, electrolytes, film, ion exchange, ionization, kinetics, liner boards, microscopy, spinning, surfaces,
swelling, tensile strength, water, water retention value.
LIST OF PAPERS
This thesis is a summary of the results presented in the papers listed below and referred to in
the text by their Roman numerals:
I
Model films of cellulose: I Method development and initial results.
Gunnars, S., Wågberg, L., Cohen-Stuart, M.A.
Cellulose 9: 239-249, 2002.
II
Model films of cellulose: II Optimization of the preparation method and
characterization of the films.
Fält, S., Wågberg, L., Vesterlind, E-L, Larsson, P.T.
Submitted to Cellulose
III
Influence of electrolytes on the swelling and strength of kraftliner-pulps.
Fält, S., Wågberg, L.
Nord. Pulp Pap. Res. J. 1: 69-74, 2003.
IV
Swelling of model films of cellulose having different charges by combination of
analysis with the quartz crystal microbalance (QCM-D) and atomic force
microscopy (AFM) and comparison to the swelling behavior of corresponding
pulps.
Fält, S., Wågberg, L., Vesterlind, E-L.
Accepted for publication in Langmuir
CONTENT
1. Introduction
1.1
Swelling of fibers
1.1.1. The origin of charges in chemical pulp fibers
1.1.2. The effect of charges on the swelling of the fibers
1.1.3. The effect of pH and electrolytes on the swelling of chemical pulps
1.1.4. The effect of cell wall stiffness on the swelling of the fiber
1.1.5. The effect of swelling on paper strength properties
1.2
Cellulose model films (surfaces)
2. Materials
2.1
Pulps
2.2
Other materials used in the preparation of cellulose model films
2.2.1. Solid substrate
2.2.2. Polymers
2.2.3. Solvents
3. Experimental techniques
3.1
WRV (Water retention value)
3.2
QCM-D (Quartz crystal microbalance with dissipation monitoring)
3.3
AFM (Atomic force microscopy)
3.4
FTIR (Fourier Transform Infrared Spectroscopy)
4. Summary of the results in paper I-IV
4.1
New cellulose model surfaces
4.1.1. Method development
4.1.2. Optimization and characterization
4.2
Swelling of kraft fibers in different electrolytes and its influence on paper strength
4.3
Properties and swelling of cellulose model films and pulps
4.3.1 Swelling of cellulose model films and corresponding dissolving pulps
4.3.2 FTIR-analysis of the amount of carboxylates in the film materials and
corresponding pulp
5. Concluding remarks
6. References
1. Introduction
The interaction between wood or fibers and water is of great importance during all pulp and
papermaking processes. Inbibition of water into the fiber leads to a debonding and separation
of the structural elements in the fiber wall and a softening of its structure. This expansion, i.e.
swelling of the structure has a direct impact on the flexibility of the fibers and on the sheet
forming, i.e. consolidation ability of the fibers during the drying of the paper (Lyne and
Gallay 1954, Forgacs et.al 1957, Hartler and Mohlin 1975, Page 1985, Lindström 1992,
Gurnagul et.al 2001). The ability of the fibers to conform to each other during the
papermaking process is greatly dependent on the swelling ability of the outer surface of the
cell wall (Barzyk 1997a, Laine and Lindström 2002).
It is tedious and difficult to study the fiber and its outer surface on a fundamental level, due to
not only the porous and rough surface but also to the small size of the fibers, that makes the
measurement of single fiber-fiber interactions time-consuming and subject to high variability.
A cellulose film that acts as a model for the outer parts of the fiber would open up new
possibilities for this type of fundamental study (Figure 1).
In the present work, a method has been developed to prepare a cellulose film that may act as a
model for the fiber. If it is to be reasonable to use this film as a representative model for the
fiber surface it should also behave like a fiber regarding its swelling behavior at different pH
and in different concentrations of different electrolytes. It was therefore decided to compare
the swelling, measured as water retention value (WRV), of both kraft fibers and
carboxymethylated dissolving pulps with the swelling of differently charged cellulose model
films in the presence of electrolytes and at different pH levels. The swelling of the model
films was determined using a quartz crystal microbalance technique (QCM-D) and atomic
force microscopy (AFM), in order to obtain not only quantitative and qualitative data but also
information about the swelling kinetics. In addition, the effect of swelling on the tensile
strength properties of the paper has been investigated.
300 nm
10 µm
Figure 1. A comparison between (left) a fiber (kraft fiber with a low hemicellulose content) surface, as
imaged by FE-SEM (field emission scanning electron microscopy) (Duchesne et. al 2001),
and (right) the new cellulose model film prepared from the NMMO-system by spin-coating,
AFM image.
1
1.1. Swelling of fibers
1.1.1. The origin of charges in chemical pulp fibers
The charges in chemical pulp fibers usually originate from the cell wall constituents, i.e.
lignin or hemicelluloses, or are introduced into the fibers during pulping and/or bleaching
actions. These charged groups may be carboxylic acid groups, sulfonic acid groups, phenolic
or hydroxyl groups (Sjöström 1981). During sulfate pulping carboxylic acid groups exposed
on the fibre are the main charged groups. Most of these groups in both softwood and
hardwood, consist of 4-O-methyl-glucoronic acid bonded to xylan. The carboxylic acid
content of the pulp decreases during the pulping due to the dissolution of polysaccarides such
as xylans. Some carboxylic acids are also introduced into the lignin during the sulfate pulping,
giving the residual kraft lignin a weakly acidic nature. The amount of charged groups in a
chemical sulfate pulp may vary depending on the cooking and bleaching conditions applied,
but it is usually between 20 and 200 µeq/g (Laine 1996). Laine (1994) also reported that two
different types of carboxylate groups may be found in a sulfate pulp by using potentiometric
titration.
1.1.2. The effect of charges on the swelling of the fibers
It is well known that the presence of charges has an effect on the swelling of the fibers in
water (Carlsson et.al 1983, Lindström 1992, Scallan 1977, Young 1985). The hydrophilic
amorphous parts of the cellulose and hemicelluloses and the ionic groups are the main factors
contributing to the extensive interaction between the fibers and the surrounding water.
Additional swelling can take place if dissociable groups attached to the fibers are ionized. The
swelling effect seen is however determined not only by the charge density but also by the
degree of crosslinking of the fibre wall and the chemical environment around the fibers. An
unlimited expansion (swelling) of the fiber wall induced by ionization of charged groups is
prevented by the 3-dimensional fibrillar network of cellulose in the different layers of the
fiber wall. This network preserves the structure of the fiber wall in the same way as chemical
crosslinks maintain the integrity of superabsorbent polymer grains (Katchalsky 1954).
The types of ionic groups present and their pKa-values also have a large effect on how the
swelling changes with pH for different pulps. The presence of phenolic groups, as in
unbleached kraft pulps gives an increase in pH where a swelling maximum is seen. This is
due to the fact that the phenolic groups have a higher pKa-value than the carboxylate groups.
If only carboxylic acids were present, it is expected that a swelling maximum would be
observed at a lower pH-value (Carlsson et.al 1983). The dissociation of the bound ionic
groups also has an effect on the apparent pKa-value. As the degree of dissociation increases,
the apparent pKa-value also increases (Flory 1953, Wågberg et.al 1991).
