Upload
others
View
8
Download
0
Embed Size (px)
Citation preview
Accepted Manuscript
Rheology and Microstructure of Aqueous Suspensions of Nanocrystalline Cel-
lulose Rods
Yuan Xu, Aleks D. Atrens, Jason R. Stokes
PII: S0021-9797(17)30172-8
DOI: http://dx.doi.org/10.1016/j.jcis.2017.02.020
Reference: YJCIS 22047
To appear in: Journal of Colloid and Interface Science
Received Date: 5 December 2016
Revised Date: 9 February 2017
Accepted Date: 9 February 2017
Please cite this article as: Y. Xu, A.D. Atrens, J.R. Stokes, Rheology and Microstructure of Aqueous Suspensions
of Nanocrystalline Cellulose Rods, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/
j.jcis.2017.02.020
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rheology and Microstructure of Aqueous Suspensions of
Nanocrystalline Cellulose Rods
Yuan Xua, Aleks D Atrens
c, Jason R Stokes
a,b*
a School of Chemical Engineering, The University of Queensland, Brisbane, Australia
bAustralian Research Council Centre of Excellence in Plant Cell Walls, The University of Queensland,
Brisbane, Australia
cSchool of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Australia
* Corresponding author:
Professor Jason Stokes
School of Chemical Engineering
The University of Queensland, Brisbane QLD 4072, Australia;
Tel: +61 7 336 54361; Fax: +61 7 336 54199;
Email: [email protected]
Abstract
Hypothesis
Nanocrystalline cellulose (NCC) is a negatively charged rod-like colloid obtained from the hydrolysis
of plant material. It is thus expected that NCC suspensions display a rich set of phase behaviour with
salt and pH because of its anisotropic shape and electrical double layer that gives rise to liquid
crystallinity and self-assembly respectively. It should thus be possible to tune the rheological
properties of NCC suspensions for a wide variety of end-use applications.
Experiments
Rheology and structural analysis techniques are used to characterise surface-sulphated NCC
suspensions as a function of pH, salinity (NaCl) and NCC concentration. Structural techniques include
atomic force microscopy, Zeta potential, dynamic light scattering, and scanning electron microscopy.
Findings
A phase diagram is developed based on the structure-rheology measurements showing various states
of NCC that form as a function of salt and NCC concentration, which go well beyond those
previously reported. This extended range of conditions reveals regions where the suspension is a
viscous fluid and viscoelastic soft solid, as well as regions of instability that is suggested to arise
when there is sufficient salt to reduce the electrical double layer (as explained qualitatively using
DLVO theory) but insufficient NCC to form a load bearing network.
Keyword: Nanocrystalline cellulose, rheology, microstructures, ionic strength, colloidal rod.
1. Introduction
Nanocrystalline cellulose (NCC) are nano-scaled (colloidal) rod-like particles of cellulose obtained
from hydrolysis of plant mass. They are attracting increasing scientific and engineering attention since
the influential works of Revol et al. [1] on nematic self-ordering of NCC in aqueous suspension and
Favier et al. [2] on NCC’s significant reinforcement of polymer composites. The advantages of NCC
as an advanced material are their high strength, abundance, biodegradability, non-toxicity, good
thermal stability and relatively low cost [3, 4]. Its large aspect ratio and the high density of surface
hydroxyl groups enable NCC suspensions to have unique rheological properties, whereby isotropic
rod-like particle suspension behaviour occurs at low concentrations, and a chiral nematic liquid
crystalline structure is observed over a certain concentration range that is followed by gelation at
higher concentrations. In addition, colloids rods are more efficient at building low-shear viscosity,
yield stress and gel strength in suspension than spherical particles[5]. Thus NCC has potential to be
used in many application fields for rheological control, including within consumer products, drilling
fluids [6], drug delivery [7], and in artificial tissue formation [8]. In order to tune their ability to
control the rheology in a wide variety of applications, it is necessary to understand its relationship to
the underlying microstructure and how this is influenced by interparticle forces between the colloidal
rods. The specific objective of this study is to map the structure and rheology of NCC suspensions in
aqueous media as a function of pH, salt, and NCC concentration.
Recent research on the rheology of NCC suspensions has focused on its liquid crystalline or self-
ordering behaviour and its gelation properties [9, 10]. Their large aspect ratio and the presence of
surface hydroxyl groups play an important role on the interactive forces between NCC particles and
thus suspension rheology [11]. Surface hydroxyl groups render NCC colloidal rods to be negatively
charged, which electrostatically stabilises the suspension in aqueous media. They assemble into a gel
structure provided there is a sufficient salt concentration to screen surface charges so as to promote
adhesive interactions to occur [12].
The rheology of NCC suspensions in aqueous media and their gelation behaviour is of interest in this
study. NCC suspensions are considered to form liquid crystalline gels at a critical NCC concentration
and a randomly entangled gel at even higher concentration[13]. The critical concentration is affected
by size, aspect ratio and surface modifications of NCC as well as the ionic strength of the solution [14,
15]. However, the mechanism about how, and to what extent, particle interactions affect the
rheological and gelation behaviour of suspensions of NCC particles is yet to be comprehensively
studied as well as the change in linear viscoelasticity of the suspension as a function of phase
behaviour in terms of its colloidal stability and microstructure. In this work, we reveal structure-
property relationships for NCC suspensions by investigating their rheology as a function of ionic
strength, and consider the corresponding effect of NCC surface properties on the structure and
morphology of NCC network via Fourier transform infrared spectroscopy (FTIR), atomic force
microscopy (AFM), scanning electron microscopy (SEM), and Zeta potential measurement.
Monovalent sodium salt has been used to alter ionic strength of solution without causing ion mediated
cross-links between the NCC particles.
2. Materials and Methods
2.1. Materials.
NCC are obtained from the primary cell wall of plants, which is made up of semicrystalline cellulose
with randomly oriented amorphous regions; these regions have a low density and are highly
accessible to ionic species during acid hydrolysis[16, 17]. The amorphous regions of the cellulose
readily decompose under controlled temperature and concentration of inorganic acid, while the
crystalline regions are left intact. The crystalline components, which are only a few nanometres in size,
grow after being released from the cell wall during hydrolytic treatment. The literature NCC colloidal
rods to have a diameter of 4 - 30 nm and length of 50 - 500 nm [9, 18]; their size and aspect ratio are
dependent on the source of primary cellulose material and the hydrolysis conditions [19, 20].
