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Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 66–73

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

hear rheology of anionic and zwitterionic modified polyacrylamides

ónica Rodrígueza, Jorge Xuea, Laura M. Gouveiaa, Alejandro J. Müllera,∗, A. Eduardo Sáezb,ulien Rigolini c, Bruno Grassl c

Grupo de Polímeros USB, Departamento de Ciencias de los Materiales, Universidad Simón Bolívar, Apartado 89000, Caracas 1080-A, VenezuelaDepartment of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USALaboratoire de Physico-chimie des Polymères, IPREM UMR CNRS/UPPA 5254, Pau, France

r t i c l e i n f o

rticle history:eceived 2 August 2010eceived in revised form8 September 2010ccepted 15 October 2010vailable online 23 October 2010

eywords:

a b s t r a c t

Two groups of copolymers were synthesized from high molecular weight polyacrylamides. One groupof copolymers consisted of sulfonated, anionic copolymers (PAM-S) of acrylamide with the sodium saltof 2-acryloamido-2-methyl-1-propane sulfonic acid, and the other consisted of zwitterionic copolymers(PAM-Z) of acrylamide with a sulfobetaine methacrylate monomer. The shear rheology of aqueous solu-tions of the copolymers and their mixtures was studied experimentally. Solutions of both copolymersexhibit shear thinning behavior in the range of concentrations explored. Solutions of mixtures of twocopolymers (PAM-Z and PAM-S) exhibited a slight viscosity synergy at high relative contents of PAM-S.

olyacrylamidewitterionic copolymersTATormlike micelles

mphiphilic polymers

Addition of a relatively high concentration of an electrolyte (0.3 M NaCl) induces decreases in viscositydue to coil contraction and eliminates the synergy of the mixtures. Mixtures of the zwitterionic copoly-mer and a cationic surfactant, cetyl trimethylammonium p-toluene sulfonate (CTAT), were also studied.These solutions exhibit a strong synergistic effect at low-shear rates when the surfactant forms wormlikemicelles. In addition, oscillatory shear measurements demonstrate that PAM-Z/CTAT mixtures are sig-nificantly more elastic than CTAT solutions, which indicates that PAM-Z is effective in promoting micelle

ed by

entanglements, as reflect

. Introduction

The rheology of polyelectrolyte solutions has been studiedxtensively due to the wide variety of practical applications ofhis type of polymer as viscosity control additives. A particularharacteristic that makes polyelectrolytes attractive is the facthat their molecular conformation can be tailored by manipu-ating pH, temperature, and addition of low-molecular weightlectrolytes or other species capable of electrostatic interactionsith the polyelectrolyte. Molecular parameters that influence rhe-

logical behavior include number, type, and distribution of chargedroups along the chain, hydrophobic/hydrophilic balance, distancef charged moieties from the backbone, and counterion type. Ionicroups present in polyelectrolyte chains may interact with otherharged groups along the same or other chains leading to, for exam-le, coil expansion due to repulsion of similar charges or binding

hrough attraction of opposite charges. Counterions act as screen-ng agents of the repulsive or attractive effects of like-charges alonghe chains, which can result in a less linear (or stretched/expanded)onformation [1].

∗ Corresponding author.E-mail address: amuller@usb.ve (A.J. Müller).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.10.024

the increase in relaxation time with PAM-Z content.© 2010 Elsevier B.V. All rights reserved.

Amphoteric water-soluble polymers contain both anionic andcationic charges. Such materials can be polymeric zwitterionswith positive and negative charges on the same pendant groups(betaines) or on the same backbone (ampholytes). Polybetaines andpolyampholytes have unusual phase behavior and solution prop-erties. Unlike polyelectrolytes, polybetaines and charge-balancedpolyampholytes are frequently more soluble and show higher vis-cosities in salt than in water solution. Hence, these polymers areknown as antipolyelectrolytes. For polyampholytes with a positiveor negative large net charge, salt addition results in a reduced vis-cosity and drop in coil size; therefore, the polyampholyte behavesas a polyelectrolyte [2].

Previous work has been conducted on acrylic, sulfonate-containing monomers. Different to others sulfonate monomers,2-acrylamido-2-methylpropanesulfonic acid (AMPSA) is hydrolyt-ically stable. Copolymers of the sodium salt of AMPS withacrylamide have potential in oil-field applications [3–5]. Polybe-taines have found utility as water and brine viscosifiers and brinedrag reduction agents [1]. These applications are aided by the

unusual interplay between positive and negative charges on thesame group or backbone, between chains, and/or between chainsand external electrolytes.

