Biomacromolecules 2001, 2, 946-951
946
Solution Rheology of K-Carrageenan in the Ordered and Disordered Conformations E. Pelletier, C. Viebke,* J. Meadows, and P. A. Williams North East Wales Institute, Centre for Water-Soluble Polymers, PP 21, Plas Coch, Wrexham LL11 2AW, U.K. Received March 21, 2001; Revised Manuscript Received May 17, 2001
The order-disorder conformational transition of κ-carrageenan, induced by both electrolyte and temperature, was found to give rise to significantly different rheological properties under shear flow, extensional flow, and small deformation oscillation regimes. Shear flow displayed only shear thinning or Newtonian behavior, depending of the chain conformation. A larger range of properties was observed in elongational flow. Strainthinning behavior was observed in the ordered conformation while strain thickening occurred in the disordered conformation. These results are discussed as a function of the chain conformation. Introduction κ-Carrageenan is a linear sulfated polysaccharide1,2 extracted from red seaweed and is used extensively in the food industry because of its ability to form gels.3 The backbone consists of a disaccharide repeat unit of β-D-galactopyranose residues linked glycosidically through the 1 and 3 positions and β-galactopyranose residues linked through the 1 and 4 positions. κ-Carrageenan has been shown to undergo a thermoreversible order-disorder conformational transition, which is strongly influenced by the presence of electrolyte.4 The ordered (helix) conformation is favored by increasing the ionic strength due to the reduction in intermolecular charge repulsion. It has been shown that certain cations, notably K+, Rb+, and Cs+, specifically bind to the helix thus promoting helix formation at higher temperature.5 The binding reduces the charge density along the helical chains and thus promotes helix aggregation and results in gelation.6 Interestingly, certain anions, notably I-, also bind specifically to the carrageenan helix7,8 resulting in an increase in the overall charge density along the helical chains and hence inhibiting chain aggregation and gelation.6 However, it has been observed9 that gels can be formed in 0.1 mol dm-3 NaI if the κ-carrageenan concentration is high enough. In examining the rheological characteristics of polymer molecules of different rigidity, it has been established that extensional flow can distinguish between flexible and rigid chains10 and thus is an appropriate tool to study the orderdisorder transition of κ-carragenan in solution. To the authors’ knowledge, experimental studies of helix-coil transitions in elongational flow have only been performed on poly(L-glutamic acid)11 and DNA12 by birefringence measurements. In this paper, we study the changes in rheological properties induced by the order-disorder transition in κ-carrageenan solutions using shear flow, elongational flow and viscoelastic measurements. Temperature variations are * Corresponding author. E-mail:
[email protected].
used to induce the order-disorder transition for a κ-carragenan solution in pure NaI. These results are also compared to those obtained in the disordered conformation induced by the use of NaCl salt. Materials κ-Carrageenan was obtained in the sodium form. The molecular mass distributions for the disordered and ordered forms were determined using asymmetric flow field flow fractionation coupled to multiangle laser light scattering (FFFF-MALLS), and the results have been reported elsewhere.13 The weight-average molecular mass, Mw, and radius of gyration, Rg, were determined to be 309 000 g mol-1 and 53 nm and 675 000 g mol-1 and 69 nm for the disordered and ordered conformations, respectively. The temperature of the coil-helix transition was determined by differential scanning calorimetry using a Setaram micro DSC II calorimeter equipped with 1 cm3 batch vessels. The samples were subjected to sequential heating/cooling cycles between 10 and 60 °C at a thermal scan rate of 0.5 °C min-1. The heat flow curves of the second heating cycle were recorded for 1.2% w/w κ-carrageenan solutions prepared in 0.1 mol dm-3 NaCl and in 0.1 mol dm-3 NaI. In both solutions, relatively broad endothermic transitions spanning a temperature range of approximately 5-10 °C were evident. The midpoint transition temperatures Tm of the coil-helix transition were determined to be 12 °C in the presence of 0.1 mol dm-3 NaCl and 38 °C in the presence of 0.1 mol dm-3 NaI. Methods Solutions of κ-carrageenan were prepared by adding the polymer (1.2% w/w) to aqueous electrolyte solutions (0.1 mol dm-3 NaI or 0.1 mol dm-3 NaCl) followed by heating to 80 °C with gentle stirring for 30 min to ensure complete dissolution. The samples were then left to equilibrate at room
