Artigo Eor Polimeros

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1560 D.A.Z.Wever et al. / Progress in Polymer Science36 (2011) 15581628

H12MDI bis(4-isocyanateocyclohexyl)methaneHDI hexamethylene di-isocyanateHEC hydroxyethyl celluloseHEUR hydrophobically modified ethoxylated ure-

thaneHHM-HEC hydrophobicallyhydrophilically modi-

fied hydroxyethyl celluloseHMPAM hydrophobically modified polyacrylamideHM-CMC hydrophobically modified carboxymethyl

celluloseHM-EHEC hydrophobically modified ethyl hydrox-

yethyl celluloseHM-HEC hydrophobically modified hydroxyethyl

celluloseHM-HPC hydrophobically modified hydroxypropyl

celluloseHM-polysaccharides hydrophobically modified

polysaccharidesHPAM partially hydrolyzed polyacrylamide

HTAC hexadecyltrimethylammonium chlorideIPBC 4-isopropylbenzyl chlorideIPDI isophorone diisocyanateKDS potassium dodecyl sulfateLCST lower critical solution temperatureMA maleic anhydrideMAA methacrylic acidMAM methacrylamideMeEAM N-methyl-N-(4-ethyl-phenyl)-

acrylamideMO methyleneoxideMWD molecular weight distributionNaA sodium acrylate

NaAMB sodium 3-acrylamido-3-methylbutanoateNaAMPS sodium 2-acrylamido-2-methylpropane-

sulfonateNaC10S sodium decyl sulfateNaC14S sodium tetradecyl sulfateNaC12(EO)2S sodium dodecyl-di(ethyleneoxide)-

sulfateNAEA 2-(1-naphthylacetyl)ethyl acrylateNAEAm 2-(1-naphthylacetamido)ethyl acrylamideNaMAMB sodium 3-methacrylamido-3-methylbu-

tanoateNBAM N-benzylacrylamideNIPAM N-isopropylacrylamide

NNDAM N,N-dimethyl acrylamidenm nanometerNMA [(1-naphthyl)methyl]acrylamideNMR nuclear magnetic resonanceNP nonylphenol ethoxylatesNPEAM N-phenethylacrylamideNRET non-radiative energy transferOG n-octyl -d-glucopyranosideOOIP original oil in placeOTG n-octyl -d-thioglucopyranosidePAA poly(acrylic acid)PAGE poly(alkylglycidyl ether)PAM polyacrylamide

PDI polydispersity indexPEO poly(ethylene oxide)PGSE pulse gradient spin echoPMAAM N-phenylmethacrylamideppm parts per millionPy pyrene

RRF residual resistance factorS-HEC hydrophilically modified hydroxyethyl cel-

luloseS-G HEUR step-growth hydrophobically modified

ethoxylated urethaneSANS small-angle neutron scatteringSEC size exclusion chromatographySDS (NaC12S) sodium dodecyl sulfateSMR surfactant to micelle ratioSOBS sodium octyl benzene sulfonateTHF tetrahydrofuranTMSPMA 3-[tris(trimethylsilyoxy)sily]propyl

methacrylate

TTAB tetradecyltrimethyl ammonium bromide

1. Introduction

Enhanced oil recovery (EOR) is a challenging field fordifferent scientific disciplines. The importance of this fieldis highlighted by the number of patents (mainly filedby multinational companies) involving polymers for EOR.Nevertheless, the limited number of patents filed in thelast 10 years (less than 25) demonstrates the difficultyof this research field as well as the relative maturity inthe scientific and technological concepts linked to rele-vant applications. Given the fact that the easily recoverableoil is running out and that much oil remains in the reser-voir after conventional methods have been exhausted, theimplementation of EOR is crucial to guarantee a contin-uing supply. In addition, alternative energy sources havenot yet proved to be capable of meeting the world energydemand, so that a mixture of different sources, includ-ing oil, is required to meet the world energy demandin the near future. According to Thomas [1], approxi-mately 7.01012 barrels of oil will remain in oilfields afterconventional methods have been exhausted; this valueconstitutes also the target recovery for EOR. Water-solublepolymers for EOR applications have been successfullyimplemented, mainly in Chinese oilfields [2,3]. The pur-pose of the water-soluble polymers in this application is toenhance the rheological properties of the displacing fluid.The oil production increases with the microscopic sweepof the reservoir and the displacement efficiency of the oil[4]. Indeed, the use of water-soluble polymers improvesthe wateroil mobility ratio [4], and leads to enhanced oilrecovery. However, given the harsh conditions present inmost oil reservoirs, new problems and limitations arisewith the use of water-soluble polymers. Besides positivelyaffectingsolution rheology, water-soluble polymers shouldwithstand high salt concentration, the presence of calcium,


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high temperatures (>70 C) and long injection times (atleast 12 months) [4,5]. High salt concentrations reduce thethickening capability of most ionic water-soluble polymerswhile the presence of calcium leads to flocculation [6].New water-soluble polymers were successfully tested athigher temperatures [7,8]. Associative water-soluble poly-mers were tested and showed promising results comparedto traditionally used polymers [9,10].

Severalstudies [1116] demonstratedthatthe oilis pro-duced faster (compared to water flooding), but also moreoil can be recovered. A mechanism based on the visco-elastic properties of the polymers has been proposed toexplain the higher oil recovery. This mechanism is mainlysupported by indirect evidence and mathematical models[1114].

Independently of the exact displacement mechanismand efficiency, the use of water-soluble polymers for EORstill constitutes a challenging field, at both industrial andacademic levels. Moreover, the broad variety of polymersstudied for EOR, at least at the academic level, clearlysuggests this research field as paradigmatic for water-soluble polymers in general. Indeed, fundamental scientificstudies linked to EOR, particularly those involving therelationship between the polymer structure and the cor-responding properties in aqueous solution (e.g. viscosity),have a conceptual character, thus providing a broad andgeneral understanding, valid also for other applicationfields. The interaction between polymer chains in aque-ous solution as well as the influence of the solute structureon the corresponding viscosity are of paramount impor-tance in EOR applications, but are studied and modeledthrough very general methods and theories. This generalinterest in structureproperty relationships is testified bythe constantly increasing amount of scientific literaturethat is dedicated to this topic [1720], the last dedicatedreview (from 1990 [21]) being already outdated by themost recent findings. As a consequence, the present reviewpaper concentrates on different water-soluble polymersfor EOR by discussing their (associative) behavior in watersolution and outlining the most general concepts thatcan be learned from the corresponding scientific litera-ture.

Section 2 presents some background information onoil recovery methods, thus defining the subject in termsof polymeric systems reviewed in this work. In Section3 the synthetic methods used for the different polymersare discussed, including the effect of the relationshipbetween the process conditions employed and the cor-responding polymer chemical structure. Subsequently,the chemical structures are linked to the correspondingrheological properties in water solution. The sensitiv-ity of the rheological behavior is also elucidated as afunction of external parameters, such as (the presenceof) mechanical shear, solution temperature, electrolytetype and concentration, pH, ionic strength and sur-factant type and concentration. Relations between thepolymer structure, i.e. chemical structure and topol-ogy, are proposed for the different polymers. Finally,in Section 4 we provide some general conclusions onthe most recent, relevant and generally accepted con-cepts.

2. Currently used EOR polymers

2.1. Polyacrylamide (PAM)

Polyacrylamidewas thefirstpolymerusedasthickeningagent for aqueous solutions. The thickening capabil-ity (increase of the corresponding solution viscosity) ofPAM resides mainly in its high molecular weight, whichreaches relatively high values (>1106 g/mol). In the gen-eral framework of EOR processes, PAM is mainly usedas the reference model system for chemical modifi-cation. Many authors have reported different attemptsto alter the chemical structure of PAM or to synthe-size new acrylamide-based copolymers with improvedproperties, i.e. shear resistance, brine compatibility andtemperature stability [2225]. The synthesis of the copoly-mer N,N-dimethyl acrylamide with Na-2-acrylamido-2-methylpropanesulfonate(NNDAMNaAMPS) was reportedby Sabhapondit et al. [22,23] and tested for its perfor-mance in EOR applications. The stability of the polymer athigh temperature was demonstrated by ageing at 120 Cfor 1 month [22]. By using a sand pack, an improvedperformance in terms of EOR for the NNDAMNaAMPScopolymer [23] as compared to unmodified partiallyhydrolyzed polyacrylamide, HPAM, was demonstrated.In another example, Song et al. [24] reported the syn-thesis of starch-graft-poly(acrylamide-co-(2-acrylamido-2-methylpropanesulfoacid)). The oil recovery rate of thesubsequent polymer solution was higher compared toHPAM, and the novel polymer displayed better temper-ature and shear stability. These two examples alreadydefine a common research theme in the general fieldof water-soluble polymers for EOR. That is, a strategicapproach involving the chemical modification of commer-cial polymers (in this case PAM) to tailor and improve thecorresponding solution properties and eventually EOR per-formance.

2.2. Partially hydrolyzed polyacrylamide (HPAM)

HPAM, by far the most used polymer in EOR applica-tions, is a copolymer of PAM and PAA obtained by partialhydrolysis of PAM or by copolymerization of sodium acry-late with acrylamide [21].

2.2.1. HPAM chemical structure

The chemical structure of HPAM is provided in Fig. 1.In most cases the degree of hydrolysis of the acrylamidemonomers is between 25 and 35% [4,26]. The fact thata relevant fraction of the monomeric units needs to behydrolyzed (lower limit of 25%) is probably related to the

Fig. 1. Chemical structure of HPAM.


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formation of the corresponding salt. According to the gen-eral theory of polyelectrolytesolutions[27],thepresenceofelectrostatic charges along a polymer backbone is respon-sible for prominent stretching (due to electric repulsion)of the polymeric chains in water and, eventually, resultsin a viscosity increase compared to the uncharged ana-logue. On the other hand, according to a study by Shupe[28] the degree of hydrolysis cannot be too high becausethe polymer solution will become too sensitive to salinityand hardness of the brine (electrolytes present in solutionhave a shielding effect on the electrostatic repulsion).

