U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`105
`
`Rheological Characterization of Aqueous Polysaccharide Mixtures
`Undergoing Shear
`
`U. Florjancic, A. Zupancic*, and M. Zumer
`Chair of Chemical, Biochemical and Ecological Engineering,
`Department of Chemical Technology, Faculty of Chemistry and
`Chemical Technology, University of Ljubljana
`Askerceva 5, SI-1000 Ljubljana, Slovenia
`tel.:+386 1 2419 529, e-mail: andreja.valant@uni-lj.si
`
`Original scientific paper
`Received: March 3, 2002
`Accepted: June 20, 2002
`
`We studied the rheological properties of aqueous polysaccharide mixtures, prepared
`by blending two compatible biopolymers,
`i.e. sodium carboxymethylcellulose and
`xanthan, under destructive and non-destructive shear conditions, in order to evaluate syn-
`ergistic/non-synergistic effects of mixed polysaccharide systems. The rheological experi-
`ments were carried out with the controlled stress rotational rheometer to detect the re-
`sponse of structured samples at
`low shear stresses. The flow behavior and the
`viscoelastic properties of polysaccharide systems under shear conditions were examined
`by applying continuous shear and oscillatory tests at ambient temperature. The empirical
`analysis of the flow behavior enabled us to evaluate the deviations in the zero shear vis-
`cosity, and the intensity of the shear-thinning behavior as determined for investigated bi-
`nary mixtures, when compared to the flow behavior of pure component solutions. The
`mechanical properties of polysaccharide mixtures in the range of the linear viscoelastic
`response were analyzed with the generalized Maxwell model and the relaxation spectra
`were determined. The examined mixtures exhibited complex rheological behavior under
`the shear conditions, regarding the flow characteristics as well as the viscoelastic proper-
`ties in the linear viscoelastic regime.
`
`Keywords:
`Aqueous polysaccharide mixtures, sodium carboxymethylcellulose, xanthan, rheology,
`shear flow, viscoelasticity
`
`Introduction
`
`Many polysaccharides play an important role
`in the field of science and technology due to their
`unique properties. These natural polymers are bio-
`degradable, nontoxical, and widely available mate-
`rials at low costs. They exhibit good compatibility
`and water solubility. The wide range of practical ap-
`plications exploits the ability of polysaccharides to
`thicken or structure many times their own mass of
`water.1 In this way biopolymers modify the proper-
`ties of aqueous environment and control the rheol-
`ogy of hydrated systems.
`Polysaccharides are widely used in food pro-
`cessing and preparation to stabilize emulsions and
`suspensions, and to improve the texture of food
`products. Within pharmaceutical and biomedical
`applications, biopolymers act as highly effective
`substances to control the release of a drug. Due to
`film-, membrane- and gel-forming properties, poly-
`saccharides have found extensive applications for
`the immobilization of proteins, enzymes and animal
`cells, and act as important components in mem-
`
`* coresponding author
`
`brane manufacturing. The water-retention proper-
`ties improve the processing of ceramics2 and the
`formulation of building materials in construction
`applications. The oil industry employs biopolymers
`due to their high swelling at low polymer concen-
`trations, high efficiency as suspending agents, high
`shear thinning behavior and extreme compatibility
`with high concentrations of various salts and tem-
`peratures. Aqueous polysaccharide solutions can be
`successfully used as model fluids in order to simu-
`late the complex rheological behavior of materials
`employed in various technological processes such
`as the mixing operation.3,4,5
`
`When different polysaccharides which exhibit a
`wide range of rheological properties, originating in
`peculiar structural properties, are mixed together, they
`are expected to have specific behavior, to improve the
`applicability of each separately used polysaccharide,
`and possibly to have cost advantages.2,6,7 In many
`cases, polysaccharide mixtures exhibit a behavior
`which is not simply the linear combination of individ-
`ual contributions. In fact, by blending different poly-
`saccharides with various structural properties, positive
`or negative effects can be observed.
`
`ALKERMES Exh. 2037
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`
`

