Langmuir 1994,10,2471-2479
2471
Solution Behavior of Hydrophobically Modified Sodium Polyacrylate C. Senan, J. Meadows, P. T. Shone, and P. A. Williams* Faculty of Science, Health and Medical Studies, North East Wales Institute, Connuh‘s Quay, Clwyd CH5 4BR, U.K. Received January 31, 1994. In Final Form: May 2, 1994@ A number of hydrophobically modified sodium polyacrylates (HMPAA’s)have been synthesized and their solution and rheologicalproperties investigated by means of capillary viscometry, small deformation oscillation, and electron spin resonance (ESR) spectroscopy studies. In the presence of added electrolyte (0.1 mol dm-3 NaBr), capillary viscometry and ESR spin probe studies indicated the existence of intramolecular hydrophobic associations at low polymer concentrations, with a gradual transition to intermolecularassociations occurringat higher polymer concentrations. In the absence of added electrolyte, only intermolecular associations were observed, and the critical polymer concentration necessary for the formation of such associations was found to show little dependence on the hydrophobe chain length and content. In contrast to typical polyelectrolytebehavior, an increase in storage modulus, G , of the HMPAA solutions was observed upon the addition of relatively low concentrations of sodium chloride, although a decrease in G was observed at higher electrolyte concentrations. The rheological properties of mixed polymer/anionicsurfactant systems were found to be strongly dependent on the level of surfactant addition with both an increase or decrease in G‘ of the system being obtainable. The maximum achievable G’ of the various HMPAAfsurfactant systems increased with increasing alkyl chain length of added anionic surfactant but showed little dependence on the nature of the anionic head group. At ‘optimum” levels of surfactant addition,the originallyviscous polymer solutions exhibited gel-likecharacteristics. The addition of relatively low concentrations of the cationic surfactant cetyltrimethylammoniumbromide (CTAB) was found to induce phase separation of the anionic polyelectrolytes, in the form of a gelatinous precipitate. The amount of CTAJ3 required to induce phase separation of the HMPAA’s was greater than that required for phase separation of the precursor sodium polyacrylate. In all instances, the level of CTAB required to induce phase separation was at least an order of magnitude less than that required for a 1:l stoichiometry of interaction between the cationic surfactant molecules and the carboxyl groups of the polymers.
Introduction Hydrophobically associating water soluble polymers (HAWSP’s) are essentially hydrophilic polymers which also contain a small proportion (up to a few mole percent) of strongly hydrophobic groups, usually in the form of pendant side chains or terminal groups. In aqueous media, neighboring hydrophobic groups associate, thereby forming a three-dimensional network of polymer ~ h a i n s . l - ~ Evidence for the presence of these associations has been gained using spectroscopicprobe HAWSP‘s exhibit novel rheological behavior and this has led to their increasing commercial exploitation as rheology modifiers in industrial water based formulations such as latex paints’ and fluids for enhanced oil recovery.8 The recent increase in research activity which has accompanied their emerging industrial significance has resulted in synthetic routes being developed for a wide range of HAWSP’sbased on a variety of water soluble polymers such as poly-
* To whom correspondence should be addressed.
* Abstract published in Advance ACSAbstracts, June 15,1994.
(1) Water Soluble Polymers: Beauty with Performance, Glass, J.E., Ed.; Advances in Chemistry Series 213;American Chemical Society: Washington DC, 1986. (2)Polymers in Aqueous Media: Performance ThroughAssociation; Glass, J.E., Ed.; Advances in Chemistry Series 223;American Chemical Society: Washington DC, 1989. (3)Yekta, A.; Duhamel, J.;Adiwidjaja, H.; Brochard, P.; Winnik, M. A. Langmuir 1995,9, 881. (4) Wang, Y.; Winnik, M. A. Langmuir 1990,6,1437. ( 5 )Winnik, F.M.; Winnik, M. A.; Ringsdorf, H.;Venzmer, J.J.Phys. Chem. 1991,95,2583. (6)Tanaka, R.; Meadows, J.; Phillips, G. 0.; Williams, P. A. Carbohydr. Polym. 1990,12,443. (7)Hall, J. E.; Hodgson, P.; Krivanek, L.; Malizia, P. J . Coatings Technol. 1986,58, 65. (8)Bock, J.;Valint, P. L., Jr.; Pace, S. J.;Siano, D. B.; Schulz, D. N.; Turner, S. R. In Water Soluble Polymers for Petroleum Recovery; Stahl, G. A., Schulz, D. N., Eds.; Plenum Publishing: New York; 1988;p 147.