The nanocrystalline cellulose used in this study was sourced from Maine University Process
Development Centre (Orono, ME) as 11.9 wt% NCC aqueous suspension with 0.9 wt% sulphur on
dry NCC sodium form which is equivalent to 4.62 SO3- per 100 anhydroglucose units [12]. The NCC
suspension was made by re-dispersing freeze-dried powder from hydrolysis of cellulose by applying
ultrasonic treatment. Sodium chloride from Merck KGaA (Darmstadt Germany) is used to investigate
the effect of salt. Hydrochloric acid and sodium hydroxide (from Ajax Finechem, NSW Australia)
were used to adjust pH values of suspensions. Diluted samples were produced using deionized water
produced by reverse osmosis water system, which has a resistivity of 18.2 MΩ•cm (Sartorius Stedim).
2.2. Preparation of NCC suspensions.
The 11.9 wt% NCC suspension was diluted to 0.1, 1, 2, 3, 4, 5, 7, and 9 wt% by addition of deionized
water. No ultrasonic treatment was further applied to sample i.e. particle size and aggregation pattern
was unchanged during sample preparation. The resultant suspensions were at pH of 6. For studies on
NCC suspension at different pH, incremental addition of 10 µL of 1 M NaOH or HCl solution were
used to achieve required pH values. The volume change due to the addition of acid or base is less than
0.17 vol%, which was considered negligible. For studies on the influence of ionic strength, solid
NaCl was added to NCC suspensions to levels required. For samples at very dilute NaCl (<0.01M),
salinity was adjusted by pipetting pre-prepared 0. 1M NaCl solution into corresponding NCC
suspensions.
2.3. Rheological Measurement.
An AR1500 rheometer (TA Instrument Inc., New Castle, DE) equipped with concentric cylinder
geometry was used to measure samples at room temperature. The inner and outer radii for the
concentric cylinder were 14 and 15 mm respectively. For fluid-like systems in this geometry, we
checked whether measurements were affected by the potential for ‘slip’ at smooth surfaces by
comparing measurements between smooth and rough surfaces; negligible differences are observed
with a result included in Supplementary Information. In contrast, a vane geometry was used for thick
and gel-like samples to avoid the effects of slip and structure modification during sample loading [21]
in the concentric cylinder; this geometry provides an accurate means in which to determine the
apparent yield stress and low-shear viscosity [21, 22]. The inner (Rv) and outer RC radii of the vane
tool was 11 and 22.5 mm respectively, and height (H) is 25 mm. Since the radius-ratio between cup
and vane is about 2 (RC/RV), the equations for Couette flow do not necessarily apply. The shear stress
(σ) and apparent shear rate ( ) is obtained from torque (M) and vane rotation rate in s-1
(Ω) using the
following equations [21, 22]:
(1)
(2)
Steady-state shear flow curves were measured in the shear range from 0.01 to 200 s-1 using shear rate
sweep tests with 25 s per data point. Unless stated otherwise, all experiments are at 25 °C and pH 6.
Measurement of the storage modulus (G’) and loss modulus (G’’) were conducted at frequency range
100 to 0.2 rad/s at controlled shear stress of 1 Pa (within the linear viscoelastic regime), for 0.01, 0.1,
1, 2, 3, and 5 wt% NCC suspensions with 0.1 M NaCl. The yield point for samples with 0.1 M salt
were measured using a shear stress sweep over the over the range of 0.001 to 200 Pa. The yielding
point was taken as the value of shear stress at the point on viscosity vs. shear stress plot where the
viscosity value started deviating from the tangent line of the linear region [21].
2.4. FT-IR spectra.
PIKE GladiATR FT-IR spectrometer (PerkinElmer, Waltham, Massachusetts) was used for
characterising the surface chemistry of NCC samples. Samples for analysis were from 5wt% water
suspensions form with pH 1.5, 6 and 12.
2.5. ζ Potential Measurement.
ZetaPals zeta potential analyser (Brookhaven Instrument, US) was used to measure the electrophoretic
mobility of NCC particles in suspension at different salt concentration. Zeta (ζ) potentials were
obtained from converting mobility values via Smoluchowski equation. Measurement of ζ potential for
the high-salinity [> 10mM] NCC suspensions was difficult due to rapid agglomeration. Results were
obtained by careful experimental technique as follows. The measurement chute was initially filled
with only saline solution, to which a drop of dilute NCC suspension was added. The first two data
values from instrument were recorded. The same procedure was repeated ten times and twenty data
values in total were obtained; the reported values are the mean of these measurements. Due to rapid
agglomeration at higher salinities, ζ potential was only measurable for concentrations up to 0.07 M
NaCl.
2.6. Dynamic Light Scattering (DLS).
DLS measurements were made using BI-200SM light scattering instrument (Brookhaven Instrument,
US). The detection was made at a scattering angle of 173°. The intensity size distributions f (Rh) were
obtained using CONTIN algorithm to analyse correlation function in the instrument software where
the hydrodynamic radius (Rh) is calculated using Stokes-Einstein relation;
(3)
where kb is Boltzmann constant, T is temperature, η0 is the viscosity of solvent, and D is translational
diffusion constant. Samples with salt water are prepared in the same way as that for ζ potential
measurements.
2.7. Scanning Electron Microscope.
The structure of 1 wt% and 5 wt% NCC suspension with salt concentration 0, 0.01 and 0.1 M was
investigated. The suspensions were freeze-dried at -68 °C. The resultant samples were foam-like
structures, and NCC suspensions at higher concentrations of salt were denser and stronger while NCC
with lower concentrations of salt was softer. Dried samples were ground to fine powder and putter-
coated with platinum and imaged using a Hitachi S-4800 ultra-high definition scanning electron
microscope (5 kV).
2.8. Atomic Force Microscope.
Samples for imaging using an AFM were produced by placing a drop of 0.05 wt% NCC aqueous
suspension onto a freshly cleaved mica surface and allowing subsequent evaporation at room
temperature. For pictures with isolated NCC particles, the method from Lahiji et al. [23] is used. This
involved placing 2 or 3 drops of 1 wt% NCC suspension onto a freshly cleaved mica plate in order to
gain better adsorption, followed by rinsing using excess deionised water before being dried and blown
by N2 gas. Samples were imaged using WI Tec Alpha 300 AFM machine.
3. Results and Discussion
3.1. Rheology of NCC Suspensions in water.
Figure 1 (a) and (b) depict the steady state flow curves of NCC dispersed in deionized water in a
concentration range from 1 to 11.9 wt%. NCC suspensions are near-Newtonian up to 3 wt%; the flow
curves show minimal shear thinning (Figure 1a, b) and a power-law response between stress and
strain rate with exponents of between 0.9 and 1 (Table 1). For samples with 4 to 7 wt%, at low-shear
rates (<10 s-1) the suspensions exhibit slight shear thinning (i.e. not quite a zero-shear viscosity
plateau) but at higher shear rates there is significant shear thinning with a power law exponent
decreasing for each sample from 0.74 to 0.29 respectively. The shape of the curve is reminiscent of
samples undergoing slip (e.g. [21]), but an identical curve is observed when comparing rough and
smooth surfaces to indicate that slip does not to play a role here, as shown in supplementary material.