There are different ways of theoretically describing the swelling of a polyelectrolyte gel
(Flory 1953). The Donnan equilibrium theory provides a platform for describing non-specific
interactions between metal ion and fiber. It is based on the difference in osmotic pressure
between the inner gel phase and the outer solution, and using the Donnan theory it is possible
to describe the swelling of the gel phase (i.e. the fibre wall ) and how this swelling depends on
the charge density of the gel, the degree of ionization of the charges, the activity of
counterions to the charged groups and the pH outside and inside the gel (Grignon and Scallan
1980). Detailed calculations of the ionic distributions over the fiber wall have also recently
been published (Towers and Scallan 1996, Räsänen and Stenius 1997, Lindgren 2000).
2
The Donnan equilibrium was first introduced in order to explain the unequal distribution of
ions on the two sides of a membrane (Donnan and Harris 1911). Later, the theory was applied
to systems without any membrane but where ions were concentrated to one part of an aqueous
system during pulp and paper production. The Donnan theory has been used to explain
different aspects of fibre behaviour and has been used for the calculation of the distribution of
ions in pulp suspensions (Scallan 1989), the prediction of cations in pulps (Towers and
Scallan 1996, Räsänen and Stenius 1997, Lindgren 2000 ) and the prediction of ionic
conductivity in pulp suspensions (Been and Oloman 1995). Several of these studies have been
combined with studies of the swelling of the pulps. In the models mentioned earlier, the
aqueous system consists of two sub-volumes, the fiber phase (f) and the outer solution (s)
(Figure 2). The fiber phase volume is referred to as the Donnan volume and is approximately
equivalent to the water retention value (WRV) (Towers and Scallan 1996). In the case of an
infinite outer solution, the chemical potential is the same inside and outside the gel:
æ (a z + ) s
ç
ç (a z + )
f
è
where
z
ö æ (a z − ) f
÷ =ç
÷ ç (a z − )
s
ø è
ö
÷
÷
ø
z
[1]
az+ and az- are the activities of the cations and anions respectively.
It is common to approximate the activities with the concentrations and to describe the
relationship between the ion concentrations in the fiber phase and outer solution by the
expression:
æ (M z+ )
f
λ = çç
+
ç (M z ) s
è
where
1/ z
ö
÷
÷÷
ø
æ ( A z− )
s
=ç
ç ( A z− )
f
è
1/ z
ö
÷
÷
ø
[2]
λ = the distribution coefficient between the two phases
M =concentration of cation
A = concentration of anion
Figure 2. Schematic picture of a fiber wall in contact with an outer solution, expressed as a Donnan
equilibrium system consisting of the fiber phase (f) and the outer solution (s). When the
bound charged groups are dissociated, they cause an unequal distribution of all mobile
ions. When these dissociated groups are anionic, they induce a higher concentration of
cations within the fiber wall than in the outer solution
(Figure adapted from Towers and Scallan 1996).
3
It has however been found (Lindgren 2000) that highly modified bleached softwood pulp
shows a specific interaction between divalent metal ions and the fiber, and that these types of
pulps will hence not follow the Donnan equilibrium model. Using a complexation model,
including two fiber sites interacting with one metal ion, the acid/base data for these pulps can
be explained.
1.1.3. The effect of pH and electrolytes on the swelling of chemical pulps
Both the pH (Jayme and Büttel 1964) and different electrolytes (Cohen et al. 1949, 1950) are
known to have an effect on the swelling of chemical pulps. These effects have been studied in
detail by several research groups (Lindström 1980, Lindström and Carlsson 1982, Lindström
and Kolman 1982, Scallan and Grignon 1979, Grignon and Scallan 1980). The differences
seen in the results reported by the different authors may be due to differences in the pulps
used in the different investigations.
It has been shown that an increase in pH increases the swelling of unbleached kraft pulp
(Figure 3), due to dissociation of carboxylate and phenolic groups (Lindström and Kolman
1982, Lindström and Carlsson 1982). The same authors found no similar swelling effects for
the bleached pulp.
The negative effect of salt on the swelling of the fibers has been discussed by several authors.
An increase in salt concentration from 0 to 0.05 M was shown to decrease the WRV from 250
to 210 for an unbleached sulfate fiber at pH 8 (Lindström and Carlsson 1978)(Figure 3). It is
suggested that this is due to a decrease in electrostatic repulsion between the charges in the
fiber wall and it has been theoretically discussed by Flory (1953) as a decrease in the
electrostatic free energy. The valency of the cation has also been show to be important for the
deswelling effect seen with different electrolytes (Nelson and Kalkipsakis 1964, Scallan and
Grignon 1979, Lindström and Carlsson 1982a) The higher the valency of the cation, the lower
is the swelling ability of the fiber. It is further shown that the pulps are increasingly swollen in
the following order:
Fe3+ < Al3+ < H+ < Ca 2+ < Mg 2+ < NH4+ < Na+
Some of the investigations reported with different electrolytes have not been performed at a
stated pH, but it can be assumed that these studies have been carried out under neutral
conditions. The very high concentrations of chemicals needed to reach a given pH-level can
also, due to the high ionic strength in the system, cause a decrease in swelling. This effect can
clearly be seen in Figure 3, which shows the swelling of unbleached sulfate pulp in deionised
water at different pH´s and where there is a deswelling of the pulp at pH>10 due to the high
concentration of NaOH needed to reach these pH-levels. In deionised water, there will be a
considerable effect of charge of the fibers on the salt concentration at a given pH. This
problem has also been addressed by Katschalsky (1954) for solutions of polyacrylic acid.
4
Figure 3. The effect of pH and NaCl concentration on the WRV of unbleached sulfate pulp, yield
53,5 %. (Lindström and Carlsson 1978)
1.1.4. The effect of cell wall stiffness on the swelling of the fiber
As was mentioned earlier, the degree of swelling of the fiber is the result of a balance between
swelling forces caused by the charges in the fiber wall and the restraining network forces of
the fibrillar fiber wall. As the yield of the pulping process decreases there will be less lignin in
the fiber wall and hence also more empty space, giving rise to a less rigid fiber wall that may
respond to the swelling forces induced by the charges in the fiber wall. During the pulping
process, the desire is to leave the carbohydrates as unaffected as possible, but there is usually
a decrease of the amount of hemicelluloses with decreasing yield. Depending on the pulping
conditions and degree of delignification, a broad range of differently charged fibers with
different stiffnesses can hence be obtained. The balance between these two entities, i.e. the
swelling forces induced by the charges in the fibre wall and the rigid fibrillar network,
determine the swelling behavior of the fiber wall. It should also be noticed that a certain
rigidity of the fibrillar network is necessary in order to restore the elasticity of the fibre wall.
This elasticity of the fiber wall and its ability to respond to swelling in different environments
as well as to offer a rigid network, is extremely important in many papermaking applications.
The effects discussed above can be seen in Figure 4, where the swelling of spruce fibers
(Carlsson and Lindström 1983) and of softwood kraft fibers (Lindström 1980) is plotted as a
function of the degree of delignification. The results show that wood fibers are too stiff to
give a swelling response in different electrolyte concentrations, whereas delignified wood or
softwood kraft is more flexible and also more sensitive to different chemical conditions. In the
figure it can also be seen that delignification of the softwood kraft pulp gradually gives a
more swollen and more responsive fiber with an observed maximum at about 50 %
delignification. At higher degrees of delignification (carboxylic acid content less than 100
µeq/g), there are few charges left in the pulp, due to dissolution of hemicelluloses, and the
swelling effect seen hence also decreases (Lindström 1980).
5
Figure 4. The WRV versus the degree of delignification under different chemical conditions. Left:
chlorite-delignified spruce fibers. Right: softwood kraft pulp (Lindström 1992).
