International Journal of Materials Science and Applications
2015; 4(2-1): 30-36
Published online December 24, 2014 (http://www.sciencepublishinggroup.com/j/ijmsa)
doi: 10.11648/j.ijmsa.s.2015040201.16
ISSN: 2327-2635 (Print); ISSN: 2327-2643 (Online)
Viscoelastic and rheological properties of carboxymethyl
cellulose /starch/graphite oxide as superabsorbent
hydrogel nano composites (SHNCs)
Auda Jabbar Braihi1, Sihama Issa Salih2, Fadhel Abbas Hashem2
1Babylon University, College of Materials Engineering, Polymer and Petrochemical Industries Department, Babylon, Iraq
2University of Technology, Materials Engineering, Baghdad, Iraq
Email address:
auda_1964@yahoo.com (A. J. Braihi), sihama_salih@yahoo.com (S. I. Salih), hashembsb@yahoo.com (F. A. Hashem)
To cite this article:
Auda Jabbar Braihi, Sihama Issa Salih, Fadhel Abbas Hashem. Viscoelastic and Rheological Properties of Carboxymethyl Cellulose
/Starch/Graphite Oxide as Superabsorbent Hydrogel Nano Composites (SHNCs). International Journal of Materials Science and
Applications. Special Issue: Steel and Direct Reduced Iron (Sponge Iron) Industry. Vol. 4, No. 2-1, 2015, pp. 30-36.
doi: 10.11648/j.ijmsa.s.2015040201.16
Abstract: Uncross-linked carboxymethyl cellulose (CMC) / starch blend used to prepare two hydrogels; the first is crosslinked
CMC / starch with aluminum sulfate octadecahydrate cross-linker and the second is SHNCs manufactured from
incorporation of 0.3wt% nano graphite oxide (GO) in the above cross-linked blend. Viscoelastic and rheological properties of
these two hydrogels were studied and compared with the uncross-linked blend. Results showed that the cross-linking process
increases the blend miscibility and converted it to miscible blend. The SHNCs hydrogel has the higher Tg and the uncrosslinked
blend has the higher Tan ?. GO addition causes increment in some viscoelastic parameters such as G*, G , and G", and
decrement in damping parameters and make the damping behavior more stable at lower frequencies, and causes increment for
all rheological parameters (?* , ?o, ?f , and GN
o). GO addition leads, also, to the absence of the shear thickening behavior and
increases both the entanglement density and the molecular weight average. Both hydrogels exhibited broad molecular weight
distribution (MWD) and highly cross-linking degree and their elastic behavior predominates; makes them rigid specially at
higher frequencies.
Keywords: Viscoelastic Properties, Rheology, Superabsorbent Hydrogel Nanocomposites (SHNCs), Graphite Oxide(GO),
Carboxymethyl Cellulose(CMC)
1. Introduction
Polymers composed of long molecular chains have unique
viscoelastic properties, which combine the characteristics of
elastic solids and Newtonain fluids [1].
Viscoelasticity makes the material s response to stressstrain
behavior time dependent and their deformation
partially reversible. Polymer melt considers as a non-
Newtonian fluid. This behavior can be attributed to its
molecular structure. Polymers consist of long molecules that
entangle with each other, forming several flexible, reversible
"joints". These enable different conformations of the
molecules by a rotation along the backbone and cause the
elastic behavior of polymer melts. The chains can also move
with respect to each other by a crawling kind of movement
called reptation. These rotation and reptation occurring above
the glass transition temperature of the polymer are called
Brownian motions, and they tend to return the molecules
towards the equilibrium, i.e., to the energetically most
preferable state, after being oriented by applying deforming
stress. This will not however, occur immediately after
removing the stress within a certain relaxation time,
dependent on the molecular characteristics of the polymer [2].
In the dynamic mechanical thermal analysis (DMTA), the
mechanical response of a viscoelastic material is studied as a
function of temperature and time, while it is subjected to a
sinusoidal strain[3]. For viscoelastic materials, DMTA and
rheological tests can be used to:
1- Determine the viscoelastic parameters (G*, G , G", tan ?,
?*, ?o) as a function of temperature, frequency or time. G* is
the complex modulus reflects the contribution of both elastic
and viscous components to the material s stiffness (eqn.1)
International Journal of Materials Science and Applications 2015; 4(2-1): 30-36 31
and measures the material s stiffness (its ability to resist
deformation).