`

`106
`
`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`The biopolymer systems, investigated in our
`study, are aqueous mixtures of sodium carboxy-
`methylcellulose (Na CMC) and xanthan. Separately
`used, both polysaccharides have found many practi-
`cal applications. As physiologically harmless mate-
`rials, they are widely used for pharmaceutical prep-
`arations, cosmetics, personal care products, and
`food products.1,8,9 Various technical applications
`can be found in oil field, agricultural, paint, and ce-
`ramic industry, and many other branches.1,10,11,12
`The wide range of Na CMC applications involves a
`wide variety of technological processes, such as
`mixing, pumping, heating, cooling, and separation
`operations13. Both polysaccharides are common
`polymers used for turbulent flow experiments14 as
`well as for studying the effects of shear-dependent
`flow properties and viscoelasticity on the hydrody-
`namics in mixing operations, performed in the lami-
`nar flow regime.3,4 Aqueous Na CMC-xanthan mix-
`tures have already found practical application as
`model fluids intended for studying power consump-
`tion in a double planetary mixer with non-Newto-
`nian and viscoelastic materials5. By applying pri-
`mary normal stress difference measurements, Zhou
`et al.5 noticed the non-linear effect of polymer con-
`centration on the elasticity of fluids. As a possible
`reason for the significantly different behavior of
`one of the examined mixtures, they suggest the
`structural configuration of the specific mixture.
`Such deviations in the rheological behavior of
`mixed polysaccharides originate from the nature of
`each polymer in the mixture and various interac-
`tions which occur between polymer chains.
`
`Sodium carboxymethylcellulose (Na CMC) is a
`water-soluble derivative of biologically degradable
`natural polysaccharide cellulose, a fundamental
`component of plant cell walls. In aqueous solution
`it represents a complex rheological system, since it
`forms aggregates and associations, and hence higher
`level structures.15 At higher polymer concentra-
`tions, extended Na CMC chains start to overlap and
`undergo the coiling process, which causes forma-
`tion of the network structure in the concentrated re-
`gime.16 With higher polymer concentration the
`polymer-polymer interactions (entanglements) be-
`come the main factor influencing the rheology of
`the Na CMC solution.8 Above the overlap concen-
`tration, aqueous Na CMC solutions exhibit more
`marked shear-thinning flow behavior and visco-
`elastic properties8,13,17 and can be seen as entangle-
`ment polymer solutions. The polymer concentra-
`tion, temperature, salt content, the presence of sur-
`factants, and the molecular structure have a consid-
`erable effect on the rheological properties of
`Na CMC.8,13,16,17,18
`
`Xanthan, an anionic exocellular microbial
`polysaccharide,
`is produced by the bacterium
`
`Xanthomonas campestris. Its primary structure con-
`sists of a linear cellulosic backbone with trisaccha-
`ride sidechains, attached to alternating backbone
`residues.1 The semi-flexible xanthan molecule un-
`dergoes a conformation transition from an ordered
`double helix to a random coil when heated, depend-
`ing on the ionic strength of the solution.19 With the
`polymer in a water-soluble ordered form, aqueous
`solutions in semi-dilute and concentrated regime
`generally have high viscosity at low shear rates and
`show characteristic weak gel properties even at low
`polymer concentrations.20,21,22 Under small-defor-
`mation conditions,
`the weak gel behaves as a
`viscoelastic solid.1 As the deformation increases,
`the weakly formed three-dimensional network
`breaks down and the material starts to flow. The
`rupture of weak junctions upon shear, and their res-
`toration upon the cessation of shear, could account
`for xanthan thixotropy. The double-stranded helix
`formation stabilizes xanthan self-association, and is
`responsible for the insensitivity of the viscosity to
`salt and pH changes.23
`The objective of this paper is to present the
`rheological characterization of aqueous Na CMC-
`-xanthan mixtures under destructive and nonde-
`structive shear conditions in order to evaluate syner-
`gistic/non-synergistic effects of blended polymers.
`Biopolymer mixtures used in our study were pre-
`pared by blending two polysaccharides with differ-
`ent rheological properties. The microbial polysaccha-
`ride xanthan, exhibiting weak gel properties in
`aqueous solutions, was added to Na CMC at differ-
`ent mass fractions, in order to investigate the differ-
`ences in the rheological behavior of aqueous
`Na CMC-xanthan mixtures in concentrated regime,
`when the mass ratio of blended polymers changes.
`The examined mixtures exhibit complex rheological
`behavior under shear conditions, regarding the flow
`characteristics as well as the viscoelastic properties
`in the linear viscoelastic regime. Due to the com-
`plex rheological behavior of examined mixtures, in-
`dicating that these systems can be regarded as struc-
`tured liquids, special attention was paid to measure-
`ments of the response at low stresses by using a
`sensitive controlled-stress rheometer.
`
`Phenomenological modeling of linear
`viscoelastic properties
`
`The quantitative interpretation of the rheologi-
`cal data, obtained with experimental tests, requires
`description of the rheological response by using
`mathematical model, providing different parameters
`with various physical meanings. The rheological
`models, based on the phenomenological approach,
`have been developed to achieve maximum agree-
`
`

