Rheological Properties of Semidilute Hydrophobically Modified Alkali

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Langmuir 2000, 16, 5600-5606

Rheological Properties of Semidilute Hydrophobically Modified Alkali-Soluble Emulsion Polymers in Sodium Dodecyl Sulfate and Salt Solutions H. Tan and K. C. Tam* School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singapore

R. D. Jenkins Union Carbide Asia Pacific Inc., Technical Center, 16 Science Park Drive, The Pasteur, Singapore 118227, Republic of Singapore Received December 31, 1999. In Final Form: March 21, 2000 The rheological properties of semidilute hydrophobically modified alkali-soluble emulsion polymers in the presence of 0.4 M NaCl and various concentrations of anionic surfactant were investigated. The viscosity profile consists of a zero-shear Newtonian region at low stresses and a shear-thickening region at intermediate stresses, followed by a catastrophic decrease in the viscosity at moderate to high stresses. The viscosity and the dynamic modulus peaked at c/cmc of ∼11, in contrast to 1 in the absence of NaCl. The activation energies determined at the zero-shear and the shear-thickening regime (denoted by Ea0 and Ea,max respectively) increase to a maximum and subsequently decrease as a function of surfactant concentration. Two critical SDS concentrations defined by the crossover of the Ea curves (at ∼0.001 and 0.009 M SDS) are related to the balance in the inter- and intramolecular association.

Introduction To enhance the performances of associative polymers in paint and coating application, a better understanding of the associative interactions between associative polymers and latexes or surfactant is needed. Depending on the nature of the additives, the thickening behavior can either decrease or increase. Over the last 20 years, most of the studies focused on the behavior of hydrophobically modified ethylene oxide urethane (HEUR) thickeners.1-13 Some studies on hydrophobically modified alkali-soluble * To whom correspondence should be addressed. Fax: (65) 7911859. E-mail: [email protected]. Currently on sabbatical leave at the Department of Mechanical Engineering, MIT. (1) Tam, K. C.; Jenkins, R. D.; Winnik, M. A.; Bassett, D. R. Macromolecules 1998, 31, 4149. (2) Hogen-Esch T. E.; Amis E. Trends Polym Sci. 1995, 3, 98. (3) Amis E. J.; Hu N.; Seery T. A. P.; Hogen-Esch T. E.; Yassini M.; Hwang F. Associating polymers containing fluorocarbon hydrophobic units. In Hydrophilic Polymers: Performance with Environmental Acceptability: Glass, J. E., Ed.; American Chemical Society: Washington, D.C. 1996; p 279. (4) Thibeault, J. C.; Sperry, P. R.; Schaller, E. J., Effect of Surfactants and Cosolvents on the Behavior of Associative Thickeners in Latex Systems. In Water-Soluble Polymers; ACS Symposium Series 213; American Chemical Society: Washington, D.C., 1986; p 375. (5) Glass, J. E. Influence of Water-Soluble Polymers on Rheology of Pigmented Latex Coatings. In Water Soluble Polymer: Beauty with Performance; Glass, J. E., Ed.; ACS Advance Chemistry Series 213; American Chemical Society: Washington, D.C., 1986, p 391. (6) Lundberg, D. L.; Glass, J. E.; Eley, R. R. J. Rheol. 1991, 35, 1255. (7) Reynolds, P. A. Prog. Org. Coatings 1992, 20, 393. (8) Ma, Z.; Kaczmarski, J. P.; Glass, J. E. Prog. Org. Coatings 1992, 21, 69. (9) Mast, A. P.; Prud′homme, R. K.; Glass, J. E. Langmuir 1993, 9, 708. (10) Mast, A. P. Prog. Colloids Polym. Sci. 1993, 93, 53. (11) Lundberg, D. L.; Ma, Z.; Alahapperruna, K.; Glass, J. E. Surfactant Influences on Hydrophobically Modified Thickener Rheology. In Polymers as Rheology Modifiers; Schulz D. N., Glass, J. E., Eds.; ACS Symposium Series 462; American Chemical Society: Washington, D.C., 1991; p 234. (12) Alahapperuna, K.; Glass, J. E. Prog. Org. Coatings 1992, 21, 53. (13) Kaczmarski, J. P.; Glass, J. E. Macromolecules 1993, 26, 5149.