Indeed, polyelectrolytes, i.e. polymers bearing charges,showsignificantly different rheologicalbehaviorcomparedto their neutral analogues [2931]. The thickening capabil-ity of HPAM lies in its high molecular weight and also in theelectrostatic repulsion between polymer coilsand betweenpolymeric segments in the same coil [4]. When poly-electrolytes are dissolved in water containing electrolytes(salts) a reduction in viscosity is observed [26,3234]. Ithas been demonstrated that the specific viscosity of HPAMsolutions depends on the amount of salt present [35]. Thiseffect is attributed to the shielding effect of the charges[4,33] leading in turn to a reduction in electrostatic repul-sion and consequently to a less significant expansion of thepolymer coils in the solution. This results in a relativelylower hydrodynamic volume, which is synonymous with alower viscosity [34]. A few decades ago, substitution of oneor both hydrogens on the amide nitrogen with alkyl groups

hasbeen presented as a solution to thesalt sensitivity of theHPAM [36,37], although the exact reasons for this behaviorhave not been fully elucidated.

The addition of monovalent NaCl leads to a reduction inthe level of aggregation. However, at higher ionic strengths(higher salt concentration) the addition of NaCl leads tomacroscopic flocculation [38]. It has also been demon-strated that multivalent cations can form polyionmetalcomplexes which affect the viscosity of the resulting solu-tion [3941]. A study byPeng and Wu [39] investigated thedependence of the self-complexation of HPAM on the Ca2+

concentration and the degree of hydrolysis of HPAM. Theydemonstrated that depending on the Ca2+ concentrationintrachain and interchain complexations take place (Fig. 2)[39].

Besides the salt dependency, other factors influencingthe viscosity of HPAM solutions are the degree of hydrol-ysis, solution temperature, molecular weight and solventquality [35]. Pressure also affects the viscosity of HPAMsolutions. According to a study by Cook et al. [42] theincrease in the viscosity of the HPAM solutions cannotsolely be accounted for by the increase in viscosity of thesolvent. The intrinsic viscosity and the radius of gyrationare both invariant with pressure, albeit with a 10% exper-imental uncertainty [42]. In principle, the dimensions ofthe polymer coils do not change while the solvent volumedecreases. Therefore the volume fraction of the polymercoil per unit volumeof thesolvent increases, hence a higher

Fig. 2. Complexation behavior of HPAM under different conditions.Reproduced with permission from [39] 1999, ACS Publishing.


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Fig. 3. Schematic presentation of behavior of HPAM coils in shear flow.

Reproduced with permission from [46] 1995, ACS Publishing.

viscosity [42]. Another parameter that affects the solutionviscosity of the polymer solution is shear [43]. Under highshear the HPAM polymer chains are reduced in size dueto chain scission, i.e. fragmentation [44]. This leads to areduction in the solution viscosity.

2.2.2. Rheological properties

HPAMis preferred in EORapplications since it cantoler-ate the high mechanical forces present during the flooding

of a reservoir. In addition, HPAM is a low cost polymerand is resistant to bacterial attack [4]. Although the HPAMsolutions display pseudoplastic behavior [4,26,32,45,46](shear thinning) in simple viscometers, it has been demon-strated that these solutions show pseudodilatant [47,48]characteristics (shear thickening) in porous media as wellas in viscometers at relatively high shear rates. Researchhas demonstrated the presence of a critical shear rate atwhich the shear thickening behavior arises in viscometers[32,33,45,46,49,50]. This critical shear rate depends on thedegree of hydrolysis of the HPAM, the solution concentra-tion, the temperature, the quality of the solvent and also onthe molecular weight of the polymer [33,45]. An increase

in the degree of hydrolysis leads to an onset of shear thick-ening at lower shear rates [45]. By decreasing the averagemolecular weight, an increase in the polymer concentra-tion results in a higher critical shear rate [45,46].

The aforementioned shear thinning of HPAM solutionsbelow a critical shear rate arises due to uncoiling of poly-mer chains and the dissociation of entanglements betweenseparate polymer coils [4]. Stiffening of the polymer back-bone has been suggested as a possible approach to controlthe dependency of HPAM polymer solutions on the shear[51]. A stiff polymer will have a lower mobility and there-

fore the entanglements, related to the solution viscosity,will be conserved as the shear increases.

The shear thickening behavior has been attributed tochanges in the molecular conformation involving the for-mation of additional links between two chains [49]. Theshear thickening behavior is observed both in laboratoryrheometers [45] (in pure water and aqueous salt solu-tions) and in porous media. According to several studiesthe shear thickening behavior in porous media arises due

to coil-stretched transitions of the polymer chains. Thestructure of a porous medium can be seen as alternatingwide openings and confined throats through which thepolymer coils have to navigate. In the wide openings thepolymer chains attain a coil structure. When these coilsthen have to pass through a narrow throat the polymercoils are forced to deform and stretch (elongational strain[32,48,52]) in order to pass. This successive contractionand expansion of the polymer coils leads to pseudodilatantbehavior of the polymer solutions [48,53,54]. This con-formational change of the macromolecules is reversiblesince it is commonly explained by formation at macro-molecular level of reversible interactions like hydrogen

bonding. Indeed, it is believed that hydrogen bonding arisesfor HPAM solutions between the carboxylic functional-ities [55]. However, this is contested due to conflictingdata [49,56] on similar polymeric solutions (e.g. dextransolution). Instead, aggregation of hydrophobic bonds hasbeen proposed [55], albeit in polymethacrylic acid, but thishas not been confirmed [57]. Hu et al. [46] proposed aschematic presentation depicting the essential behavior ofHPAM solutions in shear flow (see Fig. 3).

Another behavior that has been identified for HPAMsolutions which is important for EOR is their negative

Fig. 4. Type I and II of rheopectic behavior of HPAM solutions.Reproduced with permission from [58] 1995, Springer.


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Fig. 5. Interactions between sodium oleate and HPAM at low (A) and high (B) surfactant concentration.

thixotropic (rheopectic) property, i.e. an increase in vis-cosity with shear-time at a constant shear rate [32,5861].Researchers have identified two differenttypes of rheopec-tic behavior for HPAM solutions (Fig. 4), type I and typeII [58]. The type I effect is observed at low shearing andconsists in a slow viscosity increase with shear-time upto an asymptotic value. The type II effect is seen at highshear rates and is displayed as a steep viscosity increaseafter a given shear-time, followed by pronounced viscosityoscillation [58].

2.2.2.1. Effectof inherentparameterson rheology. ForHPAMthe degree of hydrolysis has a significant impact on therheological properties of the subsequent solution. If thehydrolysis degree of HPAM is too large, insolubility prob-lems can arise. When too small, a large dependence ofthe solution viscosity on electrolyte presence is observed.Lewandowska [45] investigated the effect of the degree ofhydrolysis on the solution viscosity in a NaCl solution. Ifthe degree of hydrolysis is increased the zero shear rateviscosity and the critical shear rate for the onset of shearthickening are both reduced. The shear rate region wherethe shear thinning behavior is observed is reducedwith theincrease in the degree of hydrolysis [45,46,62].

Another inherent parameter of HPAM is its molecularweight. Increase of themolecular weightwill lead to a morepronounced shearthinning behavior. The critical shearratefor the onset of shear thickening is also affected by themolecular weight in that it is increased with an increasein the molecular weight [45].

2.2.2.2. Effect of external parameterson rheology. The influ-ence of salt (NaCl) addition on the rheological behaviorof HPAM solutions has also been extensively studied. Itwas found that, below the critical shear rate, adding saltreduced the extent of shear thinning while above the crit-ical shear rate the amplitude of the shear thickening isincreased [32]. The zero shear rate solution viscosity is alsoreduced as the concentration of salt increases [4].

Adding salt to the solvent will reduce the extent of therheopectic behavior of the polymer solution [59]. Lookingmore closely to the effect of salt, addition of mono-valentcations (e.g. NaCl) was found to increase the onset (i.e. thecritical shear rate value) of rheopectic behavior type I whilefor type II no changes were observed [58]. When usingmultivalent cations (CaCl2 and AlCl3) the effect seen with

mono-valent cations is amplified, i.e. the effect is seen atlower cation concentrations [61]. This is due to the higherscreening capability of multivalent cations [61]. In addi-tion a reduction of the degree of hydrolysis leads to a lessprominent rheopectic behavior of type I [60].

Besides salts, also surfactants are able to interact withpolymer chains in solutions and thus display a relevantinfluence on the corresponding rheological behavior. Xinet al. [63] investigated the interaction between the surfac-tant sodium oleate and HPAM and found that the viscosityof the polymersurfactant aqueous solution depends onthe surfactant concentration. At low surfactant concentra-tion an enhancement of the viscosity is observed due tointerpolymer cross-linking of the surfactant and HPAM. Athigh surfactant concentration the repulsion between themicellar aggregates attached to the polymer increases andthis leads to a decrease in the hydrodynamic volume andthus a decrease in the solution viscosity. The authors [63]have proposed a model depicting the interactions betweenHPAM and sodium oleate (a surfactant) at low and highconcentration of the latter (Fig. 5).

Interchain cross-linking arises due to hydrogen bond-ing. At high surfactant concentration repulsion betweenthe surfactant molecules dominates and this leads tothe collapse of the network between the surfactant andHPAM, which in turn results in a reduction of the vis-cosity. Methemitis et al. [64] investigated the interactionsbetweenSDSandHPAMandconcludedthattheeffectofthesurfactant on the rheological properties of HPAM dependson the pH of the solution and the presence of electrolyte.When no effort is devoted at altering the pH of the solu-tion, a reduction in the solution viscosity is observed upto the CMC of the surfactant either in water or in saltywater. Above the CMC the solution viscosity of the ternary,system, i.e. waterpolymersurfactant, remains relativelyconstant with further addition of SDS in the salty water. Inpure water the solution viscosity still decreases with fur-theradditionofSDS.IfthepHofthesolutionisreduced(toapH of 2.5) the solution viscosity decreases until the CMC ofthe surfactant is reached, after which a significant increaseis observed with further addition of SDS. The authors havehypothesized that at low pH fixation of some protons ontothe surfactant micelles occurs [64]. This changes the ion-ization equilibrium of the carboxylic groups leading to anincrease in the surface charge density, which correspondsto an increase in solution viscosity.


