**6. Extended thermal stability**

**5. Dynamic mechanical thermo-analysis (DMTA)**

baseline and SAP network at low- and high salinity.

106 Polymer Rheology

Dynamic mechanical thermo-analysis was conducted by performing upwards temperature ramps from 282.5 (9°C) to 353.5 K (80°C) at a linear heating rate of 9 K/min and downwards temperature ramps from 353.5 (80°C) to 282.5 K (9°C) at a linear cooling rate of 7 K/min at a constant frequency (1 Hz) and % strain (20%) within the LVE range. **Figure 9** displays the oscillatory upward and downward temperature sweeps showing the *G*'- and *G*"-curves of the

The upward heating curves (**Figure 9**) show that for both systems (i.e., baseline and SAP network), the *G*'- and *G*"-values decrease as the temperature increases with the *G*'-values showing steeper curves. Similarly, the downward cooling curves show the reverse process; however, the *G*'- and *G*"-curves show hysteresis upon cooling. As temperature increases during the heating process, the motion and friction between the polymer chains increase, producing frictional heat. A fraction of this frictional heat may heat up the sample and another part may be lost to the surrounding environment; therefore, these materials exhibit irreversible deformation behavior [45] due to heat transfer. **Figure 9** also shows that hysteresis is less pronounced for both systems at higher salinities, which suggests the formation of supramolecular networks that are more rigid, stronger, and more stable at elevated temperatures.

**Figure 10** shows that the baseline system reaches the gel transition temperature (*T*gel) at approximately 334 K (60.5°C) for both brine salinities: 2.1 and 8.4 wt%. As temperature increases, the interchain interactions undergo faster rates of dissociation-association up to the vicinity of the gel transition temperature at which the rate of dissociation is much higher than the rate of association. At *T*gel, disruption of the network takes place and the system transitions from a gellike flow behavior to a fluid-like flow behavior [9, 58]. Likewise, during the cooling process as temperature decreases, the rate of interchain dissociation-association decreases and chain interactions get progressively stronger and the material becomes more elastic until *G*' = *G*" at the crossover point, the flow gel-like behavior is regained and *G*' > *G*" at lower temperatures [45]. The SAP system displays a different behavior as shown in **Figure 10**. In both brine salinities (2.1 and 8.4 wt%), the heating curves do not reach the gel transition temperatures. At low salinity, the cooling curve just reaches the crossover point (G' = G" and tan*δ* = 1) at the highest testing temperature and then rapidly transitions to a viscoelastic gel behavior upon cooling. The SAP cooling curve at a high salinity never reaches the transition temperature (*G*' = *G*"). These observations suggest that the SAP network exhibits more rigid and stronger polymer

**Figure 9.** Temperature-dependent functions of *G*' and *G*" of baseline and SAP network.

The extended thermal stability evaluation was performed at 90°C for a period of 8 weeks. **Figure 11** displays the G'- and G"-curves versus angular frequency and time of the baseline and SAP system at low- and high-salinity brines.

The *G*'- and *G*"-curves (**Figure 11**) indicate that after the second week (Week # 2) of testing, the elasticity and viscosity of the baseline and the SAP network are significantly affected. At low-salinity brine (2.1 wt%), the baseline displays a more stable performance than the SAP network during the testing period; however, this behavior is reversed at high-salinity brine concentration (8.4 wt%), in which the SAP system is noticeably more stable, particularly at week # 8, than the baseline system. Furthermore, the direct observation of the samples reveals the onset of fine solid precipitation from week # 2 for both systems and brine concentrations. Some of the samples became turbid with testing time.

The thermal deterioration of the viscoelasticity of both systems as a function of time could be attributed in part to the autohydrolysis of the amide moieties (i.e., acrylamide groups) and hydrophobic functional groups forming additional acrylate moieties at elevated temperatures (i.e., >85°C). These acrylate structures rapidly associate with divalent cations (i.e., Ca2+) causing the precipitation of polymer from the bulk of the solution, which becomes turbid. More detailed information on autohydrolysis of polymers at elevated temperature and ionic strength is provided elsewhere [43, 59]. Thermal degradation of the polymer samples can also be attributed to the induced breaking of the acrylic backbone trigger via free-radical reactions, which reduces the molecular weight of the polymer chains [59].

**Figure 11.** *G*'- and *G*"-curves as a function of angular frequency and time.

Although this extended thermal stability evaluation demonstrates the significant effect of temperature and time on the viscoelastic **flow** behavior of both systems, these results further confirm that the functionality of the SAP system is increased at higher ionic strengths due to the formation of stronger intra- and interpolymer associations that enhances its stability at 90°C.
