**1. Introduction**

Self-organization of molecular structures refers to the spontaneous assembly of individual building blocks into ordered and thermodynamically stable nanostructures by noncovalent interactions, resulting in the formation of dynamic and responsive materials. The noncovalent (reversible) interactions include electrostatic attractions, metal/ligand complexes, π/πstacking, ionic interactions, hydrogen bonding, or hydrophobic effects without the application of external energy. Therefore, self-assembly is a low-cost and high-yield process [1–6] that can be activated by external stimuli, which involves changes in temperature, pH, ionic strength, radiation, the addition of other molecules, or their combination [2, 7–9].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Supramolecular polymer networks can be formed by polymer blends (PBs) in which the reversible and highly directional noncovalent interactions are randomly distributed across the polymer chains [6]. The resulting supramolecular structure displays polymeric properties in dilute and concentrated solutions that follow well-established theories of polymer physics [9]. These networks exhibit enhanced material properties compared to the individual polymers [1], and the reversibility of noncovalent interactions provides self-healing properties to these materials [6, 10].

conformation reversal to a reordered-renatured state [11–19, 21–23, 27, 29, 30]. In saline solu-

Viscoelasticity of a Supramolecular Polymer Network and its Relevance for Enhanced Oil Recovery

moieties on the xanthan trisaccharide side chains, reducing repulsive forces between polymer chains. The side chains collapse down to the backbone causing conformation transition to a rigid rod-like shape of reduced hydrodynamic size stabilized by hydrogen bonding [23]. This conformation increases the intra- and intermolecular attraction and the xanthan chains tend to adopt a more rigid ordered conformation [13, 14]. The stability of xanthan gum is attributed to its ordered conformation which is stabilized by salts. Thus, the optimum functionality of

Xanthan gum is prone to self-association at low concentrations and has been used as a building block to generate self-assembled structures [24, 25]. The onset of macromolecular selforganization has been reported at concentrations ≥0.1 wt% [8]. Parallel packing or side-by-side associations stabilized by hydrogen bonds facilitate the formation of three-dimensional networks [8, 19, 26]. Previous research has also demonstrated the self-organization of xanthan gum through polyelectrolyte complexation with other polysaccharides, such as chitosanxanthan, β-lactoglobulin-xanthan, α-galactosidase-modified guar gum-xanthan, and sodium caseinate-xanthan, among many others [8, 26, 31]. Another example of self-aggregation through hydrophobic interactions is the intermolecular binding of xanthan gum-carob gum. These self-organizations take place in both the ordered and disordered conformations of the

Advantages of xanthan gum include non-toxicity, high water solubility, salinity tolerance, stability over a broad range of pH values, thermal stability against hydrolysis provided by its ordered conformation, high shear stability, high viscosities but a significant shear-thinning behavior on shearing at a low concentration, slight variations in viscosity with changes in temperature, availability, the ease of processing, and low manufacturing costs [11, 17, 18, 22, 26, 34, 35]. The main downside of xanthan gum is high sensitivity to microbial attack. Salt-tolerant aerobic and anaerobic microorganisms can degrade the xanthan gum chains leading to loss in solution viscosity. In practical applications, biocides are added to the xanthan gum solution to suppress the

Partially hydrolyzed polyacrylamides, HPAMs (**Figure 1(b)**), with degree of hydrolysis ranging from 25 to 35%, are the most used polymers for enhanced oil recovery (EOR) [35, 37]. HPAM is highly sensitive to mono- and divalent cation environments because of the shielding effect of the negative charges on the polymer backbone. Thus, the static repulsion between the HPAM lateral groups fades and the polymer structure collapses, which drastically reduces the viscosity of the polymer solution [35]. Divalent cations (e.g., Ca2+) can also induce the self-aggregation of HPAM through polyion-cation complexation and depending on the concentration of Ca2+ inter- and intrachain complexation takes place [36]. The molecular conformation and viscosity of HPAM solutions are also affected by the degree of hydrolysis, pH, temperature, molecular weight, pressure, and solvent quality [35, 36]. HPAM is susceptible to mechanical degradation under high shear due to chain scission reducing the polymer chain size and molecular weight, which in turns reduces the viscosity of the polymer solution. Advantages of HPAM are low manufacturing costs and resistance to bacterial attack [36].

xanthan macromolecule depending on the ionic strength [8, 20, 32, 33].

and Ca2+) condense on the ionized carboxyl

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tions, monovalent or divalent cations (e.g., Na+

xanthan gum requires the addition of salt [17].

growth of degrading microorganisms [11, 17, 36].

