Linear and non-linear flow behavior of welan gum solutions

Rheologica Acta https://doi.org/10.1007/s00397-018-1120-x ORIGINAL CONTRIBUTION Linear and non-linear flow behavior of welan gum solutions José A. Carmona 1 & Pablo Ramírez 1 & M. Carmen García 1 & Jenifer Santos 1 & José Muñoz 1 Received: 18 July 2018 / Revised: 22 October 2018 / Accepted: 30 October 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Rheological and microstructural properties of welan gum aqueous solutions were studied as a function of polymer concentration in the 0.2–0.6% (m/m) range at fixed temperature 20 °C. Welan gum is an exopolysaccharide produced by Sphingomonas sp. All the systems exhibited a shear thinning and weak gel–like behavior whose parameters were well adjusted to a power law with the concentration. Furthermore, time-concentration superposition methods were carried out to obtain two master curves, one from the flow curves and the other from the mechanical spectra, which made it possible to extend the accessible experimental range. The non-linear viscoelastic properties were also studied by means of parallel superposition tests. The superimposed shear stress induced a change from weak gel to entangled solution behavior. This latter flow behavior is characterized by a frequency crossover that has been shown to have a linear dependence on the superimposed steady-state shear rate. Finally, Cryo-SEM images revealed a network with numerous junction zones between polymer chains. Keywords Parallel superposition . Welan gum . Master curve . Cryo-SEM Introduction Welan is a non-gelling polysaccharide produced by Sphingomonas sp. Its structure is composed of a backbone chain with β-glucose, β-D-glucuronic acid, β-D-glucose, and α-L-rhamnose, with side chains containing either L-rhammose or L-mannose substituted on C3 of every 1,4-linked glucose repeating unit (Jansson et al. 1986; O’Neill et al. 1986). The welan gum exhibited a high value of viscosity at low shear rate and better thermal stability than xanthan gum (Kang et al. 1983; Sandford et al. 1984). The name of welan gum has its origin in its use in oil-field applications, such as hydraulic fracturing and secondary oil recovery processes (Kaur et al. 2014). Welan gum is also used in cement, food, and the pharmaceutics industry due to its thickening, binding, and emulsifying properties (Allen et al. 1990, 1991; O’Neill et al. 1986). Nevertheless, few studies have been carried out related to the rheological behavior of aqueous solutions of welan gum * José A. Carmona [email protected] 1 Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, C/ P. García González, 1, E41012, Seville, Spain (Xu et al. 2013) or comparisons of the rheological properties of aqueous welan gum and xanthan gum solutions (Tako and Krriaki 1990). For this reason, in order to expand the potential applications of this hydrocolloid, it is important to delve into the flow and viscoelastic properties of welan gum solutions. In addition to the more traditional rheological characterization, in the present work parallel superposition measurements have been carried out. This technique is described as a mechanical spectroscopy on flowing systems (Vermant et al. 1998), thus allowing the flow-induced microstructural changes of a polymeric network to be evaluated. Specifically, nonlinear viscoelastic properties of samples, once steady-state shear flow has been developed, can be studied by superimposing a small amplitude oscillatory shear perturbation parallel to the main flow direction according to the following equation: γ˙ ðtÞ ¼ γ˙ S þ γ˙ 0 cosωt ð1Þ where γ˙ S is the steady-state shear rate and γ˙ 0 cosωt is the superimposed small amplitude oscillatory shear perturbation. After a stationary state has been attained, the 1–2 component of the stress tensor is given by: ˙ σðtÞ ¼ ηγ S þ σ0 sinðωt þ δÞ h 0 i

} ˙ ˙ ˙ ¼ ηγ ω; γ þ γ G ω; γ cosωt ð2Þ sinωt þ G S 0 m m II II Rheol Acta where η is the viscosity of the solution at the steady-state shear rate, γ0 is the amplitude of the strain and G’|| and G”|| are the in-phase and out-of-phase components of the complex parallel superposition modulus, G*||. Nevertheless, these moduli do not have the same physical meaning as the ordinary storage and loss moduli. Moreover, the influence of the steady-state shear rate on the parallel moduli has been shown to be fully marked, especially for lower frequencies, even leading to the occurrence of negative values of the moduli. Different mathematical models have been developed to accurately explain these experimental findings, taking into account that both flows (shear and oscillatory) are actually coupled (Booij 1966; De Cleyn and Mewis 1987; Laufer et al. 1975; Vermant et al. 1998; Yamamoto 1971). Therefore, the