The slight shear thinning at low shear rates may thus indicate some weak associations between the
rods, and it has been suggested previously to be associated with variations on the disclination lines of
the polydomain texture in the liquid crystal[24]; the origin for this initial shear thinning region is still
currently in debate [13, 25, 26]. At higher concentrations, the samples exhibit shear thinning power
law behaviour at all measured shear rates, with the exponent approaching zero for 11.8% NCC to
indicate that the response is characteristic of a yield stress fluid. This phenomenon is attributed to the
formation of closely packed structure, which inhibits the particle motion due to an excessive
concentration[13, 27]. Some literature suggests that this non-linear power-law behaviour i.e. a plateau
region in a flow curve is typically expected for lyotropic liquid crystal phase formed in rods
suspensions where rods can align themselves in direction of flow at a high shear rate [13, 28]
3.2. Rheology of NCC Samples as a function of pH and Salinity.
The effect of pH and salinity on NCC suspensions was studied for the 5 wt% NCC suspension. The
effect of pH on the rheology of this NCC suspension is shown in Figure 2(a), which shows the
viscosity at shear rates of 0.1, 1 or 10 s-1
. A peak in viscosity is observed at pH 6. The viscosity
decreases steadily as pH values are lowered to below 2 or increased to above 12, beyond which
viscosity at each shear rate increases dramatically.
Salinity has a significant effect on the rheology of NCC suspensions, as shown in Figure 2(b). The
viscosities at the three shear rates shown decrease with addition of NaCl up to a concentration of 0.01
M. The viscosity values pass through a minimum and increases slightly with further increases in NaCl
until a dramatic increase occurs at a concentration of ca. 0.1 M. At this concentration, the viscosity
increases by two orders of magnitude and the suspensions visually transitions from being transparent
to opaque, as shown in Figure 2 (d). An additional experiment (Figure ii (a) in supplementary
material) showed no change in rheology for samples containing 0.1 M NaCl concentration when the
pH was varied between 1 and 13. Figure ii (b) in supplementary material shows that the pH of
solution did not cause additional changes to the ζ potential of NCC particle suspended in 0.01M NaCl
besides the increasing ionic strength due to added acid or base. It suggests that pH and salinity
influence the NCC suspension via mechanisms controlled by the ionic strength of the suspension.
NCC gels form due to high ionic strength and these show significant shear thinning as evident in
Figure 2b. The low viscosity at high shear rate suggests that the contacts between rods are relatively
weak and can be perturbed by external deformation.
Figure 3 presents the yield stress (σy) and linear viscoelastic moduli (G’ and G’’ at 6.28 rad/s) for
suspensions at 0.1 M NaCl as a function of NCC concentration. The yield stress is the minimum shear
stress required to disrupt the gel network structure to enable the system to flow. The cross-over
between G’ and G’’ occurs at a concentration of 0.83 wt% NCC, which is one indicator for the
transition point of the suspension between liquid and solid-like (gel) rheology. G’, G”, and σy each
show a power law dependency on NCC concentration (solid lines in Figure 3) with an exponent of
around 3; this exponent is commonly observed for percolating systems and colloidal gels [5, 29].
The rheological behaviour of NCC suspension in various salt concentrations can be divided into three
regions, and the transitions between them can be generally explained by DLVO theory as follows:
(1) At low ionic strength (below 10 mM of NaCl), external ions compress the surface double layer of
particles, which therefore reduces the effective volume fraction and thus viscosity. At this stage, the
original liquid crystalline gel structure is weaker but generally retained.
(2) When salt concentration is increased to about 10 mM, the inter-particle repulsion is reduced by
sufficiently high ionic strength through the electro-screening effect. This allows for potential contact
to occur between the colloidal rods and thus formation of large agglomerations[15].
(3) At NaCl concentrations at and above 0.1 M, the NCC rods agglomerate into a network gel
structure. Development of the network structure may involve a growth of aggregates into fractals due
to attractive surface forces, and subsequent percolation as the fractals inter-link. The surface forces
are attributed to van der Waals attraction between NCC rods and sufficient particle proximity for
hydrogen bond formation to occur between them. As a consequence, the suspension becomes opaque
due to the transition from an ordered-to-disordered structure, and consequently, the viscosity and gel
strength increase with increasing salt concentration.
3.3. Morphology and Colloidal Properties of NCC.
The following techniques are used to explore the dependence of the rheology on salinity, which
controls the surface properties of the NCC particles and the suspension microstructure: FTIR, to
show the surface functional group of NCC particles; ζ potential measurements to show the
electrostatic stability of suspension; and DLS, to show hydrodynamic particle size distribution and
degree of agglomeration.
3.3.1. Surface Chemistry and Charge on NCC
Figure 4 shows FTIR results for the surface functional groups on NCC under different solution
chemistry. Characteristic spectral peaks are observed at 3400 cm-1
and 2800 cm-1
that are due to
stretching of OH- and CH- bonds respectively[30]. Absorption at around 1640 cm-1 is ascribed to
bound water within NCC. Peaks at around 1371 cm-1
and 897 cm-1
originate from the O-H and C-H
vibration; while 1030 cm-1
and 665cm-1 are from stretching of C-O and C-OH bonds[31]. Peaks at
1200 to 1240 cm-1
signal the presence of sulphate groups on the surface of NCC particles[32]. NCC
suspensions at different pH values show almost identical IR spectra, indicating that no chemical
change occurs at the NCC particle surface due to high or low pH and ionic strength.
ζ potential measurements are shown in Figure 5. Colloidal suspensions are predicted to be
electrostatically stable if the colloidal particles have an absolute ζ potential above 30 mV[5]. This
value of ζ potential is obtained at between 5 and 10 mM NaCl. Figure 5 also includes the
corresponding suspension viscosity, whereby a minimum in viscosity is observed at a concentration of
10 mM NaCl. The decrease in viscosity with salinity is likely to arise from compression of surface
double layer that reduces the specific volume occupied by the NCC rods and thus the effective
volume fraction. As salt levels are increased, electrostatic repulsion is reduced and attractive particle
interactions dominate to promote aggregation. Above 10 mM, ζ potential is approximately constant
with increasing NaCl concentration up to the maximum tested concentration of 70 mM. This
independence of ζ potential with salt level suggests there is no sodium ions adsorbed onto the surface
of NCC particles above 10 mM. However, the apparent viscosity values gradually increase with salt
concentration in this regime. We attribute this to dynamic aspects of the gel formation process; that is,
an interconnected network structure forms slowly at salinities slightly above the critical point, but
more rapidly the greater the salinity. At salt concentrations above 70 mM, we visually observed that
large NCC aggregates form instantaneously.