G* = G + G" [4] (1)
G is the elastic or storage modulus, G" is the viscous or
loss modulus, which measures the energy dissipated as heat
and tan ? is the damping coefficient or loss tangent, which is
the ratio of loss and storage moduli(G /G"= tan ?) and
measures the internal friction [2]. ? is a phase angle between
stress and strain(a measure of viscous response).
Complex viscosity (?*) measures the material s overall
resistance to flow as a function of shear rate. In the range of
low shear rates, the viscosity of a polymer solution is
independent of the shear rate. This viscosity is called the
zero-shear viscosity,?0. [5].
2-Give information about shear-thinning and shear
thickening behaviors: Shear-thinning occurs in the form of
reduction in viscosity as the shear stress increases [6].This
reduction is due to molecular alignments and
disentanglements of the long polymer chains. Shear
thickening occurs due to the increasing the viscosity with the
deformation rate. Therefore shear thinning indicates rigid
systems while shear thickening reflects flexible and dilute
chains [7]. Also, more shear thinning implies a higher degree
of long chain branching [8].
3- Specify whether the molecular weight distribution
(MWD) is narrow or broad. Higher shear thinning implies
broader MWD, which in turn implies more elasticity, and this
in turn implies longer stress relaxation.
4- Identifies transition regions such as Tg, Tm, and
recognize the secondary transitions that are beyond the
resolution of DSC[9]. Tg is a reversible change of the
polymer between rubbery and glassy states, and can be
measured accurately by DMTA. In fact, DMTA is considered
the most sensitive method for measuring a material’s Tg as a
sudden change in the elastic modulus and an attendant peak
in the tan ? curve[10]. DMTA, also, tells much more about
the material before and after the Tg.
5- Determine the degree of entanglement density (N) using
equation 2:

=


[11] (2)
Where G is the elastic or storage modulus, R is the
universal gas constant and T, is the absolute temperature.
6- Differentiate between cross-linked and an uncrosslinked
polymers: The G " is much higher than the G for the
uncross-linked polymer solutions and is mainly attributed to
the higher viscous property as compared to elastic property
over the entire frequency range. While the G is higher than
the G" for the partially cross-linked polymer solutions. For
hydrogel which are highly cross-linked polymer networks,
both G and G" are very high and are nearly parallel to each
other[12].
7- Measure the rubbery plateau modulus which is much
more sensitive than is Tg for detecting, for example [10].
2. Experimental Part
2.1. Materials
Carboxymethyl Cellulose (CMC) and corn starch were
obtained from HIMMEDIA Laboratories Pvt. Ltd. Company
(India) . Aluminum Sulfate Octadecahydrate cross-linker was
obtained from REACHIM Company (USA). Nano graphite
oxide was obtained from Alibaba company (China).
2.2. Preparation of the Two Hydrogels
To prepare the cross-linked CMC/starch hydrogel, an
appropriate amount of starch dissolved in distilled water
(DW) using water bath 800C for 45 minute and added to
CMC aqueous solution. Then, the new mixture agitated for
an hour at 700C. The result paste was dried overnight at1000C,
and crushed. To prepare the SHNCs hydrogel, a solution of
nano GO/DW was mixed with sonication for 10 minutes and
added to the CMC/starch mixture (0.3 wt%) and allowed to
mix for addition 30 min.
2.3. Tests
2.3.1. Dynamic Mechanical Thermal Analysis (DMTA )Test
DMTA of the powdery sample (mesh 35-100) was
performed using a DMTA-Triton , model (Tritec 2000DMA),
England. The experiments were carried out in the
temperature range of 20- 300°C, heating rate of 5°C/min and
frequency of 1 Hz in bending mode ,with 1% mm
displacement.
2.3.2. Rheology Test
The rheological measurements of the water-swollen gels
(1.00 g sample in 5.0 mL distilled water) were performed
using a Paar-Physica Oscillatory Rheometer (Modulur
Compact Rheometer, MCR300, Germany) with parallel plate
geometry (plate diameter of 25 mm, gap of 3 mm) at 25°C.
3. Results and Discussions
3.1. DMTA Results
Figure 1 shows a temperature sweep test on CMC/ starch
blend without cross-linker. With the temperature increase, a
rapid decrease in complex modulus (G*) occurs and the tan ?
curve goes to its maximum; the polymer is in the state of
glassy transition.
The Brownian motion of an individual chain is largely
inhibited by the other molecules surrounding it. When the
temperature increased, the Brownian motion of the chains
augment and the free volume around the polymer chains
increases, which means easier flow.
Near Tg , the polymer viscoelastic properties change very
quickly both with time as well as with the changing
temperature.
