Polymer Molecular Architecture As a Tool for Controlling the

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Polymer Molecular Architecture As a Tool for Controlling the Rheological Properties of Aqueous Polyacrylamide Solutions for Enhanced Oil Recovery Diego A. Z. Wever,†,‡ Lorenzo M. Polgar,† Marc C. A. Stuart,§ Francesco Picchioni,† and Antonius A. Broekhuis*,† †

Department of Chemical Engineering-Product Technology, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG, The Netherlands Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands § Electron Microscopy-Groningen Biomolecular Sciences and Biotechnology Institute, Rijksuniversiteit Groningen, Nijeborgh 7, 9747 AG, The Netherlands ‡

S Supporting Information *

ABSTRACT: The controlled synthesis of high molecular weight branched polyacrylamide (PAM) has been accomplished by using atomic transfer radical polymerization (ATRP) of acrylamide (AM) in water at room temperature. Halogen-functionalized aliphatic polyketones acted as macroinitiators in the polymerization. The obtained branched polymers were used in water solutions to study the effect of the molecular architecture on the rheological properties. For comparison purposes, linear PAM was synthesized by using the same procedure. The intrinsic viscosities and light scattering data suggest that the 13- and 17-arm PAMs are more extended in solution compared to the linear, 4-arm, and 8-arm analogues, at equal total molecular weight. The comparison of linear and 4-, 8-, 12-, 13-, and 17-arm PAM in semidilute solutions demonstrated that the 13- and 17-arm PAM have the highest solution viscosity at equal molecular weight. Depending on the PAM molecular weight and concentration, a significant (as much as 5-fold) increase in solution viscosity (at a shear rate of 10 s−1) is observed. The elastic response of aqueous solutions containing the polymers critically depended on the molecular architecture. Both the 4- and 8-arm polymers displayed a larger phase angle value compared to the linear analogue. The 13- and 17-arm PAMs displayed a lower phase angle than the linear one. Ultimately, the rheological properties are dependent on the number of arms present. The combination of a higher hydrodynamic volume and higher entanglement density leads to an improved thickening efficiency (for N ≥ 13, N being the average number of arms). The improved thickening efficiency of the branched (N ≥ 13) PAMs makes these polymers highly interesting for application in Enhanced Oil Recovery and drag reduction.

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INTRODUCTION Polyacrylamide is a versatile industrial polymer that finds use in wastewater treatment, cosmetics, and enhanced oil recovery (EOR).1 In particular, the main purpose of using PAM (mostly in water solution) resides in the corresponding improvement of the rheological properties. Indeed, in most applications, an enhancement of the solution viscosity is required. The performance of water-soluble polymers in EOR is evaluated by the polymer’s ability to increase the solution viscosity at shear rates2 between 5 and 10 s−1, since these are the shear rates mostly encountered in the porous media where the oil resides. In addition, the comparison between polymers is performed at equal solution viscosity instead of at equal polymer concentration. This is justified by the fact that the manufacturing cost of the currently used partially hydrolyzed polyacrylamides (HPAMs) is not affected by the molecular weight of the polymers. In EOR, it has been concluded that, at equal viscosity, the viscoelasticity of the solution plays a crucial role in ensuring a high oil recovery.3−8 Such rheological behavior arises from the extremely high molecular weight (typically Mw ≈ 2 × 107 g/ mol) and the ionic character of the water-soluble polymer employed. The presence of electric charges along the backbone results (in deionized water) in the stretching of the polymer © 2013 American Chemical Society

chains/coils and ultimately in larger viscosity values. In this context, the use of partially hydrolyzed PAM (HPAM) represents the most popular choice. However, the presence of the electric charges makes these polymers highly sensitive to the presence of electrolytes, where a significant decrease in the solution viscosity is observed in salt solutions.1,9 The importance of the solution elastic response has been supposedly demonstrated3−8 by comparing a water solution of HPAM and one of glycerin in flow experiments specifically designed to simulate oil recovery processes. However, such comparison might not be completely correct since HPAM is a high molecular weight polyelectrolyte while glycerin a small molecule. Such difference in structure of the used chemicals as well as of the corresponding water solution might indeed result in differences also in other properties (e.g., interfacial tension between oil and water), thus hindering a direct correlation of the observed effect and the supposed cause, in this case the elastic behavior of the water solution. A better comparison would be between polymeric solutions where the viscoelasticity Received: Revised: Accepted: Published: 16993