1.1.5. The effect of swelling on paper strength properties
It is well established that pH and electrolytes have an effect on paper strength properties
(Jayme and Büttel 1964, Cohen et.al 1949, Edge 1944, Emerton 1957). These early
investigations did not relate the effects of pH and/or electrolytes on strength properties to the
specific swelling of the fibers but they claerly showed the importance of these variables. This
has later been studied in more detail (Nelson and Kalkipsakis 1964a and 1964b, Scallan and
Grignon 1979, Lindström 1980, Lindström and Kolman 1982, Lindström and Carlsson 1978).
Nelson and Kalkipsakis (1964a), as well as, Scallan and Grignon (1979) found a linear
relationship between the swelling of the fibers and the tensile strength properties of the paper
made from pulps in different ionic forms. In this investigation Scallan and Grignon were
using the fiber saturation point as a measure of the swelling ability of the fiber. Similarities
have been found by other workers (Lindström and Kolman 1982, Lindström and Carlsson
1982), where the effects of both pH and electrolytes have been studied for both unbleached
and bleached chemical pulps. However, no direct linear relationship between WRV and
tensile strength properties could be observed from these data (Figure 5). It is nevertheless
clear that the swelling of the fibers induced by a mechanical or chemical action has an impact
on the flexibility of the fibers and their abilities to conform to each other during sheet forming
and drying, which in turn influences the strength properties of the paper. Several authors have
tried to investigate the cell wall elasticity and its relationship to the swelling of the fiber
(Scallan and Tigerström 1992, Lindström and Westman 1980) but no simple relationship has
been found between these parameters.
6
It has moreover recently been shown that the location of the charges in the fiber wall has a
great influence on the effect of the charges (Barzyk et.al 1997a, Barzyk et.al 1997b, Laine and
Lindström 2001, Laine et al. 2002). A higher concentration of charges at the surface of the
fibers resulted in a greater effect on paper strength properties.
Few fundamental studies have however been published concerning the polyelectrolyte
behavior of cellulosic fibers and the effect of different electrolytes on the swelling and its
relation to strength parameters.
Figure 5.
Tensile index versus WRV for unbleached kraft pulps at different yields. The pulps were
beaten in their H-form in deionised water and sheets were formed at different pH-values
in deionised water (Lindström and Kolman 1982).
1.2.Cellulose model films (surfaces)
One of the first attempts to use cellulose model films for fundamental surface force
measurements was made by Neuman with spin-coated cellulose and xylan films on mica
(Neuman et.al 1993). The measurements were not successful due to the large swelling seen
with these films. The films were also shown to be weakly charged and due to the high degree
of swelling long steric forces were present. The films also showing instability and the work
was not continued at that time.
The Langmuir-Blodgett technique has recently been used by several groups to prepare
cellulose model films from TMSC (trimethylsilyl cellulose) (Holmberg et.al 1997, Poptoshev
et.al 2000). The technique was first developed by Schaub for applications within electronics
(Schaub et.al 1993) and later developed for regenerated cellulose films by Buchholz et.al
(1996). The films obtained are well oriented, thin and smooth but unfortunately thay are also
fragile and will easily detach from the solid substrate at higher pHs or in surfactant solutions.
TMCS has recently also been used to prepare spin-coated cellulose films (Torn 2000, Geffroy
2002). Such films have been used to study cellulose-surfactant interactions and
polyvinylamine adsorption by reflectometry. In the present work, a new method for the
7
preparation of cellulose model films has been developed. The raw material used for these
films is a dissolving pulp, i.e. a special pure grade of pulp fiber intended for rayon production,
which is here dissolved in N-methyl-N-morpholin-N-oxide (NMMO), and films are prepared
by the spin-coating technique.
The techniques that have been used for preparing different types of cellulose films are listed
in Table 1, which also gives a subjective overview of the most obvious advantages and
disadvantages of these techniques.
Table 1. Advantage and disadvantages of available methods for the preparation of cellulose model
films.
Surface
Advantage
Disadvantage
References
Cast-coated
Simple
Non-defined thickness
Hishikawa et.al 1999
Simple equipment needed
Thick and rough surface
Molecularly smooth
Time-consuming
Schaub et.al 1993
Well controlled thickness
Detach in surfactants
Holmberg et.al 1997
Relatively simple
Adjustment of thickness
difficult for thin surfaces
Neuman et.al 1993
films
Langmuir-Blodgett
films
Spin-coated
films
Controllable thickness
Holmberg et.al 1996
Identification of polymers
attaching the cellulose film not
trivial
Torn et.al 2000
Cellulose I
Smoothness undefined
Grey 2000
Relatively simple
No controlled thickness
Simple equipment needed
Relatively well-defined
Cast-coated Cellulose I
films
Raw material?
2. Materials
2.1. Pulps
In this work (papers I-IV), different pulps have been used and these pulps can be divided into
two groups depending on the type of study for which they been used. Dissolving pulp and
carboxymethylated dissolving pulp have been used for all studies regarding model film
preparation and model film swelling (paper I, II and IV). Unbleached softwood sulfate pulps
were however used for the swelling study of kraftliner pulps (paper III). All the pulps and
their corresponding charges are listed in Table 2, which also includes some
carboxymethylated dissolving pulps used for FTIR-analysis. The results of these latter
measurements have not been published earlier and a summary of these results are given in the
last chapter (4.3.2.) in this thesis.
The pulp used in the kraftliner swelling experiment (paper III) was an unbleached sulfate pulp
from SCA Munksund, Sweden with a kappa of 95, yield of about 50 % and a SchopperRiegler value of 18-18.5. All experiments beside the NaCl swelling experiments were
accomplished with a first batch of fibers, which had a slightly lower swelling capacity than a
8
second batch of fibers. The total charge of the pulps was measured by conductometric
titration, and the two pulp batches had total charges of 139 and 218 µeq/g respectively.
Dissolving pulp was used to prepare of cellulose model films (papers I and II). The viscosity
of the dissolving pulp was 546 g/ml. The pulp was soxhlet-extracted with acetone in order to
remove the extractives in the pulp, and the final amount of extractives in the pulp was 0.04 %.
The amount of carbohydrates apart from cellulose in the dissolving pulp was about 5.8 %,
consisting mainly of glucomannans.
In order to obtain differently charged pulps, the dissolving pulp was carboxymethylated to
different degrees using a method developed by Walecka (1956). The pulps were solventexchanged from water to isopropanol prior to the carboxymethylation in order to obtain an
open fiber structure, and thus facilitate the achievement of higher degrees of
carboxymethylation. A second batch of never-dried modified dissolving pulps was also used
for WRV measurements (paper IV). These pulps had slightly different charges, but they were
in the same range as the pulps used for the preparation of cellulose model films.
Table 2. Pulps used in the present work for the preparation of model films and WRV measurements.
Pulp
Charge
Paper
µeq/g
Unbleached sulfate pulp, Κappa 95
139
III
Unbleached sulfate pulp, Κappa 95
218
III
Dissolving pulp, film and WRV
20
I, II, IV
Carboxymethylated dissolving pulp
79
IV
Carboxymethylated dissolving pulp
286
FTIR*
Carboxymethylated dissolving pulp
409
II, IV
Carboxymethylated dissolving pulp
522
FTIR*
Carboxymethylated dissolving pulp, WRV
107
IV
Carboxymethylated dissolving pulp, WRV
329
IV
* used for FTIR-analysis, unpublished results included in this thesis.
2.2. Materials other than cellulose, used in the preparation of cellulose model films
Figure 6. Schematic picture of a cellulose model film made by the spin-coating technique. The
cellulose is dissolved in NMMO and attached to the solid substrate by an anchoring
polymer.
