3.1. Infrared Analysis of KS-3
The absorption band at 3571 cm− 1 in the infrared spectrum of the reactive hydrophobic surfactant KS-3 (Fig. 2) can be assigned to N–H stretching vibration, while that at 3007 cm− 1 is attributed to C = C–H stretching vibration. The bands at 2925 and 2855 cm− 1 correspond to saturated C–H stretching vibration. The bands at 1650, 1548, and 1370 cm− 1 originate from C = O stretching, C = C stretching, and saturated –CH3 bending vibrations, respectively. Peaks associated with the bending vibrations of = C–H are observed at 1001 and 960 cm− 1. The band at 720 cm− 1 can be attributed to long-chain out-of-plane methylene bending vibration.
3.2. NMR Analysis of KS-3
Table 1
1H NMR peaks in the spectrum of KS-3
| Peak number | δ | Atom number |
| 1 | 0.66–0.80 | 3 |
| 2 | 0.99–1.26 | 10 |
| 3 | 1.35–1.48 | 2 |
| 4 | 2.04–2.13 | 2 |
| 5 | 5.51–5.62 | 2 |
| 6 | 1.80–1.93 | 2 |
| 7 | 2.51–2.67 | 2 |
| 8 | 8.05–8.10 | 1 |
| 9 | 3.07–3.22 | 2 |
| 10 | 2.70–2.80 | 2 |
| 11 | 3.45–3.52 | 2 |
| 12 | 3.78–3.86 | 2 |
| 13 | 2.91–2.99 | 3 |
| 14 | 5.81–5.95 | 1 |
| 15 | 2.51–2.67 | 2 |
Figure 3 shows the 1H NMR spectrum of KS-3; the chemical shifts of the various protons are listed in Table 1. The peak at 8.05 ppm corresponds to the amide proton, while the single peak at 3.28 ppm is attributed to the methyl group attached to the N+ species. These results indicate the successful synthesis of KS-3 with the required double-bond quaternary ammonium salt structure.
3.3. Critical Micelle Concentration of KS-3 in Aqueous Solution
The logarithmic plots of the surface tension as a function of concentration have two different slopes on either side of the critical micelle concentration (CMC), a concentration above which surfactants can form micelles in aqueous solutions. KS-3 exhibited a CMC of 0.67 g/L and surface tension of 30.28 mN/m, whereas the CMC and surface tension of CAB were 0.0093 g/L and 29.40 mN/m, respectively (Figs. 4a and b). At concentrations greater than CMCs in aqueous solutions (Figs. 4c and d), the surfactants exhibited a uniform particle size distribution.
To optimize the synergistic effect of the surfactants, the thermodynamic relationship between the chemical potentials and activity coefficients of various components in the surface, micelle, and bulk phases was determined, and Eq. (1) was obtained [21]:
$$\text{ln}({c}_{1}^{0}/{c}_{2}^{0})=-\beta (1- 2X)$$
1
Here, X is the mole fraction of KS-3 in the surface of the surfactant (X1 = Γ1/(Γ1 + Γ2), where Γ denotes the amount of each absorbed component) and \({c}_{1}^{0}\)is the molar concentration of the surface-active agent, KS-3 in, aqueous solution. Coefficient β accounts for the difference between the mixed system and ideal state.
Based on Eq. (1), a synergistic effect between the surfactants is observed when the absolute value of \(\text{ln}({c}_{1}^{0}/{c}_{2}^{0})\) is less than the absolute value of β. As β becomes more negative β, the deviation from the ideal state, strength of the intermolecular interactions, and observed synergistic effect increase (Fig. 5). To achieve the optimal synergistic effect of the two surfactants, the systems with KS-3: CAB mixing ratios of 4:1 and 1:1 were examined.
Upon mixing the two surfactants, the particle size distribution became non-uniform (Fig. 6) owing to their synergistic effect. The diverse particle size distribution upon the addition of the dispersant CAB indicated the formation of non-uniform micellar structures.
3.4. RES Polymerization
Various aggregation models based on the CMCs of KS-3 and CAB, along with the TEM images of the monomer solution, are shown in Fig. 7. KS-3 predominantly formed spherical micelles in the system and was grafted onto the molecular chains via microblock copolymerization. Upon the addition of CAB, the two surfactants exhibited a strong synergistic effect, which caused the spherical KS-3 micelles to disperse and subsequently form wormlike micelles, which were uniformly grafted onto molecular chains, thereby promoting the uniform entanglement of these chains.