3.3.2. Morphology and Aggregation of NCC
The rod-like nature of NCC particles was evaluated using AFM, as shown in Figure 6. An ordered
structure is shown in figure 6 (a) for NCC suspension when “dropped” onto a mica plate. Figure 6 (b)
shows an example of an isolated NCC rod, obtained following methods described in experimental
section (Figure iii in suplementary material includes an AFM image of the mica plate containing
numerous isolated NCC rods). From measurements performed on twenty individual NCC particles,
the average (sd = standard deviation) length and diameter is 210 (sd = 10) and 15 (sd = 4) nm
respectively, and the average aspect ratio is 14 (range of 10-20).
Figure 7 shows the distribution of NCC particle sizes at different NaCl concentrations, measured
using DLS. Note, this does not measure the actual particle size, but rather their equivalent
hydrodynamic radius; for isolated (non-aggregating) rod-like particles with large aspect ratio, this is
comparable to its length although not a measure of it [5]. While DLS does not provide an accurate
measure of the size and morphology of the NCC rods in the same manner that is possible with AFM,
we use it to compare the effect of solution variables on the degree of aggregation.
The DLS measurement indicate that for the NCC suspension at 0 mM NaCl, there are two
characteristic peaks present, which correspond to particles with a hydrodynamic radius of mostly ca.
100 nm and a small portion of particles with diameter hydrodynamic of ca. 10 nm. The small peak
may be remnants from the hydrolysis process [33, 34]. Both characteristic peaks are present when
NaCl is increased to 10 mM, but the measured hydrodynamic radius is greater which indicates the
formation of aggregates. Further increase in NaCl causes the smaller peak to disappear and the larger
peak to increase further; the peak hydrodynamic particle radius at both 50 mM and 70 mM NaCl is
observed to be similar at a value of 200 nm. We note that it is around 10 mM that destabilisation and
aggregation is predicted to occur from ζ potential measurements in Figure 6 due to a weakening of
repulsive electrostatic forces, which is thus supported by DLS measurements.
3.4. Microscopic Analysis by Scanning Electron Microscope
Microscopic analysis was performed using SEM to determine how the microstructure of NCC
suspensions alters as a function of NaCl concentration, to support proposed mechanisms for observed
changes in rheology, surface potential and particle size (aggregation). SEM results for 5 wt% NCC
suspension in 0, 0.01 and 0.1 M NaCl, and 1wt% NCC suspension in 0.01 and 0.1 M NaCl are shown
in Figure 8 and Figure 9 respectively. Note, the size bar in the left hand images in Figure 8 and 9 is 3
microns, which is ca. 15 times greater than the length of the isolated NCC rod measured on the AFM.
Characteristic changes in microstructure are observed upon varying NCC and NaCl concentrations.
Interpretation of SEM images of the type observed are well described in the literature for various
suspensions (e.g. [35-37]). As noted in [35], the apparent structures observed in SEM are affected by
the nucleation and growth rate of ice crystals which are ultimately controlled by the freezing rate and
sub-cooling temperature[35]. Figure 8 (a) shows a layered porous lamellar structure for 5 wt% NCC
in 0 mM NaCl; layered structures are typically seen as the result of ice crystal growth during SEM
sample preparation [35]. The structure is indicative of NCC rods aligning in the direction of ice
growth, and there is no indication of fractals (percolation or aggregation) in this state. Figure 10
provides a schematic of the effect of freezing on an anisotropic suspension of rods, based on the
qualitative description proposed for a similar system [38]. It is seen in Figure 10 that the free
colloidal particles can align with the ice crystal growth whilst the fractal and gelled structure resit
such alignment, which therefore results in different structural morphologies in the SEM images.
When the salt concentration is increased to 10 mM NaCl for the 5 wt% NCC suspension, the structure
is completely altered as shown in Figure 8 (b, c). Two structures were observed in SEM images;
figure 8b shows a ‘collapsed’ porous lamellar structure, whereby the original network structure is
disrupted by a small amount of ions. The measured surface in this case was rougher, thicker with
broken pores still seen in the image. As this salt level corresponds to the point of destabilisation, the
images suggest that the aggregation of NCC rods form irregularly shaped particles prevents the
formation of the smooth layered structure along the ice growth direction. We also observe large,
spherical clusters together with collapsed lamella, as shown in figure 8c. The second schematic in
Figure 10 illustrates this process.
When NaCl concentration is increased to 100 mM NaCl for 5 wt% NCC suspension, which also
corresponds to when a very high viscosity and gel strength is observed, the SEM images display a
thick and rough layered structure consisting of large fibrils (Figure 8, d). At a higher magnification,
figure 8e shows a fractured section to comprise of densely aggregated rods. This observation is in
agreement with our proposed gelling mechanism i.e. NCC particles aggregate into larger and longer
fibrils and then these fibrils form entanglements to create a gelled network structure. We observed
that this particular suspension is a strong gel (G’ >> G’’), and thus we suggest that such a structure
resists ice crystal growth along the preferred direction in its crystallography as shown in the third
schematic of Figure 10.
Figure 9 shows the SEM images for suspensions at a lower NCC concentration, 1 wt% NCC. At 10
mM, figures 9a shows the presence of clusters, which is particularly noticeable in the less-magnified
image in figure 8b. These clusters are smaller in size than that at 5 wt% NCC sample, and no network
or lamella structure is observed. This indicates that the sample contained too few particles to form an
interconnected network structure (required for a gel), which is in agreement with the rheological
measurement that showed liquid behaviour. At the higher salt concentration of 100 mM, dual phases
were observed in the suspensions at 1wt% NCC in Figure 9(c, d). Fractured, non-aligned and rough
surface structure are observed in figure 9d, indicating small irregularly shaped aggregates and
potentially interconnections between aggregates. However, the layer is much less thick and dense than
that observed for the 5wt% NCC suspension in Figure 8 (d,e), indicating that the interactions are
weaker and less numerous. Also, large clusters are present as observed in Figure 9c. This is consistent
with our rheology measurement (Figure 3), whereby 1 wt% NCC at 0.1 M NaCl condition
corresponds to the transition point between liquid-like and gel-like rheology.
3.5 Phase diagram
The salt (NaCl)-concentration phase diagram of NCC suspension is summarised in Figure 11, based
on our rheological and structural characterisation. At this stage, the lines are indicative only of where
phase transitions are likely to occur based on our current measurements.