emulsion (HASE) polymers have recently been reported.14-23 HASE polymer can be classified as a hydrophobically modified polyelectrolyte. The polymer backbone possesses charges after neutralization with a base. Addition of salt removes the electrostatic repulsion between charges on the polymer backbone, which decreases the stiffness of the polymer chain. The conformational changes of the polymer chain from one of high to lower persistent chain length disrupt the intermolecular junctions. This causes the polymer network to collapse, reducing the hydrodynamic volume occupied by polymer chains and micellar clusters, thereby lowering the viscosity. By introducing sufficient amounts of surfactant, the active junctions can be strengthened by the adsorption of surfactant molecules onto the hydrophobic junctions. The replacement of hydrophobes in the junction with surfactant molecules decreases the functionality of polymer junctions. Consequently, a larger number of mechanically active junctions are assembled, which gives rise to enhanced rheological properties. Seng et al.14 reported on the rheological properties of HASE in the presence of cationic, anionic, (14) Seng, W. P.; Tam, K. C.; Jenkins, R. D. Colloids Surf. 1999, 154, 365. (15) Jenkins, R. D. Ph.D. Dissertation, Lehigh University, 1990. (16) Tam, K. C.; Guo, L.; Jenkins, R. D.; Bassett, D. R. Polymer 1999, 40, 6369. (17) Tirtaatmadja, V.; Tam, K. C.; Jenkins, R. D. Macromolecules 1997, 30, 1426. (18) Tirtaatmadja, V.; Tam, K. C.; Jenkins, R. D. Macromolecules 1997, 30, 3271. (19) Tirtaatmadja, V.; Tam, K. C.; Jenkins, R. D. AIChE J. 1998, 44, 12. (20) Tirtaatmadja, V.; Tam, K. C.; Jenkins, R. D. Langmuir 1999, 15, 7537. (21) Guo, L.; Tam, K. C.; Jenkins, R. D. Macromol. Chem. Phys. 1998, 199, 1175. (22) Islam, M. F.; Jenkins, R. D.; Bassett, D. R.; Ou-Yang, H. D. Macromolecules 2000, 33, 2480. (23) Tan, H. Master’s Thesis, Nanyang Technological University, Singapore, 2000.

10.1021/la991691j CCC: $19.00 © 2000 American Chemical Society Published on Web 05/20/2000

Rheological Properties of Semidilute HASE Polymer

and nonionic surfactants. They found that the addition of surfactant up to the critical concentration results in an increase in the viscosity to a maximum that is attributed to the increase in the number of mechanically active junctions and lifetime of these junctions. However, in the absence of salt, the negatively charged polyelectrolyte chains are somewhat extended, and the nature of the hydrophobic interactions are controlled by the “stiff” backbone. In addition, the electrostatic repulsion between the surfactant molecules and the charged polymer backbone will have an effect on the surfactant/polymer interactions. However, interactions between surfactant molecules and other hydrophobic molecules or surfaces tend to be energetically more favorable than between similar surfactant molecules.24 The nature of thermoreversible gelling and nongelling semidilute aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) in the presence of various amounts of SDS and different concentrations of NaCl was investigated by Nystrom et al.25 They found that in the absence of salt, the maximum value of the viscoelastic response is determined by an optimal balance between repulsive (swelling) and attractive (connecting) forces. At high surfactant concentrations, the junctions are disrupted (the connectivity is lost) and the structure of the polymer network breaks down, resulting in a sharp drop in the viscoelastic behavior. However, when salt is added, the electrostatic repulsions are screened and the junctions and the connectivity are restored and the network is regenerated. This behavior was further supported by evidence that surfactants added to the associative thickeners (AT) solution produces mixed micellar aggregates with the AT hydrophobes in the aqueous phase, which can either increase or decrease the viscosity of the solution.26 Bieleman and co-workers suggested that the net result of interactions between associative thickener and nonionic surfactants is that more micelles or physical cross-links are produced, but the strength of each micellar junction will decrease.27 Glass and co-workers have shown that the effects of surfactant concentration on the rheology of AT solutions are strongly dependent on the types of surfactant and the structure of HEUR thickeners.11-13 A critical ratio of surfactant to thickener provides the highest viscosity. In addition, Jenkins found that the amount of sodium dodecyl sulfate (SDS) required to achieve the viscosity maxima in an AT solution decreases as the molecular weight of associative thickener increases.15 Other authors observed that the total surfactant concentration needed to achieve an optimum elastic modulus is close to the cmc (∼5-7 mM) of SDS in a solution of nonmodified EHEC.28 This finding is in accordance with the results of Holmberg and Sundelof on the hydrodynamic properties and interactions between EHEC and SDS in water. They observed that the reduced viscosity reaches the maximum at an SDS concentration of ∼ 5 mM, followed by a drastic decrease with further addition of SDS.29 Similar behavior was observed for the solution viscosity of hydrophobically modified poly(acrylic acid) and hydrophobically modified hydroxyethylcellulose.30,31 The viscosity increases in the (24) Goddard, E. D. Colloids Surf. 1986, 19, 255. (25) Nystrom, B.; Kjoniksen, A.; Lindman, B. Langmuir 1996, 12, 3233. (26) Zhou, L. Ph.D. Dissertation, Lehigh University, 1995. (27) Bieleman, J. H.; Riesthuis, F. J. J.; van der Velden, P. M. In Additives for Water-Based Coatings; Karsa, D. R., Ed.; The Royal Society of Chemistry: Cambridge, 1990; p 157. (28) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1. (29) Holmberg, C.; Sundelof, L. Langmuir 1996, 12, 883.