3.5. Behavior of KS-3 in Monomer Solutions
TEM images of KS-3 and CAB in the monomer solution (Fig. 8) were obtained to identify the monomer grafting mode of KS-3 in AM solutions during the polymerization process. In the absence of CAB, KS-3 formed homogeneous micelles with uniform particle sizes. Under the stimulation of AM, KS-3 formed spherical aggregates (indicated in blue in Fig. 8) that were further polymerized. The addition of CAB reduced the electrostatic repulsion between the molecules, and the change in equilibrium induced a transition in the molecular aggregate structure from spherical micelles to larger curved aggregates. At a KS-3:CAB molar ratio of 4:1, both spherical and wormlike rod-shaped micelles with smaller sizes were formed. At a KS-3:CAB molar ratio of 1:1, the solution primarily consisted of wormlike and rod-shaped micelles (indicated in red in Fig. 8).
3.6. Temperature Changes During the Polymerization Process
Polymerization typically involves the transformation of π-bonds into σ-bonds induced by an oxidation–reduction process that results in a temperature change during the polymerization reaction. Figure 9 shows the temperature changes observed during RES polymerization. To examine the effect of the micellar structure on the resulting polymers, a KS-3 solution, 4:1 KS-3:CAB solution, and 1:1 KS-3:CAB solution were polymerized to form RES-1, RES-2, and RES-3, respectively. The steric hindrance of the wormlike micelles in RES-3 increased the rigidity of polymer chains, thereby reducing their rotational entropy. Consequently, RES-3 released more heat during the polymerization process than RES-1 and RES-2, exhibited a lower polymerization rate, and theoretically possessed superior thermal stability [22].
3.7. Critical Association Concentration of RES
The apparent viscosities of RES-1, RES-2, and RES-3 changed abruptly as the polymer concentration reached the critical association concentration (CAC), indicated by the inflection point in the corresponding viscosity–concentration curve. At solution concentrations (c) greater than the CAC, intramolecular associations change to intermolecular associations. Therefore, the associations between the KS-3 groups on polymer molecules were the dominant force in the system at higher solution concentrations, resulting in the formation of a spatial network structure with a moderately large hydraulic volume between the molecules and thereby sharply increasing the apparent viscosity. RES-3 forms a stronger intermolecular association network structure than RES-1 owing to the use of a different KS-3 grafting method (Fig. 10). Thus, RES-3 exhibits a higher apparent viscosity and forms fewer intermolecular associations than RES-1 and RES-2 at a given concentration.
3.8. TEM Analysis of RES
.
To evaluate the solubility of the polymers in aqueous solutions, the dissolved RES-1, RES-2, and RES-3 polymers were observed by TEM (Fig. 11). RES-1 formed many small spherical beads in the aqueous solution with spherical micelles of KS-3 grafted onto the polymer. The association between various KS-3 groups enhanced the intermolecular cohesion in the solution. In contrast, RES-2 and RES-3 formed spatial network structures with a hydraulic volume in the aqueous solution. In addition, hydrophobic regions were observed in these solutions (Figs. 11b and c), which confirmed the successful introduction of KS-3 species.
3.9. SEM Analysis of RES
The aggregation states of the RES polymer chains were identified by SEM (Fig. 12). The polymer molecules formed a three-dimensional (3D) pseudo-spatial structure. The strong hydrophobicity of KS-3 enhanced the repulsion between the RES-1 molecules, resulting in limited molecular aggregation and the appearance of cracks in these aggregates (Fig. 12a). Concurrently, the strong cohesive effect of KS-3 was localized within the molecule, producing closed chains in an aqueous solution (Fig. 12b). Combining CAB with KS-3 at a molar ratio of 4:1 caused the cracks in the molecular aggregates to disappear as CAB molecules dispersed a fraction of KS-3 species, lowering their hydrophobicity in water. The localized cohesive effect within the molecule remained strong, causing various protrusions on the surface of the molecular aggregate (Fig. 12c). When the optimal synergy between CAB and KS-3 was achieved, KS-3 formed wormlike micelles and was uniformly grafted onto the main polymer chain, resulting in molecular aggregates with smoother and more even surfaces. In contrast, CAB did not participate in the polymerization reaction and was dispersed and adsorbed by the polymer molecules during the dissolution process, forming chains that protected the molecular aggregates from the effects of shear force and external temperature.
The viscosities of 0.8% RES-1 and RES-2 polymer solutions increased during the initial stages of the shearing process (Fig. 13) as the spherical micelles were grafted onto the polymer molecules, leading to the formation of larger hydrophobic domains and intramolecular aggregation. The initial intramolecular aggregation mechanism was gradually replaced by an intermolecular aggregation mechanism under the actions of temperature and shear force, thereby increasing the fluid mechanical volume and viscosity. The viscoelastic curves of the RES polymers represent a temperature-dependent process in which the system viscosity decreases with increasing temperature. The hydrophobic groups of RES-3 formed strong lateral bonding structures, increasing the rigidity of polymer chains as well as the activation energy and temperature resistance of the system.
