Langmuir 1991, 7, 469-472
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Sodium Dodecyl Sulfate Induced Enhancement of the Viscosity and Viscoelasticity of Aqueous Solutions of Poly(ethy1ene oxide). A Rheological Study on Polymer-Micelle Interaction Josephine C. Brackman Department of Organic Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands Received March 6, 1990. I n Final Form: August 1, 1990 A large enhancement of the viscosity and the viscoelasticity of aqueous solutions of high molecular weight poly(ethy1ene oxide) (PEO) has been observed upon addition of sodium dodecyl sulfate (SDS). The changes in apparent viscosity, in the parameters for the power law model of non-Newtonian behavior, and in the viscoelasticitystart at the critical concentration for formation of polymer-bound micelles and level off above the saturation concentration. These phenomena are attributed to coil expansion of PEO induced by binding of SDS micelles onto the polymer. A t high SDS concentration and high shear rate, the normal stress difference, which is a measure of the viscoelasticity,becomes almost independent of shear rate. This peculiar behavior is proposed to arise from partial shear-induced breakdown of the polymer-micelle complexes.
Introduction The complex formation between water-soluble polymers and, in particular, anionic micelles is nowadays a wellestablished phenomenon in surfactant chemistry.lt2 Many useful applications of these systems have been found in industrial products like paints, coatings, and cosmetics and in tertiary oil recovery. The majority of the studies on polymer-micelle complexes has focused on the association between sodium dodecyl sulfate (SDS) and poly(ethy1ene oxide) (PE0),2 even though more hydrophobic polymers such as poly(vinylpyrrolidone) (PVP),3-5 poly(viny1 methyl ether) (PVME),6 poly(propy1ene oxide) (PPO),7y8and hydroxypropylcellulose (HPC)g are known to participate in much stronger complex formation. However, it is the pronounced hydrophilicity of PEO that makes PEO/SDS complexation so intriguing, since the process is thought to require bonding of polymer segments at the hydrophobic core-water interface of the micelle.1° This type of interaction favors the micellization processl1J2and must often compensate for the expected unfavorable energy for transfer of the polymer segments from the aqueous phase to the surface of the mi~e1le.l~ (1)Breuer, M.M.;Robb, I. D. Chem Ind. (London) 1972,13,530. (2)Goddard, E. D. Colloids Surf. 1986,19,255. (3)(a) Garcia Lopez de Sa, T. Br. Polym. J. 1988,20,457.(b) Garcia Lopez de Sa, T.;Allende Riario, J. L.; Garrido, L. M. Eur. Polym. J. 1988, 54,493. (4)Lange, H. Colloid Polym. Sci. 1971,243, 101. (5)Perron, G.; Francoeur, J.; Desnoyers, J. E.; Kwak, J. C. T. Can. J. Chem. 1987,65,990. (6)(a) Brackman, J. C.; Engberta,J. B. F. N., J. Colloid.Interface Sci. 1989.132.250. (b) Brackman. J. C.: Eneberts, J. B. F. N.. submitted for publication in Langmuir. (7)Witte, F. M.;Engberts, J. B. F. N. J. Org. Chem. 1987,52,4767. (8) Witte, F. M.; Engberts, J. B. F. N. Colloids Surf. 1989,36, 417. (9)Winnik, F. M.;Winnik, M. A.; Tazuke, S. J.Phys. Chem. 1987,91, 594. .. (10)Cabane, B. J. Phys. Chem. 1977,81,1639. (11)(a) Nagarajan, R. J.Chem. Phys. 1989,90.1980.(b) Nagarajan, R.; Kalpacki, B. In Microdomains in Polymer Solution; Dubin, P., Ed., Plenum Press: New York, 1987;p 371. (12)Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987,3,382. (13)Brackman, J. C.; van Os, N. M.; Engberts, J. B. F. N. Langmuir 1988,4, 1266. ~
0743-7463f 91f 2407-0469$02.5Of 0
