Synthesis of aminated polystyrene and its self- assembly with nanoparticles at oil/water interface

Nanoparticle (NP)–surfactants formed by the self-assembly of NPs and endfunctionalized polymers at the hydrophilic/hydrophobic interface have a wide range of Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 March 2020 doi:10.20944/preprints202003.0061.v1 © 2020 by the author(s). Distributed under a Creative Commons CC BY license. applications in many fields. In this study, the influence of density of amino groups, NPs dimension and pH on the interaction between end-functionalized polymers and NPs were extensively investigated. Single amino-terminated polystyrene (PS-NH2, Mw ≈ 0.6k, 2.5k, 3.5k, 3.9k) and diamino-terminated polystyrene (H2N-PS-NH2, Mw ≈ 1.1k, 2.8k) were prepared using reversible addition–fragmentation chain transfer polymerization and atom transfer radical polymerization. NPs with different dimensions (zero-dimensional carbon dots with sulfonate groups, one-dimensional cellulose nanocrystals with sulfate groups and two-dimensional graphene with sulfonate groups) in the aqueous phase were added into the toluene phase containing the aminated PS. The influence of pH and the molecular weight of amino-terminated PS on the interfacial tension between two phases were investigated. The results indicate that aminated PS exhibited the strongest interfacial activity after compounding with sulfonated NPs at a pH of 3. Terminating PS with amino groups on both ends leads to better performance in in reducing the water/toluene interfacial tension than modifying the molecular structure of PS on a single end. The dimension of sulfonated NPs also contributed significantly to the reduction of the water/toluene interfacial tension. The minimal interfacial tension was 4.49 mN/m after compounding PS-NH2 with sulfonated zero-dimensional carbon dots. Molecular dynamics simulation on the evolution of the water/toluene interface in the presence of sulfonated carbon dots and H2N-PS-NH2 revealed that these opposite charged substances moved towards the interface in an extreme short time and orderly assembled in a thermodynamic equilibrium.


