3.1 Relationship between the HLB value and emulsion stability
The key to reverse emulsion polymerization is the formation of microscale monomer droplets. Microdroplet formation requires surfactants with good surface activities and low interfacial tensions. Surfactants should have strong adsorption and a suitable HLB value, which is an important indicator of the emulsification performance [19]. The overall HLB value of the system was obtained using Eq. (1), as shown in Fig. 1.
\({\text{H}\text{L}\text{B}}_{\text{A}\text{B}}=\frac{{\text{H}\text{L}\text{B}}_{\text{A}}\times {W}_{\text{A}}+{\text{H}\text{L}\text{B}}_{\text{B}}\times {W}_{\text{B}}}{{W}_{\text{A}}+{W}_{\text{B}}}\) (1),
where WA and WB are the weight percentages of surfactants A and B, respectively.
S-80/T-80, isoamyl caprylate, and aqueous AM were used as the emulsifier, external phase, and internal phase, respectively. Microscale droplets were formed under the high-shear action of the homogenizer. In the continuous water phase, smaller droplets exhibited an interfacial area loss greater than that observed for larger droplets, and the decrease in the surface energy was the driving force for the degradation of smaller droplets. Therefore, determining the appropriate HLB value can aid in enhancing the stability of the emulsion interface, suppressing the diffusion of monomers from sub-micron-sized droplets, and achieving the dominant position of droplet nucleation.
In a reverse emulsion system, ultra-low interfacial tension is favorable for emulsification. The interfacial tension was tested at different HLB values, as shown in Fig. 2. When the HLB value was between 5.3–6.5, isoamyl caprylate and water exhibited ultra-low interfacial tension. Ultra-low interfacial tension indicates strong interfacial adsorption, and the arrangement of the emulsifying agent molecules at the oil–water interface is highly compact. The density of the adsorption layer is also higher, resulting in a tighter molecular arrangement. This phenomenon prevented droplet coalescence and improved the stability of the formed emulsions.
The adsorption of emulsifiers on the interface is closely related to the stability of the emulsions. While testing the interfacial tension, the oil droplets formed by isoamyl caprylate spontaneously migrated from the wall into the solution when a 2.2% aqueous emulsifier solution was used (Fig. 3). Therefore, the emulsifier solution exhibited a strong adsorption capacity for isoamyl caprylate.
The overall HLB of the system was calculated according to Eq. (1). The stabilities of the emulsions under different HLB conditions were tested, and the results are shown in Fig. 4b. The monomers diffused from the inside to the outside of the droplets owing to Brownian motion. As the droplet size gradually increased, according to Stokes’ law, the emulsion monomers underwent stratification, which can occur within a few seconds to a few minutes. Isoamyl caprylate is insoluble in the aqueous phase and cannot be separated, and the emulsifier increases the interfacial free energy. When the HLB value reached 5.72, the emulsion was maintained without stratification for 10 h or longer. The state of the emulsion was observed using an electron microscope, as shown in Fig. 4a. After high-shear emulsification, the emulsions formed oil-in-water droplets. The emulsifier balanced the reduction in the interfacial area caused by Ostwald ripening and improved the stability of the emulsion.
3.2 Impact of temperature on emulsion stability
An emulsion is a thermodynamically unstable system in which droplets tend to coalesce spontaneously. The polymerization of AM involves the conversion of π to σ bonds, which is highly exothermic. Therefore, studying the high-temperature stability of these emulsions is necessary. The time taken for 1 mL of the aqueous phase to separate from 10 mL of the emulsion was measured at varying polymerization temperatures to assess stability, as shown in Fig. 5. As the temperature increased, the molecules exhibited Brownian motion and were stimulated by the monomer solution, resulting in a loose interface film and decreased mechanical strength. This resulted in instability and separation of the emulsion. When the HLB value was 5.72, the emulsion exhibited high thermodynamic stability. At temperatures below 35 ℃, oil–water separation is slow. Therefore, the S-80/T-80 ratio with an HLB value of 5.72 was chosen as the emulsifier system for the reaction.