The influence of PEO on micellar properties2v5J4 like aggregation number,8@-17 critical micelle concentration,2 counterion binding,15 and solubilising powerl*Jg has been studied in some detail. It is also known,2 that above a molecular weight of 4000 PEO/SDS association is independent on molecular weight. Furthermore Nagarajan" and Ruckenstein12have both proposed quantitative models for polymer-micelle association, with which at least PEO/ SDS interaction is described satisfactorily. Surprisingly, the influence of SDS on the properties of the polymer remains rather unexplored, despite the wealth of research on PEO rheology.20$21 Several authors have reported rheological studies on the influence of SDS (among other surfactants) on the viscosity of aqueous solutions22 of PVP3J8JgJ!5 or PE0.11J8926,27 However nearly all these studies have been performed by using capillary (Ubbelohde) ~iscometry,~~~~~~8~1 which 9 ~ 2 5 ~is2 7best ~ 2 8 suited for the measurement of the viscosity of fluids under Newtonian flow. As a consequence little information has been obtained about the changes in viscoelasticity, which is a property of non-Newtonian fluids. The only exception, (14)Shirahama,K.; Himuro, A.;Takisawa, N. ColloidPolym. Sci. 1987, 265,96. (15)Zana, R.; Lang, J.; Lianos, P. In Microdomains in Polymer Solution; Dubin, P., Ed.; Plenum Press: New York, 1987;p 357. (16)Gilanyi, T.; Wolfram, E. In Microdomains in Polymer Solution; Dubin, P., Ed.; Plenum Press: New York, 1987;p 383. (17)Lissi, E. A.; Abuin, E. J. Colloid Interface Sci. 1985,105, 1. (18)Saito, S. J.Biochem. 1957,154,19. (19)Lange, H.Colloid Polym. Sci. 1971,243,101. (20)Bailey,F. E.; Koleske,J. V.Poly(ethyleneoxide);Academic Press: New York, 1976. (21)Braun, D. B.; De Long, D. J. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Eckroth, D., Eds.; Wiley: New York, 1982;Vol. 18,p 616. (22)Several authors have studied the influence of SDS on the rheology of cellulose derivatives like methyl cellulose23 and ethyl(hydroxyethyl) cellul0se.~4. In these cases the changes in viscosity are more complicated due to polymer aggregation in aqueous solution. (23)Sakamoto, N. Polymer 1987,28,288. (24)Carlsson, A.; Karlstrom, G.; Lindman, B.; Stenberg, 0. Colloid Polym. Sci. 1988,266,1031. (25)Fishman, M.L.; Eirich, F. R. J. Phys. Chem. 1975,79,2740. (26)Lance-Gomez, E. T. J . Appl. Polym. Sci. 1986,31, 333. (27)Franqois, J.; Dayantis, J.; Sabbadin, J. Eur. Polym. J. 1985,21, 165. (28)Bahadur, P.; Sastry, N. V.; Rao, Y. K.; Riess, G. Colloids Surf. 1988,29,343.
0 1991 American Chemical Society
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470 Langmuir, Vol. 7, No. 3, 1991
that was found, is a study by Lance-Gomez,26who used stress relaxation after cessation of steady-state flow to investigate the viscoelastic properties of PEO in aqueous solutions in the presence of salt or surfactant. He found, depending on the molecular weight fraction of the polymer, an increase or a decrease in stress relaxation (at a shear rate of 5 SI), upon addition of an alkyl benzenesulfonate. In 1957 Saito18reported the increase in viscosity of a PVP solution upon addition of surfactant measured by capillary viscometry. He proposed adsorption of mutually repelling surfactant molecules onto the polymer chain to explain the results. Later Langelg modified the model, and proposed adsorption of micelles, to account for the cooperative nature of PVP/SDS association. An increase in the viscosity upon SDS addition has also been found for PEO s o l ~ t i o n s . ~ ~ J ~ ~ ~ ~ The present paper describes a study of the rheology of an aqueous solution of 0.25% (w/w) high molecular weight (5 X lo6)PEO at various SDS concentrations using coneand-plate rheometry. The polymer concentration has been chosen with care to be well below the overlap concentration, but high enough to ensure a reasonable concentration range for polymer-micelle interactions2 Our equipment allowed the measurement of not only the shear rate dependence of the viscosity but also the viscoelasticity, which is a great advantage over capillary viscometry. Apart from the known increase in viscosity upon SDS addition, we find a concomitant increase in viscoelasticity. The power-law model proved adequate to describe the shear rate dependence of the viscosity. Furthermore the viscoelasticity data revealed a partial breakdown of the polymer-micelle complexes above a critical shear stress, which was not apparent from the corresponding viscosity data.
