5.1. Short-term thermal stability

The AP polymers and the corresponding optimum SAP-AP systems were subjected to dynamicmechanical thermo-analysis or DMTA. In this analysis, the dynamic temperature sweep was conducted by using a linear heating rate (8.65�C/min) from 9 to 81�C (�0.2�C) as an upward ramp immediately followed by a linear cooling rate (7.3�C/min) from 81 to 9�C (�0.2�C) as a downward ramp. In these tests, the angular frequency (ω) was fixed at 7 rad/s, while the strain was fixed at 20% (within the LVE range). Figure 11 displays the temperature-dependent functions of G<sup>0</sup> , G00, and tanδ for polymer AP1 and the SAP-AP1 system for low (2.1 wt%)- and high (8.4 wt%)-concentration salinity; while Figure 12 presents the temperature-dependent functions of G<sup>0</sup> , G00, and tanδ for polymers AP2 and AP3 and their corresponding SAP-AP systems in 8.4 wt % brine concentration.

At low-ionic strength (i.e. 2.1 wt% brine), the SAP-AP1 system shows higher structural strength in terms of G<sup>0</sup> and G<sup>00</sup> relative to polymer AP1. The SAP-AP1 system displays a tanδ < 1 (i.e. G<sup>0</sup> > G00) in the entire range of temperature that is consistent with the behavior of network structures. At low temperatures, the tanδ-curve of the AP1 polymer shows tanδ < 1, as temperature increases the crossover point (G<sup>0</sup> = G00) is reached and the tanδ-curve is shifted to a value of tanδ > 1 at higher temperatures, which corresponds to the behavior of viscoelastic liquids. The cooling curves of tanδ, G<sup>0</sup> , and G<sup>00</sup> as a function of temperature demonstrate a decreasing structural strength of the polymer systems relative to the corresponding heating curves. According to Mezger, as temperature increases, "the [polymer] molecules are able to move along one another which results in an increasing number of disentanglements. With increasing temperature, more motion occurs between the molecules which again cause an increase in the amount of the frictional forces [that produces] frictional heat that afterward is lost for the sample in the form of thermal energy … This process can be observed as a decreasing G<sup>00</sup> value" [22]. The cooling temperature curve displays indeed lower G00-values. Likewise, for the SAP-AP1 system, with increasing temperature, dissociation and/or disassembling of the

physical interactions takes place. Therefore, "the superstructure [network] yields more and more, breaking increasingly into smaller parts, turning into a state of flexibility. Later, during the cooling process, the physical bonds are reformed, and the network structure regains

, G00, and tanδ for polymer AP1 and SAP-AP1 system.

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, G00, and tanδ for polymers AP2 and AP3 and systems SAP-AP2 and

Figure 11. Temperature-dependent functions of G<sup>0</sup>

Figure 12. Temperature-dependent functions of G<sup>0</sup>

SAP-AP3.

Figure 11. Temperature-dependent functions of G<sup>0</sup> , G00, and tanδ for polymer AP1 and SAP-AP1 system.

based on the very fast connection, disconnection, and re-connection" of the physical bonds such as the decomplexation and complexation of host-guest interactions [7, 22], that results in improved structural strength. "Therefore, when tension is imparted on these networks, the force is distributed homogeneously across the whole network [due to the dynamic disassembling and re-assembling of the physical interactions]" protecting the macromolecules

Thermal degradation of polymer leads to chemical changes of the polymer structure. For instance, in the case of polymers derived from polyacrylamides, high temperatures induce the hydrolyzation of the acrylamide group to the acrylate moiety. These hydrolysis reactions are strongly correlated to temperature; thus, the higher the temperature, the higher the hydrolysis [2]. In this study, both short- and long-term thermal stabilities of the AP polymers and the

The AP polymers and the corresponding optimum SAP-AP systems were subjected to dynamicmechanical thermo-analysis or DMTA. In this analysis, the dynamic temperature sweep was conducted by using a linear heating rate (8.65�C/min) from 9 to 81�C (�0.2�C) as an upward ramp immediately followed by a linear cooling rate (7.3�C/min) from 81 to 9�C (�0.2�C) as a downward ramp. In these tests, the angular frequency (ω) was fixed at 7 rad/s, while the strain was fixed at 20% (within the LVE range). Figure 11 displays the temperature-dependent func-