In this work, we study the shear rheology of two different acry-lamide copolymers: acrylamide copolymerized with sodium salt

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2

2

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2

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M. Rodríguez et al. / Colloids and Surfaces

f AMPS (PAM-S), and acrylamide copolymerized with a sulfobe-aine methacrylate monomer (PAM-Z). Copolymer concentrationnd ionic strength effects were studied for both systems. Addition-lly, the effect of formulating novel mixtures of PAM-S and PAM-Zn aqueous solutions was studied in the semi-dilute regime.

Solutions of polymer and surfactant mixtures are commonn many industrial formulations. It is well known that certainolymer/surfactant mixtures may exhibit significant rheologicalynergies, which can be utilized to create efficient formulationsor applications requiring viscosification. Cooperative interactionshat lead to viscosity synergies may result from: (i) hydrophobicssociations between the polymer backbone and the hydropho-ic tail of the surfactant [6], (ii) hydrophobic associations betweenolymer pendant side chains and the surfactant tail [7–12], or (iii)ttractive columbic forces between opposite charges [13]. In recentears, there have been well documented cases of polyelectrolytesnd oppositely charged surfactants that exhibit associative phaseeparation originating from attractive electrostatic forces betweenhe two species [14]. A particular type of interaction that has gainedttention involves mixtures of water soluble polymers with sur-actants that form wormlike micelles [15–24]. At high surfactantoncentrations, physical networks between polymer moleculesnd wormlike micelles take place, inducing a significant increasef the viscosity of the solutions. Here, we explore the possible rhe-logical synergy of mixtures between wormlike micelles and thewitterionic copolymer of acrylamide synthesized in this work.

. Materials and methods

.1. Materials

Acrylamide (AM) (purity >99%) was purchased from ABCR.-Acrylamido-2-methyl-1-propanesulfonic acid sodium salt

n a 50 weight% water solution, [3-(methacryloylamino)ropyl]dimethyl(3-sulfopropyl) ammonium hydroxide inner saltMPDSA, 96%), potassium persulfate (KPS, >99%), cetyltrimethy-ammonium p-toluenesulfonate (CTAT, 99%) and NaCl (99%) werebtained from Aldrich. All chemicals were used without furtherurification. Deionized water, with an electrical conductivity of8 M� cm at 25 ◦C, was filtered through a 0.22 �m Millipore filterrior to use.

.2. Polymer synthesis

Each copolymer was synthesized using the gel polymerizationrocess; i.e. free radical polymerization with a high monomer con-entration (>30 wt%) [25]. For example, a zwitterionic copolymeras prepared by mixing a given amount of acrylamide (57 g) andPDSA (12.34 g) in Milli-Q water (120.66 g) followed by a 30-min

urified nitrogen purge. A 10 mL solution with 800 mg of KPS andhe whole reaction mixture was placed in a 250 mL water-jacketedylindrical reactor and heated to 50 ◦C. Once the reaction finished,he gel obtained was cut into small pieces, which were precipitatedn an excess of ethanol with a Waring laboratory blender 7009G toet a fine powder. After three washings in ethanol, the powder wasried in a vacuum oven.

.3. Polymer characterization

The weight-average molar mass (Mw) and polydispersity of

he copolymers were determined by size exclusion chromatog-aphy (SEC-MALS), using a Waters Alliance 2690 chromatographquipped with four Shodex OHpak columns (SB-807HQ, SB-806HQ,B-804HQ and SB-802.5HQ) and three online detectors: a differen-ial refractometer, a UV–visible (UV) photodiode spectrometer and

sicochem. Eng. Aspects 373 (2011) 66–73 67

a DAWN® HELEOSTM II utilizing a 120-mW solid-state laser operat-ing at 658 nm and fitted with a K5 cell. A 0.1-M solution of NaNO3was used as the eluent at a flow rate of 0.5 mL/min. The weight-average molar mass and polydispersity were obtained from datacollected using the ASTRA SEC-software (version 5.3, Wyatt Tech-nology Corp., USA). The calculation of molar mass was carried outaccording to the Zimm fit method [26]. The refractive index incre-ment, dn/dc, was calculated with a linear mixing rule accordingto the dn/dc values of respective homopolymers: 0.184, 0.132 and0.159 mg/mL for PAM, PAM-S and PAM-Z respectively. The molarcomposition of each copolymer was determined by 1H NMR (BrukerAVANCE 400 MHz) in D2O at 25 ◦C. The purity of the final productwas also confirmed.

