2.1. Effect of β-CD addition

Figure 1 displays the results of the oscillatory tests for β-CD/polymer blends at different β-CD concentrations and fixed concentration of polymer (0.5 wt%). Figure 1(a–c) corresponds to formulations using polymers AP1, AP2, and AP3, respectively. Figure 1(a–c) indicates that the addition of different concentrations of β-CD did not improve the frequency-dependent function of the baseline polymers; on the contrary, the addition of β-CD makes these polymers more inflexible and rigid. In all cases, tanδ increases, while G<sup>0</sup> and G<sup>00</sup> decrease relative to the baseline polymers.

Figure 1. Oscillatory tests for β-CD/polymer blends at different β-CD concentrations at 0.5 wt% polymer solution in 2.1 wt% brine.

### 2.2. Effect of surfactant addition

UK) equipped with parallel-plate measuring geometry (gap between the plates of 1000 μm) and solvent trap to avoid evaporation and/or drying effects. First, amplitude sweeps were run to determine the limit of the linear viscoelastic (LVE) range of the samples at 25�C; followed by frequency sweeps to establish the time-dependent deformation behavior [22].

Figure 1 displays the results of the oscillatory tests for β-CD/polymer blends at different β-CD concentrations and fixed concentration of polymer (0.5 wt%). Figure 1(a–c) corresponds to formulations using polymers AP1, AP2, and AP3, respectively. Figure 1(a–c) indicates that the addition of different concentrations of β-CD did not improve the frequency-dependent function of the baseline polymers; on the contrary, the addition of β-CD makes these polymers more inflexible and rigid. In all cases, tanδ increases, while G<sup>0</sup> and G<sup>00</sup> decrease relative to the

Figure 1. Oscillatory tests for β-CD/polymer blends at different β-CD concentrations at 0.5 wt% polymer solution in

), and |η\*| were plotted as a function of the angular frequency (ω) in

G0

, G00, tanδ (G00/G<sup>0</sup>

204 Cyclodextrin - A Versatile Ingredient

baseline polymers.

2.1 wt% brine.

logarithmic scales on both axis.

2.1. Effect of β-CD addition

Figure 2(a–c) demonstrates the effect of the addition of surfactant on the viscoelastic properties of polymers AP1, AP2, and AP3. These plots reveal interactions (noncovalent associations) among the surfactant and polymers AP1, AP2, and AP3. The addition of surfactant increases the elasticity (tanδ decreases and G<sup>0</sup> increases) and viscosity of the samples (G<sup>00</sup> and |η\*| increase) relative to the respective baseline polymers. However, these plots also indicate that there is no a clear relationship between surfactant concentration and the improvement of the viscoelastic properties. For instance, the surfactant concentrations that render the best viscoelastic properties for surfactant-AP1 ranged from 30 to 70 ppm; for surfactant-AP2 was 70 ppm, and for surfactant-AP3 ranged from 30 to 90 ppm.

## 2.3. Effect of the simultaneous addition of surfactant and β-CD

Figure 3(a–c) demonstrates that the simultaneous addition of surfactant and β-CD produces strong noncovalent interactions and robust self-assembling. Self-aggregation significantly increases the viscoelastic properties of the different systems, specifically for the SAP-AP systems formulated using polymers AP2 (Figure 3(b)) and AP3 (Figure 3(c)). Furthermore, the

Figure 2. Oscillatory tests for surfactant/AP blends at different surfactant concentrations at 0.5 wt% polymer solution in 2.1 wt% brine.

The remarkable gain of viscoelastic properties achieved by these SAP-AP systems is attributed to self-association through β-CD host-guest interactions and other noncovalent interactions (i.e. hydrogen bonding and hydrophobic interactions, among others). These observations agree with previous research in which self-aggregation through intermolecular noncovalent associa-

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In this work, only 70 ppm (0.007 wt%) of surfactant and 70 ppm (0.007 wt%) of β-CD were added to the AP polymers dissolved in brine. β-CD rapidly forms inclusion complexes with various hydrophobic guest moieties and polymeric chains [7]. The hydrophobic pendant groups from the associating polymers and the hydrophobic tails of the surfactant are typical guest moieties that can be spontaneously included into the β-CD cavity through both faces: the primary and secondary faces. While the hydrophilic end (i.e. polar part) of the anionic surfactant can interact with the hydrophilic pendant groups (i.e. amide groups contained in the associating polymers) through hydrogen bonding (i.e. H-bridges) or with the carboxylate pendant groups of the polymer backbone via electrostatic interactions through divalent

Figure 4 displays a hypothetical network structure formed through self-assembly. This 3D supramolecular structure is responsive and reversible because these physical bonds are not rigid [22]. Furthermore, the resulting high-order macromolecule displays increased hydrophilicity

Figure 4. Hypothetical network structure via self-assembly among β-CD, associating polymer, and anionic surfactant in

tions increased solution viscosity [7].

brine solution.

cation-bridges (i.e. Ca2+ or Mg2+) present in the brine.