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Fig. 6. Chemical structure of xanthan gum.

2.3. Xanthan gum

Xanthan gum is a polysaccharide, which is producedthrough fermentation of glucose or fructose by different

bacteria [65]. The most efficient xanthan gum producer istheXanthomonas campestris bacterium [65,66]. The chem-ical structure of xanthan gum(Fig. 6) displays the presenceoftwoglucoseunits,two mannose unitsand one glucuronicacid unit with a molar ratio of 2.82.02.0 [67]. The back-bone of xanthan gum is similar to cellulose. The side chainsof the polymer contain charged moieties, i.e. acetate andpyruvate groups, and the polymer is thus a polyelectrolyte.However the classic polyelectrolyte behavior according towhich the solution viscosity decreases with the addition ofsalt is not displayed in this case. The thickening capabilityof xanthan gum lies in its high molecular weight, whichranges from 2 to 50106 g/mol [67,68] and in the rigidity

of the polymer chains.It has been demonstrated that upon addition of salt

(mono or divalent) the xanthan gum chains undergo acooperative conformational transition from a disorderedconformation to an ordered and more rigid structure[6972] (Fig. 7).

The temperature and the ionic strength (the amountof electrolyte) of electrolyte, of the solution are triggersfor the conformational transition. When testing at lowshear, the rheology of the polymer solution is dependenton the conformation with the disordered conformationdisplaying higher solution viscosities [73]. Polymeric solu-

tions employing xanthan gum display high viscosity atlow shear rates [74] and thus the disordered conforma-tion predominates at low shear rates. At high shear ratesboth conformations display similar rheological behaviors

[73]. In addition, pseudoplastic behavior is observed forthe polymer solutions [75]. Unlike HPAM, xanthan gumdisplays good resistance to high temperatures. It wasdemonstrated that the solution viscosity of a polymericsolution employing a commercial xanthan gum remainedrelatively constant for more than 2 years at 80C [76]. Lossof solution viscosity occurs at temperature above 100 C.Several studies [7780] have investigated the temperaturedependence of the apparent viscosityof xanthan gum solu-tions. In order to display resistance to temperatures upto 90 C, the conventional understanding for xanthan gumsolutions is that the ionic strength of the solution has to berelatively high. Another positive property of xanthan gum

is its ability to withstand high shear forces. Unlike HPAMthe solution viscosity does not decrease at relatively highshear stresses [47]. Especially the ordered structure, i.e. inthe presence of salt, can withstand high shear forces [73](up to a shear rate of 5000s1).

A disadvantage of xanthan gum is its susceptibilityto bacterial degradation. It has been demonstrated thatsalt tolerant aerobic and anaerobic microorganisms candegrade the xanthan gum chains which leads to the lossin solution viscosity [8184]. Biocides are used to suppressthe growthof the xanthan gum degrading microorganisms.In most cases formaldehyde is the most efficient biocide

Fig. 7. Conformational transition of xanthan gum.


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[83,84]. However the use of biocides to protect the xanthangum renders the low environmental impact of the polymerobsolete.

Combinations of xanthan gum with surfactants havealso been studied. It has been demonstrated that the com-bination can be beneficial. According to Taugbol et al. [85]more than50% of the residual oil (after a waterflood) can berecovered using xanthan gum and an alkyl propoxy-ethoxysulfate (C1215(PO)4-(EO)2-OSO3

Na+) as the surfactant.The recovery of theresidual oilusing only a surfactant solu-tion was lower. However it has been demonstrated thatthe combination of xanthan gum and dodecyl-o-xylenesulfonate recovered less of the residual oil compared tothe case where only a surfactant solution was used [86].According to the authors a possible explanation is the for-mation of large micellar aggregates, which have a negativeeffect on the flow performance of the surfactant throughthe porous media.

3. Recent developments in water-solublethickeningpolymers

The limited number of available commercial poly-mers currently employed in EOR has been the subjectof recent developments aimed at improving their perfor-mance. Indeed, an alternative concept has been studied inthe last four decades, and involves the association betweenhydrophobic groups that are incorporated in the back-bone of the polymers [87]. Through these associations ahigher thickening capability can be achieved compared tothe traditional polymers [87]. Several different types ofassociating polymers have been studied. These include the

hydrophobically modified polyacrylamide (HMPAM) [88],ethoxylated urethane (HEUR) [89], hydroxyethylcellulose(HMHEC) [90] and alkali-swellable emulsion (HASE) [91].Alsocombinations of associative polymers with surfactantshave been developed for EOR [92]. It has been demon-strated that the addition of small amounts of surfactants

can increase the viscosity of the aqueous solution contain-ing hydrophobically modified polymers significantly [90].

Other polymers that posses interesting properties,such as high molecular weight and intrinsic viscosity,have been developed for EOR and are known as rigidrod water-soluble polymers [93]. One study comparedhydrophobically modified polyacrylamide (HMPAM) withpolyacrylamide (PAM) in a simple core flood test anddemonstrated that the residual resistancefactor (RRF) afterthepolymerflood ismuch higherfor theHMPAMcomparedto PAM [94]. All these modification strategies, togetherwith new kinds of water-soluble systems, have been exten-sively reported in the literature and will be discussed in thenext paragraph.

As mentioned earlier, a relatively new class of water-soluble polymers is the one constitutedby hydrophobicallyassociative polymers [87]. The first hydrophobically asso-ciative polymers were synthesized almost fifty years ago[95,96], albeit for a different purpose than EOR. Indeed, theresearch on these types of polymers has been primarilyfueled by the coating industry [87], where improvementin the rheology of the coating systems was required. Dur-ing the 1980s when the oil crisis hit, a lot of research wasperformed on EOR. From the many patents [97102] thathave been filed during those years it is evident that thisaccelerated the development of hydrophobically associa-tive polymers for use in EOR applications.

Hydrophobically associative polymers contain, inmost cases, a small number of hydrophobic groups,i.e. 818 carbon atoms moieties [103106], distributedalong the main backbone [20,107,108]. These hydropho-bic groups can be distributed randomly or block-like[88,91,103,107,109120], and coupled at one or bothends [104,121131]. Above a given polymer concentration(dependent on the molecular structure) the hydropho-bic groups associate, when the polymer is dissolvedin water, to form hydrophobic micro-domains (intra orintermolecular liaisons) [88,89,104,105,107,108,132135].These lead to an increase in hydrodynamic volume, which

Fig. 8. Intra- and intermolecular associations.Reproduced with permission from [132] 1996, ACS Publishing.


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in turn yields a polymer with a much better thickening(higher viscosity [107]) capability compared to its non-associative analogue [88]. Depending on the concentration,intra- or intermolecular associations are formed, which areschematically illustrated in Fig. 8.

When the hydrophobic elements are distributed in ablock-like fashion along the backbone of a water-solublecopolymer, the intramolecular associations are strongercompared to randomly or discretely distributed hydropho-bic groups [104,121].

The temperature dependence of the solution viscos-ity is an interesting property of hydrophobically modifiedpolymers for EOR applications. It has long been acceptedthat increasing the temperature of the polymer solutionwill lead to a reduction in viscosity [103,133,136140].This since an increase in temperature implies a decreaseof the association strength of the hydrophobes. Increas-ing the temperature of the solution leads to a reductionof the solvent viscosity and hence an increase in themobility of the polymer chains while the solubility of thepolymer will increase with temperature. However, manydifferent aqueous systems have been demonstrated todisplay an increase in viscosity upon increasing the tem-perature [141154]. Indeed, a temperature increase willdecrease the solubility of one of the components (lowercritical solution temperature [LCST]-groups) of the poly-mers. These less soluble components will self-aggregatewith the hydrophobic groups of the polymers, which leadsto an increase in viscosity [119]. Several researchers haveproposed a concept for thermo-associative polymers basedon the switch, i.e. the transition between low and hightemperature, of the polymers characterized by a lowercritical solution temperature [140,151,152]. The conceptinvolves a highly water-soluble polymer containing blocksor side chains of LCST groups. Upon heating of the polymersolution,theseLCST groupswillsegregate. A schematicpre-sentation of this behavior has been presented by Hourdetand coworkers [151] and is depicted in Fig. 9.

Above the critical overlap chain concentration this tran-sitionwill lead to an increase in theviscosity of the solutionthrough intermolecular associations.

Fundamental research on different polymers, in binary(polymerwater) and ternary (polymerwatersurfactant)systems, has been performed using different techniqueswhich include 13C NMR [155158] (solution or solid-state), 1H NMR [109], 23Na NMR [159,160], 19F NMR [161], NMR self-diffusion [126,162165] potentiome-try [166168], static and dynamic laser light scatter-ing [128,130,131,146,163,167172], UV-spectroscopy forpolymers bearing chromophores [88,110,111,173181],small-angle neutron scattering (SANS) [182], non-radiativeenergy transfer (NRET) studies [178,183185], size exclu-sion chromatography (SEC) [170,186189] and surfacetension [131,133,162,163,165].

Several different associative hydrophobically modifiedpolymers have been developed which include polyacry-lamides (HMPAM), ethoxylated urethanes (HEUR), alkaliswellable emulsions (HASE), and polysaccharides (HM-polysaccharides). Their synthesis, rheological behavior andadsorption on surfaces will be discussed in the followingsections.

