Polymerization Behavior and Rheological Properties of a Surfactant-Modified Reactive Hydrophobic Monomer

Abstract

The structures and properties of hydrophobic association polymers can be controlled using micelles. In this work, we synthesize a reactive hydrophobic surfactant monomer, KS-3, from oleic acid, N,N-dimethylpropylenediamine, and allyl chloride. A strong synergistic effect between KS-3 and cocamidopropyl betaine in aqueous solution enhances the hydrophilic dispersibility of KS-3, thereby transforming spherical micelles into cylindrical micelles. KS-3 was grafted onto a polyacrylamide chain via aqueous free-radical polymerization to obtain RES, a hydrophobic association polymer. Structural analysis revealed that the RES polymers assembled in wormlike micelles were more tightly arranged than those assembled in spherical micelles, resulting in a compact network structure in water, smooth surface, and high thermal stability. Rheological tests revealed that the synthesized polymers with wormlike and spherical micelles exhibited shear-thinning properties along with different structural strengths and viscoelasticities. Therefore, controlling the micellar state can effectively regulate the polymer properties. The polymers obtained through wormlike micelle polymerization have potential applications in fields with high demands, such as drug release, water purification, and oilfield development.

1. Introduction

Surfactants are amphiphilic substances that form various types of aggregates, such as micelles, vesicles, and liquid crystals, via self-assembly [1,2]. The micellar structures formed by surfactant molecules in aqueous solutions play an important role in chemical synthesis, because of which surfactants can serve as catalysts and reaction media [3,4]. Because micellar chains are formed via noncovalent interactions between surfactant molecules, they possess excellent self-healing properties and exhibit good response to external triggers in dynamic systems [5,6].

Hydrophobically associated polyacrylamides can be efficiently modified by the addition of functional hydrophobic surfactants during the synthetic procedure [7,8]. Micelles act as polymer templates and dispersants to effectively control the structure and properties of polymer systems, resulting in viscoelastic or gel-like characteristics [9,10,11]. Polymer networks consisting of micellar aggregates and polymers exhibit unique rheological properties ranging from viscoelasticity to Newtonian fluidity owing to several cooperative effects, including electrostatic and hydrophobic interactions and hydrogen bonding [11,12]. Hydrophobically associated polyacrylamides have potential applications in various fields such as drug delivery, nanomaterial synthesis, water treatment, and the petrochemical industry owing to their molecular structures and properties [13,14,15,16,17,18,19]. These micelles can switch their structures according to external stimuli, thus changing their macroscopic properties [20].

So far, few studies have attempted to control the aggregation state of micelles and investigate the effects of the aggregation state on the resulting polymer. In this study, we analyzed the micellar states of a hydrophobic surfactant, KS-3, in aqueous solutions and examined their effect on the synthesis and structure of acrylamide polymers. Spherical KS-3 micelles were effectively transformed into wormlike micelles via the synergistic effect between the surfactant and dispersant, thereby allowing polymer formation to be controlled by the assembly of the hydrophilic and hydrophobic structures [21]. Wormlike micelles can act as templates to provide the structured aggregates required for polymer growth and thus promote the formation of more ordered polymeric structures. In addition, the dispersant may modify the polymer surface and thereby increase the spatial tension of the main chain, enhancing the internal energy of the polymer and altering its physicochemical properties and application performance. The structure and properties of the surfactant and resulting polymers were fully characterized using various analytical techniques. These worm micelles can be switched by electrical, optical, thermal or pH stimulation, and have a wide range of applications in oil fields, biomedicine, cleaning processes, and other fields [5].

2. Experimental Section

2.1. Materials

Analytically pure (99%) oleic acid (OA), N,N-dimethyl-1,3-propanediamine (DMAPA), and allyl chloride (AC) were used in this study, along with industrial-grade acrylamide (AM; >98%) and cocamidopropyl betaine (CAB, 35%), a coconut oil-based amphoteric surfactant. The solvent was removed from raw CAB using a rotary evaporator to achieve a CAB concentration >98%. Sodium hydroxide, ammonium persulfate (APS), azobisisobutyronitrile (AIBN), ascorbic acid (Vc), sodium formate, ethanol, and acetone were also used in the study, AR (>99%) Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Distilled water was used in all experiments.

The following instruments were used in this study: Haake Mars 40 rotational rheometer (Carl Zeiss Thermoelectric, Karlsruhe, Germany), Avater370 Fourier transform infrared spectrometer (Perkin Elmer, Waltham, MA, USA), ADVANCEIII 400 MHz nuclear magnetic resonance (NMR) spectrometer (Brooke Spectral Instruments, Berlin, Germany), JEM-200CX transmission electron microscope (Japan Electronics JEOL, Tokyo, Japan), Quanta 450 environmental scanning electron microscope (National manufacturer FEI Co., Ltd., Hillsboro, OR, USA), ZNN-D6II electric six-speed viscometer; QBZY automatic surface tensiometer (Shanghai Fangrui Instrument Co., Ltd., Shanghai, China), RE-52A rotary evaporator (Shanghai Yarong Biological Instrument Co., Ltd., Shanghai, China), LGJ-12 vacuum freeze dryer (Beijing Songyuan Huaxing Technology Development Co., Ltd., Beijing, China), and XGJ-S digital high-speed stirrer (Qingdao Hongyulin Petroleum Instrument Co., Ltd., Qingdao, China).