The diagram shows that as the concentration of NCC is increased (lowest NaCl level), the suspension
transitions from being a viscous fluid, where NCC behave as isotropic colloidal rods, to an anisotropic
liquid with an ordered structure (i.e. ‘anisotropic suspension’), where the approximated boundary is
indicated by line E in Figure 11. Further increase in NCC results in the transition (line F) to a
structure that behaves as a viscoelastic soft solid (G’>G’’). These transition boundaries are
anticipated to be quantifiable in future work by considering the specific volume (and thus the effective
phase volume) of the NCC rods in different solution environments.
At a constant NCC concentration (1 and 5wt% NCC in this study), as salt concentration is increased,
the electrical double layer on NCC rods is reduced that leads to aggregation and an unstable regime
(labelled ‘destabilised suspension’). Suspensions in this region show a lower viscosity than without
salt at the same NCC concentration, which is attributed to phase separation that results in the
formation of micron-sized aggregates. However, at larger NaCl concentration, the NCC rods come
into close contact to form an interconnected network structure that has rheological characteristics of a
viscoelastic soft solid (i.e. G’> G’’ over an appreciable frequency range and an apparent yield stress).
Line A is the estimated boundary for gelation (liquid-solid transition) based on our experimental
observations. A goal of future work is to predict this phase boundary using computational models [39]
of the percolation process and/or more detailed rheological characterisation[40]. We consider that
line B corresponds to the point at which clusters of rods begin to form due to attractive interactions,
but interconnections between rods is not extensive enough to form a percolated network structure.
Therefore the region bounded by line A and B has the potential for microphase separation [41, 42]
and is in a transient state between liquid and solid. Macroscopic phase separation is not observed due
to the density of cellulose being close to that of the solvent, water.
The viscoelastic soft solid phases at either high NaCl or high NCC concentrations (above line F) are
characterised by G’> G’’ over an appreciable frequency range, as well as an apparent yield stress and
shear thinning under non-linear shear conditions. Line C and D separate the solid states of NCC
suspension formed at low and high ionic strengths. We believe that there can be structural difference
between these two solids because the particle interaction is repulsive below line D and is attractive
above it according to ζ potential results shown here and DLVO theory [43, 44]. This is a fascinating
area in which to pursue future work to quantify the convoluted structural states arising from a
combination of interactive forces between colloidal rods and crowding.
The phase diagram is based on the experimental results of the NCC we used, but the respective
positions of the proposed boundaries are anticipated to depend on the particular source of NCC used,
including their size, aspect ratio, and surface functional groups. In comparison to the monovalent
sodium salt used in this work, which is relatively well understood in terms of electrostatic screening
effect common in many charged colloidal systems, we should note that the structure-property
relationships for multivalent ions may be quite different due to the potential for ion-mediated cross-
links between NCCs [10].
4. Conclusions
This study builds on previously reported literature on the shear rheology, colloidal stability and
gelation behaviour of NCC suspensions[10, 11, 13, 45, 46], by significantly expanding the range of
examination in terms of concentration of NCC and NaCl, as well as pH. A key finding from the work
is a phase diagram (figure 11), which specifically utilises our rheological and structural
characterisation to provide insight into the state of the suspension. The concentration of NCC and
NaCl in the phase diagram extends to more than double that of previous works [10], and thus we show
many additional phases that arise due to both NCC’s anisotropy and its surface charge. Our phase
diagram demonstrates clear domains where suspensions are a stable viscoelastic solid and viscous
liquid, whilst also showing a region in NCC and NaCl concentration where it was unstable between
these two extremes. The region of instability is triggered by phase separation due to reduced
repulsion between particles, as observed in other similar cellulose and non-spherical colloidal systems
[47-49] although it has not been previously reported experimentally for other colloidal rods systems
that we are aware. In this unstable region, we hypothesise that salt levels are sufficient to decrease the
double layer and thus decrease repulsive forces between NCC rods, but there is insufficient NCC to
form an interconnected network structure.
It is anticipated that the rich set of phenomena observed in this NCC system as a suspension of
colloidal rods will inspire fundamental investigations into their dynamics under a range of conditions.
This includes their dynamics as colloidal glasses and gels, and development of specific rheology-
microstructural relationships in each phase domain and utilisation of DVLO theory with
computational studies to predict the boundaries between them.
The results from this work also provide a basis for controlling the rheology of this NCC in a variety of
current and emerging applications, whilst also establishing a basis for comparison in future work that
examines the effect on the structure-rheology phase diagram by altering the properties of the NCC.
NCC properties of high interest include: (i) surface functionalisation and/or interactions with
surfactants [12, 50]; (ii) examining NCC from a variety of feedstocks and processing methodologies
to vary aspect ratio, crystallinity, hemicellulose content and surface charge, with an example being
recently reported high aspect ratio cellulose nanofibers that are obtained from spinifex grasses[51, 52].
Current and emerging applications for NCC where its rheology-structure is relevant include its use in
fast moving consumer goods such as food, personal care, pharmaceutical and coating products [53-55],
as well as during processing towards the application of NCC as unique fillers in advanced materials
and biomaterials [56], whilst an emerging application is in developing novel nanostructured materials
using so-called freeze-casting process [35, 38]. In addition, our future work seeks to build on the
foundation set in this paper and examine the use of NCC within drilling fluids [6] for accessing
underground energy resources (oil, gas, geothermal), where precise rheology control and thermal
stability is needed under variable environmental conditions such as salt/pH as well as high pressure
and temperatures.
Acknowledgements
Yuan Xu thanks the University of Queensland (UQ) for an International (UQI) Scholarship. This
work was performed in part at the Queensland node of the Australian National Fabrication Facility
(ANFF-Q), a company established under the National Collaborative Research Infrastructure Strategy
to provide nano and micro-fabrication facilities for Australia’s researchers. We thank: Kinnari Shelat
from ANFF-Q for her assistance in making AFM measurements; Xiaotong Li from School of
Chemistry and Molecular Bioscience (UQ) for assistance in measuring infra-red spectra; Gleb
Yakubov from School of Chemical Engineering (UQ) for the helpful discussion about AFM, cellulose
and colloid science; and Jian Xu from Department of Chemistry and Chemical Engineering,
Shandong University, China for the advice on experiment design and data interpretation. The research
is partially supported by the Australian Research Council Centre of Excellence in Plant Cell Walls
(CE110001007).
References
1. Revol, J.F., et al., Helicoidal self-ordering of cellulose microfibrils in aqueous suspension. International Journal of Biological Macromolecules, 1992. 14(3): p. 170-172.
2. Favier, V., H. Chanzy, and J.Y. Cavaille, Polymer Nanocomposites Reinforced by Cellulose Whiskers. Macromolecules, 1995. 28(18): p. 6365-6367.