Peaks in tan ? and in G* can be associated with the Tg ,
which corresponds to the ability of chains to move past each
other.Therefore, based on the tan ? pattern, the uncrosslinking
CMC/starch blend shows two (Tg s) at 234.6 and at
32 Auda Jabbar Braihi et al.: Viscoelastic and Rheological Properties of Carboxymethyl Cellulose /Starch/Graphite Oxide as
Superabsorbent Hydrogel Nano Composites (SHNCs)
265.380C respectively, which indicates that this blend
without cross-linker is a partially miscible blend .
Secondary transitions (?) observed as well in the range
between 61.530C to 80.77 0C due to the local motion of the
polymer chains as opposed to large scale co-operative motion
that accompanies the Tg.
As shown in Figure 2, cross-linking process increases the
blend miscibility and converted the blend from partially
miscible ( has two Tg s; 234.60C and 265.380C) to miscible
blend with one Tg, which is 2480C. This gel displays a glassy
state at room temperature.
Incorporation of 0.3wt% GO in the cross-linked
CMC/starch blend; in SHNCs (Figure 3) increases Tg from
2480C to 2730C) . The reason can be attributed to higher
stiffness of GO in comparison with the neat blend. GO, also
has many functional groups which having the tendency for
inter- and intra-molecular interactions which restrict chain
movements.
Tan ? value before Tg is considerably higher in uncross -
linked CMC / starch blend than in cross-linked blend. This
indicates that the energy dissipation ability is decreased with
incorporation of the cross-linker to the gel structure. The
chain movements of uncross-linked CMC /starch blend are
significantly easier than those of cross-linked blend. The
same conclusion can be observed for the addition of GO
sheets, which can be attributed to the fact that the additional
interactions in the SHNCs limit the energy dissipation ability.
Fig (1). Complex Modulus (G*) and Damping factor (tan ?) for the CMC/starch blend without cross-linker versus temperature (oC)
Fig (2). G* and tan ? for the cross-linked CMC/starch hydrogel versus temperature (oC)
International Journal of Materials Science and Applications 2015; 4(2-1): 30-36 33
Fig (3). G* and tan ? for the SHNCs hydrogel versus temperature (oC)
Fig (4). G ,G" and ?* for the cross-linked CMC/starch hydrogel versus angular frequency
34 Auda Jabbar Braihi et al.: Viscoelastic and Rheological Properties of Carboxymethyl Cellulose /Starch/Graphite Oxide as
Superabsorbent Hydrogel Nano Composites (SHNCs)
Fig (5). G ,G" and ?* for the SHNCs hydrogel versus angular frequency
Above the glass-rubber transition temperature; Tg, large
parts of the chain are free to move; their thermal energy is
high enough to overcome the interaction forces, and the free
volume increases with increasing temperature. The polymer
is , however, not yet in the liquid condition; the coiled chains
are mutually entangled. Though the chain entanglements are
not permanent, because they are being disrupted with
increasing temperature and also with increased time of
loading, they act as temporary, physical cross-links[12].
3.2. Rheology Results
Figure 4 shows G ,G" and ?* for the cross-linked
CMC/starch blend. The plateau modulus (entanglement
modulus ;GN
o) extends over angular frequency up to about 10
s-1, then the blend exhibits glass to rubber transition , and
finally shows glassy state at higher frequencies[2].
The complex viscosity ?* decreased from the zero-shear
viscosity (?o =10000Pa.s) at low frequency (?= 0.1s-1) to 70
Pa.s at intermediate frequency (?=35s-1) and then increased
to ?f =120 Pa.s at higher frequency (?=100 s-1). Initially (at
low frequencies), the chains may be highly tangled and
intertwined (highly mutual wrapping of polymer chains
around each other) , thus the Brownian motion is insufficient
to overcome the entanglement forces between the polymer
chains, and with increasing the frequencies up to 35s-1 ,
motion increased and the chain begin to untangle and
straighten( became uncoiled ,more alignment and more apart
from others); slips easily, therefore, the viscosity decreased.
At higher frequencies, chains haven’t enough time to relax,
thus became more disentangle; viscosity decreased[2].
Generally, Figure 4 shows the strong effect of temperature
upon the viscosity (decreased from 10000 to 120 Pa.s), which
can be attributed to the molecular structure. Usually,
complexes structures such as NaCMC exhibit strong affect
by temperature due to its rings, pendant groups, and branches.
It is clear from figure 4 that G is higher than the G" and
both G and G" are very high and are nearly parallel to each
other, which indicates that this hydrogel is highly crosslinked
.Both G and G" tended to increase at higher
frequencies, but the increment in G is higher than in G",
which means that the hydrogel tend to be more elastic.