9
2.2.1. Solid substrate
Silicon wafers with a top silicon oxide layer were used as the base substrate for the cellulose
film. The silicon oxide top layer, which provides a negatively charged surface above pH ∼5, is
obtained by oxidizing the silicon wafer at ambient atmospheric pressure in an oven at 1000 °C
for 30 minutes.
2.2.2. Polymers
An anchoring polymer was used to attach the cellulose to the silicon wafer.
The polymers used were:
• Chitosan, Seacure 343, Pronova Biopolymer Inc., USA.
• Polyvinylamine (PVAm), Catiofast PR 8106, 100% hydrolyzed polyvinyl formamide,
BASF, Germany with an active content of 11.9 %.
• Glyoxalated-Polyacrylamide (G-PAM), Parez 631 NC, Cytec, Germany with an active
content of 6 %.
These polymers were used since they are cationic, cellulose-reactive and known to give initial
wet strength to paper (Laleg and Pikulik 1992, Roberts 1996, Crisp 1997, Linhard and Auhorn
1992) (Figure 7).
H CH2OH
H
H
O
Cl-
H
NH3+
H
HO
O
O
O
HO
H
NH3+
Cl-
H
H
H
O
n
H CH OH
2
Chitosan
CH
CH2
CH
CH2
C
NH2
O
H2C
15
80
C
CH
CH
CH2
5
O
H2C
CH2
-Cl +N
NH
H3C
HC
OH
HC
O
CH3
O-
Me
N
+
G-PAM
O
NMMO
n/2
NH3+ Cl-
+
NH3 Cl-
PVAm
Figure 7. Chemical structure for the cellulose solvent, NMMO, and the polymers used for attaching
cellulose to the silicon oxide; Chitosan, G-PAM (glyoxalated polyacrylamide) and PVAm
(polyvinylamine)
10
2.2.3. Solvents
NMMO (N-methylmorpholine-N-oxide), 50 wt.%-solution in water, supplied by Aldrich,
Sweden, was used to dissolve the pulp (Figure 7).
DMSO (dimethyl sulfoxide), dried, supplied by Kebo Lab, Sweden, was used to dilute the
solution and adjusting the viscosity of the NMMO-cellulose solution.
3. Experimental techniques
3.1. WRV (Water retention value)
The swelling properties of the fibers were measured as a water-retention value, WRV
according to a standard method, SCAN C 62:00. The degree of swelling, WRV, is a measure
of the amount of water bound to the fiber after a standardized centrifugation of the sample
(Lindström 1986). When small changes in the swelling capacity are measured by this method,
care should be taken to ensure that the salt concentration and the pH of the rinse water are
always kept the same as in the sample.
3.2. Quartz crystal microbalance with dissipation monitoring (QCM-D )
The development of the QCM technique began with the work of Sauerbrey in the late 1950’s
(Sauerbrey 1959). He found that AT-cut quartz could be used as a sensitive tool for small
mass adsorption measurements and that the frequency shift of the crystal was proportional to
the added mass. The technique was further developed for application in a gaseous phase or
vacuum (Warner and Stockbridge 1963, King 1964, Alder and McCallum 1983, Krozer and
Kasemo 1990) with high sensitivity. The breakthrough for the QCM technique came in 1980
when Nomura showed that the crystal could also be used for adsorption measurements when
immersed in a liquid (Nomura and Hattori 1980). Rodahl et.al (1995) further developed this
technique to include the simultaneous measurement of both frequency and damping factor, i.e.
the QCM-D technique. This enabled both the adsorption mass and properties of the adsorbed
material such as viscoelasticity or conformational changes of the adsorbed material to be
measured.
Computer
Temperature
controlled
environment
Electronics
Quartz crystal
Figure 8. Schematic illustration of the QCM-D measurement system.
11
The piezoelectric quartz crystal microbalance consists of a thin disk of single crystal quartz,
with metal electrodes deposited on each side of the disk. The crystal can be made to oscillate
at its resonance frequency, f, when connected to an external circuit. A temperature-insensitive
AT-cut crystal is often used, which oscillates perpendicular to the electric field, in a shear
mode. When mass is added or removed from the crystal a frequency shift, ∆f, is induced,
which is related to a certain change in mass, ∆m. The QCM is very sensitive to small changes
in frequency and hence also to very small mass changes (< 1 ng/cm2). Sauerbrey (1959) found
that the uptake of small masses was linearly related to the shift in resonant frequency of the
crystal according to the expression:
2
2f
∆f = 0 ∆m
ρ q vq
where
[3]
C= mass sensitivity constant
f0= resonance frequency
ρq=density
υq=shear velocity of sound in quartz
m= adsorbed mass
The application of the equation assumes that there is a uniform layer adsorbed onto the
surface of the crystal and that the adsorbed mass is much smaller than the weight of the quartz
crystal, i.e. ∆f/f«1. The mass should further be rigidly attached and not move in relation to the
crystal while the crystal oscillates.
The dissipation factor, D, i.e. the factor used to determine the viscous properties of the
adsorbed layer, is the sum of all the mechanisms that dissipate energy from the oscillating
system, such as friction and viscous losses. The dissipation describes how the oscillator is
damped. This is achieved by fitting the decay envelope of the amplitude of the crystal when
the crystal is not being driven. When the crystal is covered with soft and highly viscous
material, the oscillator is damped in a rapid manner. The dissipation, D, obtained is related to
the quality factor of the oscillator by:
D=
1 Edissipated
=
Q 2πEstored
where
[4]
Edissipated.= the energy dissipated during one period of oscillation
Estored = the energy stored in the oscillating system
Q = quality factor of the oscillator
From the relationship in [eq 4], the viscous changes in the system can be followed.
In order to use the QCM-D technique to measure the swelling of cellulose, cellulose model
films were attached to a quartz (SiO2) crystal before mounting it into the chamber cell. The
swelling changes in films with different charges and the change in stiffness properties of the
film could thus be detected (Paper II), as well as the influence of different electrolytes and
different pH’s on the swelling behavior of the swollen cellulose film (Paper IV). The
sensitivity of the QCM-D technique also made it possible to follow the kinetics of the
12
swelling/deswelling of these cellulose films. The QCM-measurements have been evaluated
with data recorded on the third overtone (15 MHz).
The considerable swelling of the cellulose model films made it difficult to use the Sauerbrey
equation as a tool for calculating the mass, but it was however used in this work in order to
obtain an approximation of the amount of water taken up by the cellulose film.
3.3. AFM (Atomic force microscopy)
The AFM technique was developed by Binnig et.al (1986) as an imaging tool. There are
several advantages of this technique over other microscopic high-resolution methods; there is
no need for surface coating and the measurements can be performed under ambient
conditions, or even when the sample is immersed in water. Parallel to surface topography
measurements, the AFM technique also provide information about the material properties of
the sample. Today, a broad range of publications describe the use of AFM for studies of
cellulose and other type of surfaces. Information about e.g. forces of interaction (Razatos et
al. 2000, Furuta and Gray 1998), friction (Saundararajan and Bharat 2001, Bagdanovic et al.
2000, Israelachvili 2001), adhesion (Mahlberg et al. 1999), and crystalline structure (Kuutti et
al. 1995) of the studied material can be obtained. These examples are only a minor part of all
the papers found on this topic.
Figure 9. Schematic picture of an AFM with an optical deflection system to measure the cantilever
displacement (adapted from Hanley and Grey 1995).
The two imaging modes that are used are the tapping mode and the contact mode. Both modes
have been used in the present work.
Tapping mode imaging is a topographical imaging technique for soft samples where the
tapping mode eliminates lateral and shear forces. The measurement tip taps the surface while
resonating at a certain frequency. Besides giving information about the topography of the
surface, the phase shift between the driving oscillator and the detector signal also gives
information about the rigidity (viscoelasticity) of the surface material.