September 14, 2013 October 24, 2013 October 24, 2013 October 24, 2013 dx.doi.org/10.1021/ie403045y | Ind. Eng. Chem. Res. 2013, 52, 16993−17005

Industrial & Engineering Chemistry Research

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Scheme 1. Synthesis of the Macroinitiators

of arms and to investigate the effect of the architecture on the rheological properties of the corresponding water solutions. To the best of our knowledge, this represents the absolute novelty, in terms of synthetic strategy as well as structure−property relationship, of the present paper. In this context, it is worthwhile stressing how the rheological characterization has been mainly carried out according to the corresponding application field (e.g., in the choice of the shear rate for viscosity comparisons). This is necessary also in connection with the structure of the prepared polymers, in between those of comb-like and star polymeric materials, for which the rheological behavior can provide useful indications. A complete rheological characterization is outside the scope of the paper.

is systematically changed. However, for water-soluble PAM a systematic change in the elastic response without affecting other properties (i.e., molecular weight and dispersity) is difficult. One approach can be the controlled synthesis of the PAM. However, the monomer itself (acrylamide) represents a difficult candidate to polymerize in a controlled fashion.10 Controlled synthesis of branched PAM has only limitedly been reported in literature. In the past, high conversion and high temperature in conventional free radical polymerization was demonstrated to lead to uncontrolled branched polyacrylamide.11−16 By increasing the reaction temperature (from room temperature to 90 °C) and the conversion level of acrylamide, more branches could be obtained.12 The properties of the uncontrolled branched PAM were evaluated with respect to its performance as flocculant, and it was concluded that linear PAM performed better than the uncontrolled branched PAM. This was attributed to the inherent lower hydrodynamic volume of the branched PAM. This is not surprising when making allowances for the fact that the hydrodynamic volume of a branched polymer is lower than that of the linear homologue both for regular and uncontrolled branching.15,16 Nevertheless, given the uncontrolled nature of the polymerization procedure, a mixture of products is synthesized with no well-defined structure. Controlled radical polymerization for the preparation of hyperbranched PAM has been recently reported.17 The hyperbranched PAMs were synthesized by using reversible addition−fragmentation chain transfer (RAFT) polymerization. Although the polymerization is a controlled one, the branching occurs randomly.17 Therefore the control in architecture of the PAM is limited and no correlation between molecular architecture and rheological properties can be obtained. Recently controlled synthesis of PAM has been reported in water−ethanol mixtures18 and, by our group, in water.19 In a water−ethanol mixture, linear PAM (with molecular weights up to >350 000 g/mol and dispersities as low as 1.10) could be synthesized.18 The molecular weights of PAM reached values >150 000 g/mol (with dispersities as low as 1.39) in water with use of the same catalyst/initiation system.19 Also thermoresponsive polymeric materials have been synthesized with use of this system.20,21 With the advent of ATRP of acrylamide, the controlled preparation of branched PAM can be envisaged. This enables the systematic study of the structure−property relationships of PAM (with different topologies) in water solutions. The aim of this work is to prepare in a controlled fashion branched PAM with varying numbers (and molecular weight)

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EXPERIMENTAL SECTION Chemicals. Acrylamide (AM) (electrophoresis grade, ≥99%), PAM (M w = 5−6 × 10 6 g/mol), tris[2(dimethylamino)ethyl]amine (Me6TREN), 2,2-bipyridine (bpy), copper(I) chloride (CuCl, 98%), copper(I) bromide (CuBr, 98%), methyl 2-chloropropionate (MeClPr, 97%), 3chloropropylamine hydrochloride (98%), and sodium hydroxide (pellets) were purchased from Sigma Aldrich. CuCl and CuBr were purified by stirring in glacial acetic acid (Aldrich), washing with glacial acetic acid, ethanol, and diethyl ether (in that order), and then drying under vacuum. All solvents were reagent grade and used without further purification. The alternating polyketones 30 mol % ethylene content (PK30, Mn = 2800 g/mol, PDI = 1.74) was synthesized according to the published procedure.22,23 Macroinitiators. The PK30 functionalization was performed according (Scheme 1) to the published method.24 The reactions were performed in a sealed 250-mL roundbottomed glass reactor with a reflux condenser, a U-type anchor impeller, and an oil bath for heating. For the preparation of PK30-Cl12 (e.g.), 3-chloropropylamine hydrochloride (9.89 g, 53.6 mmol) was dissolved in methanol (90 mL) to which an equimolar amount of sodium hydroxide (2.15 g, 53.6 mmol) was added. After the polyketone (10 g) was preheated to the liquid state at the employed reaction temperature (100 °C), the amine was added dropwise (with a drop funnel) into the reactor in the first 20 min. The stirring speed was set at a constant value of 500 rpm. During the reaction, the mixture of the reactants changed from the slight yellowish, low viscous state, into a highly viscous brown homogeneous paste. The product was dissolved in chloroform and the obtained solution was washed with demineralized water. The two phases (organic and water) were separated in a 16994