Experimental Section Materials. SDS (BDH, especially pure) and PEO (Aldrich, weight-averaged mol wt 5 x lo6) were used as received. Water was deionized and distilled twice. Rheological Measurements. Solutions were prepared several hours before the measurements by adding appropriate amounts of SDS to a0.25A (w/w) aqueous solution of PEO. The overlap concentration of PEO of mol w t 5 X lo6 is nearly twice as high as the employed concentration. During the 2 weeks that are necessary to ensure complete dissolution of the polymer, some degradation of the polymer will be unavoidable. Therefore, the actual molecular weight will be somewhat lower: thus the actual overlap concentration will be even higher. Rheological measurements were performed on a Brabender Rheotron rheometer with cone-and-plate geometry, equipped with a Normal F sensor, which allows the measurement of first normal stress differences. Although some destruction of the polymer chains was observed a t higher shear rates, the corresponding effects on the viscometric data are negligible compared to the overall effects of SDS addition. Every PEO/SDS solution was only used once. All measurements were performed a t 25 "C.
Results and Discussion A. Viscosity. The effect of SDS on the apparent viscosity (e.g. shear stress divided by shear rate) of the PEO solution at fixed shear rates is depicted in Figure 1. The curves clearly show three distinct regions, similar to previous results obtained by using capillary v i s c ~ m e t r y , whether ~ ~ , ~ ~at *~ low ~ (168.39-l) or high (2689.4 s-l) shear rate. In region I below the critical concentration for formation of polymer-bound micelles (cmpc,5.4 mM7) the viscosity changes only slightly upon addition of SDS. Besides polymer coils only free surfactant ions are present in the solution. Above the cmpc,in region 11, the viscosity increases considerably as electrostatic repulsion between the anionic micelles bound to the polymer causes the coils
App
VISC
.cP
8
2ol~. 15
0
0
0
I
0
I
,
10
20
40
30
50
[SDS] . mM Figure 1. Apparent viscosity as a function of the SDS concentration at different shear rates: ,. shear rate 168.3 s-l; 0,shear rate 2689.4 s-l. Table I. Non-Newtonian Parameters K and n of a n Aqueous PEO Solution (0.25% (w/w)) at Different SDS Concentrations at 25 'C ISDSI, mM K , (Pa n r 0 2.2 3.9 5.8 8.1 14.4 20.2 29.5 39.5 45.9
7.7 14.7 8.9 5.6 41.5 64.4 185.2 97.1 90.6 93.1
0.90 0.84 0.91 0.98 0.75 0.74 0.64 0.74 0.74 0.74
0.9982 0.9997 0.9992 0.9984 0.9983 0.9988 0.9987 0.9986 0.9982 0.9995
to expand. This increase of the viscosity can also be observed visually when swirling the solutions gently. Above 20 mM SDS (region 111)the viscosity levels off and even shows asmall decrease upon further addition of SDS. This concentration corresponds to the saturation concentration (cwt), at which the maximum number of SDS micelles is bound to the polymer. This concentration is in excellent agreement with cset determined from conductivity meas~rements.~ Further addition of SDS results in the formation of regular micelles. The slight decrease in viscosity was explained by FranGois et al.27in terms of the contraction of the extended coils, due to a decrease in electrostatic repulsion between the micelles as a result of the higher ionic strength. The apparent viscosity of the PEO solution a t a fixed SDS concentration drops with increasing shear rate especially at SDS concentrations above the cmpc. This is indicative of non-Newtonian behaviora29 Analysis of the data according to the power-law model (eq 1),27in which 7 represents shear stress, y is shear rate, and K and n are fitting parameters, shows a good fit (Table I). It should 7
= Krn
be noted that K and n exhibit extremes at 6 and 20 mM of SDS, corresponding to the cmpc and cset, respectively. The dependence of K on SDS concentration is similar to the concentration dependence of the apparent viscosity at fixed shear rate. The parameter n, which has a value of 1 for Newtonian liquids, decreases even further below (29) Bird, R. B., Curtiss, C. F., Armstrong, R. C., Hassager, O., Eds. Dynamics of Polymeric Liquids, 2nd ed.; Wiley: New York, 1987; Vols. I and 11.