(ethylene ~ x i d e )acrylamide ~J~ based copolymers,ll-ls and polysac~harides.~~J~ The tendency for HAWSP’s to undergo association and, hence, exhibit their novel rheological behavior has, within the limits of water solubility, been shown to increase with increasing hydrophobe content and length11J6 and also with increasing “blockiness” of distribution of the hydrophobe along the backbone.” The increased surface activity of these polymers due to the presence of the hydrophobic groups also results in stronger interactions with other species such as surfactantsleJg and polymer particles20s21which has important implications for their use in complex colloidal formulations such as latex-based paints. In this study, we have used electron spin resonance spectroscopy and small deformation oscillation measurements to investigate the associative and rheological properties of a series of hydrophobically modified sodium (9)Fonnum, G.; Bakke, J.;Hansen, F. K. Colloid Polym. Sci. 1995, 271. 380. (10)Binana-Limbele, W.; Clouet, F.;Francois, J. Colloid Polym. Sci. m a , 271,748. (11)McCormick, C. L.; Nonaka, T.; Johnson,C. B. Polymer 1988,29,
.--.
721
(12)Biggs, S.;Selb, J.; Candau, F.Polymer 1998,34,580. (13)Schulz, D.N.;Kaladaa, J.J.;Maurer, J.J.; Bock, J.;Pace, S. J.; Schulz, W. W. Polymer 1@87,28,2110. (14)Landoll, L. M. J. Polym. Sci. A Polym. Chem. 1982,20,443. (15)Carre,M. C.;Delestre,C.;Hubert,P.;Dellacherie,E.Carbohydr. Polym. 1991,16,367. (16)Kuo, P.L.; Hung, M. N.; Lin, Y.H. J.Appl. Polym. Sci. 1995, 47,1295;1993,48, 1571. (17)Hill, A.; Candau, F.;Selb, J. Macromolecules 1998,26,4521. (18)Winnik, F.M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991,7 , 905,912. (19)Tanaka, R.; Meadows, J.; Williams, P. A,; Phillips, G. 0. Macromolecules 1992,25,1304. (20)Jenkins, R.D.; Durali,M.;Silebi, C. A;El-Aasser, M. S.J.Colloid Interface Sci. 1992,154,502. (21)Tanaka, R.;Williams, P. A.; Meadows, J.;Phillips, G. 0.Colloids Surf. 1992,66,63.
0743-7463/94/2410-2471$04.50/00 1994 American Chemical Society
Senan et al.
2472 Langmuir, Vol. 10, No. 7, 1994 polyacrylates of constant hydrophilic backbonebut varying hydrophobe content and length. The effects of the addition of electrolyte a n d various anionic and cationic surfactants o n t h e rheological properties of the polymers have also been investigated.