dx.doi.org/10.1021/ie403045y | Ind. Eng. Chem. Res. 2013, 52, 16993−17005


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solution was first diluted with demineralized water before being precipitated. The polymer was isolated by filtration and subsequently dried in an oven at 65 °C. Characterization. The acrylamide conversion was measured by using gas chromatography (GC). Several different samples directly taken from the reaction mixtures were dissolved in acetone (polymer precipitates) and injected on a Hewlett-Packard 5890 GC with an Elite-Wax ETR column. The total molecular weight (Mn,tot) is calculated by using the acrylamide conversion (monomer−initiator ratio multiplied by the conversion). The span molecular weight (Mn,SPAN) is calculated by using the Mn,tot and is defined25 as two times the molecular weight of one arm (star PAM) or two times the molecular weight of one arm plus the molecular weight of the macroinitiator (comb PAM). The Mn,SPAN is the longest linear span in the polymer. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Mercury Plus 400 MHz spectrometer. For analysis chloroform was used as the solvent. The hydrodynamic radius (Rh,DLS) of the different polymers was measured by using a Brookhaven ZetaPALS zeta potential and particle size analyzer. Dilute (polymer concentration 4) saturates and the molecular weight of the arms determines the viscosity.25 However, recently it has been demonstrated that comb-like polyethylenes have η0 much higher than their linear and long chain branched analogues.48 Nevertheless, these measurements are performed in the melt and thus the highest possible “concentration” is measured. Another explanation might be that the branched PAMs with a high number of arms (N ≥ 13) act as soft colloidal particles and the ones with a low number of arms (N ≤ 8) as polymer stars. Earlier studies on spherical polymeric brushes have demonstrated this.39,49,50 Viscoelasticity, Effect of the Number of Arms (at Equal Concentration). The effect of the number of arms on

Figure 4. η0 as a function of the molecular architecture at Mn,th ≈ 1.6 MDa and a polymer concentration of 5c*.

model for comb-shaped polymers in the melt38 that the highest η0 are obtained with comb polymer having a low number of long arms. However, the results in aqueous solution (Figures 4 and 5) contradict these predictions. The discrepancy might lie in the difference in concentration regime (melt vs semidilute), and the fact that associations26 can arise in the aqueous solution due to the hydrophobic backbone. In addition, unlike in the melt, in water solution hydrogen bonding (between the solvent and polymer) might play a significant role in rheological properties. The impact of the number of arms has also been demonstrated for spherical polymeric brushes and micelles in solution.39 Solution Viscosity, Effect of the Number of Arms (at Equal Concentration). The effect of the number of arms on the solution viscosity has been evaluated. The solution viscosity (at γ = 10 s−1) as a function of concentration has been measured, while maintaining the molecular weight constant (Figure 6). The choice of such shear rate value is justified by the requirements for EOR (vide supra). As can be observed, the solution viscosity of the 13- and 17arm branched PAM is systematically the highest at all molecular weights. The PAM polymers can also be compared as a function of shear rate. In Figure 7A, such a comparison is made

Figure 5. The G′ and G′′ (A) and the phase angle (B) as a function of the frequency for the different polymers at a polymer concentration of 5c*. 16999



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Figure 6. Viscosity as a function of concentration for different total molecular weights (A, 0.6 MDa; B, 1.6 MDa; and C, 2.6 MDa).

Figure 7. (A) Viscosity function for PAMs (entries 2, 5, 7, 10, and 13; polymer concentration of 3.85 wt %); lines correspond to fits of the “Carreau−Yasuda” model. (B) The relaxation time for entries 2, 5, 7, 10, and 13.

