Langmuir 1991, 7, 1928-1934
1928
Revisit to the Intrinsic Viscosity-Molecular Weight Relationship of Ionic Polymers. 3. Viscosity Behavior of Ionic Polymer Latices in Ethylene Glycol/Water Mixtures Junpei Yamanaka,? Hideki Matsuoka,i Hiromi Kitano,t Norio Ise,*it Takuji Yamaguchi,? Susumu Saeki,t and Masakazu Tsubokawat Department of Materials Science, Fukui University, Fukui 910, Japan, and Department of Polymer Chemistry, Kyoto University, Kyoto 606, Japan Received November 2, 1990. In Final Form: April 1, 1991 The reduced viscosity of aqueous and ethylene glycol (EG)/water suspensions of ionic latex particles was measured in the presence and absence of a simple salt. The viscosity showed substantial shearthinning effects in both the water and binary systems. The reduced viscosity was generally much higher than the Einstein prediction and increased with increasing volume fraction of latex (4). The viscosity decreased with increasingsalt concentration. In the binary solventsthe viscosity decreased with increasing EG content (up to 60%). The fraction of free counterions (H+) was found to decrease, though slightly, with increasingEG content. This was also confirmed by the mobility measurements. We criticallyexamined existing interpretations of the concentration dependence of the reduced viscosity. It was demonstrated that there exists no direct correlation between the Debye length and the reduced viscosity. Furthermore, the possibility was recalled that, in low-salt macroionic and colloidal solutions, the Debye length loses its physical meanings as originallyfound for simple ionic systems. The intrinsic viscosity in water and binary solvents containing 10-4-10-5M KCl was in good agreement with that calculated by Booth's theory on the first-orderelectroviscouseffect,while the agreement was not satisfactoryat much lower KCl concentrations.
I. Introduction In parts 11and 22of this series, we reported the viscosity and dissociation behaviors of dilute aqueous suspensions of ionic polymer latices, and dilute aqueous solutions of sodium poly(styrenesu1fonate)(NaPSS). We revealed that (1)the viscosity of dilute aqueous suspensions of the ionic latices was much larger than Einstein's theoretical value because of the electroviscous effect, (2) the reduced viscosity increased with increasing concentration, and after reaching a maximum it decreased rapidly both for the aqueous suspensions of latices and for the aqueous solutions of linear polyelectrolytes, (3) there exists no distinct relation between the concentration dependence of reduced viscosity, qsp/4(4, volume fraction), and that of the fraction of free counterions, f , for aqueous suspensions of ionic polymer latices, and (4) the exponent a of the relation [ q ]= KMU ( K ,a constant;M, molecular weight; [ q ] , intrinsic viscosity) was not 2, at variance with the results reported earlier, but 1.2-1.6 for salt-free aqueous solutions of NaPSS,2 which suggests that linear polyelectrolytes such as NaPSS are not stretched out due to the electrostatic repulsive intramacroion interaction between the ionizable groups. It should be stressed that the conclusionthat flexible ionic polymer chains do not expand to a rodlike configuration in the absence of salt is not widely accepted. I t would be fair to mention that EisenbergSadvanced a negative view against the rodlike model. Most recently, Valleau showed that the rodlike configuration is not at all borne out by a Monte Carlo ~imulation.~ Furthermore, on the basis of Odijk's expression for the electrostatic component of the persistence length in the
* To whom correspondence should be addressed. + Fukui Universitv.
* Kyoto Universiiy.