Introduction
Nanoparticles (NPs) with a responsive behavior to external forces, e.g. magnetic fields [1] , electric fields [2] and ultraviolet radiation [3] , have been widely studied for their ability of self-assembly at oil/water (O/W) interfaces. It was also found that the formulation of NPs and end-functionalized polymers, which can be individually used as surfactants, can strengthen the O/W interfacial activity [4] by forming complex NPs/surfactants structures driven by electrostatic structures. The orderly assembly of NPs/surfactants at O/W interface improve the compatibility between the O/W phases, allowing promising applications in fields including medical pharmaceuticals [5] , food processing [6] , enhanced oil recovery [7] , and inkjet printing [8] .
The O/W interfacial energy can be regulated by the pH of the aqueous solution, the molecular weight, the concentration of the end-functionalized polymer and the external electric field [9] . Huang et al. [10] investigated the effect of pH on the in situ formation of NPs (carboxylated polystyrene, PS-COOH)/surfactants (aminated polydimethylsiloxane, PDMS-NH2) and found an optimal pH of 4.66. Liu et al. [4] used CNCs and PS-NH2 (Mw = 1.5 k, 2.5 k, 13 k, 25 k, 40 k) as NPs in water and as the surfactant in toluene, respectively, and reached the conclusion that the molecular weight of PS-NH2 exerted a considerable influence on the interfacial behavior of NPs/surfactants. When Mw = 1.5 k, PS-NH2, interacting with CNCs, reduced the O/W interfacial tension to below 7 mN/m. As the Mw increased, the interfacial activity of NPs/surfactants decreased; the O/W interface was seldom affected when the Mw was higher than 25 k. The molecular weight of PS-NH2 also influenced the formation process of NPs/surfactants. Although PS-NH2 (Mw = 13 k) and PS-NH2 (Mw = 2.5 k) exhibited a similar ability to reduce the O/W interfacial tension in the presence of CNCs, it took a longer time for the former/CNCs to reach interfacial equilibrium. The assembly of carboxylated SiO2 as NPs in water and PDMS-NH2 as the surfactant in silicon oil also reduced the O/W interfacial tension, according to Chai et al [11] . The introduction of salt increased the ionic strength of water and further decreased the O/W interfacial tension. The observation of high-speed image analysis experiments indicated that the addition of NaCl enhanced the packing density of NPs at the O/W interface and therefore improved the assembly of NPs/surfactants. Toor et al. [12] used carboxylated silica as NPs in water and PDMS-NH2 as surfactants in N,N-Dimethylformamide (DMF) to investigate the effect of polymer concentration on the interfacial properties.
The polar/nonpolar interfacial activity of NPs/surfactants benefited from the increase in polymer concentration, and the interfacial tension was reduced to 10 mN/m. They also demonstrated that PDMS-NH2 exhibited little interfacial activation in the absence of carboxylated NPs [13] . Cui et al. [14] used PS-COOH as NPs in water and PDMS-NH2 in the silicone oil as the surfactants. By analyzing the shape of the droplets, the authors revealed the formation process of NPs/surfactants at the O/W interface in an external electric field, including the diffusion of NPs and surfactants to the O/W interface as well as the electrostatic interaction mechanism.
The above research achieved a considerable reduction in O/W interfacial tension via a series of methods. Most of the end-functionalized polymers reported in these papers were aminated. Adjusting the pH of water had a profound influence on both the protonation of amino groups and the electrostatic interaction between oppositely charged groups of NPs and polymers. However, the effect of two terminal functionalized polymers on the assembly of NPs/surfactants, and the relationship between the dimension of NPs and the interaction between NPs and end-functionalized polymers are not discussed in the previous papers. In addition, to the best knowledge of the authors, little research has been done on the simulation of the arrangement of NPs/surfactants in the O/W interface.
To reveal the influence of the number of amino groups in end-functionalized polymers and the dimensional of NPs, two kinds of aminated PS and three kinds of NPs (zero-dimensional sulfonated carbon dots (CDs-SO3H)), one-dimensional sulfated cellulose nanocrystals (CNCs-OSO3H), and two-dimensional sulfonated graphene oxides (GOs-SO3H) were used to test the O/W interfacial tension in this study.
Molecular simulations on the interaction between aminated PS and sulfonated carbon dots were also carried out to reveal the assembly of NPs/surfactants at the O/W interface.

Sample Preparation
Triethylamine, 1-butanethiol, N-(bromomethyl)phthalimide, tributylstannane, potassium phthalimide, 2,2'-bipyridine, citric acid and sulfanilic acid were purchased from TCI and used as received. Copper (I) bromide was obtained from Aldrich and was purified by stirring in acetic acid, washed with ethanol and then stored in argon. Aladdin nm) were obtained from Shanghai ScienceK Nanotechnology Co., LtD (China) and Shandong Jincheng Graphene Technology Co., LtD (China), respectively. Other agents were purchased from Aladdin and used as received.

Synthesis of chain transfer agent (1)
The compound was synthesized following the procedures reported by Postma [15]. A mixture of carbon disulfide (6.07 g) and 1-butanethiol (3.57 g) in chloroform (25 mL) was prepared with stirring, and triethylamine (8.21 g) was then added. The reaction lasted for 3 h at room temperature, and N-(bromomethyl)phthalimide (9.59 g) was then added in portions over 0.5 h. The mixture was stirred at room temperature for 16 h.
Chloroform (20 mL) was added and the organic layers were washed successively with water, H2SO4, water and brine. Then, the organic layers were dried over magnesium sulfate, and the solvent was subsequently filtered and removed to provide a crude solid that was crystallized from methanol. Yield = 11.87 g (88.28%).