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presence of SDS originate from the increase in the number of mechanically active junctions and not from the increase in coil dimensions caused by electrostatic repulsions. When the polymer chains are extended under shear, the number of hydrophobic groups available for interchain association increases, owing to the breakdown of intramolecular associations.32 On the other hand, Jenkins and co-workers have shown that the maximum viscosity of a 2.5 wt % solution of associative polymer based on linear poly(ethylene oxide) with hexadecyl end groups occurs when the concentration of hydrophobes in the associative polymer is stoichiometrically equal to the concentration of SDS solution.33 The viscosity maxima for HEUR/ surfactant system of different hydrophobic end groups occurs at 1.7 and 5.2 surfactant molecules for each hydrophobe observed for SDS and nonionic nonylphenol (C9H19OH) surfactant. This indicates that the viscosity maxima is related to the size of the mixed aggregates and that the surfactant enhances the strength or lifetime of the association junction.34 The temperature effect on the associative polymer in the presence of surfactant was recently reported by Tirtaatmadja et al.20 and Sarrazin-Cartalas et al.35 They examined the interaction of hydrophobically modified poly(sodium acrylate) and HASE with a series of oligoethylene glycol monododecyl ether surfactants. They observed that association with surfactants possessing the shortest ethylene oxide chain (C12E4) exhibits unusual thermal behavior. The thermal behavior is related to the transformation of surfactant aggregates from micelles to vesicles at higher temperatures. Carlsson and co-workers observed similar behavior for EHEC A in 10 mM of cetyltrimethylammonium bromide (CTAB), where the viscosity increases with temperature up to 60 °C and then decreases thereafter. For EHEC B with the same CTAB concentration, the viscosity increases with temperature between 25 and 45 °C. This is attributed to the stronger attraction between surfactant and EHEC since the hydrophobicity of the macromonomer increases with temperature.36 The present paper examines the combined effects of model HASE polymer at a fixed salt concentration of 0.4 M but at various SDS concentrations. We seek to examine the effects of salt and SDS on the steady shear flow, viscoelastic properties, and thermal behavior of 1 wt % model HASE polymer with C20 hydrophobe (designated as RDJ31-5). The choice of this system is based on earlier studies by Tam et al. where interesting shear-thickening behavior was observed for 1 wt % RDJ31-5 in salt solutions.14,16 The correlation of such behavior with a quantitative measure of the strength of associative junctions is performed by conducting measurements over a range of temperatures. Experimental Section Materials. The model associative polymer used in the present study is an emulsion copolymerization product of methacrylic acid (MAA), ethyl acrylate (EA), and a macromonomer capped (30) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617. (31) Sau, A. C.; Landoll, L. M., Synthesis and Solution Properties of Hydrophobically Modified (Hydroxyethyl) Cellulose, Glass, J. E., Ed.; ACS Advances in Chemistry Series 223; American Chemical Society: Washington, D.C., 1989; p 343. (32) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838. (33) Jenkins, R. D.; Bassett, D. R.; Silebi, C. A.; El-Aasser, M. S. J. Appl. Polym. Sci. 1995, 58, 209. (34) Hulden, M. Ph.D. Dissertation, Abo Akademi University, Finland, 1994. (35) Sarrazin-Cartalas, A.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1994, 10, 1421. (36) Carlsson, A.; Karlstrom, G.; Lindman, B. Colloids Surf. 1990, 47, 147.