The comparison at equal polymer concentration demonstrates that the 13- and 17-arm PAM display a more pronounced elastic response (lower phase angle) irrespective of the molecular weight. However, the results can be masked by the difference in viscosity (and in EOR the comparison is performed at equal viscosity); therefore the comparison is also made at equal η0 (and thus at different concentrations). Viscoelasticity, Effect of the Number of Arms (at Equal η0). The results of the comparison between the different

the viscoelasticity of a water solution was probed by oscillation experiments. The results are displayed in Figure 8, where the polymer concentrations of the solutions were kept constant for each comparison. Viscoelastic fluids display at low frequencies (i.e., terminal zone) a G′′ that is directly proportional to the frequency (ω) with a slope of 1 and G′ proportional to ω2 (a slope of 2).27 As can be observed in the Figure 8, all samples display this behavior at low frequencies. 17000



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Figure 8. (A1) G′ and G′′ of the PAMs with Mtot = 0.6 MDa and (A2) their respective phase angles (polymer concentration = 5.66 wt %). (B1) G′ and G′′ of the PAMs with Mtot = 1.6 MDa and (B2) their respective phase angles (polymer concentration = 2.91 wt %). (C1) G′ and G′′ of the PAMs with Mtot = 2.6 MDa and (C2) their respective phase angles (polymer concentration = 1.96 wt %).

PAMs at equal η0 are displayed in Figure 9. The comparison at equal η0 reveals that the 13- and 17-arm PAMs display lower phase angles at low frequencies irrespective of the molecular weight. As the frequency is increased (Mn,tot = 0.6 MDa) to above 10 rad/s, the phase angles of the 4- and 8-arm PAM decrease to lower values than that of the linear and 12-arm. Given the different concentrations required to reach the same viscosity, the number of polymeric chains in the solution also differs. For the 4- and 8-arm PAM a concentration of 3.85 and

4.76 wt % (respectively) is required. Compared to the linear and 12-arm PAM (polymer concentration of 2.91 and 1.96 wt %, respectively), more polymeric chains are present in the 4and 8-arm solutions. In addition, the length of the arms of the 4- and 8-arm PAMs is larger than that of the 12-arm. The combination of longer arms (a higher arm molecular weight leads to a more pronounced elastic behavior in the melt51) and higher number of polymeric chains in solution (an increase in the concentration leads to a more pronounced elastic behavior 17001



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Figure 9. (A1) G′ and G′′ of the PAMs with Mtot = 0.6 MDa and (A2) their respective phase angles (equal η0). (B1) G′ and G′′ of the PAMs with Mtot = 1.6 MDa and (B2) their respective phase angles (equal η0). (C1) G′ and G′′ of the PAMs with Mtot = 2.6 MDa and (C2) their respective phase angles (equal η0).

for polystyrene in chlorinated diphenyl27,52) might explain the more pronounced elastic behavior of the solutions containing 4- and 8-arms. Another explanation might be that more arms leads to more steric hindrance and therefore less hydrophobic associations between the hydrophobic polyketone backbones. The 4- and 8-arms PAM display more hydrophobic associations, given the less steric hindrance, and more/stronger hydrophobic associations are known to lead to a more pronounced elastic response.53−56

Nevertheless, further studies (currently being carried out) are required to fully elucidate the mechanism behind the observed behavior. Viscoelasticity, Effect of the Length of the Arms (at Equal Concentration). The effect of the length of the arms on the viscoelasticity of a water solution was investigated by oscillation experiments. The results for the 13-arm PAM are displayed in Figure 10. As can be observed in Figure 10, the increase in length of the arms leads to an increase in both the loss and storage modulus. The transition from the terminal to 17002



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Figure 10. The loss (A) and storage (B) modulus, the phase angle (C) as a function of the frequency of the 13-arm PAM with different length of the arms (polymer concentration = 2.91 wt %), and the η0 as a function of the DParm (D).