Experimental Section Materials. Polyacrylic acid (average molecular weight 250 000; powder form), dodecylamine, and octadecylamine were obtained from Aldrich Chemicals, Ltd., U.K., and were used as supplied. The anionic surfactants sodium octanoate, sodium decanoate, and sodium dodecanoate were obtained from Sigma Chemicals, Ltd., U.K. Sodium dodecyl sulfate (specially pure grade) and cetyltrimethylammonium bromide were obtained from BDH Chemicals, Ltd., U.K., and sodium decyl sulfate was obtained from Lancaster Synthesis,Ltd., U.K. All surfactants were used as supplied. The stable nitroxide free radicals 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl(4-amino-Tempo) and 5-doxylstearic acid (5DSA) were obtained from Sigma and were used as supplied. Methods. Preparation of Hydrophobically Modified Sodium Polyacrylates. A number of hydrophobically modified sodium polyacrylates (HMPAA's) were prepared in a manner based on that described by Wang et al.22 Samples were prepared containing either 3 or 6 mol % incorporation of either dodecylor octadecyl side chains. A typical example of such a preparation is as follows: Polyacrylic acid (50 g) was dissolved in the aprotic solvent 1-methyl-2-pyrrolidinone (MPD) (1000 cm3;BDH) a t 60 "C, and the solution was continuously stirred, while maintaining this temperature, for a period of 24 h. Dodecylamine (7.53 g) and dicyclohexylcarbodiimide(CDI)(7.58 g, Aldrich) were separately dissolved in 100 cm3 of MPD and introduced successively into the reaction vessel under vigorous stirring. The reaction mixture was then maintained a t 60 "C with continuous stirring for a further period of 24 h. The mixture was then cooled and the dicyclohexylureacrystals formed by CDI during the reaction were removed by filtration. The HMPAA was precipitated by neutralization with concentrated sodium hydroxide solution (40% (w/v)). The precipitate was washed with hot MPD (60 "C) and methanol. The crude product was dissolved in water and twice precipitated in methanol before being initially dried overnight a t 50 "C under vacuum and finally for several hours a t 105 "C. Nomenclature. An example of the nomenclature used in the followingtext is that HMPAA-6-ClPrepresents a hydrophobically modified sodium polyacrylate derivative containing 6 mol % incorporation of the grafted dodecyl (Clz) side chains. Analysis ofPolymers. The degree ofhydrophobe incorporation of the various HMPAA's was determined using a LECO CHN elemental analyzer. In all instances, the analytical data confirmed the modification reaction to be essentially 100% emcient. Using carbon-13 NMR, Magny et al.23have recently shown this modification reaction to result in an entirely random distribution of hydrophobes along the polymer backbone. Preparation of Solutions. Polymer solutions were prepared by dissolving the appropriate amount of solid polymer in water and adjusting the resulting solution to pH 9.0 f 0.5 with dilute sodium hydroxide solution, thereby ensuring all experiments were performed with the polymers in the form of their sodium salts. For mixed polymer/surfactant solutions, equal amounts of various aqueous surfactant solutions of known concentration were added to approximately 5 g of aqueous polymer solutions. The mixtures were then tumbled overnight to ensure complete homogeneity. Controlled Stress Rheology. Small deformation oscillatory measurements were recorded at 20 "C using a Carrimed CSlOO controlled stress rheometer (TA Instruments (U.K.) Ltd.) fitted with either a 4 cm 2 deg or a 2 cm 2 deg cone and plate measuring system. All measurements were performed using a fixed amplitude of oscillation of 6 mrad over a frequency range of 0.1(22) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988,20, 577. (23) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151.