practical application of parallel superposition measurements is not straightforward. However, it has been shown that the frequencies where the loss tangent become 90° are related to the steady-shear rate, and for polymer melts this is also a function of their molecular weights (Booij 1966; Laufer et al. 1975). Furthermore, it has been shown that the time-temperature superposition principle also applies for parallel superposition measurements (Somma et al. 2007). Thus, a careful analysis of parallel superposition data can provide useful information regarding the non-linear viscoelastic properties. The overall objective of this article was the study of the effect of polymer concentration on the rheological properties of welan gum aqueous solution. For this proposal, stepwise and creep measurements were used to investigate the flow behavior. Also, parallel superposition tests were carried out to study the evolution of viscoelastic properties when a shear stress is applied simultaneously. Alternatively, cryo-scanning electron microscopy techniques were applied, which enabled an analysis of the microstructure of welan aqueous solution without the need to remove its water. Materials and methods Materials A commercial powder welan, K1A96 “industrial grade” welan gum, kindly donated by CP kelco, was used. Ultrapure water, from a Milli-Q water osmosis system, was also used. Rheological measurements All measurements were carried out in a control stress rheometer (DHR3, TA instrument, USA). A serrated plate and plate geometry with 40 mm diameter was employed. All rheological tests were performed at 20 °C, using a glass cover to inhibit evaporation. All the tests were repeated three times. Stepwise flow curves from 0.01 to 20 Pa were run, selecting a steady-state approximation of 0.01% and a maximum measuring time of 2 min per point. The experimental data fitted well to the Carreau model (Carreau 1972) (R2 > 0.99). η¼ η0 1þ ðγ˙ c γ˙ Þ2 ð 1−n 2 Þ ð3Þ where γ˙ c is the critical shear rate for the onset of shearthinning response, n the flow index, and η0 the zero-shear viscosity. Linear and non-linear creep tests were carried out at different shear stress values for an experimental time which ranged from 15 to 30 min, depending on the applied shear stress. Parallel superposition tests were carried out in three steps: (1) samples were pre-sheared at a constant shear-stress value until a steady state was reached, (2) then an oscillatory shearstress sweep at a fixed frequency of 6.28 rad/s was superimposed in the same direction as the perturbation described in step 1 in order to determine the linear viscoelastic range (LVR), (3) subsequently, a frequency sweep within the LVR from 30 to 0.1 rad/s was carried out. Cryo-SEM images The Cryo-SEM was taken using a scanning electron microscope (ZEISS EVO SEM, ZEISS, Oberkochen, Germany) with an accelerating voltage of less than 5 kV at − 120 °C. The samples of welan gum solutions (0.2 and 0.4% (m/m)) were rapidly frozen into an open bath of liquid nitrogen at 77 K. These frozen samples were then transferred to the preparation chamber (Leica model ACE600) where they were fractured and sublimated (− 90 °C for 7 min). After the sublimation process, the samples were coated with a thin gold layer by sputtering. Solution preparation Welan gum solutions with concentrations of 0.2, 0.3, 0.4, 0.5, and 0.6% (m/m) were prepared by stirring at room temperature for 3 hours. The samples were then heated to 70 °C for 45 min and finally cooled to room temperature. All samples contained 0.05% (m/m) of sodium azide. The welan gum solutions were stored at 5 °C and allowed to rest for at least 1 day before the rheological characterization. Results and discussion Flow curves and creep tests Figure 1 shows the flow curves of the welan gum solutions at different polymer concentration. The data shown in these flow curves were obtained by two different methods. Creep tests Rheol Acta 0.1 Pa 0.2 Pa 0.3 Pa 0.5 Pa 0.8 Pa 1.2 Pa 1.5 Pa 2 Pa 3 Pa 4 Pa 4.5 Pa 5.5 Pa 6.5 Pa 8 Pa 2 10 J (1/Pa) 101 100 10-1 300 600 900 1200 1500 1800 time (s) Fig. 1 Shear rate dependence of steady-state viscosity obtained by combining creep compliance tests (closed symbols) and multistep flow curves (open symbol) for welan gum solutions with different gum concentration at 20 °C. The error bars shown correspond to standard deviation (closed symbols) were carried out for the lower shear rate values, whereas stepwise flow test (open symbols) were conducted for higher shear rates. All concentrations presented a shear thinning flow behavior with a Newtonian plateau for the lower values of shear rate and a shear-thinning region with increasing shear rate. The viscosity values from creep measurements were