i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 524–529
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Viscoelastic characterization of a new guar gum derivativecontaining anionic carboxymethyl and cationic2-hydroxy-3-(trimethylammonio)propyl substituents
Li Wanga, Li-Ming Zhanga,b,∗
a School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, Chinab Key Laboratory of Cellulose and Lignocellulosic Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China
a r t i c l e i n f o
Article history:
Received 28 August 2008
Received in revised form
13 October 2008
Accepted 14 October 2008
Keywords:
Amphoteric guar gum
Viscoelastic properties
Dynamic rheology
Relaxation time
a b s t r a c t
A new guar gum derivative (CMHTPG) containing anionic carboxymethyl and cationic 2-
hydroxy-3-(trimethylammonio)propyl substituents was characterized with the help of a
stress-controlled rheometer for its linear viscoelastic behavior in aqueous systems. The
frequency-dependent elastic modulus (G′) and viscous modulus (G′′) curves for 0.5, 1.0, 1.5
and 2.0 g/dl of aqueous CMHTPG solutions were found to cross at a given frequency. The
crossover frequency value decreased with the increase of CMHTPG concentration. At 25 ◦C,
the longest relaxation time was obtained to be 5.556 s for aqueous 2.0% CMHTPG solu-
tion while the shortest relaxation time to be 0.027 s for aqueous 0.5% CMHTPG solution,
showing a strong concentration dependence on the viscoelastic properties. Moreover, the
complex viscosity (�*) of aqueous CMHTPG solution was found to increase with the increase
of CMHTPG concentration, and to decrease with the increase of frequency. By investigating
the viscoelastic properties of aqueous CMHTPG salt solutions containing various concen-
Activation energy trations of NaCl, it was observed that the addition of NaCl could lead to a slight increase
in the G′, G′′ or �* value. Temperature was confirmed to have an important influence on the
viscoelastic properties of aqueous CMHTPG solution. For aqueous 1.0% CMHTPG solution,
the activation energy reflecting the temperature sensitivity of the complex viscosity was
determined at the frequency of 1.0 rad/s and found to be 16.94 kJ/mol.
Up to now, some studies have dealt with native guar gums
1. Introduction
Guar gum consisting of a 1,4-linked �-d-mannopyranose back-bone with the branches of 1,6-linked �-d-galactopyranose andits derivatives with various functional groups hold importantpotential applications in numerous industries such as food,paints and pigments, oil field, mining, paper, water treat-
ment, personal care, pharmaceutical and agriculture (Englystet al., 1983; Feddersen and Thorp, 1993; Roberts et al., 1996;Picout et al., 2002; Blackburn, 2004; Urdiain et al., 2005; Singh
∗ Corresponding author at: School of Chemistry and Chemical EngineChina. Tel.: +86 20 84112354; fax: +86 20 84112354.
et al., 2006; Zhao et al., 2006; Yi and Zhang, 2007). For suchapplications, they function usually as thickeners, emulsionstabilizers, gelling agents, film formers or texture modifiers.Therefore, an understanding of the rheological behavior ofthese naturally occurring polymers in aqueous systems is ofconsiderable interest.
ering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275,
as well as their nonionic and anionic derivatives for the rheo-logical properties. For example, Garcia-Ochoa and Casas (1992)investigated the viscosity of locust bean gum solutions in
arious shear rates, and confirmed their shear-thinning char-cteristics; Venkataiah and Mahadevan (1982) investigatedhe aqueous solutions of guar gum as well as its nonionicydroxypropyl and anionic sodium carboxymethyl derivativesor their flow properties in the range of low to moderatelyigh shear rates, and observed the transition of the rheo-
ogical behavior from Newtonian fluid to pseudoplastic fluid;ientjes et al. (2000) studied the linear viscoelastic behavior of
uar gum solutions as a function of frequency, temperature,olymer concentration, and molecular weight, and revealedhe importance of different relaxation mechanisms like repta-ion or the breakup of physical bonds; Wunderlich et al. (2000)arried out the shear and extensional rheological investiga-ions for aqueous solution of the polyacrylamide-grafted guarum, and found that the viscosity of the grafted guar gum wasigher than that of unmodified guar gum; Sharma et al. (2004)
nvestigated the rheological properties of carbamoylethyl guarum solutions, and found that the apparent viscosity of theseolutions decreases with the increase in the nitrogen contentf the product; Aubry and Moan (1994) focused on the rheo-
ogical effect of hydrophobically modified hydroxypropyl guarum, and demonstrated that the linear and nonlinear rheo-ogical behaviors of such associating systems were dependentn the nature, concentration, lifetime, and distribution alonghe chains of the hydrophobic junctions. Recently, we studiedhe semi-dilute solutions of hydroxypropyl guar gum (HPGG)ith respect to their viscosity, yield stress and thixotropy, and
ound that the viscosity changes under various HPGG concen-rations, added salts and temperatures could be well describedy the Cross viscosity model (Zhang et al., 2007). However, weould find no reports on the rheological behavior of ampho-eric guar gum derivatives except our preliminary work, inhich the shear-dependent viscosity behavior of an ampho-
eric guar gum derivative was investigated and compared withhose of anionic and cationic guar gum derivatives (Zhang etl., 2005). In comparison with nonionic, anionic or cationicuar gum derivatives, amphoteric guar gum derivatives mayave unique rheological characteristics due to the presence ofoth anionic and cationic groups along the macromolecularackbone.