Contact mode imaging is also a topographical imaging technique. In contrast to the tapping
mode, the tip is scanned in contact over the studied surface. This is a technique that is
13
commonly used for hard surfaces with an atomic resolution. The non-contact mode is a type
of contact mode imaging where the tip is scanned closely over the surface, but at a distance
where it still interacts with the surface. This permits the imaging of very soft systems.
Both tapping and contact mode imaging have been used for thickness measurements of
cellulose model films under wet (contact mode) and dry (tapping mode) conditions. In order
to obtain this information, a thin sharp incision was made in the cellulose film and the height
difference between the top surface and the solid silicon dioxide wafer is measured. The
thickness under wet conditions has also been measured for different salt concentrations, and
this gives a quantitative measure of the swelling behavior of the cellulose model films.
3.4. ATR-FTIR (Attenuated total reflection, Fourier Transform Infrared Spectroscopy)
FTIR analysis was used to identify carboxylate groups in the cellulose film material and to
compare to the amounts of carboxylic acid groups in the film material as measured by
conductometric titration (Katz et.al 1984). This comparison was made in order to ensure that
all the carboxylate groups in the film raw material were preserved during the preparation of
the cellulose film, i.e. that there was no breakdown of the carboxylate groups during the
dissolution of the pulp.
Infrared (IR) spectra were recorded with a FTIR-spectrophotometer, Nicolet Magna 750,
fitted with an ATR diamond crystal, single reflectance. The cellulose film material was
prepared by dissolution, addition of DMSO, precipitation in a water bath and subsequent
washing with water. The material was further solvent-exchanged from water to pentane over
ethanol in order to obtain a material free from water. The pulp was measured as such. All
analyzed samples were dried in air overnight before analysis, and all the samples were
analyzed at least twice on two different days in order to exclude possible experimental
artifacts. Absorption at ≈1590 cm-1 was chosen for calibration (C=O stretching) and a peak at
1315 cm-1 was used as an internal reference (Tatsumi et.al 1995).
4. SUMMARY OF THE RESULTS IN PAPERS I-IV
4.1. New Cellulose model films
The need for cellulose model films to serve as models for the outer surface of chemical or
cellulose fibers has been the driving force for the development of a new type of cellulose
model film. Beside providing a relevant model for the fibers, the aim has also been to find a
fast and simple way of producing well characterized model films with different thicknesses,
which are stable in different chemical environments.
In order to prepare a model film that actually acts as a model for the outer surface of the fiber,
wood fibers were chosen as the cellulose raw material. Great attention was paid to the
characterization work, not only on the film dimensions by using different preparation
approaches and on the film properties in general but also on the cellulose dissolution where it
was considered very important to ensure that there was no severe breakdown of the cellulose
chains during the chosen dissolution process.
14
4.1.1. Method development (Paper I)
The development of a new method for the preparation of model films of cellulose can be
summarized in the following way:
NMMO (N-methylmorpholine-N-oxide) was used to dissolve acetone-extracted dissolving
pulp and DMSO (dimethyl sulfoxide) was added to control the viscosity of the cellulose
solution. A thin layer of the cellulose solution was spun by a spin-coating technique onto a
silicon oxide wafer precovered with a saturated polymer layer, and the cellulose was then
precipitated in deionised water and further washed in a fresh batch of deionised water.
At an early stage, it was found that a polymer was needed to attach the cellulose to the silicon
oxide wafer in order to obtain homogeneous and stable cellulose films. The choice of polymer
was identified as a critical factor in order to obtain films that would be stable in different
types of chemical environments, such as different pH’s and temperatures and in the presence
of different types of chemicals.
The polymer used as an anchoring polymer should thus not only adsorb spontaneously onto
the silicon oxide layer over a wide pH range, but it should also adsorb in a thin layer on the
silicon oxide substrate and show a sufficient interaction with the cellulose to create a waterresistant link between the silicon oxide and the cellulose. In order to meet these requirements,
it was decided to test cationic polymers that were known to create temporary or permanent
wet strength effects and that would give a complete coverage of the SiO2 surface.
A knowledge of the adsorption of the polymer on the SiO2 surface at different pH’s, was
therefore essential in order to quantify how different polymers interacted with the silicon
oxide surfaces. The adsorption of chitosan, G-PAM and PVAm was measured by
reflectometry at pH 4, 6 and 8. The results of these measurements, shown in Figure 10,
indicate that G-PAM gave the highest adsorption at all pH-values. G-PAM was also used in
all further work as the anchoring polymer, since no trace of impurities was found on the
surface after the adsorption and the polymer offered a system where the cellulose film
prepared was seen to be very stable at both high and low pH-values.
Γ
2
(mg/m )
1.8
1.6
G-PAM
PVAm
1.4
Chitosan
1.2
1
0.8
0.6
0.4
0.2
0
3
4
5
6
7
8
9
pH
Figure 10. Saturation adsorption, Γ, for the polymers used plotted as a function of solution pH.
Polymer concentration was 30 mg/l (Paper I).
15
The thicknesses of the cellulose film prepared from different systems were measured by
ellipsometry. Apart from affecting the thickness by the thickness of the polymer layer as such,
the results show that the type of polymer chosen also affects the anchoring of the cellulose
during spin-coating. In Figure 11 the layer thickness is plotted as a function of cellulose
concentration in solution for systems where both G-PAM and PVAm are used as anchoring
polymers. In the figure, the polymer layer thickness is subtracted from the total layer
thickness.
Thickness (nm)
60
PVAm
50
G-PAM
40
30
20
10
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Cellulose concentration (%)
Figure 11. Summary of thickness measurements by ellipsometry on cellulose films prepared with
PVAm and G-PAM. The thickness of the film, d, is plotted against the cellulose
concentration in the solution (Paper I).
4.3. Optimization and characterization (Paper II)
From the initial results, it was clear that clean films with thicknesses in the range of 20-270
nm could be prepared with the new film preparation method. There was however still a need
for more studies regarding repeatability and control of both surface roughness and thickness
of the film as well as a deeper understanding of the structure and characterization of the films.
In this optimization work, both the film thickness and surface roughness were analyzed with
AFM. In order to be able to determine the thickness of the cellulose films, a new method was
developed, where the height difference between the top of the cellulose film and the silicon
dioxide wafer was measured by step height image analysis at an incision in the film (Figure
12). The fresh and cut cellulose films are shown in Figure 13a and Figure 13b. This method
was further developed to include thickness measurements of water swollen cellulose films
(Figure 13c).
16
Figure 12. Profiles of a dry and subsequently swollen cellulose film in deionised water at pH 8
(charge 409 µeq/g) measured by AFM, stepheight analysis. This example shows how the
height difference between the top cellulose film and the silicon dioxide could be
measured in both the dry state (upper figure) and the wet state (lower figure). The peak in
the figure represents a ridge of material pushed sideways when the incision is made.
a)
b)
c)
Figure 13. AFM image (tapping mode) of a dry cellulose film (a). In (b) an incision has been made in
the model film in order to measure the height difference between the top surface and
silicon oxide wafer. As can be seen, a barrier of cellulose material is built up close to the
incision. (c) After preparation of a swollen cellulose film. An incision is made in the dry
model film before exposure to deionised water (pH 8). It can be seen from the image that
the cellulose film swells considerably in water (Paper II).
17
An optimization study of the preparation method showed that the thickness of the film is
directly dependent on the cellulose concentration in the solution, i.e. on the dilution factor.
