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Emulsion Polymerization with a Biosurfactant Aya Kurozuka, Shohei Onishi, Takuto Nagano, Katsumi Yamaguchi, Toyoko Suzuki, and Hideto Minami Langmuir, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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Emulsion Polymerization with a Biosurfactant

Aya Kurozuka,† Shohei Onishi,† Takuto Nagano,‡ Katsumi Yamaguchi,§ Toyoko Suzuki,† Hideto Minami†*

†

Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501,

Japan ‡

New Business Development Division, Kaneka Corporation, 2-3-18, Nakanoshima,

Kita-ku, Osaka 530-8288, Japan §

High Performance Polymers Division, Kaneka Corporation, 1-8, Miyamae-cho,

Takasago, Hyogo 676-8688, Japan

E-mail: [email protected], TEL&FAX: (+81) 78 803 6197

KEYWORDS Emulsion polymerization; Surfactin; Biosurfactant; Mechanism of polymerization

ACS Paragon Plus Environment

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Abstract Emulsion polymerization of styrene was conducted using a biosurfactant (i.e., sodium surfactin, hereinafter called just “surfactin”) having very low critical micelle concentration (CMC, 2.9 ×

10−3 mmol/L) and biodegradability

characteristics. The nucleation mechanism was investigated by comparing with a conventional surfactant (i.e., sodium dodecyl sulfate, SDS) system. Unlike the emulsion polymerization systems using conventional surfactants, nucleation mechanisms changed above CMC in the presence of a biosurfactant. At low concentrations of surfactin (above CMC), the polystyrene (PS) particles are likely generated via a soap-free emulsion polymerization mechanism. In contrast, at high surfactin concentrations, the PS particles would be synthetized by following a micellar nucleation mechanism. However, the slope (0.23) of the logNp versus logCs plot (Np: number of particles; Cs: concentration of surfactin) did not obey the Smith– Ewart theory (0.6), this probably being produced by the high adsorbability of surfactin.

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Introduction Emulsion polymerization is one of the most important and developed processes for manufacturing polymers such as adhesives, paints, coatings and films materials.1–4 Emulsion polymerization achieves high molecular weight polymers at higher rates as compared to other polymerization methods. Emulsion polymerization is environmentally friendly because it uses water as a reaction medium without employing large amounts of volatile organic compounds. In addition, emulsion polymerization provides nano- and submicron-sized polymer particles, which have recently received significant attention as particulate materials.5–8 Despite playing an essential role in nucleating and stabilizing the particles obtained

during

emulsion

polymerization,

surfactants

are

responsible

for

environmental pollution and originate poor water-resistant films from the emulsion.9 Thus, decreasing the amount of surfactants during emulsion polymerization is required. Soap-free emulsion polymerization is one of the eco-friendly alternative methods by which highly monodispersed particles can be obtained without the addition of surfactant.10–12 However, soap-free emulsion polymerization is significantly difficult to implement in the industry because of the low polymerization 2


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rates and poor stability of the obtained emulsion. Moreover, lower solid contents is required to obtain stable emulsion as compared to conventional emulsion polymerization.6 With the aim to decrease the amount of surfactant during emulsion polymerization, we employed herein sodium surfactin (i.e., surfactin). Surfactin is a biosurfactant produced by strains of bacillus subtillis13 and composed of a cyclic peptide formed by seven amino acids (hydrophilic part) and a carbon hydride (hydrophobic part).14,15 Surfactin has distinctive properties such as biodegradability (surfactin degrades in a sea within one week), low skin irritation, and a very low critical micelle concentration (CMC) value of 2.9 × 10−3 mmol/L, which is calculated from catalog value: CMC 0.0003 wt%. Therefore, the utilization of surfactin during emulsion polymerization is expected to drastically decrease the amount of surfactant while maintaining high polymerization rates and stability of the obtained emulsion. Emulsion polymerization using surfactin as an emulsifier was conducted, and the polymerization mechanism was discussed by comparing with a conventional surfactant (i.e., sodium dodecyl sulfate, SDS) system.

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Experimental part Materials Styrene (Nacalai Tesque Inc., Kyoto, Japan) was purified by distillation under reduced pressure in a nitrogen atmosphere. Analytical grade potassium persulfate (KPS, Nacalai Tesque Inc., Kyoto, Japan) was purified by recrystallization in water. Sodium surfactin (surfactin, Kaneka Co., Japan, Figure 1), sodium dodecyl sulfate (SDS, 95%, Wako Pure Chemical Industries, Ltd., Japan), sodium hydrogen carbonate (NaHCO3, 99.5%, Nacalai Tesque Inc., Kyoto, Japan), deuterium oxide (D2O,

99.90%,

Nacalai

Tesque

Inc.,

Kyoto,

Japan),

and

3-(trimethylsilyl)-1-propaneslufonic acid sodium salt (DSS, Nacalai Tesque Inc., Kyoto, Japan) were used as received. The deionized water used in all experiments was obtained from an Elix® UV (Millipore, Japan) purification system and had a resistivity of 18.2 MΩ cm.