3.3 Emulsion particle size test
Reverse emulsion polymerization occurs within the microdroplets. The formation of microdroplets requires external forces to pre-emulsify the two immiscible phases. Following emulsification, isoamyl caprylate was encapsulated in an aqueous solution of the polymerized monomers through the action of emulsifiers, providing a “reaction warehouse” for polymerization. Figure 6a shows the sizes of the pre-emulsified droplets with an average diameter of 12.91 µm. Because several AM monomers remained outside the droplets, the droplet size distribution was relatively scattered. By the time polymerization was complete, all the free monomers had entered the “reaction warehouse” and been uniformly polymerized inside the droplets to form polymer particles. The volume decreased following polymerization, and the average particle diameter was 5.837 µm, as shown in Fig. 6b. The resulting structure was more uniform, exhibiting a normal particle size distribution.
3.4 Emulsion polymerization mechanism
The polymerization mechanism of isoamyl caprylate as a continuous phase in Pickering emulsions can be divided into three stages: the initiation, steady, and termination stages, as shown in Fig. 7. During initiation, the initiator undergoes a redox reaction and the pre-polymer free radicals are continuously transferred to the solubilized micelles. This transfer is driven by the equilibrium swelling of the latex particles, initiation of monomers, and nucleation of polymerization, resulting in an increased polymerization rate. When all the free monomers have entered the monomer droplets, the polymerization rate becomes relatively constant, and the reaction enters the steady stage. At this stage, only the latex particles and liquid droplets are present in the system. This is the ideal period for AM polymerization, during which the reaction is highly exothermic. The decomposition of AIBN was stimulated by heating, and the resulting free radicals initiated a hierarchical reaction mechanism. Hence, the accumulation of numerous free radicals was prevented. However, polymerization termination resulted in a low-molecular-weight polymer. Sodium formate was added as a chain-transfer agent to render the polymer structure regular. The steady stage lasted for a long time. Finally, the reaction entered the termination stage, in which the residual monomers continued to react. Finally, the polymerization rate decreased significantly, eventually reaching zero when all the active monomers were converted into polymers.
3.5 Temperature changes during the polymerization process
Emulsion polymerization is a complex process. Smith and Ewart conducted quantitative studies on emulsion polymers and formulated an emulsion theory, which can be encapsulated by the following equation [20].
\({R}_{\text{p}}={k}_{\text{p}}[M{]}_{\text{p}}{N}_{\text{p}}{\stackrel{-}{n}/N}_{\text{A}}\) (2),
where \({R}_{\text{p}}\)is the reaction rate for the polymerization reaction; \({k}_{\text{p}}\)is the rate constant for polymerization, which represents the rate constant of the reaction; \([M{]}_{\text{p}}\) represents the monomer concentration, which indicates the initial concentration of monomers in the polymerization reaction; \(\stackrel{-}{n}\)is the reaction order, indicating the extent of the direct polymerization reaction; \({N}_{\text{p}}\) represents the number of polymer chain segments, which is the number of monomers in the polymer; and \({N}_{\text{A}}\) is Avogadro’s constant.
An NTC thermistor was used to record the temperature changes during the RSE polymerization reaction, as shown in Fig. 8a. The entire reaction was highly exothermic. To prevent the dispersion system from being destroyed at high temperatures, which can result in a runaway polymerization process, the reaction must be performed in a cold-water bath at 0–5 ℃ to control the temperature within the range of 38–45 ℃. Because different functional initiators were used at different stages of the reaction, the polymerization process required 4 h to produce the high-molecular-weight RSE polymer.
The nucleation efficiency (a dimensionless particle number) is defined as the ratio of the number of latex particles to the number of droplets in the emulsion. Steady-state nucleation particle formation occurs as a function of the reaction time and temperature. The nucleation efficiency during the emulsion reaction was calculated using Eq. (2), as shown in Fig. 8b. Particle generation was limited by the concentration of the monomer droplets in the emulsion. The active free radicals generated via oxidation-reduction reactions were captured by the droplets. Therefore, as the temperature increased, the emulsion gradually completed the nucleation reaction.