2.4. Solution preparation

An appropriate amount of polymer powder was dissolved inwater to obtain concentrated stock polymer solutions. The powderwas pre-hydrated. This was followed by gentle magnetic stirringfor several days until complete dissolution. Final desired concen-trations of polymer solutions were obtained by diluting the stocksolution with water. Solutions of polymer/NaCl and polymer/CTATwere prepared by mixing appropriate volumes of stock solutions.The solutions were allowed to stand unstirred for at least 2 h beforeeach experiment to relax any structure formed during stirring(uncontrolled shear history). The CTAT solutions were maintainedat 25 ◦C temperature to keep them above the Krafft point of CTAT(22.5 ◦C).

2.5. Shear rheometry

The steady shear and dynamical rheological experiments wereconducted using a Rheometrics ARES shear rheometer equippedwith a double-wall Couette fixture. All tests were performed at25 ◦C and a shear rate interval of 1–800 s−1. The frequency spectrawere conducted in the linear viscoelastic regime of the samples,as determined in preliminary tests by dynamic stress sweep mea-surements.

3. Results and discussion

Two different modified polyacrylamides (PAMs) were syn-thesized: a copolymer of acrylamide with the sodium salt of2-acrylamide-2-methylpropanesulfonate (AMPS), and a copolymerof acrylamide with a sulfobetaine methacrylate monomer. Theamount of charged hydrophobic groups present in each copolymer,as determined by 1H NMR, is shown in Table 1. Molar mass deter-minations by means SEC-MALS showed differences in molecularweight for the four modified polyacrylamide samples synthesized(Table 1). The chemical structures of both polyelectrolytes aredepicted in Fig. 1. The PAM-Z copolymers are zwitterionic poly-mers, with the anionic and cationic groups on the same monomericunit. The PAM-S copolymers have only an anionic group (sulfonate)on each lateral chain (Fig. 1). In the PAM-Z copolymers, the ioniccharges in each lateral chain balance each other, while in the PAM-S copolymers the charges are not balanced within the chain. ThePAMs copolymers are identified as PAM-X-Y, where X refers to thecharged group present in the polyacrylamide (Z: zwitterionic mod-ified polyacrylamide; S: sulfonate modified polyacrylamide) and Ycorresponds to molecular weight in MDa.

The rheological behavior of aqueous solutions of PAM-Z-1.5 isshown in Fig. 2a. The solutions follow the typical behavior of non-associating polymer solutions: at low concentrations, the behaviortends to be Newtonian, while at higher polymer concentrations,shear thinning is evident, with onset shear rates for shear thinning

68 M. Rodríguez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 66–73

Table 1Properties of the polyacrylamide copolymers and their aqueous solutions. 1H NMR and SEC were used to determine structural properties. Viscometric parameters (at 1 s−1

shear rate) were obtained from fits of Eq. (1) to experimental data (aqueous solutions at 25 ◦C) in the concentration range specified.

Sample Mw × 10−6 (Da) Polydispersity index Comonomer content (mol%) ˛ (ppm−1) Concentration range (ppm)

PAM-S-2.3 2.3 4.1 5 0.0147 <10,0006

tpcddobimoebiwiZc

adStwsda(is

PAM-S-1.6 1.6 6.4 4.PAM-Z-1.5 1.5 4.0 5PAM-Z-1.8 1.8 5.5 5

hat decrease as polymer concentration increases. The zwitterionicolyacrylamide used here has hydrophobic lateral chains with aationic and an anionic charge along each. The presence of twoifferent charges on each PAM-Z lateral group may lead to twoifferent effects. First, the charges increase the hydrophilic naturef the modifications, thus preventing hydrophobic associationsetween lateral chains. As a consequence, the solution viscosity

s expected to be lower than what would be expected for poly-ers with similar but uncharged hydrophobic side chains. On the