3.1. Hydrophobically modified polyacrylamide (HMPAM)

HMPAM constitutes the most popular basic structurefor the synthesis of new water-soluble polymers for EOR.Indeed, as given in Table 1, many types have been pub-lished.

There are different methods of synthesizing HMPAMsuch as micellar [88,113,135], homogeneous [88,248,249]and heterogeneous [88] copolymerization. Polyacrylamideis usually prepared via a free radical polymerization inaqueous solution [88,250]. However, as is evident fromthe name, HMPAM cannot be synthesized using this tech-nique as the hydrophobic monomer is not soluble inwater. In order to disperse the hydrophobic monomer,it is dissolved using a co-solvent (homogeneous copoly-merization) or a surfactant (micellar copolymerization) ordispersed without any additives (heterogeneous copoly-merization) [88,135].

Amongthese different methods, micellar copolymeriza-tion has been mostly studied. It generally involves the useof a hydrophobic monomer (soluble in micelles stabilizedby a surfactant) and a hydrophilic one which is solublein the water phase. The first reports on micellar copoly-merization appeared simultaneously some 25 years ago[251256]. A schematic presentation of this polymeriza-tion technique is presented in Fig. 10.

When synthesizing polymer 17 (Table 1) Changand McCormick [220] noted that the use of micellarcopolymerization leads to a block-like distribution of thehydrophobes whereas solution copolymerization leadsto a random distribution. Both polymers exhibit com-pletely different rheological properties. The same behaviorwas observed when synthesizing polymer 29 (Table 1)using both (micellar and solution) techniques [110,111].The most popular surfactants in micellar copolymeriza-tion are sodium dodecyl sulfate (SDS) [92,103,104,109,112,121,134,135,138,173179,203,229,230,241,257260]and cationic hexadecyltrimethylammonium bromide(CTAB) [113,133,220,238]. Many different hydropho-bic monomers have been used, such as acrylate ormethacrylate-derivatives, alkyl groups with vary-ing number of carbons and different topologies[99,100,103,229,261264], aryl or alkylaryl functionalities[88,110,111,113,173177,230,260,265268], fluorocarboncontaining agents [132,241,242,269271] and zwitterionicgroups [196199,201203,234,272274]. By incorporatingwater-soluble spacers between the hydrophilic backboneand the hydrophobic group an enhancement of the viscos-ity can be achieved compared to systems without spacers[199,241]. According to Hwang and Hogen-Esch [241] theformation of hydrophobic micro-domains is effectivelypromoted by increasing the lengths of the spacers.

Parameters affecting the properties of the polymersprepared by micellar copolymerization are the typeand concentration of the hydrophobic and hydrophilicmonomer [109,174,175,258], the molar ratio betweenthe two monomers, the content and type of surfactant[174,275], the content and type of initiator and the tem-perature of the reaction [135]. Another parameter that hasbeen identified is the molar ratio betweenthe hydrophobicmonomer and surfactant, i.e. the number of hydrophobic


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Table 1

Structure of different water-soluble polymers, HMPAM.

Polymer Structure

Polyelectrolyte

Copolymer of acrylamide (AM) and sodium 2-acrylamido-2-methylpropanesulfonate (NaAMPS, R = 2)

or sodium 3-acrylamido-3-methylbutanoate (NaAMB, R = 1) or

(2-acrylamido-2-methylpropyl)dimethylammonium chloride (AMPDAC, R = 3)

Polyelectrolyte

Copolymer of acrylamide (AM) and sodium 3-methacrylamido-3-methylbutanoate (NaMAMB)

Polyelectrolyte, zwitterionicmonomer

Copolymer of 3-(2-acrylamido-2-methylpropane-dimethylammonio)-1-propanesulfonate

(AMPDAPS) with 4-(2-acrylamido-2-methylpropyldimethylammonio) butanoate (AMPDAB)


Copolymer of acrylamide (AM) with

3-(2-acrylamido-2-methylpropane-dimethylammonio)-1-propanesulfonate (AMPDAPS)


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Table 1 (Continued)

Polymer Structure


Copolymer of AM with 2-(2-acrylamido-2-methylpropyldimethylammonio) ethanoate (AMPDAE,

N= 1) or AMPDAB, N= 3 or 6-(2-acrylamido-2-methylpropyldimethylammonio) hexanoate

(AMPDAH, N= 5)


Terpolymer of AM with acrylic acid (AA) and AMPDAPS


Terpolymer of AM with sodium acrylate (NaA) and AMPDAB


Copolymer of AM with [(dimethylammonioethoxy)dicyanoethenolate]propyl-methacrylamide

(DADPMA)


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Polyelectrolyte, zwitterionic backbone

Terpolymer of AM and AMPDAC (R1 =H) with NaAMPS (R2 =1) or AMPTAC (R1 = CH3) with NaAMPS

(R2 =1) orAMPTAC (R1 = CH3) with NaAMB (R2 = 2)


Terpolymer of AM with AMPDAC and sodium 3-acrylamido-3-methylbutanoate (NaAMB)


Copolymer of 2-acrylamido-2-methylpropanesulfonate (NaAMPS) with

(2-acrylamido-2-methylpropyl)-dimethylammonium chloride (AMPDAC, R = H) or

[2-(acrylamido)-2-methylpropyl]trimethylammonium chloride (AMPTAC, R = CH3)

Polyelectrolyte, amphiphilic

Copolymer of AM (R1= NH2) and sulfonate containing monomer (R2= 1)


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Table 1 (Continued)

Polymer Structure

Copolymer of NaA (R1= ONa+) and alkylchains (R2= 2) or C8F ( R 2=3) orC10F(R2= 4) or3-PDCA

(R2= 5)


Copolymer of NaA and

A, Dodecylacrylamide (C12AM), R= NH, M=11, N= 0 o r

A, Octadecylacrylamide (C18AM), R= NH, M=17, N= 0 o r

B, Substituted methacrylate (DEmMA), N= 2 , 6 or 25) or

Copolymer of NaAMPS and

B, Substituted methacrylate (DEmMA), N= 2 , 6 or 25)


Terpolymer of AM (R1= NH2) and A A ( R 2=OH) with

N-[(hexyl)phenyl]acrylamide (R3= 2, N= 5 ) o r

N-[(decyl)phenyl]acrylamide (R3= 2, N=10) or

Terpolymer of AA (R2=OH) and NaAMPS (R2= 1) with

PEO chain (R3= 4 ) or

Terpolymer of AM (R1= NH2) and NaAMPS (R2= 1) with

N,N-Dihexylacrylamide (DiHexAM, R = 5 andM= 5)


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Terpolymer of AA and methacrylamide (MAM) with

(DiC6AM, N= 4) N,N-dihexylacrylamide or

(DiC8AM, N= 6) N,N-dioctylacrylamide or

(DiC10AM, N= 8) N,N-didecylacrylamide or

(DiC12AM, N=10) N,N-didodecylacrylamide or

(DiC14AM, N=12) N,N-ditetradecylacrylamide or

(DiC16AM, N=14) N,N-dihexadecylacrylamide

Polyelectrolyte amphiphilic

Terpolymer of maleic anhydride (MA), ethyl vinyl ether (EVE) and 4-butylaniline (4-BA)


Copolymer of AM (R1= NH2) and dimethyldodecyl(2-acrylamidoethyl)ammonium bromide (DAMAB,

R2= 3, R 3= H and M= 2) or dimethyldodecyl(2-methacrylamidopropyl)ammonium bromide(DMAMAB, R2= 3, R 3= CH3and M= 3 ) o r

Copolymer of AA (R1=OH) and 2-ethylhexyl (R2= 1) orn-alkyl (R2= 2)


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Table 1 (Continued)

Polymer Structure


Copolymers ofN-isopropylacrylamide (NIPAM, R2= 1) o r A A ( R 2=OH) with

2-(N-ethylperfluorosulfoamido)ethyl acrylate (FX-13), R1= H or

2-(N-ethylperfluoro-octane/sulfoamido)ethyl methacrylate (FX-14), R1= CH3

Polyelectrolyte, zwitterionicmonomer and amphiphilic

Copolymer of 3-(N,N-diallyl-N-methylammonio)propane-sulfonate (DAMAPS, R2 =1) with

N,N-diallyl-N,N-dimethylammonium chloride (DADMAC, R1 = CH3) or N,N-diallyl-N-methylamine

chloride (DAMA, R1 = H)

Copolymer of DADMAC, (R1 = CH3) with

N,N-Diallyl-N-hexylbeznyl-N-methylammonium chloride (R2 = 2 ) o r

N,N-Diallyl-N-octylbeznyl-N-methylammonium chloride (R2 = 3)


Terpolymer of AM and n-decylacrylamide (C10AM) with


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NaA, R= ONa+ or

NaAMB, R= C7H17N2O2Na+ or

NaAMPS, R= C4H9NSO3Na+


Terpolymer of AM and N-(4-butyl)phenylacrylamide (BPAM) with

NaA, R = ONa+ or

NaAMB, R= C7H17N2O2Na+ or

NaAMPS, R= C4H9NSO3Na+


1. Terpolymer of sulfur dioxide, N,N-diallyl-N-carboethoxymethylammonium chloride with

N,N-diallyl-N-alkylammonium chloride (R2 = 1) or dendritic quadruple-tailed hydrophobic group

(R2= 2)


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N-hexylacrylamide (HexAM, R = 2) or

N-methyl-N-hexylacrylamide (MeHexAM, R = 3) orN,N-dihexylacrylamide (DiHexAM, R = 4 and N= 5 ) o r

di-n-propylacrylamide (DPAM, R = 4 andN= 2 ) o r

di-n-octylacrylamide (DOAM, R = 4 andN= 7 ) o r

N-(4-ethyl-phenyl)acrylamide (EAM, R = 5) or

N-methyl-N-(4-ethyl-phenyl)-acrylamide (MeEAM, R = 6 ) or

BPAM, R= 7

Amphiphilic

Copolymer of AM and

N-benzylacrylamide(NBAM, R1= H and R 2=(CH2)1)