calculated by means of the following equation: ð4Þ where J is the shear compliance. The shear rate was calculated by the expression: γ˙ ¼ τ η0 ð5Þ Figure 2 shows the evolution of J as a function of time for 0.6% (m/m) welan gum solution. In the shear-thinning region, the viscosity values obtained by both creep compliance and stepwise flow tests coincide within the experimental error range. Therefore, we can conclude that the latter corresponded to steady-state values. Nevertheless, the viscosity values in the Newtonian plateau were a little higher than the viscosity values obtained from the stepwise test, indicating that the steady state has not been reached at the prescribed time for shear rates below 0.01 s−1 in the stepwise tests (Santos et al. 2013). Therefore, for low shear rates, the viscosity values obtained from creep compliance tests have been used to fit the flow Fig. 2 Compliance as a function of time for various shear stresses for a 0.6% (m/m) welan gum solution at 20 °C. The error bars shown correspond to standard deviation curve to the Carreau model (Carreau 1972) (R2 > 0.99), the values of the parameters of which are given in Table 1. The zero-shear viscosity η0 has a power law dependency on the concentration with an exponent of 3.7 ± 0.3 and this value corresponds to a semi-dilute regime, in agreement with the theoretical model of Dobrynin (ɳ ≅ C15/4) (Dobrynin et al. 1995; Rubinstein et al. 1994) for polyelectrolyte in solution. Furthermore, the exponent agrees with that obtained for xanthan gum 3.75 and 4.2 (Cuvelier and Launay 1986; Carmona et al. 2015; Milas et al. 1990; Rodd et al. 2000; Wyatt et al. 2011). In addition, the power law index, n, decreased with increasing welan gum concentration. This increase in the consistency of the system may be due to the increase in interaction between the polymer chains as a result of increasing the concentration of biopolymer (Wyatt et al. 2011). The higher values of viscosity and thickening capacity of welan gum compared to similar gums are a consequence of its higher water retention in the double helix structure (Pourchez et al. 2006; Sonebi 2006). From creep tests, the equilibrium compliance parameter, J°e, can also be obtained. This parameter provides information on the elasticity of the system (Barnes 2000) and can be obtained by a linear extrapolation to zero time of the compliance data once the steady state was reached (Fig. 2). The intercept is J°e, which can be related to the elastic elements by: J 0e ¼ γ0 1 ¼ ∑ni¼1 σ Gi ð6Þ Figure 3 shows the variation of the η0 and the Je0 obtained from the creep results. The increase in polymer concentration produces an increase in interactions between the polymer chains, which produces an increase in viscosity and elasticity expressed Rheol Acta Table 1 Carreau model fitting parameters for welan gum solutions at 20 °C (R2 > 0.99) Conc. % (m/m) ɳ0 (Pa s) γ˙c (s−1) N 0.2 0.3 0.4 0.5 0.6 31.5 151 409.6 1338 2355 9.5 × 10−03 ± 7 × 10−04 2.27 × 10−03 ± 5 × 10−05 1.92 × 10−03 ± 4 × 10−05 8.55 × 10−04 ± 1.2 × 10−05 6.69 × 10−04 ± 1.8 × 10−05 0.307 0.304 0.264 0.227 0.238 ± ± ± ± ± 0.3 7 2.3 40 60 as the inverse of Je0. Both parameters were fitted with a power law model whose values are given in the insets of Fig. 3. As a similar flow, behavior was observed in all the concentrations studied (Fig. 3), a master flow curve could be obtained as shown in Fig. 4. The 0.4% concentration (m/m) was taken as the reference concentration, Cref. The master curve is obtained by representing the normalized viscosity, i.e., η/η0, where η is the apparent viscosity obtained from the creep and/or steady-state flow tests and, η0 is the zero-shear viscosity (see Table 1) versus the shear rate multiplied by a factor of displacement, ac, which is a function of concentration. A potential dependence of the ac parameter was obtained with respect to the polymer concentration, C: ac ~ (C/Cref)1.73. The master curve thus obtained provides information at lower and higher shear values than those that are experimentally accessible (Calero et al. 2010). SAOS (small amplitude oscillatory shear) and parallel tests The mechanical spectrum of the welan gum suspensions is shown in Fig. 5. A weak gel–like behavior was observed for all concentrations with the storage modulus G’ greater than the loss modulus G” in the whole frequency range studied and Fig. 3 Viscoelastic parameters (Je0 and η0) vs. welan concentration at 20 °C. The error bars shown correspond to standard deviation. The line shows data fitting to the power law model whose parameter is given in the inset R2 ± ± ± ± ± 0.017 0.008 0.004 0.003 0.004 0.995 0.994 0.996 0.992 0.993 with a small dependency of both moduli on the frequency. An increase of concentration leads to