Linear Rheology of Guar Gum Solutions.docx

1、Linear Rheology of Guar Gum SolutionsLinear Rheology of Guar Gum SolutionsRoland H. W. Wientjes,* Michel H. G. Duits, Rob J. J. Jongschaap, and Jorrit MellemaRheology Group, Department of Applied Physics, University of Twente, (member Twente Institute of Mechanics and J. M. Burgers Centre), P.O. Box

2、 217, 7500 AE Enschede, The NetherlandsReceived June 20, 2000; Revised Manuscript Received October 17, 2000ABSTRACT: We have investigated the linear viscoelastic behavior of guar gum solutions as a function of frequency, temperature, polymer concentration, and molecular weight. This was done to sort

3、 out the importance of different relaxation mechanisms like reptation or the breakup of physical bonds. In the kilohertz regime, Rouse behavior is observed. At lower frequencies, two storage modulus plateau zones were found, indicating two additional relaxations. One is operative between 1 and 100 H

4、z and gives rise to a very broad relaxation spectrum, even for monodisperse guar. Describing the dependencies of the relaxation time and low-shear viscosity on concentration and molecular weight with power laws resulted in unusually high coefficients. The second relaxation becomes manifest below 0.0

5、1 Hz and has not been earlier reported. Here the temperature dependence is very strong whereas all other dependencies are weak. Analyzing the experiments with existing models for transient polymer networks revealed that at best a partial decription of the experimental dependencies can be obtained. I

6、t was concluded that at least two different relaxation mechanisms must play a role, classical reptation not being one of these. Best overall predictions were obtained with a model assuming two types of associations. However, also the picture of star polymer-like structures held together via bonds wi

7、th a long lifetime could give comparable predictions. For a further distinction between these mechanisms, more information about the mesoscopic structure is needed.10.1021/ma001065p CCC: $19.00 2000 American Chemical SocietyPublished on Web 12/01/20001. IntroductionGalactomannans are water-soluble p

8、olysaccharides found in the seed endosperm of a variety of legumes. They consist of a (14)-linked /3-D-mannopyranosyl backbone partially substituted at O-6 with a-D-galac- topyranosyl side groups.6 One galactomannan which is widely used as an industrial hydrocolloid is guar gum which has a mannose:g

9、alactose ratio of 1.55. In connection to its use as a thickener in food products, several research groups have investigated the rheology of guar gum solutions.8,9,23,26,27In a rheological study performed by Ross-Murphy,28 where start shear behavior and the validity of the Cox- Merz rule were investi

10、gated, it was concluded that guar gum solutions behave like an entangled solution, as described by Doi and Edwards.7 This conclusion was drawn despite earlier observations of Richardson and Ross-Murphy26 who noted the onset of a transition at low shear rates (0.01 s-1), although the Newtonian low- s

11、hear plateau had already been reached. Robinson et al.27 mentioned a strong nonlinear dependence of the specific viscosity upon concentration. From this it was concluded that not only purely topological entanglements, but also specific attractive polymerpolymer interactions must play a role. Indicat

12、ions for this were also obtained by Goycoolea et al.15 and by Gidley et al.,13 who attributed a crucial role to the a-D-galactose side groups in the process of network cross-linking by semi helixhelix aggregation.From this short overview, it is clear that the rheological behavior of guar gum solutio

13、ns is still incompletely understood and that specific polymer polymer interactions might play a role in the observed rheological behavior as well as reptation phenomena. To sort out the importance of different relaxation mechanisms, we have systematically investigated the linear viscoelastic behavio

14、r as a function of frequency, concentration, temperature, and molecular weight. To support interpretation we have characterized the molecular properties by using GPC, intrinsic viscosity measurements, and several microscopy techniques. In this paper, we will compare the linear viscoelastic behavior

15、with predictions from existing microrheological models that take into account topological constraints and/or physical bonds.The paper is further organized as follows. In section 2, the preparation of the solutions, the microscopic characterizations and the rheological measurement techniques are disc

16、ussed. In section 3, the experimental results are shown. These results will be compared to rheological models in section 4, followed by a discussion in section 5, after which conclusions will be drawn in section 6.2. Experimental Section2.1. Materials. Guar gum (Meyhall) was purified from a commerci

17、al flour using a modification of the method of McCleary et al.4 Crude guar gum (10 g) was treated with 200 mL of boiling, aqueous 80% ethanol for 10 min. The obtained slurry was collected on a glass filter (no. 3) and washed successively with ethanol, acetone, and ether. This material was added to 1