Figure 14 shows this relationship, between the cellulose concentration in the initial solution
and the thickness of the cellulose films measured with AFM.
D (nm)
180
160
140
120
100
80
60
40
50°C
20
100°C
0
0.4
0.5
0.6
0.7
0.8
0.9
Cellulose concentration (%)
1
1.1
Figure 14. Thickness of the cellulose film as a function of cellulose content in the initial solution, for
different cellulose solution temperatures at the start of the spin-coating (Paper II).
Even though the thickness of the films seemed to be independent of the solution temperature,
an effect on the surface roughness could be seen when the temperature of the cellulose
solution is changed (Figure 15). The reason for this temperature-dependence is probably the
temperature gradient in the film during spinning.
RMS (nm)
16
14
12
10
8
6
4
50°C
2
100°C
0
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Cellulose concentration (%)
Figure 15. Surface roughness (RMS) of the cellulose films as a function of cellulose content in the
initial solution, with different solution temperatures at the start of the spin-coating (Paper II).
18
In order to obtain a better understanding of the structure of the cellulose films, these were
further characterized in several different ways.
The evaluation of the molecular weight distribution of the cellulose film material and the
corresponding dissolving pulp revealed that there was some breakdown of the cellulose
chains, mainly by cleavage of the largest molecular weight fractions. This can be seen in
Table 3, where it is shown that the remaining material still had an average molecular weight
of about 160 000 and was more homogeneous than the initial raw material (i.e. a lower
polydispersity index) (Table 3). The absence of low molecular mass cellulose material is also
very important for the applicability of the model cellulose films.
Table 3. Molecular weight average and polydispersity of the dissolving pulp used and the cellulose film
material
Sample
Molecular weight
Std dev
Polydispersity
Std dev.
%
%
Dissolving pulp
410000
10
8.9
12
Film material
160000
5
2.3
1
The crystallinity of the cellulose film material was studied on thin Lyocell fibers made from
the dissolving-NMMO system, using CP-MAS 13C-NMR. A direct utilization of the spincoated films in the NMR investigation was not possible due to the large amount of cellulose
material needed for this type of measurement. The Lyocell fibres were used since these are
oriented during preparation and since the model films are oriented during the spin-coating
procedure, which involves high shear. These results showed that the cellulose material
consists mainly of Cellulose II and that about 50% of the sample consisted of crystalline
material (Figure 16). The size of the crystalline part of the sample was obtained by
comparison with a fully amorphous sample.
120
110
100
90
80
70
60
50
40
δ (ppm)
13
Figure 16. CP-MAS C-NMR spectrum of fibers (1.3 dtex) made from dissolving pulp dissolved in
NMMO (Paper II).
The cleanliness of the cellulose model films was evaluated using dynamic contact angle
measurements. These measurements showed that the contact angle between the cellulose
surface and water was too low to be detected with the DAT equipment. The detection limit for
19
the apparatus is around 20°, i.e. the cellulose model films had contact angles against water
lower than 20°. These very low contact angles showed that these films were clean from
solvents and they were also in agreement with earlier measurements on cellulose (Wågberg et
al. 2001, Holmberg et al. 1997, Huang et al. 1995).
4.2. Swelling of kraft fibers in different electrolytes and its influence on paper strength
(Paper III)
Although the swelling behavior of different pulps has been studied earlier, the effect of
electrolytes on the swelling and strength has not yet been fully explored. In the present work,
the effect of common electrolytes such as NaCl, Na2SO4, CaCl2 on both the swelling and the
tensile strength properties of a kraftliner pulp was investigated for a broad range of electrolyte
concentrations at pH 5. This pH was chosen because kraftliner production is still performed
under acidic conditions due to the low cost of sizing chemicals. Moreover, there is today an
increasing use of recycled pulp within these paper grades, and this leads to a higher
concentration of dissolved calcium carbonate at acidic pH’s, and the effect of the electrolytes
present in the system will be more pronounced.
The results show that there was an initial increase in the water retention value (WRV) of the
fibers when the salt concentration was increased, but that at higher salt concentrations, the
WRV decreased again. This behavior was most pronounced for Na2SO4, but the increase was
smaller for NaCl and for CaCl2 (Figure 17).
The relationship between swelling, pH and electrolytes could be explained by assuming that
the fibers act like a polyelectrolyte gel in accordance with the Donnan theory. According to
this theory, the maximum in swelling was due to an increase in pH of the interior of the fiber
wall inducing a subsequent increase in dissociation of the carboxylate groups due to small
amounts of electrolytes in the suspension. As the electrolyte concentration was increased
further, the difference in osmotic pressure between the interior of the fiber wall and the
surrounding solution decreased.
It was also found that there was no unique relationship between WRV and strength of the
paper formed from the fibers treated with different electrolyte concentrations (Figure 18).
20
WRV, norm
(%)
12
10
8
6
4
Na2SO4
2
CaCl2
0
NaCl
-2
-4
-6
-6
-5
-4
-3
-2
-1
0
1
log concentration
Figure 17. Normalized WRV of the unbleached kraft pulp as a function of salt concentration at pH =5.
-5
The value at 10 M is representing the value for deionised water (Paper III).
Tensile index norm.
(kNm/kg)
10
9
8
7
6
Na2SO4
5
CaCl2
4
NaCl
3
2
1
0
-6,0
-4,0
-2,0
0,0
2,0
4,0
6,0
8,0
10,0
12,0
WRV, norm. (%)
Figure 18. Tensile index versus WRV normalized with respect to the WRV in deionised water for
different molar concentrations of CaCl2, NaCl and Na2SO4 in a kraftliner furnish. The zero-3
-2
-1
level refers to the untreated pulp, followed by 10 M, 10 M and 10 M respectively (last
0
point 10 M only for Na2SO4) (Paper III).
21
4.3. Properties and swelling of cellulose model films and dissolving pulps
4.3.1. Swelling of cellulose model films and corresponding dissolving pulps (Paper IV)
Effect of different electrolytes at pH 5
The use of model films of cellulose for fundamental studies of cellulose fiber interactions
requires that the model film and the fiber show similarities regarding the parameters to be
studied. An understanding of the swelling characteristics of the films is important in order to
understand the behavior of these films in different solution environments. The swelling
changes due to the presence of electrolytes and at different pH’s of the film is also important
because small changes in fiber swelling are known to have a significant influence of the
strength of the prepared paper. These small changes are probably due to the changes that
occur at the external surface of the fiber, and it would be preferable if the model film could
simulate events that are occurring at the fiber wall level.
Spin-coated cellulose films were used for QCM-D swelling measurements in order to trace
the changes in swelling of these films in the presence of electrolytes and at different pH’s.
Initial studies of the swelling of these model films in the QCM-D showed that there was no
desorption of material from the film during the measurements and that the films showed high
stability during measurements (Paper II). The swelling behavior of these model films was also
compared to the swelling behavior of corresponding fibers, measured as WRV.
The QCM-D makes it possible to follow the subsequent water uptake by the films (change in
frequency) and viscous changes of the film (change in dissipation). The swelling kinetics
were described by a first order exponential function and the rate of swelling/deswelling was
evaluated by fitting the changes in frequency to the function [4]:
F (t ) = yo + Ae( −t / b )
where
F:
t:
yo :
yo + A :
b:
[4]
frequency
time
F at t = ∞
F at t = 0
decay constant governing the rate of swelling/deswelling
The results of the swelling experiment in NaCl solutions of progressively increasing
concentrations with cellulose films having different charges are shown in Figure 19a, as the
total change in frequency and in Figure 19b, as the total change in dissipation. These
measurements are began from a film swollen in deionised water and by subsequent treatment
with different concentrations of electrolytes. The results shown that the swelling increased
with increasing charge of the film, and this could be expected from theories describing the
swelling of polyelectrolyte gels (Flory 1953, Grignon et.al 1980). The films with different
charges also showed a swelling at low NaCl concentrations and deswelling at high salt
concentrations. The effect was most pronounced for the film which had a very high charge.