ther hand, the repulsion between the ionic groups encourages coilxpansion, which could increase the solution viscosity. A balanceetween these two effects determines the polymer solution behav-

or and its capacity to associate with other species. A comparisonith a similar molecular weight PAM homopolymer can be found

n Fig. 3. It is clear that the introduction of the comonomers (i.e.,or S) increases the shear viscosity of the solution at equivalent

oncentrations.The solutions of PAM-S-1.6 are shear thinning at concentrations

s low as 50 ppm (Fig. 2b). Even though the molecular weight andegree of substitution are similar between PAM-Z-1.5 and PAM--1.6 (Table 1), comparison of Fig. 2a and b clearly shows thathe sulfonated polyacrylamide solutions have higher viscosities,hich is a consequence of a more expanded coil due to electro-

tatic repulsion between the sulfonate side chains. The viscosity

ifference between solutions of the synthesized polymers can beppreciated in the comparison of low-shear rate viscosities in Fig. 3data for a PAM homopolymer of similar molecular weight wasncluded for comparison purposes, as indicated above). The lowhear rate viscosities for each polymer were well correlated with

Fig. 1. Chemical structure of the sulfonated (a) and

0.0150 <20,0008.61 × 10−4 <50000.00177 <10,000

polymer concentration using the equation

� = �s(1 + ˛c) (1)

where c is polymer concentration, �s is the water viscosity(0.91 mPa s at 25 ◦C), and ˛ is a constant (results not shown). Val-ues of this parameter for the four polymers used in this work arereported in Table 1. These values have been calculated at a shearrate of 1 s−1 (results not shown). Note that ˛, as expected, increaseswith molecular weight. Differences between the two polymers canbe attributed to the higher degree of coil expansion in the sul-fonated copolymers, as discussed above.

The side groups in the modified PAMs can influence solutionviscosity, not only through their effect on coil expansion, but alsothrough possible interactions between side chains of different poly-mer molecules. This effect potentially can be more significantin solutions of mixtures of polymers. We investigated mixturesbetween the sulfonated and zwitterionic polymers synthesized inthis work. The mixtures were prepared at a fixed total polymerconcentration. Assuming that molecules of the two polymers donot interact in a way that is substantially different from interac-tions between molecules of the same species, a generalization ofEq. (1) for the mixtures yields

�m = �s(1 + ˛1c1 + ˛2c2) (2)

Let c = c1 + c2 be the total polymer concentration. The measuredvalues of the low-shear rate viscosity of solutions of each purepolymer are

�i = �s(1 + ˛ic), i = 1, 2 (3)

zwitterionic (b) modified polyacrylamides.

M. Rodríguez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 66–73 69

10001001010.1

1E-3

0.01

0.1 20000 ppm

10000 ppm

8000 ppm

4000 ppm

2000 ppm

500 ppm

γ (s-1)

.

-1.10001001010.1

1E-3

0.01

0.1

η (P

a.s

)η

(Pa.s

)

10000 ppm

6000 ppm

4000 ppm

1000 ppm

250 ppm

50 ppm

b

a

F(

t

�

Piraimts

FPh

100806040200

0.01

0.1

η (P

a s

)

[PAM-S] %

PAM-Z-1.5 + PAM-S-2.3

PAM-Z-1.8 + PAM-S-1.6

γ (s )

ig. 2. Shear viscosity as a function of polymer concentration for (a) PAM-Z-1.5, andb) PAM-S-1.6 aqueous solution at 25 ◦C. Legend shows polymer concentration.

Combination of Eqs. (2) and (3) leads to a linear mixing rule forhe mixture viscosity

m = c1

c�1 + c2

c�2 (4)

Fig. 4 shows low shear viscosities of solutions of mixtures ofAM-S and PAM-Z at a total concentration c = 10,000 ppm. Theres a slight but noticeable positive deviation from the linear mixingule except at low PAM-S content for both mixtures. Interestingly,t PAM-S contents between 80 and 100%, the viscosity is approx-

mately constant. The slight synergy exhibited by these mixtures

ight be a consequence of molecular interactions between thewo polymers. At high PAM-S content, entanglement networks inolution are dominated by the larger coils of this copolymer. The

1000010001001E-3

0.01

0.1

PAM-Z

PAM-S-2.3

PAM-S-1.6

PAM-Z-1.5

PAM-Z-1.8

PAM-1.7

η (P

a s

)

C (ppm)

PAM

PAM-S

ig. 3. Shear viscosity (at �̇ = 10 s−1) as a function of polymer concentration forAM-Z, PAM-S and PAM aqueous solutions at 25 ◦C. Lines are visual guides. PAMomopolymer data from Ref. [13].