3. Azizi Samir, M.A., F. Alloin, and A. Dufresne, Review of recent research into cellulosic whiskers, their properties and their application in nanocomposite field. Biomacromolecules, 2005. 6(2): p. 612-26.
4. Eichhorn, S.J., Cellulose nanowhiskers: promising materials for advanced applications. Soft Matter, 2011. 7(2): p. 303-315.
5. Mewis, J. and N.J. Wagner, Colloidal Suspension Rheology. 2012, New York: Cambridge University Press.
6. Li, M.C., et al., Cellulose nanoparticles as modifiers for rheology and fluid loss in bentonite water-based fluids. ACS Appl Mater Interfaces, 2015. 7(8): p. 5006-16.
7. Wallace, D., Collagen gel systems for sustained delivery and tissue engineering. Advanced Drug Delivery Reviews, 2003. 55(12): p. 1631-1649.
8. Klemm, D., et al., Nanocelluloses as Innovative Polymers in Research and Application. 2006. 205: p. 49-96.
9. Habibi, Y., L.A. Lucia, and O.J. Rojas, Cellulose nanocrystals: chemistry, self-assembly, and applications. Chem Rev, 2010. 110(6): p. 3479-500.
10. Chau, M., et al., Ion-Mediated Gelation of Aqueous Suspensions of Cellulose Nanocrystals. Biomacromolecules, 2015. 16(8): p. 2455-62.
11. Shafiei-Sabet, S., W.Y. Hamad, and S.G. Hatzikiriakos, Rheology of nanocrystalline cellulose aqueous suspensions. Langmuir, 2012. 28(49): p. 17124-33.
12. Eyley, S. and W. Thielemans, Surface modification of cellulose nanocrystals. Nanoscale, 2014. 6(14): p. 7764-79.
13. Ureña-Benavides, E.E., et al., Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions. Macromolecules, 2011. 44(22): p. 8990-8998.
14. Hirai, A., et al., Phase separation behavior in aqueous suspensions of bacterial cellulose nanocrystals prepared by sulfuric acid treatment. Langmuir, 2009. 25(1): p. 497-502.
15. Shafiei-Sabet, S., W.Y. Hamad, and S.G. Hatzikiriakos, Ionic strength effects on the microstructure and shear rheology of cellulose nanocrystal suspensions. Cellulose, 2014. 21(5): p. 3347-3359.
16. de Souza Lima, M.M. and R. Borsali, Rodlike Cellulose Microcrystals: Structure, Properties, and Applications. Macromolecular Rapid Communications, 2004. 25(7): p. 771-787.
17. Thielemans, W., C.R. Warbey, and D.A. Walsh, Permselective nanostructured membranes based on cellulose nanowhiskers. Green Chemistry, 2009. 11(4): p. 531.
18. Kontturi, E. and T. Vuorinen, Indirect evidence of supramolecular changes within cellulose microfibrils of chemical pulp fibers upon drying. Cellulose, 2008. 16(1): p. 65-74.
19. Kim, J.-H., et al., Review of nanocellulose for sustainable future materials. International Journal of Precision Engineering and Manufacturing-Green Technology, 2015. 2(2): p. 197-213.
20. Cai, J., et al., Science of Cellulose Materials. 2015, Beijing: Chemical Industry Press. 21. Stokes, J.R. and J.H. Telford, Measuring the yield behaviour of structured fluids. Journal of
Non-Newtonian Fluid Mechanics, 2004. 124(1-3): p. 137-146. 22. Dzuy, N.Q. and D.V. Boger, Direct Yield Stress Measurement with the Vane Method. Journal
of Rheology, 1985. 29(3): p. 335-347. 23. Lahiji, R.R., et al., Atomic force microscopy characterization of cellulose nanocrystals.
Langmuir, 2010. 26(6): p. 4480-8.
24. Walker, L. and N. Wagner, Rheology of region I flow in a lyotropic liquid‐crystal polymer: The effects of defect texture. Journal of Rheology, 1994. 38(5): p. 1525-1547.
25. Burghardt, W.R., Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers. Macromolecular Chemistry and Physics, 1998. 199(4): p. 471-488.
26. Hongladarom, K. and W.R. Burghardt, Molecular orientation, "Region I" shear thinning and the cholesteric phase in aqueous hydroxypropylcellulose under shear. Rheologica Acta, 1998. 37(1): p. 46-53.
27. Liu, D., et al., Structure and rheology of nanocrystalline cellulose. Carbohydrate Polymers, 2011. 84(1): p. 316-322.
28. Shigharu, O. and A. Tadahiro, Rheology and Rheo-Optics of Polymer Liquid Crystals, in Proceeding of the eighth international congress on rheology. 1980, Plenum Press: New York. p. 127-147.
29. van der Aerschot, E. and J. Mewis, Equilibrium properties of reversibly flocculated dispersions. Colloids and Surfaces, 1992. 69(1): p. 15-22.
30. Tang, Y., et al., Preparation and characterization of nanocrystalline cellulose via low-intensity ultrasonic-assisted sulfuric acid hydrolysis. Cellulose, 2013. 21(1): p. 335-346.
31. Li, J., et al., Homogeneous isolation of nanocellulose from sugarcane bagasse by high pressure homogenization. Carbohydr Polym, 2012. 90(4): p. 1609-13.
32. Zeng, X., et al., Characterization of sodium cellulose sulphate/poly-dimethyl-diallyl-ammonium chloride biological capsules for immobilized cultivation of microalgae. Journal of Chemical Technology & Biotechnology, 2013. 88(4): p. 599-605.
33. Lu, A., et al., Investigation on metastable solution of cellulose dissolved in NaOH/urea aqueous system at low temperature. J Phys Chem B, 2011. 115(44): p. 12801-8.
34. Cai, J., et al., Dynamic Self-Assembly Induced Rapid Dissolution of Cellulose at Low Temperatures. Macromolecules, 2008. 41(23): p. 9345-9351.
35. Deville, S., et al., Freezing as a path to build complex composites. Science, 2006. 311(5760): p. 515-8.
36. Deville, S., E. Saiz, and A.P. Tomsia, Freeze casting of hydroxyapatite scaffolds for bone tissue engineering. Biomaterials, 2006. 27(32): p. 5480-9.
37. Deville, S., E. Saiz, and A.P. Tomsia, Ice-templated porous alumina structures. Acta Materialia, 2007. 55(6): p. 1965-1974.
38. Dash, R., Y. Li, and A.J. Ragauskas, Cellulose nanowhisker foams by freeze casting. Carbohydrate Polymers, 2012. 88(2): p. 789-792.