This study focuses on the formulation and characterization of a supramolecular polymer network based on reversible interactions among the main- and side chains of a mixture of xanthan gum, HPAM, and HMPAM or associating polymer (AP) in brine. Xanthan gum and HPAM are the most commonly used polymers in the field for enhanced oil recovery (EOR) [11]. **Figure 1** displays the chemical structures of these polymers.

Xanthan gum is a high-molecular weight (≈2–3 × 10<sup>6</sup> g/mol) anionic polysaccharide derived from the fermentation of the bacterium *Xanthomonas campestris*. Its chemical structure contains a cellulose backbone consisting of five monosaccharides to form a pentasaccharide repeating unit. The cellulosic backbone is substituted at C-3 on alternate β-1,4-D-glucopyranosyl residues with charged trisaccharide side chains of β-D-rhamnopyranosyl β-1,4-D-glucuronopyranosyl, and α-1,2-D-mannopyranosyl. The two charged functional groups, COO− , are found in the α-Manp and β-GlcAp residues (**Figure 1(a)**) [8, 11–25].

Xanthan gum is a stereoregular polymer and displays order–disorder transition in solution that depends on polymer concentration, ionic strength, and temperature [11–14, 17, 19, 24, 26]. A high polymer concentration and an increased ionic strength support ordered structures, while elevated temperatures favor disorder structures [11]; therefore, xanthan gum is a thermally responsive biopolymer [27]. At low temperatures, the native state of xanthan gum shows an ordered and rigid conformation as a single helical or a double-strand helix stabilized by inter- and intramolecular hydrogen bonds. The ordered conformation of the xanthan molecule is semi-flexible with a hydrodynamic length ranging from 600 to 2000 nm and a hydrodynamic diameter of 2 nm [20]. At elevated temperatures (≥60°C) [12, 16, 24] and low ionic strength, the native state, which displays an order state, transitions to a more flexible disordered-denatured state [13]. Optical rotation measurements have demonstrated that xanthan gum dissolved in distilled water at 25°C displayed the nonordered conformation [24]. However, a decrease in temperature (<40°C) [16, 24, 28] and the simultaneous increase in the ionic strength cause

**Figure 1.** Generic chemical structures of (a) xanthan gum, (b) HPAM, and (c) associating polymers (**Figure 1(a)** was adapted from reference [12] and **Figure 1(b)** and **(c)** were adapted from reference [13]).

conformation reversal to a reordered-renatured state [11–19, 21–23, 27, 29, 30]. In saline solutions, monovalent or divalent cations (e.g., Na+ and Ca2+) condense on the ionized carboxyl moieties on the xanthan trisaccharide side chains, reducing repulsive forces between polymer chains. The side chains collapse down to the backbone causing conformation transition to a rigid rod-like shape of reduced hydrodynamic size stabilized by hydrogen bonding [23]. This conformation increases the intra- and intermolecular attraction and the xanthan chains tend to adopt a more rigid ordered conformation [13, 14]. The stability of xanthan gum is attributed to its ordered conformation which is stabilized by salts. Thus, the optimum functionality of xanthan gum requires the addition of salt [17].

Supramolecular polymer networks can be formed by polymer blends (PBs) in which the reversible and highly directional noncovalent interactions are randomly distributed across the polymer chains [6]. The resulting supramolecular structure displays polymeric properties in dilute and concentrated solutions that follow well-established theories of polymer physics [9]. These networks exhibit enhanced material properties compared to the individual polymers [1], and the reversibility of noncovalent interactions provides self-healing properties to

This study focuses on the formulation and characterization of a supramolecular polymer network based on reversible interactions among the main- and side chains of a mixture of xanthan gum, HPAM, and HMPAM or associating polymer (AP) in brine. Xanthan gum and HPAM are the most commonly used polymers in the field for enhanced oil recovery (EOR)

Xanthan gum is a high-molecular weight (≈2–3 × 10<sup>6</sup> g/mol) anionic polysaccharide derived from the fermentation of the bacterium *Xanthomonas campestris*. Its chemical structure contains a cellulose backbone consisting of five monosaccharides to form a pentasaccharide repeating unit. The cellulosic backbone is substituted at C-3 on alternate β-1,4-D-glucopyranosyl residues with charged trisaccharide side chains of β-D-rhamnopyranosyl β-1,4-D-glucuronopyranosyl,