(1)Yamanaka, J.;Matauoka,H.;Kitano,H.;Ise,N. J.Colloidlnterface Sci. 1990, 134, 92. (2) Yamanaka. J.: Matauoka. H.: Kitano. H.: Hasegawa. M.: Ise. N. J. Am. Chem. SOC.1990,112, 587. (3) Eisenberg, H. Biophys. Chem. 1977, 7, 3. (4) Valleau, J. P. Chem. Phys. 1989, 129, 163. '
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rodlike limit,6 Schmitz demonstrated that a flexible coil has a large probability of undergoing large bends in the absence of salt? However, the view that flexible macroions are not fully stretched has been the minority opinion. Various theories and interpretations have been proposed for linear polyelectrolytes on the assumption that macroions have a rodlike shape.' Clearly our finding, a # 2, shows that these theories cannot be the first principle in the discussion on physicochemical properties of polyelectrolyte solutions. Since the volume and the shape of latex particles are expected not to vary largely with changing latex concentration, the fact that the concentration dependence of the reduced viscosity of latex suspensions and linear polyelectrolyte solutions is similar, suggests that the concentration dependence of the reduced viscosity of the linear polyelectrolyte solutions may not be attributed only to a conformation change of the macroions. Instead, the firstand second-order electroviscous effects seem to play an essential role in the concentration dependence of reduced viscosity. In light of the rather surprising nature of the findings, we judged it important to confirm our viscosity data for other solvent systems. In this paper, we report the experimental results in ethylene glycol/water systems and discuss the solvent effects on the viscosity and dissociation behaviors for the latex suspensions. To our knowledge, no reports have been made on the electroviscous effect in latex suspensions of water/organic solvent systems. 11. Experimental Section A. Materials. Table I shows the properties of the latices we investigated. 1P30was purchasedfrom Dow Chemicals (Midland, MI) and N-100 was a product of Sekisui Chemical Co., Osaka, ( 5 ) Odijk, T. J. Polym. Sci., Polym. Phys. Ed. 1977, 15, 477. (6) Schmitz, K. S. Polymer 1990, 31, 1823. (7) As examples of representative theories along this line, we refer to only four works. For other articles, see ref 3. (a) Alfrey, T., Jr.; Berg, P. W.; Morawetz, H. J. Polym. Sci. 1951,7,543. (b) Fuoss, R. M.; Katchalsky, A.; Lifson, S. h o c . Natl. Acad. Sci. U.S.A. 1951, 37, 579. (c) Manning, G. S. J.Chem. Phys. 1969,51,924. (d) de Gennes, P. G.; Pincus, P.; Velasco, R. M. J.Phys. (Paris) 1976, 37, 1461.
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Langmuir, Vol. 7, No.9, 1991 1929
Viscosity-Molecular Weight Relationships Table I. Characteristics of Latices Studied diameter, charge no., charge density, latex le m (l@/particle) 10" C/cm2 1P30 0.11 0.5 0.21 N-100 0.12 13.0 4.61 Japan. Viscosity and conductivity measurements were performed for the 1P30 latex. Mobility measurements were carried out with the N-100 latex. Both were polystyrene-based latices with ionizable groups. The diameters are those reported by each producer. The surface charge density and charge number were determined by conductometric titrations. The presence of weak acid groups was confirmed to be negligible. The latices were purified by ultrafiltration and ion exchange as was described in part 1. Ultrafiltration of 1P30 was carried out by an Amicon Model 202 ultrafilter and a VM membrane (pore size 0.05 X 104 m, Amicon Co., Lexington, MA). N-100 was ultrafiltrated with a Model UHP-76 and a UK-10 membrane (exclusion limit M, = 10 000, Advantec Co., Tokyo). As deionization proceeded during the ion-exchange process, the 2.21 vol 76 1P30suspensionbecame iridescent, and the floating phenomenon of ion-exchange resin beads was observed, similar to the case for MS-5 and 1B76 latices reported in part 1. These observations suggest that ordered structures were formed in the 1P30 suspension. The concentration determinations of the stock suspension and the preparation of suspensions were performed in a similar way as described in part 1. The ion-exchange resin beads in the suspensions were eliminated by a nylon mesh, instead of a glass filter, in order to minimize contamination. The meshes were washed in an ultrasonic bath with methanol and water before use. The water was purified by ion exchange and distillation with an Aquarius GS-2ON System (Toyo Scientific, Tokyo) and subsequently by freshly and thoroughly washed cation- and anionexchangeresin beads (Amberlite MB-3, Organo,Tokyo)just prior to use. The water thus obtained had a specific conductivity of (0.5-0.6) X 1O-g S cm-l, which was practically the same as that of the water used in parts 1 and 2 (0.5 X 1O-g S cm-l), although the purification procedures were different. Amino acid analysis grade