N-phenethylacrylamide(NPEAM, R1= H and R 2= (CH2)2)

N-phenylmethacrylamide(PMAAM, R1= CH3and R2= NH)

Amphiphilic

Copolymers of acrylic acid (AA) and

3-[tris(trimethylsilyoxy)sily]propyl methacrylate (TMSPMA)


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Table 1 (Continued)

Polymer Structure

Amphiphilic

Copolymer of AM and

1,1-Dihydroperfluorobutyl acrylate (2a) or

1,1-Dihydroperfluorooctyl acrylate (2b) or

2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate, FOSA (3a) or

2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate, FOSM (3b) or

1,1-Dihydroperfluorooctylmono-(ethyleneoxy) acrylate (4a) or

1,1-Dihydroperfluorooctylbis-(ethyleneoxy) acrylate (4b) or

1,1-Dihydroperfluorooctyltris-(ethyleneoxy) acrylate (4c) or

Dodecyl acrylate (5a) or

Dodecylmono-(ethyleneoxy) acrylate (5b) or

Dodecylbis-(ethyleneoxy) acrylate (5c) or

Dodecyltris-(ethyleneoxy) acrylate (5d) or

Poly(propylene oxide) methacrylate (6)


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Amphiphilic

1. Copolymer of AM with

A monosubstituted monomer derived from 4,4-azobis(4-cyanopentanoic acid, ACVA), R1= A or

A disubstituted monomer derived from 4,4-azobis(4-cyanopentanoic acid, ACVA), R1= B

2. Terpolymer of AM and DHAM with

A monosubstituted monomer derived from 4,4-azobis(4-cyanopentanoic acid, ACVA), R2= C

Model polymer

Copolymer of (AA, R1 =H) or(MAA, R1 = CH3) with 2-(1-naphthylacetyl)ethyl acrylate (NAEA) or

2-(1-naphthylacetamido)ethyl acrylamide (NAEAm)

Modelpolymer

Copolymer of AM withN-[(1-pyrenylsulfonamido)ethyl]acrylamide (APS)


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Table 1 (Continued)

Polymer Structure

Model polymer

Terpolymer of AM and AA withN-[(1-pyrenylsulfonamido)ethyl]acrylamide (APS)

Modelpolymer

Terpolymer of AM and SA with [(1-naphthyl)methyl]acrylamide (NMA) or APS

Modelpolymer

Copolymer ofN-isopropylacrylamide (NIPAM) with N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide


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Fig. 9. Thermal induced micro-domains [151].

Fig. 10. Schematic presentation of micellar copolymerization [88,135].


monomers per micelle (NH) [88,109,134,175,258,276]. It

was first thoughtthat the solubilization of thehydrophobicmonomer in a surfactant micelle, would cause an increasein the incorporation rate of the hydrophobic monomer(into the polymer backbone) [109,174]. A NH increase willchange the randomness, i.e. the regularity of the num-ber of hydrophobic units incorporated in the polymer as afunction of time [109,174]. However two studies [109,277]demonstrated that by using a disubstituted acrylamide asthe hydrophobic monomer no drift in copolymer com-position was observed. Therefore, according to the study[109], the previously thought dissolution of the hydropho-bic monomer in the surfactant micelle no longer holdstrue. The observed behavior can be then attributed to

the difference in polarity between the bulk and micel-lar phase, which modifies the reactivity of hydrophobes[109]. Micelle copolymerization canalso be used to synthe-size a more randomly distributed copolymer. This can beachieved by using a molar ratio where approximately onehydrophobic unit is solubilized in one micelle [138,275].

Micellar copolymerization remains the most usedmethod for synthesizing HMPAM [135]. Nevertheless,

another method that is closely related to the micel-

lar copolymerization technique seems promising, but itinvolves using a micelle-forming polymerizable surfac-tant [114,115,178,179,220,278280]. Although identifyinga correct polymerizable surfactant for the desired molecu-lar structure is difficult, the technique offers the advantagethat purification (i.e. removal of surfactants as is the casewith micellar copolymerization) of the reaction mixtureno longer would be required. Yet another method, whichrecently has beendeveloped,is template copolymerization.The structure of the copolymer is defined by the templatethat is used. A schematic presentation illustrating the tem-plate copolymerization is given in Fig. 11.

The advantage of this technique with respect to the

otherones is thatthe block-likedistribution of hydrophobicgroups is better controlled. According to several differ-ent studies [281285] a longer sequence distribution ofhydrophobic groups can be achieved with this technique.

The molecular weights of the polymers influence theirbehavior in solutions. The molecular weight of HMPAMremains difficult to determine since intramolecular associ-ations still exist even at very low polymer concentrations.

Fig.11. Schematic presentation of the template copolymerization technique.


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Nevertheless, several different methods have been devised.These include using the MarkHouwinkSakurada rela-tion for polymers dissolved in water [110,113,114] orunmodified polymers in the exact same conditions as theirmodified analogues [103,229,230,286,287]. However, allthese methods introduce errors in the molecular weightdetermination and it would be better to find a suitable sol-vent in which the polymer is molecularly dispersed [135].Several authors reported on formamide as a suitable sol-vent for the molecular weight determination and measuredit by using light scattering experiments [88,109,174].

Another popular strategy to increase the thickeningability of polyacrylamide polymers is related to the pres-ence of electrical charges along the backbone. Indeed,many different polyelectrolytes based on acrylamide havebeen synthesized. These polyelectrolytes can be dividedinto polyampholytes which include zwitterionic backbonepolymers 911, having positive and negative charges inseparate building blocks in the backbone, and zwitterionicmonomers 38having positive and negative charges in thesame building block, and traditional polyelectrolytes 1 and2. The charges can be induced by controlling the pH of thesubsequent polymer solution when,for example, AA is usedas a co-monomer. In all cases it is reported that the solu-tion viscosity is a function of the chemical structure andthe solution characteristics (i.e. ionic strength).

From the discussion on the polymers 111 one canconclude that there aremany differentadvantages of incor-porating electronic charges into water-soluble polymers.One such advantage is the ability to control the interactionbetween the polymer chains by altering the pH or ionicstrength of the polymer solution. Another is the solubil-ity in water, which makes the corresponding solutions lesssensitive, in terms of viscosity, to theexternal temperature.

On the other hand if water-soluble polymers arerequired whose rheological properties are indepen-dent of the pH, polyelectrolytes bearing only one typeof charge can be used [201]. For applications whereconcentrated salt solutions are used (such as EOR),polyampholytes are suitable. They have been demon-strated to display an enhancement in the solution viscosityupon addition of low molecular weight electrolytes[197,201,204,209211,288,289]. This behavior has beenattributed to the shielding of intramolecular Coulom-bic attraction rather than the intermolecular interactions.With careful molecular design water-soluble polymerscontaining electronic charges can be synthesized exhibit-ing the required rheological behavior. Peiffer andLundberg[289] reported that in order to control the physical prop-erties of the zwitterionic polymers it is crucial to separatethe oppositely charged monomers using neutral ones.

Derivatives of polyelectrolytes have been investigatedwith amphiphilic (hydrophobic) moieties incorporatedalong with charges, either positive or negative. These acry-lamide based amphiphilic polyelectrolytes, 1217, havebeen synthesized by many different research groups insearch for better thickening polymers in different appli-cations.

Polymers where both charges are incorporated alongwith hydrophobic groups have also been studied. Thesepolymers can be classified as zwitterionic amphiphilics,

1921. The properties of the subsequent polymer solutionscan be tailored by careful design of the polymer structureand composition. The polymers can be designed to be salt-tolerant or responsive to changes in the salt concentrationor ionic strength.

Amphiphilics, 2328, without electronic charges havebeen studied extensively for their salt-tolerance. Additionof salt will not affect the viscosity of the polymer solu-tion since its thickeningcapabilityarises from hydrophobicassociations and not from electronic interactions.

The polymers 2933 have been synthesized in order toenable fluorescence studies of hydrophobically associatingpolymers. This technique allows studying the associatingbehavior of these polymers through their photo-physicalbehavior in response to changes in the system. It hasbeen demonstrated thatpolymers of acrylamidecontainingpyrene functionalities exhibit increased in excited-statedimer (excimer) formation and viscosity [110,111,179].

Variation in type of polymers causes different behaviorof the subsequent polymeric solutions. However there areother parameters that also affect the structure and associ-ations of the polymer solution: the chemical structure, thesynthesis method, the temperature, the type and concen-tration of salt and the pH (ionic strength). The followingdiscussion summarizes the effect of these parameters.

3.1.1. Chemical structure

Generally speaking the first macroscopic effect of thepolymer chemical structure is observed from the cor-responding solubility in water. Indeed, the solubility ofpolymer3 is dependenton the level of AMPDAPS monomerincorporated [196]. This has a clear influence on the vis-cosity of the solution [196] as observed for polymer 5 asfunction of the AMPDAE and AMPDAH monomers incor-poration level [199,200]. The same considerations hold forpolymer 6 as function of the monomer composition (inthis case AMPDAPS and AA) [201] as well as polymer 8as function of the charged monomer (DADPMA) intake[203]. The same can be said for polymer 19 where phaseseparation is observed at low incorporation levels of theDAMAPS monomer. These trends are easily understand-able on the basis of simple considerations regarding theoverall polarity of the polymeric chains. However, in con-trast to this, polymer (8) is no longer soluble in waterwhen itcontainsmore than1 mol% of DADPMA.In a similarexample, McCormick and Johnson [208] observed hydrogelformation when the incorporation level of charged groupssurpassed1 mol% in polymer10. Similar to polymer8, poly-mer (10) forms hydrogels and is no longer water-solubleabove 1 mol% incorporation of the chargedgroups. The car-boxylate groups lead to strong ionic interactions whichprevents the dissolution of polymer 10 [208]. It seems thusthat the nature of the charge (e.g. carboxylate versus, sul-fonate anion in Table 1) exert a clear influence on theassociation behavior and ultimately on the solubility of thepolymer. This phenomenon is never observed in the caseof a zwitterionic backbone. Indeed, at low incorporation ofeither monomer, AMPDAC or NaAMPS, in the correspond-ing copolymer (polymer series 11) classic polyelectrolytebehavior is exhibited by the solution. However, poly-meric solution employing polymers containing equimolar


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amounts of the AMPDAC and NaAMPS monomers displayhigh salt tolerance [209]. A similar behavior is observedwhen, instead of AMPDAC, AMPTAC is used as the sec-ond monomer [211]. Similar to polymer 11, polymer 19exhibits classic polyelectrolyte behavior when the incor-poration rate of DAMAPS is below 40mol% [226]. However,in dilute solutions of polymer 19 (both DADMAC-1 or 2copolymer) primarily intra-molecular hydrophobic associ-ations are present [228].