39. Coniglio, A., et al., Percolation, gelation and dynamical behaviour in colloids. Journal of Physics: Condensed Matter, 2004. 16(42): p. S4831-S4839.
40. Winter, H.H. and M. Mours, Rheology of Polymers Near Liquid-Solid Transitions. 1997. 134: p. 165-234.
41. Zaccarelli, E., Colloidal gels: equilibrium and non-equilibrium routes. Journal of Physics: Condensed Matter, 2007. 19(32): p. 323101.
42. Charbonneau, P. and D.R. Reichman, Phase behavior and far-from-equilibrium gelation in charged attractive colloids. Phys Rev E Stat Nonlin Soft Matter Phys, 2007. 75(5 Pt 1): p. 050401.
43. Saunders, J.M., et al., A Small-Angle X-ray Scattering Study of the Structure of Aqueous Laponite Dispersions. The Journal of Physical Chemistry B, 1999. 103(43): p. 9211-9218.
44. Secor, R.B. and C.J. Radke, Spillover of the diffuse double layer on montmorillonite particles. Journal of Colloid and Interface Science, 1985. 103(1): p. 237-244.
45. Lu, A., et al., Unique viscoelastic behaviors of colloidal nanocrystalline cellulose aqueous suspensions. Cellulose, 2014. 21(3): p. 1239-1250.
46. Prathapan, R., et al., Modulating the zeta potential of cellulose nanocrystals using salts and surfactants. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2016. 509: p. 11-18.
47. Appaw, C., et al., Phase separation and heat-induced gelation characteristics of cellulose acetate in a mixed solvent system. Cellulose, 2010. 17(3): p. 533-538.
48. Takahashi, M., M. Shimazaki, and J. Yamamoto, Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions. Journal of Polymer Science Part B: Polymer Physics, 2001. 39(1): p. 91-100.
49. Cummins, H.Z., Liquid, glass, gel: The phases of colloidal Laponite. Journal of Non-Crystalline Solids, 2007. 353(41-43): p. 3891-3905.
50. Hu, Z., et al., Tuning cellulose nanocrystal gelation with polysaccharides and surfactants. Langmuir, 2014. 30(10): p. 2684-92.
51. Raj, P., et al., Gel point as a measure of cellulose nanofibre quality and feedstock development with mechanical energy. Cellulose, 2016. 23(5): p. 3051-3064.
52. Amiralian, N., et al., Easily deconstructed, high aspect ratio cellulose nanofibres from Triodia pungens; an abundant grass of Australia's arid zone. RSC Adv., 2015. 5(41): p. 32124-32132.
53. Gómez H, C., et al., Vegetable nanocellulose in food science: A review. Food Hydrocolloids, 2016. 57: p. 178-186.
54. Qing, W., et al., The modified nanocrystalline cellulose for hydrophobic drug delivery. Applied Surface Science, 2016. 366: p. 404-409.
55. Li, F., et al., Multi-functional coating of cellulose nanocrystals for flexible packaging applications. Cellulose, 2013. 20(5): p. 2491-2504.
56. Yu, H.-y., Z.-y. Qin, and Z. Zhou, Cellulose nanocrystals as green fillers to improve crystallization and hydrophilic property of poly(3-hydroxybutyrate-co-3-hydroxyvalerate). Progress in Natural Science: Materials International, 2011. 21(6): p. 478-484.
Table 1 A list of the Power law ( n) parameters obtained from fitting regions of the flow
curves presented in figure 1.
NCC wt% 1 2 3 4 5 7 9 11.9
K (Pasn) 0.002 0.0052 0.011 0.043 0.13 2.2 6.7 51.5
n 1 0.96 0.91 0.74 0.64 0.29 0.22 0.085
List of Figures
Figure 1. Shear rheology of 1 to 11.9 wt% NCC dispersed in water, showing (a) apparent viscosity as
a function of shear rate and (b) shear stress against shear rate. Solid lines are power law fits to regions
of the flow curves (see Table 1).
Figure 2. Steady-state viscosity at 3 shear rates for 5 wt% NCC suspension as a function of (a) pH, (b)
salt concentration, and (c) as a function of NCC wt% at 0.1 M NaCl. (d) Shows the relative
translucency of suspensions containing 5 wt% NCC at 0, 0.01 and 0.1 M of NaCl respectively.
Figure 3. Apparent yield stress (σy), storage (G’) and loss (G’’) modulus at frequency of 6.28 rad/s as
a function of NCC concentration (c, wt%) at 0.1 M NaCl for the solid-like suspensions (G’ > G’’).
Solid lines are power law fits to the data, according to: G’=8.6 c2.7
; G’’= 1.6 c2 ; and σy=0.5 c
3.1 .
Figure 4. IR spectra for NCC at different pH values. (Vertical axis was not scaled)
Figure 5. Zeta potential of NCC particles (dilute concentrations) and shear viscosity at 1 s-1
for 5 wt%
NCC suspensions as a function of NaCl concentration. Error bars are standards deviations. Dashed
lines are included to guide the eye. The vertical grey line corresponds to the NaCl concentration at
which the particle potential is ca. -30 mV, above which the suspension is no longer electrostatically
stable due to the electrostatic screening effect.
Figure 6.AFM images for NCC on mica substrate, showing: (a) suspension of NCC colloidal rods; (b)
an isolated NCC colloidal rod with a measure of its length.
Figure 7. Measurement of particle size distribution of dilute NCC suspensions for varying NaCl
concentrations. The vertical axis is the distribution function based on intensity of scattered light and is
calculated from CONTIN analysis; and the horizontal axis is hydrodynamic radius for an equivalent
sphere.
Figure 8. SEM images of 5 wt% NCC suspensions in: (a) water, (b,c) 0.01 M NaCl, and (d, e) 0.1 M
NaCl.
Figure 9. A representative sample of SEM images for 1 wt% NCC suspensions in: (a, b) 0.01 M NaCl,
and (c, d) 0.1M NaCl.
Figure 10. Schematic of NCC rods (short lines) before and after freezing (short dashed arrows
showing the direction of ice crystal growth) arising from SEM procedures.
Figure 11. A schematic of the phase behaviour of NCC suspension with different concentration of
NaCl concentration in water based on the microstructure and rheological measurements performed in
this work (circles). Although dashed lines are used to indicate the boundaries between phases, further
experiments are still required to substantiate these as well as the properties of each phase. A to F are
labels of boundary lines. Note, the axes are not scaled.