Synthesis of modified PS via RAFT (2a)
Styrene (30.12 g), chain transfer agent 1 (2.92 g) and AIBN (1.71 g) were added into a three-neck flask. The flask was sealed and pumped to vacuum (= 10 -3 Torr), and the mixture was purged with argon for 30 min. The mixture was stirred at 60 °C for various durations (4, 8, 12, 24 h). After the reaction, the mixture was cooled to room temperature and added to methanol to obtain yellowish modified PS. The 1 H NMR spectrum of 2a is shown in Figure 1.

Removal of Trithiocarbonate (2b)
Modified PS 2a (3.03 g), tributylstannane (5.24 g), AIBN (0.39 g) and toluene (20 mL) were added to a three-neck flask. The flask was purged with argon (20 min) three times and heated in an oil bath at 70 °C for 3 h. After the mixture was cooled to room temperature, it was concentrated by rotary evaporation. The concentrated solution was poured into methanol to obtain the product 2b. The product was dried at 50 °C under vacuum (= 10 -3 Torr).

Synthesis of PS-NH2 (2c)
Modified PS 2b (1.35 g), hydrazine hydrate (0.34 g) and DMF (25 mL) were added to a three-neck flask. The reaction was carried out with stirring at 80 °C for 12 h in the protection of argon. The mixture was added into methanol. The precipitate was washed with deionized water and then with brine, and its color changed from yellow to white.
Finally, the product 2c was obtained. The chemical structure of 2a, 2b and 2c was characterized by 1 H NMR, and the spectra are shown in Figure 1. previous papers [15] .

Synthesis of PS via ATRP (3a)
The synthesis of PS via ATRP was carried out following a previous paper [16] . CuBr (0.83 g), styrene (15 g), 2,2'-bipyridine (1.78 g) and N-(bromomethyl)phthalimide (2.35 g) were added into a three-neck flask. The flask was purged with argon (20 min) three times and was then heated in an oil bath at 110 °C for various durations (1.5, 3 h). The mixture was cooled to room temperature, diluted with THF (25 mL), and passed through a neutral alumina column to remove the copper catalyst. Then, the product was precipitated from methanol and dried under vacuum (= 10 -3 Torr). A mixture of potassium phthalimide (2.13 g), PS 3a (2.51 g) and DMF (30 mL) was heated at 80 °C for 12 h under argon. Then, the mixture was added to water, and the product 3b was prepared by filtration [17] .

Synthesis of H2N-PS-NH2 (3c)
Product 3b (1.83 g), hydrazine hydrate (1.05 g) and DMF (25 mL) were added to a three-neck flask. The mixture was stirred at 80 °C for 16 h under argon. The mixture was added to methanol, and the precipitate was washed with deionized water and brine.
Then, H2N-PS-NH2 3c was obtained. The chemical structure of 3a, 3b and 3c was characterized by 1 H NMR, and the spectra are shown in Figure 2.

Synthesis of CDs-SO3H
Citric acid (8.01 g) was added to deionized water (100 mL). The mixture was sonicated and transferred to a hydrothermal kettle. The reaction was carried out at 200 Gel permeation chromatography (GPC) analyses were performed on a Waters 1515 instrument equipped with a MIXED 7.5×50 mm PL column, two MIXED-C 7.5×300 mm columns and a differential refractive index detector. Tetrahydrofuran (THF, HPLC grade) was used as the eluent at 35 °C with a flow rate of 1 mL min -1 .
The O/W interfacial tension was measured by a KRÜSS DSA25 using the pendant drop method. CDs-SO3H (10 mg/mL, pH = 3, 9), CNCs-OSO3H (10 mg/mL, pH = 2, 3,4,5,6,9) and GOs-SO3H (10 mg/mL, pH = 2, 3, 4, 5, 6,9) were added to deionized water as the aqueous phase, and PS-NH2 (1 mg/mL) and H2N-PS-NH2 (1 mg/mL) were dissolved in toluene as the oil phase. When the aqueous droplet contacted the oil phase, the interfacial tension was measured and the time was marked as 0 s. The intervals of the subsequent tests were every 15 s in the first 1 min and every 30 s after that. The experiments were carried out at 25 °C.