Fig. 4. Shear viscosity (at �̇ = 1 s−1) of PAM-S/PAM-Z aqueous solutions at a totalpolymer concentration of 10,000 as a function of PAM-S content at 25 ◦C. Solid linescorrespond to Eq. (4).

strength of these networks is not affected by the replacement inthe solution of PAM-S molecules with PAM-Z molecules at first,as evidenced by the near constant viscosity, which suggests thatthe smaller PAM-Z coils are participating in the networks of PAM-S coils. The viscosity difference observed between the two PAM-Zsolutions, which can be attributed to molecular weight differences,loses importance as the PAM-S content increases.

An increase in the ionic strength of a solution containing apolyelectrolyte is known to yield decreases in viscosity of poly-electrolytes due to the screening of charges along the chain ofthe polymer by free counterions, which results in coil contraction.Eventually, this effect saturates with an increase in ionic strength.For the polymer solutions studied here, the viscosity decreasesmonotonically with NaCl addition until a constant viscosity value isachieved at NaCl concentration around 0.3 M (results not shown).The suppression of electrostatic effects in the presence of an elec-trolyte means that viscosity values will be determined by molecularweight and comonomer content if no other species are present.Fig. 5 shows viscosity variation with shear rate for PAM-S-1.6 andPAM-Z-1.8 solutions of different concentration in solutions con-taining 0.3 M NaCl. Solutions of both copolymers exhibit Newtonianbehavior at low concentrations and a lower degree of shear thinningat higher concentrations than in the absence of salt (e.g. compareFigs. 2b and 5b).

The effect of salt addition on low shear rate viscosity of thesolutions is shown in Fig. 6. As expected, NaCl addition to PAM-S solutions (Fig. 6b) results in a substantial drop in viscosity due tothe polyelectrolytic nature of this copolymer. This drop is more pro-nounced than the effect of molecular weight in the range explored.The viscosity decrement in the presence of salt is due to sulfonatecharge screening by sodium ions, which causes coil contractionand a consequent decrease in hydrodynamic volume. In the pres-ence of salt, both PAM-S copolymers have similar viscosities below2000 ppm, but a clear departure from this behavior is present athigher concentrations, where the viscosity becomes more sensitiveto molecular weight.

The effect of salt addition on the viscosity of zwitterionic poly-mer solutions is strongly related to the distribution of charges alongthe polymer chain. Specifically, copolymers that contain differentside groups carrying opposite charges might adopt a compact coilconformation in the absence of other electrolytes due to attrac-

tion between oppositely charged side chains. The coil would thenexpand as the attracting charges are shielded by presence of ionsprovided by a low molecular weight salt. As a consequence, saltaddition would result in an initial increase in viscosity. Whenboth opposite charges are present in the same side chain, as is

70 M. Rodríguez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 66–73

1000100101

1E-3

0.01

0.120000 ppm

10000 ppm

8000 ppm

4000 ppm

2000 ppm

500 ppm

1000100101

1E-3

0.01

10000 ppm

8000 ppm

4000 ppm

1000 ppm

250 ppm

50 ppm

b

a

γ (s-1)

.

γ (s-1)

.

η (P

a.s

)η

(Pa.s

)

Fig. 5. Shear viscosity as a function of polymer concentration for (a) PAM-Z-1.8, and(b) PAM-S-1.6 aqueous solutions with 0.3 M NaCl, at 25 ◦C. Legend shows polymerconcentration.

100001000

0.001

0.010

C (ppm)

PAM-Z-1.5 PAM-Z-1.5 + NaCl PAM-Z-1.8 PAM-Z-1.8 + NaCl

a

b

100001000100

0.001

0.010

0.100 PAM-S-2.3

PAM-S-1.6

η (P

a s

)η

(Pa s

)

C (ppm)

+ NaCl 0.3 M

Without NaCl

Fig. 6. Shear viscosity (�̇ = 10 s−1) of (a) PAM-Z/NaCl 0.3 M, and (b) PAM-S/NaCl0.3 M aqueous solutions, as a function of polymer concentration at 25 ◦C. Lines arevisual guides.