2.2. Methods

2.2.1. Synthesis of Surfactant

The KS-3 reactive functional surfactant monomer was synthesized from OA, DMAPA, and AC via a two-step reaction (Figure 1). This synthesis was performed in an acetone–ethanol mixture at a solvent concentration of 70 wt.% at an OA-to-DMAPA molar ratio of 1:1.1 in the presence of a KF/γ-Al₂O₃ catalyst (0.1 mmol). The reaction was conducted at 120 °C under a protective N₂ atmosphere for 12 h to obtain an oleic acid amide propyl dimethyl tertiary amine (PKO) intermediate. Subsequently, the system was cooled to 80 °C and subjected to a quaternization process in which AC was added dropwise to the mixture. The AC:OA molar ratio was maintained to be the same, and the reaction continued for 8 h. Evaporation of the solvent yielded the reactive functional surfactant monomer KS-3.

2.2.2. Polymer Synthesis

An aqueous AM solution (30 wt.%) was prepared and added to the 1 wt.% KS-3 monomer, using CAB as a dispersant. The pH of the system was adjusted to 6.5–7, and the temperature was reduced to 0–5 °C before the system was deoxygenated with N₂ for 30 min. Sodium formate was added as a chain transfer agent to produce an oxidation–reduction initiation system with a m(HCOONa):m(APS):m(Vc):m(AIBN) weight ratio of 3:4:6:3. After 2 min of deoxygenation, the reaction proceeded under adiabatic conditions for 6 h to yield a transparent gel block. This block was ground using a homogenizer and dried at 75 °C for 4 h, extracted and purified with a 30% ethanol solution for 12 h, and subsequently dried at 75 °C to obtain the RES polymer. When there is no interaction between CAB and KS-3, that is, the actual reaction is KS-3 and AM, the reaction product is RES-1. When CAB and KS-3 are partially mixed, the product is named RES-2. When CAB and RES-3 reach a proper ratio in the mixing process (such as CAB:RES-3 = 1:4 or 1:1), the product is named RES-3.

2.3. Characterization

2.3.1. Rheological Analysis

Thixotropicity Measurements: Rheological measurements were conducted using a HAAKE Mars40 rheometer to evaluate the strength of the polymer network structure. An aqueous solution (0.8%) of the RES polymer was prepared and tested in both the upward and downward shear modes over a shear rate range of 0–500 s⁻¹ to characterize the thixotropic behavior of the polymers.

Viscoelasticity Measurements: The steady-state and dynamic rheological properties of the polymers were investigated at concentrations of 0.2%, 0.5%, and 0.8% using a cone fabricated from standard ETC steel. The cone diameter was 40 mm, cone angle was 2°, and the gap between the cone center and the plate was 48 mm. Stress scans (0.1–10 Pa) were conducted to determine linear viscoelastic regions. Frequency scans were obtained under a fixed shear stress over a frequency range of 0.1–10 Hz to further examine the linear viscoelasticity regions. The analyzed samples were equilibrated on the plate for 5 min before measurements to ensure sufficient accuracy, and the temperature was maintained at 30 °C throughout the experiment [1].

2.3.2. Microstructural Analysis

Transmission Electron Microscopy of RES and Surfactant: The micellar state of the surfactant in the monomer solution and the microstructure of the hydrophobically associated RES in an aqueous solution were examined at a concentration of 0.3%. The samples were dried on a 200-mesh copper grid and characterized by transmission electron microscopy (TEM). An analytical basis for the unique rheological properties of the studied materials was established by comparing the structures of the monomer and polymer.

Scanning Electron Microscopy of RES: The morphology of the RES specimens was observed by environmental scanning electron microscopy (ESEM). Samples were prepared by stirring RES in pure water at a shear rate of 170 s⁻¹ for 90 min, following which a drop of the sample was placed on a conductive adhesive surface. This system was freeze-dried using liquid nitrogen. The frozen surfaces of the samples were observed by ESEM at an accelerating voltage of 20 kV [7].

3. Results and Discussion

Based on literature research, we found that researchers usually use aqueous free-radical polymerization to graft hydrophobic functional monomers with acrylamide and study the aqueous solution properties, and at the same time add surfactants to the aqueous solution of hydrophobic aggregates to study the aggregation behavior between the two; in this study, based on the literature research and experimental work, the synergistic effect between the surfactants and the coconut oil was utilized. This study is based on literature research and experimental work. Using the synergistic effect between surfactants, cocoamidopropyl betaine (CAB) can solubilize the synthesized hydrophobic monomer KS-3 solution from spherical micelles to form worm micelles. Studying the changes of two surfactants in the micellar conversion and polymerization process in aqueous solutions of acrylamide monomers, using surfactants for modification of hydrophobic-conjugated polymers, and studying the differences in the polymer rheology will put forward a new idea for the research on the modification of hydrophobic-conjugated polymers.