3.6 RSE average molecular weight testing
Employing 1 mol/L sodium chloride solution as the reference liquid, the flow time of the RSE polymer solution was measured using a non-dilution capillary viscometer. The characteristic viscosity [η] of the polymer was calculated using Eq. (3), and the molecular weight of the polymer was obtained using Eq. (4). The results are displayed in Table 1. The molecular weight of the continuous phase for synthesizing the polymer in isoamyl caprylate was 18 million. In contrast, it was 16 million in white oil.
Table 1
Molecular weight determination of polymers.
Continuous phase | Continuous phase molecular weight | Flow time (s) for RSE solution(s) | Flow time (s) for a 1 mol/L NaCl solution | Effective RSE concentration c (mg/L) | Relative viscosity ηr | Viscosity index [η] cm3/g | Mν |
Isoamyl caprylate | 214.3 | 342 | 124 | 507 | 2.76 | 2329 | 18.4×106 |
White oil | 250–450 | 384 | 124 | 624 | 3.1 | 2148 | 16.3×106 |
\(\left[\eta \right]=\frac{\sqrt{\frac{2-\text{l}\text{n}3}{{\eta }_{\text{r}}-\text{l}\text{n}{\eta }_{\text{r}}-1}}({\eta }_{\text{r}}-1)-2\sqrt{\frac{{\eta }_{\text{r}}-\text{l}\text{n}{\eta }_{\text{r}}-1}{2-\text{l}\text{n}3}}}{c(\sqrt{\frac{2-\text{l}\text{n}3}{{\eta }_{\text{r}}-\text{l}\text{n}{\eta }_{\text{r}}-1}}-1)}\) (3).
\({M}_{{\nu }}=802[\eta {]}^{1.25}\) (4).
White oil is a mixture of normal alkanes and iso-alkanes, with carbon chain lengths ranging from C16 to C31. When white oil is used as the continuous phase, the penetration of the initiator into the reaction nucleus results in a higher resistance, quenching active free radicals. In contrast, isoamyl caprylate has a simpler structure and affords more uniform oil-in-water droplets than white oil. Isoamyl caprylate has a low molecular weight and carbonyl groups that serve as diffusion channels for free radicals, enhancing their penetration through the droplet membrane. This enables the active free radicals to rapidly enter the “reactor,” which improves the efficiency of free-radical transfer, resulting in high-molecular-weight polymers.
By prolonging the shear time through pre-emulsification, the emulsion viscosity of the system increased, and the average particle size and polymer molecular weight of the emulsion gradually decreased, as shown in Fig. 9. From a kinetic perspective, each oil-in-water droplet acts as a small, independent reactor. Thus, as the particle size decreased, the interfacial area of the droplets increased, and a minimal amount of material exchange occurred between the droplets. Simultaneously, the high viscosity of the mixed system hindered the release of the polymerization heat. The heat energy promoted the opening of the π bond in the AM monomer, and excessive active radicals resulted in bimolecular termination, thereby reducing the molecular weight of the polymer. Conversely, when the emulsion viscosity was low, and the concentration of the encapsulated monomers in the droplets was high, high-molecular-weight polymers were obtained. However, droplet aggregation and runaway polymerization tended to occur during this reaction. Therefore, controlling the emulsion viscosity in the rof 900–100 cp range was beneficial for effective polymerization.
3.7 SEM analysis of the RSE
SEM images illustrating the molecular aggregation state of the polymer, as shown in Fig. 10a, reveal that the RSE polymer chains in the aqueous solution intertwine, forming a complex three-dimensional network structure. This increased the fluid volume and viscosity of the solution. In Fig. 10b, the large molecular chains are intertwined with each other, forming chain entanglements, which enhance the spatial structure stability of the polymer. This renders the molecular chains less prone to open under shear stress, resulting in excellent shear resistance. Additionally, small branches protruded from the main chains, enhancing the interaction between the molecular chains. Scaly fragments were dispersed on the molecular chains; these were the emulsifiers adsorbed on the molecular chains after dissolution and emulsification. This enhanced the strength of the molecular chains and prevented chain rupture.