`

`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`107
`
`the strain of a particular material varies linearly
`with the stress, the response of a single Maxwell
`element can be written in a differential form:25,27
`
`(6)
`
`=
`
`h g
`&,
`
`t
`t
`
`d d
`
`t
`
`+
`
`l
`
`where t is the shear stress, l is the relaxation time, t
`is the time, h is the shear viscosity, and &g is the
`shear rate. By using a single Maxwell element to
`describe the linear viscoelastic response of the liq-
`uid under oscillatory shear conditions,
`the fre-
`quency-dependent dynamic moduli can be mathe-
`matically expressed as follows:
`
`G
`
`¢ =
`w
`(
`)
`
`G
`
`wl
`)
`(
`+
`wl
`1
`
`(
`
`2
`
`2
`
`)
`
`G
`
`
`(
`)
`
`¢¢ =w
`
`G
`
`wl
`wl
`
`(
`
`2
`
`)
`
`+
`1
`
`,
`
`.
`
`(7)
`
`(8)
`
`ment between the predicted and experimental rheo-
`logical properties of a fluid, by taking into account
`only the principles of continuum mechanics, regard-
`less of structural characteristics.1 Despite the fact
`that for structured systems, such as weak gels, satis-
`factory rheological modeling has not been devel-
`oped yet due to their peculiar structural characteris-
`tics and physical properties, we examined the linear
`viscoelastic properties by using the generalized
`Maxwell model, based on the phenomenological
`approach.
`A number of small-deformation experiments
`are used to measure the linear viscoelastic response
`of the material.24,25 When the small-amplitude oscil-
`latory shear technique is used, the sample is sub-
`jected to a sinusoidal shear strain, g, and the result-
`ing oscillatory shear stress, t, is measured. As a re-
`sponse of the material to the oscillating strain input,
`the shear stress will also oscillate sinusoidally at the
`same oscillation frequency, w, but in general it will
`be shifted by the phase angle, d, with respect to the
`strain wave as described with the following mathe-
`matical expressions:25
`
`g
`
`
`
`
`
`wg= °sin t,
`
`t
`
`
`
`t= °
`
`sin (
`
`w
`
`t
`
`+
`
`d
`
`,
`)
`
`(1)
`
`(2)
`
`where g° is the shear strain amplitude, and t° is the
`shear stress amplitude. By decomposing the stress
`wave into two waves of the same frequency, two
`dynamic moduli, the storage modulus, G', and the
`loss modulus, G'', are introduced:
`
`G
`
`sin
`
`t
`
`G
`
`cos
`
`.
`t
`
`(3)
`
`The rheological behavior of many liquids in the
`linear viscoelastic regime is often too complex to be
`successfully described by using a single Maxwell
`element model. Therefore, the generalized Maxwell
`model
`is frequently used to describe the linear
`viscoelastic response of the liquid. When the model
`is described in terms of a discrete set of Maxwell
`elements, it is possible to incorporate a discrete
`relaxation times, lk, and relaxation
`range of
`strengths, gk, within the model. The complete set of
`(gk, lk) is called the spectrum of the relaxation
`times.27 If the generalized Maxwell model with a
`limited number of Maxwell elements is used to de-
`scribe the dynamic response of the viscoelastic liq-
`uid in the linear viscoelastic regime, the discrete re-
`laxation spectra can be determined from experimen-
`tally obtained dependencies of the storage modulus,
`G', and the loss modulus, G'', on the oscillation fre-
`quency, w:25
`
`,
`
`2
`
`.
`
`2
`
`(9)
`
`(10)
`
`(
`
`wl
`
`2
`
`)
`
`k
`wl )
`k
`
`+
`1
`
`(
`
`wl
`
`k
`wl )
`k
`
`+
`1
`
`(
`
`g k
`
`g k
`
`N
`
`=å
`
`k
`
`1
`
`N
`
`=å
`
`k
`
`1
`
`G
`
`w
`¢ =
`(
`)
`
`G
`
`¢¢ =
`w
`(
`)
`
`To fit our experimental data, we used a model
`with five Maxwell elements. First we selected a set
`of relaxation times, lk, evenly spaced on a log scale,
`i.e. one per decade, and then we ran the fitting pro-
`cedure to determine the relaxation strengths, gk, us-
`ing the least-squares regularization method that
`minimizes the sum of squared relative deviations
`between the calculated values and the experimental
`data.25
`
`t
`
`
`
`g= °
`
`¢
`
`w
`
`
`
`g+ °
`
`¢¢
`
`w
`
`The storage modulus, G', and the loss modulus,
`G'', are the real and the imaginary component of the
`complex modulus, G*, respectively:
`
`2
`= ¢+ ¢¢= ¢ + ¢¢
`G
`G
`iG
`G
`
`2 ,
`
`(4)
`
`° °
`t g
`
`G
`
`*=
`
`and the ratio between the dynamic moduli can be
`written as follows:
`
`(5)
`
`=
`
`tan d.
`
`¢¢
`¢
`
`G G
`
`When the mechanical properties of the material
`the linear visco-
`are experimentally determined,
`elastic behavior can be analyzed by using a proper
`mechanical model. The generalized Maxwell me-
`chanical model can be represented as a parallel se-
`ries of Maxwell elements, consisting of
`linear
`springs and dashpots.26 Each Maxwell element has
`a characteristic relaxation time, l, defined as the ra-
`tio between the shear viscosity, h, and the elastic
`modulus, G. In the linear viscoelastic region, where
`
`