10.1021/bm010060c CCC: $20.00 © 2001 American Chemical Society Published on Web 06/23/2001
Solution Rheology of κ-Carrageenan
temperature and used within 24 h. The critical polymer overlap concentration, C*, calculated from the molecular masses and radii of gyration (C* ≈ (Mw/Na)/(4/3πRg3) where Na is the Avogadro number) is around 0.08% for solutions containing disordered and ordered molecules. Hence solutions were in the semidilute regime. Shear flow viscosity and small deformation oscillation measurements were performed using a Carrimed CSL2 500 controlled stress rheometer. The rheometer was fitted with a 4 cm 2° cone and plate for the solution in NaI at 25 °C while a 5 cm 2° cone and plate was used at 7.5 °C. The measurements on the solutions in the disordered conformation were performed by using a double concentric geometry (rotor outer radius, 21.96 mm; rotor inner radius, 20.38; stator outer radius, 20 mm; cylinder immersed height, 20.5 mm). The samples were loaded onto preheated rheometer geometry and allowed equilibrating at the temperature of measurement for 5 min. The viscosity shear rate profiles of the samples at various temperatures were recorded over the range 0.04-2000 s-1 using a logarithmic stress ramp of 20 min duration. A strain sweep was performed prior to all oscillatory measurements in order to determine the appropriate linear viscoelastic region for each sample. The principle of frequency/temperature superposition14 was applied to the results obtained from small deformation oscillation measurements. All data was brought to the same temperature To by plotting GT/o ) G/TToFo/TF as a function of ωaT where F is the density of the polymer solution, and aT is the frequency shift factor needed to superimpose the curves. The subscript, o, indicates the reference temperature, which was set to 25 °C in our measurements. We have neglected any variation of the density of the polymer solutions, F, with the temperature. The extensional viscosity -strain rate profiles were determined using an RFX fluids analyzer (Rheometrics Inc., NJ), which generates extensional flow fields through use of opposing jets. In contrast to filament stretching types of extensional rheometers, opposing jet instruments enable extensional viscosity measurements to be performed on fluids that cannot sustain a continuous filament. The RFX instrument has been described in detail elsewhere,15 and only a short description of the experimental set up and evaluation of data is given below. A series of jet diameters (0.5, 1, 2, 4, and 5 mm) were used in this study to enable the extensional flow characteristics of the polymer solutions to be determined over approximately 4 decades of extensional strain rates (∼10-1 to 103 s-1). In all measurements the jet separation was set equivalent to the jet diameter.16 The sensitivity of the instrument enables apparent extensional viscosity of between approximately 10-1 to 104 Pa‚s to be measured. The temperature of the sample solution was controlled to within ( 0.1 °C by means of a thermostated water jacket. A comparison of the shear and extensional viscosity of the polymer solutions was achieved through calculation of the Trouton ratio (Tr) which may be defined as the ratio of the extensional viscosity at any given strain rate, ˘ , to the shear viscosity at a corresponding shear rate (γ˘ ) x3˘ ).
Biomacromolecules, Vol. 2, No. 3, 2001 947
Figure 1. G′ (full symbols) and G′′ (empty symbols) vs frequency for κ-carrageenan solutions in 0.1 mol dm-3 NaCl (b) and 0.1 mol dm-3 NaI (9) at 25 °C. The lines indicate the slope of 1 and 2.