In the present study, we investigated further the rheo-
ogical properties of the amphoteric guar gum derivative,-carboxymethyl-O-2-hydroxy-3-(trimethylammonio)-propyluar (CMHTPG), with respect to its linear viscoelastic behaviorn aqueous systems. In particular, the effects of CMHTPG
Fig. 1 – The sketch map for the chemica
t s 2 9 ( 2 0 0 9 ) 524–529 525
concentration, added NaCl and temperature on the dynamicstorage and loss moduli as well as the complex viscosity werestudied by using a stress-controlled rheometer.
2. Materials and methods
The amphoteric CMHTPG used in this study is a commer-cial product and was kindly supplied by the China Agent ofEconomy Polymers and Chemicals Company in USA. Fig. 1gives the sketch map for its chemical structure. The contentof the anionic carboxymethyl groups was determined to be6.3 × 10−3 mol/g by UV analysis at corresponding maximumadsorption of 204 nm. The content of the cationic quaternaryammonium groups was determined to be 5.42 × 10−4 mol/gby elementary analysis. The isoelectric point (IEP), where thepositive charges ascribable to the quaternary ammonium sub-stituent and the negative charges ascribable to carboxymethylsubstituent become almost equal within a CMHTPG chain, wasdetermined to be pH 5.9 by viscometry.
Dynamic viscoelastic properties of aqueous CMHTPG solu-tions without and with added salt (NaCl) were studied bydynamic oscillatory measurements using a stress-controlledRheometrics ARES rheometer. Depending on the sample vis-cosity, the plate–plate tool with the diameter of 50 mm andthe gap of 1.0 mm or the cylinder geometry with the innerdiameter of 34 mm, the height of 33 mm and the gap of 5 mmwas used. The measurements for the elastic and loss mod-uli were performed in the angular frequency range from 0.1to 100 rad/s. The test temperature was controlled by a circu-lating bath. All measurements were performed in the linearviscoelastic regime, which was determined from strain sweeptests at constant frequency and found to extend up to 10%strain for aqueous CMHTPG solutions without and with addedsalt.
3. Results and discussion
The changes of dynamic elastic modulus (G′) and viscous mod-ulus (G′′) as a function of frequency for aqueous CMHTPGsolutions with various CMHTPG concentrations at 25 ◦C are
shown in Fig. 2. Depending on the frequency, aqueous solu-tions of amphoteric CMHTPG could show liquid-like behavior,where the G′ value was smaller than the G′′ value, or solid-likebehavior, where the G′ value was larger than the G′′ value. The
l structure of amphoteric CMHTPG.
526 i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 524–529
y forratur
that the �* value increased obviously with the increase ofCMHTPG concentration at a given frequency. In particular, the�* value decreased with the increase of the frequency, showinga non-Newtonian shear-thinning behavior. The extent of devi-
Fig. 2 – Elastic and viscous moduli as a function of frequenc(a) 0.5 g/dl, (b) 1.0 g/dl, (c) 1.5 g/dl and (d) 2.0 g/dl. Test tempe
G′ and G′′ curves were observed to cross at a given frequency,at which the G′ and G′′ values are equal. In the case of 0.5 g/dl,the crossover modulus (Gc) and crossover frequency (wc) werefound to be 1.96 Pa and 36.84 rad/s, respectively. In the caseof 1.0 g/dl, the Gc and wc values were found to be 7.81 Pa and4.25 rad/s, respectively. In the case of 1.5 g/dl, the Gc and wc
values were found to be 13.96 Pa and 1.19 rad/s, respectively.In the case of 2.0 g/dl, the Gc and wc values were found to be24.73 Pa and 0.18 rad/s, respectively.