`

`108
`
`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`Experimental
`
`Materials and preparation
`
`Sodium carboxymethylcellulose (Na CMC),
`used in our study, with the trade name BLANO-
`SE® 7HF cellulose gum, is a commercial product of
`Aqualon France, Hercules. Determined by the pro-
`ducer, the molar mass of this purified Na CMC
`powder is 4.35 · 105 g mol–1, with the degree of sub-
`stitution in the range of 0.65–0.90, pH of 6.5–8.5,
`and sodium fraction of 7.0–8.9 %. Its purity is
`99.5 % minimum. Xanthan, known as KELTROL®,
`was supplied by CP Kelco. The lot number of
`xanthan sample used was 8LO68OV.
`Water-based salt-free solutions of Na CMC and
`xanthan (X), having a polymer mass fraction of w =
`1.0 % were prepared by dissolving a known amount
`of polymer powder in distilled water at room tem-
`perature. All samples were stirred by hand for few
`minutes and stored in a refrigerator at 4 °C in cov-
`ered glass beakers, allowing the powder to hydrate
`for several days to ensure that the sample had com-
`pletely dissolved.
`The mixtures of Na CMC-xanthan (CX) with a
`variable mass ratio between two polymers and a
`constant total polymer mass fraction of w = 1.0 %
`were prepared by blending appropriate amounts of
`the two polysaccharide solutions at room tempera-
`ture. The rheological behavior of aqueous Na CMC-
`-xanthan mixture was investigated in three different
`mass ratios.
`During the mixture preparation, the storage pe-
`riod, and the measurements no stability problems
`occurred.
`
`Measurement techniques
`
`The rheological measurements used in this
`study were carried out by using a HAAKE RS 150
`controlled stress rheometer. A cone and plate geom-
`etry of the sensor system was used to ensure a con-
`stant shear rate in the sample. The cone diameter
`was 60 mm, with the cone angle of 2°. All experi-
`ments were carried out at a temperature of 20 ±
`0.3 °C. The temperature was externally controlled
`by circulator HAAKE DC5-K20.
`As a large deformation technique, the continu-
`ous-shear experiments were applied to determine
`the flow behavior under destructive shear condi-
`tions when the structure of the material is broken
`down.
`The viscoelastic response of aqueous poly-
`saccharide systems was examined by carrying out
`oscillatory measurements. The stress-sweep tests at
`a frequency of 1 Hz enabled us to follow the behav-
`ior of dynamic moduli, while the shear stress ampli-
`
`tude was changing in order to determine the limit of
`linear viscoelastic response.
`Hence, the mechanical spectra in the range of
`the linear viscoelastic regime, when non-destructive
`shear conditions enable the material to preserve the
`structure, were obtained with frequency tests at a
`constant strain amplitude of 3 %.
`
`Results and discussion
`
`Flow behavior under shear conditions
`
`The first part of our paper represents the results
`of the continuous-shear experiments under the con-
`ditions of large shear deformations for aqueous
`Na CMC-xanthan mixtures at total polymer mass
`fraction of w = 1.0 %. The experimental tests were
`repeated at least two times, and the results found to
`be reproducible in the range of experimental error.
`The Na CMC-xanthan mixtures exhibit com-
`plex time-dependent non-Newtonian flow behavior
`of the shear-thinning type, as shown in Figure 1.
`Pure Na CMC solution exhibits the first Newtonian
`plateau at low shear-stresses and moderate shear
`thinning time-independent flow behavior (Figure
`1a), whereas pure xanthan solution shows a signifi-
`cant effect of shear history and more pronounced
`shear thinning behavior (Figure 1b). At low shear
`conditions, the ‘up flow curve’ of the xanthan solu-
`tion exhibits the first Newtonian plateau and high
`values of shear viscosity, indicating strong resis-
`tance of the material to flow, while the ‘down flow
`curve’ indicates continuously decreasing viscosity
`(Figure 1b). The ‘up flow curve’ and the ‘down
`flow curve’ represent the viscosity as a function of
`increasing and decreasing shear stress, respectively.
`As the shear stress increases, a strong decrease of
`shear viscosity is detected in a narrow shear stress
`range. Such behavior of the xanthan solution shows
`existence of a three-dimensional network structure,
`consisting of long polymer chains associated into
`higher level structural formations as observed for
`weak gels,19,20 while the Na CMC solution behaves
`as an entanglement polymer solution in a concen-
`trated regime8 (Figure 2).
`Under destructive shear conditions our atten-
`tion is primarily focused on the effect of the shear
`action during dynamic process of the structural
`breakdown in the samples. The flow behavior of
`aqueous Na CMC-xanthan mixtures when the shear
`stress increases (‘up flow curves’) is illustrated in
`Figure 1a. When xanthan at mass fraction of 0.25 is
`present in the Na CMC solution, the zero-shear vis-
`cosity remains unchanged, while the shear-thinning
`behavior becomes more pronounced at higher shear
`stresses. The presence of xanthan at mass fraction
`
`