The addition of 0.3wt% GO sheets causes the viscosity to
increase in all frequencies(Figure 5 and Table 1),which
indicates the high interaction between GO sheets and the neat
blend components, therefore, the entanglement modulus(GN
o)
extends over larger angular frequencies range; up to about 12
s-1( in the expense of the glass to rubber region).
In view point the molecular weight, GO addition increases
the molecular weight average since there is a linear relation
between the intrinsic viscosity and the molar mass(M)
according to the Mark-Houwink relation:
[?] = k . Ma [13] (3)
in which k and a are constants for a given combination of
polymer, solvent and temperature[13].
GO addition, also, increases the zero-shear viscosity( ?o)
International Journal of Materials Science and Applications 2015; 4(2-1): 30-36 35
six times due to the increment in the intermolecular
interactions[5], and disappeared the variations in G" at lower
frequencies ; make the damping behavior more stable.
Since that the degree of entanglement density(N) can be
calculated from storage modulus data (equation 2) ,it can be
concluded from Table 1 that the value of the entanglement
density(N) for SHNCs hydrogel is 4.99 times higher than its
value for the same blend but without GO, which can be
attributed to the contributing of the functional groups (those
available on the GO sheets surfaces) to interface with the
functional groups inherently exit in the CMC/starch blend,
which leads to increasing the entanglement between the
homopolymers(CMC, and starch).
GO sheets addition, increases the complex modulus 5.41
times because the additional chain entanglement (4.99 times)
inhibits the relative chain motion. It should be noted that
even though secondary intermolecular(e.g. Van der Waals)
bonds are much weaker than the primary covalent ones, a
significant intermolecular force result from the formation of
large number of Van der Waals interchain bonds.
GO sheets leads, also to the absence of the shear
thickening behavior due to the strain hardening effect
because with the increasing density of nanoparticles, the
SHNCs hydrogel become stiffer and more fragile.
This behavior suggests that the breaking of the
nanoparticles interactions dominates the onset of shear
thinning[8].For both hydrogels, since G is higher than G" in
the whole frequency range of the linear viscoelastic (LVE)
range, the elastic behavior of the both hydrogels
predominates over its viscous behavior and the swollen
samples becomes increasingly rigid. In addition, with
increasing frequency, i.e. low relaxation time, the samples
flexibility is diminished and the swollen samples becomes
increasingly rigid[13].
Also, for both hydrogels, it can be concluded that these
hydrogels exhibited broad MWD because of the gradual
transition of the complex viscosity curve towards the shear
thinning region[2].
Table 1. Viscoelastic and rheological properties for the hydrogels at ? = 10 s-1
Property Without GO With GO Increment
G (Pa) 1611 8050 4.99
G" (Pa) 210 1800 8.5
G* (Pa) 1821 9850 5.41
Tan ? 7.67 4.47 0.58
?(degree) 82.57 77.83 0.94
?* (Pa.s) 125 850 6.8
?o (Pa.s) a 10000 60000 6
?f (Pa.s) b 120 200 1.66
GN
o(s-1) c Extends up to ?
=10 s-1
Extends up to
? =12 s-1 1.2
a at ? = 0.1 s-1 b at ? = 100 s-1 c at different frequencies
4. Conclusions
1- Cross-linking process increases the blend miscibility
and converted it from partially miscible with two Tg s to
miscible blend with one Tg.
2- Incorporation of GO in the cross-linked CMC/starch
blend; in SHNCs increases Tg from 2480C to 2730C.
3- Tan ? value before Tg in uncross-linked CMC/starch
blend is higher than in both the cross-linked blend or in the
SHNCs.
4- GO addition causes increment in some viscoelastic
parameters such as G*, G , and G" (5.41, 3.99, and 8.5 times
respectively), and decrement in damping parameters such as
? and Tan ? and make the damping behavior more stable at
lower frequencies.
5- All rheological parameters (?* , ?o , ?f , and GN
o )
increased due to the GO addition( 6.8, 6 , 1.66 , and 1.2 times
respectively).
6- G and G" profiles indicate that both hydrogels are
highly cross-linked.
7- GO addition leads to the absence of the shear thickening
behavior and increases both the entanglement density;
N(about five times) and the molecular weight average.
8- Both hydrogels exhibited broad MWD and in the whole
frequency of the LVE range, the elastic behavior
predominates over the viscous behavior and the swollen
hydrogels becomes rigid.
9- With increasing frequency, hydrogels flexibility is
diminished and the swollen hydrogels becomes increasingly
rigid.
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