Table 2Viscometric parameters for the copolymers used in this work obtained from fits ofEq. (5) to experimental data (at 25 ◦C and shear rate 1 s−1) in the concentration rangespecified. All fits cover a concentration range up to 10,000 ppm. All solutions wereprepared in 0.3 M NaCl.

Sample ˛ (ppm−1) ˇ (ppm−2)

−5 −7

PAM-S-2.3 5.05 × 10 7.37 × 10PAM-S-1.6 6.01 × 10−6 2.41 × 10−7

PAM-Z-1.5 4.12 × 10−4 1.18 × 10−7

PAM-Z-1.8 5.34 × 10−4 6.59 × 10−8

the case for the PAM-Z copolymers synthesized in this work,effects of ionic strength on coil conformation may be more com-plex [27–29]. Mahon and Zhu [29] studied salt presence effectson poly(2-methacyloyl oxyethyl phosphorylcholine) (PMPC) solu-tions. This polymer has similar structure to the PAM-Z copolymerssince the side chain has both a positive and a negative charge. Themain difference is that the negative charge in PMPC is the one clos-est to the backbone. Using ions with various sizes, Mahon and Zhufound that the negative charge group is less important with regardsto changes in hydrodynamic volume than the positive charge groupsituated at the end of the side chain. The main effect observed wasa decrease in the coil size when small anions were added. Theyhypothesized that anions acted as a bridging point between cationsof two different side chains, thereby contracting the coil. In our case(Fig. 6a), different qualitative trends are obtained with salt additionfor the PAM-Z copolymers: for the higher molecular weight copoly-mer (PAM-Z-1.8) a substantial decrease in viscosity is observedupon salt addition, which would be consistent with the mecha-nism proposed by Mahon and Zhu. However, PAM-Z-1.5 does notexhibit appreciable viscosity variations. It could be argued that coilcontraction by bridging is more effective as the molecular weightincreases since there would be a higher probability of intrachainbridging.

A distinguishing characteristic of the copolymer solutions in thepresence of 0.3 M NaCl is the nonlinear nature of the viscosity vs.concentration relation in the same range in which linearity pre-vailed in the absence of salt. The low shear rate viscosities (Fig. 6)can be correlated with copolymer concentration only if quadraticterms are introduced,

� = �s(1 + ˛c + ˇc2) (5)

Values of ˛ and ˇ for the four copolymers used in this work arereported in Table 2 for solutions containing 0.3 M NaCl. Comparisonbetween Tables 1 and 2 shows that, while excess salt decreases ˛for the zwitterionic copolymer, changes of much higher magnitudeare observed for the sulfonated copolymer, which is consistent withthe expected trend in coil contraction.

Salt addition effects were studied for PAM-S and PAM-Z mix-tures at a fixed total concentration of 10,000 ppm. A mixing rulebased on Eq. (5) applied to each polymer is

�m = �s(1 + ˛1c1 + ˇ1c21 + ˛2c2 + ˇ2c2

2) (6)

Fig. 7 shows that the viscosity of PAM-S-1.6/PAM-Z-1.8 mixtureas a function of PAM-S-1.6 content is well described by this mixingrule, which leads to conclude that the two copolymers do not inter-act in the presence of such a relatively high concentration of NaCl.For the PAM-S-2.3/PAM-Z-1.5, the mixing rule represents the gen-eral trend of the data, although the experimental change between40 and 60% PAM-S seems more abrupt than the model values.

The results presented show that mixtures of the two copolymers

synthesized in this work do not exhibit significant rheological syn-ergy due to lack of interaction between the two species in solution,even as their conformation changes with variations in salt content.In what follows, we explore viscosification synergy in mixturesof the copolymers with a cationic surfactant capable of forming

M. Rodríguez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 66–73 71

100806040200

0.01

0.1η

(Pa s

)

PAM-Z-1.5 + PAM-S-2.3 PAM-Z-1.8 + PAM-S-1.6

[PAM-S] %

Fpl

w(

wtsaTcsfcftwwgpScfstoltc

lmracutmitusr

tPcoC

10001001010.1

0.01

0.1

1

10 5000 ppm

3500 ppm

2500 ppm

2000 ppm

1500 ppm

800 ppm

500 ppm

250 ppm

100 ppm

CTATη (P

a s

)