As shown in Fig. 3, the Gc value has a rapid increase whenthe CMHTPG concentration (Cp) increases from 0.5 to 2.0 g/dl.The dependence of the Gc on the Cp could be well describedwith the regression coefficient of 0.992 by the following expo-nential growth relationship:
Gc = 1.98 e1.27Cp (1)
On the other hand, the wc value corresponds to a characteristicrelaxation time (�) according to Larson’s theory (Larson, 1999)and can be determined according to the following relationship:
� = 1(2)
wc
Corresponding to the CMHTPG concentration of 0.5, 1.0, 1.5and 2.0 g/dl, the � value was determined to be 0.027, 0.235,0.840 and 5.556 s, respectively. As indicated by these data, the
aqueous CMHTPG solutions with various concentrations:e: 25 ◦C.
characteristic relaxation time has a continuous increase whenthe Cp value increases from 0.5 to 1.5 g/dl, and has a rapidincrease with a further increase of the Cp to 2.0 g/dl.
Fig. 4 gives the plots of the complex viscosity (�*) versusfrequency (w) for aqueous CMHTPG solutions. It was found
Fig. 3 – The change of crossover modulus with CMHTPGconcentration. Test temperature: 25 ◦C.
i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 524–529 527
Fig. 4 – Dynamic complex viscosity as a function offc
ardc
�
Ifiao1CubqwN
spe(diimwhnt
ssesofaic
Fig. 5 – Effects of added salt on the elastic modulus (a), loss
requency for aqueous CMHTPG solutions with variousoncentrations. Test temperature: 25 ◦C.
tion from Newtonian flow behavior can be semi-qualitativelyelated to the shear-thinning exponent (n), which could beetermined by fitting the frequency-dependent complex vis-osity to a power law relationship (Larson, 1999):
∗ ∼ w−n (3)
n this way, the n value and corresponding regression coef-cient were respectively determined to be 0.30 and 0.902 forqueous 0.5 g/dl CMHTPG solution, 0.45 and 0.982 for aque-us 1.0 g/dl CMHTPG solution, 0.56 and 0.990 for aqueous.5 g/dl CMHTPG solution, 0.68 and 0.996 for aqueous 2.0 g/dlMHTPG solution. It is known that an ideal Newtonian liq-id should have the shear-thinning exponent equal to zeroecause its viscosity is independent on shear rate or fre-uency (Larson, 1999). Therefore, aqueous CMHTPG solutionith a higher concentration is characteristic of stronger non-ewtonian shear-thinning behavior.
The above-mentioned results demonstrate that there istrong concentration dependence for the linear viscoelasticroperties of aqueous CMHTPG solutions. In general, a vari-ty of structures may appear in aqueous CMHTPG solutions:i) linear structure involving one or a few bonded chains inilute solution; (ii) branched structure involving several chains
n semi-dilute solution; (iii) large aggregated structure involv-ng many chains in concentrated solution. These structures
ay occur both as free units and as part of a space-filling net-ork. In the case of high CMHTPG concentration, therefore,igh G′, G′′ and �* values, long relaxation time as well as strongon-Newtonian shear-thinning behavior may be attributed tohe formation of large aggregated network structure.
Further investigation was focused on the effect of addedalt on the linear viscoelastic properties of aqueous CMHTPGolution. Fig. 5 shows the changes of the rheological param-ters (G′, G′′ and �*) with the frequency for aqueous saltolutions consisting of 1.0 g/dl CMHTPG and different amountsf added NaCl. An interesting phenomenon was observed
or such investigation. That is, the addition of NaCl leads ton increase in the G′, G′′ and �* values at a given frequencynvestigated, although the amount of added NaCl affects thehanges of G′, G′′ and �* with the frequency. This behavior
modulus (b) and complex viscosity (c) for aqueous 1.0 g/dlCMHTPG solution. Test temperature: 25 ◦C.