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Tan et al. The Contraves LS40 controlled rate rheometer was used to perform the rheological experiments. It was fitted with the MS 41S/1S concentric cylinder measuring system consisting of a cup (12 mm diameter) and a bob (11 mm diameter) of length 8 mm. The Carri-Med CSL500 controlled stress rheometer was used to check the reproducibility of the results obtained from Contraves LS-40. A 4 cm, 2° cone and plate and a double concentric cylinder measuring system were used. The dynamic data were obtained from the Rheometric ARES fluids rheometer, which is fitted with double-wall Couette measuring geometry. All of the experiments were carried out over a temperature range of 10-35 ( 0.1 °C. The calorimetric data were obtained using a Microcal isothermal titration calorimeter (Microcal ITC). This power compensation, differential instrument was previously described in detail by Wiseman et al..38 It has a reference cell and a sample cell of 1.35 mL and the cells are both insulated by an adiabatic shield. The titration was carried out by injecting concentrated surfactant solution (in aliquots of 0.1-10 µL) from a 250 µL injection syringe into the sample cell filled with a known concentration of sample polymer solution. The syringe is tailormade such that the tip acts as a blade-type stirrer to ensure continuous mixing efficiency at 400 rpm. An injection schedule was automatically carried out using interactive software after setting up the number of injections, volume of each injection and time between each injection. The measurement of the cmc was performed at a constant temperature (25.0 ( 0.02 °C).

Results and Discussion

Figure 1. (a, top) Chemical structure of HASE polymer. (b, bottom) Microstrusture of model HASE polymer in SDS/0.4 M NaCl solution. with a hydrophobic group through polyethylene oxide (PEO) chains. The general structure of HASE polymer is shown in Figure 1a. X, Y, Z represents the mole fraction of each segment having the ratio of 0.5/0.49/0.01. The HASE polymer was designated as RDJ31-5 since this nomenclature is similar to the ones used in our previous publications.17,18 The PEO chains, p, and hydrophobic group, R, were kept constant at 31 mol and 20 carbons, respectively. The network structure of model HASE polymer with addition of salt and SDS in aqueous solution is described in Figure 1b. The molecular weight of the macromonomers were calculated by adding the molecular weight of meta-TMI (MW ) 201.25) to the hydroxyl number average molecular weights of the surfactant precursors. The synthetic procedures and GPC output of the macromonomer can be found elsewhere.18,37 For RDJ31-5, the molecular weight as determined by static light scattering is ∼180 000.22 The result agrees with that determined from intrinsic viscosity for the polymers in 0.01 and 0.1 M NaCl solution, where the molecular weight was estimated to be in the range of 170 000190 000 Da. Hence the average number of hydrophobes on each polymer chain is estimated to be 16∼18. The intrinsic viscosity of RDJ31-1 in 0.1 M NaCl was found to be 3.3 dl/g.21 Thus at 1.0 wt %, c[η] ) 3.3, which suggests that the polymer solutions are in the semidilute region. Measurement Techniques. A stock solution containing 3 wt % polymer latexes dispersion was diluted with an appropriate amount of sodium chloride (NaCl) to a concentration of 1 wt %, and the pH was then adjusted to 9.0 ( 0.1 by small amounts of concentrated 2-amino- 2-methyl-1-propanol (AMP) solution. After 3 days, the correct amount of SDS was added to the system and it was kept for another 3 days prior to testing. (37) Jenkins R. D.; DeLong L. M.; Bassett D. R. In Hydrophilic Polymers: Performance with Environmental Acceptability; Glass, J. E., Ed.; Advances in Chemical Series 248; American Chemical Society: Washington, D.C., 1996, p 425.

Steady Shear and Viscoelastic Properties of RDJ31-5 in SDS/0.4 M NaCl Solutions. Previous studies on RDJ31-5 polymer in various salt concentrations showed that the solutions exhibit a shear-thickening behavior at moderate shear rates.21,23 Such behavior is attributed to the conversion to intermolecular associations at the expense of intramolecular associations. However, detailed studies using various techniques such as rheometry, light scattering, neutron scattering, fluorescence spectroscopy, and pulse gradient NMR to elucidate the nature of the network structure and its strength with added anionic surfactant have not been carried out. Our focus in this study is to use rheometry to probe the nature of the network structure of the polymer/surfactant solutions. The addition of salt disrupts the associative junctions owing to the shielding of polyelectrolyte backbones by cationic charges of salt, causing the associative network to detach and form smaller clusters with predominantly intramolecular associations.16,21 When SDS is introduced to the system, SDS molecules bind to hydrophobic junctions forming mixed micellar aggregates.24 The proportion of SDS bound to the hydrophobic junctions is currently not known and should be a rich area for future research. Techniques such as pulse gradient NMR and isothermal titration calorimetry may be used to determine the binding characteristics of SDS to the hydrophobic junctions. The negatively charged polyelectrolyte backbones are shielded by the positive charges from the salt; hence, SDS molecules can more readily bind onto the hydrophobic junctions. This results in the increase in the junction functionalities. The shear viscosity profiles of 1 wt % RDJ31-5 in various SDS/0.4 M NaCl solutions are shown in Figure 2. Generally, the viscosity profile exhibits three distinct regions: Newtonian behavior at low stresses, shearthickening at moderate stresses, and shear-thinning at high stresses. The abrupt drop in the viscosity after the shear-thickening region becomes more severe at higher SDS concentrations, often showing a discontinuity in the viscosity profile. As SDS concentration increases, the zero(38) Wiseman, T.; Williston, S.; Brandts, J. F.; Lin, L. N. Analy. Biochem. 1989, 28, 131.