Xanthan gum is a stereoregular polymer and displays order–disorder transition in solution that depends on polymer concentration, ionic strength, and temperature [11–14, 17, 19, 24, 26]. A high polymer concentration and an increased ionic strength support ordered structures, while elevated temperatures favor disorder structures [11]; therefore, xanthan gum is a thermally responsive biopolymer [27]. At low temperatures, the native state of xanthan gum shows an ordered and rigid conformation as a single helical or a double-strand helix stabilized by inter- and intramolecular hydrogen bonds. The ordered conformation of the xanthan molecule is semi-flexible with a hydrodynamic length ranging from 600 to 2000 nm and a hydrodynamic diameter of 2 nm [20]. At elevated temperatures (≥60°C) [12, 16, 24] and low ionic strength, the native state, which displays an order state, transitions to a more flexible disordered-denatured state [13]. Optical rotation measurements have demonstrated that xanthan gum dissolved in distilled water at 25°C displayed the nonordered conformation [24]. However, a decrease in temperature (<40°C) [16, 24, 28] and the simultaneous increase in the ionic strength cause

**Figure 1.** Generic chemical structures of (a) xanthan gum, (b) HPAM, and (c) associating polymers (**Figure 1(a)** was

adapted from reference [12] and **Figure 1(b)** and **(c)** were adapted from reference [13]).

, are found in the

[11]. **Figure 1** displays the chemical structures of these polymers.

α-Manp and β-GlcAp residues (**Figure 1(a)**) [8, 11–25].

and α-1,2-D-mannopyranosyl. The two charged functional groups, COO−

these materials [6, 10].

96 Polymer Rheology

Xanthan gum is prone to self-association at low concentrations and has been used as a building block to generate self-assembled structures [24, 25]. The onset of macromolecular selforganization has been reported at concentrations ≥0.1 wt% [8]. Parallel packing or side-by-side associations stabilized by hydrogen bonds facilitate the formation of three-dimensional networks [8, 19, 26]. Previous research has also demonstrated the self-organization of xanthan gum through polyelectrolyte complexation with other polysaccharides, such as chitosanxanthan, β-lactoglobulin-xanthan, α-galactosidase-modified guar gum-xanthan, and sodium caseinate-xanthan, among many others [8, 26, 31]. Another example of self-aggregation through hydrophobic interactions is the intermolecular binding of xanthan gum-carob gum. These self-organizations take place in both the ordered and disordered conformations of the xanthan macromolecule depending on the ionic strength [8, 20, 32, 33].

Advantages of xanthan gum include non-toxicity, high water solubility, salinity tolerance, stability over a broad range of pH values, thermal stability against hydrolysis provided by its ordered conformation, high shear stability, high viscosities but a significant shear-thinning behavior on shearing at a low concentration, slight variations in viscosity with changes in temperature, availability, the ease of processing, and low manufacturing costs [11, 17, 18, 22, 26, 34, 35]. The main downside of xanthan gum is high sensitivity to microbial attack. Salt-tolerant aerobic and anaerobic microorganisms can degrade the xanthan gum chains leading to loss in solution viscosity. In practical applications, biocides are added to the xanthan gum solution to suppress the growth of degrading microorganisms [11, 17, 36].

Partially hydrolyzed polyacrylamides, HPAMs (**Figure 1(b)**), with degree of hydrolysis ranging from 25 to 35%, are the most used polymers for enhanced oil recovery (EOR) [35, 37]. HPAM is highly sensitive to mono- and divalent cation environments because of the shielding effect of the negative charges on the polymer backbone. Thus, the static repulsion between the HPAM lateral groups fades and the polymer structure collapses, which drastically reduces the viscosity of the polymer solution [35]. Divalent cations (e.g., Ca2+) can also induce the self-aggregation of HPAM through polyion-cation complexation and depending on the concentration of Ca2+ inter- and intrachain complexation takes place [36]. The molecular conformation and viscosity of HPAM solutions are also affected by the degree of hydrolysis, pH, temperature, molecular weight, pressure, and solvent quality [35, 36]. HPAM is susceptible to mechanical degradation under high shear due to chain scission reducing the polymer chain size and molecular weight, which in turns reduces the viscosity of the polymer solution. Advantages of HPAM are low manufacturing costs and resistance to bacterial attack [36].