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 (Figure 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

Figure 3 shows the ¹H NMR spectrum of KS-3; the chemical shifts of the various protons are listed in

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

. 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 (Figure 4a,b). At concentrations greater than CMCs in aqueous solutions (Figure 4c,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 Equation (1) was obtained [22]:

$$\ln (c_1^0/c_2^0)=-\beta \left(1-2X\right)$$

(1)

Here, $X$ is the mole fraction of KS-3 in the surface of the surfactant $X$₁ = 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 $\beta$ accounts for the difference between the mixed system and ideal state.

Based on Equation (1), a synergistic effect between the surfactants is observed when the absolute value of $\ln (c_1^0/c_2^0)$ is less than the absolute value of $\beta$. As $\beta$ becomes more negative $\beta$, the deviation from the ideal state, strength of the intermolecular interactions, and observed synergistic effect increase (Figure 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.

As shown in Figure 6, when the mixing ratio of the two surfactants is 4:1 (Figure 6a) and 1:1 (Figure 6b), the particle size distribution becomes uneven due to their synergistic effect. The different particle size distributions after the addition of dispersant CAB showed that the inhomogeneous micelle structure was formed.

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 Figure 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 (Figure 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 Figure 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 Figure 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 [23].

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 (Figure 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 (Figure 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 (Figure 11b,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 (Figure 12). The polymer molecules formed a three-dimensional (3D) pseudo-spatial structure [24]. 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 (Figure 12a). Concurrently, the strong cohesive effect of KS-3 was localized within the molecule, producing closed chains in an aqueous solution (Figure 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 (Figure 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 viscosity-temperature properties of 0.8%RES polymer are tested. As shown in the Figure 13, in the initial stage, the viscosity of black curve RES-1 and pink curve RES-2 increases with the progress of shearing. The reason is that the spherical micelle is grafted on the polymer molecule, which is easy to form large hydrophobic micro-region intramolecular association. With the action of temperature and shear force, the intramolecular association gradually changes to intermolecular association, and the hydrodynamic volume increases. Macroscopically, the viscosity increases. The viscosity-temperature curve of RES shows that the polymer has a certain dependence on the change of temperature. When the temperature increases, the viscosity of the system decreases. Among them, the hydrophobic group in the blue curve RES-3 can form a stronger side-group association structure, which increases the rigidity of the molecular chain, so the viscous flow activation energy of the system is higher and the temperature resistance of the system is better.

3.10. Thixotropic Analysis of RES

The network structures of the studied polymers were characterized by thixotropic studies (Figure 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 (Figure 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 [25]. Specifically, G′ represents the elastic modulus and G″ represents the viscosity modulus. 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 [26]. 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 (Figure 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 < c_CAC, intramolecular binding was dominant, i.e., G′ < G″, and the molecules were mostly viscous. At c > c_CAC, 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 < c_CAC), intramolecular bonds were transformed into intermolecular bonds under the action of external stress, thereby increasing the degree of elasticity. When c > c_CAC, 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 (Figure 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 [27]. 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.

4. Conclusions

The aggregation behavior and micellar state of KS-3 and CAB in aqueous solution and AM monomer solution were studied. The two surfactants showed good synergistic effect. In particular, CAB increased the hydrophilic dispersion of KS-3, and the micelle morphology could be changed from spherical to worm-shaped when the ratio of CAB and PVA reached 1:1 or 1:4. By studying the aggregation of different micelles to form RES polymers, the microstructure of two kinds of surfactants was discussed. Compared with the polymer formed by spherical micelle, the steric hindrance of worm micelle increases the rigidity of polymer chain, and the increase in steric hindrance decreases the entropy of intermolecular rotation, which increases the heat of polymerization and improves the thermal stability of the product.

By studying the polymers formed by micelles with different aggregation forms, according to their unique microstructure and properties, the results are as follows: there is a strong hydrophobic repulsive force between monomers and cracks in the aggregates formed by spherical micelles; in contrast, the polymers synthesized by wormlike micelles have lower CACs and form a more compact spatial network structure. In the shear test, although the shear rates of the three RES polymers were different, all polymers produced shear thickening rings and formed a spatial network structure in aqueous solution. Polymer fluids have obvious viscoelasticity, and different states of polymerization form polymers with different structural strength and viscoelasticity.

Based on the above analysis, we can find that the surfactant significantly affects the polymerization and the properties of the polymer. Therefore, adjusting the synergistic effect of surfactants and polymers represents a new polymer modification strategy.