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Figure 3. Dynamic moduli (measured at 2 rad/s) versus SDS concentrations of 1 wt % RDJ31-5 solutions.

Figure 2. Shear viscosity profiles of behavior of 1 wt % RDJ31-5 in SDS/0.4 M NaCl solutions.

shear viscosity and the shear-thickened viscosity increases, reaching a maximum at 0.005 M SDS, and then decreases (also see Figure 6 below). This decrease is attributed to the excess surfactant micelles saturating the hydrophobe, which disrupts the hydrophobic junctions. The optimum SDS concentration, c*, at 0.005 M was also observed by several workers on interactions of SDS with EHEC, hydrophobically modified poly(acrylic acid), and hydrophobically modified hydroxyethylcellulose.28-31 These authors reported similar behavior where the viscosity exhibited a Newtonian behavior at low deformation stress followed by shear-thickening at moderate stresses and sudden decrease at higher stresses. The shear-thickening behavior was reported by various researchers and is attributed to the conversion of intra-to intermolecular associations.1,14,39 According to the transient network theory, shear thickening is a result of polymer chain expansion caused by shear deformation that results in the free energy gain of the polymer segment.11,29 Van den Brule and Hoogerbrugge correlated the thickening behavior to the reassociation of the polymer hydrophobes that do not have sufficient time to completely relax to the previous network configuration.40 Besides, shear-thickening may also be caused by the multiplication of active junction by SDS micelles. When the deformation force is applied, polymer hydrophobic junctions fragment and “free hydrophobes” reassociate with other junctions to form larger number of active junctions. In the absence of SDS molecules, the active junctions are re-formed at the expense of intramolecular association. However, when SDS molecules are present in the system, the functionality of hydrophobic junctions decreases owing to the absorbed SDS molecules on the junction, displacing the hydrophobes, which are freed to form additional active junctions. The viscosity reduction at high stresses is due to the destruction of active junctions when the applied stress is greater than the strength of the network. The viscosity profiles of the polymer in 0.002-0.012 M SDS solutions exhibit a sharp decrease after the maximum viscosity is reached. This is related to the fragmentation of active junctions with a narrowed distribution of aggregation number and whose strength corresponds to the critical (39) Ballard, M. J.; Buscall, R.; Waite, F. A. Polymer 1988, 1287. (40) Van den Brule, B. H. A. A.; Hoogerbrugge, P. J. J. Non-Newtonian Fluid Mech. 1995, 60, 303.

applied stress.23 The zero-shear and maximum viscosities increase by about 10 and 100 times, respectively, when the SDS concentration is at the optimum (i.e., 0.005 M). The addition of surfactant increases the number of active junctions by reducing the aggregation number of the polymer hydrophobes (i.e., decreasing the functionality) at each active junction owing to the formation of mixed micellar aggregates, which are more stable. Adsorption of SDS molecules onto the hydrophobic junction effectively increases the “aggregation number” of the mixed micellar junctions (i.e., SDS molecules plus polymer hydrophobes). However, the polymer functionality is reduced since the aggregation number mainly consists of SDS micelles with a few polymer hydrophobes. Such deduction is borne out of the experimental evidence that the storage modulus measured at 2 rad/s for the polymer in 0.4 M NaCl increases sharply with the addition of SDS until a maximum and thereafter it decreases (Figure 3). The storage modulus can be correlated to the number of mechanically active junctions in the polymer system as described by the transient network theory proposed in 1946 by Green and Tobolsky.41 The theory was based on the extension of classical rubber elasticity theories to transient networks, which was first introduced to account for entanglements or reversible physical bonds. The theory predicts a constant steady shear viscosity of

η(γ˘ ) ) η0 ) λG∞

(1)

where the relaxation time λ is the reciprocal of the bond breaking and re-formation rate, and G∞ is the highfrequency or plateau modulus given by