is very different from those observed for aqueous salt solu-tions of anionic or nonionic guar gum derivatives, where theaddition of salts weakens usually the viscoelastic properties(Venkataiah and Mahadevan, 1982; Wunderlich et al., 2000).In the aqueous solution without the added salt, CMHTPG hasextensive the intrachain association, which derives from theinteractions between cationic quaternary ammonium groupand anionic carboxymethyl group from the same CMHTPG
chain, and the interchain association, which are attributed tothe interactions between the quaternary ammonium groupand the carboxymethyl group from different CMHTPG chains.These interactions may lead to a collapsed conformation. In
r o d u c t s 2 9 ( 2 0 0 9 ) 524–529
528 i n d u s t r i a l c r o p s a n d p
aqueous salt solutions, however, the addition of NaCl neutral-izes the negative and positive charges of amphoteric CMHTPG,and then disrupts the intra- and intermolecular associa-tions. As a result, the macromolecular chains of amphotericCMHTPG may adopt an extended conformation, which hasa positive action for the enhancement of the viscoelasticproperties.
To understand the influence of temperature, the frequency-dependent G′ ,G′′ and �* values were measured for aqueous1.0 g/dl CMHTPG solution under various temperatures of 25,
30, 40, 50 and 60 ◦C, as shown in Fig. 6. At the frequency rangeinvestigated, all the rheological parameters were found todecrease when the temperature increased. This phenomenoncould be attributed to the Brownian motion (Tiziani and
Fig. 6 – Effects of temperature on the elastic modulus (a),loss modulus (b) and complex viscosity (c) for aqueous1.0 g/dl CMHTPG solution. Test temperature: 25 ◦C.
Fig. 7 – The plot of ln �T versus 1/T for aqueous 1.0 g/dlCMHTPG solution. Frequency: 1.0 rad/s.
Vodovotz, 2005), which is favorable for the relaxation ofCMHTPG macromolecular chains in aqueous medium. It maybe hypothesized that the increase of temperature affectsthe entanglement structure of aqueous CMHTPG solutionby weakening the hydrophilic and hydrophobic interactionsamong CMHTPG macromolecular chains. In order to bettercorrelate the effect of temperature on the complex viscosity ofaqueous CMHTPG solution, we fit the complex viscosity data,which were collected at 1 rad/s, to the following Arrheniustype model (Lapasin and Pricl, 1995):
�T = A exp[
Ea
RT
](4)
where �T (Pa s) is the viscosity of a fluid at the absolute tem-perature T (K), R (J K−1 mol−1) is the universal gas constant, Ea
(J mol−1) is the energy of activation of flow process, and A is acharacteristic constant. Based on the regression of ln (�T) ver-sus the inverse of absolute temperature (Fig. 7), the activationenergy was calculated to be 16.94 kJ mol−1 with the regressioncoefficient of 0.973.
4. Conclusion
A stress-controlled Rheometrics ARES rheometer wasused to investigate the dynamic viscoelastic proper-ties of an amphoteric guar gum derivative (CMHTPG)containing anionic carboxymethyl and cationic 2-hydroxy-3-(trimethylammonio)propyl substituents in aqueous systems.With the increase of CMHTPG concentration from 0.5 to2.0 g/dl, the dynamic elastic (G′) and viscous (G′′) moduli aswell as the complex viscosity (�*) of aqueous CMHTPG solutionand its characteristic relaxation time were found to have anobvious increase due to the formation of large aggregatednetwork structure. Among four concentrations of aqueousCMHTPG solutions investigated, aqueous 2.0 g/dl CMHTPGsolution has the longest relaxation time and strongest non-
Newtonian shear-thinning behavior. The addition of NaClleads to a slight increase in the G′, G′′ or �* value in the saltconcentration range investigated, which could be attributedto the weakening of added NaCl to the intra- and intermolec-
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hydroxypropyl guar gum: viscosity behaviour and thixotropic
i n d u s t r i a l c r o p s a n d p r o
lar associations between cationic quaternary ammoniumroup and anionic carboxymethyl group from the CMHTPGhains. With the increase of temperature, the viscoelasticroperties of aqueous CMHTPG solution became weak.
cknowledgements
his work is supported by the Natural Science Foundation ofhina (Grant Nos. 20676155 and 20874116) and the Natural Sci-nce Foundation of Guangdong Province (Grant Nos. 6023103nd 8151027501000004) as well as the Key Laboratory of Cellu-ose & Lignocellulosic Chemistry, Academia Sinica (Grant No.CLC-2007-702).
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