`

`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`109
`
`F i g . 1 – The flow behavior of pure Na CMC solution
`(CMC), pure xanthan solution (X), and aqueous Na CMC-xan-
`than mixtures (CX) at different mass ratios and a total polymer
`mass fraction of w = 1.0 %, determined at temperature of
`20 ± 0.3 °C and represented as a) up flow curves, b) up and
`down flow curves
`
`of 0.25 only enhances the shear-thinning behavior,
`without changing the type of the ‘up flow curve’,
`which is controlled by Na CMC properties. As the
`mass fraction of xanthan increases up to 0.5, the
`zero shear viscosity increases, and the ‘up flow
`curve’ changes its shape more pronouncedly. It
`seems that the gradual decrease of shear viscosity
`with increasing shear stress passes over two flow
`regimes. With a further increase of mass fraction of
`xanthan the effect is even more pronounced. Figure
`1b illustrates the ‘up and down flow curves’ (i.e. the
`shear stress first increases and then decreases dur-
`
`F i g . 2 – Shematic representation of
`the structural con-
`ditions of a) concentrated Na CMC solution,
`b) weak gel network of xanthan.
`
`ing experimental test). A double S-shape of the flow
`curves found for the mixtures is only observed with
`the ‘up flow curves’, indicating that two flow re-
`gimes of the Na CMC-xanthan mixtures originate in
`two structural mechanisms and are not caused by
`slippage effects.
`
`We can conclude that at low shear stresses the
`presence of small quantities of xanthan (up to mass
`fraction of 0.25) doesn’t change the flow properties
`of the Na CMC solution, which shows that the
`
`

`

`110
`
`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`structure of the polysaccharide mixture at low shear
`is defined with the structural conditions of Na CMC.
`We assume that the concentration of xanthan mole-
`cules which are present as ordered helices and prob-
`ably linked through intermolecular interactions, is
`too low to contribute to the entanglement structure
`formed by Na CMC polymer chains.
`
`When the Na CMC-xanthan mixture is sub-
`jected to higher shear stresses, the shear viscosity of
`the mixture decreases more rapidly compared to
`pure Na CMC solution. The latter observation
`shows that the structure of the mixture is more sen-
`sitive to higher stresses than the structure of pure
`Na CMC.
`
`At higher mass fractions of xanthan (above
`0.5),
`the zero-shear viscosity has significantly
`higher values, indicating that by the additional for-
`mation of the network structure xanthan helices
`contribute to higher resistance to flow. Under the
`conditions of flow regime, the shear-thinning be-
`havior becomes slightly more pronounced. The
`zero-shear viscosity reaches almost the same value
`as with the pure xanthan solution when the weight
`fraction of xanthan increases to 0.75, and the inten-
`sity of the shear-thinning behavior is similar as for
`the xanthan solution. The structure of the mixture
`breaks down at much lower shear stresses and the
`‘up flow curve’ passes over two flow regimes. In
`the intermediate region of
`the examined shear