The RFX instrument has been found to yield Trouton ratios of approximately 3.5-4 for Newtonian fluids.15,16 This slightly, but consistently, higher value than the theoretical value of 3 can probably be attributed to the fact that the measured torque is not solely due to the generated extensional flow. In particular, Schunk et al.16 have shown there to be a momentum transport from outside the region between the opposing jets. Results Small Deformation Oscillatory Measurements. The variation of G′ and G′′ as a function of frequency for 1.2% w/w κ-carrageenan solutions in the presence of 0.1 mol dm-3 NaCl and 0.1 mol dm-3 NaI at 25 °C are presented in Figure 1. The values of both moduli obtained for the κ-carrageenan in the ordered conformation (NaI) are at least 1 order of magnitude greater across all the frequency range studied than those corresponding moduli obtained with the polymer in its disordered conformation (NaCl). In addition, whereas G′′ dominates G′ throughout the frequency spectrum in the disordered conformation, the parameters exhibit a crossover point at an angular frequency of approximately 0.6 rad‚s-1 for the ordered conformation. The characteristic frequency of the G′/G′′ crossover corresponds to a relaxation time, τ, of approximately 1.6 s for the ordered conformation in solution. For the disordered conformation (NaCl) slopes of 1 and 2 are obtained for the frequency dependence of G′′ and G′ respectively enabling the characteristic parameters of the terminal relaxation area to be determined.14 The zero shear viscosity, ηo, is defined by the relationship: ηo ) lim ωf0 G′′/ω. An approximation of the longest relaxation time is determined at the characteristic angular frequency where the extension at high frequency of G′ ∝ ω2 and G′′ ∝ ω1 overlap. The zero shear viscosity of the 1.2%w/w solution of κ-carrageenan in 0.1 mol dm-3 NaCl at 25 °C was determined to be 0.047 Pa‚s with the characteristic relaxation time of the system being 3.4 × 10-2 s. The effect of temperature on the variation of G′ and G′′ with frequency of oscillation for the 1.2% κ-carrageenan
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Biomacromolecules, Vol. 2, No. 3, 2001
Figure 2. Storage (full symbols) and loss (open symbols) modulus as a function of frequency for different temperatures, (9) 7.5, (b) 25, (2) 40, and (1) 50 °C, of κ-carrageenan (0.1 mol dm-3 NaI). The storage modulus could not be obtained at 40 and 50 °C.
Figure 3. Master curve of the storage (full symbols) and loss modulus (open symbols) as a function of frequency, brought to 25 °C for κ-carrageenan solution in 0.1 mol dm-3 NaI. The different initial temperatures are indicated as follow: (9) 7.5, (b) 25, (2) 40, and (1) 50 °C.
solution in the presence of 0.1 mol dm-3 NaI is presented in Figure 2. It can be seen that significantly different viscoelastic characteristics are observed depending upon whether the carrageenan is in its ordered or disordered conformation. At temperatures lower than the coil helix transition ( 1. Table 1 presents various parameters characterizing the lowest observable data of apparent strain thickening behavior of the solutions of disordered κ-carrageenan. It can be seen that the calculated Reynolds numbers are significantly less than 1 with the exception of the measurements performed in NaI at 50 °C which display a value slightly higher than 1. This indicates that the observed strain thickening is a real reflection of the behavior of the chains within the solution. For κ-carrageenan solutions at 25 °C, the deviation of the Trouton ratio from the Newtonian value occurs at a lower critical strain rate, ˘ c, for the ordered conformation (0.3 s-1) than for the disordered conformation (20 s-1); see Figure 8. This is a reflection of the longer relaxation time for the polymer chains in the ordered conformation than in the disordered conformation. This behavior was also observed in shear measurements where relaxation times of 1.6 and 3.4 × 10-2 s were obtained for the ordered and disordered conformation of κ-carrageenan, respectively. These values are consistent with each