(a)
Shear Rate (s-1
)
10-2 10-1 100 101 102 103 104
Vis
co
sity (
Pa.s
)
0.001
0.01
0.1
1
10
100
1000
10000water
1 wt% NCC
2 wt% NCC
3 wt% NCC
4 wt% NCC
5 wt% NCC
7 wt% NCC
9 wt% NCC
11.9 wt% NCC
(b)
Shear Rate (s-1
)
10-2 10-1 100 101 102 103 104
She
ar
Str
ess (
Pa)
0.001
0.01
0.1
1
10
100
Figure 1. Shear rheology of 1 to 11.9 wt% NCC dispersed in water, showing (a) apparent viscosity as
a function of shear rate and (b) shear stress against shear rate. Solid lines are power law fits to regions
of the flow curves (see Table 1).
NaCl concentration M
0.001 0.01 0.1 1 10
Vis
co
sity (
Pa.s
)
0.1
1
10
100
1000
wt% of NCC
0 1 2 3 4 5
Vis
co
sity (
Pa.s
)
0.1
1
10
100
1000
a
(c)(d)
pH
0 2 4 6 8 10 12 14
Vis
co
sity (
Pa.s
)
0.1
1
10
100
1000
0.1 s-1
1 s-1
10 s-1
(a) (b)
Figure 2. Steady-state viscosity at 3 shear rates for 5 wt% NCC suspension as a function of (a) pH, (b)
salt concentration, and (c) as a function of NCC wt% at 0.1 M NaCl. (d) shows the relative
translucency of suspensions containing 5 wt% NCC at 0, 0.01 and 0.1 M of NaCl respectively.
wt% of NCC
0 1 2 3 4 5 6
G', G
'' o
r y(
Pa)
0.1
1
10
100
1000
G'
G''
y
Figure 3. Apparent yield stress (σy), storage (G’) and loss (G’’) modulus at frequency of 6.28 rad/s as
a function of NCC concentration (c, wt%) at 0.1 M NaCl for the solid-like suspensions (G’ > G’’).
Solid lines are power law fits to the data, according to: G’=8.6 c2.7
; G’’= 1.6 c2 ; and σy=0.5 c
3.1 .
Figure 4. IR spectra for NCC at different pH values. (Vertical axis was not scaled)
NaCl Concentration (M)
0.00 0.02 0.04 0.06 0.08 0.10
Ze
ta P
ote
ntia
l (m
V)
-50
-40
-30
-20
-10
0
Vis
co
sity
(Pa
.s)
0.01
0.1
1
10
100
Viscosity
Zeta Potential
NaCl at = -30 mV
Figure 5. Zeta potential of NCC particles (dilute concentrations) and shear viscosity at 1 s-1
for 5 wt%
NCC suspensions as a function of NaCl concentration. Error bars are standards deviations. Dashed
lines are included to guide the eye. The vertical grey line corresponds to the NaCl concentration at
which the particle potential is ca. -30 mV, above which the suspension is no longer electrostatically
stable due to the electrostatic screening effect.
Figure 6.AFM images for NCC on mica substrate, showing: (a) suspension of NCC colloidal rods;
(b) an isolated NCC colloidal rod with a measure of its length.
Rh, apparent
(nm)
1 10 100 1000
f (R
h)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 M NaCl
0.01 M NaCl
0.05 M NaCl
0.07 M NaCl
Figure 7. Measurement of particle size distribution of dilute NCC suspensions for varying NaCl
concentrations. The vertical axis is the distribution function based on intensity of scattered light and is
calculated from CONTIN analysis; and the horizontal axis is hydrodynamic radius for an equivalent
sphere.
Figure 8. A representative sample of SEM images for 5 wt% NCC suspensions in: (a) water, (b,c)
0.01 M NaCl, and (d, e) 0.1 M NaCl.
Figure 9. A representative sample of SEM images for 1 wt% NCC suspensions in: (a, b) 0.01 M NaCl,
and (c, d) 0.1M NaCl.
Figure 10. Schematic of NCC rods (short lines) before and after freezing (short dashed arrows
showing the direction of ice crystal growth) arising from SEM procedures.
Figure 11. A schematic of the phase behaviour of NCC suspension with different concentration of
NaCl concentration in water based on the microstructure and rheological measurements performed in
this work (circles). Although dashed lines are used to indicate the boundaries between phases, further
experiments are still required to substantiate these as well as the properties of each phase. A to F are
labels of boundary lines. Note, the axes are not scaled.
Rheology and Microstructure of Aqueous Suspensions of
Nanocrystalline Cellulose Rods
Supplementary material
List of Figures
Figure i – Comparison between rough (emery paper) and smooth parallel plates (diameter = 50 mm) at
1 mm gap for 7 wt% NCC, in comparison to measurements on smooth concentric cylinders in Figure
1. There is negligible difference between the 3 measurements under the conditions tested.
Figure ii – (a) pH effects on viscosity of 5wt% NCC suspension with 0.1 M NaCl and zeta potential of
NCC particle in 0.01M NaCl. (b) pH effects on zeta potential of NCC in 0.01M NaCl plotted as
equivalent NaCl concentration. Dotted line is the zeta potential change due to increasing salinity at
constant pH, and is for reference.
Figure iii – The AFM picture of mica plate with isolated NCC rods. Note that the length scale of these
images is considerably greater than those in figure 6.
Figure i – Comparison between rough (emery paper) and smooth parallel plates (diameter =
50 mm) at 1 mm gap for 7 wt% NCC, in comparison to measurements on smooth concentric cylinders in Figure 1. There is negligible difference between the 3 measurements under the
conditions tested.
0.001
0.01
0.1
1
10
0.001 0.01 0.1 1 10 100 1000 10000
Vis
cosi
ty P
a.s
Shear rate s-1
Data in Figure 1
1.0 mm rough
1.0 mm smooth
Figure ii – (a) pH effects on viscosity of 5wt% NCC suspension with 0.1 M NaCl and zeta potential
of NCC particle in 0.01M NaCl. (b) pH effects on zeta potential of NCC in 0.01M NaCl plotted as
equivalent NaCl concentration. Dotted line is the zeta potential change due to increasing salinity at
constant pH, and is for reference.
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
1
10
100
1000
0 5 10 15
Zeta
Po
ten
tial
mV
Vis
cosi
ty P
a.s
pH
0.1 s-1
1 s-1
10 s-1
Zeta potential
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
0.001 0.01 0.1
Zeta
po
ten
tial
mV
Equivalent NaCl concentration M
0.01M NaCl with varied pH
pH=6.8 with varied salinity
a
b
Figure iii – The AFM picture of mica plate with isolated NCC rods. Note that the length scale of
these images is considerably greater than those in figure 6.
20
15
10
5
0
µm
20151050
µm
-4
-2
0
2
4
nm
5
4
3
2
1
0
µm
543210
µm
-4
-2
0
2
4
nm
a
b
Graphical abstract