`stresses, the Na CMC fraction significantly influ-
`ences the flow pattern of the mixture. At high shear
`stresses the flow behavior is again similar to that of
`the pure xanthan solution.
`
`In order to evaluate the synergistic/non-syner-
`gistic effect of the Na CMC-xanthan mixtures in
`terms of flow behavior, the experimental data were
`empirically analyzed in two flow regimes. The zero
`shear viscosity of the Na CMC-xanthan mixtures is
`lower than it would be expected from the linear re-
`lationship drawn through the flow behavior of pure
`components (considering h0 or log h0 vs. mixture
`composition), indicating that the structure of the
`mixture is less resistant to flow.
`
`The intensity of the shear thinning behavior
`was quantitatively examined by applying the empir-
`ical parameter D (log h)/D (log t). This empirical
`parameter is a measure of decrease of the shear vis-
`cosity within the shear stress range, limited with
`two values of shear stress. The lower limit is the
`critical shear stress above which shear viscosity de-
`creases sharply, and is determined as the shear
`stress at which shear viscosity decreases by 5 %.
`The upper limit is chosen as the maximum shear
`stress experimentally reached for all examined sys-
`tems and has the value of 40 Pa. As shown in Fig-
`ure 3, a smooth non-linear increase of intensity of
`
`F i g . 3 – The intensity of the shear-thinning as a function
`of the mass fraction of xanthan in aqueous Na CMC-xanthan
`mixtures at a total polymer mass fraction of w = 1.0 %, evalu-
`ated from the up flow curves at 20 ± 0.3 °C
`
`the shear-thinning behavior for the Na CMC-xan-
`than mixtures, is observed.
`
`The synergistic effect of the Na CMC-xanthan
`mixtures, observed as deviations from the flow be-
`havior of pure polysaccharide components, can be
`explained in terms of specific molecular interac-
`tions between unbranched Na CMC chains and
`multi-stranded helices of xanthan.
`
`The weak gel nature of pure xanthan solution
`at polymer fraction of w = 1.0 % originates from
`the intermolecular interactions that lead to physical
`cross-linking and hence the formation of junction
`zones, resulting in a three-dimensional network20
`(Figure 2). When xanthan is mixed with Na CMC,
`the presence of linear unbranched Na CMC chains
`inhibits formation of extended junction zones and
`induces formation of small clusters of xanthan as a
`disperse phase, surrounded with entangled Na CMC
`chains (Figure 4). At a high content of xanthan, the
`dispersion of small gel clusters displays solid-like
`behavior at low stresses, and its rheological proper-
`ties cannot be easily distinguished from those of
`weak gels.1
`
`When Na CMC chains dominate in the Na
`CMC-xanthan mixture, the rheological properties
`are ruled more pronouncedly by the properties of
`Na CMC as a continuous phase. At higher mass
`fractions of xanthan, more progressive shear-thin-
`ning behavior of the Na CMC-xanthan mixture is
`observed and the ‘up flow curves’ exhibit a
`regi-
`double-step behavior,
`indicating different
`mes in the structural breakdown due to the shear ac-
`tion.
`
`