Hydrophobically modified polyacrylamides (HMPAMs) are water-soluble polymers containing both hydrophobic groups (e.g., methyl or ethyl acrylates, alkyl vinyl ethers, styrene comonomers, or alkyl acrylamides) and weakly charged pendant groups (e.g., carboxylic acid pendant groups) directly attached to the polymer backbone (**Figure 1(c)**). The hydrophobic groups can be distributed in a block-like fashion, randomly, or discretely distributed along the backbone. In aqueous solutions, these hydrophobic groups aggregate through intra- and intermolecular associations that increase the hydrodynamic volume of the polymer, which increases the viscosity of the polymer solution [36, 38]. The alkyl chain length and charge density determine the hydrophobicity of these polymers [39]. The presence of counter-ions in the aqueous media reduces the repulsion between charged groups within the polymer chain, which favors inter- and intrachain hydrophobic interactions [39, 40]. Benefits of HMPAM include salt tolerance and the enhancement in solution viscosity in the presence of low-molecular-weight electrolytes because of the shielding of intramolecular Coulombic attractions rather than the intermolecular hydrophobic interactions [36]. The inter- and intramolecular hydrophobic interactions are dynamic and reversible. Therefore, under high shear rate, these associations are interrupted and the viscosity of the solution decreases, but as soon as the high mechanical shear is removed, the hydrophobic groups re-associate and the viscosity of the solution returns to its original value [36].

were mixed using a magnetic stirrer under strong mixing conditions to prevent the formation of lumps (e.g., fish eyes) at room temperature. The viscoelastic behavior, DMA, and DTMA of the baseline polymers and SAP were characterized through oscillatory rheology at 25°C using a Bohlin Geminin HR Nano Rheometer manufactured by Malvern (Worcestershire, UK) equipped with a parallel-plate measuring geometry (gap: 1000 μm) and a solvent trap to prevent drying effects during measurements. The amplitude sweep was performed in the strain range from 1 to 1000% at a fixed frequency of 1 rad/s, while the frequency sweep was run from 0.01 to 100 rad/s at fixed strain within the linear viscoelastic (LVE) range as determined from

Viscoelasticity of a Supramolecular Polymer Network and its Relevance for Enhanced Oil Recovery

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**Figure 2** displays the frequency sweeps as the curve functions of the loss factor (tan*δ*), storage modulus (*G*'), loss modulus (*G*"), and complex viscosity (|*η*\*|) for the baseline polymers B, HPAM, and AP at concentrations of 0.2, 0.2, and 0.1 wt%, respectively, prepared in 2.1 wt% brine. The frequency sweeps (**Figure 2**) demonstrate that all polymers follow the characteristic shear-thinning flow behavior of polymers, thus viscosity decreases with increasing shear rate. At the same salinity (2.1 wt% brine), temperature, and angular frequency range, polymer B at 0.2 wt% solution concentration shows a significantly higher viscoelasticity than the HPAM at 0.2 wt% solution and the AP at 0.1 wt% solution. The behavior of polymer B (i.e., xanthan gum) results from its molecular-ordered aggregation behavior in saline solutions.

Regarding the loss factor (tan*δ*), which is useful to describe the viscoelastic behavior of materials in the low shear range (*ω* ≤ 1 s−1) [12, 45]; **Figure 2** shows that polymer B behaves different from the HPAM and AP polymer solutions. Polymer B exhibits a flow transition from a viscoelastic liquid behavior (tan*δ* > 1) to a viscoelastic weak-gel behavior (tan*δ* < 1) as angular frequency increases. At low angular frequencies (i.e., *ω* ≤ 1 rad/s), the polysaccharide molecules are entangled with neighboring macromolecules forming aggregates stabilized

**2.1 4.2 8.4**

**Figure 2.** Frequency sweeps as curve functions of tan*δ*, *G*', *G*', and |*η*\*|for baseline polymers prepared in 2.1 wt% brine.

NaCl 1.72 3.45 6.9 MgCl2 0.04 0.09 0.18 CaCl2 0.33 0.65 1.30

SO<sup>4</sup> 0.01 0.02 0.04

the preceding amplitude sweep.

**Salts Brine compositions (wt%)**

Na2

**Table 1.** Brine compositions (wt%).

The goal of this exploratory research was to formulate a supramolecular polymer network with enhanced properties (i.e., viscoelasticity, mechanical stability, salt tolerance, and thermal stability), taking advantage of the synergistic combination of the beneficial properties of the individual polymers in the blend. The formation and characterization of the supramolecular polymer network was demonstrated through frequency (time) and temperature-dependent oscillatory rheology [4, 5]; the use of other analytical techniques for structural characterization of the supramolecular system is beyond the scope of this work.

In this chapter, the formulation and characterization of the supramolecular self-assembled polymer network (SAP) is first described, and then the effect of ionic strength on the viscoelastic behavior of the SAP is discussed. The next section describes the structural strength of the SAP through dynamic mechanical analysis (DMA), followed by the evaluation of the SAP thermal stability by dynamic mechanical thermo-analysis (DMTA). In the following section, the long-term thermal stability of the SAP prepared in brines having different ionic strengths is discussed. Finally, the performance of the SAP in displacing heavy oil employing conventional sad-pack displacement tests at simulated oil reservoir conditions is presented.