10 Hz. These measurement conditions were determined to be within the linear viscoelastic region of the samples. Capillary Viscometry. The reduced viscosities of the various polymers in 0.1 mol dm-3 aqueous sodium bromide solutions a t 20 "C were determinedusinga Cannon-Ubbelohde 75M capillary viscometer. For each sample, the flow times were recorded until three consecutive measurements agreed to within f0.5%. Electron Spin Resonance (ESR) Spectroscopy. Spin-Label Studies. Aportion of each of the polymers was spin labeled with 4-amino-Tempo using the method of Cafe and R0bb.2~ The ESR spectra of aqueous solutions of the spin-labeled polymers in the absence and presence of various surfactant additions were recorded a t 20 "C on a Bruker ESP300 ESR spectrometer (Bruker Spectrospin, Ltd., U.K.) using a quartz cell suitable for aqueous solutions. Spin Probe Studies. All solutions were prepared by dissolving the appropriate amount of polymer in a slightly alkaline (pH 9) aqueous solution of 5 x mol dm-3 5-DSA. The polymer was completely solubilized by stirring continuously overnight. The ESR spectra of the 5-DSA in the various polymer solutions were recorded as described above. Use ofESR Spectroscopy To Study Interpolymer Associations. Nitroxide free radicals give rise to well-characterized three-lined ESR spectra. The relative shapes and intensities ofthe lines are a reflection of the mobility of the nitroxide moiety. If the motion of the nitroxide radical is unrestricted, then the three lines are relatively narrow and of similar intensities due to isotropic tumbling. However,as the mobility of the free radical is reduced, line broadening occurs due to anisotropic effects. For spin-label experiments, it is inferred that the mobility of the spin label strongly reflects the mobility of the polymer segments to which it is attached. Specifically selected nitroxide spin probes can be used to monitor the formation of regions of microheterogeneity within bulk systems. In this instance, the spin probe experiments were carried out using 5-doxylstearicacid (5-DSA)which is amphiphilic in nature and, thus, can be expected to preferentially associate with any microregions of hydrophobicity present within a bulk aqueous environment. The decrease in rotational mobility experienced by the probe as a consequence of preferential association can be monitored through the resultant changes in the shape of its ESR spectrum. In the following spin probe studies, composite spectra were obtained for 5-DSA in a number of HMPAA solutions indicating a partitioning of the probe molecules between two differing environments. Such spectra (an example of which is given in Figure l a ) were quantitatively resolved into two components, i.e. a broad "immobile" anisotropic component (Figure lb) attributed to those 5-DSAmolecules which are closely associated with the regions of hydrophobic association and a "mobile" isotropic component (Figure IC) arising from those 5-DSA molecules residing in the bulk solvent phase. This resolution was achieved by subtracting increasing proportions of the isotropic spectrum obtained for the 5-DSAin solution alone from the composite spectra of interest, until all composite character had been removed. By use of such an analysis, the specimen composite spectrum shown in Figure l a was determined to comprise of 85% of the anisotropic component and 15% of the isotropic component. The rotational correlation time, re, of the amino-Tempo and doxy1nitroxide moieties undergoing rapid isotropictumbling were calculated from the equations described by Zhao et aLZ6and Martinie et al.,26 respectively. For slow anisotropic tumbling, the values of zc were estimated using the methods and spectra reported by Freed.27
Results The reduced viscosities of HMPAA-6-C12 a n d t h e precursor sodium polyacrylate (NaPAA) in 0.1 mol dm-3 (24) Cafe, M. C.; Robb, I. D. Polymer 1979,20, 513. (25)Zhao, F.; Rosen, M. J.; Yang, N. L. Colloids Surf. 1984,11, 97. (26)Martinie,J.; Michon, J.;Rassat, A. J.Am. Chem. SOC. 1976,97, 1818.
(27)Freed, J. H.In Molecular Biology Spin Labelling: Theory and Applications, Berliner, L. J.,Ed.;Academic Press: London, 1976;p 53.
Solution Behavior of Sodium Polyacrylate
Langmuir, Vol. 10, No. 7, 1994 2473
n
Figure 1. ESR spectra of (a) 5 x mol 5-DSA in a 1%aqueous solution of HMPAA-6412, and ita resolved (b) anisotropic and (c) isotropic components.
.-v
c;: 0
oa
w
; c;
Figure 2. Plots of reduced viscosity against concentration for (0)NaPAA and (0) HMPAA-8C12 in 0.1 mol dm-3 aqueous sodium bromide solution at 20 "C.