Preparation of polystyrene (PS) particles by emulsion polymerization Emulsion polymerization of styrene was carried out at 70 °C for 8 h under a nitrogen atmosphere in a four-necked 200 mL round-bottom flask equipped with an 4


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inlet of nitrogen gas and a reflux condenser. Water (115 g), the emulsifier (surfactin: 3.2 × 10−3 mM ~ 23.5 mM or SDS: 8.0 mM ~ 48 mM)) and NaHCO3 (0.05 g) were added to the reactor, which was subsequently stirred with a half-moon type stirrer at 240 rpm under a nitrogen atmosphere. NaHCO3 was used to adjust the pH between 7.2 and 8.0 since surfactin operates effectively at pH values between 6.5 and 8.0. Once water reached a temperature of 70 °C, styrene (6 g) was poured into the reactor. The mixture was deoxygenated by flowing nitrogen gas for 15 min while stirring at 240 rpm. Subsequently, a solution of KPS (72 mg) in water (5 g) was added to the reactor to initiate polymerization. The conversion was measured by a gravimetric method.

Preparation of PS particles by soap-free emulsion polymerization Soap-free emulsion polymerization was carried out at 70 °C for 8 h under a nitrogen atmosphere in a four-necked 1,000 mL round-bottom flask equipped with an inlet of nitrogen gas and a reflux condenser. Water (538 g) was added to the reactor, which was stirred with a half-moon type stirrer at 240 rpm under a nitrogen

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atmosphere. After the water was heated to 70 °C, styrene (42 g) was poured into the reactor. The mixture was deoxygenated with flowing nitrogen for 15 min while stirring at 240 rpm, after which a solution of KPS (1.0 g) in water (20 g) was added to the reactor to initiate polymerization.

Characterization of the PS particles The PS particles were characterized by transmission electron microscopy (TEM, JEM-1230, JEOL Ltd., Japan, 100 kV) observations. A drop of each emulsion was placed onto a carbon-coated copper grid and allowed to dry at room temperature in a desiccator. The number-average diameter (Dn) and coefficient of variation (Cv) were determined by counting over 100 particles in the TEM images by using an image analysis software (WinROOF, Mitani Co., Ltd., Japan). The numbers of particles (Np) were determined from average particle volume (which was calculate from Dn) and monomer conversion.

The pH values of the emulsion before and after

polymerization were measured by a pH meter (HM-55, DKK-TOA Co., Japan). The zeta potential of the obtained particles was determined using a zeta potential

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analyzer (ELSZ, Otsuka Electronics Co., Ltd., Osaka, Japan)

Measurement of the amount of adsorbed emulsifier The medium used for soap-free emulsion polymerization of PS emulsions was replaced with D2O (+0.02% DSS) via centrifugation, and varying amounts of surfactin or SDS were added to the emulsion. The emulsion was left overnight, and the medium was separated by centrifugation. The concentration of free (i.e., unadsorbed) emulsifiers in the medium was measured by 1H nuclear magnetic resonance (NMR) spectroscopy, and the amount of adsorbed emulsifier was calculated by subtracting the amount of free emulsifiers detected by 1H NMR from the amount of emulsifier added.

+ 3/2 Na+ + 1/2 H+

Figure 1.

Structure of surfactin.

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Results & Discussion The CMC of surfactin is significantly low (2.9 × 10−3 mmol/L) as compared to conventional surfactants used for emulsion polymerization such as SDS (ca. 8.0 mmol/L). When emulsion polymerization was carried out at surfactin concentrations close to the CMC (4.8 × 10−3 mmol/L), highly monodispersed and submicron-sized PS particles were obtained (Figure. 2a). The obtained emulsion was colloidally stable without coagulation for at least 2 years (The obtained particles exhibited zeta potential of −25mV).