G∞ ) νeffRT

(2)

where νeff is the number density of effective or elastic chains, R is the gas constant, and T is the absolute temperature. From the plateau modulus, the mechanically active junctions in the system may be estimated. As a consequence of this, the viscosity is directly proportional to the elastic modulus. In excess of 0.012 M SDS, the number of active junctions is roughly the same as the solution without SDS addition as shown in Figure 3. The plot of zero- and maximum shear viscosities at various SDS concentrations at room temperature is shown in Figure 4a. Both viscosities exhibit similar trends; i.e., the viscosity increases to a critical concentration of 0.005 M and then decreases. However, the increase in the maximum shear viscosity is more pronounced than the zero- shear viscosity and the largest difference occurs at (41) Green, M. S.; Tobolsky, A. V. J. Chem. Phys. 1946, 14, 80.

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Figure 5. Plot of maximum and zero-shear viscosities versus c/cmc.

Figure 4. Comparison of (a, top) maximum and zero-shear viscosities; (b, bottom) critical stress of 1 wt % RDJ31-5 in SDS/ 0.4 M solutions.

c*. At Csds ) 0, the viscosity ratio (ηmax/η0) is 2.8, and it increases to 8.8 at c* and then decreases to 1.5 at high SDS concentrations. This difference is due to the creation of an increasing proportion of active junctions initiated by shear. The role of SDS is to strengthen the associative junctions by increasing the aggregation number of the mixed micellar junction, which contributes to the increase in the viscosity and relaxation time of the network structure. The critical deformation stress prior to the large decrease in the viscosity, σcrit, was plotted at various SDS concentrations as shown in Figure 4b. On the basis of similar observations by Aubry and Moan, the kink after the maximum viscosity is related to the disruption of junctions of similar size or strength.42 The σcrit increases until c* and then decreases to a constant value when the SDS concentration exceeds 0.01 M. The Critical Micellar Concentration of SDS in 0.4 M NaCl. The critical micellization concentration for SDS in 0.4 M NaCl was determined using the Microcal ITC system. The enthalpy versus the normalized SDS concentration (c/cmc) is provided in the Supporting Information (Figure S1). From the enthalpy data, a large drop in the enthalpy is evident at ∼ 0.5 mM, corresponding to the cmc of SDS in 0.4 M NaCl solution. Comparing the results to the cmc of SDS in water (8.3 mM), the onset concentration for micellization is decreased by at least 17 times when the electrostatic repulsion is removed by the addition of salt. The cmc determined for SDS in 0.4 M solution was used in quantifying the ratio c/cmc in subsequent plots. The plateau and maximum viscosities were plotted against the normalized concentration of SDS (c/cmc) as shown in Figure 5. Both the viscosity curves show that the maximum point occurs at approximately 11 times the critical micellar concentration, cmc, of SDS. This is in contrast to the results for HASE/SDS interactions in the absence of salt, where the maximum point occurs at c/cmc close to unity.14 Various (42) Aubry, T.; Moan, M. J. Rheol. 1994, 38, 1681.

Figure 6. Viscosity ratio (ηmax/η0) versus SDS concentrations at 15 (0), 20 (b), 25 °C (O).

researchers have observed a maximum viscosity at c/cmc close to unity, 1, for other polymer-SDS systems.28-31 Two comments can be deduced from this result: (a) The delay in the (c/cmc)* from 1 to 11 can be correlated to the larger number of SDS molecules absorbing onto the hydrophobic junctions that are induced by the large decrease in the electrostatic repulsion between the negative charges on the SDS molecules. This strengthens the junctions as depicted by the large increase in the activation energy (to be discussed later). (b) Since the strength of the junction is dictated by the absorbed SDS molecules, the catastrophic drop in the viscosity beyond the critical applied stress as evident in Figure 2 is defined by the characteristic of the junctions. Temperature Dependence of the HASE-NaClSDS System. The effects of temperature on the shear viscosity profiles of 1.0 wt % RDJ31-5 in 0.4 M NaCl containing SDS concentrations ranging from 0 to 0.02 M, respectively, are documented in the Supporting Information (Figure S2). The viscosity profiles at other SDS concentrations show similar trends. The shear viscosities decrease when the temperature is increased. Detailed observation reveals that the shear viscosities of 1 wt % RDJ31-5 in 0.002 and 0.007 M SDS concentrations exhibit a catastrophic drop beyond the maximum viscosity region. By decreasing the temperature, the kink becomes even more apparent, which suggests that at low temperature the Brownian dynamics of the hydrophobic clusters are not as dominant; hence, the network is controlled by the character of the associative junctions. The ratio of maximum to zero-shear viscosity at various temperatures was plotted against SDS concentrations in Figure 6. The viscosity ratio provides information on the relative number of active junctions that are produced by (a) shear deformation, (b) aggregation with SDS micelles, and (c) increase in the hydrophobicity at higher temperatures. At SDS concentration of less than 0.01 M, the