Figure 3. Oscillatory tests for surfactant/β-CD/AP blends at different surfactant/β-CD concentrations at 0.5 wt% polymer solution in 2.1 wt% brine.

experimental results show a clear and consistent relationship between surfactant/β-CD concentration and the improved viscoelastic properties of the SAP-AP systems. The selfassembling systems displaying the most enhanced viscoelasticity were obtained at a surfactant concentration of 70 ppm and β-CD concentration of 70 ppm (Figure 3(a–c)). The viscoelastic properties achieved by the SAP-AP systems at a surfactant/β-CD concentration of 70 ppm overlapped the viscoelastic performance corresponding to surfactant/β-CD concentrations of 90 and 110 ppm; therefore, a surfactant/β-CD concentration of 70 ppm was selected as the optimum concentration from the technical and cost-effective standpoint.

All the self-assembling systems display a decrease of the loss factor (tanδ), which indicates that these SAP-AP systems are more elastic (i.e. improved reversible deformation behavior) relative to the baseline AP polymers. This observation agrees with the significant increase of G<sup>0</sup> shown by the SAP-AP systems, especially for the SAP-AP2 with a percentage increase of G<sup>0</sup> = 310% and the SAP-AP3 with a percentage increase of G<sup>0</sup> = 220%. Likewise, the loss modulus (G00) and the complex viscosity (|η\*|) increased significantly. For instance, the SAP-AP2 shows a percentage increase of G<sup>00</sup> = 61% and of |η\*| = 253%; while the SAP-AP3 displays a percentage increase of G = 694% and of |η\*| = 414%.

The remarkable gain of viscoelastic properties achieved by these SAP-AP systems is attributed to self-association through β-CD host-guest interactions and other noncovalent interactions (i.e. hydrogen bonding and hydrophobic interactions, among others). These observations agree with previous research in which self-aggregation through intermolecular noncovalent associations increased solution viscosity [7].

In this work, only 70 ppm (0.007 wt%) of surfactant and 70 ppm (0.007 wt%) of β-CD were added to the AP polymers dissolved in brine. β-CD rapidly forms inclusion complexes with various hydrophobic guest moieties and polymeric chains [7]. The hydrophobic pendant groups from the associating polymers and the hydrophobic tails of the surfactant are typical guest moieties that can be spontaneously included into the β-CD cavity through both faces: the primary and secondary faces. While the hydrophilic end (i.e. polar part) of the anionic surfactant can interact with the hydrophilic pendant groups (i.e. amide groups contained in the associating polymers) through hydrogen bonding (i.e. H-bridges) or with the carboxylate pendant groups of the polymer backbone via electrostatic interactions through divalent cation-bridges (i.e. Ca2+ or Mg2+) present in the brine.

Figure 4 displays a hypothetical network structure formed through self-assembly. This 3D supramolecular structure is responsive and reversible because these physical bonds are not rigid [22]. Furthermore, the resulting high-order macromolecule displays increased hydrophilicity

experimental results show a clear and consistent relationship between surfactant/β-CD concentration and the improved viscoelastic properties of the SAP-AP systems. The selfassembling systems displaying the most enhanced viscoelasticity were obtained at a surfactant concentration of 70 ppm and β-CD concentration of 70 ppm (Figure 3(a–c)). The viscoelastic properties achieved by the SAP-AP systems at a surfactant/β-CD concentration of 70 ppm overlapped the viscoelastic performance corresponding to surfactant/β-CD concentrations of 90 and 110 ppm; therefore, a surfactant/β-CD concentration of 70 ppm was selected as the

Figure 3. Oscillatory tests for surfactant/β-CD/AP blends at different surfactant/β-CD concentrations at 0.5 wt% polymer

All the self-assembling systems display a decrease of the loss factor (tanδ), which indicates that these SAP-AP systems are more elastic (i.e. improved reversible deformation behavior) relative to the baseline AP polymers. This observation agrees with the significant increase of G<sup>0</sup> shown by the SAP-AP systems, especially for the SAP-AP2 with a percentage increase of G<sup>0</sup> = 310% and the SAP-AP3 with a percentage increase of G<sup>0</sup> = 220%. Likewise, the loss modulus (G00) and the complex viscosity (|η\*|) increased significantly. For instance, the SAP-AP2 shows a percentage increase of G<sup>00</sup> = 61% and of |η\*| = 253%; while the SAP-AP3 displays a percentage

optimum concentration from the technical and cost-effective standpoint.

increase of G = 694% and of |η\*| = 414%.

solution in 2.1 wt% brine.

206 Cyclodextrin - A Versatile Ingredient

Figure 4. Hypothetical network structure via self-assembly among β-CD, associating polymer, and anionic surfactant in brine solution.

because the molecular locations containing the β-CD complexations become more hydrophilic in nature [7].