3.10. Thixotropic Analysis of RES
The network structures of the studied polymers were characterized by thixotropic studies (Fig. 14). The shear stress gradually increased with the shear rate; however, when the shear rate was gradually reduced, the polymer could not instantly restore its initial network structure, resulting in a lower shear stress during down-shear than during up-shear. All RES polymers exhibited thixotropic loops, indicating the presence of spatial network structures, and the number of laterally bonded network structures increased from RES-1 to RES-3. As the polymer chains became more tightly packed, their deformation was suppressed and more energy was required to break the bonding network structure, thereby increasing the polymer rigidity.
The RES polymers also exhibited good shear-thinning properties (Fig. 14d). The original conformation of the polymer chains changed under an externally applied force, which caused the chains to align in the flow direction and thus reduced the system viscosity. The observed differences between the flow curves reflected the differences in the molecular chain structures and fluid hydrodynamics. In addition, polymers may be fractured in a shear flow field, reducing both the molecular weight and system viscosity.
3.11. Viscoelasticity Analysis of RES
Viscoelasticity describes both the viscous and elastic behaviors of a material during deformation [23]. Polymer chains can deviate from their equilibrium positions while moving with the center of mass; thus, polymer fluids undergo not only permanent deformation but also recoverable elastic deformation during flow. Tensile stress or shear force can align molecular chains along the flow direction. Flexible polymer chains are oriented along the flow direction under the action of external tensile stress, which reduces the conformational entropy of the system. The conformational entropy of the system partially recovers upon removal of the external stress, demonstrating elastic behavior. Thus, the strengths of the intermolecular forces between polymer chains determine the fluid elasticity of the polymer. Consequently, the viscoelastic behavior of polymer fluids is closely related to their molecular conformation, polymer chain flexibility, and strength and distribution of intermolecular forces. Viscoelastic behavior can also be observed in solid polymer materials owing to the shift in the balance between the elastic and viscous responses under the applied external stress or during temperature changes. Viscoelasticity is particularly important in polymer processing, material design, and drug delivery applications.
Strain sweep curves of the three polymers were obtained at various concentrations (Fig. 15). Dilute solutions contained primarily isolated single polymer chains or clusters with no overlap, resulting in a predominantly viscous solution. As the polymer concentration increased, the fluid formed a supramolecular network structure, and the corresponding curve exhibited a linear viscoelastic region.
When the strain exceeded the CAC, the structure of the system was destroyed, and its modulus began to decrease, exhibiting the characteristic shear-thinning behavior. The three polymers exhibited different viscoelasticity changes owing to the different modes of grafting KS-3 onto the main chain under the synergistic effect of CAB. In RES-1, molecular chains of KS-3 underwent local polymerization. At c < cCAC, intramolecular binding was dominant, i.e., G′ < G″, and the molecules were mostly viscous. At c > cCAC, intermolecular binding dominates, i.e., G′ > G″ and the molecules were mostly elastic; however, large hydrophobic clusters were formed between the molecules, resulting in a low shear resistance. In RES-3, KS-3 was uniformly grafted onto the main molecular chain. At low polymer concentrations (c < cCAC), intramolecular bonds were transformed into intermolecular bonds under the action of external stress, thereby increasing the degree of elasticity. When c > cCAC, intermolecular binding was enhanced, forming a roughly uniform entanglement network on chain segments throughout the molecule, thus increasing the shear resistance.
The frequency sweep curves of all polymers (Fig. 16) show that their elastic and viscous moduli increase with increasing frequency. In the low-frequency region, the molecular chains were in a low-energy state, resulting in a long relaxation time and a correspondingly slow deformation. Most of the energy was dissipated via the viscous flow, which reduced the elastic modulus. As the frequency increased, the supramolecular network structure deformed, and the short deformation time prevented slippage. The ability of the network structure to store energy was enhanced continuously, gradually increasing the elastic modulus G’.
The network structure exhibited relaxation characteristics under stress owing to its microstructure. The 3D network formed via the molecular aggregation of RES-3 was more compact and contained more crosslinked points produced through intermolecular binding than RES-1 and therefore generated a stronger elastic response. At low polymer concentrations, the high-frequency region of the viscoelastic curve became chaotic because the 3D network structure of the polymer could no longer store energy, resulting in the destruction of the system structure. As the polymer concentration increased, the overlap, entanglement, and number of crosslinking interactions between the polymer chains increased, which consequently increased the elastic modulus.