other since ˘ cτ is expected to be 1/ x3 ) 0.5827 and the obtained values are 0.48 and 0.62, for the order and disorder conformation, respectively. The data obtained at 50 °C in NaI solution are an extension of the values obtained for the disordered κ-carrageenan molecules in 0.1 mol dm-3 NaCl at 25 °C but to higher strain rate (Figure 8). This can be taken as an indication that, at this higher temperature, all the molecules have attained their coil conformation. Conclusion The order-disorder transition of κ-carrageenan has been investigated using rheological measurements. The order-disorder conformational transition of κ-carrageenan was found to give rise to significantly different
Solution Rheology of κ-Carrageenan
rheological properties under shear flow, extensional flow and small deformation oscillation regimes. In this respect, the extensional flow behavior of the two conformational states was particularly divergent, with strain thinning behavior being observed for the κ-carrageenan in the ordered state while strain thickening was observed for the molecules in the disordered state. These measurements show that the chain rigidity of the κ-carrageenan molecules is significantly affected by the conformational transition, which supports the idea largely demonstrated in the literature of the disordering process being from semirigid helix toward flexible chains. To our knowledge, that is the first time that such evidence has been shown by elongational measurements. References and Notes (1) Rees, D. A.; Morris, E. R.; Thom, D.; Madden, J. K. Shapes and interactions of carbohydrate chains. In The Polysaccharides 1; Aspinall, G. O., Ed.; Academic Press: New York, 1982; pp 195290. (2) Painter, T. J. Algal polysaccharides. In The Polysaccharides 2; Aspinall, G. O., Ed.; Academic Press: New York, 1983; pp 195285. (3) Guiseley, K. B.; Stanley, N. F.; Whitehouse, P. A. In Industrial Gums; Davidsson, R. L., Ed.; McGraw-Hill: New York, 1980; Chapter 5. (4) Piculell, L. Gelling Carrageenans. In Food Polysaccharides and their Applications; Stephens, A. M.; Ed.; Marcel Dekker: New York, 1995. (5) Piculell, L.; Zhang, W.; Turquois, T.; Rochas, C.; Taravel, F. R.; Williams, P. A. Carbohydrate Res. 1995, 265. (6) Viebke, C.; Borgstro¨m, I.; Carlsson, I.; Piculell, L.; Williams, P. A. Macromolecules 1998, 31, 1833. (7) Zhang, W.; Piculell, L.; Nilsson, S. Macromolecules 1992, 25, 6165.
Biomacromolecules, Vol. 2, No. 3, 2001 951 (8) Grasdalen, H.; Smidsrød, O. Macromolecules 1981, 14, 1843. (9) Chronakis, I. S.; Piculell, L.; Borgstro¨m, I. Carbohydr. Polym. 1996, 31, 215. (10) Odell, A. A.; Keller, A.; Atkins, E. D. T. Macromolecules 1985, 18, 1443. (11) Hayakawa, I.; Sasaki, N.; Hibichi, K. J. Appl. Polym. Sci. 1995, 56, 661. (12) Hayakawa, I.; Hibichi, K. Polymer 1998, 39, 1393. (13) Viebke, C.; Williams, P. A. Food Hydrocolloids 2000, 14, 265. (14) Ferry J. D. Viscoelastic properties of polymers; John Wiley&Sons Inc.:, New York, 1980. (15) Meadows, J.; Williams, P. A.; Kennedy, J. C. Macromolecules 1995, 28, 2683. (16) Schunk, P. R.; de Santos, J. M.; Scriven, L. E. J. Rheol. 1990, 34, 387. (17) Morse D. L. Macromolecules 1998, 31, 7044. (18) Goncalves, M. P.; Gomes, C.; Langdon, M. J.; Williams, P. A.; Viebke, C. Biopolymers 1997, 41, 657. (19) Wakabayashi, Y.; Ito, K.; Li, H.-L.; Ujihira, Y.; Hashimoto, H.; Matsui, H.; Chiba, A.; Jean, C. J. Radioanal. Nucl. Chem. 1996, 211, 119. (20) Chronakis, I. S.; Doublier, J.-L.; Piculell, L. Int. J. Biol. Macromol. 2000, 28, 1. (21) Osaki, K. Stud. Polym. Sci. 1988, 2, 185. (22) Milas, M.; Rinaudo, M.; Knipper, M.; Schuppiser, S. L. Macromolecules 1990, 23, 2506. (23) Andrews, N. C.; McHugh, A.; Schieber, J. D. J. Rheol. 1998, 42, 281. (24) Yuzuu, N. Y.; Doi, M. Polym. J. 1980, 12, 883. (25) Lapassin, R.; Prici, S. Rheology of industrial polysaccharides; Lippincott Williams & Wilkins: Philadelphia, PA, 1999. (26) Dontula, P.; Pasquali, M.; Scriven, L. E.; Macosko, C. W. J. Rheol Acta 1997, 36, 429. (27) Jones, D. M.; Walters, K.; Williams, P. R. Rheol. Acta 1987, 26, 20.
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