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Figure 7. Activation energies determined at zero-shear (b) and maximum shear (O) viscosity conditions of 1 wt % RDJ31-5 at various SDS concentrations.

viscosity ratio increases with temperature, which correlates with the increase in the hydrophobicity at high temperature. At ∼0.01 M SDS concentration, there is an inflection point, and beyond this point the viscosity ratio decreases with increasing temperature. The behavior implies that the presence of SDS molecules in the system has reached a critical point where the polymer network is weakened owing to the reduction in both the junction functionality and density. Therefore, with increasing temperatures, the active junctions are more readily disrupted owing to enhanced mobility of the chains. Activation Energy of the HASE-SDS/0.4 M NaCl System. The strength of the average network junction of the HASE-salt system at various SDS concentrations can be described by the activation energy, Ea, based on the Arrhenius equation

η0 ) Ae-Ea/RT

(3)

where η0 is the shear viscosity at low shear stress (Pa‚s), A the Arrhenius constant, Ea the activation energy (J/ mol), R the gas constant (8.314 mol/J0 K), and T the temperature (K). Two activation energies i.e., one determined from the zero-shear region (Ea0) and the other from the maximum viscosity region (Ea,max) are plotted as shown in Figure 7. The activation energies were determined on the basis of the Arrhenius expression as described by eq 3. The graph shows that both activation energies increase rapidly with SDS content, reaching a maximum at 0.008 and 0.009 M SDS for Ea0 and Ea,max respectively. They then decrease with further addition of SDS. The activation energy plot can be divided into three regimes: (I) Csds < 0.0015 M The strength of the network (as depicted by the activation energy) at the maximum viscosity region is larger than that at the zero-shear region. This suggests that the shear-induced creation of the additional active junction is easier in this region owing to the lower stability of the hydrophobic junction. (II) 0.0015 M < Csds < 0.009 M We observe that the strength of the junction at the zeroshear condition is larger than at the maximum shear-

Figure 8. Comparison of (a, top) shear viscosity of 1 wt % RDJ31-5 in 0.002 M (0) and 0.009 M (4) SDS/0.4 M NaCl solutions; (b, bottom) dynamic moduli of 1 wt % RDJ31-5 in 0.002 M [G′ (9), G′′ (0)] and 0.009 M [G′ (2), G′′ (4)] SDS/0.4 M NaCl solutions.

thickening region. More stable junctions are produced by the absorbed SDS molecules, which hinder the conversion of intramolecular to intermolecular junctions. (III) Csds > 0.009 M Beyond this point, SDS micelles saturate the hydrophobes and some of the junctions are detached by the absorbed SDS micelles as indicated by the decrease in the viscosity and the storage modulus. The higher strength of the network at the shear-thickening region compared to that at the zero-shear region suggests that under shear the probability of shear-induced creation of active junctions from the collisions of aggregates under flow contributes to the net formation of active junctions. It should be pointed out that the strength of the polymer network with excess surfactants is still higher than that in the absence of surfactant (even though the viscosity is lower), pointing to the fact that SDS molecules (or micelles) strengthen the network. Comparison between the Rheological Profiles of HASE in 0.002 and 0.009 M SDS/0.4 M NaCl Solutions. The shear viscosity profiles of 1 wt % RDJ31-5 in 0.002 M (solution A) and 0.009 M (solution B) SDS/0.4 M NaCl solutions are compared in Figure 8a. The zero-shear (∼1 Pa‚s) and the maximum shear-thickened (∼6 Pa‚s) viscosities for both solutions are identical. However, the critical stress before the catastrophic decrease in the viscosity of solution B is twice that of solution A (1.8 compared to 0.9 Pa). This means that the polymer network of Solution B is stronger and offers greater resistance to shear deformation. From the activation energy plot shown in Figure 7, Ea0 of solution B is 105 kJ/mol, while that of solution A is 80 kJ/mol. However, solution B possesses a lower modulus (see Figures 3 and 8b) compared to solution A, which means that solution B contains fewer mechanically active junctions. Thus, the stronger network found in solution B is attributed to the stabilization effects of