Molecular dynamics (MD) simulation
MD methods were used to simulate the interface between H2N-PS-NH2 in the toluene phase and CDs-SO3H in the water phase. The molecular model used in this study is illustrated in Figure 3, molecules. We also used C60 spheres to simulate quantum dots; therefore, the CDs-SO3H are represented by negatively charged C60-(SO3 -)7. Both the toluene and the water phase were set to be electronically neutral by adding counter ions of Cland H3O + .
These two phases were sandwiched by two layers of graphene, and a vacuum space of 24 Å was added between the graphene layers to minimize electrostatic interactions between neighboring images in the z-direction. MD simulations were performed in the constant-volume and constant-temperature (NVT) ensemble with a Nosé-Hoover thermostat at 298 K [18] . A cutoff radius of 15.5 Å and time step of 1 fs were used. The universal forcefield was used in this work [19] .
During the simulations, the graphene layers were fixed.

Results and Discussion
The molecular weight and polydispersity (PDI) of PS-NH2 and H2N-PS-NH2 are summarized in Table 1. The gravimetric conversion rate of aminated PS increased with the reaction time, and the PDI was well controlled.   alone had no interfacial activity, as shown in Figure 4  The results indicate that the pH of the aqueous solutions exerted a significant effect on the formation rate and interfacial activity of NP-surfactants. At pH = 9, amine groups were rarely protonated, and their interaction with negatively charged groups on the NPs through the electrostatic force was not intensive, thus failing to exhibit high interfacial activity. As the pH decreased, the protonation of amine groups increased dramatically, resulting in a strengthening interaction between positively charged surfactants and negatively charged NPs. NPs/surfactants assembled and deformed the O/W interface, and the interfacial tension decreased. Meanwhile, it can be observed that the O/W interfacial tension reached a minimum at pH = 3.
The reason for this may be that, although the ratio of primary amino protonation at pH = 2 is higher than that at pH = 3, the lower pH also inhibits the ionization of negatively charged groups in NPs partially combined with H + rather than NH 3 + , leading to a slight decrease in interfacial activity, as the pKa of methanesulfonic acid ranged CDs-SO3H, thereby reaching a lower O/W interfacial tension [9] . The dimension of NPs (CDs-SO3H of zero dimensions, CNCs-OSO3H of one dimension, GOs-SO3H of two dimensions) also affected the O/W interfacial tension, as shown in Figure 6.   Therefore, the MD simulation suggests that the system of the phase interface can return to the equilibrium state after a small perturbation. The MD simulation can serve as a proof of the thermodynamic stability of the NPs/surfactants interface but not a reflection of the dynamic process of phase interface formation in the macroscope time and length scale, the latter takes

Conclusions
In this study, PS-NH2 with different molecular weights was synthesized by RAFT followed by chemical modification. H2N-PS-NH2 was prepared by ATRP and then modified by the Gabriel method. The factors determining the O/W interfacial tension of the compounding between aminated polystyrene and nanoparticles include the pH of aqueous phase, molecular weight of aminated PS, the density of amine groups and NPs dimension. The grafting density of amine groups in polystyrene and the dimension of nanoparticles play important roles, other than the pH and the molecular weight of functionalized polymers suggested by previous works [7,9]  which is a significant improvement compared with previous results [5,7,20] . The results suggest that the density of amine groups has a positive correlation with the performance of aminated PS. NPs with a smaller dimension also contribute to the lowering of O/W interfacial tension. The molecular simulation results of the evolution of the distribution of NPs and aminated PS at the O/W interface revealed that CDs-SO3H and H2N-PS-NH2 reaching a distribution equilibrium within a considerably short time.
The fast and significant reduction in O/W interfacial tension induced by NPs/surfactants assembly could be applied in both enhanced oil recovery and inkjet printing. Based on the result of this study, we plan to carry out a further research on both increasing the number of amino groups in PS and decreasing the size of CDs-SO3H, aiming at lowering the O/W interfacial tension by more than an order of magnitude.

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