On the other hand, when emulsion

polymerization was carried out with SDS at concentrations close to its CMC value (8.0 mmol/L), unlike surfactin system, the PS particles obtained with SDS were a few tens of nanometers in size (Figure. 2b). The particles obtained by emulsion polymerization using surfactin were relatively similar to those prepared by soap-free emulsion polymerization (i.e., highly monodispersed and submicron-sized, Figure. 2c). In order to kinetically investigate the emulsion polymerization process, conversion–time plots were prepared for each system by a gravimetric method. As shown in Figure. 3, the polymerization rate during emulsion polymerization with

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SDS was very high and the polymerization was almost completed within 2 h (i.e., common emulsion polymerization behavior). On the other hand, the conversion of the monomers during both soap-free and emulsion polymerizations with surfactin was very slow. Although the polymerization of both soap-free and surfactin systems seemed to stop approximately at 75%, the residual monomer was not detected by gas chromatography after 6 h of reaction. We believe that styrene might have evaporated during polymerization as a result of the slow polymerization rate. This difference in the rate of polymerization might be produced by the different molar concentrations of the surfactants during emulsion polymerization (SDS was nearly 2,000 times more concentrated than surfactin). As surfactin has a gigantic head-group, the association number of surfactin would be smaller than that of SDS from the viewpoint of packing parameter. Even if the association number of surfactin is ten times lower than for SDS, the number of surfactin micelles would be considerably lower as compared to the SDS system around the CMC point. Therefore, emulsion polymerization

using surfactin

predominantly proceeded

via

homogeneous

nucleation (i.e., soap-free emulsion polymerization) over micelle nucleation (i.e.,

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emulsion polymerization). Indeed, the polymerization rate increased and approached to that of the SDS system while increasing the surfactin concentration. The size of the obtained particles decreased with the surfactin concentration from submicrons to nanometers (Figure. 4). These results suggest that the mechanism of particle formation shifted from “homogeneous nucleation” to “micelle nucleation” upon increasing the surfactin concentration.

Figure 2.

TEM photographs of PS particles prepared by emulsion polymerization with: (a)

surfactin (4.8 × 10−3 mM), (b) SDS (8 mM), and (c) soap-free emulsion polymerization.

−

Conversion–time plots for emulsion polymerization with surfactin (mM): (● , 4.8×10 3;

Figure 3. ▲,

0.07;

■

, 0.14; Vand

◆,

6.4), (＊) SDS (8 mM), and (×) soap-free emulsion polymerization of

styrene. 10


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Figure 4.

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TEM photographs of PS particles prepared by emulsion polymerization with various −

−

−

−

concentration of surfactin (mM): (a) 4.8×10 3 ; (b) 8.1×10 3 ; (c) 4.0×10 2; (d) 7.0×10 2 ; (e) −

9.7×10 2; (f) 0.14; (g) 0.21; (h) 0.8; (i) 6.4; and (j) 9.6.

In order to clarify the polymerization mechanism of the surfactin system, the number of particles (Np) at various concentrations of surfactin was compared with the Smith–Ewart theory.16 According to this theory, the relationship between Np and the surfactant concentration (Cs) is defined as Np∝[Cs] 0.6. Thus, the slope in the logNp versus logCs plot is approximately 0.6 in conventional emulsion polymerizations. Figure. 5 represents the logNp and the logarithm of the coefficient of variation (Cv) as a function of the logarithm of the concentration of surfactin (CSurfactin:3.0 µM–23.5mM) for a surfactin system. According to the slope of the

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logNp–logCSurfactin plot, three different mechanisms can be proposed. This behavior is very different to that of the emulsion polymerization systems using conventional surfactants. Three different slopes were obtained: 0.37 (region I), 1.48 (region II), and 0.23 (region III). In region I (i.e., surfactin concentration below 0.04 mM), submicron-sized and monodispersed PS particles were obtained, thereby indicating that the mechanism for particle formation in this region was similar to that of conventional

soap-free

emulsion

polymerization.

In

region

II

(surfactin

concentrations: 0.04–0.8 mM), the number of particle was drastically increased (the size of the obtained particles was decreased). According to the TEM observations, PS particles with two different sized were present in this region, thereby revealing a transition from homogeneous nucleation (i.e., soap-free emulsion polymerization) to micellar nucleation (i.e., emulsion polymerization) during the formation of PS particles (at the early stage). The PS particles obtained in region III (surfactin concentrations above 0.8 mM) were a few tens of nanometers in size with a broad size distribution. This distribution was similar to that obtained with SDS in conventional emulsion polymerization processes, and the polymerization should

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proceed following an emulsion polymerization mechanism. However, the slope obtained in the Np vs Cs plot was different to 0.6 (0.23), thereby implying that the emulsion polymerization mechanism using surfactin did not obey the Smith–Ewart theory.

Figure 5.

Particle number (log-scale) (● ) and coefficient of variation (○ ) versus the surfactin

concentration (log-scale).