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absorbed surfactant molecules on the hydrophobic junctions. It should be pointed out that the dynamic data could not be fitted with a single-mode Maxwell model. Mechanism of the HASE-Salt-SDS System under Shear Flow. It is evident from the present study that the addition of large amounts of salt and SDS drastically alters the rheological behavior of the polymer solution. The mechanism responsible for this phenomenon can be best described by classifying the behavior into three regime, i.e., SDS concentration below c*(0.002 M), between c* and c** (0.009 M), and above c**. On the basis of previous understanding of the microstructure of HASE polymers in solution, the evolution of the network structure in the presence of salt and SDS under a deformation flow field is proposed. (a) SDS concentrations c < c* At this level of SDS content, SDS molecules bind onto the polymer hydrophobes forming mixed micellar aggregates. Absorption of SDS molecules on the associative junctions strengthens the polymer network. When stress is applied, shear-thickening occurs owing to the conversion of intra- to intermolecular associations. The SDS molecules stabilize the additional junctions in the form of mixed micellar aggregates, which results in the increase in the activation energy. At a much higher stresses, the associative junctions fragment to yield isolated aggregates, which are stabilized by the absorbed SDS molecules. However, owing to the finite amount of SDS molecules, the polymer hydrophobe can re-establish to form intramolecular associations. (b) SDS concentrations c* < c < c** Since the content of SDS is higher, significantly more SDS molecules bind onto the hydrophobic junctions yielding a stronger polymer network. When stresses are applied, the number of active junctions increases significantly owing to two factors: (i) conversion of intra- to intermolecular association, and (ii) creation of more active junctions with fewer polymer hydrophobes in a junction. Both these factors produce the shear-thickening response observed at moderate shear stresses. Addition of SDS micelles to the system enhances the strength and stability of associative junctions. Indirectly, the addition of SDS also increases the number of active junctions through the creation of more stable associative junctions with lower functionality (i.e., mixed micellar aggregates with fewer polymer hydrophobes). At higher applied stresses, associative junctions of similar strength are fragmented, which results in a catastrophic drop in the viscosity. This results in a chain reaction; i.e., weaker junctions fragment followed by stronger ones. The disengaged hydrophobes are stabilized by SDS micelles yielding isolated polymer clusters that do not contribute significantly to the viscosity (c) SDS concentrations c > c** At such high SDS content, the associative junctions of lower aggregation number are detached by bound SDS

Tan et al.

micelles. Associative junctions with higher aggregation number are more resistant to the effects of SDS micelles and are not disrupted. When stresses are applied, conversion of intra- to intermolecular association still occurs as revealed by the shear-thickening profile (Figure 2). The shear-thickening phenomenon is however, less dramatic. At higher applied stresses, some of the associative junctions are detached and the SDS micelles saturate the mixed micellar junctions, which decreases the functionality of polymer to unity, i.e., individual hydrophobe stabilized by SDS micelles. However, the strength of the junction is higher than that without SDS, indicating that not all the junctions are disrupted. Conclusions The shear viscosity of HASE polymers in 0.4 M NaCl is either enhanced or diminished, depending on the amount of SDS in the solution. Addition of salt shields the negatively charged ionic species along the polyelectrolyte backbones, which reduces the repulsive interactions of SDS molecules and those between the polymer backbones and SDS molecules. Consequently, more SDS molecules can bind onto the polymer hydrophobe, producing stronger and a larger number of active junctions. This deduction is supported by the increase in the elastic modulus at moderate SDS content, which also results in the increase in the viscosity. The addition of SDS beyond 5 mM significantly lowers the shear viscosity and the modulus. The maximum occurs at c/cmc of ∼11, compared to 1 for the HASE/SDS system in the absence of salt. The shear-thickening is due to the conversion of intrato intermolecular associations yielding a larger polymer network induced by deformation stresses. Both activation energies increase rapidly with addition of SDS, reaching a maximum at 0.008 and 0.009 M SDS for Ea0 and Ea,max respectively. They then decrease with further addition of SDS. Acknowledgment. One of the authors (H.T.) acknowledges the financial support provided by the university. We also thank Dr. Dave Bassett for his support in this research collaboration between NTU and Union Carbide. The funding provided by the National Science and Technology Board (NSTB) and the Ministry of Education has made this research possible. Supporting Information Available: The isothermal titration calorimetric data of SDS in 0.4 NaCl solution, showing a decrease in the cmc in the presence of salt, and the viscosity profiles of 1 wt % RDJ31-5 in (a) 0.0, (b) 0.002, (c) 0.007, and (d) 0.02 M SDS measured at 10-30 °C (2 pages). This material is available free of charge via the Internet at http://pubs.acs.org. LA991691J