`

`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`111
`
`F i g . 4 – Shematic illustration of proposed structural conditions of aqueous Na CMC-xanthan mixtures
`
`The heterogeneous nature of the polysaccha-
`ride dispersion is reflected in a progressive disrup-
`tion of the weakly structured network of gel re-
`gions, surrounded by entangled Na CMC chains,
`which occur at lower stresses than observed for the
`pure xanthan solution, indicating that the structure
`of the polysaccharide mixture has weaker character
`than that of the pure xanthan solution. Further in-
`crease of the shear stress causes a formation of
`smaller flow units, having the ability to flow. With
`increasing mass fraction of Na CMC, the rheologi-
`cal properties are governed by the surrounding me-
`dium, in which small gel regions of xanthan are dis-
`persed.
`
`Viscoelastic properties under oscillatory
`shear conditions
`
`The linear viscoelastic properties of aqueous
`Na-CMC-xanthan mixtures at a total polymer mass
`fraction of w = 1.0 % were examined by applying
`oscillatory tests under small-deformation conditions
`which preserve the structure of the material.
`the limit of the linear viscoelastic re-
`First
`sponse was determined by carrying out stress-sweep
`tests at a constant frequency of 1 Hz. Figure 5 rep-
`resents the shear strain amplitude dependence of the
`dynamic moduli of aqueous polysaccharide systems
`used in our study. At low shear strain amplitudes
`pure Na CMC solution exhibits the loss modulus,
`G'', slightly higher than the storage modulus, G',
`while for the xanthan solution the elastic response
`
`predominates over the viscous one (Figure 5a). As
`the shear strain amplitude increases, the dynamic
`response becomes non-linear and the decrease of
`the moduli occurs. A significantly different behav-
`ior is observed for the loss modulus, G'', of the pure
`xanthan solution above the critical strain amplitude
`which determines the transition from the linear to
`the nonlinear regime. As shown in Figure 5a, a
`slight increase of the loss modulus, G'', is observed,
`followed by a continuous decrease which is less
`pronounced than the decrease of the storage modu-
`lus, G'. Such behavior is often found for weak gel
`structures.1,28
`
`When xanthan is blended with Na CMC, the
`characteristic behavior of the loss modulus, exhib-
`ited by pure xanthan solution, disappears, as can be
`noticed in Figure 5b. The analysis of the strain am-
`plitude independent dynamic moduli for Na CMC-
`-xanthan mixtures, within the linear viscoelastic re-
`gime shows that smaller mass fractions of xanthan
`(up to 0.5) reduce both moduli, with a stronger ef-
`fect observed for the loss modulus. The lowest val-
`ues of both moduli, G' and G'', are observed when
`xanthan is present in the Na CMC-xanthan mixture
`at a mass fraction of 0.5. As the mass fraction of
`xanthan increases above 0.5, a slight increase of the
`loss modulus and a significantly stronger increase
`of the storage modulus, are observed.
`
`Figures 6 and 7, indicating not-linear variation
`* and do with the mixture composition, show
`of G o
`that the synergistic effect of the Na CMC-xanthan
`
`