The increase in swelling was also observed in the films with lower charges but only a small
decrease was found for these films at high salt concentrations. The reason for the increased
22
swelling with increasing NaCl concentration was the higher pH inside the cellulose film,
which in turn was due to a decrease in the initial Donnan effect with increasing salt
concentration (Fält and Wågberg 2002, Grignon et.al 1980). This increase in pH inside the
film also led to an increase in the degree of dissociation of the carboxyl groups, and this gave
an increase in the swelling forces within the film. For the more highly charged films, these
swelling forces overcame the restraining forces within the film and promoted leading to an
increased water uptake. As the salt concentration increased further, the net effect of the salt
was only to reduce the osmotic pressure induced by the charged groups within the film. For
crosslinked structures, such as fibers and/or superabsorbent polymers, this decrease in the
swelling force would lead to a decrease in the water uptake but, according to Figure 19a, only
an indication of this effect was found for the different films. This indicates that the films were
held together by non-covalent linkages that were broken when the films expanded upon water
uptake. When the swelling forces were removed, those bonds did not reform in water and the
swelling thus remained almost constant at higher salt concentrations.
The decay constant in Figure 19a shows the rate of change from one salt solution to a more
concentrated one. There was a maximum in all the curves between 10-2 M and 10-1 M NaCl,
showing that the rate of swelling reached its lowest value at this concentration. According to
Grignon and Scallan (1980) there was a large decrease in the swelling force when the salt
concentration was increased above 10-3 M for NaCl and the change in the decay constants for
all three films agreed well with this. This therefore indicated that, when the swelling pressure
decreased, at salt concentrations higher than 10-3 M, there were only small changes in the
amount of water absorbed and the rate of these changes was also lower.
40000
35000
-50
30000
-100
25000
20000
-150
15000
-200
10000
5000
-250
0
-300
20 ueq/g swelling
79 ueq/g swelling
409 ueq/g swelling
20 ueq/g Decay constant
79 ueq/g Decay constant
409 ueq/g Decay constant
-5000
-5
-4
-3
-2
-1
0
Diff. in Dissipation, cum. (1e-6)
0
Decay constant
Diff. in Frequency cum. (Hz)
In Figure 19b, the subsequent changes in viscous properties are plotted as the total change in
dissipation factor, D. In the figure, it can be seen that the swelling changes seen in Figure 19a
were followed by a softening of the material, and that the deswelling was followed by a
stiffening of the film material.
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
1
-5
-4
-3
-2
-1
0
1
log conc.
log conc.
Figure 19a and 19b. (a) Cumulative change in frequency and corresponding rate of
swelling/deswelling measured with the QCM-D for charged model films of cellulose treated with
increasing concentrations of NaCl at pH 5. The higher the decay constant, the slower is the
swelling/deswelling. (b) Subsequently measured cumulative change in dissipation (Paper IV).
Figure 20a and 20b shows the results of a comparison of the swelling responses detected for
a medium charged cellulose film treated with different electrolytes at different concentrations.
These results show that there was a similar swelling response to NaCl and CaCl2 but that
23
Na2SO4 showed a slightly different swelling behavior (Figure 20a). Initially, i.e. at low salt
concentrations, Na2SO4 seemed to show a higher swelling than the other electrolytes but at the
same time much lower dissipation values, which thus indicated that this film had a more rigid
structure than the other films.
Diff. in Frequency, cum. (Hz)
-10
-20
-30
-40
NaCl
-50
CaCl2
Na2SO4
-60
Diff. in Dissipation cum (1e-6)
2,5
0
2
1,5
1
0,5
0
-0,5
-5
-4
-3
-2
-1
0
1
-5
log conc.
-4
-3
-2
-1
0
1
log conc.
Figure 20a and 20b. (a) Cumulative change in frequency and corresponding rate of
swelling/deswelling measured with the QCM-D for a medium charged (79 ueq/g) model film of
cellulose treated with increasing concentrations of different electrolytes at pH 5.
(b) Subsequently measured cumulative change in dissipation (Paper IV).
Effect of pH
The results of the studies of cellulose model films with increasing pH, starting from the dry
cellulose film in ethanol and followed by subsequent treatment of the film with deionised
water at pH 5, pH 8 and pH 10, are shown in Figures 21a and 21b. In these measurements it
was chosen to start with having the films in ethanol due to equipment limitations regarding
the possibility of performing measurements where the dry film was treated directly with
deionised water. Ethanol can be regarded as a non-swelling medium and it offers a system
that is similar to water with regard to the viscosity and can thus be used in the QCMequipment.
The results (Figure 21a) showed that there was a difference in the initial swelling between
low and highly charged cellulose at pH 5. The cellulose films, 20 and 79 µeq/g, did however
show about the same swelling response when exposed to water at pH 8 and pH 10.
As can be seen in Figure 21a, the most highly charged film exhibited a large swelling as the
pH is increased. This was also expected, since an increase in pH in solution leads to an
increased pH in the cellulose film and a higher degree of dissociation of the carboxyl groups.
It was also evident that the absolute swelling, measured as frequency change, was much
higher when the pH was increased than when the salt concentration was increased.
The decay constants in Figure 21a show that the swelling forces were significantly increased,
as the pH increased and the rate of swelling also increased. This was noticed for both the
modified materials, whereas the opposite trend was found for the unmodified cellulose film.
The dissipation changes for these experiments are summarized in Figure 21b. There was a
large increase in dissipation for the most highly charged film, indicating a change from a
rather stiff film to a more flexible film at pH 10.
24
40000
-500
35000
30000
-1000
25000
20000
-1500
20 ueq/g swelling
79 ueq/g swelling
409 ueq/g swelling
20 ueq/g Decay constant
79 ueq/g Decay constant
409 ueq/g Decay constant
15000
10000
-2000
5000
-2500
Diff. in Dissipation, cum. (1e-6)
45000
Decay constant
Diff. in Frequency cum. (Hz)
0
0
4
5
6
7
8
9
10
100
90
80
70
60
50
40
30
20
10
0
4
11
5
6
7
pH
8
9
10
11
pH
Figure 21a and 21b. (a) Cumulative change in frequency measured with the QCM-equipment for
differently charged model films of cellulose with increasing pH. (b) Subsequently measured cumulative
change in dissipation. (Paper IV). The films were swollen in deionised water from ethanol
(corresponds to the zero-level).
Since the frequency change is not a direct measure of the thickness a more direct way of
measuring the thickness change was needed. For this purpose, AFM was selected.
Measurements were made on a cellulose film (charge 409 µeq/g) with increasing salt
concentration. The consecutive salt solutions were introduced with a swelling time interval of
about 5 hours and the thickness was measured in the different salt concentrations after 5
hours. The results show that there was an initial swelling of the cellulose film with low salt
concentration at pH 5 and a decrease at high salt concentration, as demonstrated in Figure 22.
A comparison between the AFM-results and the QCM-D data (Figure 22) shows that there
was a match between the two methods, which definitely supported the interpretation that both
the QCM and the AFM method measure the swelling of the cellulose.