the same exponential dependency of the η0 on the arm molecular weight is observed.25 Schematic Model. With the available data on linear and branched PAMs a conceptual model can be devised (Figure 11)

the plateau zone is shifted to lower frequencies as the arm length increases (i.e., also the Mn,tot). In addition the plateau zone becomes longer as the arm length is increased. Both these effects are in line with results on low dispersity polystyrene in the melt,27 where constraints, due to entanglement, cause an increase in the terminal relaxation time and increases with molecular weight. In the 13- and 17-arm PAM cases, the constraints arise due to their high molecular weight and architecture. Therefore the terminal relaxation time increases with increasing arm length. One might speculate that it should increase more rapidly compared to a linear polymer (given the higher relaxation time in the melt for branched polymers38). This is then in line with the higher solution viscosity of the 13and 17-arm branched PAM compared to their linear analogues. The phase angle decreases as the arm length increases. The disentanglement of the overlapping chains (entanglements) becomes progressively more difficult as the length of the arms increase. Therefore, in essence, a stiffer solution is obtained as the length of the arms increases. The dependence of the η0 in solution on length of the arms is displayed in Figure 10D. As can be observed, the η0 increases exponentially (relatively good fit) with the increase in the length of the arms. This matches the theory in the melt where

Figure 11. Schematic model of the linear and branched PAMs.

for the branched PAMs in dilute and semidilute solutions. The hydrodynamic radius of the branched PAMs depends on the number of arms. At low number of arms (N ≤ 8) the hydrodynamic volume is slightly lower compared to that of a linear analogue. This is in line with the general view of a more compact conformation for branched polymers compared to their linear analogues,31 which leads to lower η0 for the branched polymers in unentangled solutions.57 However, at a high number of arms (N ≥ 13), the low amount of space 17003



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available for each arm will lead to an extended configuration for the arms close to the backbone. Increasing the concentration of the polymer to above the critical overlap concentration leads to entanglements. When entangled at equal polymer concentration, the branched PAMs with a higher number of arms (N ≥ 13) have a higher entanglement density compared to PAMs with few arms (N ≤ 8). The increase in entanglement density leads to a higher solution viscosity. In addition, above the critical overlap concentration, the rheology of a star-like (compared to a linear analogue) polymeric solution is governed by the arm retraction, where the arms explore new configurations through retraction and extension into new directions.37 As this is a much slower process37 compared to the reptation of linear chains,58−61 an exponential dependence of the η0 on the arm molecular weight is observed in the melt.37 For the 13- and 17-arm PAM, the combination of a higher hydrodynamic volume (due to stretching) and a higher entanglement density leads to an increased thickening efficiency compared to their linear analogues. In addition, an increase in entanglement density leads to a more pronounced shear thinning behavior, thus in striking agreement with the observed experimental behavior (Figure 7A).

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ASSOCIATED CONTENT

S Supporting Information *

GPC traces and data on some of the branched polymers. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is part of the Research Programme of the Dutch Polymer Institute DPI, Eindhoven, The Netherlands (#716). REFERENCES

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CONCLUSION The controlled synthesis of branched high molecular weight of PAM, through ATRP in water (and acetone as a cosolvent), has been accomplished. Branched PAMs of 4, 8, 12, 13, and 17 arms have been synthesized. The effect of the molecular architecture (i.e., number of arms) on the rheological properties in semidilute water solutions (solution viscosity and viscoelasticity) was investigated. The 13-arm and 17-arm PAM displayed a higher solution viscosity compared to the linear, 4-arm, and 8arm analogues irrespective of the molecular weight. The comparison between the 13-arm PAM and a linear analogue displays an as much as 5-fold increase in the solution viscosity (at a shear rate of 10 s−1). Furthermore, a more pronounced shear thinning is observed for the 13- and 17-arm PAMs. The elastic response of the 13- and 17-arm PAM in solution is more pronounced compared to their linear analogue. The 4- and 8arm PAMs though, display a lower elastic response compared to their linear analogues. The rheological properties of the branched PAMs are dependent on the average number of arms and their length. In semidilute aqueous solutions, the combination of a higher hydrodynamic volume and higher entanglement density leads to an improved thickening efficiency (for N ≥ 13) of the branched PAMs. The prepared polymers display a well-characterized structure (especially in connection with their industrial nature and applicability) and rheological behavior similar to the one of star-like polymers. The comparisons elucidated in the paper clearly indicate the suitability for EOR application with respect to the currently used linear PAM. A more fundamental understanding of the rheological behavior (outside the scope of the present work) is currently being carried out. Nevertheless, the manipulation of the rheological properties of PAM in water through smart architectural design opens new ways in designing industrial PAM-based materials for new applications where control in the rheological properties is crucial. The increased thickening efficiency of the branched PAMs, without introducing salt-sensitive moieties, makes these water-soluble polymers highly attractive for applications in EOR. 17004



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