Figure 3. (a,top)ESR spectra of 5-DSAin 0.1mol dm-s aqueous sodium bromide solution at 20 "C containing varying concentrations ofNaPAA: (a)0%;(b) 0.2%;(c) 0.5%. (b, bottom) ESR spectra of S-DSA in 0.1 mol dm-3 aqueous sodium bromide solution at 20 "C containing varying concentrations of H"AA-B-Cl2: (a) 0%; (b) 0.20%,(c) 0.37%;(d) 0.49%.
aqueous sodium bromide solutions at 20 "C are given as a function of polymer concentration in Figure 2. At low polymer concentrations (0.34%)themodified polymer exhibited ahigher reduced viscosity than the NaPAA. The intrinsic viscosities of NaPAA and HMPAA-6412 under these solvent conditions were determined to be 12.5 and 6.0 dL g-l, respectively. While the expected linear relationship between reduced viscosity and concentrationwas observed for NaPAA over the concentration range studied, the corresponding curve for HMPAA-6412 consisted of two linear sections with a distinct change in gradient being observed a t a concentration of 0.31%. The Huggins coefficients of the two linear sections of the curve for HMPAA-6412 beneath and above the "transition" concentration of 0.31%were calculated to be 0.80 and 2.94, respectively.
The ESR spectra of 5-DSA in solutions of varying concentration of NaPAA and HMPAA-6412 in 0.1 mol dm-3 aqueous sodium bromide at 20 "Care given in parts a and b of Figure 3, respectively. For NaPAA,the spectra of the 5-DSA remained purely isotropic (z, = 1.6 x s) over the entire range of concentrations studied. In contrast, the spectra of 5-DSAin the presence of HMPAA6 4 1 2 were composite in character over the entire range of polymer concentrations studied, with the broad anisotropic component (attributed to "immobilized" 5-DSA molecules in close association with regions of hydrophobicity) becoming more evident with increasing polymer goncentration. These observations are quantified in Figure 4 which gives the percentage of immobilized 5-DSA molecules in solutions of NaPAA and HMPAA-6-ClB in 0.1 mol dm-3 aqueous sodium bromide a t 20 "C as a function of polymer concentration. Above concentrations of 0.3% HMPAA-6-Cl2, the anisotropic component ac-
0.0
0.1
01
0.4
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Polymer Concentration I %
Senan et al.
2474 Langmuir, Vol. 10, No. 7, 1994
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'
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Figure 4. Percentage of immobilized 5-DSA molecules, Pbm, in the presence of varying concentrationsof (0)NaPAA and (0) HMPAA-6412 in 0.1 mol dm-3 aqueous NaBr solution.
0.16
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4
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0.20
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Polymer concentration I %
Figure 6. Plots of dynamic viscosity(w = 1Hz)against polymer concentrationfor aqueous solutions of(.) NaPAA, (A) HMPAA34218, (M) HMPAA-6-Cl8, and ( 0 )HMPAA-64312.
Polymer Concentration I
%
Figure 7. Percentage of immobilized 5-DSA molecules, Pi,-, in aqueous solutions of varying concentrations of ( 0 )NaPAA,
(A) HMPAA-3-Cl8,(M) HMPAA-6-Cl8,and (0)HMPAA-6-Cl2.
Figure 6. Resolved ESR spectra of immobilized 5-DSA molecules in the presence of (a)0.20% and (b) 0.49%HMPAA6 4 1 2 in 0.1 mol dmw3aqueous NaBr solution.