The presence of hydrophobic groups along the polymerbackbone results generally in the formation of micro-domains. Important parameters for association behaviorof the polymers are the distribution of the hydrophobicgroups and their hydrophobicity. As mentioned before,a block-like distribution of the hydrophobic groups willlead to stronger associations compared to a random dis-tribution. The surfactant to micelle ratio (SMR) duringpolymerization affects the distribution of the hydrophobicgroups. At low SMRthe block-like distribution is favoredwhile at high SMR a random distribution is preferred[175]. Polymers 13, 14 (synthesized using a high SMR)and 17 (using solution polymerization) are classic exam-ples of low level association due to random distributionof the hydrophobic groups [156,175,220] while polymers14 (synthesized with a low SMR), 17 (using micellar poly-merization) and 30 are classic examples of a block-likedistribution [111,175,220]. Although the SMR has beenidentified as an important parameter for the observedassociation behavior, its effect on the composition andmolecular weight of the polymer is minimal as demon-strated with polymer 31 [177].

The presence of hydrophobic groups suppresses thesolubility of the polymer. Following this, it is easy tounderstand that increasing the fraction of the hydrophobicgroups above a certain percentage will lead to solubil-ity issues, i.e. the polymer is no longer water-soluble.This has been demonstrated with polymer 24 where awater insoluble polymer was obtained using DiC8AM asthe co-monomer above an incorporation rate of 1.2 mol%[237].

Increasing the hydrophobicity of the hydrophobicgroups will impart better thickening capability of the poly-mer as demonstrated with the polymers 12, 15, 17, 18, 23,24and27 (2a, 2b,3ac and4ad). It hasbeen demonstratedthat only a fraction of the hydrophobic groups contributeto the formation of micro-domains which is affected by thehydrophobicity of the groups and the incorporation rate[290]. The hydrophobicity of the groups can be increasedby increasing the length of the groups [103,220,237], usingtwin-tailed hydrophobes instead of single tailed groups[185,218] or using fluorocarbons instead of hydrocarbons[212,223,242,269]. The critical polymer concentration atwhich hydrophobic associations (critical association con-centration, CAC) arise is reduced compared to classicassociating polymers [218]. These trends can be explainedby the stronger interactions between the hydrophobicgroups due to their increased hydrophobicity. The lengthof the hydrophobic groups is important for associationwhere a short hydrophobe will not lead to association asdemonstrated with polymer 24 using DiC

3AM as the co-

monomer [237]. Increasing its length, by using DiC8AM as

the co-monomer,does impart associations [237]. However,in contrast to this, increasing the hydrophobicity too muchwill lead to insolubility of the polymers as observed withpolymer 15 using longer twin-tailed groups than DiC12AM[218].

The presence of spacers, i.e. small chains linking thehydrophobic groups to the backbone, affects the associ-ation behavior of the polymers markedly. Increasing thelength of the spacers allows for easier movement of thehydrophobic group in solution which in theory shouldlead to easier formation of micro-domains and thus inter-molecular association is favored. This is demonstratedwith polymer 13 [216] (DemMA series). The associationbehavior of the polymer is not affected by the type of co-monomer that is used as long as the length of thespacer is large enough (in this case a length of 25 EOmoieties). At intermediate lengths (2 or 6 EO moieties)this is no longer true as demonstrated with the samepolymer backbone [216]. With NaA as co-monomer thenetwork formation was favorable at shorter EO spac-ers while just the opposite behavior is observed withNaAMPS as the co-monomer. The authors [216] attributethis difference to the stronger tendency of intermolecularassociation for the NaA containing copolymer. The pres-ence of a charged group further away from the polymerbackbone will disrupt the hydrophobic associations to agreater extent than when the charged group is closer tothe polymer backbone [229]. The NaAMPS co-monomerplaces the charged group further away from the backbonethus leading to a stronger disruption of the hydropho-bic group association at similar length EO spacers. Thisbehavior has been identified for the aforementioned poly-mer 13 and for polymer 20. In addition, charge densityof the polymer affects the association behavior. Usinglow charge density co-monomers (carboxylate anions)will display stronger association above the CAC due toless interference with the hydrophobic associations, whilesulfonate anions (higher charge density than carboxy-late anions) will display the opposite behavior. Polymersbearing high charge density groups are more suscepti-ble to screening effects in the presence of low molecularweight electrolytes and will therefore be less responsiveat high electrolyte concentration or low pH. The poly-mer series 21 is a classic example of this [230]. Whenusing 3 co-monomers the incorporation level of chargedspecies will also affect the association behavior of the poly-mer. At low incorporation levels of charged species nosuppression of the hydrophobic associations is expected.However, at high incorporation levels the interference withthe associations is expected to be significant. This wasdemonstrated using the polymer series 14. The polymerswith 9 or 21mol% AA display intermolecular hydrophobicassociations at low pH (


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Fig.12. Schematic model structure of HMPAM.


3.1.2. Rheological properties

Hydrophobic interactions confer HMPAM interestingrheological and solution properties. Leibler et al. [291]proposed the sticky reptation model, which explainsthe complex viscoelastic behavior [112,257,258,292] ofHMPAM polymers. The authors proposed a schematicmodel structure where a hydrophobic containing chainand the hydrophobic interactions between the chainshydrophobic groups and other chains in the vicinity arecontained in an imaginary tube (Fig. 12).

According to the model the HMPAM polymer chainsform an entangledtransientnetwork whereentanglementsand hydrophobic interactions are present. The distancebetween entanglements is shorter than the one betweenhydrophobic groups. Two different relaxationtimes, a shortand a long one, are predicted for the entangled transientnetwork. The short one corresponds to residence time ofa hydrophobic group in a hydrophobic micro-domain andthe long one corresponds to the time a chain takes to rep-tate outside its tube.

In the dilute region (at low polymer concentration)intramolecular associations dominate [108]. The hydro-dynamic volume is reduced and therefore the viscosityof the subsequent polymer solutions. When the polymerconcentration is increased the solution ideally moves tothe semi-dilute region where intermolecular associationsdominate [107,108,293]. This leads then to network-like formations (transient network) which substantiallyincreases the viscosity of the solution [107,108,293].Many publications discuss the complex nature of therheology of polymer solutions of hydrophobically mod-ified polyelectrolytes. It has been demonstrated that allthe types of non-Newtonian rheological behavior existfor these polymer solutions, i.e. pseudoplastic; pseu-dodilatant; thixotropic; and rheopectic. Application ofshear will, in classical pseudoplastic polymeric solu-tions, disrupt the hydrophobic associations, which leadsto a reduction in viscosity [88,92,103,109,132]. How-ever this process is reversible, i.e. when the shear is

removed the hydrophobic groups will form new associa-tion thus returning the viscosity of the solution up to its

original value [88,92,103,109,132134,136,138,139,242,257,278,293].

Some associative polymeric solutions display pseudodi-latant behavior, i.e. increase in viscosity with increasingshear rate [109,134,220,258,260,293,294]. According toseveral studies [109,134,220,295,296] this behavior can beinterpreted as a balance between intra- and intermolecularassociations. Above a given shear rate the intramolecu-lar associations are disrupted and the polymeric chainsare extended, which leads to more intermolecular asso-ciations [220]. A later study [258] demonstrated that thepseudodilatant behavior arisesslightly above the crossoverconcentration (C*), which is the overlap concentration ofthe polymer chains. Increasing the polymer concentrationwill lead to a polymericsolutionthat does notdisplay pseu-dodilatant behavior [109,134,258,297]. The pseudodilatantbehavior of the polymeric solutions is followed by thepseudoplastic behavior discussed earlier. Although manyauthors have demonstrated the pseudodilatant propertiesof HMPAM solutions, it has to be mentioned that there aredifferences in the observed behaviors. The most importantone being that polymers produced by post modificationonly display the pseudodilatant behavior in the presenceof salt [293,294] while polymers produced using the micel-lar copolymerization technique display the pseudodilatantbehavior also in water [109,134,220,258,260]. However,no clear explanation for this observation has been givenyet. As mentioned before, the type of distribution of thehydrophobic groups has a pronounced effect on the type ofrheology that is subsequently observed in solution. By postmodifying the polymers the distribution of the hydropho-bic groups would be different since little control is presenton which functional group is modified. The distribution ofthe hydrophobic groups is similar to block-like distributionobtained with micellar copolymerization.

Thixotropic and rheopectic (anti-thixotropic) behavior,as mentioned before, has been observed. The viscos-ity against shear rates curves obtained under increasingand decreasing shear rates are not superimposable[88,103,175,257]. Thixotropic solutions display a peculiarbehavior where the viscosities along the increasing shear


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Fig. 13. Counterion mediated interpolymer associations.


rate curve are all higher than those along the decreasingshear rate curve, i.e. the viscosity decreases with time ata constant shear rate. The application of shear disruptsthe hydrophobic associations, both intra- and interchain,which need time to re-associate in order to recover theenhanced viscosity. In the semi-dilute region interchain

associations dominate, as mentioned before, and there dis-ruption by shear will lead to a significant loss in viscosity[257]. However as the viscosity is recovered, the shearinduced reduction in viscosity is not due to polymer degra-dation [88]. Polymers prepared using the heterogeneousorhomogeneous technique do not display this behavior [88].