0
5
QCM
AFM
-20
-40
4
3
-60
-80
2
∆ F (Hz) -100
∆ D (nm)
1
-120
-140
0
-160
-1
-180
-200
-2
-5
-4
-3
-2
-1
0
1
log conc NaCl
Figure 22. Comparison between the QCM-D and AFM-results for a charged cellulose film (409 µeq/g)
with increasing concentration of NaCl at pH 5. The QCM-D data are presented as the cumulative
change in frequency and the AFM-data as the difference in thickness of the cellulose film (Paper IV).
25
In order to test whether the model films could describe the behavior of fibers, similar swelling
experiments were conducted with the fibers used as raw material in the preparation of the
films. All the pulps treated with sodium chloride (NaCl) exhibited a maximum in swelling
(WRV) at a certain electrolyte concentration. The two pulps, with low (20 µeq/g) and medium
(107 µeq/g) chargs showed a maximum in swelling at a concentration of 10-3 M. The highly
charged pulp (329 µeq/g) showed a maximum in swelling at a higher salt concentration, 10-210-1 M, above which a large decrease in swelling could be detected (Figure 23). These results
were in agreement with the results in Paper III and could be explained by the hypothesis that
the fibers act like a polyelectrolyte gel. The swelling of the fibers was due to small amounts of
electrolytes in the suspension which gave rise to an increase in pH in the interior of the fiber,
and hence an increase in the dissociation of the carboxyl groups. At high electrolyte levels,
the pressure difference between the interior of the fibers and the surrounding solution
decreased and this was seen as a deswelling of the fibers.
WRV measurements with the same type of pulp at different pH’s showed an increase in
swelling with increasing pH (Figure 24). This was also in agreement with earlier results
(Lindström and Kolman 1982, Lindström and Carlsson 1978). The low (20 µeq/g) and
medium (107 µeq/g) charged pulps exhibited a lower swelling response than the highly (329
ueq/g) charged pulp (Figure 24).
WRV (%)
100
98
20 ueq/g
96
107 ueq/g
329 ueq/g
94
92
90
88
86
84
82
-6
-5
-4
-3
-2
-1
0
1
NaCl log conc
Figure 23. WRV of differently charged carboxymethylated dissolving pulps as a function of salt
-5
concentration at pH 5. The value at 10 M is representing the value for deionised water
(Paper IV).
26
WRV (%)
98
20 ueq/g
96
107 ueq/g
329 ueq/g
94
92
90
88
86
84
4
5
6
7
pH
8
9
10
11
Figure 24. WRV of differently charged carboxymethylated dissolving pulps as a function of pH
(Paper IV).
Comparison between the swelling of cellulose films and fibers
A comparison between the swelling of model films measured as the change in frequency with
the QCM-D (Figure 19a) and the WRV measurements on carboxymethylated pulps (Figure
23) showed a similarity between the results. Swelling of both types of material occurred at
low NaCl concentrations at pH 5 and a higher initial swelling was observed for the highly
charged material than for the lower charged materials in deionised water. The same similarity
was seen with increasing pH in the case of the charged pulps and films treated with deionised
water (Figure 21a and Figure 24). A higher pH led to a greater swelling of the materials, and
the highly charged pulps and films showed the largest swelling effects. An interesting
difference between the fibers and the films could however be seen. There was a marked
deswelling of the fibers at high NaCl concentration, see Figure 23, but only a minor
deswelling was detected for the cellulose films, as discussed in Figure 19a. In connection with
Figure 19b, this lack of deswelling was ascribed to the breaking of non-covalent bonds within
the film that do not reform when the swelling forces are decreased. For the fibers, the
situation was different. Since the structure of the fiber wall was intact after swelling, the fiber
wall retracted when the swelling forces were removed and this gave rise to the pronounced
decrease in swelling shown in Figure 23. This means that the swelling was similar in the films
and in the fibers and virtually described the same type of swelling process. However, since the
restraining forces in the film and in the fibers are very different, the comparison of the
deswelling must be treated with caution. From these results it can be concluded that these
cellulose model films do indeed act as a model of the fiber for this type of swelling study.
27
4.3.2. FTIR-analysis of the amount of carboxylates in the film materials and
corresponding pulp
The charge of the cellulose film material was studied by ATR-FTIR in order to evaluate,
whether the charge of the pulp was changed during dissolution in NMMO. Figure 25 shows
that the charges of the pulps seemed to remain unchanged during dissolution. Since, the
model films prepared from these cellulose solutions showed the same charge as the
corresponding pulp raw material.
Peak height
0.4
0.35
0.3
0.25
0.2
0.15
Film material
Pulp
0.1
0.05
linear, Film material
R2 = 0.9646
linear, Pulp
R2 = 0.9436
0
0
100
200
300
400
500
600
Charge (ueq/g)
Figure 25. Determination by FTIR analysis of carboxylates in charged pulps and corresponding
cellulose model film material.
5. Concluding remarks and suggestions to future work
New cellulose model films has been successfully prepared by the spin-coating technique from
a NMMO/DMSO solvent system. These films have been found to be stable over a broad pH
range and they have also been found to show the same swelling behavior as the corresponding
fibers. Since they are easy to prepare and have a high stability, it is suggested that these films
should be used as a model of the external fiber surface for specific fundamental studies of
cellulose interactions. Such films have already been used to achieve a fundamental
understanding of deinking, adhesion studies, nanoparticle, polymer and surfactant adsorption
studies as well as for elastic modulus measurements.
Although these model films have been developed as a tool for studying cellulose interactions
within pulp- and papermaking applications, these films could be seen as a model for cellulose
interaction studies in general. There would also be an obvious use of these films within the
textile (perfume adsorption, surfactant interaction, enzyme treatment, dyeing etc) and
pharmaceutical industries (chemical modification effects, drug delivery).
28
Crosslinking of the film structure by heating the charged film or by using a crosslinking agent
would provide a covalent network which would facilitate the study of the deswelling of these
films and would also provide a more rigid structure if needed. A nanopattering of the structure
by a template as well as the preparation of porous structures would also permit studies where
the surface structure is important (method development, printability, effect of sizing
chemicals etc.)
It is recommended that future work should focus on the preparation of cellulose films which
are covalently bonded to the wafer substrate. Such a film would be temperature-insensitive
and could thus be used for fundamental studies in situ in process studies in for example a
digester or a washing machine.
Acknowledgements
I thank all the people who in one way or another have contributed to this work:
•
•
•
•
•
•
•
•
•
•
•
•
•
MidSweden University – for the financial support.
Lars W – without you no work would have been done at all!
I hope I can learn from you how to always remember to give credit to the people who
have actually done the job.
Birgitta Sundberg – who was actually the person who made this thesis work possible
by following one of her principles –“Stand by your word!”.
Folke Ö – for helping me to find a way of combining my work at SCA Packaging with
my half-time leave during 2001.
Inger Nygren and Sara Kumlin at SCA Research – you were there at the beginning and
without a nice start there would be nothing to build on.
Tomas Larsson, STFi – for NMR-measurements.
Eva-Lotta V – It has been fun to have the opportunity of working with you. Your
analytical and critical eye has been valuable in the work in the preparation of the
cellulose films.
Cedric Geffroy, Bert Torn and Joost at Wageningen Agricultural University – these
are the guys who taught me how to prepare spin-coated films. You were really nice!
Gunilla Pettersson – you’re the master of reflectometry!
Johan Lindgren - who taught me how to use WinSGW.
Boel Nilsson - for helping me out with the AFM.
Eva-Maria van Hees – for the FTIR-measurements.
Christer – for your infinite patience. Who else would agree to sleep in the car on a
Sunday night while I’m changing samples…? Our long discussions about the
objectives of basic research and practical applicability have made me see more clearly.
29
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