-1.

relatively small quantities of PAM-Z to entangled wormlike micellesolutions are a consequence of specific interactions between thesulfobetaine units and the CTAT wormlike micelles. It is possi-ble that PAM-Z chains bind to CTAT wormlike micelles through

5000400030002000100000.01

0.1

1

PAM-Z-1.5

PAM-Z-1.8

η (P

a s

)

ig. 7. Shear viscosity (at �̇ = 1 s−1) of PAM-S/PAM-Z/0.3 M NaCl solutions at a totalolymer concentration of 10,000 ppm, as a function of PAM-S content at 25 ◦C. Solid

ines correspond to Eq. (6).

ormlike micelles, cetyltrimethylammonium p-toluene sulfonateCTAT).

In general, surfactant/polymer interactions in solution occurhen surfactant aggregates attach to polymer chains due to elec-

rostatic or hydrophobic interactions. The surfactant moleculestart to bind to the polymer when their concentration exceedscritical value, the critical aggregation concentration (CAC) [30].

he CAC value is usually substantially lower than the criti-al micelle concentration (CMC), at which surfactant monomerstart aggregating to form free micelles [31]. In particular, CTATorms spherical micelles at concentrations just above the criti-al micelle concentration (CMC = 0.26 mM) [32–34]. This surfactantorms rodlike micelles beyond the critical rodlike concentra-ion (CRC = 1.97 mM) [34]. The size of rodlike micelles increasesith concentration, eventually forming wormlike micelles [35]. Inormlike micellar systems, the relatively long cylindrical aggre-

ates of self-assembled surfactant molecules behave similarly toolyelectrolytes, but they can break and re-form dynamically.olutions of these aggregates may exhibit relatively high shear vis-osities, and even can develop shear thickening behavior due to theormation of shear-induced cooperative structures (SIS) [32]. Theize and conformation of wormlike micelles change with surfac-ant concentration, ionic strength, flow field applied, and additionf other components to the solution. When an increase of micellarength is promoted (e.g. due to the addition of screening elec-rolytes) the viscosity of the solution increases, and the micellesan form an entangled multi-connected network [36–43].

Solutions of CTAT may exhibit a pseudo-Newtonian behavior atow shear rates, followed by a shear thickening regime at inter-

ediated shear rates and a shear thinning behavior at high shearates. The shear-thickening behavior is typical of concentrationsbove but near the CRC [34], at which the micelles are cylindri-al in shape but not long enough to form long-range interactionsnder static conditions, so that a minimum shear rate is neededo induce the formation of SIS. Since the length of the wormlike

icelles increases with concentration [35], the value of the crit-cal shear rate decreases as CTAT concentration increases. Abovehe semi-dilute threshold (11 mM), the micelles are fully entanglednder static conditions and therefore exhibit relatively high zero-hear rate viscosities with slight shear thickening (this behavior isepresented by the 20 mM CTAT curve in Fig. 8).

Fig. 8 shows the effect of addition of PAM-Z-1.8 to a CTAT solu-

ion at a fixed CTAT concentration of 20 mM. The measurements forAM-Z/CTAT mixtures were limited to relatively low polymer con-entrations and high CTAT concentration, since at moderate levelsf surfactant addition macroscopic phase separation was observed.opolymer addition causes significant increases in the low shear

γ (s )

Fig. 8. Shear viscosity vs. shear rate for PAM-Z-1.8 solutions in CTAT 20 mM at 25 ◦C.Legend shows polymer concentration.

viscosity of the solutions. Note that the slight shear thickening ofthe CTAT solution is suppressed at relatively low copolymer con-centrations. At copolymer concentrations higher than 2500 ppm,the low shear rate pseudo-Newtonian plateau disappears, which isa possible indication that the solutions are developing a yield stress,although this cannot be completely elucidated from the data.

The viscosity increment caused by copolymer addition goesbeyond what would be expected. A linear mixing rule such as Eq.(2) predicts in this case a mixture viscosity given by

�m = �c + �s˛c (7)

where �c is the viscosity of the CTAT solution (which includes thesolvent contribution), and ˛ is the linear correlation constant forPAM-Z (Table 1). Fig. 9 clearly shows a synergistic behavior for mix-tures of CTAT with the two synthesized zwitterionic copolymers, interms of low shear rate viscosity. Note that mixture viscosity getsto be almost two orders of magnitude higher than the predictionsof Eq. (7).