Apart from its very low CMC, surfactin has very low surface tension (27.2 mN/m at 2.7 × 10−2 mmol/L), which indicates that this compound has high adsorbability to oil/water interfaces. This characteristic of surfactin was attributed to

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the formation of a β-sheet structure via hydrogen bonding interaction as a result of the ring structure of the hydrophilic part.17,18 The different behavior in region III shown in Figure. 5 could be explained in terms of the high adsorbability of surfactin as compared to SDS. With the aim to quantify this adsorbability, the amount of unadsorbed emulsifier was measured by 1H NMR in D2O dispersing PS particles prepared by soap-free emulsion polymerization. The amount of unadsorbed emulsifier was significantly lower for surfactin as compared to SDS (Figure. 6a). The amount of adsorbed emulsifier was calculated by subtracting the amount of unadsorbed emulsifiers (Figure. 6b). Considering the occupation area of one molecule of surfactin,19 the amount of this compound required to totally cover the surface area of the PS particles was calculated to be ca. 8.0 × 10−3 mmol, thereby indicating that surfactin was mostly adsorbed on the PS particles. As mentioned above, this high adsorbability might originate from the hydrogen bonding interaction between the ring structure of the hydrophilic parts of surfactin. With the aim to clarify this hypothesis, the ring structure of surfactin was broken via hydrolysis resulting in the synthesis of linear surfactin. When using linear surfactin, the amount

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of unadsorbed emulsifier and the adoption ratio showed a similar behavior as compared to the SDS system. In addition, the slope of the logarithmic scale plots (0.61) was consistent with that dictated by the Smith–Ewart theory (0.6) (Figure. 7).

Figure 6.

Relationship between: (a) the charged and unadsorbed emulsifiers and (b) the adsorption

ratio of (●) surfactin, (○) linear surfactin, and (×) SDS. 15.5

Log(particle number) (-)

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15.0

0.61

14.5 -2.5

-2.0

-1.5

Log(concentration) (-)

Figure 7.

Log-log plots of the particle number versus the concentration of linear surfactin.

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The less significant effect of the surfactin concentration on Np, as revealed by the Np vs Cs slope of 0.23, might be also explained by the high adsorbability of this compound. During emulsion polymerization, a large number of micelles, which is influencing the number of particles, and monomer droplets exist in the aqueous medium. However, owing to its high adsorbability, the amount of surfactin adsorbed on the monomer droplets cannot be neglected, and this would induce the decrease of the number of micelles in the aqueous medium. With the aim to decrease the surface area of the monomer droplets, emulsion polymerization with surfactin was carried out at the same conditions described above, except low stirring rate (Figure. 8). At these conditions, instead of being dispersed as droplets, the styrene phase floated as a layer over the aqueous phase. At low stirring rates (50 rpm), the size of the obtained particles dramatically decreased (27 nm) as compared to that obtained at 240 rpm. These results also suggested that the high adsorbability of surfactin can affect the emulsion polymerization mechanism.

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(a) 240 rpm

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(b) 50 rpm

100 nm Dn = 46 nm

Dn = 27 nm Cv = 23%

Cv = 28%

Figure 8.

100 nm

TEM photographs of PS particles prepared by emulsion polymerization with surfactin at:

(a) 240 rpm and (b) 50 rpm.

Moreover, this high adsorbability would be expected to contribute to increase the stability of the high solid systems, which is very important for industrial applications. Emulsion polymerization of styrene at high solid contents (40 wt%) was carried out. In the case of emulsion polymerization with SDS, 80 mmol/L of this compound were required to stabilize the emulsion during polymerization and avoid coagulation. On the other hand, in the case of emulsion polymerization with surfactin, a stable emulsion was obtained with only 8 mmol/L of emulsifier (i.e., ten times lower as compared to SDS).

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Conclusion We demonstrated emulsion polymerization of styrene using surfactin as an emulsifier. When compared to SDS, the polymerization mechanism of surfactin was significantly different, and three different regions can be defined. In the region (I), the particles were formed by a conventional soap-free emulsion polymerization mechanism because of the low number of micelles. Region (II) lies in a transition area from soap-free emulsion to emulsion polymerization mechanisms. In region (III), the polymerization proceeded following an emulsion polymerization mechanism. However, this mechanism did not obey the Smith–Ewart theory likely because of the high adsorbability of surfactin. In addition, we obtained a stable emulsion with high solid contents (40 wt%) at low concentrations of surfactant via emulsion polymerization with surfactin. This process represented a significant reduction in the amount of surfactant as compared to the SDS system.

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