counted for approximately 90% of the observed spectra of 5-DSA. The resolved anisotropic ESR spectra of the 5-DSA molecules associated with the regions of hydrophobicity present in 0.20% and 0.49% solutions of HMPAA-64212 in 0.1 mol dm-3NaBr are given in Figure 5. These polymer concentrations were chosen to respectively represent points at which the HMPAAS-C12 solutions had a lower and higher reduced viscosity than the precursor NaPAA. It can be seen that the spectra are essentially identical (z, = -7 x s) with their broad anisotropic character being indicative of a strong interaction between the amphiphilic 5-DSA molecules and microregions of hydrophobicity present in the system. The dynamic viscosity, f , of aqueous solutions of various polymers are given as a function of polymer concentration in Figure 6. While the values of 7' were essentially similar for all polymers a t low concentrations, differences in 7' did become apparent at higher concentrations. The polymer concentration at which the curve for a modified polymer initially deviated from that of the precursor NaPAA decreased with increasing alkyl chain length and degree of incorporation of the bound hydrophobe, i.e. -0.35%, 0.25%,and 0.08%for HMPAAB-C12,-3-C18,and -6-C18, respectively. At any given polymer concentration above these critical values, 7' increased with increasing
degree of incorporation and alkyl chain length of the bound hydrophobe. The proportion of anisotropic component in the ESR spectra obtained for 5-DSA in aqueous solutions of the various polymers are given as a function of polymer concentration in Figure 7. For NaPAA, the ESR spectra of the 5-DSA remained purely isotropic over the entire polymer concentration range studied, indicating a complete absence of interaction between the 5-DSA and NaPAA. For the various modified polymers, while no anisotropic component was detectable in the ESR spectra of 5-DSA at low polymer concentrations, a significant proportion of anisotropic component was evident at higher polymer concentrations. For the series of HMPAA's studied, the polymer concentration at which the anisotropic component was first detected in the ESR spectra of the 5-DSA was within the range of 0.1-0.13% for all the HMPAA's studied and showed little dependence on either the degree of incorporation or alkyl chain length of the bound hydrophobes. The resolved ESR spectra of the immobilized 5-DSA molecules present in 1%aqueous solutions of the various HMPAA's are given in Figure 8. The various spectra are essentially identical (z, = -7 x s), with their broad anisotropic character again being indicative of a strong interaction between the amphiphilic 5-DSA molecules and microregions of hydrophobicity present in the various systems. In the following text, the effects of various additives on the storage modulus, G of 3% aqueous solutions of HMPAA's will be described. To assist comparison of the relative effects of the different additives on the various polymers, the data on G have been normalized with respect to the storage modulus ofthe appropriate polymer solution in the absence of the additive. Thus, with this
Solution Behavior of Sodium Polyacrylate
Langmuir, Vol. 10, No. 7, 1994 2475
v
b
-
0.08
.
. 0.04
0.08
0.08
0.10
[NaCll/ mol dm*
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Figure 9. Effectof sodium chloride on the normalized storage modulus, GIN(w = 1Hz) of 3%aqueous solutions of (0)NaPAA, (A)HMPAA-3-Cl2, (0) HMPAA-6-Cl2, and (A)HMPAA-3-Cl8.
n Figure 8. Resolved ESR spectra of immobilized 5-DSA molecules in 1%aqueous solutions of (a) I-XvIPAA-3-Cl8,(b) HMPAA-6418, and (c) HMPAA-6412.
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Table 1. Absolute Values of the Storage M d d w of Various Polymer Solutions wlvmer concentration (% w/v) G (at 1 Hz)(N m+) NaPAA 2 0.45
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0.75
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225
61 250
system, each polymer solution has a normalized storage modulus, GN,of 1 in the absence of any additives. For reference purposes, the absolute values of G' of the various polymer solutions in the absence of additives are given in Table 1. The effect of added sodium chloride on the normalized storage modulus, GNof 3% aqueous solutions of NaPAA and its various hydrophobized derivatives are given in Figure 9. For NaPAA,G N decreased with increasing added electrolyteconcentration. In contrast, the addition of relatively low concentrations of sodium chloride produced a marked increase in G N of solutions of the hydrophobically modified polymers. For these polymers, the value of G'N passed through a maximum at a sodium chloride concentrationwhich varied slightly from polymer to polymer, and then decreased at higher electrolyte concentrations. The maximum value of G N increased slightly with increasing hydrophobe length but showed a much stronger increase with increasing degree of hydrophobe incorporation. The effecta of the addition of various anionic alkyl carboxylate and alkyl sulfate surfactants on G N of 3% aqueous solutions of HMPAA-6-Cl2 are given in Figures 10 and 11, respectively. All the curves exhibit the same general trends, with G Nat first increasing with increasing surfactant concentration up to a maximum value, G'N(-). The magnitude of G'N(-) and the concentration of added [ S I , ) at which it occurred varied from surfactant ( surfactant to surfactant but, in all instances, further additions of surfactant above the level of [ SI, produced
0.05
0.10
0.15
0.20
036
[~urfactant]/ mol dm-3
Figure 10. Normalized storage modulus, GIN (w = 1Hz) of a 3%aqueous solution of HMPAA-6412 in the presence ofvarying concentrations of anionic surfactant (0)sodium dodecanoate; (A)sodium decanoate; (W) sodium octanoate.
z
L
V .