`

`112
`
`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`*, independent on shear
`F i g . 6 – The complex modulus, Go
`strain amplitude, as a function of the mass fraction of xanthan
`in aqueous Na CMC-xanthan mixtures at a total polymer frac-
`tion of w = 1.0 %, obtained from oscillatory stress-sweep tests
`at 20 ± 0.3 °C
`
`F i g . 5 – The dependence of storage modulus, G’, and loss
`modulus, G’’, on shear strain amplitude, g°, for w = 1.0 %
`aqueous polysaccharide systems, a) pure Na CMC and pure
`xanthan solution, b) the Na CMC-xanthan mixtures: oscillatory
`stress-sweep tests at 20 ± 0.3 °C
`
`mixtures is also observed for rheological properties
`measured under non-destructive shear conditions.
`* , determined for the
`The complex modulus, G o
`Na CMC-xanthan mixtures, is lower than expected,
`and simultaneously slightly higher values of the
`phase angle, d
`, are observed for the Na CMC-xan-
`°
`than mixtures.
`
`By analyzing the results of oscillatory stress-
`-sweep tests for all examined polysaccharide sys-
`tems, the strain amplitude of 3 % was chosen to ap-
`ply in the frequency-sweep tests. Figure 8 repre-
`sents the mechanical spectra of pure Na CMC and
`pure xanthan solution used in our study as obtained
`
`F i g . 7 – The phase angle, d°, independent on shear strain
`amplitude, as a function of the mass fraction of xanthan in
`aqueous Na CMC-xanthan mixtures at a total polymer fraction
`of w = 1.0 %, obtained from oscillatory stress-sweep tests at
`20 ± 0.3 °C
`
`by applying the frequency-sweep experiments in
`the linear viscoelastic regime at a constant shear
`strain amplitude of 3 %. The strong frequency de-
`pendence of dynamic moduli as observed for the
`Na CMC solution with the viscous component, ex-
`ceeding the elastic one in the whole frequency
`range examined, indicates that the Na CMC solu-
`tion behaves as an entangled polysaccharide solu-
`tion.8 The xanthan solution exhibits the viscoelastic
`
`

`

`U. FLORJANCIC et al., Rheological Characterization of Aqueous Polysaccharide Mixtures …, Chem. Biochem. Eng. Q. 16 (3) 105–118 (2002)
`
`113
`
`F i g . 8 – The mechanical spectra of w = 1.0 % aqueous
`polysaccharide solutions of Na CMC and xanthan: the compar-
`ison between the experimental data (symbols), obtained from
`frequency-sweep tests at 20 ± 0.3 °C, and the calculated spec-
`tra (lines), determined by using the generalized Maxwell
`model
`
`properties usually observed for weak gel systems:
`the elastic response predominates over the viscous
`one, both dynamic moduli show only slight varia-
`tion with oscillation frequency, and the frequency
`dependencies of dynamic moduli become paral-
`lel.20,21
`
`When xanthan is added to the Na CMC solu-
`tion, the mechanical spectra change significantly,
`indicating the transition from the entangled polymer
`solution to a structured system (Figure 9). By in-
`creasing the mass fraction of xanthan, the dynamic
`moduli become less frequency-dependent and the
`elastic contribution gradually prevails over the vis-
`cous one.
`
`At a mass fraction of 0.25, xanthan slightly re-
`duces the loss modulus in the whole frequency
`range examined, as well as the storage modulus at
`high frequencies, while at low frequencies the stor-
`age modulus slightly increases (Fig. 9a). The pres-
`ence of xanthan in Na CMC solution is first notice-
`able in the low frequency range where both dy-
`namic moduli become less dependent on the oscilla-
`tion frequency. When xanthan is present at a mass
`fraction of 0.5, the intersection of dynamic moduli
`occurs, the elastic response exceeds the viscous one
`in the range of low frequencies, and the loss mo-
`dulus becomes less frequency-dependent at
`low
`