ethylene glycol (hereafter, EG) was purchased from Wako Chemical, Osaka, Japan. EG aqueous solutions (75 vol %) were prepared and cation- and anionexchange resin beads MB-3were added therein. After the mixture was shaken for a while, the specific conductivity of the solution decreased from 0.14 X 10" to 0.04 X 10" S cm-l. However, the specific conductivity of the 100% EG subjected to the same procedure did not change substantially even after being allowed to stand for a few days. This might be due to the rapid decrease of the electric mobilities of ions with increasingEG concentration, as will be mentioned below. Potassium chloride (Merck, Darmstadt, FRG) was of a Suprapur grade. After the potassium chloride was dried at 130 "C for at least 24 h, the stock aqueous solution (0.1M) was prepared. In parts 1 and 2, we used polyethylene bottles as containers to avoid contamination by ionic impurities from the glass wall. However, in this study we used Pyrex glass bottles washed by the ultrasonic device and rinsed with purified water because we feared that EG might extract organic impurities from the polyethylene bottles. To minimize contamination by carbon dioxide in the air, the bottles were filled with nitrogen gas during the suspension preparation. Nitrogen gas was further blown into the bottles after the preparation. B. Methods. Viscosity measurements were performed by using an Ubbelohde viscometer (type OA, Kusano Scientific, Tokyo) and a variable shear capillary viscometer, which were described in parts 1 and 2. The viscometers were cleaned by sulfuric acid and rinsed with purified water. The viscometers and other apparatus were washed with purified water just prior to use, conductivity of which was measured to examine whether the cleaning was satisfactory. The temperature was 25 f 0.02 "C. The air inside the viscometers was replaced by nitrogen gas. The details of the measurements and the determination of the viscosity were as described in part 1. The latex suspensions showed a non-Newtonian behavior at low salt concentrations.
Table 11. Properties of Ethylene Glycol/Water Mixture at 26 OC W, S cm2 esuivl [EG],vol % c no, C P S WH+ WK+ Wct ~ , g / m L 0 78.5 0.89 349.8 72.4 69.3 0.997 20 72.2 1.51 47.1 46.4 1.026 40 65.6 2.60 153.0 28.0 28.0 1.055 60 57.7 4.59 68.4 16.7 16.7 1.079 80 48.5 8.52 28.5 8.7 8.7 1.100 1.117 100 37.7 16.74 24.1 4.8 4.8 ~~
20
0
40
[EG]
60
80
(V%)
Figure 1. Electric conductivity,M, of EG/water mixtures plotted against EG concentration at 25 OC. Thus we used the variable shear viscometer. However, since this viscometer required fairly large amounts of the sample, the Ubbelohde viscometer was used even for non-Newtonian cases. The approximate nature of the results obtained was taken into consideration in our discussion. Conductivity was measured with a Horiba conductivity meter DS-14 (Kyoto) and a conductance cell having a pair of parallel platinum electrodes (cell constant, 1.17 cm-l). The temperature was controlled to be 25.00 f 0.05 OC. The electric mobility of the latex particles was measured by using the Penkem Model 501 lazer zee meter described in part I (Bedford Hills, NY) at room temperature. The suspension concentration was 5 x 10-6 vol %.
111. Results and Discussion
A. Properties of EG/ Water Mixtures. Table IIgives some characteristics of the EG/water mixtures at 25 "C, where e denotes the dielectric constant, 10is the viscosity, WHt, WKt, and Wcl- are the equivalent conductivities of H+, K+, and C1-, and p is the density. The e and p values were taken from the work by Tsurumi,8 in which the dielectric constant and density were obtained by interpolation of literature values while equivalent conductivities are those reported by Erdey-Grtiz et a1.: and we measured 10with an Ubbelohde viscometer. The viscosity of ionic polymer solutions and suspensions is strongly influenced by the ionic strength, as is widely recognized and was pointed out in part 2. Furthermore the H+concentration in the EG/water mixtures is expected to change with the mixing ratio. Therefore, we first determined experimentally the change of the ionic strength with mixing ratio. Figure 1shows the electric conductivity, p, of EG/water mixtures plotted against EG concentration, [EG]. The p decreased markedly with increasing [EG], owing to the decrease of the ionic conductivity with ~~~~
~
(8) Tsurumi, M. Mastera Thesis, Kyoto University, 1987,in which the z and p values were interpolated from literaturevalues in Kagaku Benran, 3rd ed.; compiled by the Chemical Society of Japan; Maruzen Publishing Company: Tokyo, 1984, and Handbook of Chemistry and Physics, 65th ed.; Weaat, R. C., Ed.; CRC Press: Boca Ratan, FL, 1984. (9) Erdey-Grh,T.; Majthhyi, L. Acta Chim. Hang. Tr0n.q. 1969,20, 175.
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increasing [EG]. The maximum at a small [EG] (