0.02
0.04
0.08
0.08
0.10
[~ur~actant] I mol dma
Figure 11. Normalized storage modulus, G'N (w = 1Hz) of a 3%aqueous solutionofHMPAA-6-Cl2in the presence ofvarying concentrations of anionic surfactant (0) sodium dodecyl sulfak, (a)sodium decyl sulfate.
a marked decrease in GIN. In all instances, the addition of the anionic surfactants was found to have a negligible effect on the storage modulus of solutions of the precursor NaPAA. A plot of GN(") of the 3% HMPAA-6412 solutions as a function of the effective alkyl chain length, C,, of the added surfactant is given in Figure 12. It can be seen that GN(-) increases markedly with increasing C, above an apparently limitingvalue of -6 for the latter parameter. observed for It is also evident that the values of
Senan et al.
2476 Langmuir, Vol. 10,No. 7,1994
*/
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,
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.
,
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,
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Figure 12. Maximum normalized storage modulus, GN(") (w = 1 Hz) of 3% aqueous solutions of HMPAA-8C12 as a function of the effectivealkyl chain length, C,, ofvarious added anionic surfactants: ( 0 )sodium alkyl sulfates (Cn-OS03-Na+); (A)sodium alkyl carboxylates (C,-COO-Na+).
Figure 14. ESR spectra of spin-labeled HMPAA8-Cl2 at a concentration of 3% in (a) water, (b) 0.01 mol dm-3 aqueous SDS solution, and (c) 0.15 mol dm-3 aqueous SDS solution. oi 1
1
Frequency I Hz
Figure 13. Plots of G (filled symbols)and G (open symbols) against frequency of oscillation for 3% solutions of HMPAA6-C12 in (0, 0 ) water, (m,0)0.01 mol dm-3 aqueous SDS solution, and (A,A ) 0.15 mol dmW3 aqueous SDS solution.
the addition of the carboxylated and sulfated surfactants fall on the same curve, indicating that, for these two series of surfactants, the nature of the headgroup was of secondary importance to the alkyl chain length of the added surfactant in determining the rheological effects of such polymedsurfactant interaction. The effect of frequency of oscillation on the storage ( G ) and loss ( G )moduli of 3%solutions of HMPAA-6412 in the presence of varying amounts of SDS are given in Figure 13. In the absence of surfactant, the data indicate the system to be behaving as a viscous solution with G < G a t low frequencies, but with a crossover between these parameters occurring at a frequency of approximately 0.2 Hz. In the presence of 0.01 mol dmT3SDS (corresponding to [SI,,), both moduli show a reduced dependence on frequency, and G > G" over the entire frequency range studied. This is indicative of this level of SDS addition having produced a change to gel-like characteristics within the system. At the relatively high SDS concentration of 0.15 mol dm-3, both moduli show a pronounced frequency dependence, with G" > G over the entire frequency range studied, indicating that the system has reverted to a viscous solution. The actual values of G and G observed in the presence of 0.15 mol dm-3 SDS are lower than those observed in the absence of surfactant. The gradient, n, of the log-log plots of G against frequency given in Figure 13 for 3% HMPAA-6412 in the presence of 0, 0.01, and 0.15 mol dm-3 SDS were determined to be 0.56,0.20, and 0.77, respectively. The value of n of 0.20 obtained for the
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8
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Figure 15. Effect of the addition of CTAB on the normalized storage modulus, GIN (0 = 1Hz) of 2% aqueous solutionsof ( 0 ) NaPAA, (A) HMPAA-3-Cl8, and (m) HMPAA-8C18.