Rheopectic solutions display the opposite behavior, i.e.the viscosities along the increasing shear rate curve are alllower than those along decreasing shear rate curve (theviscosity increases with time at a constant shear rate). Thisbehavior involves the instant recovery of the hydrophobicassociationsafter the application of shear andthe enhance-mentof thesehydrophobic associations,which explains the

enhanced viscosity [103,175].

3.1.2.1. Effect of inherent parameters. As mentionedbefore, spacers between the hydrophobic group and thehydrophilic backbone can enhance the solution viscosity.Increasing the length of the spacer will lead to a betterenhancement of the solution viscosity. A classic example ofsuch behavior is polymer 13 (NaAMPS as the co-monomer)[215]. A subsequent study [214] demonstrated that,depending on the incorporation rate of the DEmMA, a sig-nificant increase in solution viscosity is obtained above thecritical overlap concentration. The authors hypothesizedthat the additional increase in solution viscosity is dueto the simultaneous interactions of countercations withthe EO spacers via coordination and with the polyanionvia counterion condensation. A schematic presentationillustrating this behavior is given in Fig. 13.

It has been demonstrated that the responsiveness of thepolymer solution is more pronounced when a longerspaceris used. A study of Kathmann et al. [199] demonstrated thatusing three methylene groups as spacers increased the pHresponsiveness of the polymer solution compared to whenonly one methylene unit is used. The pH responsive behav-ior of the polymers canalso be tailored by moleculardesignof the polymer. The ampholytic polymer 7 represents sucha polymer where by smart design different behavior isachieved as a result of changes in the pH. The polymer

(7) undergoes a polyanionpolyzwitterionpolycationtransition as the pH decreases [202].

The shear rate for the onset of pseudoplastic behav-ior and the viscoelastic properties are dependent on thestrength of the hydrophobic micro-domains. The strengthdepends on the aforementioned hydrophobicity of thegroups. Increasing the hydrophobicity of the groups, i.e.increasing the length of the groups [220,267], the lengthof the hydrophobic segment [220], using twin-tailed ratherthan single tailed groups [185] or fluorocarbons instead ofhydrocarbons [298], result in a reduction of the shear ratefor the onset of pseudoplasticity and an enhancement ofthe viscoelastic properties. A classic example of a polymerwhose shear rate for the onset of pseudoplasticity reduceswith increase in the hydrophobicity of the groups is poly-mer 15 [185].

The rheological properties of HMPAM can be, as men-tioned before, explained by the balance between intra- andintermolecular hydrophobic associations. The copolymer-ization process affects the distribution of the hydrophobes(vida supra). With the preference being intermolecularassociations, an enhancement in the solution viscosity isobtained whereas with intramolecular associations theexact opposite is observed [110,111]. Polymers 16 and 29are classic examples of polymers with an enhanced solu-tion viscosity due to their preference for intermolecularassociation.

The position of the hydrophobic groups along the back-bone is important [245]. From a chemical point of viewthe amount of hydrophobic groups present in the polymershould have an optimum. If the concentration is low, noassociation will arise, while, if too high, solubility issuesplay a crucial role. However it is demonstrated that plac-ing the hydrophobic group (polymer 24) at the ends of the polymer is the optimal configuration for obtaining thehighest solution viscosity [245]. According to the authorsthe placement of hydrophobic groups along the back-bone of the polymer leads to a polymer with a muchmore compact structure in solution compared to whenthe hydrophobic groups are placed at both ends. Howeverwhen combining the two extremes (telechelic with mul-tisticker) it was demonstrated that the highest thickeningcapability, i.e. solution viscosity against polymer concen-tration can be obtained [104,245]. The critical polymerconcentration for the onset of hydrophobic associationsdepends on the placement of the hydrophobic group [104].For a telechelic polymer with hydrophobic groups at bothends and a polymer with hydrophobic groups at both endsand along the backbone the critical polymer concentra-tion for the onset of hydrophobic association is the samewhereas for a multisticker polymer with only hydrophobicgroups along the backbone the concentration is higher.

The same kind of consideration, namely strong depen-dence of the rheological behavior from the chemicalstructure is applicable for fluorine containing polymers.Indeed, polymer 18 displayed shear thickening, whichaccording to Chang and McCormick [228] depends on themolecularstructure of thecopolymer. Increase in theincor-poration rate of the hydrophobe from 3 to 10mol% leads toa more pronounced shear thickening behavior. Howeverat an incorporation level of 24mol% the shear thickening


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behavior is significantly suppressed. The shear thickeningis attributed to the breakage of intramolecular hydropho-bic associations by shear. If the hydrophobic associationsare too strong (as is the case for the latter polymer), theapplied shear cannot break the associations and thus shearthickening is not observed.

The dependency of the solution viscosity on the(low molecular weight) electrolyte concentration can beadjusted as function of the chemical structure. Indeed, thesalt tolerance of terpolymer 19, 20 was examined usingthree different derivatives (NaAMPS, NaAMB and NaA) asthe third monomer. Terpolymers incorporating NaA asthe third monomer displayed the best salt tolerance ofthe three different terpolymers [229,230]. The apparentviscosity (app) of the polymer solution was significantlyhigher in high salt concentration solution compared tothe other two terpolymers. The strength of the hydropho-bic associations depends on the type of groups used. Theterpolymers incorporating carboxylate anions displayedstronger hydrophobic interactions compared to the poly-mers with sulfonate anions. In addition, the placement ofthe anions is crucial for the strength of the hydrophobicassociations. It was demonstrated that the further awayfrom the polymer backbone the anions are placed theweaker the hydrophobic associations will be. This behaviorwas attributed by the authors [229] to the interference bythe anions with the hydrophobic associations. In additionthe rheological properties of the polymer solutions dependon the charge density. Polymers with low charge densitylead to less electrostatic interference of the hydrophobicassociations [230].

The rheological properties of polymer 15, 22 and

24 depend on which hydrophobe is incorporated. Usingdisubstituted monomers leadsto a morepronounced thick-ening effect compared to monosubstituted ones. This isattributed to the proximity of two hydrophobic chains(higher density of the hydrophobic domains) that leadsto stronger hydrophobic associations [109]. In additionthe length of the hydrophobic block affects the rheologi-cal properties of the polymer. The longer the hydrophobicblocks, the stronger the hydrophobic associations will be.The onset of shear thickening shifts to lower shear ratesas the length of the hydrophobic blocks increases [109].This behavior is demonstrated using polymer 14 (R3 = 5)where the thickeningcapabilityincreases as the hydropho-bic block increases in length [299].

3.1.2.2. Effect of external parameters. For neutral associa-tive copolymers it has been demonstrated that uponadditionofsalttheviscosityofthesolutioncanbeenhanced[103,108,203]. This behavior of the neutral copolymers isattributed to a salting out effect, which arises due to achange in the solubility of the hydrophobic units [108].The solubility of the hydrophobic groups decreases withincreasing salt concentration. This leads to the formationofaggregates, which enhances the interactions between thepolymeric chains [108]. The intermolecular association isenhanced leading to a stronger network and thus increasedsolution viscosity [217]. Polymer 14 is a classic example ofthis behavior [217].

The presence of salt causes a shielding of the electro-static interactions and thus a collapse of the network withtotal loss of the solution viscosity. Polymer 1 is a classicexample of such a polyelectrolyte where phase separationis observed in the presence of multivalency cations [193].Polymer 5 displays a similar trend. However, a decreasein the pH of the polymeric solution (containing salts)improves the solution viscosity [198] due to the increasein hydrodynamic volume caused by the protonation of thecarboxylic groups at low pH [198200].

The effect of salt on polymers bearing charged moi-eties in combination with hydrophobic groups is peculiar.Indeed, an increase in the solution viscosity is observedupon the addition of different salts. The presence of saltscreens the electrostatic repulsion thus suppressing thedisruption of hydrophobic associations by the chargedgroups. This allows the formation of a stronger networkthrough hydrophobic associations [225], i.e. intermolecu-lar hydrophobic interactions are dominant rather than theelectrostatic repulsions [221], and thus an enhancementof the solution viscosity. Classic examples of this behav-ior (vida supra) are the polymers 12 [115], 17 [221] (AA-CNcopolymer, N12), 18 [225] and 25 [133,238]. Althoughnot in the same class of polymer, polymer 4 displays thesame behavior [197]. Another possibility is that the methylgroups on the AMDAPS monomer induce hydrophobicbehavior of the monomer at higher salt concentrationthereby displaying the aforementioned behavior.

Maia et al. [121] demonstrated that the method ofpreparing the HMPAM solutions affects their rheologicalbehavior in the presence of salt (NaCl). When the polymer,23 (AMDiC6AM copolymer), is dissolved in the salt (brine)solution (A, Fig. 14) the classical behavior, i.e. reduction inviscosity with increasing salt concentration, is displayed.However, if the polymer is dissolved in water first, whichis preceded by the addition of salt, a complete differentsolution behavior is observed. Diluting a polymer solu-tion (no salt) with water and adding salt afterwards (B,Fig. 14) results in a polymer solution whose viscositypassesthrough a maximum with increasing salt concentration. Ifthe polymer solution (no salt) is diluted with salt water(C, Fig. 14) the viscosity of the resulting polymer solu-tion increases with increasing salt concentration. Fig. 14presents the three different behaviors. This confirms thenon-equilibrium character of these solutions, but a betterexplanation has yet to be provided.