3.8 Rheological evaluation of the RSE shear performance
At a constant shear rate of 170 s− 1, the temperature-resistant shear properties of RSE were tested at high temperatures, as shown in Fig. 11. The flowability and stability of the polyacrylamide polymer were dependent on the ambient temperature. Under high-temperature conditions, the thermal motion of the RSE molecules intensified, and the thermal motion intensity of the ionic groups increased, reducing molecular aggregation and causing a gradual decrease in viscosity. When the ambient temperature was constant, the viscosity was not dependent on the temperature; the Brownian motion of the molecules maintained a dynamic equilibrium, so the viscosity was stable. In addition, the RSE exhibited high flexibility, a low viscoelastic activation energy, and a low barrier-overcoming ability of the flow units, resulting in high flowability and the ability to withstand high-speed shear, ensuring sustained stability at high temperatures.
After high-temperature shearing and subsequent testing of the RSE molecular microstructure,as shown in Fig. 12a, the molecular chains were found to be reoriented and rearranged owing to the transverse shear imposed during shear flow. Some of the side chains of the molecules were broken during the high-temperature shearing, as indicated by the green color in Fig. 12b. However, the main chains of the molecules remained intact, as depicted by the blue-colored structures in Fig. 12b. The spatial distribution of the molecular aggregates maintained a certain level of structural organization, which waes remained intact, as depicted by thes crucial for retaining residual shear viscosity.
At 30 ℃, the shear rate of the RSE polymer solution was switched between 170, 510, and 1022 s− 1. The results in Fig. 13 indicate that when the shear rate was increased from 170 to 1022 s− 1, the viscosity rapidly decreased. When the shear rate was constant, the viscosity remained unchanged. When the shear rate was changed back to 170 s− 1 after a period of shearing at 1022 s− 1, the viscosity returned to its initial value. These results indicate that the polymer exhibited good shear-recovery performance. Thus, at high shear rates, the RSE network structure was not completely destroyed. When the shear rate decreased, the molecular structure recovered, resulting in an apparent increase in viscosity, i.e., a return to the initial value at the macroscopic level.
The flow behavior of the polymer solutions can be described using the power-law Eq. (5):
\(\text{lg}\tau =\text{lg}\kappa +n\text{lg}\gamma\) (5),
where \(\tau\) represents shear stress, κ represents the consistency coefficient, n represents the power law index, and γ is the shear rate. As shown in Fig. 14, an R2 value of 0.99805 was obtained for n = 0.28839 (which is > 1) with a consistency coefficient of 8.01. Therefore, the RSE polymer solution is a pseudoplastic fluid that exhibits shear-thinning behavior.
3.9 Viscoelasticity analysis of the RSE
Measurements of the viscoelasticity of the RSE were conducted in an oscillatory manner, as shown in Fig. 15a. When the elastic modulus is greater than the viscosity modulus (G′ > G″), the primary contribution to the response originates from the elastic behavior of the molecules. The yield point, at which the elastic modulus started to decrease, was determined to establish the linear viscoelastic region. In this region, the applied stress was insufficient to break the structure of the material. As the strain continued to increase, the elastic modulus began to decrease, indicating entry into the plastic region. In this region, the polymer could not withstand shear stress, and its internal structure was disrupted. As the RSE concentration increased, the viscosity and elasticity increased, forming a supramolecular network structure in the flowing system. This network enhanced the resistance to external shear forces.
The shear stress was determined to be 1 Pa based on strain testing. Frequency scanning was then performed on the RSE in the 0.1–10 Hz range. As shown in Fig. 15b, the elastic and viscosity moduli increased with the frequency. In the low-frequency range, gradual deformation occurred because the relaxation time of the RSE molecular chains was long, and the chains were in a low-energy state. Most of the energy was dissipated through viscous flow, resulting in a low elastic modulus. As the frequency increased, the supramolecular network structure underwent deformation. However, the molecular chains were unable to slide, resulting in energy storage capacity enhancement for the network structure and a gradual increase in the elastic modulus G′.