The synergistic viscosity increases produced by the addition of

C (ppm)

Fig. 9. Low shear rate viscosity (�̇ = 2 s−1) as a function of polymer concentrationfor PAM-Z/CTAT 20 mM solutions at 25 ◦C. The solid line is the linear mixing rulegiven by Eq. (7) for PAM-Z-1.8.

72 M. Rodríguez et al. / Colloids and Surfaces A: Phy

1001010.1

1

G'(P

a),

G''(

Pa)

ω (s-1)

ω (s-1)

τr=5.567 s

(C=1000 ppm)

τr=0.119 s

(C=0 ppm)

a

1001010.10.010.1

1

τr=45.928 s

(C=1500 ppm)

G'(P

a),

G''(

Pa)

b

τr=217.329 s

(C=2000 ppm)

τr=0.225 s

(C=500 ppm)

FPP

hmtiwoo

taitimmGerga

4

prpsp

[

[

[

[

[

[

[

[17] J.A. Shashkina, O.E. Philippova, Y.D. Zaroslov, A.R. Khokhlov, T.A. Pryakhina, I.V.

ig. 10. G′ (open symbols) and G′′ (filled symbols) as a function of frequency forAM-Z-1.5/CTAT solutions in oscillatory shear flow. Legend: �r: relaxation time; C:AM-Z-1.5 concentration. CTAT concentration fixed at 20 mM (C = 0). T = 25 ◦C.

ydrophobic or electrostatic interactions. The presence of CTATay promote the formation of PAM-Z aggregates that attach to

he CTAT worm-like micelles, forming “sticky points” that wouldnduce intermicelle physical binding and thus a strengthening of

ormlike micelle entanglements [44]. In this regard, the zwitteri-nic polyacrylamide/wormlike micelle system behaves similar tother surfactant/polymer systems [15–24,44,45].

Fig. 10 shows the rheological behavior of PAM-Z-1.5/CTAT solu-ions under oscillatory shear at different PAM-Z concentrations andfixed CTAT concentration (20 mM). The solutions become increas-

ngly more elastic as PAM-Z is added as evidenced by the shift ofhe cross-over point between G′ and G′′ to lower frequencies. Thenverse of the crossover frequency can be considered the longest

ean relaxation time for the formation or breakup of the fluidicrostructure. In this case, the elastic behavior (G′ higher than′′) occurs when the wormlike micelles behave as a mechanicallyntangled network at frequencies above the crossover point. Theesults indicate that PAM-Z is effective in promoting micelle entan-lements, which would be consistent with the hypothesis of PAM-Zttached to the micelles that enhance intermicelle interaction.

. Concluding remarks

One zwitterionic polyacrylamide (PAM-Z) was tested in theresence of an anionically modified polyacrylamide (PAM-S). The

heological behavior shows a synergistic effect when the pro-ortion for each polymer was changed. When NaCl is added theynergistic effects disappear due to the screening of the chargesresent in both polymers.

[

sicochem. Eng. Aspects 373 (2011) 66–73

Solutions of PAM-Z and a cationic surfactant (CTAT) were alsoinvestigated. The results obtained demonstrate that amphiphilicpolyacrylamides induce a significant rheological synergy in solu-tions of cationic wormlike micelles. A considerable strengtheningof micellar entanglements and formation of cooperative structuresamong CTAT wormlike micelles can be induced by the presenceof relatively small amounts of PAM-Z. These interactions lead tosolutions with appreciably higher low shear viscosities and com-paratively higher elastic character at high frequencies. The resultsalso suggest that zwitterionic polymers micellar aggregates areformed on the CTAT wormlike micelles and they become points ofinteraction between wormlike micelles, thus strengthening physi-cal entanglements between the micelles.

Acknowledgments

We acknowledge funding received by grant DID-GID-G02. Wealso thank the PCP program (Programa de Cooperación en Post-grado, PCP project: “Rheology of dispersed systems”) for fundingexchange visits between France and Venezuela. We would also liketo thank one of the referees of this work for the pertinent anddetailed comments that allowed us to substantially improve thismanuscript.

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