18、 L of demiwater and allowed 1 h to hydrate. It was then stirred with a food blender (125 W), homogenized (1 min), and centrifuged at 2300g for 15 min. The supernatant was precipitated in two volumes of cold acetone. After redissolving in hot water, the polymer solution was ultracentrifuged at 82000g

19、 for 1.5 h at room temperature. The supernatant was precipitated with two volumes of ethanol. The precipitate was collected on a glass filter (no. 4) and washed with ethanol, acetone, and ether before freeze-drying. This lead to almost monodisperse purified guar gum. Only one batch of guar was purif

20、ied in this laborious way, to get a monodisperse system. Solutions of this material were prepared by adding known weights of the dry guar to twice distilled water, and allowing it to hydrate for extended periods (several days) to ensure that the sample had completely dissolved. This was done at aTab

21、le 1. Molecular Weights for Samples Prepared via Procedures I and IIguarMw (kD)Mw/MnHM10481.0215014001.19010001.5303501.7temperature of 277 K for concentrations between 0.4 and 2.0% (w/w). We will refer to this purification and dissolving method as procedure I.For the purpose of characterization wit

22、h Mark Houwink plots, three guar gums with different molecular weights (Meyhall) were purified with a less laborious but otherwise similar procedure. Here crude guar gum (10.0 g) was suspended in 1 L of demiwater and stirred with a food blender (125 W) for 1 min. The obtained solution was placed in

23、a refrigerator for 24 h and then centrifuged at 22000 for 5 h. Then, 800 mL of the obtained supernatant was precipitated in two volumes of cold acetone. The precipitate was collected on a glass filter (no. 4) and washed with ethanol, acetone, and ether. The so obtained purified guar gum was freeze-d

24、ried and dissolved as described in procedure I. This procedure is called procedure II.Using these procedures led to very long dissolving times. To shorten this, a third purification and dissolving procedure was used. In this procedure four different guar gums (Meyhall) with different molecular weigh

25、ts were purified by adding 10.0 g to 400 g acetate buffer of pH 4.66 (Merck). The slurry was homogenized for 75 s with a food blender (500 W) and centrifuged at 22000 for 5 h at room temperature. The supernatant (typical concentration 2% w/w) was used as a stock solution from which lower concentrati

26、ons were obtained via dilution. This is procedure III. Several control experiments revealed that the rheological behavior was not significantly changed on switching from procedure II to III.2.2. Molecular Characterization. The molecular weights of the purified guar gums using procedure I and II were

27、 determined by GPC-MALLS-RI (multiangle laser light scattering). The guar was dissolved in a 50 mM phosphate buffer with pH 8.0 to a concentration of 0.1% (w/w) and filtered through a 0.45 m filter prior to injection. At the exit of the GPC column the (instantaneous) values of M and Rg were detected

28、 on line. The results are summarized in Table 1.The mannose/galactose ratio of guar HM/150/90/30 was determined by HPLC after hydrolysis of the polymer to be 1.59 0.05.Molecular weights of the guar gums, obtained via procedure II, were obtained from intrinsic viscosity measurements. These experiment

29、s were done with an Ubbelohde capillary viscometer (Scott, type 532 01/0A).From the reduced viscosity measurements, intrinsic viscosities were obtained using the Huggins equation:n -1nred = 1Y = n + Acn2c (1)Here c is the concentration guar gum, nred is the reduced viscosity, ys is the solvent visco

30、sity and y is the intrinsic viscosity. hc is the so-called Huggins coefficient which is a polymer constant which usually lies in the range 0.5-0.8. The Huggins coefficient was determined for all the curves and turned out to be constant within the experimental error range: 0.55 0.05, which is within

31、the expected range. With the obtained intrinsic viscosities we made a Mark-Houwink plot (Figure 2) and found the Mark-Houwink constants AMH and a to be (6.7 1.1) x10-7 L/g and 1.05 0.01. This relation was used later as a calibration curve to determine the molecular weights from intrinsic viscosity m

32、easurements for the samples made by purification procedure III.The critical concentration where overlap between polymer chains starts to occur (c*) and the intrinsic viscosities for the, via preparation procedure III obtained samples, were obtainedFigure 1. Reduced viscosity measurements: (a) guar 30; () guar 30 duplo; () guar 90; (+) guar 150.Figure 3. Determination of c* of guar 150: () Ubbelohde measurements; (O) Contraves Low Shear 40 measurements.Table 2. n, Mw and c* for the Samples Prepared via Procedure IIIguarn (L/g)Mw (kD)c*(g/L)1501.2