PEO-SDS Rheology
Langmuir, Vol. 7,No. 3, 1991 471
Log ( f i r s t n o r m a l
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s t r e s s diffence, Pa
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I I
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o
I I I
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Figure 3. Viscoelasticity as indicated by the first normal stress difference as a function of the SDS concentration a t a shear rate
2.0
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of 952 s-l. First normal
t
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log ( s h e a r r a t e . s - ’ )
4 .O
t ., 14000
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12000
LO mM 30 m M 20 m M
Figure 2. Logarithm of the first normal stress difference, indicating viscoelasticity, as a function of the logarithm of shear
rate. For clarity each curve is shifted upward 0.2 with respect to the previous curve at lower SDS concentration. 0 , O mM; 2.2 mM; A,3.9 mM; A,5.2 mM; 0,8.1mM; , 14.4 mM; 0,20.2 mM; 0 , 29.5 mM; V, 39.5 mM; V, 45.9 mM SDS.
+
1 above the cmpc. It is evident that binding of SDS micelles onto the PEO polymer induces increasingly nonNewtonian behavior. B. Viscoelasticity. Apart from viscous flow an aqueous PEO solution also exhibits viscoelasticity. This is easily observed visually, for instance by viewing the recoil of trapped air bubbles when a swirling motion of the solution is abruptly stopped. The first normal stress difference that can be obtained by using a cone-and-plate viscometer is a measure of viscoelasticity. Figure 2 depicts the first normal stress difference as a function of the shear rate for various SDS concentrations. The enormous increase in first normal stress difference induced by SDS is even more obvious from Figure 3, which showsthe first normal stress difference at fixed shear rate. The same three regions as apparent from Figure 1can be distinguished. Obviously the viscoelasticity, as indicated by the first normal stress difference, is greatly enhanced when SDS micelles bind to the polymer. Another feature of Figure 2 is the almost constant first normal stress difference at the highest shear rates in the PEO solutions containing SDS concentrations near or above cast. From a plot of first normal stress difference against shear stress, Figure 4,it is apparent that this leveling off starts at about the same shear stress of 25 Pa. This phenomenon can be interpreted as a partial breakdown of the polymer-micelle complex, mediated by the shear stress or, more precisely, the hydrodynamic drag f0rce.~9 If the hydrodynamic drag force exceeds the force that keeps the micelles bound to the polymer, the micelles will be ripped off. This phenomenon induces the leveling off of the first normal stress differences. It must be emphasized that the binding of the micelles onto the polymer segments becomes weaker as the SDS concentration reaches cSat.30
10000
8000
6000
4000
2000
I
0
10
20
30
LO
50
shear s t r e s s , Pa
Figure 4. Relation between first normal stress difference and shear stress a t various SDSconcentrations. For clarity each curve is shifted upward 1000 P a with respect to the previous curve at lower SDS concentration: 0,0 mM; W, 2.2 mM; A, 3.9 mM; A, 5.2 mM; 0,8.1mM; +, 14.4 mM; 0,20.2 mM; 0,29.5 mM; V, 39.5 mM; V, 45.9 mM SDS.
The reason is that the electrostatic repulsion will increase whereas the stabilization of the hydrophobic core-water interface remains constant. A t first sight it seems surprising that the viscosity is not influenced in this region. However it is well-known that viscoelasticity is much more sensitive to shape and flexibility of polymer coils than viscosity.29 Conclusion It has been shown that SDS micelles greatly enhance the viscosity as well as the viscoelasticity of PEO in aqueous (30)Cabane, B.;Duplessix, R. Colloids Surf. 1985, 13, 19.
472 Langmuir, Vol. 7, No. 3, 1991
solution. This is attributed to coil expansion due to electrostatic repulsion between polymer-bound micelles. The solution shows increasing nonlNewtonian behavior at SDS concentrations above cmpc, as indicated by the power-law parameter n (eq 1). At SDS concentrations around and above csatthe viscoelasticity, measured as the first normal stress difference, levels off at high shear rate.
Brackman This behavior is thought to originate from a partial breakdown of the -polymer-micelle comdex. -
Acknowledgment. Professor Dr. J. B. F. N. Engberts, Dr. J. P. Sek, and Professor Dr. L. P. B. M. Janssen are gratefully acknowledged for helpful discussions. Registry No. SDS, 151-21-3; PEO, 25322-68-3.