(8.4 wt%)-concentration salinity; while Figure 12 presents the temperature-dependent functions

At low-ionic strength (i.e. 2.1 wt% brine), the SAP-AP1 system shows higher structural strength in terms of G<sup>0</sup> and G<sup>00</sup> relative to polymer AP1. The SAP-AP1 system displays a tanδ < 1 (i.e. G<sup>0</sup> > G00) in the entire range of temperature that is consistent with the behavior of network structures. At low temperatures, the tanδ-curve of the AP1 polymer shows tanδ < 1, as temperature increases the crossover point (G<sup>0</sup> = G00) is reached and the tanδ-curve is shifted to a value of tanδ > 1 at higher temperatures, which corresponds to the behavior of viscoelastic liquids.

structural strength of the polymer systems relative to the corresponding heating curves. According to Mezger, as temperature increases, "the [polymer] molecules are able to move along one another which results in an increasing number of disentanglements. With increasing temperature, more motion occurs between the molecules which again cause an increase in the amount of the frictional forces [that produces] frictional heat that afterward is lost for the sample in the form of thermal energy … This process can be observed as a decreasing G<sup>00</sup> value" [22]. The cooling temperature curve displays indeed lower G00-values. Likewise, for the SAP-AP1 system, with increasing temperature, dissociation and/or disassembling of the

, G00, and tanδ for polymers AP2 and AP3 and their corresponding SAP-AP systems in 8.4 wt

, G00, and tanδ for polymer AP1 and the SAP-AP1 system for low (2.1 wt%)- and high

, and G<sup>00</sup> as a function of temperature demonstrate a decreasing

in the network from permanent shear degradation [7].

5. Thermal stability of the SAP-AP systems

corresponding SAP-AP systems were evaluated.

5.1. Short-term thermal stability

216 Cyclodextrin - A Versatile Ingredient

tions of G<sup>0</sup>

% brine concentration.

The cooling curves of tanδ, G<sup>0</sup>

of G<sup>0</sup>

Figure 12. Temperature-dependent functions of G<sup>0</sup> , G00, and tanδ for polymers AP2 and AP3 and systems SAP-AP2 and SAP-AP3.

physical interactions takes place. Therefore, "the superstructure [network] yields more and more, breaking increasingly into smaller parts, turning into a state of flexibility. Later, during the cooling process, the physical bonds are reformed, and the network structure regains rigidity. However, due to losses of frictional heat in the form of thermal energy, a decrease of the G00-values is observed in the cooling temperature curve" [22].

Increasing the ionic strength (brine 8.4 wt%) and temperature significantly affects the structural strength of polymer AP1 and the SAP-AP1 system (see Figure 11). Similarly, the downward curve (i.e. cooling process) shows a decreasing structural strength relative to the corresponding heating curves. The SAP-AP1 temperature curve shows a higher structural strength in terms of G<sup>0</sup> -values compared to the AP1 polymer; however, in terms of G00-values, there is no difference between the AP1 polymer and the SAP-AP1 system. Furthermore, the tanδ-curve of the cooling process for the SAP-AP1 system shows a shift from tanδ-values > 1 at elevated temperatures to tanδ-values < 1 at lower temperatures. This makes evident that at elevated temperatures, the SAP-AP1 network system changes its flow behavior, showing G<sup>00</sup> > G<sup>0</sup> , performing as a viscoelastic liquid. As temperature decreases, the network becomes more rigid and the tanδ-values become < 1, showing again the consistency of a rigid supramolecular structure.

At elevated ionic strength (i.e. 8.4 wt% brine), the baseline polymers AP2 and AP3 and their corresponding SAP-AP systems show similar behavior to the baseline AP1 and SAP-AP1, discussed above. Nevertheless, it is important to mention that the SAP-AP2 system shows the highest structural strength in terms of G<sup>0</sup> and G<sup>00</sup> and the least hysteresis between the heating and the cooling curves among the systems. These observations make evident that the optimum SAP-AP2 displays enhanced structural strength at elevated temperatures and ionic strengths compared to the baseline.