the rise of both moduli and a decrease of the slope with frequency. This feature is typical of a weak gel–like behavior, which is likely to be due to the intermolecular association between the L-rhamnosyl residues of different molecules via van der Waals forces, and between side and backbone chains of different molecules via hydrogen bonds (Member and Morris 1995; Morris et al. 1996). The sample with a 0.2% (m/m) concentration of polymer is close to the crossover frequency which is related to the beginning of the terminal relaxation zone. A master curve for the linear viscoelastic tests was obtained as a function of the concentration (Fig. 6). The frequency concentration superposition allows the extrapolation of G’ and G” values at frequencies that would otherwise be inaccessible (Larson 1999). Taking as a reference the solution at 0.4% (m/m), the moduli were normalized by a factor bc = Cref/C and by a horizontal factor ac, which follows a potential dependence with an exponent of 4.1. In the parallel superposition tests, the sample is subjected simultaneously to an oscillatory shear stress and a steady shear flow with a constant shear rate γ˙s. Thus, the parallel superposition technique combines typical small amplitude oscillatory Fig. 4 Master flow curve at the reference concentration of 0.4% (m/m) welan at 20 °C. The error bars shown correspond to standard deviation. Power law model for ac shifting factor is given in the inset Rheol Acta Fig. 5 Mechanical spectra for welan gum solutions with different gum concentration at 20 °C. T = 20 °C. The error bars shown correspond to standard deviation tendency to orient themselves in the direction of shearinduced flow, weakening the entanglements and the gel structure. On the other hand, in Fig. 7b, the values of the parallel modules, G’II and G”II, as a function of frequency for the solution containing 0.4% (m/m) welan are shown. At shear rate values lower than ca. 2·10−3 s−1, i.e., within the plateau zone corresponding to the zero-shear viscosity, no differences were observed between the parallel modules and the normal ones. In this zone, no disruption of the polymer network occurs, and the viscoelastic properties remained unaltered. When parallel shear rate values exceeded those limiting the zero-shear viscosity plateau region (γ̇ > 2·10−3 s−1) the obtained viscoelastic parallel modules differed from the normal values. Moreover, the storage parallel modulus was more affected than the viscous one, this difference being more marked at low frequencies. The phase angles at low frequency present value higher than 90° which come from negative values of storage moduli (data no shown). Therefore, the data at these low frequencies are not reliable. shear experiments in the presence of a steady-state flow. Although, it takes in consideration that the parallel superposition moduli do not have the same physical meaning as ordinary storage and loss moduli (Vermant et al. 1998). Similarly, to the standard protocol for small amplitude oscillatory shear (SAOS) tests, the LVR was calculated by means of oscillatory viscoelastic stress sweeps at 1 Hz. Figure 7a shows the results obtained for the solution containing 0.4% (m/m) welan by way of example. The values of the critical shear stress and strain amplitudes that limit the LVR decreased with an increase in the parallel steady-shear stress applied. This may be the result of the fact that the polymer chains are likely to show a Fig. 6 Master curve of storage modulus and loss modulus for welan gum solutions at the reference concentration of 0.4% (m/m). Temperature 20 °C. The error bars shown correspond to standard deviation. Power law model for ac-shifted factor is given in the inset Fig. 7 a Critical stress and strain for various shear stresses as a function of parallel stress. b Parallel storage modulus G’II and parallel loss modulus G”II as a function of frequency. For a 0.4% (m/m) welan gum solution at 20 °C. The error bars shown correspond to standard deviation Rheol Acta Thus, in the present work, we have focused in the analysis of the crossover viscoelastic modules, G’II and G”II, which occur at a characteristic frequency, ωc, higher than the nonreliable negative value region. It is commonly assumed that the onset of a crossover frequency when moving towards lower frequencies is related to a change in the viscoelastic properties of the system, consistent with a shear-induced change from weak gel solution to a polymer-entangled solution. It must be noted that this structural transition shifted towards higher angular frequencies when the parallel shear rate applied increased, as previously reported for other polymers (Booij 1968; Tirtaatmadja et al. 1997). This behavior has been explained by a change in