It has been demonstrated that the following surfac-tants interact with different HMPAM polymers and give,depending on the surfactant concentration, a higherviscosity [90,92,115,138,173,287,300302]. The interac-tion between the polymers 12, 13 (both copolymers),17 (both copolymers), 18 (DADMAC-1 and DADMAC-2copolymers), 21 (copolymer AM-C10AM), 22 (terpolymer1, R2 =2), 23 (copolymers AM-EAM, AM-BPAM andAM-2 or 3 or 4), 27 (6) and 33 with several different sur-factants (SDS [90,92,120,138,173,212,228,243,301305],potassium dodecyl sulfate (KDS) [212], cetyltrimethyl-ammonium p-toluenesulfonate [CTAT] [305,306],dodecyltrimethylammonium chloride [DTAC] [290,300],hexadecyltrimethylammonium [HTAC] [303,304], CTAB[90,233], trimethyltetradecylammonium bromide [TTAB]


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Fig. 14. Different behavior of HMPAM solution dependent on the preparation method.

Adapted from Maia et al. [121]. Reproduced with permission from [121] 2005, Wiley VCH.

Fig. 15. Schematic presentation of concentration regions of HMPAM with SDS [92].

Reproduced with permissions from [92] 1992 and [132] 1996, ACS Publishing.

[302], cetyldimethylamine oxide [CDMAO] [90], alkyl-benzenesulfonates [ABS] [287], Triton X-100 [302],

n-octyl -d-glucopyranoside [OG] [303] and n-octyl-d-thioglucopyranoside [OTG] [303,304]) have beeninvestigated. The viscosity of the HMPAM solution withsurfactant passes through a maximum, which is always

just under the surfactants critical micelle concentration(CMC) [173]. The increase in viscosity is attributed to theformation of mixed micelles of surfactant and hydrophobicregions [92,117,120,154,173,300,301,303,304,307309].According to several authors [92,303,304], when addingsurfactant to a HMPAM solution, the surfactant binds tothe copolymeric regions, which leads to preferential inter-chain micro-domain formation instead of intrachain. Aschematic presentation of this behavior has been providedby Biggs et al. [92] (Fig. 15).

The surfactant concentration in region I is low. Thesurfactant associates in a non-cooperative way withthe hydrophobic groups [92]. However, this is depen-dent on the type of surfactant. Two different studies[303,304] demonstrated that the surfactants hexade-cyltrimethylammonium chloride (HTAC) and SDS bondedin a non-cooperative way but the surfactants OG and OTGbonded in a cooperative manner. Winnik et al. [304] con-cluded that ionic surfactants bind by a non-cooperativemechanism and neutral surfactants by a cooperative one.Accordingto another study there are twoclasses of interac-tions,whichare differentiated by thebridgingand viscosity

increase below or above the cmc of the surfactant [90]. Inregion II enough surfactant molecules (higher surfactantconcentration) are present to solubilize the hydrophobicgroups more effectively (mixed micelles). The formation ofmixed micelles is the onset of a significant increase in vis-cosity. Most of the mixed micelles incorporate more thanone hydrophobic region, i.e. a higher degree of overlap orlinks between separate chains is achieved. In region III thesurfactant concentration is higher, at or above the cmc,which results in solubilization of each hydrophobic regionin one micelle. This leads to a reduction in viscosity.

Transition from spherical to rod-like micelles in thesurfactant phase has been observed upon addition of potassium bromide and hexanol to respectively thepolymerCTAB and polymerCDMAO mixture [90,115].Thetransitionisbelievedtobeduetothegaininfreeenergyupon addition of the chemicals where the hydropho-bic groups no longer are exposed to the solvent butinstead are present inside the hydrophobic rods [90,115].Fig. 16 provides a schematic presentation of the rod-likemicelles.

This transition to the rod-like conformation leads tomore intermolecular bridging and thus enhanced solutionproperties. This is evident from the significantly higherviscosity of the solutions compared to the viscosity ofthe individual components [90,115]. Both, rheopectic andthixotropic, behaviors have been observed for polymersolutions in the presence of surfactant [92].


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Fig.16. Schematic presentation of rod-like micelles [90,115].

Using polymer 23 (R2 = 2 , 3 or 4 series) Gouveia andMuller [306] observed a large increase in the solution vis-cosity upon the addition of CTAT. Adding salt (NaCl) to thissolution leads to a further increase in the solution viscos-

ity of the mixture. According to the authors the worm-likemicelles increase in size by the addition of salt and at highsalt concentration salting out effects arise.

A peculiar behavior is observed for the interactionsbetween fluoro- and hydrocarbon copolymers 12 and sur-factants. At low incorporation level of the hydrophobicgroups the interactions with surfactants is selective to sur-factants with the same chemical nature as the hydrophobicgroups [212].

Different oil reservoirs possess different temperatures.The polymer solutions that are to be used must be ableto cope with the different temperatures, i.e. no loss ofviscosity with alteration in temperature. The viscosity

of polymeric solutions employing the polymers 10 (ter-polymer of AMNaAMPSAMPDAC) or 12 (Table 1) isminimally dependent on the temperature in the range3060 C [204,209,210]. In addition polymer 12 displayedgood retention of solution viscosity for a prolonged period(40 days) [209]. The viscosity of the polymer solutioncontaining the zwitterionic (monomer) polyelectrolyte,

polymer 4, displays a unique behavior when the temper-ature is increased. The intrinsic viscosity of the polymersolution increased (from 6.5 to 8.5 dl/g) when the tem-perature is increased from 25 to 60 C [197]. This has

been observed for polymer 14 in the temperature rangeof 2040 C. According to the authors [217] the increasecan be attributed to the fact that hydrophobic associa-tions are endothermic in the investigated temperaturerange, as hypothesized by McCormick et al. [103]. Usingthe latter terpolymers 14 LAlloret et al. [310] observeda significant increase in the solution viscosity when thetemperature is increased from room temperature to 80C.In addition the authors demonstrated that the thermoth-ickening behavior of the polymeric solution is morepronounced at lower salt concentration and lower shearrates. The latter effect is also observed with polymer 27(6) [243] and is attributed to the increase in PPO con-

centration in the hydrophobic micro-domains caused byan increase in the mobility of the chains at relativelyhigh temperature. At higher temperatures, the reductionin viscosity is attributed to the loss in connectivity of the network due to changes in the hydrophobic micro-domains. According to the authors the size of the micro-domains increase at higher temperatures but their number

Fig.17. Schematic illustration representing the thermally induced conformational changes.Reproduced with permission from [235] 1991, ACS Publishing.


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Fig. 18. Schematic presentation of the effect of solution pH on the network structure.


decreases and causes the aforementioned connectivityloss.

The viscosity of the AMAMPDAC copolymers in thepolymer series 1 decreased as expected with increasingtemperature [193]. The same behavior is observed for apolymeric solution employing polymer 2 [195]. Unlikepolymers 1 and 2, an intriguing property of polymer 4 isthe increase of the intrinsic viscosity with temperature (inthe range 2560 C). Another polymeric solution that dis-plays peculiar behavior with increase in the temperatureis the solution of terpolymer (9, AMNaAMPSAMPDAC),which was minimally dependent on the temperature[204].

The LCST concept, discussed in the introduction of thischapter, also applies to polymer 33 where the viscosityof the solution can increase with an increase in temper-ature. However, one important finding reported by theauthors [235] is the presence of hydrophobic associationsbelow the LCST. An increase of the solution temperature(towards the LCST)leadsto disruptionof theseassociations.However, only for long or high hydrophobicity groups dothe associations arise below the LCST. This behavior wasobserved withthe amphiphilic polymer23 (NIPAM-CNAM).Four decades ago, it was proposed that the LCST shoulddecrease with the polymer hydrophobicity [311]. However,the study by Ringsdorf et al. [235] demonstrated that theproposed trend is not followed when increasing the lengthof the hydrophobes in polymer 23 (NIPAM series). Thisis attributed to the formation of a micellar structure bythe hydrophobes, which prevents them from contributingto the LCST suppression. On the basis of this hypothesis,the authors have proposed a model representing the ther-mally induced polymer conformational changes for both,23 (NIPAM series) and 33, polymers (see Fig. 17).

In addition, increasing the temperature from 25 to55 C leads to the preferential formation of intermolecu-lar hydrophobic associations and thus an increase in theviscosity.

The pH of the polymeric solution also affects the solu-tion viscosity. The effect on classic polyelectrolyte is easilyunderstood given the charged nature of the polymers.Polyanions will have a low viscosity at low pH and highviscosity at high pH while polycations display the oppo-site behavior. Polymer 2 is a classic example of a polyanionwhere the solution viscosity decreases as the pH of thesolution is reduced. The effect of pH is more pronouncedon polyelectrolyte amphiphilics where the associations area balance between hydrophobic and electrostatic interac-

tions. Zhou et al. [225] demonstrated, using polymer 18(AA series), that increasing the pH from 4 to 5 and from11 to 12, a significant increase in the solution viscosityis observed while at a pH between 5 and 11 the solu-tion viscosity is lower. A similar behavior was observedfor polymer 17 (AA series) although the pH where theincrease in solution viscosity is observed is shifted. Theincrease is observed between a pH of 56 and 1213 [221].Although theeffect of pH on polyelectrolyteamphiphilics iscomplex (vidasupra), the investigation of the polymer net-work structures of solutions employing polymer 15 usingNRET measurements led to the development of a modelillustrating the effect of the solution pH on the confor-mation of the polymer chains and their associations [218](see Fig. 18).

Increasing the pH will lead in principle to the transi-tion towards more intermolecular associations rather thanintramolecular. The high degree of ionization (at high pH)will disrupt the intramolecular hydrophobic associationscausing a rearrangement of the network with a prefer-ence for intermolecular associations [218]. Amphiphilicsareaffected by thesolutionpH mainlydue to thecarboxylicgroups of the polymers. Polymer 26 is a classic exampleshowing what the effect of the pH is on the behavior of anamphiphilic polymer. The solution viscosity passes througha maximum as the pH rises from 4 to 12. This behavior isattributedtotwo different effects: neutralization of thecar-boxylic groups leading to (1) intramolecular electrostaticrepulsion and thus chain exten

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