HMPAAS-C12/0.01 mol dm-3 SDS indicates this system to be behaving as a physical gel (n > 0) rather than a covalent gel (n = O).2s The ESR spectra of 3%aqueous solutions of spin labeled HMPAA-6-C12in water and in the presence of 0.01 and 0.15mol dm-3 SDS are given in Figure 14. The essentially identical nature of the spectra (re= 1.4x s) indicates that the segmental motion of the polymer chains remained virtually unchanged in these systems, despite the pronounced differences in the bulk rheological characteristics of the polymerlsurfactant solutions, as illustrated by the data given in Figures 11 and 13. The effects of the addition of the cationic surfactant cetyltrimethylammonium bromide (CTAB) on G Nof 2% aqueous solutions of NaPAA, HMPAA-3-Cl8, and HMPAA-6-Cl8 are illustrated in Figure 15. The level of CTAB addition is expressed in terms of the molar ratio ([CTABY [COO-]) of CTAB molecules to acrylate residues in the ~
~
(28) Egelandsdal, B.; Fretheim, K.; Harbitz, 0. J. Sei. Food Agric. 1986,37,944.
Solution Behavior of Sodium Polyacrylate polymer backbone. For 2% aqueous solutions of the various polymers investigated in this study, the concentration of acrylate residues is approximately 0.2 mol dm-3. For NaPAA, G N was observed to increase upon the addition of relatively low concentration8 of surfactant ([CTABHCOO-] -= -0.005:l). Further additions of CTAB above this level, however, produced a gradual decrease in GIN until phase separation in the form of a gelatinous precipitate was observed at a [CTABHCOO-I ratio of approximately 0.05: 1. Both Cis-modified polymers exhibited the same general rheological trends as NaPAA toward the addition of CTAB with G N first increasing, passing through a m a x i ” , and then decreasing with increasing levels of CTAB addition. The maximum value of G N and the CTAB concentration at which it occurred increased with increasing degree of hydrophobe incorporation of the polymers. For both the modified polymers, precipitate formation was not observed until a slightlyhigher [CTABY [COO-] ratio of -0.08:l was attained.
Discussion The novel rheological properties of hydrophobically associating water soluble polymers (HAWSP’s)are largely attributed to the association of neighboring hydrophobic moieties. In dilute solution, the reduced coil dimensions of HAWSP’s compared to their nonmodified counterparts, as inferred from their lower reduced viscosities, has been taken as evidence of the existence of intramolecular hydrophobic a s s o c i a t i ~ n s . ~In~ ~contrast, ~~ at higher polymer concentrations, the increased solution viscosities of HAWSP’s have been attributed to the occurrence of intermolecular hydrophobic associations, which result in the formation of a reversibly cross-linked polymer network.lS2 Current conception envisages such intermolecular associations as occurring only once an apparently critical concentration (C*,) of polymer has been surpassed. The “sharpness” of the transition between such intramolecular and intermolecular associations is, however, a matter of considerable uncertainty and the actual interpretation of C*,, is still somewhat dependent upon the technique employed in its identification. For rheological investigations, C*,,is usually identified as the concentration at which the viscosity (or some other rheological parameter) of the hydrophobically modified polymer begins to increase above that of its nonmodified counterpart. However,if we consider the rheological data given in Figure 2, it is evident that two critical polymer concentrations may be defined for HMPAA-6412 in 0.1 mol dm-3 aqueous NaBr solution at 20 “C. These are namely C1*, at which the distinct change in gradient of the curve occurs, and C2*,at which the crossover between the curves for HMPAAB-C12 and the precursor NaPAA occurs. The ESR spin probe data given in Figures 3 and 4 indicate that regions ofhydrophobicityare present within solutions of HMPAA-6-Cl2 in 0.1 mol dm-3 aqueous sodium bromide solutions throughout the entire range (0.05-0.5%) of concentrations studied. For low polymer concentrations (