the contribution of the terms associated with longest relaxation times (Mewis and Biebaut 2001; Ianniruberto and Marrucci 2014). For comparative purposes, Fig. 8 shows the values of the crossover frequencies as a function of shear rate for all the welan gum solutions. The crossover frequency is related to the terminal relaxation time, which is a characteristic parameter of the material, i.e., shorter terminal relaxation times imply greater elasticity and more structured systems. It can be seen that regardless of the welan gum concentration, the same behavior was obtained, i.e., the crossover frequency increases linearly with the shear rate superimposed with a slope of ca. 10. Therefore, the following relationship holds: ωC ¼ a γ˙ ð7Þ where a can be defined as a viscoelastic parameter that accounts for the influence of the steady shear rate on the crossover frequency of parallel superposition measurements Fig. 8 Crossover frequencies as a function of steady-state shear rate values for all the welan gum solutions studied. The error bars shown correspond to standard deviation. The line shows data fitting to the power law model whose parameter is given in the inset for a particular material. To see to what degree Eq. (7) is valid, we have compared the influence of shear rate in the crossover frequencies for some weak gel systems: xanthan gum (unpublished work), and high molecular poly-(isobutene) (PIB) in decalin (Vermant et al. 1998). Figure 9 shows the ωC for the three systems as a function of the shear rate (for the sake of clarity only the values for 0.4% welan concentration are displayed). It is seen that the data can be fitted to Eq. (7) with values of parameter given in the table insert in the figure. These experimental results show that the study of the crossover frequencies of the parallel superposition measurements can be a convenient tool to characterize and compare the viscoelastic response in the presence of shear stress for different materials. Cryo-SEM images Finally, Cryo-SEM images were obtained. This technique enables an exploration of the natural microstructure of aqueous solutions, since it is not necessary to eliminate water, a condition that must be fulfilled in the case of other microscopy techniques. (Aston et al. 2016). Figure 10 shows the micrographs obtained by Cryo-SEM at two different concentrations of welan gum at two different micrograph magnifications. The images show a cell block–like structure in which there is a clearly visible network structure which become denser when the polymer concentration was increased, as expected from the rheological data. Fig. 9 Crossover frequencies as a function of steady-state shear rate values for (1) welan gum solutions 0.4% (m/m), (2) xanthan gum 0.4% (m/m), and (3) 4% (m/m) high molecular poly-(isobutene) (PIB) in decalin (Vermant et al. 1998). The error bars shown correspond to standard deviation. The line shows data fitting to the power law model whose parameter is given in the inset Rheol Acta Fig. 10 Cryo-SEM micrographs for welan gum solutions with different gum concentration and magnification a and b, 0.2% (m/m); c and d, 0.4%(m/m) Conclusions It has been shown that creep compliance tests make it possible to determine a more reliable zero-shear viscosity, as demonstrated for welan gum/water systems in the (0.2–0.6) % m/m range. All the studied systems exhibited a zero-shear Newtonian plateau region followed by a shear-thinning region, in which zero-shear viscosity and consistency increase with increasing polymer concentration. Moreover, parallel superposition tests provide information on viscoelastic properties in experimental conditions that are closer to real ones. Therefore, this technique is a useful tool to study the changes induced by shear on the sample. It was shown that at low superimposed shear rates the parallel viscoelastic modules G’II and G”II were unchanged. Nevertheless, for shear rates exceeding the critical shear rate, both modules vary with the superimposed shear rate and a crossover frequency can be observed. This crossover frequency, which is related to a change from weak gel to entangled solution behavior, has the same linear dependence on the applied shear rate for all the studied systems. It has been shown that the crossover frequencies of parallel superposition measurements increase linearly with the superimposed shear rate. Furthermore, this behavior has been also observed for two other weak gel type systems but differing in the value of the slope. Therefore, the analysis of crossover frequencies has been proven to be a useful tool to characterize and compare the viscoelastic response in the presence of shear stress for different materials. 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