These observations clearly indicate the combined damaging effect of salinity and elevated tem-

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219

Although the introduction of hydrophobic groups into the molecular chain is an effective way to improve polymer salt tolerance and thermal stability [3], these hydrophobic moieties such as AMPS or n-VP are still susceptible to hydrolysis at high temperatures (i.e. > 85�C). As explained by Levitt and Pope, "…hydrolysis of the [amide] group from the acrylamide moiety and/or β, β, dimethyl taurine from the AMPS moiety [takes place] forming additional acrylate moiety in the hydrolysed polymer molecule" [6]. At high salinity and hardness concentration, these acrylate moieties (i.e. hydrolyzed poly(AM-co-AMPS)) associate strongly with cations (i.e. Ca2+, M2+) and polymer precipitation takes place, which results in a rapid drop of viscosity of the polymer solution that becomes turbid [6, 31]. Precipitation of polymer due to interactions with multivalent cations increases with temperature. Furthermore, thermal degradation of polymers results in a "reduction of molecular weight because of free-radical induced scis-

This long-term thermal stability testing demonstrates that the AP and the SAP-AP systems are not stable at elevate temperatures; therefore, these SAP-AP systems are recommended for low-

In summary, we formulated advanced polymer-surfactant systems via self-assembling driven by host-guest chemistry and other physical noncovalent bonding by mixing associating

perature on the stability of the AP and SAP-AP systems.


sion of the acrylic backbone" [6].

temperature (< 90�C) applications.

6. Conclusions

Figure 13. G<sup>0</sup>

#### 5.2. Long-term thermal stability

The AP and SAP-AP systems were subjected to a thermal stability test at 90�C for a period of 8 weeks. The presence of dissolved oxygen at high temperatures might induce the formation of free radicals which degrade the polymer molecule by cleavage reducing its molecular weight and viscosifying functionality [2]. Besides, if dissolved oxygen is present in the polymer solutions together with very low concentrations of dissolved iron, it might also cause substantial polymer degradation [4, 26–30]. Therefore, to prevent chemical degradation, the AP and the SAP-AP samples were placed in a glove chamber and bubbled with nitrogen at a pressure ranging from 10 to 20 psi for a period of 30 min. Two duplicated set of samples of the AP and the SAP-AP systems were prepared in two different brine salinity concentrations: 2.1 and 8.4 wt% and placed in the oven at 90 � 0.5�C for 8 weeks. Every 2 weeks, a set of samples were taken out of the oven and subjected to rheological analysis. Figure 13 displays the G<sup>0</sup> - and G00-curves as a function of angular frequency and time for the AP and SAP-AP systems at brine salinity concentrations of 2.1 and 8.4 wt%.

The G<sup>0</sup> - and G00-plots in Figure 13 demonstrate that the AP polymers and the SAP-AP systems are significantly degraded at a temperature of 90�C after 2 weeks of testing. A dramatic drop of G<sup>0</sup> and G<sup>00</sup> is observed for all the samples in both low and high salinities.

In low-salinity brine, samples AP1, AP2, SAP-AP1, and SAP-AP2 show precipitation of solids and color change at week # 4; while in samples AP3 and SAP-AP3, the precipitation of solids was observed at week # 8. In high-salinity brine, precipitation of solids took place faster at week # 2.

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Figure 13. G<sup>0</sup> - and G00-curves vs. angular frequency, time, and brine salinity.

These observations clearly indicate the combined damaging effect of salinity and elevated temperature on the stability of the AP and SAP-AP systems.

Although the introduction of hydrophobic groups into the molecular chain is an effective way to improve polymer salt tolerance and thermal stability [3], these hydrophobic moieties such as AMPS or n-VP are still susceptible to hydrolysis at high temperatures (i.e. > 85�C). As explained by Levitt and Pope, "…hydrolysis of the [amide] group from the acrylamide moiety and/or β, β, dimethyl taurine from the AMPS moiety [takes place] forming additional acrylate moiety in the hydrolysed polymer molecule" [6]. At high salinity and hardness concentration, these acrylate moieties (i.e. hydrolyzed poly(AM-co-AMPS)) associate strongly with cations (i.e. Ca2+, M2+) and polymer precipitation takes place, which results in a rapid drop of viscosity of the polymer solution that becomes turbid [6, 31]. Precipitation of polymer due to interactions with multivalent cations increases with temperature. Furthermore, thermal degradation of polymers results in a "reduction of molecular weight because of free-radical induced scission of the acrylic backbone" [6].

This long-term thermal stability testing demonstrates that the AP and the SAP-AP systems are not stable at elevate temperatures; therefore, these SAP-AP systems are recommended for lowtemperature (< 90�C) applications.
